Anesthetic and cardiovascular effects of balanced anesthesia using constant rate infusion of midazolam-ketamine-medetomidine with inhalation of oxygen-sevoflurane (MKM-OS anesthesia) in horses

The anesthetic sparring and cardiovascular effects produced by midazolam 0.8 mg/ml-ketamine 40 mg/ml-medetomidine 0.05 mg/ml (0.025 ml/kg/hr) drug infusion during sevoflurane in oxygen (MKM-OS) anesthesia was determined in healthy horses. The anesthetic sparring effects of MKM-OS were assessed in 6 healthy thoroughbred horses in which the right carotid artery was surgically relocated to a subcutaneous position. All horses were intubated and ventilated with oxygen using intermittent positive pressure ventilation (IPPV). The end-tidal concentration of sevoflurane (ET(SEV)) required to maintain surgical anesthesia was approximately 1.7%. Heart rate and mean arterial blood pressure averaged 23-41 beats/min and 70-112 mmHg, respectively. All horses stood between 23-44 min after the cessation of all anesthetic drugs. The cardiovascular effects of MKM-OS anesthesia were evaluated in 5 healthy thoroughbred horses ventilated using IPPV. Anesthesia was maintained for 4 hr at an ET(SEV) of 1.7%. Each horse was studied during left lateral (LR) and dorsal recumbency (DR) with a minimum interval between evaluations of 1 month. Cardiac output and cardiac index were maintained between 70-80% of baseline values during LR and 65-70% of baseline values during DR. Stroke volume was maintained between 75-85% of baseline values during LR and 60-70% of baseline values during DR. Systemic vascular resistance was not different from baseline values regardless of position. MKM-OS anesthesia may be useful for prolonged equine surgery because of its minimal cardiovascular depression in both of lateral and dorsal recumbency.     

Cardiovascular and respiratory measurements in awake and isoflurane-anesthetized horses

Circulatory and respiratory function was monitored in nonmedicated, spontaneously breathing horses (n = 7) immediately before, during, and 1 hour after 85 +/- 4.1 (X +/- SEM) minutes of constant 1.57% isoflurane in O2 anesthesia. Comparison of values during anesthesia with those obtained while horses were awake revealed a significant (P less than 0.05) decrease in arterial blood pressure that was related to a slight, but insignificant, decrease in cardiac output and peripheral vascular resistance. Although isoflurane anesthesia and recumbency resulted in a significant (P less than 0.05) decrease in stroke volume, cardiac output did not decrease significantly because heart rate tended to increase. Isoflurane and recumbency also significantly (P less than 0.05) increased PaCO2, peak expiratory gas flow, total expiratory time, and PCV and significantly decreased PaO2, minute expired ventilation, and the ratio of peak inspired to expired gas flow. Differences imposed by isoflurane anesthesia were reversed by 1.5 hour after anesthesia.     

Repeatability of Doppler ultrasound measurements of hindlimb blood flow in halothane anaesthetised horses

The purpose of this study was to determine the repeatability of femoral blood flow recorded using Doppler ultrasound in anaesthetised horses. Doppler ultrasound of the femoral artery and vein was performed in 6 horses anaesthetised with halothane and positioned in left lateral recumbency. Velocity spectra, recorded using low pulse repetition frequency, were used to calculate time-averaged mean velocity (TAV), velocity of component a (TaVa), velocity of component b (TaVb), volumetric flow, early diastolic deceleration slope (EDDS) and pulsatility index (PI). Within-patient variability was determined for sequential Doppler measurements recorded during a single standardised anaesthetic episode. Within-patient variability was also determined for Doppler and cardiovascular measurements recorded during 4 separate standardised anaesthetic episodes performed at intervals of at least one month. Within-patient variation during a single anaesthetic episode was small. Coefficients of variation (cv) were <12.5% for arterial measurements and <17% for venous measurements. Intraclass correlation coefficient was >0.75 for all measurements. No significant change was observed in measurements of cardiovascular function suggesting that within-patient variation observed during a single anaesthetic episode was due to measurement error. In contrast, within-patient variation during 4 separate anaesthetic episodes was marked (cv>17%) for most Doppler measurements obtained from arteries and veins. Variation in measurements of cardiovascular function were marked (cv>20%), suggesting that there is marked biological variation in central and peripheral observed. Further studies are warranted to determine the ability of this technique to detect differences in blood flow during administration of different anaesthetic agents.     

