A Breakthrough in Respiratory Physiology: Inhaled Nitric Oxide Transported as SNO-Hb

Key Points

• Inhaled nitric oxide (NO) increases circulating levels of SNO-Hb, a bioactive form of NO

• Inhaled NO also increases circulating levels of nitrite, another NO metabolite

• The lungs might act as a reservoir of SNO-Hb, releasing it into circulation as needed

The Breathing Diabetic Summary

Inhaled nitric oxide (NO) has many systemic impacts.  An overview of these effects can be found here and here.  However, it has remained unclear how inhaled NO exerts these effects.  In general, inhaled NO is believed to react and become inactive after reaching the lungs.  Thus, conventional thinking is that the systemic effects of NO are due to improved gas exchange in the lungs, which then has positive downstream impacts.

Interestingly, despite its widespread clinical use, there have been very few studies testing this hypothesis to truly discover how inhaled NO exerts its systemic effects.  This paper sought to fill that gap.

To do this, they recruited 15 healthy volunteers.  They had them inhale NO at concentrations of 40 ppm (the maximum produced in the paranasal sinuses is on the order of ~20 ppm, but typically much less).  They inhaled the added NO for 15 minutes.  Blood samples were collected before inhalation, at the end of the 15 minutes of inhalation, and then at 5, 15, and 30 minutes post-inhalation. 

A Breakthrough in Cardio-Respiratory Physiology

The results were striking.  They found that NO inhalation significantly increased circulating levels of SNO-Hb and nitrite.  This is important because SNO-Hb plays a significant role in whole-body oxygenation.  A 2015 PNAS study discovered that SNO-Hb “senses” areas of low oxygen, and then releases bioactive NO to increase blood flow and oxygenation.  This discovery led to breathing be considered as a three-gas system involving oxygen, carbon dioxide, and NO.  Thus, if inhaling NO increases SNO-Hb, it could be playing a critical role in whole-body oxygenation.  This gets even more intriguing (see next two sections), but first, let’s cover their nitrite finding.

They also observed increases in circulating nitrite.  This is important because, like SNO-Hb, nitrite can also release bioactive NO in regions of hypoxia. However, nitrite can do this independent of the hemoglobin, thus providing a “back-up mechanism” for increasing blood flow in regions of low oxygen.

The Lungs as a Reservoir of SNO-Hb

An interesting finding from this study was that nitrite levels were most significant at the 5-min post inhalation mark.  In contrast, SNO-Hb continued rising throughout the 30 minutes.  This led the authors to believe that the lungs might be acting as an SNO-Hb reservoir, releasing it "as needed." 

Why These Findings Matter

When we breathe through our nose, we carry NO into the lungs (although not at concentrations as high as those studied here).  Based on these findings, we can now be reasonably confident this NO enters the bloodstream and is carried as SNO-Hb and nitrite.  Thus, breathing through your nose might not just improve gas exchange in the lungs.  It might also help make sure oxygen gets delivered where it is needed most throughout the body. 

Additionally, their finding that SNO-Hb levels continued increasing after NO inhalation is intriguing.  It might support the idea that nose breathing provides a baseline level of NO that keeps SNO-Hb in its normal range.  Then, when excess NO is inhaled, the body stores that "just in case."  This is speculative, but interesting to contemplate.

Finally, this is one study, and it’s relatively new.  We’ll need more to confirm/deny that NO inhalation consistently increases SNO-Hb and nitrite across different populations.  In the meantime, let’s keep breathing through our noses.  It may just be the key to whole-body oxygenation.

Abstract

Rationale: Inhaled nitric oxide (NO) exerts a variety of effects through metabolites and these play an important role in regulation of hemodynamics in the body. A detailed investigation into the generation of these metabolites has been overlooked. 

Objectives: We investigated the kinetics of nitrite and S-nitrosothiol-hemoglobin (SNO-Hb) in plasma derived from inhaled NO subjects and how this modifies the cutaneous microvascular response.

Findings: We enrolled 15 healthy volunteers. Plasma nitrite levels at baseline and during NO inhalation (15 minutes at 40 ppm) were 102 (86-118) and 114 (87-129) nM, respectively. The nitrite peak occurred at 5 minutes of discontinuing NO (131 (104-170) nM). Plasma nitrate levels were not significantly different during the study. SNO-Hb molar ratio levels at baseline and during NO inhalation were 4.7E-3 (2.5E-3-5.8E-3) and 7.8E-3 (4.1E-3-13.0E-3), respectively. Levels of SNO-Hb continued to climb up to the last study time point (30 min: 10.6E-3 (5.3E-3-15.5E-3)). The response to acetylcholine iontophoresis both before and during NO inhalation was inversely associated with the SNO-Hb level (r: -0.57, p = 0.03, and r: -0.54, p = 0.04, respectively).

Conclusions: Both nitrite and SNO-Hb increase during NO inhalation. Nitrite increases first, followed by a more sustained increase in Hb-SNO. Nitrite and Hb-SNO could be a mobile reservoir of NO with potential implications on the systemic microvasculature.

Journal Reference:

Tonelli AR, Aulak KS, Ahmed MK, Hausladen A, Abuhalimeh B, Casa CJ, Rogers SC, Timm D, Doctor A, Gaston B, Dweik RA. A pilot study on the kinetics of metabolites and microvascular cutaneous effects of nitric oxide inhalation in healthy volunteers. PLoS One. 2019 Aug 30;14(8):e0221777. doi: 10.1371/journal.pone.0221777. PMID: 31469867; PMCID: PMC6716644.

A Concise Review of Inhaled Nitric Oxide’s Systemic Impacts

Key Points

• The classical viewpoint that inhaled nitric oxide (NO) only has local effects cannot explain observations.

• For example, inhaled NO has many systemic effects, including the ability to selectively increase blood flow where it is needed most.

• SNO-Hb might be the likely candidate for how inhaled NO is transferred into the blood and transported throughout the body while retaining its bioactivity.

