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Measurement of intraocular pressure in healthy anesthetized horses during hoisting
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Veterinary Anaesthesia and Analgesia 2017, xxx, 1e7
http://dx.doi.org/10.1016/j.vaa.2016.10.001
RESEARCH PAPER
Measurement of intraocular pressure in Q5
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healthy anesthetized horses during hoisting Caroline S Monka, Dennis E Brooksa,b, Tiffany Granoneb, Fernando L Garcia-Pereirab, Alexander Meleskoc & Caryn E Plummera,b a
Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of
Florida, Gainesville, FL, USA b
Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of
Florida, Gainesville, FL, USA c
Federated Investors, New York, NY, USA
Correspondence: Caryn E Plummer, College of Veterinary Medicine, University of Florida, 2089 SW 16th Avenue, Gainesville, FL 32610, USA. Email: PlummerC@ufl.edu
Abstract Objective To measure intraocular pressure (IOP) in horses during hoisting after induction of anesthesia. Study design Prospective nonrandomized clinical study. Animals Eighteen healthy adult horses aged [mean ± standard deviation (SD)] 10 ± 4.2 years and weighing 491 ± 110 kg anesthetized for elective procedures. Methods IOP was measured in the superior eye of each horse based on planned recumbency after induction of anesthesia. Measurements were taken directly after premedication with xylazine or detomidine with butorphanol, after induction with diazepameketamine, after intubation, when suspended by the hoist and on the operating table. During hoisting, the head was supported and the eyeeheart height was measured to account for variations in head positioning among patients. IOPs were compared across time points using repeated-measures analysis of variance. Regression was used to compare IOP outcome with potential cofactors. Results Compared with measurements after premedication (17.5 ± 2.5 mmHg) (mean ± SD), hoisting significantly increased IOP (32.4 ± 15.3 mmHg) (p < 0.01). The highest recorded IOP in the hoist was 80.0 (range, 16.0e80.0) mmHg. The difference in IOP between premedication and hoisting was 15.0 ± 16.2
(range, e1.0 to 68.0) mmHg. Body weight had a significant effect on absolute IOP and change in IOP in the hoist (p < 0.01). Conclusions and clinical relevance Hoist IOP was significantly higher than post-premedication IOP with heavier horses having higher hoist IOPs and greater increases in IOP. The clinician should take this relationship into account when anesthetizing and hoisting larger horses where an increase in IOP could be detrimental. Keywords alpha2-adrenergic agonist, body weight, equine, general anesthesia, ophthalmic. Introduction Intraocular pressure (IOP) is defined by aqueous humor formation, uveoscleral outflow and episcleral venous pressure in the Goldmann equation as follows: IOP ¼ [F/C] þ PV where F denotes the aqueous fluid formation rate, C is the aqueous fluid outflow rate and PV is the episcleral venous pressure.
IOP is most commonly estimated indirectly using tonometry. The reference range for horses using tonometry is reported as 15e37 mmHg (Miller et al. 1990; Smith et al. 1990; van der Woerdt et al. 1995, 1998; Ramsey et al. 1999; Knollinger et al. 2005; Kom aromy et al. 2006; Stine et al. 2014). Maintenance of IOP within a physiologic range is vital for intraocular health, especially that of the optic nerve,
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Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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Hoisting on equine intraocular pressure CS Monk et al.
which is particularly susceptible to the pathophysiologic impact of glaucoma. The effect of head position on IOP has been previously documented in horses and humans (Kergoat & Lovasik 2005; Komaromy et al. 2006; Prata et al. 2010; Malihi & Sit 2012). When the head is inverted relative to the heart, the position changes the brain-to-heart hydrostatic gradient. When the head position was lowered below heart level in 30 horses administered detomidine, IOP increased in 52 out of 60 eyes (Komaromy et al. 2006). This change likely results from increases in intracranial pressure (ICP), cerebral perfusion pressure (CPP) and episcleral venous pressure as the head-down position increases the hydrostatic gradient of blood flow to the head (Brosnan et al. 2008). Additional factors that may also raise IOP are congestion of orbital content compressing the globe and an increase in the ocular blood volume within the uveal tract (Linder et al. 1988; Komaromy et al. 2006). Horses anesthetized for surgical or diagnostic procedures are often hoisted with the limbs supporting the body in an inverted position. In this position, the head commonly is carried below the level of the heart, and the thoracic cavity and large abdominal vessels are compressed by the weight of the colon. Additionally, a more pronounced effect of body position on ICP and CPP than in a conscious horse is induced by anesthetic drugs that interfere with normal cerebrovascular autoregulation (Brosnan et al. 2008, 2011). Increases in ICP and CPP during hoisting for general anesthesia may have an impact and increase IOP. Deep corneal ulcers, descemetoceles and penetrating foreign bodies of the eye may involve fragile corneal lesions that may rupture if IOP increases. In cases of pre-existing glaucoma, an even greater increase in IOP may further damage retinal ganglion cells (RGC). In a rat model study on glaucoma, slowly rising pressure was not overtly damaging to RGCs, but rapid, high-pressure pulses injured these cells almost immediately after insult (Resta et al. 2007). The effects of hoisting on the IOP of horses have not been published, and this effect should be determined to enable risk assessment of anesthesia for ophthalmic procedures. The aims of this study were to determine the IOP in normal horse eyes during hoisting after induction of anesthesia and to evaluate factors that had potential to influence IOP. We hypothesized that IOP would be increased during the hoisting position compared with the standing position. 2
Materials and methods This study was approved by the Institutional Animal Care and Use Committee of the University of Florida (no. 201408375). Owner consent was obtained before each horse was enrolled in the study. Eighteen horses were considered to be healthy, with no cardiovascular or respiratory abnormalities, based on a physical examination and results of laboratory tests within the normal reference ranges for the laboratory (complete blood cell count and serum biochemistry profile). Within 5 minutes after administration of drugs for premedication, an anterior segment ophthalmic examination without pharmacologic mydriasis was performed using a portable slit-lamp (Kowa SL-14; Kowa Company, CA, USA). The horse was included in the study if the anterior segment of the measured eye was free of detectable disease on direct examination. Anesthesia The anesthesia protocol was formulated for each horse by the anesthesiologist on duty on the day of the procedure. The horses were administered intravenously (IV) [mean ± standard deviation (SD)] either xylazine (0.54 ± 0.09 mg kg1; AnaSed; Lloyd Inc., IA, USA; n ¼ 9) or detomidine (0.01 ± 0.00 mg kg1; Domosedan; Orion Pharma, Finland; n ¼ 9) for premedication with butorphanol (0.02 mg kg1; Torbugesic; Zoetis Inc., MI, USA; n ¼ 14, seven horses with xylazine and seven with detomidine). Induction of anesthesia was achieved by administration of diazepam (0.07 ± 0.01 mg kg1; Diazepam hydrochloride USP; Hospira Inc., IL, USA) and ketamine (2.50 ± 0.24 mg kg1; Ketamine hydrochloride USP; Putney Inc., ME, USA) IV in direct succession. During induction, the horse was supported against the wall by personnel until the horse assumed sternal recumbency. The horse was turned to lateral recumbency and the trachea intubated with a cuffed endotracheal tube with an internal diameter 18e26 mm (SurgiVet, OH, USA). IOP measurements IOP was measured in one eye in each horse using the uppermost eye for the planned lateral recumbency during anesthesia. Within 5 minutes after administration of drugs for premedication, 0.5% tetracaine ophthalmic solution (Bausch & Lomb, NY, USA) was instilled into the eye. IOP was measured in the standing horse with the head in a position such that