MEASUREMENTS OF HINDLIMB BLOOD FLOW RECORDED USING DOPPLER ULTRASOUND DURING ADMINISTRATION OF VASOACTIVE AGENTS IN HALOTHANE-ANESTHETIZED HORSES

The purpose of the study was to determine the ability of Doppler ultrasound to detect changes in femoral blood flow during pharmacologic manipulation of arterial blood pressure. Doppler ultrasonography was performed in the femoral vessels of six halothane-anesthetized horses before and during administration of phenylephrine HCI and sodium nitroprusside. The time-averaged mean velocity and volumetric flow were calculated. The contour of the velocity waveform was assessed, and the early diastolic deceleration slope (EDDS) and pulsatility index (PI) were calculated. Administration of phenylephrine HCI resulted in increased mean aortic blood pressure (MABP) by 40% (29.3-53.0%). This caused significant decrease in cardiac output (26.8 to 13.5 l/min), femoral arterial velocity (left artery 7.20 to 4.00 cm/s; right artery 5.01 to 3.39 cm/s) and volumetric flow (left artery 556 to 221 ml/min; right artery 397 to 193 ml/min) in the femoral vessels and significant increase in systemic vascular resistance (163 to 433 dyn-s/cm5), EDDS (1a: 285 to 468: ra: 250 to 481) and PI (1a: 9.38 to 20.4; ra 17.1 to 29.1). Administration of sodium nitroprusside resulted in a decreased MABP of 27.2% (22.5-33%). This increased cardiac output (20.8 to 32.4 L/min), however, no significant changes were observed in femoral blood flow. Despite obvious changes in the waveform contour, no significant change occurred in EDDS or PI. These results suggest that Doppler ultrasound may be useful for measuring femoral blood flow in anesthetized horses. However, waveform analysis appears to be limited when multiple changes occur in central and peripheral haemodynamics.     

Time-related responses of spontaneously breathing, laterally recumbent horses to prolonged anesthesia with halothane

Cardiovascular and respiratory functions were serially evaluated in ten healthy, fasting, spontaneously breathing, laterally recumbent adult horses during five hours of constant 1.06% alveolar halothane (equivalent to 1.2 times the minimum alveolar concentration for horses). Mean carotid arterial pressure was about 25% higher after one hour of constant-dose halothane than after 30 minutes of constant-dose (P less than 0.05), and remained increased throughout the study. Mean carotid arterial pressure peaked after 90 minutes, and was about 30% higher than at 30 minutes. Total peripheral vascular resistance initially increased (20% at one hour), then gradually returned to the 0.5-hour value over the next four hours. Cardiac output progressively increased with time (P less than 0.05; 20% by two hours; nearly 40% by five hours) because of an increase in stroke volume. An increase (P less than 0.05) in mixed venous PO2 accompanied the increase in cardiac output. Heart rate did not change significantly (P greater than 0.05). Some measures of ventilation changed significantly with time (P less than 0.05). After four and five hours of constant alveolar halothane, the PaCO2, inspired gas flow, and ratio of inspired vs expired gas flow were significantly higher than the 0.5-hour values. Inspiratory time significantly decreased, beginning at three hours. All horses recovered from anesthesia and recumbency without complications.     

Cardiopulmonary function in horses during anesthetic recovery in a hydropool.

OBJECTIVE: To determine the cardiovascular and respiratory effects of water immersion in horses recovering from general anesthesia. ANIMALS: 6 healthy adult horses. PROCEDURE: Horses were anesthetized 3 times with halothane and recovered from anesthesia while positioned in lateral or sternal recumbency in a padded recovery stall or while immersed in a hydropool. Cardiovascular and pulmonary functions were monitored before and during anesthesia and during recovery until horses were standing. Measurements and calculated variables included carotid and pulmonary arterial blood pressures (ABP and PAP respectively), cardiac output, heart and respiratory rates, arterial and mixed venous blood gases, minute ventilation, end expiratory transpulmonary pressure (P(endXes)), maximal change in transpulmonary pressure (deltaP(tp)max), total pulmonary resistance (RL), dynamic compliance (Cdyn), and work of breathing (W). RESULTS: Immersion in water during recovery from general anesthesia resulted in values of ABP, PAP P(endXes), deltaP(tp)max, R(L), and W that were significantly greater and values of Cdyn that were significantly less, compared with values obtained during recovery in a padded stall. Mode of recovery had no significant effect on any other measured or calculated variable. CONCLUSIONS AND CLINICAL RELEVANCE: Differences in pulmonary and cardiovascular function between horses during recovery from anesthesia while immersed in water and in a padded recovery stall were attributed to the increased effort needed to overcome the extrathoracic hydrostatic effects of immersion. The combined effect of increased extrathoracic pressure and PAP may contribute to an increased incidence of pulmonary edema in horses during anesthetic recovery in a hydropool.     

