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.     

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.     

Evaluation of transcutaneous Doppler ultrasonography for the measurement of blood flow in the femoral artery of pigs

OBJECTIVE: To compare measurements of blood flow in the common femoral artery obtained by duplex Doppler ultrasonography (DDU) and a reference ultrasonic transit-time flow (TTF) method and to examine the impact of Doppler spectral waveform measurement techniques on volumetric estimates. ANIMALS: 5 healthy female pigs. PROCEDURES: Femoral arterial blood flow was measured simultaneously in anesthetized pigs by use of a TTF probe (left femoral artery) and transcutaneous DDU (right femoral artery). A range of flow states was induced pharmacologically by using xylazine, bradykinin, dobutamine, and isoflurane. Volumetric blood flow was calculated from DDU waveforms, using the product of the flow velocity integral (FVI), the cross-sectional vessel area, and heart rate. Three calculations of FVI were obtained by manually tracing the Doppler spectral envelopes at the outer envelope, the modal, and the inner envelope of the spectral dispersion pattern. Data analysis included calculation of Pearson correlation coefficients and Bland-Altman limits of agreement. RESULTS: Blood flow measured by DDU was more closely correlated with TTF measurements when the modal or inner envelope tracing method was used (r, 0.76 and 0.78; limits of agreement, -100 to 54.2 and -48.5 to 770 mL/min, respectively). Limits of agreement for the outer envelope tracing method were -238.5 to 64 mL/min. CONCLUSIONS AND CLINICAL RELEVANCE: Transcutaneous DDU is a reliable noninvasive technique for measuring blood flow in the femoral artery of pigs over a range of flow states. Tracing the inner envelope of the Doppler spectral dispersion pattern provided the best estimate of blood flow in this study.     

Cardiac output measured by lithium dilution, thermodilution, and transesophageal Doppler echocardiography in anesthetized horses

OBJECTIVE: To assess the suitability of lithium dilution as a method for measuring cardiac output in anesthetized horses, compared with thermodilution and transesophageal Doppler echocardiography. ANIMALS: 6 horses (3 Thoroughbreds, 3 crossbreeds). PROCEDURE: Cardiac output was measured in 6 anesthetized horses as lithium dilution cardiac output (LiDCO), thermodilution cardiac output (TDCO), and transesophageal Doppler echocardiographic cardiac output (DopplerCO). For the LiDCO measurements, lithium chloride was administered i.v., and cardiac output was derived from the arterial lithium dilution curve. Sodium nitroprusside, phenylephrine hydrochloride, and dobutamine hydrochloride were used to alter cardiac output. Experiments were divided into 4 periods. During each period, 3 LiDCO measurements, 3 DopplerCO measurements, and 3 sets of 3 TDCO measurements were obtained. RESULTS: 70 comparisons were made between LiDCO, DopplerCO, and triplicate TDCO measurements over a range of 10 to 43 L/min. The mean (+/- SD) of the differences of LiDCO - TDCO was -0.86 +/- 2.80 L/min; LiDCO = -1.90 + 1.05 TDCO (r = 0.94). The mean of the differences of DopplerCO - TDCO was 1.82 +/- 2.67 L/min; DopplerCO = 2.36 + 0.98 TDCO (r = 0.94). The mean of the differences of LiDCO - DopplerCO was -2.68 +/- 3.01 L/min; LiDCO = -2.53 + 0.99 DopplerCO (r = 0.93). CONCLUSIONS AND CLINICAL RELEVANCE: These results indicate that lithium dilution is a suitable method for measuring cardiac output in horses. As well as being accurate, it avoids the need for pulmonary artery catheterization and is quick and safe to use. Monitoring cardiac output during anesthesia in horses may help reduce the high anesthetic mortality in this species.     

