Podcast information | Dr Louise Oliver TLC

A missing jigsaw piece in perimenopausal and menopausal care

Perimenopausal and menopausal information frequently discusses the effect of changing hormones levels on skin, brain, heart, bones, muscles etc however the effect on breathing is often overlooked.

Hormones affect how we breathe

How we breathe affects our health and well-being

Breathing is generally an unconscious process, however this does not mean it is automatically efficient.

I am promoting the 3 A’s approach:

Aware – raising awareness of how breathing is altered by hormones, how this can lead to inefficient breathing and symptoms of inefficient breathing.

Assess – offering an opportunity for women to assess their breathing efficiency.

Action – encouraging and empowering women to improve their breathing efficiency.

Breathing may be a large or small piece in your perimenopausal or menopausal care jigsaw – I encourage you to read on to find out more.

Aware

Women generally have a smaller airway, ribcage size, lung volume and a shorter diaphragm than men. In addition, the movement of the ribs during an inhalation is different in women compared to men. (1) All these factors impact how easily air enters and leaves the female body.

A large population-based study showed lung function declined more rapidly among perimenopausal and menopausal women. This decline was greater than the normal expected deterioration in lung function related to aging. The study noticed the decline appeared to increase difficulties breathing in compared to breathing out. (2) It is known there is an increased loss of muscle around the menopause (3) but how often do we discuss muscle loss from the muscles involved in breathing and keeping the airway open whilst asleep? Are we encouraging women to perform muscle exercises to strengthen these muscles?

Although the exact mechanism is not clear, studies have shown an increased risk of sleep disordered breathing after menopause. (4) Sleep disordered breathing (SDB) refers to a wide spectrum of sleep-related conditions including increased resistance to airflow through the upper airway, heavy snoring, marked reduction in airflow (hypopnea), and complete cessation of breathing (obstructive sleep apnoea = OSA) (5)

The Wisconsin Sleep Cohort Study demonstrated that there is a 3 times greater risk of moderate or worse OSA in post-menopausal women compared to pre-menopausal women. (4) Women with OSA are less likely to report snoring or witnessed gaps in breathing (apnoea) but are more likely to complain of daytime fatigue, lack of energy, insomnia, morning headaches, mood disturbance and nightmares compared to men. (6)

More research is needed in how hormonal changes affect breathing however it seems female hormones generally have a protective effect on the airway and drive to breathe. It is thought the female hormones increase the tone of the upper airways muscles preventing airway collapse during sleep. Therefore, the reduction in the female hormones after the menopause may be the reason why the upper airway is more likely to collapse and increases the risk of OSA. (6)

Progesterone acts as a powerful respiratory stimulant (which means it increases how fast the individual breathes) by increasing the body’s response to low oxygen and high carbon dioxide levels. This could help the airway open up but conversely the faster breathing rate could cause a vacuum effect leading to airway collapse. Also, if the faster breathing rate leads to a reduction in carbon dioxide levels this can trigger the body to have pauses in the breathing to allow the body to increase the levels back to normal.

Weight gain is common at this time of life and unfortunately increases the amount of fat stored in the tongue and the surrounding tissues which reduces the space at the back of the throat therefore making it more difficult to breathe especially whilst asleep. Furthermore the change in body fat distribution around the menopause can lead to increased body mass index (BMI), waist circumference, and neck circumference which are all associated with higher severity of OSA (4). If breathing disrupts sleep this negatively impacts the normal 24-hour rhythm of the hormones that control hunger (ghrelin) and feeling full (leptin) leading to increased appetite and not feeling full which promotes further weight gain. The research shows the increase in sleep disordered breathing around the menopause is independent of weight however weight gain further increases the risk of sleep disordered breathing.

With all these changes the efficiency of breathing can deteriorate as women transition through to menopause. I developed snoring in my perimenopause despite a healthy weight and running three times a week. HRT helped many of my symptoms but did not improve my snoring and sleep. I improved my breathing efficiency which stopped my snoring, improved my sleep, ran with greater ease and felt calmer.

Autonomic nervous system

To understand why breathing is so important and how we can improve our breathing efficiency I would like you to understand the autonomic nervous system (ANS). Your body is amazing. From moment to moment without conscious control your body is making fine adjustments to your blood pressure, pulse, temperature, amount of saliva, gut digestion, muscle contraction, pupil size etc. There are two main parts to the ANS: the fight, flight, freeze system (sympathetic nervous system) and the ‘rest & digest’ or ‘feed & breed’ system (parasympathetic nervous system).

