Effect of low dose rate ketamine infusions on thermal and mechanical thresholds in conscious cats

Objective To determine the thermal and mechanical antinociceptive effects of two different subanesthetic constant rate infusions of racemic ketamine in cats. Study design Prospective, randomized, blinded, experimental study. Animals Eight healthy adult domestic shorthair cats (two intact females and six neutered males). Methods The thorax and the lower thoracic limbs of each cat were shaved for thermal (TT) and mechanical threshold (MT) testing and a cephalic catheter was placed. Three intravenous treatments of equivalent volume were given as loading dose (LD) followed by an infusion for 2 hours: (K5) 0.5 mg kg(-1) ketamine followed by 5 mu g kg(-1) minute(-1) ketamine infusion, (K23) 0.5 mg kg(-1) ketamine followed by 23 mu g kg(-1) minute(-1) ketamine infusion or (S) 0.9% saline solution. Effects on behavior, sedation scores, MT and TT were obtained prior to drug treatment and 0.25, 0.5, 0.75, 1, 1.5, 2, 2.25, 2.5 2.75, 3 hours then every 0.5 hours for 7 hours and 10, 12, 14 and 26 hours after loading dose administration. Results Ketamine induced mild sedation for the period of the infusion, no adverse behavioral effects were observed. Thermal threshold was significantly higher than baseline (K5: 44.5 +/- 0.7 degrees C; K23: 44.5 +/- 0.5 degrees C) at 15 minutes in the K5 group (46.8 +/- 3.5 degrees C) and at 45 minutes in the K23 group (47.1 +/- 4.1 degrees C). In the K23 group TT was significantly increased compared to S and K5 at 45 minutes. In K5 at 15 minutes MT (9.6 +/- 4.0 N) was different to baseline (6.1 +/- 0.8 N) and to the S group (5.9 +/- 2.3 N). Conclusion and clinical relevance Low dose rate ketamine infusions minimally affect thermal and mechanical antinociception in cats. Further studies with different nociceptive testing methods are necessary to assess whether ketamine could be a useful analgesic in cats.