We included all mechanically ventilated patients admitted directly from the emergency department (ED) to our ICU from August 2011 to December 2013, who also developed shock within the first 24 h of ICU stay. We chose to include only mechanically ventilated patients because these patients routinely received arterial cannulation and invasive blood pressure monitoring. Only the first ED to ICU admission was used in patients with multiple encounters during the study period. Patients with onset of shock beyond 24 h of ICU stay were excluded.
The presence of shock was defined as systolic blood pressure less than 90 mmHg or lactate greater than 4 mmol/L within the first 24 h of ICU stay. All systolic blood pressure readings less than 90 mmHg were routinely rechecked and confirmed by nurses before documentation. We did not consider a drop of more than 40 mmHg from baseline blood pressure because the latter was largely unknown in our practice. Shock Index was computed as heart rate divided by systolic blood pressure (15). Sepsis was defined according to the 1992 American College of Chest Physicians/Society of Critical Care Medicine Consensus Conference criteria, which required the presence of the systemic inflammatory response syndrome caused by infection (16).