The fight, flight, freeze system (sympathetic nervous system) gets a bad rap. We should be very grateful for this system. The fight, flight, freeze system has been developed to keep us safe and helped our species survive and develop. It makes you the best version of yourself if not constantly activated. When this system is activated, your whole body reacts so it is ready to respond to danger – e.g. your muscles will contract, sugar pours into your blood stream, your pupils widen, cortisol (steroid) and adrenaline increase in the blood, you are more likely to breathe fast, with an upper chest, take larger amounts of air through an open mouth. This means you are ready to fight, run or play dead to help stay safe. This is a perfect response to a true threat but not to an e-mail from your boss, getting stuck in traffic etc. If the human body was a car this should be regarded as the accelerator.

The wonderful news is we have an opposite system: the 'relaxation response', ‘rest & digest’ or ‘feed & breed’ system (parasympathetic nervous system). If the human body was a car this would be the brake. In this state our body doesn’t feel under threat, feels safe, can relax, digest food, libido and the ability to reproduce is encouraged. Breathing in this state is slow, gentle, through the nose and driven by the diaphragm.

The human body is amazingly complex and the ANS without our conscious control makes fine adjustments to the unconscious processes that take place throughout our body. One way to understand the ANS is regarding the human body as a car which drives with two feet: one foot on the accelerator (fight, flight, freeze system) and one foot on the brake (relaxation response). The human body needs to travel along the road of life with all the various curves, hills and valleys with one foot on the accelerator and one foot on the brake. The body has to make gentle adjustments to the accelerator and brake according to the circumstances to maintain a smooth ride and remain on the road. Using this analogy if the accelerator (fight, flight, freeze system) is being heavily used it would be more difficult to negiogate the curves on the road with the danger of coming off the road completely (resulting in burnout) This flexibility in the ANS to make fine adjustments between accelerator and brake is reduced in chronic pain (7), long covid (8) and chronic health conditions such as heart disease (8) and diabetes (9). Breathing re-education can improve the flexibility of the ANS leading to improvements in blood pressure, pulse, mood, anxiety etc. (10)

There is evidence the threat detection centre of the brain (the amygdala) has a (nerve) neuronal connection to our breathing (respiratory) centre monitoring for changes in our breathing. (11) The amygdala also has roles in emotion, learning and reward. It is thought this neuronal connection has developed through evolution to improve survival, for example if our breathing becomes fast, heavy and uses the upper chest the amygdala notices this change, perceives it as a threat to the body and sends a signal to the body to activate the fight, flight, freeze response. Therefore, if breathing during sleep is noisy and fast the amygdala will try to wake us up as the body is responding to a perceived threat which unfortunately is likely to lead to a light sleep or regular waking. In addition, during the deeper stages of sleep the muscles become relaxed which may narrow or block the airway enough to trigger a fight, flight, freeze response to take the individual out of this stage of sleep to open the airway and allow the individual to breathe more easily. The individual may or not be aware of this micro-arousal. This can lead to sleep fragmentation and reduced time in the deeper stages of sleep which is important for emotional processing and memory consolidation. A slow and quiet breathing pattern at night reduces the likelihood the fight, flight, freeze response will be triggered and therefore sleep quality can be improved.

If an individual struggles to fall asleep changing to a slow, gentle breathing pattern sends a signal to the brain to turn on the ‘rest and digest’ system aiding transition from wakefulness to sleep.

Symptoms of inefficient breathing

In 1975 Dr Lum a Respiratory Consultant at Papworth & Addenbrookes Hospital in Cambridge published a paper that described 700 patients who had been assessed but 82% had no detectable pathology and their symptoms were largely or entirely related to breathing inefficiently. (12) Table 1 in his paper lists the main symptoms observed by a general physician in 270 cases. (12) Dr Lum stated ‘symptoms may show up anywhere, in any organ, in any system’ (12) The symptoms included palpitations, dizziness, muscle pain, anxiety, exhaustion, sleep disturbance – common symptoms of the perimenopause and menopause. I would encourage anyone with these symptoms to assess their breathing efficiency (see below) in addition to seeing their health practitioner to exclude any other underlying physical cause.

