Spontaneous variability of cardiac output in ventilated critically ill patients

Objective: To define the magnitude of spontaneous cardiac output variability over time in sedated medical intensive care unit patients attached to a continuous cardiac output monitor, and to determine whether high level positive end-expiratory pressure or inverse inspiratory-to-expiratory (I:E) ratio ventilation resulted in greater variability over time than low positive end-expiratory pressure with conventional I:E ratio ventilation. Design: Prospective study. Setting: Medical intensive care unit in a tertiary medical center, Patients: A total of 22 hemodynamically stable acute respiratory failure patients with a pulmonary artery catheter inserted for hemodynamic monitoring Interventions: After being sedated, patients were randomized alternately to receive pressure control ventilation first at setting A thigh positive end-expiratory pressure [15 cm H2O] with conventional I:E ratio [1:2]) and then at setting B (low positive end-expiratory pressure [5 cm H2O] with inverse I:E ratio [2:1]), or vice versa, and then at setting C (low positive end-expiratory pressure [5 cm H2O] with conventional I:E ratio [1:2]), Each ventilation setting period lasted 1 hr. Measurements and Main Results: Cardiac output (CO) was measured continuously. The continuous CO value displayed was updated every 30-60 sees. The updated value reflected an average of the previous 3-6 mins, The coefficient of variation (CV) of CO for each setting in each patient was calculated to represent the spontaneous variability. The mean CO +/- SD and CV of each setting was 5.7 +/- 1.8 L/min and 4.4% for setting A, 5.6 +/- 1.5 L/min and 4.6% for setting B, and 5.9 +/- 1.7 L/min and 4.8% for setting C, Analysis of variance revealed no significant differences between the CVs of the three settings. The 95% confidence interval for the COs for each setting was approximately the mean CO +/- 0.1 x mean CO measured. Conclusions: In critically ill sedated medical intensive care unit patients with stable hemodynamics, the spontaneous variability of cardiac output over time was not significant. High positive end-expiratory pressure (15 cm H2O) and inverse ratio ventilation (2:1) did not contribute to increased spontaneous variability of cardiac output.