Hypoventilation following oxygen administration associated with alfaxalone-dexmedetomidine-midazolam anesthesia in New Zealand White rabbits.

OBJECTIVE

To investigate the relationship between oxygen administration and ventilation in rabbits administered intramuscular alfaxalone-dexmedetomidine-midazolam.

STUDY

DESIGN

Prospective, randomized, blinded study.

ANIMALS: A total of 25 New Zealand White rabbits, weighing 3.1-5.9 kg and aged 1 year.

METHODS

Rabbits were anesthetized with intramuscular alfaxalone (4 mg kg-1), dexmedetomidine (0.1 mg kg-1) and midazolam (0.2 mg kg-1) and randomized to wait 5 (n = 8) or 10 (n = 8) minutes between drug injection and oxygen (100%) administration (facemask, 1 L minute-1). A control group (n = 9) was administered medical air 10 minutes after drug injection. Immediately before (PREoxy/air5/10) and 2 minutes after oxygen or medical air (POSToxy/air5/10), respiratory rate (fR), pH, PaCO2, PaO2, bicarbonate and base excess were recorded by an investigator blinded to treatment allocation. Data [median (range)] were analyzed with Wilcoxon, Mann-Whitney U and Kruskal-Wallis tests and p < 0.05 considered significant.

RESULTS

Hypoxemia (PaO2 < 88 mmHg, 11.7 kPa) was observed at all PRE times: PREoxy5 [71 (61-81) mmHg, 9.5 (8.1-10.8) kPa], PREoxy10 [58 (36-80) mmHg, 7.7 (4.8-10.7) kPa] and PREair10 [48 (32-64) mmHg, 6.4 (4.3-8.5) kPa]. Hypoxemia persisted when breathing air: POSTair10 [49 (33-66) mmHg, 6.5 (4.4-8.8) kPa]. Oxygen administration corrected hypoxemia but was associated with decreased fR (>70%; p = 0.016, both groups) and hypercapnia (p = 0.016, both groups). Two rabbits (one per oxygen treatment group) were apneic (no thoracic movements for 2.0-2.5 minutes) following oxygen administration. fR was unchanged when breathing air (p = 0.5). PaCO2 was higher when breathing oxygen than air (p < 0.001).

CONCLUSIONS AND CLINICAL RELEVANCE: Early oxygen administration resolved anesthesia-induced hypoxemia; however, fR decreased and PaCO2 increased indicating that hypoxemic respiratory drive is an important contributor to ventilation using the studied drug combination.