Consequences of sleep disordered breathing:

Sleep disordered breathing has consequences which affect health and well-being (4):

Fatigue – SDB makes it less likely the ‘rest and digest’ system is activated whilst asleep. Also, SDB uses more of the body’s energy requirement as the breathing pattern is less efficient.
Reduced concentration or ‘brain fog’ - SDB alters the transition through the various stages of sleep therefore affecting the ability of the brain to think.
Raised blood pressure – often individuals with SDB do not use the nose to breathe at night. Nasal breathing makes it much more likely the diaphragm is used. When the diaphragm is used to breathe less work is required by the heart. The diaphragm acts as a ‘second heart’ pumping the blood around the body resulting in lower blood pressure and resting pulse. Mouth breathing does not fully utilise the ‘second heart’.
Increased risk of heart attack, heart failure and stroke – due to higher blood pressure and SDB maintaining the fight, flight, freeze system whilst asleep.
Increased risk of anxiety and depression – research has shown there is an association between OSA and depression. Persistent activation of fight, flight, freeze system fuels anxiety.
Increased risk of accidents – due to fatigue. Unfortunately, some accidents are fatal or life changing.
Increased risk of memory problems – MRI studies of the brain have reported damage of the hippocampus (area of the brain involved in memory) (13). Autopsy brain tissue from people with OSA found as OSA severity increased, the amount of damage to the hippocampus increased. (14)
Increased weight gain and diabetes – persistent activation of fight, flight, freeze system leads to higher glucose and cortisol levels in addition to alteration to the hormones controlling appetite and feeling full. This increases the risk of gaining weight.

Assess (click here if you prefer this in video format)

Effective breathing is in and out of the nose, slow, gentle, the exhale is longer than the inhale, there is a short pause after the exhale, and it is driven by the diaphragm (lower ribs expand sideways on the inhale). Over the next week check in with your breathing intermittently to assess whether it matches this description, especially whilst you are distracted or moving.

Breathing inefficiency (dysfunctional breathing) lacks a rigorous definition and a clear method of assessment. Kiesel and colleagues (15) explored different assessment methods which led to them proposing a breathing screening procedure consisting of measuring breath hold time (BHT) and answering four questions. This does not diagnose dysfunctional breathing however it is a useful tool to indicate if you may benefit from improving your breathing efficiency.

Why would measuring the time you can hold your breath indicate inefficient breathing? At some point after you start holding your breath your brain sends a signal to breathe. Frequently individuals believe this is due to their oxygen levels dropping however this is not the case. By holding your breath you stop breathing out carbon dioxide, so the levels of carbon dioxide rise. Generally there is enough oxygen in your lungs and blood to maintain oxygen levels during a breath hold of this type. You have chemoreceptors in your breathing centre that have a specific tolerance to carbon dioxide that is personal to you. When the carbon dioxide level reaches your tolerance level your body will send a signal to breathe, and you will feel ‘air hunger’ which is a feeling you either need to take more air in or a feeling of suffocation.

Measure your breath hold time (BHT)

Check this after waking and before eating.

Sit upright.

Keep your mouth closed throughout the measurement.

Take a small silent breath in through your nose.

Allow a small silent breath out through your nose

Hold your nose with your fingers to prevent air from entering your lungs

Using a stopwatch record the number of seconds until you feel the first involuntary muscle spasm – you will feel this as a contraction in the neck or diaphragm (bottom of your ribs). This is your brain sending a signal to your body to breathe as you have reached your carbon dioxide sensitivity level. We are testing comfortable breath hold time not testing willpower therefore your breathing should be calm afterwards.

Do you notice any of the symptoms below often or very often:

Do you feel tense?

Do you feel a cold sensation in your hands or feet?

Do you notice yourself yawning?

Do you notice breathing through your mouth at night? (this may need to be assessed by another person. A very dry mouth in the morning, noisy breathing or snoring is suggestive of this)

If you scored less than 25 seconds and answered often or very often to one of the questions I would encourage and empower you to take action to improve your breathing efficiency.

Action

Improving breathing efficiency with breathing re-education (18) (19) consists of two aspects:

The first is ensuring the air way is open, strong and combined with nasal breathing.

