Definition of Hyperventilation and Its Physiology

How to define hyperventilation? The most common medical definition of hyperventilation is: Hyperventilation (or overbreathing) is the state of breathing that is faster and/or deeper than normal. This mechanical definition of hyperventilation is based on calculations of normal minute ventilation (which is 6 L/min at rest for a 70-kg man) and can be found in many sources (Wikipedia, National Institute of Health, WebMD, and many medical textbooks). Although this hyperventilation definition works in most situations, it is not suitable for some cases described below.

In contrast, Dr. Buteyko's definition of hyperventilation (or what he implied in relation to hyperventilation) is based on the pathological physiological effects that are caused by reduced CO2 levels in the alveoli of the lungs due to hyperventilation.

Indeed, the above medical hyperventilation definition incorrectly includes numerous positive situations, for example:
- Breathing gets much deeper and faster during physical exercise, however, alveolar and arterial CO2 increase for nose breathing (in and out) during exercise
- Fire breath in hatha yoga is an example of very fast breathing, but due to a small tidal volume, which is close to dead volume, arterial and alveolar CO2 may get even higher during this special breathing practice
- Buteyko reduced breathing exercise (where CO2 accumulation in the alveoli can be achieved by frequent and small breathing for students with less than 20 s CP) also leads to CO2 increase
- Breathing exercises with various breathing devices (the Frolov breathing device, Samozdrav device, Cosmic Breath, Amazing DIY breathing device, etc.) can be accompanied by increased minute ventilation (depending on the amount of additional volume) and low breathing frequency, but with increased CO2 levels in the alveoli of the lungs due to CO2 getting trapped in the device
- Breathing CO2-rich air (like carbogen and other mixtures) increases minute ventilation (leading to faster and deeper breathing), but alveolar and arterial CO2 concentrations usually become higher. (They could become lower, if a person is in a state of panic after starting to breathe CO2-rich air).

During all these situations, the levels of CO2 in the arterial blood and cells get higher than before these situations, while classical overbreathing (or voluntary hyperventilation with normal air) reduces cell O2 and CO2 content.

Many medical textbooks suggest defining hyperventilation based on arterial hypocapnia. The most common example of this is: Hyperventilation is a physiological state when the partial pressure of arterial CO2 is less than 35 mm Hg. However, people with ventilation-perfusion mismatch normally have elevated minute ventilation (e.g., over 10-12 L/min at rest for bronchitis, COPD, cystic fibrosis, etc.) with obvious alveolar hypocapnia (reduced CO2 in the alveoli of the lungs), but their arterial CO2 can be very high (e.g., up to 50 mm Hg and more). Physiologically, it is obvious that these patients require more CO2 in order to restore airways and lung tissues due to CO2 bronchodilating effects (expansion of airways) and the abilities of CO2 to heal alveoli. Breathing less for all these groups of patients leads to a reduction of the abnormally high arterial CO2 and an increase in arterial O2 content due to the reduced ventilation-perfusion abnormality. This effect can be easily confirmed using finger oximeters and other types of devices that measure oxygen content of the blood.

How to define hyperventilation

Hence, the most logical and physiologically strict way to define hyperventilation is the following. Hyperventilation is the physiological state of the human organism characterized by alveolar hypocapnia (CO2 deficiency in the alveoli of the lungs). This definition of hyperventilation is based on an abnormally low concentration of CO2 (carbon dioxide) in the functioning alveoli of the lungs, causing reduced oxygen transport, tissue hypoxia (low O2 in cells) and other pathological effects that intensify breathing.

This hyperventilation definition satisfies various practical situations with no exceptions due to its physiological (or biochemical) basis: abnormally low CO2 tension in the alveoli of the lungs. It has an important therapeutic value since it is based on known physiological and biochemical effects of carbon dioxide on airways, lung tissue, blood vessels, and other organs and tissues of the human body.

However, since alveolar CO2 is very difficult to measure, in most situations, the "mechanical" definition of hyperventilation works fine and this is the reason why many tables on this site quote minute ventilation. Indeed, when minute ventilation is above 10 L/min at rest, while metabolic rate might be only slightly above normal, it is obvious that this "mechanical" hyperventilation causes physiological hyperventilation (alveolar hypocapnia).

