Card: Permissive Hypercapnia

Permissive hypercapnia - or controlled mechanical hypoventilation - is a ventilation strategy, whereby priority is given to the prevention or limitation of severe pulmonary hyperinflation over the maintenance of normal alveolar ventilation, provided oxygenation is preserved. Tidal volumes are adjusted to prevent hyperinflation and increased airway pressures. Allowing alveolar hypoventilation will invariably lead to hypercarbia and respiratory acidosis. Several investigators have reported that hypoventilation is safe and may improve outcome. The resulting acidosis is less severe and the elevation in arterial PCO₂ is generally well tolerated, when carbon dioxide levels are allowed to increase gradually.

Physiologic effects of hypercapnia:

a) At cellular level:

CO₂ readily diffuses across cell membranes and leads to the formation of carbonic acid and generation of hydrogen ions. Fortunately, intracellular buffering is rapid, reaching 90% completion within three hours after the onset of hypercapnia.

b) Cardiac effects:

Acute hypercapnia increases sympathetic nervous system discharge. As a result, plasma levels of epinephrine and norepinephrine rise, leading to increased myocardial contractility and cardiac output but also increased risk for cardiac arrhythmias.

c) Effects on central nervous system

Hypercapnia leads to cerebral vasodilation, increased intracranial pressure, and lowering of the seizure threshold.

Alveolar hypoventilation has the potential for lowering the arterial PO₂. Although the use of supplemental oxygen allows for substantial reduction of minute ventilation without jeopardizing oxygenation. In addition, hypercapnic acidosis shifts the oxyhemoglobin dissociation curve to the right, thereby promoting oxygen release at the tissue level.

Current uncontrolled studies suggest that permissive hypercapnia is an acceptable strategy for limiting airway pressures and avoiding volutrauma in patients with acute lung injury. However, controlled randomized trials are lacking. How low a pH and how high a pCO₂ can be safely tolerated is unknown. A pH >7.24 is widely accepted, but pHs as low as 7.05 have been accepted without obvious adverse effects. Hypercarbia is generally not harmful, provided that oxygenation is preserved and conditions of increased susceptibility of respiratory acidosis are avoided. Because of its effect on cerebral vascular tone and seizure threshold, permissive hypercapnia is contraindicated in patients with cerebrovascular disease, trauma, space-occupying intracranial lesions and seizure disorders. Further data are needed concerning the safety of hypercapnia in patients with coronary artery disease and heart failure.