Effects of halothane, isoflurane, enflurane, thiopental, and fentanyl on blood gas values in rats exposed to hypoxia

In rats pretreated with phenobarbital breathing 10% oxygen, subanesthetic doses of halothane, isoflurane, enflurane, thiopental, and fentanyl caused hepatic injury. Because hypoxia per se can produce such injury, we hypothesized that the anesthetic-induced injury resulted from increased hypoxemia secondary to respiratory depression. Male Sprague-Dawley rats were pretreated with phenobarbital; half of the rats were fed and the other half were deprived of food for the 24 h before study. Isoflurane anesthesia was given for the placement of a catheter into the femoral artery. After 1 h of recovery, the rats were exposed to 10% oxygen. Control samples were obtained and halothane, isoflurane, enflurane, thiopental, or fentanyl was administered. Rats given food had higher PaCO2 and lower pH values than starved rats. Also, arterial oxygen saturation (SaO2) tended to be lower in rats given food. At concentrations of 0.15-0.2 MAC or higher, halothane, isoflurane, and enflurane slightly increased PaCO2 values relative to values for a control group exposed only to hypoxia. However, SaO2 and PaO2 did not show significant drug-induced changes. Fentanyl transiently decreased PaO2 and SaO2. Thiopental caused no changes. Thus, we conclude that subanesthetic doses of anesthetics may depress the ventilatory response to hypoxia but that this depression is inconsistent and appears to be too small to cause hepatic damage.