The Breathing Diabetic Summary

This paper presented a concise review of inhaled NO’s systemic effects.  So, I’ll keep the summary brief as well. 

The classical view that inhaled NO only has local effects in the airways and lungs is not supported by observations.  It turns out that inhaled NO has many systemic effects.  Notably, inhaled NO selectively increases blood flow where it is needed most.  Thus, our bodies have a way of using inhaled NO other than just in the airways and lungs.  It can also be transported to distant regions where blood flow is restricted, resulting in vasodilation and increased blood flow.  This was also shown in the Cannon et al. (2001) study.   

Here, as in that study and others, the precise mechanism for how this is done is unknown.  However, there is one pathway that has been brought up repeatedly, which is SNO-Hb.  As we learned in a 2015 PNAS study, SNO-Hb is critical to blood flow regulation and oxygen delivery.  It “senses” regions of hypoxia, releases bioactive NO, and improves blood flow to get more oxygen to the tissues.

The authors suspect that this is also the mechanism by which inhaled NO is selectively improving blood flow, stating that this pathway “likely represents an important mechanism by which inhaled NO can cause systemic effects.”  The difficulty is that SNO-Hb is hard to measure; therefore, there have been no conclusive studies to show that this is the mechanism by which inhaled NO works. 

Altogether, this paper shows that the traditional view of inhaled NO is not adequate to explain its systemic effects.  It’s selective vasodilating effects suggest that SNO-Hb is the mechanism by which inhaled NO is transported throughout the body.  Still, more studies are needed to support this hypothesis.

Abstract

Many effects of inhaled nitric oxide (NO) are not explained by the convention that NO activates pulmonary guanylate cyclase or is inactivated by ferrous deoxy- or oxyheme. Inhaled NO can affect blood flow to a variety of systemic vascular beds, particularly under conditions of ischemia/reperfusion. It affects leukocyte adhesion and rolling in the systemic periphery. Inhaled NO therapy can overcome the systemic effects of NO synthase inhibition. In many cases, these systemic-NO synthase-mimetic effects of inhaled NO seem to involve reactions of NO with circulating proteins followed by transport of NO equivalents from the lung to the systemic periphery. The NO transfer biology associated with inhaled NO therapy is rich with therapeutic possibilities. In this article, many of the whole-animal studies regarding the systemic effects of inhaled NO are reviewed in the context of this emerging understanding of the complexities of NO biochemistry.

Journal Reference:

Gaston B. Summary: systemic effects of inhaled nitric oxide. Proc Am Thorac Soc. 2006 Apr;3(2):170-2. doi: 10.1513/pats.200506-049BG. PMID: 16565427.

Evidence of Systemic Transport and Delivery of Inhaled Nitric Oxide

Key Points

• Inhaled nitric oxide (NO) is traditionally thought to only have local effects in the upper airways and lungs.

• However, this study found that inhaled NO can improve blood flow in distant regions when endothelial NO is suppressed. The measurements were consistent with systemic transport and delivery of inhaled NO.

• The effects of inhaled NO on systemic blood flow might be important in diseases that disrupt endothelial-derived NO (such as diabetes).

The Breathing Diabetic Summary

There are two primary sources of nitric oxide (NO) in the body: inhaled NO and endothelial-derived NO.

Inhaled NO is produced in the paranasal sinuses.  When you breathe through your nose, you bring this NO into your lungs, where it aids in blood flow redistribution and increases oxygen uptake.  However, it is traditionally thought that this NO only affects the airways and lungs; it is said to immediately react and lose its bioactivity.  Although there are many benefits of inhaled NO in the lungs, its journey ends there. 

Endothelial-derived NO, on the other hand, has systemic effects in the body, including improving whole-body blood flow and, especially, blood flow to the heart.  However, it is thought that there is a complete disconnect between these two sources of NO: Inhaled NO does not have systemic effects. 

But several studies suggest otherwise (see review here).  The reported systemic effects of inhaled NO imply it is somehow retaining its bioactivity and being transported throughout the body.  But, it’s now quite sure how. 

This study did something interesting to try to find out. They administered L-NMMA, which inhibits endothelial-derived NO from being produced. Then, they measured what happened to forearm blood flow under several conditions:

1. When participants breathed normal air.

2. During a handgrip exercise (which should increase blood flow).

3. During inhalation of extra NO (at 80 ppm) and repeat the two measurements (sitting still and the handgrip exercise). Note that 80 ppm is much higher than what is produced in the paranasal sinuses, which maxes out around 25 ppm.

4. Lastly, they had participants inhale the added NO without using L-NMMA, which, as we will see, turns out to be a critical measurement.

The results were quite fascinating.  First, when NO was inhaled without the L-NMMA administered, nothing happened to forearm blood flow.  Therefore, under normal conditions, inhaling extra NO doesn’t seem to impact blood flow.  But things got interesting when L-NMMA was administered.  Inhaling NO counteracted the blood flow reduction due to L-NMMA.

Thus, under normal conditions, inhaled NO doesn’t have much impact on systemic blood flow.  But, when endothelial-derived NO is suppressed (the L-NMMA case), the inhaled NO “takes over,” compensating for the missing NO.  This opens up the blood vessels and increases blood flow.  This effect was most marked during the handgrip exercise.

Moreover, by looking at arterial-to-venous gradients in different gases, which show how gases change from when the blood leaves the lungs versus when it returns to the heart, they found evidence of NO transport and delivery.  This led them to conclude:

“The most fundamental and important observation of this study is that NO gas introduced to the lungs can be stabilized and transported in blood and peripherally modulate blood flow.” 

This study was groundbreaking in that it showed, for the first time, evidence of inhaled NO being transported throughout the body while maintaining its bioactivity.  These results might be significant to diabetics because we suffer from reduced endothelial-derived NO and reduced blood flow.  Thus, the results might provide more support for nose-breathing (although again, NO concentrations in the nose are far less than what was administered here).

