As premedication, we administered medotomidine and medazolam (MM) to 48 dogs classified as grade I or II according to the Classification of the American Society of Anesthesiologists. Following endotracheal intubation, anesthesia was maintained by administration of isoflurane. We measured the arterial blood gases of all dogs during spontaneous ventilation (pre-stage). After that, the ventilation of 35 dogs was changed to artificial ventilation (Group 1), and the remaining 13 dogs were left unchanged (Group 2). After 15 to 30 minutes, arterial blood gases in the dogs of Groups 1 and 2 were measured again (post-stage). The measured values were compared and analyzed. In Group 1, post-stage values of pH, PaCO
2, PaO
2/FIO
2 (P/F ratio) and alveolar-arterial oxygen difference (P (A-a) O
2) had been significantly improved compared with pre-stage values. In Group 2, there were no significant differences in pH, PaCO
2, P/F ratio, or P (A-a) O
2 between pre-and post stages. In both groups, there were no significant differences in saturated base excess, lactate, or electrolytes. Consequently, it was concluded that inhalation during spontaneous ventilation after preanesthetic medication with MM may cause an increase in PaCO
2 and a lowering of oxygenation of the lung, and that artificial ventilation can improve these symptoms. In conclusion, during the inhalation anesthesia of a dog with isoflurane and preanesthetic MM, artificial ventilation should be the procedure of choice.
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