A comparison of romifidine and xylazine when used with diazepam/ketamine for short duration anesthesia in the horse.

The purpose of this study was to compare and evaluate sedation with intravenous xylazine (1.1 mg/kg bodyweight [BW]) versus intravenous romifidine (100 micrograms/kg BW) followed by induction of anesthesia with intravenous diazepam (0.04 mg/kg BW) and ketamine (2.2 mg/kg BW). Twelve healthy horses were used in a blinded, randomized, cross-over design. Heart rate, presence of 2nd degree atrioventricular heart blocks (2 degrees AVB), respiratory rate, arterial blood pressures, blood gases, packed cell volume, total serum proteins, and duration of anesthesia and recumbency were recorded. Induction and recovery quality was evaluated using a 0 to 4 score. Response to stimulation with noise, pressure, and cutaneous electrical stimulation was assessed at 5 minute intervals during recumbency to evaluate the depth of anesthesia. Heart rate was lower and 2 degrees AVB more frequent in the romifidine group, while blood pressure was lower in the xylazine group. Duration of anesthesia was longer in the romifidine group (mean 20.8, s mean 2.3 min) versus the xylazine group (mean 15.8, s mean 1.6 min), while induction and recovery were excellent in both groups. Respiratory rates, blood gas values, packed cell volumes, and total protein levels did not differ between groups. The results indicate that romifidine premedication followed by diazepam and ketamine is a very satisfactory regime for short duration intravenous anesthesia in horses.     

Duplex ultrasonography of the common carotid artery and external jugular vein of cows

OBJECTIVE: To determine blood flow patterns in the common carotid artery and external jugular vein in cows before and after sedation achieved by administration of xylazine hydrochloride. ANIMALS: 30 clinically normal Swiss Braunvieh cows. PROCEDURE: A 5.0-MHz sector transducer was used to examine the common carotid artery and external jugular vein before and after cows were sedated by administration of xylazine. Several variables were calculated, including diameter of the blood vessels, blood flow velocity, and flow-time volume. RESULTS: The common carotid artery before sedation had a maximum systolic velocity of 89 +/- 8.5 cm/s, maximum diastolic velocity of 36 +/- 6.0 cm/s, mean velocity of 35 +/- 5.4 cm/s, and flow-time volume of 28.2 +/- 3.48 cm3/s. In all cows, sedation achieved by administration of xylazine resulted in a significant decrease in velocity of arterial blood flow and flow-time volume. The external jugular vein before sedation had a maximum velocity of 65 +/- 8.3 cm/s, maximum velocity of retrograde venous blood flow of 20 +/- 6.6 cm/s, and flow-time volume of 29.7 +/- 5.42 cm3/s. These values decreased significantly after cows were sedated by administration of xylazine. CONCLUSIONS AND CLINICAL RELEVANCE: Doppler ultrasonography is particularly suitable for evaluation of blood flow patterns in the common carotid artery and external jugular vein of healthy cows. The results reported here provide a basis for use in examination of cows with cardiac and blood vessel disease.     

Effect of a 30-minute infusion of dobutamine hydrochloride on hind limb blood flow and hemodynamics in halothane-anesthetized horses