To conclude, I’ll borrow a line from the abstract, which succinctly states how the findings of this study could be particularly important to diabetics: 

“These results indicate that inhaled NO during blockade of regional NO synthesis can supply intravascular NO to maintain normal vascular function. This effect may have application for the treatment of diseases characterized by endothelial dysfunction.”

Abstract

Nitric oxide (NO) may be stabilized by binding to hemoglobin, by nitrosating thiol-containing plasma molecules, or by conversion to nitrite, all reactions potentially preserving its bioactivity in blood. Here we examined the contribution of blood-transported NO to regional vascular tone in humans before and during NO inhalation. While breathing room air and then room air with NO at 80 parts per million, forearm blood flow was measured in 16 subjects at rest and after blockade of forearm NO synthesis with NG-monomethyl-l-arginine (l-NMMA) followed by forearm exercise stress. l-NMMA reduced blood flow by 25% and increased resistance by 50%, an effect that was blocked by NO inhalation. With NO inhalation, resistance was significantly lower during l-NMMA infusion, both at rest and during repetitive hand-grip exercise. S-nitrosohemoglobin and plasma S-nitrosothiols did not change with NO inhalation. Arterial nitrite levels increased by 11% and arterial nitrosyl(heme)hemoglobin levels increased tenfold to the micromolar range, and both measures were consistently higher in the arterial than in venous blood. S-nitrosohemoglobin levels were in the nanomolar range, with no significant artery-to-vein gradients. These results indicate that inhaled NO during blockade of regional NO synthesis can supply intravascular NO to maintain normal vascular function. This effect may have application for the treatment of diseases characterized by endothelial dysfunction.

Journal Reference:

Cannon RO 3rd, Schechter AN, Panza JA, et al. Effects of inhaled nitric oxide on regional blood flow are consistent with intravascular nitric oxide delivery. J Clin Invest. 2001;108(2):279-287. doi:10.1172/JCI12761

A Review of Nasal Nitric Oxide's Powerful Effects

Key Points

• Nasal nitric oxide (NO) acts as our first line of defense against airborne pathogens by sterilizing incoming air and enhancing cilia movement

• Nasal NO plays a role in warming the air we breathe as it travels into the lungs

• Humming significantly increases nasal NO and could be used as a test for sinus disorders
  

The Breathing Diabetic Summary

I spend a lot of time reading about nitric oxide (NO).  But, the more I learn, the more interested I become. It seems to pop up everywhere I look. Sometimes I wonder what it can’t do.

Nitric oxide’s physiological relevance was discovered in 1987, the same year I was born. Its effects were known several years prior, but it wasn’t until two separate papers (both in prestigious journals, PNAS and Nature) were published that NO’s benefits became “official.”

In the respiratory system, the primary source of NO is the upper airways. The paranasal sinuses, in particular, produce ~90% of the NO measured in exhaled air.  

Previously, we have learned that NO acts as our first line of defense against airborne pathogens by sterilizing incoming air.  The breathing community often touts aspect of NO.  Here, we learn there is more to it: nitric oxide also increases the cilia motility.  

Cilia are tiny hairs lining the back of your nose and respiratory tract. They oscillate back and forth to move mucus out of the upper and lower airways, bringing pathogens and other potentially harmful agents along for the ride.  Cilia are your lung’s first defense against inhaled particles and nitric oxide enhances their activity.  

Nitric oxide also plays a role in warming incoming air. The precise mechanism is unclear, but increased nasal NO release is associated with increased temperature in the nasal airways.

Here is my speculation: NO increases blood flow in your nose, which warms the nasal passages and airways. As air travels through, it extracts this warmth before entering the lungs. Makes sense, but is just a hypothesis and likely oversimplifies what is going on…

Nasal nitric oxide also redistributes blood flow in the lungs when in the upright position, leading to better oxygen uptake. (Nasal NO might even be an adaptation to gravity, allowing us to walk upright.)  

Finally, humming causes a significant increase in nasal NO. However, some sinus disorders inhibit this enhanced NO release. Therefore, the measurement of nasal NO after humming might be a way to test for sinus disorders.

To summarize, nasal nitric oxide is a powerful gas. It acts as our first line of defense against airborne pathogens by sterilizing incoming air and by improving cilia motility. Additionally, NO helps warm the air we breathe as it travels into our lungs. NO also redistributes blood flow in the lungs, resulting in better oxygen uptake. Lastly, humming increases NO significantly and might provide a way to test for sinus disorders.

Abstract

Exhaled nitric oxide (NO) originates from the upper airways, and takes action, to varying extents, in regulation, protection and defense, as well as in noxious processes. Nitric oxide retains important functions in a wide range of physiological and pathophysiological processes of the human body, including vaso-regulation, antimicrobial activity, neurotransmission and respiration. This review article reports the ongoing investigations regarding the source, biology and relevance of NO within upper respiratory tract. In addition, we discuss the role of NO, originating from nasal and paranasal sinuses, in inflammatory disorders such as allergic rhinitis, sinusitis, primary ciliary dyskinesia, and cystic fibrosis.

Journal Reference:

Maniscalco M, Bianco A, Mazzarella G, Motta A.  Recent Advances on Nitric Oxide in the Upper Airways.  Curr Med Chem. 2016;23(24):2736-2745.

The Importance of Carbon Dioxide for Sleep-Disordered Breathing

Key Points

• Sleep-disordered breathing is common, even among healthy individuals

• The normal inputs to and reflexes of the respiratory system are dampened during sleep

• Added carbon dioxide (CO2) might be the best treatment for sleep-disordered breathing

The Breathing Diabetic Summary

Sleep-disordered breathing is common, even among healthy individuals. As we have discussed before, breathing during sleep is shallow and irregular. Not exactly what we would expect. But, the word “disordered” could be a misconception. It is only disordered relative to wakefulness. It might be completely normal for sleep.

I digress. This paper examines the mechanisms behind sleep-disordered breathing at large, and for more serious issues such as central sleep apnea.

Our normal breathing system does not work correctly during sleep. For example, while awake, our bodies do amazing things to make sure blood gases stay within a specific range and that the breathing muscles do as little work as possible.