Doppler ultrasonographic features of thoracic limb arteries in clinically normal horses

OBJECTIVES: To determine blood flow velocities and indices from spectral waveforms obtained by use of Doppler ultrasonography of thoracic limb arteries of horses and to assess interobserver and patient variability associated with the technique. ANIMALS: 9 clinically normal adult horses. PROCEDURE: Left thoracic limb arteries of 8 nonsedated horses were examined at 5 sites by use of pulsed-wave Doppler ultrasonography to determine a range of values for peak systolic, end diastolic, and mean velocities and resistive and pulsatility indices. Interobserver and patient variabilities were determined by 2 operators repeating similar measurements on 1 horse 8 times at weekly intervals. RESULTS: A range of values for each variable measured at the 5 selected sites was obtained. For each variable, strong positive correlations (R > or = 0.7) were detected for > 70% of the site-to-site comparisons made (excluding the coronary band). Among horses, resistive index varied least, whereas over time, mean velocity varied least. Waveform characteristics were consistent with resistive (n = 5) or nonresistive (4) patterns. In the single-horse experiment, waveform characteristics were consistent throughout the 8 weeks, and operator effects were not detected. CONCLUSIONS AND CLINICAL RELEVANCE: Doppler ultrasonography of no one site resulted in more reliable measurements of blood flow characteristics in thoracic limb arteries of horses. Mean velocity and resistive index were the least variable measurements made. Pulsed-wave Doppler ultrasonography may be a useful technique for evaluating diseases that alter normal thoracic limb arterial blood flow in horses.     

Evaluation of the Doppler ultrasonic method of measuring systolic arterial blood pressure in cats.

The accuracy of the Doppler technique for indirect systolic blood pressure measurement was assessed in 16 anesthetized cats. Eight cats were anesthetized with isoflurane and 8 were anesthetized with halothane. Anesthetic depth and mode of ventilation were varied to obtain a wide range of arterial blood pressure. A Doppler transducer was placed on the palmer surface of the left forelimb over the common digital branch of the radial artery to detect blood flow, and a blood pressure monitoring cuff with a width 37% the limb circumference was placed half way between the elbow and the carpus. To enable direct arterial pressure measurements, the left femoral artery was catheterized and the blood pressure waveforms recorded simultaneously. Systolic blood pressure measured by use of the Doppler ultrasonic technique was significantly lower than that obtained from the femoral artery catheter. Using linear regression, we determined a clinically useful calibration adjustment for Doppler indirect blood pressure measurement in cats: femoral systolic pressure = Doppler systolic pressure + 14 mm of Hg.     

Measurement of Muscle Surface Capillary Blood Flow by Laser Doppler Flowmetry

Muscle surface capillary blood flow was measured in the biceps femoris and lateral head of the triceps brachii muscles in six horses before and during halothane anesthesia by using laser Doppler flowmetry. During 90 minutes of anesthesia, muscle surface capillary blood flow was reduced to 20% to 40% of preanesthetic values. Muscle surface capillary blood flow tended to be lower in dependent muscles than in nondependent muscles, and this disparity was greater in the forelimbs than in the hind limbs.     

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.     

Effects of sedation with acepromazine maleate and buprenorphine hydrochloride on femoral artery blood flow in healthy dogs

The purpose of this study was to qualify and quantify the femoral artery blood flow by duplex Doppler ultrasonography (DDU) in healthy dogs, before and after the administration of a combination of acepromazine maleate and buprenorphine hydrochloride (ACP-BPN). Seven healthy adult mongrel dogs and three adult beagles were used. Heart rate, arterial blood pressure and measurement of femoral artery blood flow by DDU were also recorded. The DDU measurements were: femoral artery diameter (FAD), peak systolic velocity (PSV), early retrograde (EDV) and end diastolic velocities (EnDV), mean velocity (BMV), pulsatility index (PI), flow velocity integral (FVI) and femoral blood flow (FBF). After 30 min, combination ACP-BPN was administered intramuscularly, and all the measurements were recorded again. The ACP-BPN protocol induced a significant decrease in systolic, mean, and diastolic arterial blood pressure, and heart rate. A significant increase in peak systolic velocity and integral flow velocity integral of the femoral blood were obtained. The Doppler spectra of the blood flow in the femoral artery revealed a spectral dispersion pattern after ACP-BPN administration in all the dogs. These results demonstrate that despite quantitative and qualitative changes, the overall femoral blood flow (FBF) is not significantly modified.     

