Pulmonary gas exchange in anaesthetised horses mechanically ventilated with oxygen or a helium/oxygen mixture

Reason for performing study: It is unknown whether administration of gas-mixtures high in inspired fraction of oxygen (FiO(2)) under general anaesthesia may increase formation of pulmonary atelectasis and impair gas exchange. Objective: To evaluate the effects of different FiO(2) on pulmonary gas exchange in isoflurane-anaesthetised horses breathing a helium/oxygen (He/O-2) mixture. Methods: Thirty healthy mature horses were sedated with i.v. acepromazine (0.02 mg/kg bwt), detomidine (0.002 mg/kg bwt) and xylazine (0.2-0.4 mg/kg bwt). General anaesthesia was induced with i.v. 5% guaifenesin to effect, diazepam (0.1 mg/kg bwt) and ketamine (2 mg/kg bwt), and maintained with isoflurane. Fifteen horses (Group HX) were ventilated mechanically with gas mixtures of successively increasing FiO(2) (0.25-0.30, 0.50-0.55, > 0.90), obtained by blending O-2 with Heliox (70% He/30% O-2). The other 15 horses (Group O) were ventilated immediately with 100% O-2 (FiO(2)> 0.90). After 20 min of ventilation at the different FiO(2) levels in Group HX and after 60 min in Group O, PaO2 and PaCO2 were measured and the alveolar to arterial PO2 gradient (P(A-a)O2) was calculated. Data analysis included robust categorical regression with clustering on horse (P < 0.05). Results: Inhalation of a He/O-2 mixture with FiO(2) as low as 0.25-0.30 ensured adequate arterial oxygenation and was associated with a smaller P(A.a)O2 gradient than inhalation of pure O-2 (P < 0.05). In Group HX, PaO2 increased with each rise in FiO(2) and so did P(A-a)O2 (P < 0.05). The PaO2 was significantly lower and the P(A-a)O2 higher in Group O compared to Group HX at a FiO(2)> 0.90 (P < 0.05). Conclusions and potential relevance: Administration of a He/O-2 gas mixture low in FiO(2) can better preserve lung function than ventilation with pure oxygen. A step-wise increase of FiO(2) using a He/O-2 gas mixture might offer advantages with respect to pulmonary gas exchange over an immediate exposure to 100% O-2.