During sleep, however, we commonly experience respiratory acidosis. Additionally, the upper airways often get narrower because the breathing muscles are not recruited in the same way they are while awake. Thus, both chemical and mechanical breathing deficiencies occur during sleep.

There are several reasons for these deficiencies.

To begin with, sleep reduces input to the breathing muscles. This reduction causes the airways to narrow (these muscles act to keep them open) and increases breathing resistance.

Sleep also reduces chemoreflexes, meaning that your body is less sensitive to high CO2 and low O2. This can cause respiratory acidosis or blood gas imbalance.

On the more severe side, many patients with central apnea syndrome have chronic hypocapnia (low CO2), which might predispose them to apnea. For example, if you are chronically low on CO2, your body might try to compensate for this during sleep. It might compensate by reducing breathing or even stopping it. (That’s my speculation, not the paper’s.)

So, what does all of this mean for you? Interestingly, this “Sleep and Breathing State-of-the-Art Review” concludes that CO2 might be the best treatment for SDB. Added CO2 will stimulate the respiratory muscles and therefore prevent narrowing of the upper airways. Additionally, CO2 is a smooth muscle dilator, which will help increase blood flow to the muscles.

There are two simple ways we can “add” our own CO2 tonight. First, you can tape your mouth while sleeping. This will reduce breathing volume and increase CO2. Paradoxically, nose breathing also reduces upper airway resistance during sleep, while it increases breathing resistance during the day.

Second, we can practice light breathing during the day, and before sleep, to increase our tolerance to CO2. By doing this consistently, we can reset our baseline CO2 back to normal values and improve our breathing during sleep. 

Abstract

We present a view of the neuromechanical regulation of breathing and causes of breathing instability during sleep. First, we would expect transient increases in upper airway resistance to be a major cause of transient hypopnea. This occurs in sleep because a hypotonic upper airway is more susceptible to narrowing and because the immediate excitatory increase in respiratory motor output in response to increased loads is absent in non-REM sleep. Secondly, sleep predisposes to an increased occurrence of ventilatory "overshoots", in part because abruptly changing sleep states cause transient changes in upper airway resistance and in the gain of the respiratory controller. Following these ventilatory overshoots, breathing stability will be maintained if excitatory short-term potentiation is the prevailing influence. On the other hand, apnea and hypopnea will occur if inhibitory mechanisms dominate following the ventilatory overshoot. These inhibitory mechanisms include: a) hypocapnia-if transient, will inhibit carotid chemoreceptors and cause hypopnea, but if prolonged will inhibit medullary chemoreceptors and cause apnea; b) a  persistent inhibitory effect from lung stretch; c) baroreceptor stimulation, from a transient rise in systemic blood pressure immediately following termination of apnea or hypopnea may partially suppress the accompanying hyperpnea; d) depression of central respiratory motor output via prolonged brain hypoxia. Once apneas are initiated, reinitiation of inspiration is delayed even though excitatory stimuli have risen well above their apneic thresholds, and these prolonged apneas are commonly accompanied by tonic EMG activation of expiratory muscles of the chest wall and upper airway.

Journal Reference:

Dempsey JA, Smith CA, Harms CA, Chow C, Saupe KW.  Sleep-Induced Breathing Instability.  Sleep.  1996;19(3):236-47.

Nasal Airflow Activates Broad Regions of the Olfactory Bulb

Key Points

• Nasal breathing information is “encoded” by olfactory sensory neurons in the olfactory bulb

• Breathing activates broad regions of the olfactory bulb, with the intensity changing as total breathing volume changes

• The neural activity stimulated by nasal breathing helps explain how breathing can have immediate physiological effects throughout the body

The Breathing Diabetic Summary

The olfactory bulb (OB) and its neurons (olfactory sensory neurons, OSNs) can sense both odors and airflow.  However, previous studies have rarely examined how airflow is actually encoded by the OB and OSNs.  This study used fMRI and local field potential to figure out how airflow activity is mapped in the OB. 

They studied mice under different airflow stimulation (these can be thought of as different breathing patterns because the mechanical stimulation was occurring through their noses).  The different breathing paradigms included changing respiratory rate, changing tidal volume, or changing both rate and tidal volume while keeping the total airflow the same (that is, keeping Rate X Volume = Constant). 

The results showed that airflow stimulation activated broad regions of the OB.  Odor stimulation, on the other hand, had more localized activity maps.  Furthermore, the overall structure of the activity maps was similar regardless of which breathing paradigm was being studied.  Only the intensity of the signal changed with total airflow.  Greater total volume led to more intense activity in the OB. 

Another interesting result was that nasal airflow affected the physiological state of the mice.  Their resting heart and breathing rates slowed, and EEG power declined in specific ranges. 

These results are important because they show for the first time that nasal breathing information is encoded in the olfactory bulb.  We know that breathing can directly influence emotional state, for example, providing a calming effect.  And, there have been several studies showing a direct correlation between breathing, brain activity, memory, and behavior.  Here, we see why.   

Nasal breathing information is imprinted in the OB.  The OB then projects onto the limbic system, which regulates emotions, olfaction, and the autonomic nervous system.  This helps explain the wide-ranging benefits of slow breathing and how breathing can have such immediate effects on our physiological state.

To summarize, this is the first study to show that nasal airflow elicits broad activity maps in the olfactory bulb.  The patterns are robust and change only in intensity when total airflow is altered.  The effects of nasal respiration on the olfactory bulb are then projected on the limbic system, helping explain how breathing can quickly impact physiological state.

