Hemodynamic effects of nitrous oxide in isoflurane anesthetized cats

Abstract

Nitrous oxide (N2O) can be used as an anesthetic adjuvant in cats in an attempt to decrease the cardiovascular depression induced by potent inhalant anesthetics. However, its cardiovascular effects in anesthetized cats have not, to our knowledge, been reported. Twelve healthy adult domestic short-haired cats were studied. Anesthesia was induced and maintained with isoflurane (ISO) in oxygen; ventilation was spontaneous. After instrumentation, end-tidal ISO concentration was set at 1.25 times the individual minimum alveolar concentration (MAC), determined in a previous study, and that concentration was maintained for the duration of the study. In addition, N2O was randomly administered at 0, 30, 50, and 70%, according to a Latin square design. Twenty-five-minutes of stabilization were allowed after each change of concentration. The ECG (lead II) and heart rate, arterial blood pressure, pulmonary arterial blood pressure, central venous pressure, core body temperature, and inspired and end-tidal O2, ISO and N2O concentrations were continuously monitored and recorded. At selected times, pulmonary capillary wedge pressure was measured, cardiac output was determined in triplicate and averaged, and arterial and mixed venous blood samples were collected for pH and blood gas analysis, packed cell volume, total protein determinations, and lactate concentration measurement. Cardiac index, stroke index, systemic and pulmonary vascular resistance indices, rate-pressure product, and left and right ventricular stroke work indices were calculated. Data were analyzed by a repeated measures Latin square followed by a Tukey test for two-by-two comparisons. Mean arterial pressure, central venous pressure, mean pulmonary arterial pressure, packed cell volume, PaCO2, rate–pressure product, and systemic and pulmonary vascular resistance indices increased significantly with 70% N2O, compared to 0 and, for most variables, 30%. Arterial and mixed venous pH decreased significantly with 70% N2O, compared to any other concentrations. The PaO2 decreased dose-dependently, according to FIO2. For 70% N2O, PaO2 was 122.5 ± 6.6 and PaCO2 was 43 ± 3.5 mm Hg (mean ± SE). The cardiac and stroke indices did not change. In conclusion, in cats anesthetized with 1.25 MAC of ISO, addition of 70% N2O induced an increase in systemic and pulmonary arterial pressures, which appeared to be related to a vasoconstrictive effect. Lower N2O concentrations did not significantly improve arterial pressure.