The relationship between arterial Po2 and mixed venous Po2 in response to changes in positive end‐expiratory pressure in ventilated patients

Abstract

The response of arterial PO(2) (P(a)O(2)) to airway pressure has been used as a measure of recruitment in mechanically ventilated patients. We hypothesised that mixed venous PO(2) (P(mv)O(2)) directly affects P(a)O(2). Sixteen patients with acute lung injury (ALI, lung injury score > or = 1) on volume-controlled mechanical ventilation (F(I)O(2) 0.40) were studied. Positive end-expiratory pressure (PEEP) was increased and decreased. Incremental PEEP increased median values of P(a)O(2), diminished venous admixture (Q(va)/Q(t)) and cardiac index, but maintained arterial PCO(2) and tissue O(2) uptake. These changes were reversed during decremental PEEP. However P(a)O(2) did not increase in 37% of PEEP steps and changes in P(a)O(2) correlated to those in P(mv)O(2) (r(s) = 0.45, p < 0.001). Changes in P(mv)O(2) contributed to changes in Q(va)/Q(t) in determining changes in P(a)O(2) (p < 0.05). P(mv)O(2) may be an independent determinant of P(a)O(2) during mechanical ventilation for ALI, so that dosing PEEP to recruit the lung should not be guided by arterial blood oxygenation alone. Arterial hypoxaemia with increasing PEEP may improve by reducing PEEP (or increasing tissue O(2) delivery), when the fall in P(mv)O(2) is greater than about 0.133 kPa.