Abstract

Olfactory sensory neurons (OSNs) can sense both odorants and airflows. In the olfactory bulb (OB), the coding of odor information has been well studied, but the coding of mechanical stimulation is rarely investigated. Unlike odor-sensing functions of OSNs, the airflow-sensing functions of OSNs are also largely unknown. Here, the activity patterns elicited by mechanical airflow in male rat OBs were mapped using fMRI and correlated with local field potential recordings. In an attempt to reveal possible functions of airflow sensing, the relationship between airflow patterns and physiological parameters was also examined. We found the following: (1) the activity pattern in the OB evoked by airflow in the nasal cavity was more broadly distributed than patterns evoked by odors; (2) the pattern intensity increases with total airflow, while the pattern topography with total airflow remains almost unchanged; and (3) the heart rate, spontaneous respiratory rate, and electroencephalograph power in the β band decreased with regular mechanical airflow in the nasal cavity. The mapping results provide evidence that the signals elicited by mechanical airflow in OSNs are transmitted to the OB, and that the OB has the potential to code and process mechanical information. Our functional data indicate that airflow rhythm in the olfactory system can regulate the physiological and brain states, providing an explanation for the effects of breath control in meditation, yoga, and Taoism practices.

SIGNIFICANCE STATEMENT Presentation of odor information in the olfactory bulb has been well studied, but studies about breathing features are rare. Here, using blood oxygen level-dependent functional MRI for the first time in such an investigation, we explored the global activity patterns in the rat olfactory bulb elicited by airflow in the nasal cavity. We found that the activity pattern elicited by airflow is broadly distributed, with increasing pattern intensity and similar topography under increasing total airflow. Further, heart rate, spontaneous respiratory rate in the lung, and electroencephalograph power in the β band decreased with regular airflow in the nasal cavity. Our study provides further understanding of the airflow map in the olfactory bulb in vivo, and evidence for the possible mechanosensitivity functions of olfactory sensory neurons.

Journal Reference:

Wu R, Liu Y, Wang L, Li B, Xu F.  Activity patterns elicited by airflow in the olfactory bulb and their possible functions.  J Neurosci. 2017;37(44):10700-10711. doi: 10.1523/JNEUROSCI.2210-17.2017.

Nitric Oxide Might Outweigh All Other Benefits of Nose Breathing

Key Points

• Nasal nitric oxide (NO) acts as our body’s first line of defense against airborne pathogens

• Nasal NO reduces blood pressure, redistributes blood flow, and increases gas exchange

• The humidifying effects of the nose might not be as important as NO

The Breathing Diabetic Summary

Nitric oxide (NO) has, somewhat quietly, become a staple of breathing science.  NO is produced in the nasal airways and carried into the lungs with each (nasal) breath we take.  This review discussed nasal NO, its origins, and its physiological effects in the body.

The general consensus is that NO is produced in the paranasal sinuses and is continuously released into the nasal airways.  Because of this continuous release, NO’s concentration is dependent on flow rate.  A lower flow rate will allow more NO to build up, thus bringing higher concentrations down into the lungs with each breath.   

This could be yet another benefit of slow breathing: Slower flow rates will increase NO. Each breath then brings in a higher concentration of NO, redistributing blood flow, increasing gas exchange, and potentially increasing infection-fighting capabilities.

Which brings us to the next physiological effect of nasal NO: Host defense.  Some bacteria die when NO concentrations are as low as 100 parts per billion (ppb).  In the paranasal sinuses, the concentration can be as great as 30,000 ppb(!).  Thus, nasal NO might be the first line of defense against airborne bacteria, acting to sterilize the incoming air and reduce infection. 

Nasal NO also increases arterial oxygenation and reduces blood pressure in the lungs.  For example, one study showed that nasal breathing increased tissue oxygenation by 10% when compared to mouth breathing.  That’s pretty remarkable.

For example, one study showed that nasal breathing increased tissue oxygenation by 10% when compared to mouth breathing.

Another study showed that when mouth breathers were given supplemental NO, arterial oxygenation increased and and lung blood pressure decreased similar to nose breathing.  Interestingly, if the mouth breathers were just given moistened air (without NO), these effects did not occur.  Thus, the main benefits of nasal breathing might be due to NO, not the warming and humidifying effects that are typically touted (although they clearly help).

Finally, widening the nostrils via nasal tape also increases arterial oxygenation during breathing at rest.  This could partially be due to an increased delivery of NO to the lungs.  We can naturally unblock our noses using simple breath hold techniques or use something like Intake Breathing for assistance.

Overall, this study highlighted several important aspects of nasal NO.  It acts as our body’s first line of defense against airborne pathogens by sterilizing incoming air.  Then, as NO travels into the lungs, it reduces blood pressure, redistributes blood flow, and increases gas exchange, leading to greater arterial oxygenation.  Finally, we learned that the humidifying effects of the nose might not be as important as NO. 

I am continually amazed by the many roles of nitric oxide in the body.  I believe it might be the most important aspect of nasal breathing. 

Journal Reference:

Lundberg JO, Weitzberg E.  Nasal nitric oxide in man.  Thorax.  1999;54(10):947-52.

Nasal Nitric Oxide: Nature’s Answer to Gravity?

Key Points

• Nitric oxide redistributes blood flow in the lungs to be more uniform

• Nitric oxide increases gas exchange in the lungs

• Nasal nitric oxide might be an evolutionary adaptation to counter gravity

The Breathing Diabetic Summary

Blood flow in the lungs is essential for gas exchange and defense against infections. However, lung blood flow is not as uniform as we might think. And, although several factors account for this, gravity plays a significant role.  Gravity acts to focus blood flow toward the bottom of the lungs.  

Interestingly, in humans and higher primates, a large amount of nitric oxide (NO) is released in the nasal passages. As we learned, NO is critical for blood flow and whole-body oxygenation. The authors of this study wondered if nasal NO might also redistribute blood flow in the lungs, thus countering the effects of gravity and increasing gas exchange in the lungs. This adaptation would have allowed us to evolve into the bipedal mammals we are today.

To test this, they examined how different breathing protocols affected lung blood flow. Participants were injected with a radionuclide that acted as a passive tracer of blood flow, which could then be imaged to show relative “heat maps” of blood flow in the lungs.

Fourteen participants were broken into three groups. The first group served as a control to ensure the radiotracer imaging technique worked as intended. The second group was used to see how nasally produced NO affected lung blood flow. These participants sat in an upright position and breathed through their mouths for 20 min. The tracer was injected, and their lung blood flow was imaged. Then, they switched to nasal breathing for 10 min. Tracer was again injected imagery was taken.