B‐mode and power Doppler ultrasonography of the equine suspensory ligament branches: A descriptive study on 13 horses

Ultrasonography is routinely used to achieve the diagnosis of equine suspensory ligament desmopathy. In human medicine, power Doppler ultrasonography has also been found to be useful for the diagnosis of tendon/ligament injuries. The aim of this prospective, pilot study was to assess the presence or absence of power Doppler signal in suspensory ligament branches and compare B‐mode findings with power Doppler findings in suspensory ligament branches of lame and non‐lame limbs. Thirteen horses were used (eight lame horses, with lameness related to pain in the suspensory ligament branches, and five non‐lame horses). Ten lame limbs and 24 sound limbs were assessed by B‐mode and power Doppler ultrasonography. The severity of power Doppler signal was scored by two independent readers. The B‐mode ultrasonographic examination revealed abnormalities in branches of lame limbs and in branches of sound limbs. Suspensory ligament branches that were considered normal in B‐mode showed no power Doppler signal. However, power Doppler signal was detected in suspensory ligament branches that were abnormal in B‐mode, both in lame and sound limbs. Power Doppler scores were subjectively higher in suspensory ligament branches of lame limbs and in branches with more severe B‐mode changes. Findings supported the use of power Doppler as an adjunctive diagnostic test for lame horses with suspected suspensory desmopathy.     

Effects of intramuscular administration of acepromazine on palmar digital blood flow, palmar digital arterial pressure, transverse facial arterial pressure, and packed cell volume in clinically healthy, conscious horses

Objective— To determine the magnitude and duration of effects of acepromazine administered intramuscularly (IM) on digital and systemic hemodynamic variables in clinically healthy horses.
Study Design— Experimental study.
Animals— Healthy adult horses (n=12).
Methods— An ultrasonic Doppler flow probe was surgically implanted around the medial palmar digital artery before the study. Catheters were inserted in the transverse facial artery, lateral palmar digital artery, and jugular vein. A treatment group (n=6) was administered 0.04 mg/kg body weight of acepromazine IM; control horses (n=6) were administered an equivalent volume of saline IM. Palmar digital blood flow, and digital and facial arterial pressures were measured at baseline and for 6 hours after administration. Venous blood was collected for measurement of packed cell volume (PCV).
Results— Horses administered acepromazine had significantly lower facial arterial pressure compared with control horses administered saline. Palmar digital arterial blood flow in acepromazine-treated horses was not significantly different from that in control horses but increased significantly post-administration, compared with the respective baseline values for acepromazine-treated horses. PCV significantly decreased in horses administered acepromazine compared with their respective baseline value.
Conclusion— IM acepromazine causes hypotension and increases palmar digital blood flow over time but the magnitude of the effect on digital blood flow was not sufficient to yield differences compared with saline-treated horses.
Clinical Relevance— IM acepromazine has a modest effect on palmar digital blood flow, facial arterial pressures and PCV in healthy horses with minimal sedation.     

Effects of acetylpromazine on the hemodynamics of the equine metatarsal artery, as determined by two-dimensional real-time and pulsed Doppler ultrasonography

Heart rate, blood velocity, volumetric blood flow, and arterial diameter for 10 horses given acetylpromazine were determined from measurements of the dorsal metatarsal artery 3 (the great metatarsal artery), using 2-dimensional real-time and gated pulsed Doppler ultrasonography. Acetylpromazine induced significant increases in arterial diameter (P less than 0.01) and volumetric flow rate (P less than 0.05)--all compatible with adrenergic blockade. There was a trend indicating that there was increased blood velocity. Heart rate was unchanged.     

