Endotracheal suctioning of intubated patients is associated with hemodynamic complications including arterial hypoxemia, cardiac arrhythmias, hypotension and even death. Prior investigations of this subject focus primarily on arterial hypoxemia. Our observations of ETS revealed significant falls in the mixed SvO2 and we postulated that alterations in VO2 or CO must be occurring. This study was then designed to determine the alterations in CO, VO2, SaO2 and the resulting effect on SvO2 during ETS. Ten critically ill intubated patients with pulmonary artery catheters in place, were studied during routine ETS by the nursing staff. We found that ETS produced a significant decrease in SvO2 which was predominantly due to an increased VO2 accompanied by an inadequate rise or even fall in CO. Alterations in SaO2 appeared to be modest and were insensitive indicators of alterations in SvO2.