The effects of blood pressure variations on end-tidal and arterial CO2 pressure differences in patients undergoing coronary artery bypass graft

Capnography, the measurement of CO2 in respiratory gases has become an integral part of anaesthesia monitoring. A decrease in end tidal CO2 is seen with a decrease in blood pressure and cardiac output if ventilation remains constant. Theaim of this study was to prospectively assess the effect of changes in the blood pressure (20% above or below from baseline) on arterial-to-end tidal carbon dioxide gradient [P(a-ET)CO2] and their relationship during general anesthesia for coronary artery bypass graft surgery.

Fifty patients undergoing coronary artery bypass graft surgery were selected. A standard anaesthetic procedure was followed for all cases. Systoloic and diastolic arterial blood pressure were recorded as baseline after induction ofgeneral anesthesia, 20% above or below from baseline was calculated in all patients. The end tidal (ET) CO2 and PaCO2 were recorded at basline arterial blood pressure and during 20% above or below from baseline.

A significant increase in P(a-ET)CO2 was noted from 20% above of baseline and baseline to 20% below of baseline. (4.1±3.3 vs. 6.4±4.7 vs. 7±3 respectively).The Pearson correlation coefficient was 0.5, 0.4 and 0.5 respectively. There was a significant correlation between PaCO2 andETCO2 in each of the three conditions. (p<0.001)

In conclusion, the clinical practice of predicting PaCO2 from ETCO2 must be tempered by recognition of the potential magnitude of P(a-ET) CO2 gradient, which is higher than normal during hypotension. Further investigations are recommended.