Effects of norepinephrine and fluid administration on diaphragmatic O2 consumption in septic shock

The effects of norepinephrine infusion and fluid administration on diaphragmatic O2 consumption during endotoxic shock were assessed in spontaneously breathing anesthetized dogs. Blood flow was measured with the microsphere technique, and diaphragmatic venous blood was obtained via a catheter inserted into the left inferior phrenic vein. One group of dogs (n = 6) received 10 mg/kg Escherichia coli endotoxin intravenously (E group). In the second and third groups, blood pressure after endotoxin injection was restored by continuous infusion of norepinephrine tartrate (N group) or by infusion of normal saline and dextran infusion (F group). The animals were observed for 2 h after endotoxin injection. Cardiac output fell significantly in the E and N group, whereas it was restored in the F group. Minute ventilation and diaphragmatic pressure-time index rose twofold in the three groups of dogs. Diaphragmatic O2 consumption (VO2 di) increased substantially in the E group to a mean value of 3.46 ml X 100 g-1 X min-1, which was achieved by higher blood flow and by an increase in O2 extraction. In the N group, VO2 di was higher than control but was lower than that of the E group (mean value of 1.43 ml X 100 g-1 X min-1), which was achieved solely by increasing O2 extraction. In the F group, VO2 di was also lower than that of the E group (mean value of 1.51 ml X 100 g-1 X min-1), which was achieved by high diaphragmatic blood flow. Thus, at any given diaphragmatic task, the diaphragm consumed less O2 in the N and F group than in the E group.