OBJECTIVE: To evaluate the hemodynamic effects of dobutamine hydrochloride (0.5 microg/kg of body weight/min) in halothane-anesthetized horses. ANIMALS: 6 adult Thoroughbred horses. PROCEDURE: Anesthesia was induced by use of romifidine (100 microg/kg) and ketamine (2.2 mg/kg), IV. Anesthesia was maintained by halothane (end-tidal concentration 0.9 to 1.0%). Aortic, left ventricular, and right atrial pressures were measured, using catheter-mounted strain gauge transducers. Cardiac output (CO), velocity time integral, maximal aortic blood flow velocity and acceleration, and left ventricular preejection period and ejection time were measured from aortic velocity waveforms obtained by transesophageal Doppler echocardiography. Velocity waveforms were recorded from the femoral vessels, using Doppler ultrasonography. The time-averaged mean velocity and early diastolic deceleration slope (EDDS) were measured. Pulsatility index (PI) and volumetric flow were calculated. Microvascular perfusion was measured in the semimembranosus muscles by laser Doppler flowmetry. Data were recorded 60 minutes after induction of anesthesia (control) and at 15 and 30 minutes after start of an infusion of dobutamine (0.5 microg/kg/min). RESULTS: Aortic pressures were significantly increased during the infusion of dobutamine. No change was observed in the indices of left ventricular systolic function including CO. Femoral arterial flow significantly increased, and the PI and EDDS decreased. No change was observed in the femoral venous flow or in microvascular perfusion. CONCLUSIONS AND CLINICAL RELEVANCE: At this dosage, dobutamine did not alter left ventricular systolic function. Femoral blood flow was preferentially increased as the result of local vasodilatation. The lack of effect of dobutamine on microvascular perfusion suggests that increased femoral flow is not necessarily associated with improved perfusion of skeletal muscles.     

Occlusion of the internal carotid artery by means of microcoils for preventing epistaxis caused by guttural pouch mycosis in horses.

Occlusion of the internal carotid artery by insertion of intravascular platinum microcoils for guttural pouch mycosis was experimentally evaluated in 9 healthy adult Thoroughbred horses. The internal carotid artery was ligated to its origin, and an arteriotomy was made distal to the ligature, which was then occluded by insertion of the microcoil approximately 13 cm distal to its origin. Cessation of blood flow was determined visually and by angiography at the arteriotomy site. Six horses were evaluated for complication clinically and by endoscopy after surgery. One horse was necropsied after 30 days of surgery for histological evaluation of artery thrombus formation. In the other 3 horses, the blood flow of the right internal carotid artery was monitored, before and after microcoil occlusion of the left internal carotid artery. One or 2 microcoils stopped blood flow within a few minutes. No other abnormal findings were observed clinically. Thrombus was observed in the occluded segment of 1 horse 30 days after insertion; but no abnormalities were detected. The blood flow in the right internal carotid artery increased by approximately 28-58% after occlusion of the left internal carotid artery. This microcoil vascular occlusion technique causes an effective thrombosis, and based on experimental studies and clinical application in 2 horses with epistaxis due to guttural pouch mycosis, this technique would appear to be safe and efficacious.     

Cardiopulmonary effects of romifidine/ketamine or xylazine/ketamine when used for short duration anesthesia in the horse

The cardiovascular changes associated with anesthesia induced and maintained with romifidine/ketamine versus xylazine/ ketamine were compared using 6 horses in a cross over design. Anesthesia was induced and maintained with romifidine (100 microg/kg, IV)/ketamine (2.0 mg/kg, IV) and ketamine (0.1 mg/kg/min, IV), respectively, in horses assigned to the romifidine/ ketamine group. Horses assigned to the xylazine/ketamine group had anesthesia induced and maintained with xylazine (1.0 mg/kg, IV)/ketamine (2.0 mg/kg, IV) and a combination of xylazine (0.05 mg/kg/min, IV) and ketamine (0.1 mg/kg/min, IV), respectively. Cardiopulmonary variables were measured at intervals up to 40 min after induction. All horses showed effective sedation following intravenous romifidine or xylazine and achieved recumbency after ketamine administration. There were no significant differences between groups in heart rate, arterial oxygen partial pressures, arterial carbon dioxide partial pressures, cardiac index, stroke index, oxygen delivery, oxygen utilization, systemic vascular resistance, left ventricular work, or any of the measured systemic arterial blood pressures. Cardiac index and left ventricular work fell significantly from baseline while systemic vascular resistance increased from baseline in both groups. The oxygen utilization ratio was higher in the xylazine group at 5 and 15 min after induction. In conclusion, the combination of romifidine/ketamine results in similar cardiopulmonary alterations as a xylazine/ketamine regime, and is a suitable alternative for clinical anesthesia of the horse from a cardiopulmonary viewpoint.     

