Oxygen and carbon dioxide in the cerebral circulation during progression to brain death

The authors propose that for a moderate reduction of perfusion during progressive irreversible ischemia, oxygen extraction increases to maintain aerobic metabolism, and arteriojugular oxygen difference (AJDo2) increases. Because of reduced carbon dioxide washout, venoarterial difference in carbon dioxide tension (DPco2) increases, with no change in the DPco2/AJDo2 ratio. With further reduction of cerebral perfusion, the aerobic metabolism will begin to decrease, AJDo2 will decrease while DPco2 will continue to increase, and the ratio will increase. When brain infarction develops, the metabolism will be abated, no oxygen will be consumed, and no carbon dioxide will be produced.

The authors studied 12 patients with acute cerebral damage that evolved to brain death and collected intermittent arterial and jugular blood samples.

Four patterns were observed: (1) AJDo2 of 4.1 +/- 0.7 vol%, DPco2 of 6.5 +/- 1.9 mmHg, and a ratio of 1.55 +/- 0.3 with cerebral perfusion pressure of 62.5 +/- 13.4 mmHg; (2) a coupled increase of AJDo2 (5.8 +/- 0.7 vol%) and DPco2 (10.1 +/- 1.0 mmHg) with no change in ratio (1.92 +/- 0.14) and cerebral perfusion pressure (57.9 +/- 5.8 mmHg); (3) AJDo2 of 4.7 +/- 0.4 vol% with an increase in DPco2 (11.8 +/- 1 mmHg) and correspondingly higher ratio (2.7 +/- 0.2); in this phase, cerebral perfusion pressure was 39.7 +/- 10.5 mmHg; (4) immediately before diagnosis of brain death (cerebral perfusion pressure, 17 +/- 10.4 mmHg), there was a decrease of AJDo2 (1.1 +/- 0.1 vol%) and of DPco2 (5.3 +/- 0.6 mmHg) with a further ratio increase (5.1 +/- 0.8).

Until compensatory mechanisms are effective, AJDo2 and DPco2 remain coupled. However, when the brain's ability to compensate for reduced oxygen delivery is exceeded, the ratio of DPco2 to AJDo2 starts to increase.