Effects of an endothelin receptor antagonist and nitroglycerin on digital vascular function in horses during the prodromal stages of carbohydrate overload-induced laminitis

OBJECTIVE: To evaluate changes in digital vascular function in horses with carbohydrate overload (CHO)-induced laminitis and determine the effects of an endothelin (ET) receptor antagonist and nitroglycerin on laminitis-associated vascular dysfunction. ANIMALS: 20 adult horses without abnormalities of the digit. PROCEDURES: Hemodynamic variables were recorded before (baseline) and hourly after all horses were administered a CHO ration via nasogastric tube. In 4 groups of 5 horses each, saline (0.9% NaCl) solution or ET receptor antagonist (10(5)M in digital blood) was administered into the digital arterial circulation according to 1 of 2 schedules. During anesthesia, blood flow; arterial, venous, and capillary pressures; and total, precapillary, and postcapillary resistances were measured in an isolated perfused digit of each horse. In all groups, nitroglycerin was infused (10(5)M in digital blood), and digital microvascular assessments were repeated. RESULTS: The CHO caused a significant decrease in right atrial pressure by 14 hours that was not affected by administration of saline solution or ET receptor antagonist. In isolated digits of anesthetized horses, CHO resulted in a significant decrease in digital blood flow associated with a significant increase in total and postcapillary resistances. Treatment with the ET receptor antagonist and nitroglycerin caused a significant decrease in total resistance. Postcapillary resistance was significantly decreased following treatment with the ET receptor antagonist but was not altered by treatment with nitroglycerin. CONCLUSIONS AND CLINICAL RELEVANCE: Treatment with an ET receptor antagonist and nitroglycerin resulted in significant improvement in vascular resistance in isolated perfused digits of anesthetized horses with CHO-induced laminitis.     

DOPPLER SONOGRAPHIC EVALUATION OF THE DIGITAL BLOOD FLOW IN HORSES WITH LAMINITIS OR SEPTIC PODODERMATITIS

The aim of this study was to evaluate the use of Doppler ultrasonography to monitor vascular blood flow dynamics in defined diseases of the equine digit in a noninvasive way. Doppler sonography was used to evaluate medial digital artery blood flow in eight horses with septic pododermatitis and four horses with laminitis in comparison with 10 horses of a control group. Doppler sonographic measurement and lameness examinations were performed in lame horses before treatment (day 0) and at 3, 6, and 9 days following treatment. Before treatment, blood flow velocities, arterial diameter, and flow volume were significantly higher in the lame horses, while pulsatility indices (PIs) were significantly lower (P < 0.05). Compared with the control group, there was no significant difference of resistive indices (RI) in the septic pododermatitis group, but these indices were significantly lower in the laminitis group. No significant difference between the two lame groups was recorded. After treatment, blood flow velocities and flow volume decreased significantly and PIs increased significantly in both lame groups, while RI and arterial diameter did not change. No correlation between Doppler parameters and the degree of lameness was found. Doppler sonography can be used to monitor vascular blood flow dynamics in horses suffering from septic pododermatitis and laminitis, while blood flow velocities and PI can serve as parameters for monitoring the course of the disease.     

The relationship between digital perfusion pressure and hoof lamellar blood flow in isoflurane-anesthetized horses

Digital perfusion pressure (DPP) equals mean arterial pressure (MAP) at the hoof coronet minus digital interstitial pressure (DIP) within the hoof. To test whether lamellar blood flow (LBF) changes proportionately to DPP, anesthesia was induced and maintained with isoflurane in six horses to target a MAP of 60 mmHg. Arterial, venous, and hoof interstitial pressures were measured in each pelvic limb. LBF was measured using fluorescent microspheres during dobutamine infusions targeting either 60 (low), 80 (medium), or 100 (high) mmHg MAP. Following euthanasia, hoof lamina was collected for microsphere isolation. To reduce intra-individual variability, medium and high pressures and flows were divided by their respective low pressure and flow baseline values, yielding indexed variables of ΔLBF and ΔDPP. The ΔLBF correlated negatively with the ΔDPP. We conclude that LBF was not solely determined by passive pressure-flow relationships and that systemic hypertension may not effectively increase dermal LBF in horses.     

