Objective

To present the preliminary results of the cardiovascular responses of ephedrine (0.2 mg/kg IV) to 5 client-owned dogs undergoing routine surgeries to treat hypotension (mean arterial blood pressure, MAP < 60 mmHg) under isoflurane anesthesia.

Materials & Methods

Healthy dogs (ASAII) that required orthopedic or soft tissue surgical procedures at the Veterinary Teaching Hospital of Ontario Veterinary College were recruited. All dogs were judged healthy based on physical examination with no clinical or laboratory evidence of dehydration. Routine anesthetic protocols were not altered for the study. A catheter was inserted in the dorsal pedal artery to monitor direct blood pressure (BP). Dogs were monitored and entered into the study before surgery. All measurements were completed without adjusting anesthetic depth. When hypotension unrelated to bradycardia, excessive anesthetic depth or blood loss developed, baseline (T0) cardiopulmonary parameters were determined, including lithium dilution cardiac output (CO). Ephedrine at 0.2 mg/kg IV was administered and direct BP monitored every 2.5 minutes until 10 minutes (T10). If MAP at T10 were < 70 mmHg, a second dose of ephedrine at 0.2 mg/kg IV were administered and BP were monitored every 2.5 minute until the next 10 minutes (T20). CO was determined at T10 & T20. ANOVA for repeated measures (p=0.05) was used to determine cardiopulmonary changes over time.

Results

Five dogs (3M, 2F), 2-5 years of age, 31.5±5.4 kg bwt, requiring surgical repair of ruptured cranial cruciate ligament were treated with ephedrine for hypotension. All dogs were premedicated with acepromazine (0.02-0.05 mg/kg IM) and hydromorphone (0.05-0.1 mg/kg IM). Two dogs did not receive glycopyrrolate while three had glycopyrrolate (0.005-0.01 mg/kg IM). Induction agents included thiopental (4.5-10 mg/kg IV) with or without diazepam (0.2 mg/kg IV) and ketamine-diazepam (1:1 mixture, 0.1 ml/kg IV). All dogs received epidural bupivacaine 0.5% (1 ml/5 kg) with morphine (0.15-0.2 mg/kg) or hydromorphone (0.1 mg/kg). End-tidal isoflurane when hypotension developed ranged from 0.6-1.2%. All dogs required the second dose of ephedrine at T10. Mean arterial pressure (mean±SD mmHg) increased from 57±4 at T0 to 80±14 at T2.5 but declined rapidly (69±14, 61±14 and 56±9 at T5, T7.5 and T10 respectively). Repeated dose of ephedrine increased MAP slightly (63±10, 63±12, 61±11 and 61±11 at T12.5, T15, T17.5 and T20 respectively) but were not significant statistically. In general, systolic and diastolic BP followed similar trends. Cardiac index tended to increase (83.68±10.54, 96.85±10.98 and 106±24.07 ml/kg/min at T0, T10 and T20 respectively) although not significant (p=0.158). PaO₂ was slightly higher (554.56±26.23 mmHg) at T10 compared to baseline (502.32±43.02 mmHg). There were no significant changes over time in heart rate, stroke volume, PaCO₂, hemoglobin concentration or oxygen delivery.

Conclusions

These preliminary results indicate that treatment with ephedrine (0.2 mg/kg IV) in hypotensive dogs under isoflurane anesthesia caused an increase in MAP for less than 5 minutes.

A second dose of ephedrine did not elicit an obvious increase in MAP, although CI tended to increase.