Computed tomographic analysis of the effects of two inspired oxygen concentrations on pulmonary aeration in anesthetized and mechanically ventilated dogs

Objective-To compare the effect of 2 concentrations of oxygen in inspired gas (fraction of inspired oxygen [Flo(2)] 1.0 or 0.4) on pulmonary aeration and gas exchange in dogs during inhalation anesthesia. Animals-20 healthy dogs. Procedures-Following administration of acepromazine and morphine, anesthesia was induced in each dog with thiopental and maintained with isoflurane in 100% oxygen (100% group; n = 10) or a mixture of 40% oxygen and air (40% group; 10). Dogs were placed in dorsal recumbency and were mechanically ventilated. After surgery, spiral computed tomography (CT) of the thorax was performed and Pao(2), Paco(2), and the alveolar-arterial oxygen tension difference (P[A-a]O2) were assessed. The lung CT images were analyzed, and the extent of hyperinflated (-1,000 to -901 Hounsfield units [HUs]), normally aerated (-900 to -501 HUs), poorly aerated (-500 to-101 HUs), or nonaerated (-100 to +100 HUs) areas was determined. Results-Compared with the 100% oxygen group, the normally aerated lung area was significantly greater and the poorly aerated and nonaerated areas were significantly smaller in the 40% oxygen group. The time to CT (duration of surgery) was similar in both groups. Although Paco(2) was similar in both groups, Pao(2) and P((A-a))o(2) were significantly higher in the 100% oxygen group. In both groups, pulmonary atelectasis developed preferentially in caudal lung fields. Conclusion and Clinical Relevance-In isoflurane-anesthetized dogs, mechanical ventilation with 40% oxygen appeared to maintain significantly better lung aeration and gas exchange than ventilation with 100% oxygen.