Bronchial circulation during prolonged exercise in ponies.

Tracheal, bronchial, and renal flow were studied in 8 healthy ponies at rest and during exercise performed on a treadmill at a speed setting of 20.8 km/h and 7% grade (incline) for 30 minutes. Blood flow was determined with 15-microns-diameter radionuclide-labeled microspheres that were injected into the left ventricle when the ponies were at rest, and at 5, 15, and 26 minutes of exertion. Heart rate and mean aortic pressure increased from resting values (40 +/- 2 beats/min and 124 +/- 3 mm of Hg, respectively) to 152 +/- 8 beats/min and 133 +/- 4 mm of Hg at 5 minutes of exercise, to 169 +/- 6 beats/min and 143 +/- 5 mm of Hg at 15 minutes of exercise, and to 186 +/- 8 beats/min, and 150 +/- 5 mm of Hg at 26 minutes of exercise. Tracheal blood flow at rest and during exercise remained significantly (P less than 0.05) less than bronchial blood flow. Tracheal blood flow increased only slightly with exercise. Vasodilation caused bronchial blood flow to increase throughout exercise. Pulmonary arterial blood temperature of ponies also increased significantly (P less than 0.05) with exercise and a significant (P less than 0.005) correlation was found between bronchial blood flow and pulmonary arterial blood temperature during exertion. At 5 minutes of exercise, renal blood flow was unchanged from the resting value; however, renal vasoconstriction was observed at 15 and 26 minutes of exercise. We concluded that bronchial circulation of ponies increased with exercise in close association with a rise in pulmonary arterial blood temperature.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regional blood flow changes in response to near maximal exercise in ponies: a review

In recent years, increasing attention has been focused on the physiological responses of the horse to maximal exercise. Cardiovascular response in near maximally exercised galloping ponies (heart rate 225 +/- 7 beats/min; whole body oxygen consumption 122 +/- 12 ml/min/kg) comprised a marked increase in blood flow to the cerebellum, myocardium, diaphragm and the working muscles, while renal blood flow decreased precipitously. Cerebral and brainstem perfusion did not vary from resting values. Transmural homogeneity of myocardial blood flow persisted during near maximal exercise. It was reported that tachycardia of exercise contributed about one-third of the total increment in left ventricular coronary blood flow. Considerable unutilised coronary vasodilator capacity was also demonstrated in near maximally exercised ponies and it was suggested that maximally exercising ponies were not limited from further exertion because of the coronary circulation.     

Regional brain blood flow during prolonged submaximal exercise in ponies.

Experiments were carried out on 8 healthy ponies to examine the effects of prolonged submaximal exercise on regional distribution of brain blood flow. Brain blood flow was ascertained by use of 15-microns-diameter radionuclide-labeled microspheres injected into the left ventricle. The reference blood was withdrawn from the thoracic aorta at a constant rate of 21.0 ml/min. Hemodynamic data were obtained with the ponies at rest (control), and at 5, 15, and 26 minutes of exercise performed at a speed setting of 13 mph on a treadmill with a fixed incline of 7%. Exercise lasted for 30 minutes and was carried out at an ambient temperature of 20 C. Heart rate, mean arterial pressure, and core temperature increased significantly with exercise. With the ponies at rest, a marked heterogeneity of perfusion was observed within the brain; the cerebral, as well as cerebellar gray matter, had greater blood flow than in the respective white matter, and a gradually decreasing gradient of blood flow existed from thalamus-hypothalamus to medulla. This pattern of perfusion heterogeneity was preserved during exercise. Regional brain blood flow at 5 and 15 minutes of exercise remained similar to resting values. However, at 26 minutes of exercise, vasoconstriction resulted in a significant reduction in blood flow to all cerebral and brain-stem regions. In the cerebellum, the gray matter blood flow and vascular resistance remained near control values even at 26 minutes of exercise. Vasoconstriction in various regions of the cerebrum and brainstem at 26 minutes of exertion may have occurred in response to exercise-induced hypocapnia, arterial hypertension, and/or sympathetic neural activation.     