© 2017 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e7
Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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the eye was approximately at the level where the jugular veins enter the thoracic inlet. This position was to approximate a zero heart-to-eye hydrostatic gradient (Ferreira et al. 2013). IOP was measured by gentle tapping of the tonometer (Tono-Pen AVIA; Reichert Ophthalmic Instruments, NY, USA) on the axial cornea until a reading was obtained. The instrument collects data from each tap and produces a final number with a calculated confidence interval. The first and only IOP reading with a 95% instrument-calculated confidence interval was used to mitigate bias. The same tonometer was used by one examiner (CSM) for all IOP measurements. IOP was measured after premedication (baseline), approximately 2 minutes after induction of anesthesia at the time of lateral recumbency, after intubation (n ¼ 9), at hoisting in dorsal recumbency and in final position on the table for surgery. For hoisting, hobbles were attached around the pasterns and the horse was inverted into dorsal position and suspended off the ground. The head was supported in a position on line with the thoracic and abdominal vertebrae. The nose was held so that the dorsal aspect of the head was parallel with the ground (Fig. 1). In addition to measurement of IOP at that moment, the distance from the ground to the medial canthus of the
eye and from the ground to the olecranon was measured in centimeters. The difference between these measurements was used as an estimation of the position of the eye relative to the heart to account for variability in head positioning (Fig. 1). Time from injection of the induction agents to measurement of IOP during hoisting was also recorded. Horses were then immediately transported to the surgery table. A final IOP measurement was obtained once the horse was positioned for the intended procedure in either dorsal or lateral recumbency but before connecting the endotracheal tube to the anesthesia delivery system. Positioning occurred immediately after hoisting, and the time between measurement in hoist and measurement in the final position was less than 2 minutes. Statistical analysis Data were analyzed using commercial software (Microsoft Excel 2013; WA, USA). Similarities in drug dosages were confirmed using a Student t test. A descriptive analysis for IOP by time point was performed. Differences in IOP between time points were verified using a Student t test, and further confirmed by use of repeated-measures analysis of variance with
Q3 Figure 1 Anatomic measurements for calculation of the eyeeheart height in anesthetized horses during hoisting after induction of anesthesia. The endotracheal tube is not pictured. A, ground to medial canthus; B, ground to olecranon; C, BeA ¼ eyeeheart height. © 2017 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e7
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Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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Hoisting on equine intraocular pressure CS Monk et al.
a set alpha of 5% and a power of 80%. Potential comorbidities influencing positional IOP changes were investigated. The following variables were tested using linear and multivariate regression in comparison to hoist IOP and difference in IOP from premedication to hoisting: signalment (age, sex, breed), body weight, drug dose (detomidine, xylazine, butorphanol, ketamine, diazepam) and eyeeheart height during hoisting. A NeweyeWest estimator was applied to account for heteroskedasticity in the data.
IOP ¼ 10.727 þ 0.052 weight). Absolute IOP in the hoist also was significantly correlated with body weight (R2 ¼ 17%, confidence ¼ 99%, p < 0.01). A simple linear regression showed that increasing body weight correlated to increasing IOP when in the hoist (IOP ¼ 4.301 þ 0.057 weight). None of the remaining investigated factors (signalment, medication dosage, eyeeheart height during hoisting) had a statistically significant relationship with outcome (hoist IOP or IOP difference) at a 95% confidence level. Discussion
Results Horses The study population consisted of six Warmbloods, five Quarter Horses, three ponies, two Thoroughbreds and two Paint Horses. There were 10 geldings, seven mares and one stallion, mean ± SD age was 10 ± 4.2 years and mean weight was 491 ± 110 kg. These animals were a mixture of client-owned horses undergoing anesthesia for an elective surgical procedure (n ¼ 6) or diagnostic imaging (n ¼ 9), or horses anesthetized for research purposes not related to this study (n ¼ 3). IOP was not different between horses administered detomidine or xylazine for premedication at any time point; therefore, the data were combined. IOPs measured in 18 horses were after premedication (18 ± 2.5 mmHg), after induction (17 ± 4.5 mmHg), during hoisting (32 ± 15.3 mmHg) and on the surgery table (20 ± 7.1 mmHg). A smaller subset of nine horses also had an IOP assessment after intubation (17 ± 4.6 mmHg). The difference in IOP between premedication and hoisting was 15.0 ± 16.2 (range, 1.0 to 68.0) mmHg. IOP during hoisting was significantly higher than at other time points (p < 0.01). Cofactors All planned anatomic measurements were obtained from all horses in the study. Eyeeheart height ranged from 51 to 117 cm (96 ± 17.5 cm). Body weight was found to be the best predictor of absolute hoist IOP and numeric change in IOP (R2 ¼ 13%, confidence ¼ 99%, p < 0.01) based on linear regression with a NeweyeWest Error Correction. A simple linear regression showed that for every 1 kg increase in weight, the IOP was expected to rise by 0.06 mmHg from post-sedation IOP when in the hoist (IOP ¼ intercept þ coefficient weight þ error; 4
The results of this study showed that hoisting during general anesthesia has a significant effect on IOP in normal horse eyes. The increase was not sustained, as the final measurement taken within 2 minutes of hoist IOP was not significantly different from IOP before hoisting. Similar to a previous study relating IOP to head position in horses, there was considerable variability in the magnitude of the change in IOP (Kom aromy et al. 2006). A fourfold increase was measured in some horses, whereas others sustained a negligible change. Studies in humans have documented a moderately unpredictable effect of position on IOP with interindividual variation postulated to be the result of factors such as age and vascular health (Prata et al. 2010). In some instances, the magnitude of IOP change due to position was found to be greatest in glaucomatous eyes (Buys et al. 2010; Prata et al. 2010; Kim et al. 2013). Body weight resulted in a significant linear increase in both change from baseline IOP and absolute IOP in the hoist. Body weight is likely representative of other key anatomic and physiologic variables. None of the remaining cofactors were associated with outcome of IOP in the hoist using linear and multivariate regression. Previous studies have failed to identify consistent risk factors in similar positionbased studies (Kom aromy et al. 2006; Prata et al. 2010). The effect of body weight was differentiated from a simple increase in hydrostatic gradient owing to measurement of the distance from the eye to the heart. Heavier horses theoretically would have an increased hydrostatic pressure on the eye from having longer necks and deeper chests, resulting in the eye being farther from the heart during hoisting. Because the head was supported approximating dorsal midline, vertical variation in the distance from the eye to the heart was not as wide as the variation in weights of the horses. The range of eyeeheart
© 2017 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e7
Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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Hoisting on equine intraocular pressure CS Monk et al.