The final group was used to see if NO was, in fact, the driver of lung blood flow redistribution. These participants breathed through their mouths but were given supplemental NO. If NO was the driver, mouth breathing with additional NO should result in similar blood flow redistribution as nasal breathing.

They found that nasal breathing redistributed blood flow both vertically and horizontally in the lungs, making it more uniform. The same occurred when mouth breathing with supplemental NO. Thus, NO, whether produced naturally in the nasal passages or supplemented, acts to redistribute blood flow and increase gas exchange in the lungs.

The authors hypothesize that the NO produced in the nasal passages is an evolutionary adaptation to walking upright.  The NO acts to make blood flow and gas exchange more uniform, thus countering the effects of gravity.

In summary, nasal nitric oxide counteracts the effects of gravity and makes lung blood flow more uniform in the upright position. Interestingly, this only occurs in humans and higher primates. Thus, NO production in the upper airways might have been a critical evolutionary adaptation that allowed us to walk upright.

Abstract

There are a number of evidences suggesting that lung perfusion distribution is under active regulation and determined by several factors in addition to gravity. In this work, we hypothesised that autoinhalation of nitric oxide (NO), produced in the human nasal airways, may be one important factor regulating human lung perfusion distribution in the upright position. In 15 healthy volunteers, we used single-photon emission computed tomography technique and two tracers (99mTc and 113mIn) labeled with human macroaggregated albumin to assess pulmonary blood flow distribution. In the sitting upright position, subjects first breathed NO free air through the mouth followed by the administration of the first tracer. Subjects then switched to either nasal breathing or oral breathing with the addition of exogenous NO-enriched air followed by the administration of the second tracer. Compared with oral breathing, nasal breathing induced a blood flow redistribution of approximately 4% of the total perfusion in the caudal to cranial and dorsal to ventral directions. For low perfused lung regions like the apical region, this represents a net increase of 24% in blood flow. Similar effects were obtained with the addition of exogenous NO during oral breathing, indicating that NO and not the breathing condition was responsible for the blood flow redistribution. In conclusion, these results provide evidence that autoinhalation of endogenous NO from the nasal airways may ameliorate the influence of gravity on pulmonary blood flow distribution in the upright position. The presence of nasal NO only in humans and higher primates suggest that it may be an important part of the adaptation to bipedalism.

Journal Reference:

Sánchez Crespo A, Hallberg J, Lundberg JO, Lindahl SG, Jacobsson H, Weitzberg E, Nyrén S.  Nasal nitric oxide and regulation of human pulmonary blood flow in the upright position.  J Appl Physiol.  2010;108:181–188.

Your breathing is shallow and irregular for 1/3 of your life

Key Points

• Breathing volume decreases between 6% and 16% during sleep

• Breathing is shallow and irregular during sleep

• We experience relative hypoxia and hypercapnia during sleep

The Breathing Diabetic Summary

To understand sleep-related breathing disorders, we first have to understand normal breathing during sleep.  That was the goal of this study.

Experiments were conducted with 19 subjects (8 males, 11 females) that had no history of sleep complaints.  Additionally, they were all nocturnal sleepers.  The researchers studied them between 10 PM and 7 AM.  They studied one subject on 3 nights, 9 subjects on 2 nights, and 9 subjects on 1 night.

Baseline measurements were obtained while the patients were lying in bed either before falling asleep or after waking up (using EEG-confirmed wakefulness).  Theses recordings were subsequently averaged to produce the “awake” value. 

For measurements of breathing during different sleep stages, the subjects had to stay in that sleep stage continuously for at least 2 minutes.  Additionally, there could not be any detectable leaks within the breathing mask they were wearing.

Comparison of the awake versus sleeping parameters revealed that breathing volume reduced significantly during sleep.  For non-REM sleep, breathing volume decreased between 6% and 8%.  During REM sleep, ventilation reduced by ~16%.  Interestingly, the breathing rates of these subjects were slightly faster during sleep than while awake, suggesting that breathing becomes shallower during sleep.

Because the participants were breathing less, they became significantly more hypoxic (low O2) and hypercapnic (high CO2) while asleep compared to while awake.

The researchers used this information, along with assumptions regarding lung dead space and dead space due to the breathing mask, to estimate the change in gas exchange occurring in the lungs.  These calculations revealed a reduction in gas exchange between 19% and 39% during sleep, helping explain why the participants experienced hypoxia and hypercapnia.

Lastly, during non-REM sleep, breathing rates were somewhat regular (although a few patients still showed irregular rates during non-REM).  In REM sleep, all participants exhibited shallow and irregular breathing patterns. 

Overall, these results show that breathing volume is reduced during all stages of sleep. The greatest reductions occur during REM sleep, which is also when breathing rate is the most irregular and unstable. The reduction in breathing leads to relative hypoxia and hypercapnia. Interestingly, these breathing patterns are normal and are part of the natural physiological changes our bodies makes during sleep.

Abstract

Respiratory volumes and timing have been measured in 19 healthy adults during wakefulness and sleep. Minute ventilation was significantly less (p less than 0.05) in all stages of sleep than when the subject was awake (7.66 +/- 0.34(SEM) 1/min), the level in rapid-eye-movement (REM) sleep (6.46 +/- 0.29 1/min) being significantly lower than in non-REM sleep (7.18 +/- 0.39 1/min). The breathing pattern during all stages of sleep was significantly more rapid and shallow than during wakefulness, tidal volume in REM sleep being reduced to 73% of the level during wakefulness. Mean inspiratory flow rate (VT/Ti), an index of inspiratory drive, was significantly lower in REM sleep than during wakefulness or non-REM sleep. Thus ventilation falls during sleep, the greatest reduction occurring during REM sleep, when there is a parallel reduction in inspiratory drive. Similar changes in ventilation may contribute to the REM-associated hypoxaemia observed in normal subjects and in patients with chronic obstructive pulmonary disease.