Distribution of blood flow during moderate and strenuous exercise in ponies (Equus caballus)

Blood flow to the brain, heart, kidneys, diaphragm, and skeletal muscles was studied at rest and during graded treadmill exercise, using radionuclide-labeled microspheres (15 microns diameter), in 11 healthy adult ponies. Hemodynamic changes brought about by exercise included marked increases in cardiac output, mean aortic pressure, left ventricular end-diastolic pressure, and right ventricular systolic and end-diastolic pressures. Blood flow to the brain stem and cerebral hemispheres was unchanged during both moderate exercise (heart rate = 154 +/- 3 beats/min) and severe exercise (heart rate = 225 +/- 7 beats/min). Despite marked hypocapnia during severe exercise, cerebellar blood flow increased by 32% above control value (94 +/- 7 ml/min/100 g). Myocardial blood flow increased transmurally with both levels of exercise. The endo:epi (inner:outer) perfusion ratio for the left ventricle and the interventricular septum decreased during exercise. It was, however, not different from unity. During severe exercise, renal blood flow decreased to 19% of its control value. Blood flow to the diaphragm exceeded that to the skeletal muscles during both intensities of exercise. Blood flow to the exercising muscles of the brachium and thigh increased by 31- to 38-fold during moderate exercise and by 70- to 76-fold during severe exercise. It is concluded that the cardiovascular response to strenuous exercise in the pony included an increase in blood flow to the cerebellum, myocardium, diaphragm, and exercising skeletal muscles, while blood flow was diverted away from the kidneys. It would appear that the pony's cardiovascular response to severe exercise is similar to that of persons.     

Effect of furosemide administration on systemic circulation of ponies during severe exercise

Systemic distribution of blood flow was studied in 11 healthy adult grade ponies, using radionuclide-labeled microspheres (15 micron diameter) that were injected into the left ventricle. Measurements were made at rest, during severe exercise (SE) without furosemide, as well as during SE at 10 minutes and 120 minutes after furosemide administration (1.0 mg/kg, IV). During SE, heart rate, cardiac output, mean aortic pressure, and whole body O2 consumption were 220 +/- 4 beats/min, 720 +/- 44 ml/min/kg, 169 +/- 4 mm of Hg, and 126 +/- 9 ml of O2/min/kg, respectively. With SE performed after furosemide administration, mean aortic pressure decreased from prefurosemide SE value (P less than 0.05), but heart rate, cardiac output, and whole body O2 consumption remained similar to values during SE without furosemide. During SE, blood flow to cerebellar gray matter, pons, and medulla oblongata increased despite marked hypocapnia, but in other regions of the brain, blood flow was unchanged. As arterial O2 content increased by 58% with SE, O2 delivery to all brain regions increased. With SE, adrenal gland blood flow increased, but intense vasoconstriction in the kidneys, spleen, pancreas, small intestine, and colon caused blood flow to plummet. During SE, blood flow in the diaphragm, gluteus medius, biceps femoris (muscles of propulsion), and triceps brachii muscles increased to a similar level, indicating that metabolic requirements of the diaphragm during exercise may not be less than those of other vigorously contracting muscles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electromyography of some respiratory muscles in the horse

To investigate activity in respiratory muscles, insulated wire electrodes were used to record electromyographic activity in the costal diaphragm and in the intercostal, serratus ventralis, internal abdominal oblique, transversalis and rectus abdominis muscles in conscious horses and in the same animals when anaesthetised. Electromyographic activity was related to respiratory phases as recorded by a stethograph around the chest wall. The costal diaphragm showed tonic and inspiratory activity in both conscious and anaesthetised animals. The principal muscle actively involved in expiration was the transversalis muscle. Intercostal muscle activity, and any increased activity in the second part of either inspiration or expiration recorded in the conscious animal, was absent under anaesthesia. The very marked tonic activity found in the serratus ventralis muscle in standing horses disappeared during anaesthesia. It was concluded that any stabilisation of the chest wall contributed by activity in the serratus ventralis and intercostal muscles in conscious, standing horses is greatly reduced during anaesthesia.     