height was 50.8e109.2 (95.8 ± 17.5, mean ± SD) cm, and attempts were made to position the horses similarly. When converting those distances to approximate pressure (mean 72; range, 39e83 mmHg), the numbers are so large that a pressure of this magnitude would cause serious and documentable disease to the eye. However, these hydrostatic variations did not yield a relationship with IOP outcome or body weight in this study. Therefore, the influence of body weight is possibly independent of the influence of neck length and its variation in hydrostatic gradient. Of course, hydrostatic gradient, being a vertical measurement relative to gravity, might not be the only factor. Length of neck may make venous return more challenging, resulting in increased ICP and IOP. Length of neck was not documented in this study. Another mechanism to consider behind the weight-related increase in IOP is blood pressure. In general, mean arterial pressure (MAP) decreases when a horse is turned into dorsal recumbency or hoisted, with minimal effect described in foals weighing 220 kg (Braun et al. 2009) but with a much greater effect recorded in heavy horses (Steffey & Howland 1980; Brosnan et al. 2011). As decreases in MAP would be predicted to decrease IOP in heavier horses, alternate possibilities for the hemodynamics in hoisting should be considered. Instead of changes to MAP, increases in central venous pressure (CVP) may be present. CVP could increase as a result of compression of gastrointestinal contents on the diaphragm and large abdominal vessels. Measurement of MAP and CVP during the hoisting event and modeling of the cardiopulmonary system would provide a more descriptive understanding. The main purpose of this study was to measure the effects of posture on IOP in horses to provide insight into ocular health of horses during anesthesia. Owing to the necessity of anesthesia and the observational nature of the study, confounding factors could not be removed to isolate the effects of the hoist. Specifically, injectable medications given to the patient, as well as the act of intubation, have been shown to alter IOP (Bechara et al. 1998; Safavi & Honarmand 2008; Ferreira et al. 2013). In the study presented here, the drugs used for induction of anesthesia did not increase IOP, as no time points other than hoisting were significantly different. Ketamine is the most common induction agent used in horses, and is generally thought to increase IOP in horses and other species by increasing extraocular muscle tone (Hofmeister et al. 2006;
Ferreira et al. 2013). However, variable reports of ketamine's effects on IOP exist even within species including horses (Trim et al. 1985; Smith et al. 1990; Ghaffari et al. 2010; Ferreira et al. 2013; Kovalcuka et al. 2013). The effects are rarely outside the normal reference range. Effects of premedication, specifically a2-adrenergic agonists, are more consistent. These medications have been shown to decrease IOP (van der Woerdt et al. 1995; Brosnan et al. 2008; Kom aromy et al. 2006; Holve 2012; Stine et al. 2014), occasionally after a transient increase (Rauser et al. 2012). All horses in this study had what is considered clinically normal IOPs after administration of premedication agents. This measurement was used as baseline IOP in this study to mitigate the confounding effect of a2-adrenergic agonist sedation on IOP. Because IOP was not measured prior to administration of the premedication, conclusions cannot be drawn about the effects of detomidine or xylazine on IOP in this patient population. Therefore, it is possible that the magnitude of increase from baseline to hoisting is not representative of the change measured if hoisting IOP were to be compared with a baseline IOP obtained in the absence of sedation. Nonetheless, the absolute IOP measurements obtained in this study were representative of a clinical situation. Laryngoscopy and tracheal intubation are associated with tachycardia and a rise in MAP resulting from sympathetic stimulation in humans and dogs (Kayhan et al. 2005; Thompson & Rioja 2016). However, the administration of an a2-adrenergic agonist for premedication or lidocaine IV have been shown to prevent this response (Yavascaoglu et al. 2008; Richa 2009; Thompson & Rioja 2016). No study evaluating the effect of intubation on heart rate and MAP in horses was found in a search of the literature. All horses were administered an a2adrenergic agonist as part of the premedication protocol. IOP measured directly after intubation in a subset of horses was not significantly different from any time point other than the hoisting IOP. Weaknesses of this study mainly relate to its observational nature. In order to minimize the time each horse was suspended in the hoist, mimicking the clinical situation, precise anatomic positioning among the horses was not achieved. The measurement of eyeeheart height was intended to account for this variability in approximate head positioning. The IOP may have been different had the head of each horse been raised to heart level during hoisting. Results of previous studies investigating the impact of
© 2017 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e7
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Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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Hoisting on equine intraocular pressure CS Monk et al.
