A Balancing Act for Carbon Dioxide

Jun 26

Carbon dioxide has an importance in the body far beyond that of a mere by-product of metabolism. Released from cells during the production of energy from glucose, carbon dioxide diffuses rapidly into the bloodstream and hence into the gas spaces in the lungs. Here the rising concentration of carbon dioxide is diluted with each fresh breath, finding a steady state in equilibrium with arterial blood which circulates back to the body.

A proportion of the carbon dioxide dissolved in plasma is hydrated to form carbonic acid, H2 CO3 which lowers plasma pH. Chemoreceptors in the brain stem detect any change in pH and take corrective action by signaling the respiratory control centre. A fall in pH leads to hyperventilation which lowers the concentration of CO2 in the lung and hence the blood. Less carbonic acid is formed and the pH rises back to normal.

These compensatory mechanisms are demonstrated by the rapid increase in breathing during exercise. Cellular processes depend on enzymes which are exquisitely sensitive to changes in pH, such that a fall from the normal 7.4 to below 7 may prove rapidly fatal.

During anesthesia, the patients’ breathing is often controlled by a mechanical ventilator, short circuiting the normal pH control mechanism. Over or under ventilation by the ventilator will disturb the patient’s plasma pH with potentially serious consequences in the unfit or elderly, producing for instance, irregularities of the heart rhythm. These hazards are avoided by the use of capnograph.

Gas sampled at the end of exhalation comes from deep within the lung and has apartial pressure of carbon dioxide close to that of arterial blood. This end-tidal carbon dioxide is shown as the plateau of the capnograph waveform in the figure. Ventilation is adjusted to maintain a normal end-tidal carbon dioxide and hence arterial pH.

Capnography also warns of several uncommon but serious complications of anesthesia. A sudden fall in end-tidal CO2 occurs when blood flow to the lung is obstructed, for instance when air accidentally enters the circulation and heart chambers. A rapid rise is a sign of malignant hyperpyrexia, a very rare but potentially fatal reaction to certain anesthetic drugs. Early recognition allows an antidote to be administered.

The capnograph was designed as a physiological monitor. Its added role, in the detection of technical mishaps such as disconnection of the breathing circuit, is part of the new concept of patient safety monitoring.

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