Pressures in the right side of the heart and esophagus (pleura) in ponies during exercise before and after furosemide administration

Pressures in the right side of the heart and esophagus (pleural) have not been determined in the exercising equine subjects. In the present study, 8 healthy ponies were examined to determine the changes in these variables caused by 2 degrees of exercise done on a treadmill (heart rate:183 +/- 5 beats/min [trot] and 220 +/- 6 beats/min [canter]). Measurements were also made during both degrees of exertion 10 minutes and 120 minutes after furosemide (1.0 mg/kg) administration. It was observed that both gaits resulted in significant increases in pulmonary artery, right ventricular, and right atrial pressures. The pulmonary artery systolic, mean, and diastolic pressures during strenuous exertion were 306%, 252%, and 242% of the respective resting values. At canter, when respiratory frequency (138 +/- 4 breaths/min) is synchronized with stride frequency, the delta esophageal pressure approached 30.4 +/- 2.86 cm of water. During exercise 10 minutes after furosemide administration, the increment in right atrial pressure was markedly attenuated. During strenuous exertion 120 minutes after furosemide administration, the right atrial and pulmonary arterial pressures increased, but to a significantly lower level than did the prefurosemide values. However, the mean pulmonary artery pressure was still 240% of the resting value. It is concluded that marked pulmonary hypertension is a consistent feature of moderate, as well as strenuous, exertion in the pony. Although furosemide administration attenuated the pulmonary hypertension somewhat, the significance remains unclear.     

Antagonism of xylazine-pentobarbital anesthesia by yohimbine in ponies

Effects of yohimbine on xylazine-pentobarbital anesthesia were evaluated in ponies. Five minutes after the IV injection of xylazine (1.1 mg/kg of body weight), pentobarbital sodium (12.7 mg/kg, IV) and additional xylazine (2.2 mg/kg, IM) were given and produced anesthesia in 12 ponies for 64.0 +/- 16.4 minutes (mean +/- SD) as well as immobilization for 89.8 +/- 34.2 minutes. Eleven ponies were given yohimbine (0.1 mg/kg, IV) 50 minutes after pentobarbital dosing. In these 11 ponies, durations of anesthesia and immobilization were shorter, 52.0 +/- 1.4 and 65.5 +/- 14.8 minutes, respectively. The xylazine-pentobarbital combination caused bradycardia that was reversed by yohimbine injection. Xylazine-pentobarbital produced a small, but steady, decrease of mean arterial blood pressure, which was compounded by yohimbine administration and was evident for approximately 2 minutes. Within a minute after yohimbine injection, the ponies' respiratory rate decreased and the length of inspiration and expiration and thoracic breathing increased. This lasted approximately 2 to 3 minutes and was followed by an increase in respiratory rate. The anesthesia also produced a decrease in PaO2 that gradually returned to base line in 12 control ponies, but was more pronounced in 11 ponies given yohimbine. The PaCO2, although remaining moderately high in control ponies, returned to base line after yohimbine injection. An increased pHa was seen 60 minutes after induction of anesthesia and was especially noticeable after yohimbine administration. Decreases in the number of WBC, hemoglobin content, PCV, plasma protein and serum aspartate transaminase resulting from xylazine-pentobarbital were reversed by yohimbine. Conversely, serum glucose values and creatine kinase activities were increased by xylazine-pentobarbital.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gastrointestinal ultrasonography in normal adult ponies

