Risk factors contributing to the relatively high equine morbidity and mortality rate associated with general anesthesia have been extensively investigated over the last several decades. A multitude of risk factors have been reported including the relative contribution of hypotension and hypoxemia both intraoperatively and during recovery (1, 2). Volatile anesthetics and patient position adversely affect cardiovascular and respiratory systems (3, 4). Equine patients are especially susceptible to oxygenation impairment under anesthesia as a result of marked ventilation/perfusion (V/Q) mismatching secondary to increased pulmonary atelectasis formation and intrapulmonary shunting (5). Cardiovascular parameters including arterial blood pressure and cardiac output of spontaneously breathing anesthetized ponies have been shown to be affected by body positioning, with the most pronounced decrease of those parameters occurring with animals in dorsal recumbency (6). Further, a decrease in PaO₂ and increase in the P_A−aO₂ gradient, encountered in anesthetized horses (7, 8) is well-documented and likely caused more by recumbency than the anesthetic agents used (9), although anesthetics may influence the magnitude of those changes. Post-anesthetic hypoxemia has been documented in the recovery period even in horses that had shown only mild arterial oxygenation impairments during anesthesia (10–12).

To the best knowledge of the authors, there has only been one investigation into the effects of hoisting on arterial blood pressures and oxygenation in anesthetized horses. Braun et al. studied cardiovascular and oxygenation changes in foals under total intravenous anesthesia (TIVA) before and after hoisting to the ipsilateral or contralateral recumbency (13). The study revealed a significant decrease in oxygenation; however, no significant difference in cardiovascular parameters was evident after hoisting between foals placed in the same or contralateral recumbency (13).

Many equine hospitals and private practice surgical facilities utilize a hobble-hoist system to facilitate movement and positioning of equine patients after induction of anesthesia on a surgery table, and—after surgery—back onto the floor of the recovery stall. However, to the authors' knowledge, there have been no studies determining in adult horses the effects of hoisting on cardiovascular and respiratory system function.

The objective of this study was to investigate effects of hoisting on hemodynamic function and pulmonary gas exchange in isoflurane-anesthetized horses. Based on previous clinical observations, we hypothesized that hoisting will cause primarily a significant decrease in arterial blood pressures and oxygen tension (P_aO₂) and that those changes will continue into the period following hoisting.

In this study, dynamic and significant changes to both cardiovascular and respiratory parameters were noted during and after hoisting. Heart rate and arterial blood pressures changed markedly from baseline during hoisting.

An initial period of hypotension with an increase in HR immediately after hoisting was consistently followed by a rapid and progressive increase in arterial blood pressures. The results of this study revealed an initial mixed systolic/diastolic hypotension immediately after hoisting with a return to baseline within 1 min. Changes in DAP are most commonly due to a change in the conducting vessels' resistance (i.e., vasomotor tone) under isoflurane anesthesia (15). Changing position from lateral to dorsal recumbency during hoisting may initially exacerbate hypotension secondary to shifting weight of abdominal organs onto the caudal vena cava, thereby decreasing venous return to the heart. After the initial decrease in arterial blood pressures, the rapid increase observed was likely due to a central redistribution of peripheral blood because of a change in gravitational forces, increase in preload, and increase in arterial pressure secondary to the Frank-Starling mechanism (16). Horses under halothane anesthesia displayed cardiovascular changes associated with positional changes. The greatest decrease in cardiovascular system function in both spontaneously and mechanically ventilated ponies can be seen in dorsal recumbency with a decrease in myocardial contractility while maintaining systemic arterial blood pressures (6). Total peripheral resistance has been shown to change insignificantly in lateral recumbency and to increase with horses in dorsal recumbency, which may be the result of compensatory vasoconstriction in this position (6).

In addition to the physiologic response, horses were maintained in a light plane of anesthesia before hoisting with an end-tidal isoflurane of ~1.1%, which is below the reported MAC of 1.3% for the equine (17). The tension hobbles exert on the underlying musculoskeletal structures during hoisting is likely stimulating nociceptors. The slip rope design of the hobble may cause a tourniquet effect, which has been shown to lead to hypertension in anesthetized horses (18, 19). In this population of horses, 5/6 horses developed hypertension for at least 1 min during hoisting. Although horses in this study were maintained on isoflurane/oxygen while being hoisted, some horses developed slow nystagmus and one horse exhibited slight movement. Therefore, it could be considered that horses with orthopedic or musculoskeletal pathology such as arthritis could experience an increase in noxious stimulation. Further investigation is warranted to clarify the role of musculoskeletal pathology in stimulation during hoisting. Cardiovascular parameters recorded in this study were limited to HR and arterial blood pressures. Cardiac output and pulmonary artery pressure measurements during hoisting would have provided further information about pulmonary perfusion changes during hoisted flexion as well changes to global perfusion when considering more critically sick patients.

