Anesthesiology
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Studies with ethanol have indicated that dihydropyridine-sensitive calcium (Ca++) channels may be involved in the adaptation to prolonged exposure to ethanol. This study investigated the effects, in mice, of the dihydropyridine Ca++ antagonist, nitrendipine, on acute tolerance to nitrous oxide after 60 min exposure to anesthetizing concentrations, and also the withdrawal syndrome which occurred following removal from nitrous oxide. Control mice were anesthetized by nitrous oxide concentrations in the range 1.28-1.51 atmospheres. ⋯ Nitrendipine diminished or prevented nitrous oxide withdrawal seizures, in a dose-dependent manner (P less than 0.05 for nitrendipine 50 and 100 mg.kg-1). These results support the importance of the role of dihydropyridine-sensitive Ca++ channels in the mechanism of tolerance and dependence to central depressant drugs. They also suggest that acute and chronic tolerance to sedative drug action may share some common pathways, and that tolerance and physical dependence may share a common mechanism through voltage-operated Ca++ channels.
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The performance of three commercially available pulse oximeters was assessed in five anesthetized dogs in which increasing levels of methemoglobin were induced. Hemoglobin oxygen saturation in each dog was monitored with three pulse oximeters (Nellcor N-100, Ohmeda 3700, and Novametrix 500) and a mixed venous saturation pulmonary artery catheter (Oximetrix Opticath). Arterial and mixed venous blood specimens were analyzed for PaO2, PaCO2, and pHa using standard electrodes. ⋯ Plots of SpO2 versus functional saturation (oxyhemoglobin/reduced hemoglobin plus oxyhemoglobin) show an improved but still poor relationship (regression slopes from 0.32 to 0.46). The Oximetrix Opticath pulmonary artery catheter behaves similarly but provides somewhat better agreement with functional saturation than do the pulse oximeters in the presence of methemoglobinemia. Pulse oximetry data (SpO2) should be used with caution in patients with methemoglobinemia.
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An endotracheal tube with a 5 mm diameter ultrasonic transducer incorporated into its distal end was inserted into the tracheas of six anesthetized dogs. The device provides for ultrasound measurement of aortic diameter and blood velocity in the ascending aorta to obtain cardiac outputs. Because measurements are made proximal to the origins of the aortic arch vessels, total cardiac output is obtained. ⋯ R2 = .82. The results from this preliminary assessment of the feasibility of measuring cardiac output in dogs using transtracheal Doppler suggest that additional studies should be pursued. Additional issues that require investigation include confirmation of constancy of human anatomy and demonstration of lack of tracheal trauma from the Doppler equipped endotracheal tube.