The objective of this study was to determine the characteristics based on ultrasonographic examination of the stomach, duodenum, jejunum, cecum, and peritoneal fluid in normal adult ponies. Abdominal ultrasonographic examination was performed in nine unsedated standing ponies. The duodenum was examined at three sites and the jejunum in 12 regions. Wall thickness, contractility, distention, and luminal contents were recorded. Stomach wall thickness and location, cecal wall thickness, and peritoneal fluid location and character were recorded. Statistical analysis was performed. Wall thicknesses (in cm) were 0.431 +/- 0.069 for the stomach, 0.188 +/- 0.033 for the duodenum (at all sites), 0.195 +/- 0.031 for the jejunum (at all regions), and 0.179 +/- 0.031 for the cecum. Duodenal contractions per minute were 3.78 +/- 1.10. The stomach spanned 5.14 +/- 0.9 intercostal spaces, with the 8th intercostal space being the most cranial and the 15th intercostal space being the most caudal space through which the stomach was identified. It was possible to identify the jejunum in all ponies dorsal to the left dorsal colon and from the ventral abdominal wall. Peritoneal fluid was identified in six ponies. Peritoneal fluid was usually seen transiently and most commonly in the ventral aspect of the abdominal cavity or around the duodenum. Overall, the ponie's abdominal ultrasonographic examinations revealed wall thicknesses that were less than the published normal ranges for horses. It appears that ponies may have increased duodenal contractility than horses and that the conformation of ponies may change the locations for imaging the stomach.     

Cerebral, renal, adrenal, intestinal, and pancreatic circulation in conscious ponies and during 1.0, 1.5, and 2.0 minimal alveolar concentrations of halothane-O2 anesthesia

Blood flow to the brain, kidneys, adrenal glands, pancreas, and small intestine was studied in 8 healthy ponies while awake (control) and during 1.0, 1.5, and 2.0 minimal alveolar concentrations (MAC) of anesthesia produced, using halothane vaporized in oxygen. During the anesthesia steps, intermittent positive-pressure ventilation was used to ensure isocapnia. Organ blood flow was determined with 15-micron (diameter) radionuclide-labeled microspheres, after allowing 30 minutes of equilibration at each of the 3 preestablished end-tidal halothane concentrations. The sequence of 1.0, 1.5, and 2.0 MAC levels of anesthesia (0.90, 1.35, and 1.80% end-tidal halothane) was randomized for every animal. In the awake ponies, cerebral blood flow in the cortical (106 +/- 15 ml/min/100 g) and deep gray (103 +/- 12 ml/min/100 g) matter was approximately 5-fold of that in the white matter (22 +/- 3 ml/min/100 g). In the brain stem, there was a decreasing gradient of blood flow from the cranial (thalamohypothalamus: 65 +/- 8 ml/min/100 g) to caudal regions (medulla: 34 +/- 5 ml/min/100 g). Vasodilatation occurred in all regions of the brain with halothane-O2 anesthesia; the decrease in vascular resistance reached its nadir at 1.5 MAC. In the medulla and pons, blood flow increased above control values, with each of the 3 concentrations of halothane, but in the midbrain and thalamohypothalamus, it remained similar to the control value. In the cerebral white matter and cerebellum, blood flow increased with 1.0 and 1.5 MAC of halothane anesthesia, whereas mean aortic pressure decreased to 91% and 74% of the control value. Blood flow in the cerebral cortex was not different from the control value, even at 2.0 MAC of halothane, despite a 49% reduction in perfusion pressure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ultrasound-guided superficial serratus plane block in dog cadavers: an anatomical evaluation and volume dispersion study