REPEATABILITY OF DOPPLER ULTRASONOGRAPHIC MEASUREMENT OF EQUINE DIGITAL BLOOD FLOW

The repeatability and sensitivity of Doppler ultrasonographic measurement of lateral digital arterial and venous blood flow has not been previously determined. Doppler ultrasonography was used to measure blood flow within the forelimb lateral digital vessels in one normal adult Thoroughbred horse on six occasions and in six normal adult Thoroughbred horses on three occasions, each occasion being at least 1 h apart, to determine the within- and between-horse variation. The values obtained from the right and left lateral digital vessels did not differ significantly. The within-horse coefficients of variation (CV) for arterial and venous measurements were all acceptable (< 11%); the between-horse CV were acceptable (< 7%) for all parameters except TaVa (average velocity of first peak of arterial waveform) and TaVb (average velocity of remainder of arterial waveform). The within-horse intraclass correlation coefficients (ICC) demonstrated excellent repeatability (> or = 0.71) for all parameters except venous diameter; the between-horse ICC demonstrated good to excellent repeatability (> or = 0.67) for all parameters except TaVb. Doppler ultrasonography can detect differences of 0.005 and 0.01 ml/ min in digital arterial and venous flow, respectively, using measurements from six horses on three occasions (80% power; P < 0.05). Thus, the technique is sufficiently repeatable and sensitive to be able to detect changes in flow during different physiological or pathological states or following pharmacologic intervention.     

Comparison of detomidine and romifidine as premedicants before ketamine and halothane anesthesia in horses undergoing elective surgery

OBJECTIVE: To compare detomidine hydrochloride and romifidine as premedicants in horses undergoing elective surgery. ANIMALS: 100 client-owned horses. PROCEDURE: After administration of acepromazine (0.03 mg/kg, IV), 50 horses received detomidine hydrochloride (0.02 mg/kg of body weight, IV) and 50 received romifidine (0.1 mg/kg, IV) before induction and maintenance of anesthesia with ketamine hydrochloride (2 mg/kg) and halothane, respectively. Arterial blood pressure and blood gases, ECG, and heart and respiratory rates were recorded. Induction and recovery were timed and graded. RESULTS: Mean (+/- SD) duration of anesthesia for all horses was 104 +/- 28 minutes. Significant differences in induction and recovery times or grades were not detected between groups. Mean arterial blood pressure (MABP) decreased in both groups 30 minutes after induction, compared with values at 10 minutes. From 40 to 70 minutes after induction, MABP was significantly higher in detomidine-treated horses, compared with romifidine-treated horses, although more romifidine-treated horses received dobutamine infusions. In all horses, mean respiratory rate ranged from 9 to 11 breaths/min, PaO2 from 200 to 300 mm Hg, PaCO2 from 59 to 67 mm Hg, arterial pH from 7.33 to 7.29, and heart rate from 30 to 33 beats/min, with no significant differences between groups. CONCLUSIONS AND CLINICAL RELEVANCE: Detomidine and romifidine were both satisfactory premedicants. Romifidine led to more severe hypotension than detomidine, despite administration of dobutamine to more romifidine-treated horses. Both detomidine and romifidine are acceptable alpha2-adrenoceptor agonists for use as premedicants before general anesthesia in horses; however, detomidine may be preferable when maintenance of blood pressure is particularly important.     

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.     

Hemodynamic Effects of Atropine, Dobutamine, Nitroprusside, Phenylephrine, and Propranolol in Conscious Horses

The authors investigated the cardiovascular effects of low doses of nitroprusside, dobutamine, and phenylephrine and a beta-adrenergic blocking dose of propranolol in conscious, healthy horses with and without prior atropine administration. A parasympathetic blocking dose of atropine produced significant increases in heart rate and arterial pressures, and decreased stroke volume, ejection fraction, pulse pressure, and right-ventricular end-diastolic pressure and volume. Cardiac output was not changed by atropine administration. Nitroprusside reduced arterial pressures to a greater extent in atropinized horses but increased heart rate in both atropinized and non-atropinized horses. Dobutamine increased mean arterial pressure in both non-atropinized and atropinized horses but increased heart rate, diastolic arterial pressure, and systemic vascular resistance only in atropinized horses. Propranolol did not affect any of the hemodynamic variables that were measured. Phenylephrine, in the presence of beta-adrenergic blockade, increased mean arterial pressure and reduced cardiac output. This study showed that low doses of nitroprusside, dobutamine, and phenylephrine produce significant hemodynamic effects in conscious, healthy horses and that these effects are modified by prevailing parasympathetic tone.