Permissive hypercapnia is a ventilation strategy that allows carbon dioxide levels to rise more than normal, which can benefit patients with certain lung problems. It may have a protective effect on the lungs and other organs, but is not recommended for patients with brain or heart and circulatory problems.
Permissive hypercapnia is a strategy that is sometimes used during artificial ventilation of patients who are having difficulty breathing. Hypercapnia means there is too much carbon dioxide in the blood. Normally, when patients are given artificial ventilation, doctors aim to keep blood levels of oxygen and carbon dioxide within normal limits. For some patients with certain lung problems, this can mean that the pressure and volume of air entering the lungs becomes high enough to stretch and damage the fragile lung tissues. Permissive hypercapnic ventilation (PHV) is an alternative approach, which allows carbon dioxide levels to rise more than normal, so that ventilation can be delivered at lower, less potentially harmful pressures and volumes.
When carbon dioxide levels in the blood rise more than normal, this changes the body’s pH, making it lower and more acidic than usual. The body tries to compensate for this by breathing more deeply, and the kidneys regulate bicarbonate levels in an effort to make the blood more alkaline. Traditionally, when permissive hypercapnia was performed, doctors would give the patient something like bicarbonate to bring the pH up, but more recently it has been thought that a low pH may have a protective effect on the lungs and other organs. This protection could arise because hypercapnia prevents some cells from releasing inflammatory substances.
While permissive hypercapnia is not suitable for all patients, it has been suggested that it may benefit some patients with a condition where fluid and protein leak into the lungs, causing sudden difficulty breathing. This condition, known as adult respiratory distress syndrome, occurs more often in people with chest infections. It can also affect patients with a number of other problems, including serious accidental injuries. Other patients who may benefit from permissive treatment of hypercapnia include those suffering from severe asthma attacks and those suffering from lung disease, emphysema and bronchitis.
Permissive hypercapnia is not recommended for some patients, such as those with brain injury. This is because one of the effects of hypercapnia is the widening of blood vessels in the brain. This enlargement increases blood flow and pressure within the brain, making bleeding in the brain more likely.
Patients with heart and circulatory problems may also be at risk from lax treatment for hypercapnia, as higher levels of carbon dioxide can affect the strength and rhythm of the heartbeat. This could cause complete collapse of circulation. Because of its potential risks, permissive hypercapnia is used only for patients in whom the risks from normal ventilation are greater than the risks from hypercapnia.
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