On the other hand, ventilation-perfusion abnormalities are confined to only a small portion of people with emphysema, severe asthma, severe bronchitis, COPD, cystic fibrosis, and some other conditions. Hence, arterial hypocapnia, as a definition and criterion of hyperventilation, is also a sensible idea because, for most people, in most situations, arterial hypocapnia means deep and/or fast breathing with low CO2 levels in the alveoli. This YouTube video (on the right side) provides the definition and info about prevalence of hyperventilation: Hyperventilation.

References

Dr. Buteyko's words about physical exercise and its relation to CO2 changes and physiology:
"Next is about physical activity, labor, and sports. Here again the fact that in our press and everywhere else, there are people who are illiterate in physiology. They have imposed upon us a thought, and again contrary to the truth, that physical activity, sport and labor deepen our breathing. This is quite the opposite! It is wrong to consider any function bureaucratically, as a fact detached from life. After all, breathing is done to ensure metabolism. Therefore, breathing must be considered in parallel with metabolism. It turns out that physical labor, sports, and workouts increase metabolism, i.e., they increase production of carbon dioxide and carbon dioxide increases, during exercise, in the blood, while oxygen is reduced. This is what physical exercise does." Dr. Buteyko's Lecture at the Moscow State University

Reference pages: Breathing norms and medical facts:
- Breathing norms: Parameters, graph, and description of the normal breathing pattern
- 6 breathing myths: Myths and superstitions about breathing and body oxygenation (prevalence: over 90%)
- Hyperventilation: Definitions of hyperventilation: their advantages and weak points
- Hyperventilation syndrome: Western scientific evidence about prevalence of chronic hyperventilation in patients with chronic conditions (37 medical studies)
- Normal minute ventilation: Small and slow breathing at rest is enjoyed by healthy subjects (14 studies)
- Hyperventilation prevalence: Present in over 90% of normal people (24 medical studies)
- HV and hypoxia: How and why deep breathing reduces oxygenation of cells and tissues of all vital organs
- Body-oxygen test (CP test) : How to measure your own breathing and body oxygenation (two in one) using a simple DIY test
- Body oxygen in healthy: Results for the body-oxygen test for healthy people (27 medical studies)
- Body oxygen in sick : Results for the body-oxygen test for sick people (14 medical studies)
- Buteyko Table of Health Zones: Clinical description and ranges for breathing zones: from the critically ill (severely sick) up to super healthy people with maximum possible body oxygenation
- Morning hyperventilation: Why people feel worse and critically ill people are most likely to die during early morning hours

References: pages about CO2 effect:
- Vasodilation: CO2 expands arteries and arterioles facilitating perfusion (or blood supply) to all vital organs
- The Bohr effect: How and why oxygen is released by red blood cells in tissues
- Cell oxygen levels: How alveolar CO2 influences oxygen transport
- Oxygen transport: O2 transport is controlled by vasoconstriction-vasodilation and the Bohr effects, both of which rely on CO2
- Free radical generation: Reactive oxygen species are produced within cells due to anaerobic cell respiration caused by cell hypoxia
- Inflammatory response: Chronic inflammation in fueled by the hypoxia-inducible factor 1, while normal breathing reduces and eliminates inflammation
- Nerve stabilization: People remain calm due to calmative or sedative effects of carbon dioxide in neurons or nerve cells
- Muscle relaxation: Relaxation of muscle cells is normal at high CO2, while hypocapnia causes muscular tension, poor posture and, sometimes, aggression and violence
- Bronchodilation: Dilation of airways (bronchi and bronchioles) is caused by carbon dioxide, and their constriction by hypocapnia (low CO2)
- Blood pH: Regulation of blood pH due to breathing and regulation of other bodily fluids
- CO2: lung damage: Elevated carbon dioxide prevents lung injury and promotes healing of lung tissues
- CO2: Topical carbon dioxide can heal skin and tissues
- Synthesis of glutamine in the brain, CO2 fixation, and other chemical reactions
- Deep breathing myth: Ignorant and naive people promote the idea that deep breathing and breathing more air at rest is beneficial for health
- Breathing control: How is our breathing regulated? Why hypocapnia makes breathing uneven, irregular and erratic.

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