body position on IOP suggest that the increase in IOP would be less if the head was supported closer to heart level (Linder et al. 1988; Brosnan et al. 2002, 2008; Hirooka & Shiraga 2003; Kergoat & Lovasik 2005; Buys et al. 2010; Malihi & Sit 2012). Further direction for this research is IOP at the conclusion of anesthesia, when hoisting is necessary to return the horse to the recovery stall. The general anesthesia protocol for surgery frequently includes an inhalation agent such as isoflurane or sevoflurane that is maintained until the conclusion of surgery. Inhalation agents have a more substantial effect on cerebrovascular autoregulation by decreasing MAP, blunting baroreflex activity and causing vasodilation (Brosnan et al. 2008). Ophthalmologic surgeries, such as phacoemulsification or for penetrating keratoplasty, create a fragile globe. For these horses, an increase in IOP postoperatively may jeopardize the surgical outcome, and anesthetic management should be adjusted to avoid a spike in IOP during hoisting into the recovery stall. In conclusion, IOP was significantly increased during hoisting in 18 horses anesthetized with xylazine or detomidine with butorphanol and diazepameketamine when compared with IOP measured after premedication. The magnitude of the increase was variable among individuals. Increased body weight was identified as a risk factor for a more substantial increase in change of IOP and absolute IOP. Further research is needed using different head heights in relation to the heart and cardiovascular measurements to elucidate the mechanisms behind the increase in IOP. Acknowledgements The authors acknowledge Renata Velloso Ramos, College of Veterinary Medicine, University of Florida, for her assistance with data collection and Gil BenShlomo, College of Veterinary Medicine, Iowa State University, for his assistance with the study design. Authors’ contributions CM, TG, FGP: study design, data collection, data interpretation, preparation of the manuscript. DB, CP: study design, data interpretation, preparation of the manuscript. AM: study design, statistical analysis, data interpretation, preparation of the manuscript. Q2
Conflict of interest statement Authors declare no conflict of interest. 6
References Bechara JN, Barros PSM, Fantoni DT et al. (1998) Intraocular pressure evaluation of equines anaesthetized with romifidine, tiletamine/zolazepam, halothane and vecuronium. Ciencia Rural 28, 59e64. Braun C, Trim CM, Eggleston RB (2009) Effects of changing body position on oxygenation and arterial blood pressures in foals anesthetized with guaifenesin, ketamine, and xylazine. Vet Anaesth Analg 36, 18e24. Brosnan RJ, Steffey EP, LeCouteur RA et al. (2002) Effects of body position on intracranial and cerebral perfusion pressures in isoflurane-anesthetized horses. J Appl Physiol 92, 2542e2546. Brosnan RJ, Esteller-Vico A, Steffey EP et al. (2008) Effects of head-down positioning on regional central nervous system perfusion in isoflurane-anesthetized horses. Am J Vet Res 69, 737e743. Brosnan RJ, Steffey EP, LeCouteur RA et al. (2011) Effects of isoflurane anesthesia on cerebrovascular autoregulation in horses. Am J Vet Res 72, 18e24. Buys YM, Alasbali T, Jin Y et al. (2010) Effect of sleeping in a head-up position on intraocular pressure in patients with glaucoma. Ophthalmology 117, 1348e1351. Ferreira TH, Brosnan RJ, Shilo-Benjamini Y et al. (2013) Effects of ketamine, propofol, or thiopental administration on intraocular pressure and qualities of induction of and recovery from anesthesia in horses. Am J Vet Res 74, 1070e1077. Ghaffari MS, Rezaei MA, Mirani AH, Khorami N (2010) The effects of ketamineemidazolam anesthesia on intraocular pressure in clinically normal dogs. Vet Ophthalmol 13, 91e93. Hirooka K, Shiraga F (2003) Relationship between postural change of the intraocular pressure and visual field loss in primary open-angle glaucoma. J Glaucoma 12, 379e382. Hofmeister EH, Mosunic CB, Torres BT et al. (2006) Effects of ketamine, diazepam, and their combination on intraocular pressures in clinically normal dogs. Am J Vet Res 67, 1136e1139. Holve DL (2012) Effect of sedation with detomidine on intraocular pressure with and without topical anesthesia in clinically normal horses. J Am Vet Med Assoc 240, 308e311. Kayhan Z, Aldemir D, Mutlu H, O güs¸ E (2005) Which is responsible for the haemodynamic response due to laryngoscopy and endotracheal intubation? Catecholamines, vasopressin or angiotensin? Eur J Anaesthesiol 22, 780e785. Kergoat H, Lovasik J (2005) Seven-degree head-down tilt reduces choroidal pulsatile ocular blood flow. Aviat Space Environ Med 76, 930e934. Kim KN, Jeoung JW, Park KH et al. (2013) Effect of lateral decubitus position on intraocular pressure in
© 2017 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e7
Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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Hoisting on equine intraocular pressure CS Monk et al. glaucoma patients with asymmetric visual field loss. Ophthalmology 120, 731e735. Knollinger AM, La Croix NC, Barrett PM, Miller PE (2005) Evaluation of a rebound tonometer for measuring intraocular pressure in dogs and horses. J Am Vet Med Assoc 227, 244e248. Kom aromy AM, Garg CD, Ying GS, Liu C (2006) Effect of head position on intraocular pressure in horses. Am J Vet Res 67, 1232e1235. Kovalcuka L, Birgele E, Bandere D, Williams DL (2013) The effects of ketamine hydrochloride and diazepam on the intraocular pressure and pupil diameter of the dog's eye. Vet Ophthalmol 16, 29e34. Linder BJ, Trick GL, Wolf ML (1988) Altering body position affects intraocular pressure and visual function. Invest Ophthalmol Vis Sci 29, 1492e1497. Malihi M, Sit AJ (2012) Effect of head and body position on intraocular pressure. Ophthalmology 119, 987e991. Miller PE, Pickett JP, Majors LJ (1990) Evaluation of two applanation tonometers in horses. Am J Vet Res 51, 935e937. Prata TS, De Moraes CGV, Kanadani FN et al. (2010) Posture-induced intraocular pressure changes: considerations regarding body position in glaucoma patients. Surv Ophthalmol 55, 445e453. Ramsey DT, Hauptman JG, Petersen-Jones SM (1999) Corneal thickness, intraocular pressure, and optical corneal diameter in Rocky Mountain Horses with cornea globosa or clinically normal corneas. Am J Vet Res 60, 1317e1321. Rauser P, Pfeifr J, Proks P, Stehlik L (2012) Effect of medetomidineebutorphanol and dexmedetomidineebutorphanol combinations on intraocular pressure in healthy dogs. Vet Anaesth Analg 39, 301e305. Resta V, Novelli E, Vozzi G et al. (2007) Acute retinal ganglion cell injury caused by intraocular pressure spikes is mediated by endogenous extracellular ATP. Eur J Neurosci 25, 2741e2754. Richa F (2009) Dexmedetomidine blunts haemodynamic and intraocular pressure responses to tracheal intubation. Eur J Anaesthesiol 26, 444e445.
