Hypocapnia is a condition defined by the arterial partial pressure of CO2 being below 35mmHG which in turn causes blood plasma pH to rise above 7.45… resulting in respiratory alkalosis… pH that is too high! Hypocapnia that is a function of dysfunctional breathing (breathing too deep or too fast to offset metabolic requirements) is known as behavioural hypocapnia.

Behavioural hypocapnia is a condition where the partial pressure of arterial blood CO2 levels (PaCO2) falls below 35mmHg from over-breathing. This, in turn, pushes pH of interstitial fluids beyond their thresholds as per the Henderson-Hasselbach equation listed above. Behavioural hypocapnia is often learned and reinforced subconsciously, so can be difficult to undo without professional assistance and the discovery process of personal reinforcements that will unknowingly keep a dysfunctional breathing habit in place. Consequences of long-term hypocapnia (symptoms) are again outlined in figure 1.1. The figure below shows a fMRI of a brain that has less blood-flow due to vasoconstriction… one result of over-breathing. For every 1mm Hg below 40mmHg of CO2 partial pressure, the average blood flow reduction is 2%. Many people who overbreadth have CO2 levels down at  25 – 35mmHG… an oxygen/blood flow reduction of 20% to 30%!

HYPOCAPNIA is a PaCO2 deficit.

↑ pH = [HCO3¯] ÷ PCO2↓

Effects of 1 minute of voluntary hyperventilation on brain oxygen levels
(vasoconstriction due to a lack of CO2

From: Laffey, J. & Kavanagh, B. Hypocapnia. New England Journal of Medicine. 2002.

“…Extensive data from a spectrum of physiological systems indicate that hypocapnia has the potential to propagate or initiate pathological processes. As a common aspect of many acute disorders, hypocapnia may have a pathogenic role in the development of systemic diseases.”

“Increasing evidence suggests that hypocapnia appears to induce substantial adverse physiological and medical effects.”

“Hypocapnia has been clearly linked to the development of arrhythmias, both in critically ill patients and in patients with panic disorder.”

“…Further lowering of the partial pressure of arterial CO2 – even for a short duration -such ass during anesthesia for cesarean section -may have serious adverse effects on the fetus.”

“Hypocapnia is a common finding in patients with sleep apnea and may be pathogenic.”

“The causative role of hypocapnia in postoperative cognitive dysfunction is underscored by the finding that exposure to an elevate partial pressure of arterial carbon dioxide [I.e, normalising CO2 levels] during anesthesia appears to enhance postoperative neuropsychologic performance .”

NEJM article, Hypocapnia