Augmented arterial to end-tidal PCO2 difference during laparoscopic CO2 insufflation in man

Acid-base status of the blood and tissue fluids and pulmonary gas exchange were continuously observed during intra-abdominal CO2 insufflation for 2 h in 8 paralyzed and artificially ventilated patients who underwent laparoscopic cholecystectomy or resection of the sigmoid colon. Carbon dioxide pressure in the arterial and mixed venous blood as well as in end-tidal air started to increase already at 5 min CO2 insufflation. On the other hand, appreciable elevation in VCO2 and tissue PCO2 (PTCO2) was noted only after 15 min. In vivo buffer value (beta) was well within the normal physiological range at 5 min and gradually declined up to 2 h. These observations indicated that chemical buffering to CO2 and redistribution of buffer base among the blood and tissue fluids were slowly developed during entire period of observation. In reflecting these experimental findings, arterial to end-tidal PCO2 difference (a-ADCO2) and respiratory gas exchange ratio (R) were promptly and significantly increased at 5 min and maintained slow increment up to 2 h. We conclude that these profiles of a-ADCO2 and R can be explained by initially rapid and subsequently slow augmentation in ventilation-perfusion ratio (VA/Q) during the specific type of respiratory acidosis elicited in this study.