Effect of temperature on volatile anesthetic depression of myocardial contractions

Anesthetics are studied and used under conditions of hypothermia both experimentally and clinically. To determine if myocardial contractile depression by anesthetics is equivalent at different temperatures, depressant effects on guinea pig papillary muscle contractions were compared at 37 degrees C vs 30 degrees C. Stimulation after rest and at rates from 0.1 to 3 Hz was used. After control measurements were established at one temperature, effects due to equi-anesthetic concentrations (equivalent to approximately 0.9 minimum alveolar concentration) of halothane, isoflurane, or enflurane were determined, followed by recovery measurements. The same procedure was then applied at the other temperature. Hypothermia itself had positive inotropic effects: contractions at 30 degrees C developed about twice the tension as at 37 degrees C; the maximum rate of tension development (dT/dt-max) at 30 degrees C was about 50% greater than at 37 degrees C (1-3 Hz stimulation). When anesthetic contractile depression was assessed by the fractional decrease in dT/dt-max, the anesthetics were more depressant at 30 degrees C than at 37 degrees C, an effect which was most prominent at 0.5 Hz. In contrast to the anesthetics, 1 microM ryanodine which decreases sarcoplasmic reticulum (SR) Ca2+ release, and 0.1 microM nifedipine which decreases Ca2+ entry, both depressed dT/dt-max less at 30 degrees C than at 37 degrees C. Furthermore, ryanodine, nifedipine, and a decrease in external [Ca2+] all exhibited distinct rate-dependent contractile depression at 37 degrees C (typically greatest at 2-3 Hz) not seen with anesthetics.(ABSTRACT TRUNCATED AT 250 WORDS)