Oxygen transport and venous admixture in the extremely obese. Influence of anaesthesia and artificial ventilation with and without positive end-expiratory pressure.

Eight extremely obese patients (mean weight 136 kg) were studied when awake and breathing air, and during anaesthesia with controlled ventilation (oxygen fraction in inspirate (FIO2): 0.5). During anaesthesia, the atients were first studied with zero end-expiratory pressure (ZEEP) ventilation. Then two different positive end-expiratory pressures (PEEP) were applied, 10 cmH2O and 15 cmH2O, in order to study the effect of an increase in functional residual capacity (FRC). Arterial oxygenation and oxygen availability, as well as cardiac output (QT) and venous admixture (QS/QT) were studied. With the institution of anaesthesia and ZEEP, the alveolar arterial oxygen tension difference (P(A-a)O2) rose from 3.5 +/- 1.1 to 28.4 +/- 2.6 kPa, and the oxygen availability fell from 1346 +/- 222 to 1039 +/- 239 ml/min, due to the additive effect of an increase in QS/QT from 10 +/- 4 to 21 +/- 5% and a fall in QT, from 7.7 +/- 1.2 to 5.5 +/- 1.1 1/min. With increasing levels of PEEP, despite a fall in P(A-a)O2, there was a reduction in oxygen availability. This was due to simultaneous reduction in QS/QT and QT. At a PEEP of 15cmH2O, the P(A-a)O2 was 21.2 +/- 7.1 kPa, oxygen availability 862 +/- 170 ml/min, QS/QT 13 +/- 4 and QT 4.4 +/- 0.6 1. It is concluded that PEEP ventilation significantly reduces QS/QT in extremely obese patients during anaesthesia and should be used in these patients if there is arterial hypoxemia despite a high FIO2.