Objective To determine the influence of a low-dose constant rate infusion (LCRI; 50 µg kg−1 minute−1) and high-dose CRI (HCRI; 200 µg kg−1 minute−1) lidocaine infusion on the minimum alveolar concentration (MAC) of isoflurane (I) in dogs.
Study design Prospective experimental study.
Animals Ten mongrel dogs (four females, six males), weighing 20–26.3 kg.
Methods Dogs were anesthetized with I in oxygen and their lungs mechanically ventilated. Baseline MAC was determined using mechanical or electrical stimuli. Lidocaine (2 mg kg−1 IV) was administered over 3 minutes, followed by the LCRI and MAC determination commenced 30 minutes later. Once MAC was determined following LCRI, the lidocaine infusion was stopped for 30 minutes. A second bolus of lidocaine (2 mg kg−1, IV) was administered, followed by the HCRI and MAC re-determined. Concentrations of lidocaine and its metabolites were measured at end-tidal I concentrations immediately above and below MAC. Heart rates and blood pressures were measured.
Results Minimum alveolar concentration of I was 1.34 ± 0.11 (%; mean ± SD) for both types of stimulus. The LCRI significantly reduced MAC to 1.09 ± 0.13 (18.7% reduction) and HCRI to 0.76 ± 0.10 (43.3% reduction). Plasma concentrations (ng mL−1, median; value below and above MAC, respectively) for LCRI were: lidocaine, 1465 and 1537; glycinexylidide (GX), 111 and 181; monoethylglycinexylidide (MEGX), 180 and 471 and for HCRI were: lidocaine, 4350 and 4691; GX, 784 and 862; MEGX, 714 and 710. Blood pressure was significantly increased at 30 minutes after high dose infusion.
Conclusion and clinical relevance Lidocaine infusions reduced the MAC of I in a dose-dependent manner and did not induce clinically significant changes on heart rate or blood pressure.