Effect of Halothane on Critical Levels of Oxygen Transport in the Anesthetized Newborn Lamb

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Abstract
A critical level of oxygen transport has been defined as the level which is required to maintain tissue oxygen uptake (.ovrhdot.VO2). If halothane reduces the susceptibility to hypoxia, it should lower the critical levels of both O2 delivery (.ovrhdot.DO2) and arterial oxygen tension (pO2). To test this hypothesis, 12 newborn lambs were anesthetized with either fentanyl and pancuronium (control group) or fentanyl, pancuronium, and 1.1% (1 MAC) halothane (halothane group). Baseline measurements of hemoglobin, cardiac output (CO), arterial and mixed-venous pO2, and saturation were obtained on FIO2 1.0, and repeated with FIO2 .21, .15, and .10. O2 delivery (CO .times. CaO2) and O2 consumption were calculated from measured parameters. Critical levels were selected using a system of repetitive linear regression. Halothane decreased baseline O2 consumption (12.1 .+-. 0.7 to 8.4 .+-. 0.4 cc .cntdot. kg-1 .cntdot. min-1, x .+-. SEM, P < .001, unpaired t test), but caused similar reductions in cardiac output (235 .+-. 15 to 132 .+-. 15 cc .cntdot. kg-1 .cntdot. min-1, P < .001) and O2 delivery (29.2 .+-. 2.9 to 20.2 .+-. 1.6 cc .cntdot. kg-1 .cntdot. min-1, P < .05). Addition of halothane decreased the critical level of O2 delivery from 17.9 to 14.3 cc .cntdot. kg-1 .cntdot. min-1, but had no effect on the critical level of arterial PO2 (control group, 47 mmHg halothane, 46 mmHg). Peripheral oxygen utilization was mildly reduced during halothane anesthesia, as evidenced by a decrease in oxygen extraction (control group O2 extraction rate = 0.63; halothane group O2 extraction = 0.51, P < 0.5, unpaired t test). The reduction in critical .ovrhdot.DO2 suggests that halothane, by reducing tissue oxygen requirements, may be protective in hypoxemic patients.