Introduction

Previous studies indicate anti-nociceptive effects of tramadol and lidocaine infusions. However, the effects of intravenous infusion of both tramadol and lidocaine on minimum alveolar concentration (MAC) sparing effects of sevoflurane in dogs remain unclear.

Objectives

To compare the effects of continuous rate infusion (CRI) of tramadol and tramadol-lidocaine on MAC of sevoflurane and entropy indices in dogs.

Methods

Eight, healthy German Shepherds were induced and maintained with sevoflurane. A standard tail-clamp technique was used for MAC determination. The MAC was determined with only sevoflurane for MAC baseline (MAC_B), during infusions of tramadol (intravenous loading dose of 1.5 mg/kg and CRI of 2.6 mg/kg/h; MAC_T), and tramadol-lidocaine (tramadol CRI of 2.6 mg/kg/h and intravenous loading dose of 1.0 mg/kg lidocaine and CRI of 6 mg/kg/h; MAC_TL). Stated entropy (SE), response entropy (RE), and RE-SE difference were recorded at five minutes prior to and during tail-clamping.

Results

The MAC_B was 2.4±0.2%. Tramadol and tramadol-lidocaine decreased MAC to 2.2±0.3% and 1.7±0.3%, respectively. The MAC-sparing effect of tramadol-lidocaine was greater than tramadol alone (8.2±8.9% vs. 30.1±10.7%; p<0.01). All of SE, RE, and most RE-SE difference were increased (all p<0.05) when subjects responded purposefully to the noxious stimulation. While no response occurred, all of entropy indices did not change compared to pre-stimulation data.

Conclusions

In dogs, a combination of tramadol-lidocaine infusion can reduce anesthetic requirements in higher degree than tramadol alone. Entropy indices may change associated with nociceptive responses in anesthetized animals.