Permissive hypercapnia

Overview
Permissive hypercapnia is hypercapnia, (i.e. high concentration of carbon dioxide in blood), in respiratory insufficient patients in which oxygenation has become so difficult that the optimal mode of mechanical ventilation (with oxygenation in mind) is not capable of exchanging enough carbon dioxide. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs.

In acute respiratory distress syndrome (ARDS), permissive hypercapnia (allowing increased CO2 retention) by decreasing the tidal volume on the ventilator (usually 10-15 mL/kg/min) to 8 mL/kg/min may decrease barotrauma by decreasing ventilatory peak airway pressures and leads to improved respiratory recovery. The permissive hypercapnia leads to respiratory acidosis which has negative side effects, but given that the patient is in ARDS, improving ventilatory function is more important.

Since hypoxemia is a major life threatening condition and hypercapnia is not, one might choose to accept the latter. Hence the term, "permissive hypercapnia."

Symptoms
Symptoms of early hypercapnia (i.e. where PaCO2 is elevated but not extremely so) include flushed skin, full pulse, extrasystoles, muscle twitches, hand flaps, and possibly a raised blood pressure. In severe hypercapnia (generally PaCO2 greater than 10 kPa or 75 mmHg), symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.