High inspired oxygen concentrations increase intrapulmonary shunt In anaesthetized horses

Objectives To compare pulmonary function and gas exchange in anaesthetized horses during and after breathing either O-2-rich gas mixtures or air. Animals Six healthy standard bred trotters (age range 3-12 years; mass range 423-520 kg), four geldings and two mares. Study design Randomized, cross-over experimental study. Methods Horses were anaesthetized on two occasions with tiletamine-zolazepam after pre-anaesthetic medication with acepromazine, romifidine and butorphanol. After endotracheal intubation and positioning in left lateral recumbency, animals were allowed to breathe spontaneously. One of two, randomly allocated inspired gas treatments was provided: either i) room air (fractional concentration of inspired O-2 [FIP2] = 0.21) provided throughout anaesthesia; or ii) an O-2-rich gas mixture (FIO2 = >0.95) for 15 minutes, followed by room air. The alternative treatment was delivered at the second anaesthetic. Respiratory and haemodynamic variables and the distribution of ventilation-perfusion (V-A/Q) ratios (using the multiple inert gas elimination technique) were determined in the standing conscious horse (baseline) after sedation and during anaesthesia. Results Breathing O-2-rich gas was associated with a decreased respiratory rate (p = 0.015) increased PaCO2 (p < 0.001) and increased PaO2 (p = 0.004) compared with breathing air. All horses developed intrapulmonary shunt during anaesthesia, but shunt was significantly greater (13 +/- 5%) when O-2-rich gas was delivered compared with air breathing (5 +/- 2%; p = 0.013). Ten minutes after 0,rich gas was replaced by air, shunt remained larger in horses that had initially received oxygen compared with those breathing air (p = 0.042). Mixed venous oxygen tensions were significantly lower during sedation than at baseline (p < 0.001) and during anaesthesia (P < 0.001). Conclusions During dissociative anaesthesia, arterial oxygenation was greater when horses breathed gas containing more than 95% oxygen, compared with when they breathed air. However, breathing O-2-rich gas increased intrapulmonary shunt and caused hypoventilation. The intrapulmonary shunt created during anaesthesia by high inspired O-2 concentrations remained larger when FIO2 was reduced to 0.21, indicating that absorption atelectasis produced during O-2-rich gas breathing persisted throughout anaesthesia. Clinical relevance In healthy horses undergoing short-term dissociative anaesthesia, air breathing ensures a level of oxygen delivery that meets tissue demand. There is no benefit to horses in breathing O-2-rich gas after the gas supply is discontinued. On the contrary, the degree of shunt induced by breathing O-2-rich gas persists. The clinical relevance of this during recovery requires investigation.