Influence of the mode of ventilation on ketamine/xylazine requirements in rabbits


  • The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense (AR 360-5).

Michael Dubick, US Army Institute of Surgical Research, 3400 Rawley E. Chambers Avenue, Fort Sam Houston, San Antonio, TX 78234, USA. E-mail:


Objective  To evaluate the effect of the mode of mechanical ventilation (MV) on the dose of intravenous anesthetic during 3 hours of ketamine/xylazine anesthesia.

Study design  Prospective laboratory study.

Animals  Sixty-one adult male New Zealand White rabbits.

Methods  Rabbits were anesthetized (ketamine/xylazine 35 + 5 mg kg−1, IM), the trachea was intubated and randomized to four groups – (1) CMV-1 (n = 14), ventilated with traditional conventional volume-cycled MV [VT = 12 mL kg−1, RR = 20, positive end-expiratory pressure (PEEP) = 0 cmH2O]; (2) CMV-2 (n = 13), ventilated with a modern lung-protective regimen of volume-cycled MV (VT = 6 mL kg−1, RR = 40, PEEP = 5 cmH2O); (3) HFPV (n = 17) ventilated with high-frequency percussive ventilation [high-frequency oscillations (450 minute−1) superimposed on 40 minute−1 low-frequency respiratory cycles, I:E ratio = 1:1], oscillatory continuous positive airway pressure (CPAP) of 7–10 cmH2O, and demand CPAP of 8–10 cmH2O. (4) A fourth group, spontaneously ventilating (SV, n = 17), was anesthetized, intubated, but not ventilated mechanically. FiO2 in all groups was 0.5. Anesthesia was maintained at a surgical plane by IV administration of a ketamine/xylazine mixture (10 + 2 mg kg−1, as necessary) for 3 hours after intubation. Total dose of xylazine/ketamine administered and the need for yohimbine to facilitate recovery were quantitated.

Results  The total dose of xylazine/ketamine was significantly higher in the HFPV and SV groups compared with CMV-1 (p < 0.01). Fewer animals required yohimbine to reverse anesthesia in the HFPV than CMV-1 group (p < 0.05).

Conclusions  The HFPV mode of MV led to higher doses of ketamine/xylazine being used than the other modes of MV.

Clinical relevance  In rabbits, anesthetic dose for the maintenance of anesthesia varied with the mode of MV used. Investigators should be aware of the possibility that changing the mode of ventilation may lead to an alteration in the amount of drug required to maintain anesthesia.