High inspired oxygen concentrations increase intrapulmonary shunt in anaesthetized horses.
OBJECTIVE
To compare pulmonary function and gas exchange in anaesthetized horses during and after breathing either O2-rich gas mixtures or air.


METHODS
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 O2 [FIO2] = 0.21) provided throughout anaesthesia; or ii) an O2-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 (VA/Q) ratios (using the multiple inert gas elimination technique) were determined in the standing conscious horse (baseline) after sedation and during anaesthesia.


RESULTS
Breathing O2-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 O2-rich gas was delivered compared with air breathing (5 +/- 2%; p = 0.013). Ten minutes after O2-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 O2-rich gas increased intrapulmonary shunt and caused hypoventilation. The intrapulmonary shunt created during anaesthesia by high inspired O2 concentrations remained larger when FIO2 was reduced to 0.21, indicating that absorption atelectasis produced during O2-rich gas breathing persisted throughout anaesthesia.


CONCLUSIONS
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 O2-rich gas after the gas supply is discontinued. On the contrary, the degree of shunt induced by breathing O2-rich gas persists. The clinical relevance of this during recovery requires investigation.
Marntell
Stina
S
Department of Large Animal Clinical Sciences, Faculty of Veterinary Medicine, Swedish University of Agricultural Sciences, Uppsala, Sweden. stina.marntell@oriopharma.com
Nyman
Görel
G
Hedenstierna
Göran
G
D000758:Anesthesia / Q000662:veterinary*; D000778:Anesthetics, Dissociative / Q000494:pharmacology*; D000818:Animals; D016292:Procedural Sedation / Q000662:veterinary*; D018592:Cross-Over Studies; D005260:Female; D006736:Horses / Q000502:physiology*; D008297:Male; D010100:Oxygen / Q000008:administration & dosage / Q000097:blood*; D010101:Oxygen Consumption / Q000187:drug effects / Q000502:physiology*; D010313:Partial Pressure; D011659:Pulmonary Gas Exchange / Q000187:drug effects
D000778:Anesthetics, Dissociative; D010100:Oxygen