An evaluation of acid-base changes following aortic cross-clamping using transcutaneous carbon dioxide monitoring.

To investigate the cause of acidosis following release of an aortic cross-clamp, we measured tissue PCO2 using a transcutaneous (TC) CO2 monitor placed below the level of the cross-clamp in 10 patients undergoing aortic arch surgery. Following placement of the aortic cross-clamp, the TC CO2 value from the lower extremity increased from 41 +/- 4 to 92 +/- 41, whereas there was no change in the TC CO2 value from the upper extremity. With release of the cross-clamp, end-tidal CO2 increased by 6.2 +/- 1.9 mmHg, the upper TC CO2 increased by 8.4 +/- 4.8 mmHg, and the lower extremity TC CO2 value returned to baseline. During cross-clamping, there was an increase in the base deficit of 4.3 +/- 2.9 when comparing the baseline arterial blood gas value with the one obtained after cross-clamp release (p = 0.0004). These data demonstrate that the acidosis occurring during aortic cross-clamping is a mixed metabolic and hypercarbic acidosis. Appropriate treatment includes the provision of adequate minute ventilation to ensure CO2 removal and the use of sodium bicarbonate based on the degree of metabolic acidosis demonstrated by arterial blood gas analysis.