Oxygen tension decreased significantly during hoisting and remained below baseline when horses returned back to left lateral recumbency. Horses are commonly subject to a marked V/Q mismatching under inhalant anesthesia due to changes in lung volume and function when placed in different recumbences and due to interference of volatile anesthetics with the hypoxic pulmonary vasoconstriction response. Reduced oxygen tensions in anesthetized horses has been documented in both laterally recumbent and dorsally recumbent horses with a larger alveolar-arterial (A-a) gradient observed in dorsal recumbency (5, 20–24). The decrease in functional residual capacity and atelectasis formation are most pronounced in dorsal recumbency as abdominal organs shift cranially to push against the diaphragm and thus compress a larger portion of both lungs leading to an increase in collapsing alveolar units (25). Equine lungs have minor collateral ventilation; however, not enough to contribute to ventilation of associated alveoli and therefore this mechanism does not contribute to improved oxygenation as seen in other species (26). In this study, the FiO₂ remained at least at ≥ 90% for at least 40 min before the first ABG analysis was performed, which is enough time for significant denitrogenation to occur which promotes formation of absorption atelectasis. A single hoist system necessarily pulls fore and hind limbs together to one central point, thereby increasing intra-abdominal as well as transpulmonary pressures, which likely exacerbates compression atelectasis and hence further impairs gas exchange. Spinal flexion posture, such as occurs during hoisting, has been shown to significantly decrease maximal inspiratory and expiratory pressures, as well as decreased respiratory muscle strength compared to the upright posture when investigated in adults (27). When suspending spontaneously breathing horses, the additive effect of compression atelectasis, decreased functional residual capacity, and increased respiratory muscle effort required to expand the thoracic cavity against abdominal pressure appears to lead to a significant decrease in PaO₂.

Increased pulmonary shunting has also been observed during anesthesia involving a higher fraction of inspired oxygen (FiO₂) anesthesia (23, 28, 29). As the FiO₂ did not change during the experiment, the contribution of high FiO₂ induced intrapulmonary shunting would have been minimal. Furthermore, previous studies failed to show an improvement in alveolar dead space ventilation or shunt fraction in dorsally recumbent horses by reducing the FiO₂ to 0.5 (30). The shunt fraction increased significantly from baseline solely after hoisting and decreased mildly once returned to lateral recumbency. Arterial oxygenation in horses can worsen over time but usually stays in acceptable limits if conditions do not change (31). In mechanically ventilated horses positioned in dorsal recumbency, PaO₂ measured over time does not significantly decrease from baseline. Therefore, we assume that the drop in PaO₂ was related to the change in positioning and not related to any time effect.

The mean PaCO₂ pre-hoisting was 62 mmHg indicating mild to moderate hypoventilation during baseline spontaneous ventilation. While this is another common cause of decreased PaO₂ that can be treated by increasing the FiO₂, it is unlikely the main contributing factor (32). In this study, adequate ventilation during hoisting was indicated by the decrease in PaCO₂ and mild increase in respiratory rate, though neither parameter changed statistically significantly.

This study was performed under controlled experimental conditions, however there were several limitations that should be recognized. Although arterial catheter placement was successful in each horse, placement of the catheter varied between experiments. Catheters were initially placed in the transverse facial artery adjacent to the halter strap during the first three experimental procedures. During hoisting, the pressure exerted by the halter may have compressed the artery and excessively dampened the arterial pressure wave resulting in lower than accurate systolic, mean, and diastolic pressure readings for those 5 min. Although these catheters subjectively seemed compressed and the waveform dampened, they remained patent, a distinguished arterial waveform was displayed on the monitor, and arterial pressures increased at a similar rate and magnitude compared to the situation of catheters that were not under pressure of the halter. During subsequent experiments, the arterial catheter was positioned to be away from the pressure points of the halter. The horses anesthetized in this study were underwent a single hoisting. In a clinical setting, horses are hoisted onto and off of a surgical table and therefore might lead to potential worsened oxygenation during the second hoist. All horses in this experiment continued to spontaneously breathe an oxygen rich gas mixture during and after hoisting. Although no horse was considered hypoxemic based on blood gas analysis data, in practice, horses are not commonly provided oxygen supplementation during hoisting. Interpretation of the changes of PaO₂ should take this into consideration and horses may become hypoxemic during those times.

Despite the administration of pure oxygen, hoisting an adult horse during or at the end of isoflurane anesthesia carries the risk of a precipitous, though short-lived (1–2 min), drop in arterial blood pressures and a persistent decrease in arterial oxygenation. While in systemically healthy animals the observed functional impairments were not life threatening, they may be more severe and longer lasting in systemically compromised horses and therefore much attention must be paid to monitoring arterial blood pressures and oxygenation when hoisting sick equine patients.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal study was reviewed and approved by International Animal Care and Use Committee.

MC was responsible for data collection, interpretation, manuscript writing, and submission. BD was responsible for overseeing experimental protocol, data analysis, and manuscript editing. HD was responsible for data acquisition. KH was responsible for conception of study, supervision of collection, data analysis, and manuscript editing. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.