The second is ensuring the breathing volume matches the requirements of the body, using the correct breathing muscles, slowing the speed of breathing to influence the autonomic functioning of the body and bring calmness to the mind.

I would like you to think of breathing like drinking a thick milkshake through a paper straw. If I offered you a choice of a thick, thin, straight or bent paper straws choosing the thick, straight paper straw would increase your chance of drinking the milkshake with ease. Therefore, it is important we work to ensure our airway is as open as possible.

Ensuring the airway is open, strong and combined with nasal breathing

Tongue position

You may be surprised to hear the position of the tongue is crucial to keeping the airway open. The back of the tongue forms part of the airway. If the tongue is in the correct position, it opens the airway. Click here if you would like to see a video of me demonstrating this. The correct position for the tongue is flat against the roof of the mouth, tip of the tongue on the ridge above the back of the front teeth, lips sealed and not clenching your teeth. I find leaving a tiny gap between my teeth prevents me from clenching. The teeth can be in light contact if you are able to do this without clenching. If the mouth is open the tongue naturally sits in the lower part of the mouth and narrows the airway. If your tongue is not normally in this position it will take 3 months to make nerve connections and build up the strength in your muscles of your face, upper airway, and tongue so this becomes an unconscious habit. I would recommend going through the tongue routine every time you fill up a glass of water, switch on the kettle, go to the toilet and wash your hands.

Throat (oropharyngeal) exercises (16)

As I have highlighted it is thought the tone of the upper airways muscles decreases after the menopause. This could cause increased resistance to air passing through the upper airway or the throat to vibrate (resulting in snoring) or collapse (resulting in OSA). We can decrease the chance of this happening by practising throat (oropharyngeal) exercises to improve muscle strength. Teaching these exercises is one of the roles of a myofunctional therapist however this is not available within the NHS at present. Mr Vik Veer is an ENT Consultant and lead for the Sleep Surgery department at the Royal National ENT Hospital, London. He has created a YouTube video describing throat (oropharyngeal) exercises which have been shown in studies (20) to improve snoring and sleep apnoea. I would encourage you to watch the video several times and when you are confident you will be able to practice these quickly without watching the video. Tag this to an everyday activity so you don’t forget to practice once daily e.g. when you first switch on the kettle or before brushing your teeth at night. Be patient building muscle strength takes time.

Nasal breathing

The next step in effective breathing is breathing should occur through the NOSE. This includes at rest, when moving and whilst sleeping. The nose is designed to pressurise, warm, moisten, and filter out impurities and invaders from the air we breathe – the mouth is not. Please let this information sink in ……despite all my medical training this was a wake-up moment for me when I suddenly thought if I mouth breathe, I inhale viruses, bacteria, pollutants, pollen straight from the environment into my lungs without involving the first line defence of my nose? It makes no sense at all. The mouth has no role in breathing. Also if you breathe through the mouth, it brings dry, cold, unpressurised air into the lungs which can trigger spasm of the breathing tubes in the lungs (exacerbating asthma or causing wheeze) or trigger the cough reflex.

One study (17) looked at individuals with OSA whilst under sedation and whether breathing through the nose or mouth whilst asleep affected the percentage of time with an oxygen saturation less than 90% (this level is too low). The individuals mouth breathing spent 36.41% of the time with their oxygen levels less than 90%, those breathing with mouth and nose were 15.97% and those nasal breathing 5.76%. I appreciate this study is in individuals with OSA under sedation however it highlights nasal breathing is less likely to cause a drop in oxygen levels whilst asleep. Remember if the mouth is open the tongue is in the bottom of the mouth compromising the airway. I came across an xray picture by an organisation called HealthyStart® comparing the airway size with the mouth open and closed. I have found the xray is worth a thousand words. When individuals observe the xray, they can see the compromised airway with an open mouth, and in my experience helps them understand the importance of nasal breathing.

