Carbon dioxide during and after the apnoea test – an illustration of the Haldane effect

Paco2 becomes greater than Pvco2 during apnoea testing for brain death diagnosis

“…This finding has been previously reported in arterial and mixed venous samples by Rigg and Cruickshank [4], who investigated CO 2 kinetics in two patients undergoing apnoea testing for brain death. They postulated that the reversal of the normal P a co 2 and co 2 (mixed venous) relationship during apnoea to be a consequence of the Haldane effect: the CO 2 dissociation curves are different in reduced and oxygenated haemoglobin.…”

“…5 shows the changes in one of these patients, with a normal gradient of beginning of the test and inversion from the second minute . Unlike Rigg and Cruikshank [4] we could not take mixed venous samples and our venous samples were from the central venous catheter. The repeated observation of this phenomenon in 20 patients of two hospitals over 18 months suggests that it was not a laboratory mistake.…”

Apnea Testing for the Determination of Brain Death: A Systematic Scoping Review

“…In non-ECMO patients, PvCO 2 is higher than PaCO 2 before AT, but after AT, PaCO 2 is higher than PvCO 2 [78]. This is likely due to CO 2 production by the lungs without elimination [79].…”

Apnea testing for the diagnosis of brain death

“…During apnea the normal relationship of venous PCO 2 being greater than arterial PCO 2 is reversed (32). Based on empirical findings derived from physiological observations, a PaCO 2 of about 60–70 mmHg would seem ideal as there is a rather linear increase in respiratory minute volume up to this level and a decrease thereafter (33).…”