The effect of experimentally induced hypothyroidism on the isoflurane minimum alveolar concentration in dogs.

OBJECTIVE

To determine the effect of experimentally induced hypothyroidism on isoflurane (ISO) minimum alveolar concentration (MAC) in dogs.

STUDY DESIGN

Prospective experimental study.

ANIMALS

Eighteen adult female mongrel dogs, age 2-4 years and weighing 8.2-13.1 kg.

METHODS

Hypothyroidism was induced in nine dogs by the intravenous administration of 1 mCi kg(-1) of (131) Iodine. The remaining nine dogs served as controls. Dogs were studied 9-12 months after the induction of hypothyroidism. Anesthesia was induced with ISO in oxygen via a mask. The trachea was intubated, and anesthesia was maintained using ISO in oxygen using a semi-closed rebreathing circle system. The dogs were mechanically ventilated to maintain an end-tidal carbon dioxide concentration between 35 and 45 mmHg. End-tidal ISO concentrations were measured with an infrared gas analyzer. The MAC was determined in duplicate using a tail clamp technique. The mean values for the groups were compared using a two sample t-test.

RESULTS

The mean ± SD MAC of isoflurane in the hypothyroid and euthyroid dogs was 0.98 ± 0.31% and 1.11 ± 0.26%, respectively. The mean MAC of isoflurane in hypothyroid dogs was not significantly different from the mean MAC of isoflurane in the control dogs (p = 0.3553).

CONCLUSION AND CLINICAL RELEVANCE

The MAC of ISO in dogs was not significantly affected by experimentally induced hypothyroidism. The dose of ISO in dogs with hypothyroidism does not need to be altered.