Journal Reference:

Douglas NJ, White DP, Pickett CK, Weil JV, Zwillich CW.  Respiration during sleep in normal man.  Thorax.  1982;37(11):840-844.

Our somewhat unusual breathing patterns during sleep

Key Points

• Breathing volume is reduced by as much as 16% during sleep

• Breathing rate is variable during sleep, especially in REM

• Hypoxic and hypercapnic responses are reduced by as much as 66% during sleep

The Breathing Diabetic Summary

We spend approximately 1/3 of our life sleeping.  And although sleep science is still relatively new, it’s undeniable that sleep is a key component of achieving optimal health.  Which begs the question, if sleep is so restorative, what is happening to our breath during this time?

Published in 1984, this review study found that breathing is significantly reduced during all stages of sleep.  This reduction can be as great as 16%.

Somewhat surprisingly, our breathing rate is extremely variable during sleep.  I expected that our breathing would become rhythmic and deep.  However, research shows that the opposite is true.  We breathe shallower and our breathing rate remains the same, or even increases slightly.

Additionally, it differs for different stages of sleep.  During non rapid eye movement sleep (non-REM), our breathing volume reduces and we sometimes achieve a steady rhythm.  In REM sleep, however, our breathing volume reduces even more, but our rate becomes more sporadic.

We also experience relative hypoxia (low O2) and hypercapnia (high CO2).  In fact, our tolerance to CO2 increases dramatically.  One study suggested that during non-REM, CO2 tolerance increases by ~33%.  During REM sleep, it increases by about 66%.  That’s fairly remarkable.

So, to summarize, here is what happens to breathing during sleep:

• Breathing volume reduces

• Breathing rate is variable

• Hypoxic and hypercapnic responses are reduced

The processes occurring during sleep clearly serve a purpose in restoring health.  If we interrupt these processes, we will not harness the full power of sleeping.

Therefore, if you are breathing with an open mouth during sleep, you are probably breathing too much and not supporting restorative sleep.Luckily, it’s an easy fix.Simply taping your mouth at night is the first step toward achieving optimal breathing volumes during sleep.

Journal Reference:

Douglas NJ.  Control of Breathing during Sleep.  Clin Sci (Lond).  1984;67(5):465-471.

Nasal breathing synchronizes brain wave activity and improves cognitive function

Key Points

• Nasal breathing synchronizes brain wave oscillations in the piriform cortex, amygdala, and hippocampus

• Nasal breathing improves cognitive function when compared to mouth breathing

• Breathing affects emotional and mental state, shifting the paradigm for why we breathe

The Breathing Diabetic Summary

It is established that emotions and mental state affect breathing.  When you’re anxious, you breathe faster and shallower.  When you’re relaxed, you breathe quiet and light.  Intuitively, I think we all know that the opposite is true too: Your breathing can affect your emotions and mental state.  However, the brain mechanisms behind this shift have remained elusive.

This study sheds light on the issue.  Intracranial EEG (iEEG) was used to assess how breathing impacts electrical oscillations in different regions of the brain.  Then, emotional recognition and memory tests were used to see how breathing impacts cognitive function.

The results showed that oscillations in the piriform cortex are directly related to nasal breathing. The piriform cortex is associated with the nose through smell, so it makes sense that nasal breathing would cause oscillations in this region (although the participants were breathing odorless air).

Interestingly, two other regions of the brain also showed these oscillations: the amygdala and hippocampus.  When breathing was switched to the mouth, however, this brainwave activity became disorganized.  Thus, nasal breathing is critical to synchronizing electrical brainwave oscillations.

If nasal breathing affects these regions of the brain, it follows that it would potentially impact cognition.  And that’s exactly what they found.

They showed participants faces expressing either fear or surprise and had them quickly decide which one it was.  When breathing through the nose, the response times were faster than when breathing through the mouth.  Additionally, the participants identified fearful faces faster during inhalation than exhalation.  This effect wasn’t present when mouth breathing. 

Next, they had the participants perform a memory task involving picture recognition.  They found that their memory retrieval was more accurate during nasal inhalation, which was not observed for mouth breathing.  However, there was not a statistically significant difference in the overall accuracy between nose and mouth breathing.

Taken together, the iEEG measurements and cognitive tasks suggest that nasal breathing promotes coherent brainwave oscillations in the piriform cortex, amygdala, and hippocampus.  This coherence leads to improved cognitive function, especially during nasal inhalation.

“We also found that the route of breathing was critical to these effects, such that cognitive performance significantly declined during oral breathing.”

We’ve already established that breathing can no longer be thought of as a 2-gas system.  Now, we might have to extend beyond gases altogether.  Breathing acts to synchronize brain activity and enhance cognitive function…but only when performed through the nose.

I think that bears repeating.  Nasal breathing synchronizes brainwave activity and enhances cognitive function.  Pretty remarkable.

Abstract

The need to breathe links the mammalian olfactory system inextricably to the respiratory rhythms that draw air through the nose. In rodents and other small animals, slow oscillations of local field potential activity are driven at the rate of breathing (∼2-12 Hz) in olfactory bulb and cortex, and faster oscillatory bursts are coupled to specific phases of the respiratory cycle. These dynamic rhythms are thought to regulate cortical excitability and coordinate network interactions, helping to shape olfactory coding, memory, and behavior. However, while respiratory oscillations are a ubiquitous hallmark of olfactory system function in animals, direct evidence for such patterns is lacking in humans. In this study, we acquired intracranial EEG data from rare patients (Ps) with medically refractory epilepsy, enabling us to test the hypothesis that cortical oscillatory activity would be entrained to the human respiratory cycle, albeit at the much slower rhythm of ∼0.16-0.33 Hz. Our results reveal that natural breathing synchronizes electrical activity in human piriform (olfactory) cortex, as well as in limbic-related brain areas, including amygdala and hippocampus. Notably, oscillatory power peaked during inspiration and dissipated when breathing was diverted from nose to mouth. Parallel behavioral experiments showed that breathing phase enhances fear discrimination and memory retrieval. Our findings provide a unique framework for understanding the pivotal role of nasal breathing in coordinating neuronal oscillations to support stimulus processing and behavior.