Safavi M, Honarmand A (2008) Influence of head flexion after endotracheal intubation on intraocular pressure and cardio-respiratory response in patients undergoing cataract surgery. Ghana Med J 42, 105e109. Smith PJ, Gum GG, Whitley RD et al. (1990) Tonometric and tonographic studies in the normal pony eye. Equine Vet J Suppl 10, 36e38. Steffey EP, Howland D (1980) Comparison of circulatory and respiratory effects of isoflurane and halothane anesthesia in horses. Am J Vet Res 41, 821e825. Stine JM, Michau TM, Williams MK et al. (2014) The effects of intravenous romifidine on intraocular pressure in clinically normal horses and horses with incidental ophthalmic findings. Vet Ophthalmol 17, 134e139. Thompson KR, Rioja E (2016) Effects of intravenous and topical laryngeal lidocaine on heart rate, mean arterial pressure and cough response to endotracheal intubation in dogs. Vet Anaesth Analg 43, 371e378. Trim CM, Colbern GT, Martin CL (1985) Effect of xylazine and ketamine on intraocular-pressure in horses. Vet Rec 117, 442e443. van der Woerdt A, Gilger BC, Wilkie DA, Strauch SM (1995) Effect of auriculopalpebral nerve block and intravenous administration of xylazine on intraocularpressure and corneal thickness in horses. Am J Vet Res 56, 155e158. van der Woerdt A, Gilger BC, Wilkie DA et al. (1998) Normal variation in, and effect of 2% pilocarpine on, intraocular pressure and pupil size in female horses. Am J Vet Res 59, 1459e1462. Yavascaoglu B, Kaya FN, Baykara M et al. (2008) A comparison of esmolol and dexmedetomidine for attenuation of intraocular pressure and haemodynamic responses to laryngoscopy and tracheal intubation. Eur J Anaesthesiol 25, 517e519. Received 29 March 2016; accepted 13 October 2016. Available online xxx
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Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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Veterinary Anaesthesia and Analgesia 2017, xxx, 1e7
http://dx.doi.org/10.1016/j.vaa.2016.10.001
RESEARCH PAPER
Measurement of intraocular pressure in Q5
Q4
healthy anesthetized horses during hoisting Caroline S Monka, Dennis E Brooksa,b, Tiffany Granoneb, Fernando L Garcia-Pereirab, Alexander Meleskoc & Caryn E Plummera,b a
Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of
Florida, Gainesville, FL, USA b
Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of
Florida, Gainesville, FL, USA c
Federated Investors, New York, NY, USA
Correspondence: Caryn E Plummer, College of Veterinary Medicine, University of Florida, 2089 SW 16th Avenue, Gainesville, FL 32610, USA. Email: PlummerC@ufl.edu
Abstract Objective To measure intraocular pressure (IOP) in horses during hoisting after induction of anesthesia. Study design Prospective nonrandomized clinical study. Animals Eighteen healthy adult horses aged [mean ± standard deviation (SD)] 10 ± 4.2 years and weighing 491 ± 110 kg anesthetized for elective procedures. Methods IOP was measured in the superior eye of each horse based on planned recumbency after induction of anesthesia. Measurements were taken directly after premedication with xylazine or detomidine with butorphanol, after induction with diazepameketamine, after intubation, when suspended by the hoist and on the operating table. During hoisting, the head was supported and the eyeeheart height was measured to account for variations in head positioning among patients. IOPs were compared across time points using repeated-measures analysis of variance. Regression was used to compare IOP outcome with potential cofactors. Results Compared with measurements after premedication (17.5 ± 2.5 mmHg) (mean ± SD), hoisting significantly increased IOP (32.4 ± 15.3 mmHg) (p < 0.01). The highest recorded IOP in the hoist was 80.0 (range, 16.0e80.0) mmHg. The difference in IOP between premedication and hoisting was 15.0 ± 16.2
(range, e1.0 to 68.0) mmHg. Body weight had a significant effect on absolute IOP and change in IOP in the hoist (p < 0.01). Conclusions and clinical relevance Hoist IOP was significantly higher than post-premedication IOP with heavier horses having higher hoist IOPs and greater increases in IOP. The clinician should take this relationship into account when anesthetizing and hoisting larger horses where an increase in IOP could be detrimental. Keywords alpha2-adrenergic agonist, body weight, equine, general anesthesia, ophthalmic. Introduction Intraocular pressure (IOP) is defined by aqueous humor formation, uveoscleral outflow and episcleral venous pressure in the Goldmann equation as follows: IOP ¼ [F/C] þ PV where F denotes the aqueous fluid formation rate, C is the aqueous fluid outflow rate and PV is the episcleral venous pressure.
IOP is most commonly estimated indirectly using tonometry. The reference range for horses using tonometry is reported as 15e37 mmHg (Miller et al. 1990; Smith et al. 1990; van der Woerdt et al. 1995, 1998; Ramsey et al. 1999; Knollinger et al. 2005; Kom aromy et al. 2006; Stine et al. 2014). Maintenance of IOP within a physiologic range is vital for intraocular health, especially that of the optic nerve,
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Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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Hoisting on equine intraocular pressure CS Monk et al.
which is particularly susceptible to the pathophysiologic impact of glaucoma. The effect of head position on IOP has been previously documented in horses and humans (Kergoat & Lovasik 2005; Komaromy et al. 2006; Prata et al. 2010; Malihi & Sit 2012). When the head is inverted relative to the heart, the position changes the brain-to-heart hydrostatic gradient. When the head position was lowered below heart level in 30 horses administered detomidine, IOP increased in 52 out of 60 eyes (Komaromy et al. 2006). This change likely results from increases in intracranial pressure (ICP), cerebral perfusion pressure (CPP) and episcleral venous pressure as the head-down position increases the hydrostatic gradient of blood flow to the head (Brosnan et al. 2008). Additional factors that may also raise IOP are congestion of orbital content compressing the globe and an increase in the ocular blood volume within the uveal tract (Linder et al. 1988; Komaromy et al. 2006). Horses anesthetized for surgical or diagnostic procedures are often hoisted with the limbs supporting the body in an inverted position. In this position, the head commonly is carried below the level of the heart, and the thoracic cavity and large abdominal vessels are compressed by the weight of the colon. Additionally, a more pronounced effect of body position on ICP and CPP than in a conscious horse is induced by anesthetic drugs that interfere with normal cerebrovascular autoregulation (Brosnan et al. 2008, 2011). Increases in ICP and CPP during hoisting for general anesthesia may have an impact and increase IOP. Deep corneal ulcers, descemetoceles and penetrating foreign bodies of the eye may involve fragile corneal lesions that may rupture if IOP increases. In cases of pre-existing glaucoma, an even greater increase in IOP may further damage retinal ganglion cells (RGC). In a rat model study on glaucoma, slowly rising pressure was not overtly damaging to RGCs, but rapid, high-pressure pulses injured these cells almost immediately after insult (Resta et al. 2007). The effects of hoisting on the IOP of horses have not been published, and this effect should be determined to enable risk assessment of anesthesia for ophthalmic procedures. The aims of this study were to determine the IOP in normal horse eyes during hoisting after induction of anesthesia and to evaluate factors that had potential to influence IOP. We hypothesized that IOP would be increased during the hoisting position compared with the standing position. 2