Teaching your body to nasal breathe whilst awake will make it more likely you will nasal breathe whilst asleep however if you are mouth breathing during sleep your body may need to be re-educated. There are various products available to keep the mouth closed whilst asleep. The product I have used is called MyoTape® (5% discount code available using this link and the code LOUISEOLIVER5) that can help you retrain your body to nasal breathe whilst asleep. MyoTape® does not cover the mouth and is available in small, medium, large size and for beards - Amazon links. (To help support my work, in particular the information I provide without cost, I am linked with affiliate programmes. Click to read my affiliate disclosure). I used the large size. MyoTape® is placed around the mouth and gently brings the lips together with a light, elastic tension that helps to maintain lip closure and nasal breathing however the body is able to breathe using the mouth if necessary. I do not recommend using a product that covers the lips because getting the mouth closed does not correct over breathing and the individual may need to occasionally mouth breathe until breathing volume is normalized. It is sensible to get used to wearing the product whilst awake and when you are ready start wearing it whilst asleep. Do not use this product if under the influence of alcohol or sedative drugs. It takes around 3 months of daily use to retrain the nerve circuits, so this becomes a natural unconscious habit. Getting the mouth closed whilst asleep can lead to a profound improvement in energy levels and concentration however the other aspects of improving breathing efficiency are also important.

Ensuring the mouth remains closed whilst asleep will stop mouth snoring however some individuals may still nasal snore. For many individuals nasal snoring can be eliminated by restoring nasal breathing during wakefulness and sleep, increasing breath hold time to above 25 seconds, normalising breathing volume, using the correct breathing muscles and slowing the speed of breathing. If nasal snoring continues, I recommend seeing a dentist or myofunctional therapist who has experience in assessing and treating the dental arch and tongue position and therefore airway size.

Breathing volume matches the requirements of the body, using the correct breathing muscles, slowing the speed of breathing to influence the autonomic functioning of the body and bring calmness to the mind

If we return to the milkshake and paper straw analogy, we have ensured the paper straw (airway) is as open and as strong as possible. Now we need to look at the volume of milkshake coming through the straw. If we stuck fast and hard to drink a large volume of milkshake it is much more likely the paper straw (airway) will collapse. However, if we suck slowly and gently, we are able to drink the milkshake without the straw collapsing. Therefore, we need to look at how much air we are breathing. Living in a modern-day society makes it more likely individuals breathe in excess of what their body requires resulting in over breathing. In my clinical and personal experience wearing masks during the pandemic and COVID infection has further increased over breathing. Breathing uses a proportion of the energy requirements of the body therefore over breathing results in using up more energy than is needed and can lead to symptoms described by Dr Lum (12).

I want to dispel a myth that we need to take bigger breaths to take in more oxygen. 75 percent of the oxygen you take in is exhaled during rest. Even during physical exercise, when the muscles need a much greater supply of oxygen, as much as 25 percent of inhaled oxygen is exhaled. The issue is not getting oxygen into the lungs it is getting the blood cells to release the oxygen to our cells. Oxygen doesn’t dissolve very well in blood, so it is carried bound to haemoglobin in our red blood cells. What you want is to increase the rate at which haemoglobin in the red blood cells releases oxygen to the cells. How do you do that….carbon dioxide. Carbon dioxide in the environment causes global warming but, in our body, it helps release oxygen to the tissues. When performing the breath hold time, I was testing your tolerance to carbon dioxide. I asked you to hold your breath after an exhale. Carbon dioxide was building up and you let go when your muscle caused an involuntary muscle contraction to breathe. A breath hold time of less than 25 seconds suggests you have become intolerant to the build-up of carbon dioxide however you can train yourself to become more tolerant. A breath hold time of above 40 seconds is ideal however for every 5 second improvement in your breath hold test you will feel benefit. Please note the breath hold time is just a measurement and not a breathing exercise. As you become more tolerant to carbon dioxide your haemoglobin will release oxygen more easily to your cells and you will breathe slower and gentler, feel less breathless and exercise becomes easier. In addition, carbon dioxide strengthens the nerve which controls the ‘rest and digest’ system therefore decreasing sensitivity to carbon dioxide improves mental health. Carbon dioxide also opens up blood vessels therefore improving circulation in the body. (12)

I would normally spend 5 sessions helping someone to re-educate their body to breathe efficiently as there is a lot to learn and needs to be adapted according to the breath hold time of the individual and their medical history. A very gentle approach is required if the breath hold time is less than 15 seconds. In my experience it is more beneficial to begin with breathing exercises that normalise breathing volume. Naturally as this happens the correct breathing muscles start to be used and breathing becomes slower.

