Effects of two fractions of inspired oxygen on lung aeration and gas exchange in cats under inhalant anaesthesia.

OBJECTIVE

To compare the effects of two fractions of inspired oxygen (FiO(2)) (0.4 and 1) on lung aeration and gas exchange during general anaesthesia in cats.

STUDY DESIGN

Randomized, blinded, controlled study.

ANIMALS

Thirty healthy, mixed breed, client owned female cats.

MATERIALS AND METHODS

Cats were premedicated intramuscularly with acepromazine (0.03 mg kg(-1)) and medetomidine (0.015 mg kg(-1)). Anaesthesia was induced with propofol (5 mg kg(-1)) and, after orotracheal intubation, maintained with isoflurane carried by either 100% oxygen (G100, n=15) or an oxygen-air mixture with 40% oxygen (G40, n=15). All cats were placed in dorsal recumbency and breathed spontaneously throughout the entire procedure. Following surgery (ovariectomy), a spiral computed tomography (CT) of the thorax was performed, arterial oxygen (PaO(2)) and carbon dioxide (PaCO(2)) tensions were measured and alveolar-arterial gradient of oxygen [P(A-a)O(2) ] calculated. The CT images were analysed for lung aeration by the analysis of radiograph attenuations (Hounsfield units, HU), according to the following classification: hyperinflated area (-1000 to -900 HU), normally aerated area (-900 to -500 HU), poorly aerated area (-500 to -100 HU) and non-aerated area (-100 to +100 HU). The groups were compared using one-way anova.

RESULTS

Compared to G100, the normally-aerated lung area was significantly greater and the poorly-aerated and non-aerated areas were significantly smaller in G40. PaCO(2) was similar in both groups. PaO(2) and P(A-a)O(2) were significantly higher in G100. In both groups, pulmonary atelectasis developed preferentially in the caudal lung fields.

CONCLUSION

In cats anaesthetised with isoflurane, the administration of an FiO(2) of >0.9 significantly impaired lung aeration and gas exchange as compared to an FiO(2) of 0.4.

CLINICAL RELEVANCE

An FiO(2) of 0.4 may better preserve lung aeration and gas exchange in anaesthetised spontaneously breathing cats but monitoring is essential to ensure oxygenation is adequate.