What Is Permissive Hypercapnia?

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  • Written By: H. Colledge
  • Edited By: Heather Bailey
  • Last Modified Date: 16 September 2019
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Permissive hypercapnia is a strategy which is sometimes used while artificially ventilating patients having difficulty breathing. Hypercapnia means that there is too much carbon dioxide in the blood. Normally, when patients are artificially ventilated, 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 the air entering the lungs becomes high enough to stretch and damage fragile lung tissues. Permissive hypercapnia ventilation (PHV) is an alternative approach, which allows carbon dioxide levels to rise higher than normal, so that ventilation can be carried out at lower, less potentially damaging, pressures and volumes.

When the levels of carbon dioxide in the blood rise higher 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 harder, and the kidneys adjust bicarbonate levels in an attempt to make the blood more alkaline. Traditionally, when carrying out permissive hypercapnia, doctors would give the patient something like bicarbonate to bring the pH back up, but more recently it has been thought that a low pH might have a protective effect on the lungs and other organs. This protection could arise because hypercapnia prevents some cells from releasing inflammatory substances.


Although permissive hypercapnia is not suitable for all patients, it has been suggested that it could benefit some patients with a condition in which fluid and proteins leak into the lungs, causing sudden breathing difficulties. This condition, known as adult respiratory distress syndrome, most often occurs in people with chest infections. It can also affect patients with a range of other problems, including serious accidental injuries. Other patients who might benefit from permissive hypercapnia treatment include those experiencing severe asthma attacks and those who are suffering from the lung diseases emphysema and bronchitis.

Permissive hypercapnia is not recommended for certain patients, such as those with brain injuries. This is because one of the effects of hypercapnia is widening of the blood vessels in the brain. Such widening increases the flow of blood and the pressure inside the brain, making brain hemorrhages more likely.

Patients with heart and blood circulation problems may also be at risk from permissive hypercapnia treatment, as higher levels of carbon dioxide can affect the strength and rhythm of the heart beat. This could cause the circulation to collapse completely. Due to its potential risks, permissive hypercapnia is only used for patients where the risks from normal ventilation are greater than the risks of hypercapnia.


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