Materials and methods This study was approved by the Institutional Animal Care and Use Committee of the University of Florida (no. 201408375). Owner consent was obtained before each horse was enrolled in the study. Eighteen horses were considered to be healthy, with no cardiovascular or respiratory abnormalities, based on a physical examination and results of laboratory tests within the normal reference ranges for the laboratory (complete blood cell count and serum biochemistry profile). Within 5 minutes after administration of drugs for premedication, an anterior segment ophthalmic examination without pharmacologic mydriasis was performed using a portable slit-lamp (Kowa SL-14; Kowa Company, CA, USA). The horse was included in the study if the anterior segment of the measured eye was free of detectable disease on direct examination. Anesthesia The anesthesia protocol was formulated for each horse by the anesthesiologist on duty on the day of the procedure. The horses were administered intravenously (IV) [mean ± standard deviation (SD)] either xylazine (0.54 ± 0.09 mg kg1; AnaSed; Lloyd Inc., IA, USA; n ¼ 9) or detomidine (0.01 ± 0.00 mg kg1; Domosedan; Orion Pharma, Finland; n ¼ 9) for premedication with butorphanol (0.02 mg kg1; Torbugesic; Zoetis Inc., MI, USA; n ¼ 14, seven horses with xylazine and seven with detomidine). Induction of anesthesia was achieved by administration of diazepam (0.07 ± 0.01 mg kg1; Diazepam hydrochloride USP; Hospira Inc., IL, USA) and ketamine (2.50 ± 0.24 mg kg1; Ketamine hydrochloride USP; Putney Inc., ME, USA) IV in direct succession. During induction, the horse was supported against the wall by personnel until the horse assumed sternal recumbency. The horse was turned to lateral recumbency and the trachea intubated with a cuffed endotracheal tube with an internal diameter 18e26 mm (SurgiVet, OH, USA). IOP measurements IOP was measured in one eye in each horse using the uppermost eye for the planned lateral recumbency during anesthesia. Within 5 minutes after administration of drugs for premedication, 0.5% tetracaine ophthalmic solution (Bausch & Lomb, NY, USA) was instilled into the eye. IOP was measured in the standing horse with the head in a position such that
© 2017 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e7
Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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the eye was approximately at the level where the jugular veins enter the thoracic inlet. This position was to approximate a zero heart-to-eye hydrostatic gradient (Ferreira et al. 2013). IOP was measured by gentle tapping of the tonometer (Tono-Pen AVIA; Reichert Ophthalmic Instruments, NY, USA) on the axial cornea until a reading was obtained. The instrument collects data from each tap and produces a final number with a calculated confidence interval. The first and only IOP reading with a 95% instrument-calculated confidence interval was used to mitigate bias. The same tonometer was used by one examiner (CSM) for all IOP measurements. IOP was measured after premedication (baseline), approximately 2 minutes after induction of anesthesia at the time of lateral recumbency, after intubation (n ¼ 9), at hoisting in dorsal recumbency and in final position on the table for surgery. For hoisting, hobbles were attached around the pasterns and the horse was inverted into dorsal position and suspended off the ground. The head was supported in a position on line with the thoracic and abdominal vertebrae. The nose was held so that the dorsal aspect of the head was parallel with the ground (Fig. 1). In addition to measurement of IOP at that moment, the distance from the ground to the medial canthus of the
eye and from the ground to the olecranon was measured in centimeters. The difference between these measurements was used as an estimation of the position of the eye relative to the heart to account for variability in head positioning (Fig. 1). Time from injection of the induction agents to measurement of IOP during hoisting was also recorded. Horses were then immediately transported to the surgery table. A final IOP measurement was obtained once the horse was positioned for the intended procedure in either dorsal or lateral recumbency but before connecting the endotracheal tube to the anesthesia delivery system. Positioning occurred immediately after hoisting, and the time between measurement in hoist and measurement in the final position was less than 2 minutes. Statistical analysis Data were analyzed using commercial software (Microsoft Excel 2013; WA, USA). Similarities in drug dosages were confirmed using a Student t test. A descriptive analysis for IOP by time point was performed. Differences in IOP between time points were verified using a Student t test, and further confirmed by use of repeated-measures analysis of variance with
Q3 Figure 1 Anatomic measurements for calculation of the eyeeheart height in anesthetized horses during hoisting after induction of anesthesia. The endotracheal tube is not pictured. A, ground to medial canthus; B, ground to olecranon; C, BeA ¼ eyeeheart height. © 2017 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e7
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Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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a set alpha of 5% and a power of 80%. Potential comorbidities influencing positional IOP changes were investigated. The following variables were tested using linear and multivariate regression in comparison to hoist IOP and difference in IOP from premedication to hoisting: signalment (age, sex, breed), body weight, drug dose (detomidine, xylazine, butorphanol, ketamine, diazepam) and eyeeheart height during hoisting. A NeweyeWest estimator was applied to account for heteroskedasticity in the data.
IOP ¼ 10.727 þ 0.052 weight). Absolute IOP in the hoist also was significantly correlated with body weight (R2 ¼ 17%, confidence ¼ 99%, p < 0.01). A simple linear regression showed that increasing body weight correlated to increasing IOP when in the hoist (IOP ¼ 4.301 þ 0.057 weight). None of the remaining investigated factors (signalment, medication dosage, eyeeheart height during hoisting) had a statistically significant relationship with outcome (hoist IOP or IOP difference) at a 95% confidence level. Discussion
Results Horses The study population consisted of six Warmbloods, five Quarter Horses, three ponies, two Thoroughbreds and two Paint Horses. There were 10 geldings, seven mares and one stallion, mean ± SD age was 10 ± 4.2 years and mean weight was 491 ± 110 kg. These animals were a mixture of client-owned horses undergoing anesthesia for an elective surgical procedure (n ¼ 6) or diagnostic imaging (n ¼ 9), or horses anesthetized for research purposes not related to this study (n ¼ 3). IOP was not different between horses administered detomidine or xylazine for premedication at any time point; therefore, the data were combined. IOPs measured in 18 horses were after premedication (18 ± 2.5 mmHg), after induction (17 ± 4.5 mmHg), during hoisting (32 ± 15.3 mmHg) and on the surgery table (20 ± 7.1 mmHg). A smaller subset of nine horses also had an IOP assessment after intubation (17 ± 4.6 mmHg). The difference in IOP between premedication and hoisting was 15.0 ± 16.2 (range, 1.0 to 68.0) mmHg. IOP during hoisting was significantly higher than at other time points (p < 0.01). Cofactors All planned anatomic measurements were obtained from all horses in the study. Eyeeheart height ranged from 51 to 117 cm (96 ± 17.5 cm). Body weight was found to be the best predictor of absolute hoist IOP and numeric change in IOP (R2 ¼ 13%, confidence ¼ 99%, p < 0.01) based on linear regression with a NeweyeWest Error Correction. A simple linear regression showed that for every 1 kg increase in weight, the IOP was expected to rise by 0.06 mmHg from post-sedation IOP when in the hoist (IOP ¼ intercept þ coefficient weight þ error; 4
