Objective—To evaluate the isoflurane-sparing effects of lidocaine and fentanyl administered by constant rate infusion (CRI) during surgery in dogs.
Design—Randomized prospective study.
Animals—24 female dogs undergoing unilateral mastectomy because of mammary neoplasia.
Procedures—After premedication with acepromazine and morphine and anesthetic induction with ketamine and diazepam, anesthesia in dogs (n = 8/group) was maintained with isoflurane combined with either saline (0.9% NaCl) solution (control), lidocaine (1.5 mg/kg [0.68 mg/lb], IV bolus, followed by 250 μg/kg/min [113 μg/lb/min], CRI), or fentanyl (5 μg/kg [2.27 μg/lb], IV bolus, followed by 0.5 μg/kg/min [0.23 μg/lb/min], CRI). Positive-pressure ventilation was used to maintain eucapnia. An anesthetist unaware of treatment, endtidal isoflurane (ETiso) concentration, and vaporizer concentrations adjusted a nonprecision vaporizer to maintain surgical depth of anesthesia. Cardiopulmonary variables and ETiso values were monitored before and after beginning surgery.
Results—Heart rate was lower in the fentanyl group. Mean arterial pressure did not differ among groups after surgery commenced. In the control group, mean ± SD ETiso values ranged from 1.16 ± 0.35% to 1.94 ± 0.96%. Fentanyl significantly reduced isoflurane requirements during surgical stimulation by 54% to 66%, whereas the reduction in ETiso concentration (34% to 44%) observed in the lidocaine group was not significant.
Conclusions and Clinical Relevance—Administration of fentanyl resulted in greater isoflurane sparing effect than did lidocaine. However, it appeared that the low heart rate induced by fentanyl may partially offset the improvement in mean arterial pressure that would be expected with reduced isoflurane requirements.
Objective—To evaluate the effects of 2 remifentanil infusion regimens on cardiovascular function and responses to nociceptive stimulation in propofol-anesthetized cats.
Animals—8 adult cats.
Procedures—On 2 occasions, cats received acepromazine followed by propofol (6 mg/kg then 0.3 mg/kg/min, IV) and a constant rate infusion (CRI) of remifentanil (0.2 or 0.3 μg/kg/ min, IV) for 90 minutes and underwent mechanical ventilation (phase I). After recording physiologic variables, an electrical stimulus (50 V; 50 Hz; 10 milliseconds) was applied to a forelimb to assess motor responses to nociceptive stimulation. After an interval (≥ 10 days), the same cats were anesthetized via administration of acepromazine and a similar infusion regimen of propofol; the remifentanil infusion rate adjustments that were required to inhibit cardiovascular responses to ovariohysterectomy were recorded (phase II).
Results—In phase I, heart rate and arterial pressure did not differ between remifentanil- treated groups. From 30 to 90 minutes, cats receiving 0.3 μg of remifentanil/kg/min had no response to noxious stimulation. Purposeful movement was detected more frequently in cats receiving 0.2 μg of remifentanil/kg/min. In phase II, the highest dosage (mean ± SEM) of remifentanil that prevented cardiovascular responses was 0.23 ± 0.01 μg/kg/min. For all experiments, mean time from infusion cessation until standing ranged from 115 to 140 minutes.
Conclusions and Clinical Relevance—Although the lower infusion rate of remifentanil allowed ovariohysterectomy to be performed, a CRI of 0.3 μg/kg/min was necessary to prevent motor response to electrical stimulation in propofol-anesthetized cats. Recovery from anesthesia was prolonged with this technique.