 SIGNIFICANCE STATEMENT:

Animal studies have long shown that olfactory oscillatory activity emerges in line with the natural rhythm of breathing, even in the absence of an odor stimulus. Whether the breathing cycle induces cortical oscillations in the human brain is poorly understood. In this study, we collected intracranial EEG data from rare patients with medically intractable epilepsy, and found evidence for respiratory entrainment of local field potential activity in human piriform cortex, amygdala, and hippocampus. These effects diminished when breathing was diverted to the mouth, highlighting the importance of nasal airflow for generating respiratory oscillations. Finally, behavioral data in healthy subjects suggest that breathing phase systematically influences cognitive tasks related to amygdala and hippocampal functions.

Journal Reference:

Zelano C, Jiang H, Zhou G, Arora N, Schuele S, Rosenow J, Gottfried JA.  Nasal Respiration Entrains Human Limbic Oscillations and Modulates Cognitive Function.  J Neurosci. 2016;36(49):12448-12467.

How slow breathing improves physiological and psychological well-being (hint: it might be in your nose)

Key Points

• Slow breathing increases heart rate variability, respiratory sinus arrhythmia, and alpha brain wave activity

• These physiological changes lead to improved behavioral outcomes

• The nose links slow breathing to these positive physiological and psychological outcomes

The Breathing Diabetic Summary

I think this paper wins “Best Title Ever” award!

This was a review study that pulled together all of the scientific literature on slow breathing and psychological/behavioral outcomes.  They were trying to answer the following question: What physiological changes are common to all slow breathing studies that have shown improvements in stress and anxiety?

After using some rather rigorous criteria for their literature search, they reduced 158 potential papers down to only 15. 

The physiological outcome parameters they focused on were heart rate variability (HRV), respiratory sinus arrhythmia (RSA), and brain wave activity.  The studies they examined also used several different subjective questionnaires to assess stress, anxiety, depression, and well-being.

As it is with science, there was a lot of nuance and many contradictory findings.  However, several common results did emerge.

First, slow breathing was associated with increases in HRV, particularly in the low frequency (LF) band.  Second, it was associated with increases in RSA.  Finally, slow breathing was associated with increases in alpha brain wave activity (brain waves associated with “flow”) and decreases in theta brain wave activity. 

All of these common physiological changes observed during/after slow breathing were associated with improved psychological and behavioral outcomes.  For example, several studies showed reductions in anxiety, improvements with depression, reduced anger, and increased relaxation.

Thus, slow breathing consistently increases HRV, RSA, and alpha brain wave activity.  These physiological changes then improve psychological and behavioral outcomes.

From a practical perspective, all of the studies used breathing rates of 3-6 breaths/min.  With practice, we can use an app (such as Breathing Zone) to achieve these rates.

Lastly, they examined the importance of the nose.  They reviewed studies showing that nasal breathing has a direct relationship with brain activity, which goes away when the nasal cavity tissue is numbed.  Moreover, certain areas of the brain follow oscillations that match breathing…but only with nasal respiration.  In fact, simply puffing air into the nostrils activates the brain at those “puff” oscillations (independent of actually breathing).

The authors hypothesize that the nose is the link between slow breathing, brain and autonomic functioning, and positive emotional outcomes.

From all of this, we find that slow breathing through the nose at 3-6 breaths/min (Principle 1) has positive effects on HRV, RSA, and brain wave activity.  These benefits then lead to improved psychological and behavioral outcomes.

Abstract

Background: The psycho-physiological changes in brain-body interaction observed in most of meditative and relaxing practices rely on voluntary slowing down of breath frequency. However, the identification of mechanisms linking breath control to its psychophysiological effects is still under debate. This systematic review is aimed at unveiling psychophysiological mechanisms underlying slow breathing techniques (<10 breaths/minute) and their effects on healthy subjects. Methods: A systematic search of MEDLINE and SCOPUS databases, using keywords related to both breathing techniques and to their psychophysiological outcomes, focusing on cardio-respiratory and central nervous system, has been conducted. From a pool of 2,461 abstracts only 15 articles met eligibility criteria and were included in the review. The present systematic review follows the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Results: The main effects of slow breathing techniques cover autonomic and central nervous systems activities as well as the psychological status. Slow breathing techniques promote autonomic changes increasing Heart Rate Variability and Respiratory Sinus Arrhythmia paralleled by Central Nervous System (CNS) activity modifications. EEG studies show an increase in alpha and a decrease in theta power. Anatomically, the only available fMRI study highlights increased activity in cortical (e.g., prefrontal, motor, and parietal cortices) and subcortical (e.g., pons, thalamus, sub-parabrachial nucleus, periaqueductal gray, and hypothalamus) structures. Psychological/behavioral outputs related to the abovementioned changes are increased comfort, relaxation, pleasantness, vigor and alertness, and reduced symptoms of arousal, anxiety, depression, anger, and confusion. Conclusions: Slow breathing techniques act enhancing autonomic, cerebral and psychological flexibility in a scenario of mutual interactions: we found evidence of links between parasympathetic activity (increased HRV and LF power), CNS activities (increased EEG alpha power and decreased EEG theta power) related to emotional control and psychological well-being in healthy subjects. Our hypothesis considers two different mechanisms for explaining psychophysiological changes induced by voluntary control of slow breathing: one is related to a voluntary regulation of internal bodily states (enteroception), the other is associated to the role of mechanoceptors within the nasal vault in translating slow breathing in a modulation of olfactory bulb activity, which in turn tunes the activity of the entire cortical mantle.

Journal Reference:

Zaccaro A, Piarulli A, Laurino M, et al.  How Breath-Control Can Change Your Life: A Systematic Review on Psycho Physiological Correlates of Slow Breathing.  Front Hum Neurosci.  2018;12:353.