The results of this study showed that hoisting during general anesthesia has a significant effect on IOP in normal horse eyes. The increase was not sustained, as the final measurement taken within 2 minutes of hoist IOP was not significantly different from IOP before hoisting. Similar to a previous study relating IOP to head position in horses, there was considerable variability in the magnitude of the change in IOP (Kom aromy et al. 2006). A fourfold increase was measured in some horses, whereas others sustained a negligible change. Studies in humans have documented a moderately unpredictable effect of position on IOP with interindividual variation postulated to be the result of factors such as age and vascular health (Prata et al. 2010). In some instances, the magnitude of IOP change due to position was found to be greatest in glaucomatous eyes (Buys et al. 2010; Prata et al. 2010; Kim et al. 2013). Body weight resulted in a significant linear increase in both change from baseline IOP and absolute IOP in the hoist. Body weight is likely representative of other key anatomic and physiologic variables. None of the remaining cofactors were associated with outcome of IOP in the hoist using linear and multivariate regression. Previous studies have failed to identify consistent risk factors in similar positionbased studies (Kom aromy et al. 2006; Prata et al. 2010). The effect of body weight was differentiated from a simple increase in hydrostatic gradient owing to measurement of the distance from the eye to the heart. Heavier horses theoretically would have an increased hydrostatic pressure on the eye from having longer necks and deeper chests, resulting in the eye being farther from the heart during hoisting. Because the head was supported approximating dorsal midline, vertical variation in the distance from the eye to the heart was not as wide as the variation in weights of the horses. The range of eyeeheart
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Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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height was 50.8e109.2 (95.8 ± 17.5, mean ± SD) cm, and attempts were made to position the horses similarly. When converting those distances to approximate pressure (mean 72; range, 39e83 mmHg), the numbers are so large that a pressure of this magnitude would cause serious and documentable disease to the eye. However, these hydrostatic variations did not yield a relationship with IOP outcome or body weight in this study. Therefore, the influence of body weight is possibly independent of the influence of neck length and its variation in hydrostatic gradient. Of course, hydrostatic gradient, being a vertical measurement relative to gravity, might not be the only factor. Length of neck may make venous return more challenging, resulting in increased ICP and IOP. Length of neck was not documented in this study. Another mechanism to consider behind the weight-related increase in IOP is blood pressure. In general, mean arterial pressure (MAP) decreases when a horse is turned into dorsal recumbency or hoisted, with minimal effect described in foals weighing 220 kg (Braun et al. 2009) but with a much greater effect recorded in heavy horses (Steffey & Howland 1980; Brosnan et al. 2011). As decreases in MAP would be predicted to decrease IOP in heavier horses, alternate possibilities for the hemodynamics in hoisting should be considered. Instead of changes to MAP, increases in central venous pressure (CVP) may be present. CVP could increase as a result of compression of gastrointestinal contents on the diaphragm and large abdominal vessels. Measurement of MAP and CVP during the hoisting event and modeling of the cardiopulmonary system would provide a more descriptive understanding. The main purpose of this study was to measure the effects of posture on IOP in horses to provide insight into ocular health of horses during anesthesia. Owing to the necessity of anesthesia and the observational nature of the study, confounding factors could not be removed to isolate the effects of the hoist. Specifically, injectable medications given to the patient, as well as the act of intubation, have been shown to alter IOP (Bechara et al. 1998; Safavi & Honarmand 2008; Ferreira et al. 2013). In the study presented here, the drugs used for induction of anesthesia did not increase IOP, as no time points other than hoisting were significantly different. Ketamine is the most common induction agent used in horses, and is generally thought to increase IOP in horses and other species by increasing extraocular muscle tone (Hofmeister et al. 2006;
Ferreira et al. 2013). However, variable reports of ketamine's effects on IOP exist even within species including horses (Trim et al. 1985; Smith et al. 1990; Ghaffari et al. 2010; Ferreira et al. 2013; Kovalcuka et al. 2013). The effects are rarely outside the normal reference range. Effects of premedication, specifically a2-adrenergic agonists, are more consistent. These medications have been shown to decrease IOP (van der Woerdt et al. 1995; Brosnan et al. 2008; Kom aromy et al. 2006; Holve 2012; Stine et al. 2014), occasionally after a transient increase (Rauser et al. 2012). All horses in this study had what is considered clinically normal IOPs after administration of premedication agents. This measurement was used as baseline IOP in this study to mitigate the confounding effect of a2-adrenergic agonist sedation on IOP. Because IOP was not measured prior to administration of the premedication, conclusions cannot be drawn about the effects of detomidine or xylazine on IOP in this patient population. Therefore, it is possible that the magnitude of increase from baseline to hoisting is not representative of the change measured if hoisting IOP were to be compared with a baseline IOP obtained in the absence of sedation. Nonetheless, the absolute IOP measurements obtained in this study were representative of a clinical situation. Laryngoscopy and tracheal intubation are associated with tachycardia and a rise in MAP resulting from sympathetic stimulation in humans and dogs (Kayhan et al. 2005; Thompson & Rioja 2016). However, the administration of an a2-adrenergic agonist for premedication or lidocaine IV have been shown to prevent this response (Yavascaoglu et al. 2008; Richa 2009; Thompson & Rioja 2016). No study evaluating the effect of intubation on heart rate and MAP in horses was found in a search of the literature. All horses were administered an a2adrenergic agonist as part of the premedication protocol. IOP measured directly after intubation in a subset of horses was not significantly different from any time point other than the hoisting IOP. Weaknesses of this study mainly relate to its observational nature. In order to minimize the time each horse was suspended in the hoist, mimicking the clinical situation, precise anatomic positioning among the horses was not achieved. The measurement of eyeeheart height was intended to account for this variability in approximate head positioning. The IOP may have been different had the head of each horse been raised to heart level during hoisting. Results of previous studies investigating the impact of
© 2017 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved., ▪, 1e7
