CIRCULATORY AND RESPIRATORY RESPONSES OF SPONTANEOUSLY BREATHING, LATERALLY RECUMBENT HORSES TO 12 HOURS OF HALOTHANE ANESTHESIA

Cardiovascular and respiratory changes that accompany markedly long pe riods (12 hours) of halothane anesthesia were characterized. Eight spo ntaneously breathing horses were studied while they were positioned in left lateral recumbency and anesthetized only with halothane in oxyge n maintained at a constant end-tidal concentration of 1.06% (equivalen t to 1.2 times the minimal alveolar concentration for horses). Results of circulatory and respiratory measurements during the first 5 hours of constant conditions were similar to those previously reported from this laboratory (ie, a time-related significant increase in systemic a rterial blood pressure, cardiac output, stroke volume, left ventricula r work, PCV, plasma total solids concentration, and little change in r espiratory system function). Beyond 5 hours of anesthesia, arterial bl ood pressure did not further increase, but remained above baseline. Ca rdiac output continued to increase, because heart rate significantly ( P < 0.05) increased. Peak inspiratory gas flow increased significantly (P < 0.05) in later stages of anesthesia. There was a significant dec rease in inspiratory time beginning at 4 hours. Although PaO2 and PaCO 2 did not significantly change during the 12 hours of study, Pv(O2)BAR increased significantly (P < 0.05) and progressively with time, begin ning 6 hours after the beginning of constant conditions. Metabolic aci dosis increased with time (significantly [P < 0.051 starting at 9 hour s), despite supplemental IV administered NaHCO3. Plasma concentrations of eicosanoids: 6-keto-prostaglandin F1alpha (PGF1alpha, a stable met abolite of PGI2), PGF2alpha, PGE, and thromboxane (TxB2, a stable meta bolite of TxA2) were measured in 5 of the 8 horses before and during a nesthesia. Significant changes from preanesthetic values were not dete cted. Dynamic thoracic wall and lung compliances decreased with time.