Effect of Ligation on Internal Carotid Artery Blood Pressure in Horses

The effect of a single ligature on back pressure in the internal carotid artery was studied in nine horses. In six anesthetized horses, one internal carotid artery was catheterized 2 cm from its origin and blood pressure was recorded continuously. Then the artery was ligated, pressure was recorded again, and the horses were euthanatized. In another three anesthetized horses, indwelling catheters were placed in both internal carotid arteries and a loose ligature was placed proximal to one catheter. After horses recovered from anesthesia, the ligature was tied and blood pressure was recorded in both arteries on that day and 3 days later, then these horses were euthanatized. The anatomy of the cerebral arteries was examined in all nine horses. Blood pressure in the internal carotid arteries did not change after ligation. Subjectively, collateral channels considered most likely to maintain blood pressure in the ligated internal carotid artery were the caudal intercarotid artery and the cerebral arterial circle. We concluded that ligation of the internal carotid artery would not prevent severe hemorrhage from the internal carotid artery in horses with guttural pouch mycosis until the ligated artery thrombosed to the level of the lesion.     

Effect of body posture on cardiopulmonary function in horses during five hours of constant-dose halothane anesthesia

Cardiovascular and respiratory functions were serially characterized in 7 healthy, spontaneously breathing, adult horses (from which food had been withheld) during 5 hours of constant 1.06% alveolar halothane (end-expired halothane concentration of 1.06%; equivalent to 1.2 times the minimal alveolar anesthetic concentration for horses). To enable comparison of temporal results in relation to 2 body postures, horses were studied in lateral recumbency (LR) and dorsal recumbency (DR) on separate occasions. Temporal changes in results of measures of circulation previously reported from this laboratory for horses in LR were confirmed (ie, a time-related increase in systemic arterial blood pressure, cardiac output, stroke volume, and PCV). During DR, systemic arterial blood pressure was initially significantly (P less than 0.05) greater and pulmonary artery pressure less than results at comparable periods during LR. Differences ceased to exist with duration of anesthesia. Except for a greater heart rate at hour 5 of DR, no other significant differences in circulation were found between LR and DR. In general, except for PaO2, measures of ventilation did not change with time in either LR or DR. The PaO2 was significantly greater during LR, compared with DR, but the average did not change significantly with time in either body posture.     

Colour-coded and Pulsed Doppler Sonography of the Canine Testis, Epididymis and Prostate Gland: Physiological and Pathological Findings

Two-dimensional ultrasound was used in combination with colour-coded and pulsed Doppler sonography to study the blood flow of the testes and prostate gland in a total of 30 male dogs. After detection of the vessels by colour-coded Doppler sonography, the blood flow patterns were determined by pulsed Doppler sonography measuring and describing the systolic and diastolic peak velocity (SPV, DPV), the end-diastolic velocity, the time-averaged maximum velocity (TAMAX), the pulsatility and resistance index, as well as the ratios of the systolic peak velocity and end-diastolic velocity and of the systolic and diastolic peak velocities. The blood flow of the testicular artery was measured within the pampiniform plexus and the marginal location. The prostatic blood supply was measured in the artery of the deferential duct (cranial), the prostatic artery outside (lateral) and within the gland (subcapsular). The physiological testicular flow pattern was monophasic with a high diastolic flow. Testes with neoplastic alterations showed a significant increase of SPV and TAMAX. The epididymal vessels could not be detected. Under physiological conditions the prostatic blood flow pattern was biphasic in the cranial and lateral location and monophasic in the subcapsular location. Benign prostatic hyperplasia was characterized by a significant increase of SPV, DPV and TAMAX. The results of the present investigation demonstrate that the colour-coded and pulsed Doppler sonography give additional valuable information which improves the andrological diagnostics in the dog.     

PULSED WAVE-DOPPLER ULTRASONOGRAPHIC EVALUATION OF THE COMMON CAROTID ARTERY IN THE RESTING HORSE: PHYSIOLOGIC DATA

A pulsed wave-Doppler ultrasonographic evaluation of common carotid arterial blood flow was carried out on 63 healthy Italian Saddlebred horses. Vessel diameter and tracing morphology were evaluated were evaluated and blood flow parameters (systolic, diastolic and mean velocity, acceleration and deceleration of the systolic wave, cartid pulse volume) were calculated and correlated with class variables (sex, age and body weight). On the basis of the presence of an incisure in the ascending branch of the systolic curve, subjects were divided in two groups: one with a bifid systolic curve and the other with a monophasic aspect. Correlations between: 1) diameter of the vessel and body weight and 2) carotid pulse volume and flow velocity (systolic, diastolic and mean) were found. A greater systolic pulse volume was found in male subjects, in subjects with greater body weight and in those which had a monophasic systolic wave.     

