Therapeutic hyperventilation

 Q: In "therapeutic hyperventilation", what is the rate of decrease in cerebral blood flow (CBF) per 1 mm Hg decrease in PCO2?

Answer: 1 mmHg decrease in PaCO2 avails about 3 percent drop in cerebral blood flow (CBF). 

 Hyperventilation has been utilized in acute situations to lower the Intra-Cranial Pressure (ICP). In acute situations, hyperventilation can be instituted to bring PaCO2 down to 26-30 mmHg. This results in vasoconstriction, and so decreases the intracranial blood volume. An additional benefit of this therapeutic alkalosis is to achieve some shield against post-injury acidosis. 

Said that this effect last somewhere from 1 to 24 hours. More importantly, is to be careful in reversing therapeutic hyperventilation. The normal ventilatory rate should be brought back over hours to avoid the rebound effect. 

Therapeutic hyperventilation should be used as an acute rather desperate intervention when elevated ICP threatens life or organ with cerebral edema, bleed, or mass. It should never be employed as a prophylactic measure. It should be used by an experienced person and should be used judiciously particularly in patients with traumatic brain injury (TBI) or acute stroke. Vasoconstriction due to respiratory alkalosis may cause a critical decrease in local cerebral perfusion and may worsen the injury.

#neurology

#neuruosurgery

#trauma

#surgical-critical-care

References:

1. Marik PE, Varon J, Trask T. Management of head trauma. Chest 2002; 122:699. 

2. Laffey JG, Kavanagh BP. Hypocapnia. N Engl J Med 2002; 347:43. 

3. Yundt KD, Diringer MN. The use of hyperventilation and its impact on cerebral ischemia in the treatment of traumatic brain injury. Crit Care Clin 1997; 13:163. 

4. Stocchetti N, Maas AI, Chieregato A, van der Plas AA. Hyperventilation in head injury: a review. Chest 2005; 127:1812. . 

5. Coles JP, Minhas PS, Fryer TD, et al. Effect of hyperventilation on cerebral blood flow in traumatic head injury: clinical relevance and monitoring correlates. Crit Care Med 2002; 30:1950. 

6. Imberti R, Bellinzona G, Langer M. Cerebral tissue PO2 and SjvO2 changes during moderate hyperventilation in patients with severe traumatic brain injury. J Neurosurg 2002; 96:97.