Sunday August 26, 2007
Hypercapnia - Bench to bedside !


In this month of 'Critical Care Medicine' (September 2007) 1, an interesting lab study published from Japan, evaluating the permissive range of Paco2. Study examined how Paco2 affects microvascular changes, hemodynamics, and cardiac output in rabbits.

Background: Permissive hypercapnia is a potent vasoactive stimulus, adequate tissue perfusion and oxygen delivery to dilated microvessels may be restored. Diffusion of CO2 to tissues may alter intracellular or extracellular hydrogen-ion concentrations, influencing calcium channels. An indirect action of CO2 is activation of the sympathetic nervous and adrenal systems.

Method: A total of 31 white rabbits were anesthetized with pentobarbital. An ear chamber was prepared to examine blood vessels by intravital microscopy. The rabbits were mechanically ventilated with air, oxygen, and CO2. The values of Paco2 were adjusted to about 20 (hypocapnia), 40, 60, 80, 100, 125, 150, and above 250 mm Hg. After stabilization at each Paco2 level, microvascular changes were recorded with a microscope-closed video camera to permit analysis of arteriolar diameter and blood flow.

Results:

• The pH and heart rate decreased and mean blood pressure increased progressively as the Paco2 was increased.
• When Paco2 was increased from 20 to 80 mm Hg, vessel diameter, blood-flow velocity, and blood-flow rate increased markedly. Cardiac output increased slightly.
• When Paco2 exceeded 100 mm Hg, all of these variables decreased.
• When Paco2 exceeded 150 mm Hg, all variables were significantly lower than the control values.

Reference: Click to get abstract/article

1. Permissive range of hypercapnia for improved peripheral microcirculation and cardiac output in rabbits - Critical Care Medicine. 35(9):2171-2175, September 2007.