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Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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body position on IOP suggest that the increase in IOP would be less if the head was supported closer to heart level (Linder et al. 1988; Brosnan et al. 2002, 2008; Hirooka & Shiraga 2003; Kergoat & Lovasik 2005; Buys et al. 2010; Malihi & Sit 2012). Further direction for this research is IOP at the conclusion of anesthesia, when hoisting is necessary to return the horse to the recovery stall. The general anesthesia protocol for surgery frequently includes an inhalation agent such as isoflurane or sevoflurane that is maintained until the conclusion of surgery. Inhalation agents have a more substantial effect on cerebrovascular autoregulation by decreasing MAP, blunting baroreflex activity and causing vasodilation (Brosnan et al. 2008). Ophthalmologic surgeries, such as phacoemulsification or for penetrating keratoplasty, create a fragile globe. For these horses, an increase in IOP postoperatively may jeopardize the surgical outcome, and anesthetic management should be adjusted to avoid a spike in IOP during hoisting into the recovery stall. In conclusion, IOP was significantly increased during hoisting in 18 horses anesthetized with xylazine or detomidine with butorphanol and diazepameketamine when compared with IOP measured after premedication. The magnitude of the increase was variable among individuals. Increased body weight was identified as a risk factor for a more substantial increase in change of IOP and absolute IOP. Further research is needed using different head heights in relation to the heart and cardiovascular measurements to elucidate the mechanisms behind the increase in IOP. Acknowledgements The authors acknowledge Renata Velloso Ramos, College of Veterinary Medicine, University of Florida, for her assistance with data collection and Gil BenShlomo, College of Veterinary Medicine, Iowa State University, for his assistance with the study design. Authors’ contributions CM, TG, FGP: study design, data collection, data interpretation, preparation of the manuscript. DB, CP: study design, data interpretation, preparation of the manuscript. AM: study design, statistical analysis, data interpretation, preparation of the manuscript. Q2
Conflict of interest statement Authors declare no conflict of interest. 6
References Bechara JN, Barros PSM, Fantoni DT et al. (1998) Intraocular pressure evaluation of equines anaesthetized with romifidine, tiletamine/zolazepam, halothane and vecuronium. Ciencia Rural 28, 59e64. Braun C, Trim CM, Eggleston RB (2009) Effects of changing body position on oxygenation and arterial blood pressures in foals anesthetized with guaifenesin, ketamine, and xylazine. Vet Anaesth Analg 36, 18e24. Brosnan RJ, Steffey EP, LeCouteur RA et al. (2002) Effects of body position on intracranial and cerebral perfusion pressures in isoflurane-anesthetized horses. J Appl Physiol 92, 2542e2546. Brosnan RJ, Esteller-Vico A, Steffey EP et al. (2008) Effects of head-down positioning on regional central nervous system perfusion in isoflurane-anesthetized horses. Am J Vet Res 69, 737e743. Brosnan RJ, Steffey EP, LeCouteur RA et al. (2011) Effects of isoflurane anesthesia on cerebrovascular autoregulation in horses. Am J Vet Res 72, 18e24. Buys YM, Alasbali T, Jin Y et al. (2010) Effect of sleeping in a head-up position on intraocular pressure in patients with glaucoma. Ophthalmology 117, 1348e1351. Ferreira TH, Brosnan RJ, Shilo-Benjamini Y et al. (2013) Effects of ketamine, propofol, or thiopental administration on intraocular pressure and qualities of induction of and recovery from anesthesia in horses. Am J Vet Res 74, 1070e1077. Ghaffari MS, Rezaei MA, Mirani AH, Khorami N (2010) The effects of ketamineemidazolam anesthesia on intraocular pressure in clinically normal dogs. Vet Ophthalmol 13, 91e93. Hirooka K, Shiraga F (2003) Relationship between postural change of the intraocular pressure and visual field loss in primary open-angle glaucoma. J Glaucoma 12, 379e382. Hofmeister EH, Mosunic CB, Torres BT et al. (2006) Effects of ketamine, diazepam, and their combination on intraocular pressures in clinically normal dogs. Am J Vet Res 67, 1136e1139. Holve DL (2012) Effect of sedation with detomidine on intraocular pressure with and without topical anesthesia in clinically normal horses. J Am Vet Med Assoc 240, 308e311. Kayhan Z, Aldemir D, Mutlu H, O güs¸ E (2005) Which is responsible for the haemodynamic response due to laryngoscopy and endotracheal intubation? Catecholamines, vasopressin or angiotensin? Eur J Anaesthesiol 22, 780e785. Kergoat H, Lovasik J (2005) Seven-degree head-down tilt reduces choroidal pulsatile ocular blood flow. Aviat Space Environ Med 76, 930e934. Kim KN, Jeoung JW, Park KH et al. (2013) Effect of lateral decubitus position on intraocular pressure in
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Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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Hoisting on equine intraocular pressure CS Monk et al. glaucoma patients with asymmetric visual field loss. Ophthalmology 120, 731e735. Knollinger AM, La Croix NC, Barrett PM, Miller PE (2005) Evaluation of a rebound tonometer for measuring intraocular pressure in dogs and horses. J Am Vet Med Assoc 227, 244e248. Kom aromy AM, Garg CD, Ying GS, Liu C (2006) Effect of head position on intraocular pressure in horses. Am J Vet Res 67, 1232e1235. Kovalcuka L, Birgele E, Bandere D, Williams DL (2013) The effects of ketamine hydrochloride and diazepam on the intraocular pressure and pupil diameter of the dog's eye. Vet Ophthalmol 16, 29e34. Linder BJ, Trick GL, Wolf ML (1988) Altering body position affects intraocular pressure and visual function. Invest Ophthalmol Vis Sci 29, 1492e1497. Malihi M, Sit AJ (2012) Effect of head and body position on intraocular pressure. Ophthalmology 119, 987e991. Miller PE, Pickett JP, Majors LJ (1990) Evaluation of two applanation tonometers in horses. Am J Vet Res 51, 935e937. Prata TS, De Moraes CGV, Kanadani FN et al. (2010) Posture-induced intraocular pressure changes: considerations regarding body position in glaucoma patients. Surv Ophthalmol 55, 445e453. Ramsey DT, Hauptman JG, Petersen-Jones SM (1999) Corneal thickness, intraocular pressure, and optical corneal diameter in Rocky Mountain Horses with cornea globosa or clinically normal corneas. Am J Vet Res 60, 1317e1321. Rauser P, Pfeifr J, Proks P, Stehlik L (2012) Effect of medetomidineebutorphanol and dexmedetomidineebutorphanol combinations on intraocular pressure in healthy dogs. Vet Anaesth Analg 39, 301e305. Resta V, Novelli E, Vozzi G et al. (2007) Acute retinal ganglion cell injury caused by intraocular pressure spikes is mediated by endogenous extracellular ATP. Eur J Neurosci 25, 2741e2754. Richa F (2009) Dexmedetomidine blunts haemodynamic and intraocular pressure responses to tracheal intubation. Eur J Anaesthesiol 26, 444e445.
Safavi M, Honarmand A (2008) Influence of head flexion after endotracheal intubation on intraocular pressure and cardio-respiratory response in patients undergoing cataract surgery. Ghana Med J 42, 105e109. Smith PJ, Gum GG, Whitley RD et al. (1990) Tonometric and tonographic studies in the normal pony eye. Equine Vet J Suppl 10, 36e38. Steffey EP, Howland D (1980) Comparison of circulatory and respiratory effects of isoflurane and halothane anesthesia in horses. Am J Vet Res 41, 821e825. Stine JM, Michau TM, Williams MK et al. (2014) The effects of intravenous romifidine on intraocular pressure in clinically normal horses and horses with incidental ophthalmic findings. Vet Ophthalmol 17, 134e139. Thompson KR, Rioja E (2016) Effects of intravenous and topical laryngeal lidocaine on heart rate, mean arterial pressure and cough response to endotracheal intubation in dogs. Vet Anaesth Analg 43, 371e378. Trim CM, Colbern GT, Martin CL (1985) Effect of xylazine and ketamine on intraocular-pressure in horses. Vet Rec 117, 442e443. van der Woerdt A, Gilger BC, Wilkie DA, Strauch SM (1995) Effect of auriculopalpebral nerve block and intravenous administration of xylazine on intraocularpressure and corneal thickness in horses. Am J Vet Res 56, 155e158. van der Woerdt A, Gilger BC, Wilkie DA et al. (1998) Normal variation in, and effect of 2% pilocarpine on, intraocular pressure and pupil size in female horses. Am J Vet Res 59, 1459e1462. Yavascaoglu B, Kaya FN, Baykara M et al. (2008) A comparison of esmolol and dexmedetomidine for attenuation of intraocular pressure and haemodynamic responses to laryngoscopy and tracheal intubation. Eur J Anaesthesiol 25, 517e519. Received 29 March 2016; accepted 13 October 2016. Available online xxx
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Please cite this article in press as: Monk CS, Brooks DE, Granone T et al. Measurement of intraocular pressure in healthy anesthetized horses during hoisting, Veterinary Anaesthesia and Analgesia (2017), http://dx.doi.org/10.1016/ j.vaa.2016.10.001
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