Evaluation of cardiopulmonary effects of total intravenous anesthesia using a drug combination of medetomidine,lidocaine,butorphanol and propofol (MLBP-TIVA) with or without controlled ventilation and effect of positioning in mechanically ventilated horses

The purpose of this study was to evaluate cardiopulmonary effects of MLBP-TIVA with or without controlled ventilation in horses and to determine effects of positioning on cardiovascular function in mechanically ventilated horses.Five healthy adult horses were anesthetized with MLBP-TIVA for 2-hours on 3-occasions at 4-week interval with (CMV-group) or without controlled mechanical ventilation (SB-group) and in lateral (LR-group) or dorsal (DRgroup) recumbency.Anesthesia was induced with lidocaine (1 mg/kg: IV) and propofol (3 mg/kg: IV) following premedications with medetomidine (0.005 mg/kg: IV) and butorphanol (0.02 mg/kg: IV),and then maintained by propofol infusion combined with constant rate infusions of medetomidine (0.0035 mg/kg/h),lidocaine (3 mg/kg/h)and butorphanol (0.024 mg/kg/h).Data were compared between groups using repeated-measures ANOVA.The level of significance was set at P < 0.05.Cardiovascular parameters were maintained within acceptable ranges in SB-group.However,severe hypercapnia with insufficient oxygenation was observed (PaCO2: 83 ~103 mmHg,arterial O2pressure [PaO2]: 155 ~171 mmHg).In CMV-group,significant decreases in heart rate (29 ~ 31 beats/min,P = 0.020),cardiac output (17 ~ 21 L/min,P= 0.005) and PaCO2 (42 ~ 50 mmHg,P = 0.001) and a significant increase in PaO2 (395 ~ 419 mmHg,P =0.005) were observed compared to SB-group.Mean pulmonary arterial pressure and mean atrial blood pressure were significantly decreased in DR-group compared to LR-group (18 ~ 20 mmHg vs 27 ~ 31 mmHg,P = 0.001 and 8 ~ 9 mmHg vs 15 ~ 17 mmHg,P = 0.003,respectively).Arterial O2pressure was significantly lower in DR-group than in LR-group (171 ~301 mmHg vs 385 ~416 mmHg,P =0.043).Cardiovascular functions were maintained within acceptable range during MLBP-TIVA in horses.Controlled ventilation is useful for improving hypercapnia and oxygenation.Also,MLBP-TIVA provides a preservation of cardiovascular function leading to compensation for decreased preload in horses positioned in dorsal recumbency.