I have found if individuals with a low breath hold time begin with ‘belly’ breathing (beware it is possible to ‘belly’ breathe and not actually move the diaphragm) and/ or focusing on breathing at 6 breaths a minute they tend to over breathe which can exacerbate or cause the symptoms described by Dr Lum (12). I would recommend speaking to your health professional if you still have symptoms of sleep disordered breathing after 3 months of working on improving your breathing efficiency. If an individual is diagnosed with OSA and treated with a continuous positive airway pressure (CPAP) machine nasal CPAP is more effective and better tolerated than full mask CPAP (21). An individual generally needs to be a nasal breather to use a nasal CPAP machine therefore in my opinion trying to retraining the body for 3 months to nasal breathe whilst awake, asleep and moving is sensible before embarking on a sleep study.

I run functional breathing programmes for the perimenopause and menopause which teach individuals how to improve breathing efficiency. Please see this page.

My 3 top tips

I want women to be aware that hormones affect how we breathe and how we breathe affects our health and well-being.
I would like women to consider assessing their breathing efficiency.
I encourage and empower women to act to improve their breathing efficiency by dedicating time over a number of weeks and months. Over time they will not need to spend as much time on the exercises, it becomes part of how they naturally breathe, they develop a mind, body, breath connection and they develop a method to self-regulate their autonomic nervous system.

