Response of critically ill patients to increased oxygen demand: hemodynamic subsets.

OBJECTIVE

To ascertain how patients with different abnormalities of oxygen transport at rest respond to an acute increase in oxygen demand.

DESIGN

Observational study with retrospective assignment to subgroups, based on resting oxygen extraction ratio or increased cardiac output.

SETTING

University hospital surgical intensive care unit (n = 96).

PATIENTS

Postoperative, mechanically ventilated, critically ill patients (n = 96).

INTERVENTION

Chest physical therapy.

MEASUREMENTS AND MAIN RESULTS

Metabolic, hemodynamic, and respiratory measurements were made during an initial rest period and then during chest physical therapy. During chest physical therapy, patients (n = 10) having low resting oxygen extraction ratios (< or = 0.20) increased oxygen extraction, without changing oxygen delivery (DO2); while those patients (n = 19) with high resting oxygen extraction ratios (> or = 0.30) increased DO2, but not oxygen extraction. Patients (n = 46) with oxygen extraction ratios between 0.2 and 0.3 had an intermediate response; both DO2 and oxygen extraction increased. The group (n = 19) with increased resting cardiac output (> 9 L/min) and associated low resting oxygen extraction ratios and high DO2 values, increased their extraction of oxygen during chest physical therapy.

CONCLUSIONS

The response to an acute increase in oxygen demand was influenced by resting conditions and was characterized by the use of "reserve" capacity. Patients with a resting hyperdynamic state (high DO2 and low oxygen extraction) were able to further increase oxygen extraction during the increase in oxygen demand.