FRANK J. MILNE STATE-OF-THE-ART LECTURE


include limiting duration of anesthesia to 1 hour unless oxygen can be supplemented and ventilation can be provided, if necessary. If propofol is more widely adopted, a method for the support of ventila- tion will be required, particularly in out-of-hospital environments.


14. Inhalant Anesthesia in the 21st Century


Significant changes have occurred in the 50 years since the introduction of halothane and closed inhal- ant anesthetic systems.6,11–13 Halothane has been replaced by the halogenated ethers isoflurane, sevo- flurane, and desflurane.72,73,74 The development of more sophisticated surgical procedures requiring longer durations of anesthesia have resulted in the consistent use of mechanical ventilation, an in- creased level of monitoring, a variety of strategies to support cardiovascular function, and modifications in the methods used in recovery. Mechanical ventilation is used to restore normo-


the target level of PaO2 be?47 The question is posed because most techniques of mechanical ventilation


capnea and provide consistent delivery of the inhal- ants but standard mechanical ventilation strategies may not restore “normal” arterial oxygen tensions in horses with suboptimal tensions.43,44,47 This has led to the development of a number of strategies for increasing arterial oxygen tensions that are not con- sistently effective or only effective for short time periods.47 A continuing question is, what should


used to increase PaO2 do so at the expense of blood flow and arterial blood pressure. Further, studies


assessing the effects of less than ideal PaO2 tensions in anesthetized horses have not shown increased morbidity/mortality.75 A recent paper by Hopster et al76 suggests that intestinal oxygenation and per- fusion may not be significantly compromised until arterial oxygen saturations fall below 80% and mean arterial blood pressures fall below 51 mm Hg, re-


spectively. If a PaO2 greater than 60 mm Hg is associated with 80% saturation of hemoglobin, how


hard should we work to increase PaO2 above 60 mmHg? The author would argue that the mainte- nance of sufficient arterial blood pressure and car- diac output to deliver oxygen to the tissues is of greater importance than employing methods to in-


crease PaO2 greater than 70 mm Hg. Monitoring recommendations include the use of


direct monitoring of arterial blood pressures in all horses anesthetized with inhalants.58 Strategies to counteract hypotension include the use of inotropes such as dobutamine and the co-administration of anesthetic adjuncts (partial intravenous anesthesia) to allow a reduction in inhalant concentrations.77–79 The currently used halogenated ethers are less soluble in blood than halothane so recovery from anesthesia occurs more rapidly.52,72–74 This has led to the development of equipment, methods, and protocols designed to produce safe recovery from anesthesia.


160 2020  Vol. 66  AAEP PROCEEDINGS 15. Recovery from Anesthesia in the 21st Century


Reports on morbidity and mortality rates indicate considerable variation with some reports suggesting that mortality rates remain at 1% and others point- ing to lower rates.7–9 A recent publication reported no intraoperative deaths (approximately 30% of deaths in previous reports) and a shift to fractures in recovery as the primary source of mortality.9 A number of recovery strategies have been developed including the use of head and tail ropes, inflatable pads, swimming pools, slings and drugs, both seda- tives and anesthetics.80–85 Clearly improvements are still required. Potential solutions are compli- cated by the difficulty in determining which horses will have problems in recovery. Many of the inves- tigated strategies are labor, time, and equipment/ facility intensive and none eliminate the potential for complications. To which cases can they or should they be applied? The author’s best guess is that some combination of sedation followed by phar- macologic reversal and physical assistance will pro- vide the most practical method of enhancing safety until some, yet-unknown method or technique, is manifest.


16. Anesthesia of the Neonate and the Young Horse in the 21st Century


Horses are more mature at birth than are most of the domestic species and come in varying sizes and temperaments. Approaches to handling foals are frequently influenced by the husbandry practices of the caretakers, the breed, degree of prior human interaction, and other learned behaviors.86–88 The age, attitude, and behavior of the foal should be assessed prior to the administration of any sedative or analgesic medication with sick foals requiring minimal sedation to accomplish tasks such as cath- eter placement. Foals requiring sedation and/or anesthesia because of temperament or the need for more extensive procedures should receive a complete physical examination prior to drug administration. The cardiovascular system of the neonatal horse


has less reserve than that of the adult, making the foal more dependent on heart rate to maintain car- diac output.89 Foals have a larger surface-to-body weight ratio than adult horses and higher total body water, higher extracellular fluid volume, lower body fat, lower total protein, and a larger volume of distribution for some drugs.90,91 Foals breathe at higher rates than adult horses and have a higher minute ventilatory volumes based on weight.92 In addition, foals are insensitive to changes in oxy- gen and carbon dioxide tension, thus they hypoven- tilate under anesthesia. Hepatic oxidative capacity for drugs is lower in 4-day-old foals than in older animals but it appears to increase rapidly, reaching adult levels at 3 to 4 weeks of age. Protein binding appears to be less in the foal than in the adult horse which could mean that drugs that are highly protein bound would be more active. Withholding of food in


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