ObjectiveTo evaluate the anatomy of the serratus plane in dogs to establish the optimal landmarks for a superficial serratus plane (SSP) block and evaluate ropivacaine–methylene blue solution dispersion with three volumes of injection.Study designProspective experimental cadaveric study.AnimalsA formaldehyde solution-preserved dog cadaver and 15 frozen/thawed adult dog cadavers.MethodsThe thoracic wall of the formaldehyde-preserved cadaver was dissected. An SSP injection was performed on each hemithorax of the cadavers, with the ultrasound transducer placed over the fourth and fifth ribs, at the level of the shoulder joint. A needle was inserted in-plane in a caudocranial direction until it could be visualized between the serratus ventralis thoracis and latissimus dorsi muscles. Dog cadavers were injected with a ropivacaine–methylene blue solution at 0.3, 0.6 and 1.0 mL kg^–1 and were dissected to determine the spread of the dye.ResultsThe thoracic wall muscles identified in the formalinized cadaver were the cutaneous trunci, latissimus dorsi, external abdominal oblique, serratus ventralis thoracis, scalenus, serratus dorsalis cranialis and external intercostal. The nerves identified in the SSP included the lateral cutaneous branches of intercostal nerves, intercostobrachial nerves and long thoracic nerve. The solution was successfully injected at the SSP in 26 of 29 (89.7%) attempts. Dermatomal dye spread, median (range), was 4 (3–6), 4 (2–5) and 5 (4–8) for 0.3, 0.6 and 1.0 mL kg^–1, respectively, with no significant difference among them.Conclusions and clinical relevanceInjections for an SSP block were easily performed under ultrasound guidance, using the fourth and fifth ribs at the level of the shoulder joint as reference landmarks. An injected volume of 0.3 mL kg^–1 may be sufficient for hemithorax analgesia in dogs. Further studies in dogs are required to determine the utility of this technique.     

Detomidine-Propofol Anesthesia for Abdominal Surgery in Horses

OBJECTIVE: To evaluate propofol for induction and maintenance of anesthesia, after detomidine premedication, in horses undergoing abdominal surgery for creation of an experimental intestinal adhesion model. STUDY DESIGN: Prospective study. ANIMALS: Twelve horses (424 +/- 81 kg) from 1 to 20 years of age (5 females, 7 males). METHODS: Horses were premedicated with detomidine (0.015 mg/kg i.v.) 20 to 25 minutes before induction, and a propofol bolus (2 mg/kg i.v.) was administered for induction. Propofol infusion (0.2 mg/kg/min i.v.) was used to maintain anesthesia. The infusion rate was adjusted to maintain an acceptable anesthetic plane as determined by muscle relaxation, occular signs, response to surgery, and cardiopulmonary responses. Oxygen (15 L/min) was insufflated through an endotracheal tube as necessary to maintain the SpO2 greater than 90%. Systolic (SAP), mean (MAP), and diastolic (DAP) arterial pressures, heart rate (HR), electrocardiogram (ECG), respiratory rate (RR), SpO2 (via pulse oximetry), and nasal temperature were recorded at 15 minute intervals, before premedication and after induction of anesthesia. Arterial blood gas samples were collected at the same times. Objective data are reported as mean (+/-SD); subjective data are reported as medians (range). RESULTS: Propofol (2.0 mg/kg i.v.) induced anesthesia (mean bolus time, 85 sec) within 24 sec (+/-22 sec) after the bolus was completed. Induction was good in 10 horses; 2 horses showed signs of excitement and these two inductions were not smooth. Propofol infusion (0.18 mg/kg/min +/- 0.04) was used to maintain anesthesia for 61 +/- 19 minutes with the horses in dorsal recumbency. Mean SAP, DAP, and MAP increased significantly over time from 131 to 148, 89 to 101, and 105 to 121 mm Hg, respectively. Mean HR varied over time from 43 to 45 beats/min, whereas mean RR increased significantly over anesthesia time from 4 to 6 breaths/min. Mean arterial pH decreased from a baseline of 7.41 +/- 0.07 to 7.30 +/- 0.05 at 15 minutes of anesthesia, then increased towards baseline values. Mean PaCO2 values increased during anesthesia, ranging from 47 to 61 mm Hg whereas PaO2 values decreased from baseline (97 +/- 20 mm Hg), ranging from 42 to 57 mm Hg. Muscle relaxation was good and no horses moved during surgery: Recovery was good in 9 horses and acceptable in 3; mean recovery time was 67 +/- 29 minutes with 2.4 +/- 2.4 attempts necessary for the horses to stand. CONCLUSIONS: Detomidine-propofol anesthesia in horses in dorsal recumbency was associated with little cardiovascular depression, but hypoxemia and respiratory depression occurred and some excitement was seen on induction. CLINICAL RELEVANCE: Detomidine-propofol anesthesia is not recommended for surgical procedures in horses if dorsal recumbency is necessary and supplemental oxygen is not available (eg, field anesthesia).     

