Less invasive indicators of changes in thermodilution cardiac output by ventilatory changes after cardiac surgery

We studied whether changes in less invasive, noncalibrated pulse-contour cardiac output (by modified ModelFlow, COmf) and derived stroke volume variations (SVV), as well as systolic and pulse pressure variations, predict changes in bolus thermodilution cardiac output (COtd), evoked by continuous and cyclic increases in intrathoracic pressure by increases in positive end-expiratory pressure (PEEP) and tidal volume (Vt), respectively.

Prospective study on 17 critically ill postcardiac surgery patients on full mechanical ventilatory support, in the intensive care unit.

In contrast to systolic pressure variation and pulse pressure variation, SVV increased from (mean +/- SD) 6.2 +/- 4.4 to 8.1 +/- 5.6 at PEEP 10 cmH2O (P = 0.064) and to 7.8 +/- 3.5% at PEEP 15 cmH2O (P = 0.031), concomitantly with a 12 +/- 7 and 11 +/- 8% decrease in COmf and COtd (P < 0.001), respectively. For pooled data, changes in COmf correlated with those in COtd (r = 0.55, P = 0.002), but changes in SVV did not. Variables did not change when Vt was increased up to 50%.

A fall in COmf is more sensitive than a rise in SVV, which is more sensitive than systolic pressure variation and pulse pressure variation, in tracking a fall in COtd during continuous (and not cyclic) increases in intrathoracic pressure, in mechanically ventilated patients after cardiac surgery. This suggests a reduction in biventricular preload as the main factor in decreasing cardiac output and increasing SVV with PEEP.