Comparison of cardiorespiratory variables in dorsally recumbent horses anesthetized with guaifenesin-ketamine-xylazine spontaneously breathing 50% or maximal oxygen concentrations

This study compared cardiorespiratory variables in dorsally recumbent horses anesthetized with guaifenesin-ketamine-xylazine and spontaneously breathing 50% or maximal (> 90%) oxygen (O₂) concentrations. Twelve healthy mares were randomly assigned to breathe 50% or maximal O₂ concentrations. Horses were sedated with xylazine, induced to recumbency with ketamine-diazepam, and anesthesia was maintained with guaifenesin-ketamine-xylazine to effect. Heart rate, arterial blood pressures, respiratory rate, lithium dilution cardiac output (CO), inspired and expired O₂ and carbon dioxide partial pressures, and tidal volume were measured. Arterial and mixed-venous blood samples were collected prior to sedation (baseline), during 30 minutes of anesthesia, 10 minutes after disconnection from O₂, and 30 minutes after standing. Shunt fraction, O₂ delivery, and alveolar-arterial O₂ partial pressures difference [P(A-a)O₂] were calculated. Recovery times were recorded. There were no significant differences between groups in cardiorespiratory parameters or in P(A-a)O₂ at baseline or 30 minutes after standing. Oxygen partial pressure difference in the 50% group was significantly less than in the maximal O₂ group during anesthesia.     

Influence of detomidine on echocardiographic function parameters and cardiac hemodynamics in horses with and without heart murmur

30 warmblood horses were examined before and after sedation with 20 micrograms/kg BW detomidine, to determine changes of cardiac function parameters, using B-mode, M-mode and Doppler echocardiography. 15 horses showed a heart murmur, but no clinical signs of cardiac heart failure, 15 horses had neither a heart murmur nor other signs of cardiac disease. After sedation with detomidine we could recognise a significant increase of end-diastolic left atrium diameter, an increase of end-systolic left ventricular diameter and aortic root diameter. The end-systolic thickness of papillary muscle and interventricular septum showed a decrease. Fractional shortening and amplitude of left ventricular wall motion was decreased after sedation. The mitral valve echogram revealed a presystolic valve closure and an inflection in the Ac slope (B-notch) in xy horses before sedation. Both increased after sedation with detomidine. Doppler echocardiography showed a decrease of blood flow velocity and velocity time integral (VTI) in the left and right ventricular outflow tract after sedation. Regurgitant flow signals were intensified following sedation in xy horses, especially at the mitral valve.     

Effects of head-down positioning on regional central nervous system perfusion in isoflurane-anesthetized horses

OBJECTIVE: To test the hypothesis that head-down positioning in anesthetized horses increases intracranial pressure (ICP) and decreases cerebral and spinal cord blood flows. ANIMALS: 6 adult horses. PROCEDURES: For each horse, anesthesia was induced with ketamine hydrochloride and xylazine hydrochloride and maintained with 1.57% isoflurane in oxygen. Once in right lateral recumbency, horses were ventilated to maintain normocapnia. An ICP transducer was placed in the subarachnoid space, and catheters were placed in the left cardiac ventricle and in multiple vessels. Blood flow measurements were made by use of a fluorescent microsphere technique while each horse was in horizontal and head-down positions. Inferential statistical analyses were performed via repeated-measures ANOVA and Dunn-Sidak comparisons. RESULTS: Because 1 horse developed extreme hypotension, data from 5 horses were analyzed. During head-down positioning, mean +/- SEM ICP increased to 55+/-2 mm Hg, compared with 31+/-2 mm Hg during horizontal positioning; cerebral perfusion pressure was unchanged. Compared with findings during horizontal positioning, blood flow to the cerebrum, cerebellum, and cranial portion of the brainstem decreased significantly by approximately 20% during head-down positioning; blood flows within the pons and medulla were mildly but not significantly decreased. Spinal cord blood flow was low (9 mL/min/100 g of tissue) and unaffected by position. CONCLUSIONS AND CLINICAL RELEVANCE: Head-down positioning increased heart-brain hydrostatic gradients in isoflurane-anesthetized horses, thereby decreasing cerebral blood flow and, to a greater extent, increasing ICP. During anesthesia, CNS regions with low blood flows in horses may be predisposed to ischemic injury induced by high ICP.