References

LoMauro A, Aliverti A. Sex differences in respiratory function. Breathe (Sheff). 2018 Jun;14(2):131-140. doi: 10.1183/20734735.000318. PMID: 29875832; PMCID: PMC5980468.
Triebner K, Matulonga B, Johannessen A, Suske S, Benediktsdóttir B, Demoly P, Dharmage SC, Franklin KA, Garcia-Aymerich J, Gullón Blanco JA, Heinrich J, Holm M, Jarvis D, Jõgi R, Lindberg E, Moratalla Rovira JM, Muniozguren Agirre N, Pin I, Probst-Hensch N, Puggini L, Raherison C, Sánchez-Ramos JL, Schlünssen V, Sunyer J, Svanes C, Hustad S, Leynaert B, Gómez Real F. Menopause Is Associated with Accelerated Lung Function Decline. Am J Respir Crit Care Med. 2017 Apr 15;195(8):1058-1065. doi: 10.1164/rccm.201605-0968OC. PMID: 27907454.
Greising SM, Baltgalvis KA, Lowe DA, Warren GL. Hormone therapy and skeletal muscle strength: a meta-analysis. J Gerontol A Biol Sci Med Sci. 2009 Oct;64(10):1071-81. doi: 10.1093/gerona/glp082. Epub 2009 Jun 26. PMID: 19561145; PMCID: PMC2737591.
Young T, Skatrud J, Peppard PE. Risk factors for obstructive sleep apnea in adults. JAMA. 2004 Apr 28;291(16):2013-6. doi: 10.1001/jama.291.16.2013. PMID: 15113821.
American Thoracic Society https://qol.thoracic.org/sections/specific-diseases/sleep-disordered-breathing.html
Saaresranta T, Anttalainen U, Polo O. Sleep disordered breathing: is it different for females? ERJ Open Res. 2015 Nov 3;1(2):00063-2015. doi: 10.1183/23120541.00063-2015. PMID: 27730159; PMCID: PMC5005124.
Koenig J, Loerbroks A, Jarczok MN, Fischer JE, Thayer JF. Chronic Pain and Heart Rate Variability in a Cross-Sectional Occupational Sample: Evidence for Impaired Vagal Control. Clin J Pain. 2016 Mar;32(3):218-25. doi: 10.1097/AJP.0000000000000242. PMID: 25924095.
Goldenberg I, Goldkorn R, Shlomo N, Einhorn M, Levitan J, Kuperstein R, Klempfner R, Johnson B. Heart Rate Variability for Risk Assessment of Myocardial Ischemia in Patients Without Known Coronary Artery Disease: The HRV-DETECT (Heart Rate Variability for the Detection of Myocardial Ischemia) Study. J Am Heart Assoc. 2019 Dec 17;8(24):e014540. doi: 10.1161/JAHA.119.014540. Epub 2019 Dec 16. PMID: 31838969; PMCID: PMC6951049.
Benichou T, Pereira B, Mermillod M, Tauveron I, Pfabigan D, Maqdasy S, Dutheil F. Heart rate variability in type 2 diabetes mellitus: A systematic review and meta-analysis. PLoS One. 2018 Apr 2;13(4):e0195166. doi: 10.1371/journal.pone.0195166. PMID: 29608603; PMCID: PMC5880391.
Steffen PR, Austin T, DeBarros A, Brown T. The Impact of Resonance Frequency Breathing on Measures of Heart Rate Variability, Blood Pressure, and Mood. Front Public Health. 2017 Aug 25;5:222. doi: 10.3389/fpubh.2017.00222. PMID: 28890890; PMCID: PMC5575449.
Homma I, Masaoka Y. Breathing rhythms and emotions. Exp Physiol. 2008 Sep;93(9):1011-21. doi: 10.1113/expphysiol.2008.042424. Epub 2008 May 16. PMID: 18487316.
Lum LC. Hyperventilation: the tip and the iceberg. J Psychosom Res. 1975;19(5-6):375-83. doi: 10.1016/0022-3999(75)90017-3. PMID: 1214233.
Macey PM. Damage to the hippocampus in obstructive sleep apnea: a link no longer missing. Sleep. 2019 Jan 1;42(1):zsy266. doi: 10.1093/sleep/zsy266. PMID: 30615182; PMCID: PMC6765131.
Owen JE, BenediktsdÓttir B, Gislason T, Robinson SR. Neuropathological investigation of cell layer thickness and myelination in the hippocampus of people with obstructive sleep apnea. Sleep. 2019 Jan 1;42(1). doi: 10.1093/sleep/zsy199. PMID: 30346595.
Kiesel K, Rhodes T, Mueller J, Waninger A, Butler R. DEVELOPMENT OF A SCREENING PROTOCOL TO IDENTIFY INDIVIDUALS WITH DYSFUNCTIONAL BREATHING. Int J Sports Phys Ther. 2017 Oct;12(5):774-786. PMID: 29181255; PMCID: PMC5685417.
Carrasco-Llatas M, O'Connor-Reina C, Calvo-Henríquez C. The Role of Myofunctional Therapy in Treating Sleep-Disordered Breathing: A State-of-the-Art Review. Int J Environ Res Public Health. 2021 Jul 8;18(14):7291. doi: 10.3390/ijerph18147291. PMID: 34299742; PMCID: PMC8306407.
Hsu YB, Lan MY, Huang YC, Kao MC, Lan MC. Association Between Breathing Route, Oxygen Desaturation, and Upper Airway Morphology. Laryngoscope. 2021 Feb;131(2):E659-E664. doi: 10.1002/lary.28774. Epub 2020 May 30. PMID: 32473063.
McKeown P, O'Connor-Reina C, Plaza G. Breathing Re-Education and Phenotypes of Sleep Apnea: A Review. J Clin Med. 2021 Jan 26;10(3):471. doi: 10.3390/jcm10030471. PMID: 33530621; PMCID: PMC7865730.
Hsu B, Emperumal CP, Grbach VX, Padilla M, Enciso R. Effects of respiratory muscle therapy on obstructive sleep apnea: a systematic review and meta-analysis. J Clin Sleep Med. 2020 May 15;16(5):785-801. doi: 10.5664/jcsm.8318. Epub 2020 Feb 6. PMID: 32026802; PMCID: PMC7849810.
Camacho M, Certal V, Abdullatif J, Zaghi S, Ruoff CM, Capasso R, Kushida CA. Myofunctional Therapy to Treat Obstructive Sleep Apnea: A Systematic Review and Meta-analysis. Sleep. 2015 May 1;38(5):669-75. doi: 10.5665/sleep.4652. PMID: 25348130; PMCID: PMC4402674.
Genta PR, Kaminska M, Edwards BA, Ebben MR, Krieger AC, Tamisier R, Ye L, Weaver TE, Vanderveken OM, Lorenzi-Filho G, DeYoung P, Hevener W, Strollo P. The Importance of Mask Selection on Continuous Positive Airway Pressure Outcomes for Obstructive Sleep Apnea. An Official American Thoracic Society Workshop Report. Ann Am Thorac Soc. 2020 Oct;17(10):1177-1185. doi: 10.1513/AnnalsATS.202007-864ST. PMID: 33000960; PMCID: PMC7640631.