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

Abstract

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.