The effect of submaximal exercise on the peripheral blood pressure of untrained ponies

A significant increase in systolic and diastolic peripheral blood pressure was recorded after 8 minutes submaximal exercise in untrained ponies. Blood pressure was recorded by a modified auscultatory method from the coccygeal artery.     

Cardiopulmonary and anesthetic effects of medetomidine-ketamine-butorphanol and antagonism with atipamezole in servals (Felis serval).

Seven (three male and four female) 4-7-yr old captive servals (Felis serval) weighing 13.7 +/- 2.3 kg were used to evaluate the cardiopulmonary and anesthetic effects of combined intramuscular injections of medetomidine (47.4 +/- 10.3 microg/kg), ketamine (1.0 +/- 0.2 mg/kg), and butorphanol (0.2 +/- 0.03 mg/kg). Inductions were smooth and rapid (11.7 +/- 4.3 min) and resulted in good muscle relaxation. Significant decreases in heart rate (85 +/- 12 beats/min) at 10 min after injection and respiratory rate (27 +/- 10 breaths/min) at 5 min after injection continued throughout the immobilization period. Rectal temperature and arterial blood pressure did not change significantly. The PaO2 decreased significantly, and PaCO2 increased significantly during immobilization but remained within clinically acceptable limits. Hypoxemia (PaO2 < 60 mm Hg) was not noted, and arterial blood oxygen saturation (SaO2) was greater than 90% at all times. Relative arterial oxygen saturation (SpO2) values, indicated by pulse oximetry, were lower than SaO2 values. All animals could be safely handled while sedated. Administration of atipamezole (236.8 +/- 51.2 microg/kg half i.v. and half s.c.), an alpha2 antagonist, resulted in rapid (4.1 +/- 3 min to standing) and smooth recoveries.     

Skeletal muscle perfusion during prolonged 2.03% end-tidal isoflurane-O2 anesthesia in isocapnic ponies

Effects of 1.55 minimum alveolar concentration isoflurane O2 anesthesia (2.03% end-tidal isoflurane) on blood flow in the up-(nondependent) and down-(dependent) positioned skeletal muscles were studied at 60, 120, and 180 minutes in 6 healthy isocapnic ponies in right lateral recumbency on a nonpadded hardwood floor. Measurements were made, using 15-micron diameter radionuclide-labeled microspheres injected into the left ventricle, and comparisons were made with data obtained from ponies in the conscious state. Isoflurane administration caused a sharp reduction in cardiac output and systemic pressure (P less than 0.01), but total peripheral resistance did not change significantly. In the triceps brachii, gluteus medius, biceps femoris, and vastus lateralis of both sides, blood flow decreased significantly during 1.55 minimum alveolar concentration isoflurane anesthesia (P less than 0.01), and fluctuations did not occur with increasing duration of anesthesia. In masseter muscles, perfusion values during the 3 hours of anesthesia were not significantly different from values in awake ponies. Despite the fact that 4 ponies developed marked edema of the dependent masseter muscle, 1 pony without masseter edema developed postanesthetic forelimb lameness and 2 of the 4 ponies with masseter edema had generalized hind limb weakness after anesthesia; significant differences in blood flow between up- and down-positioned muscles were not observed. During isoflurane-O2 anesthesia in ponies, a sharp significant decrease in skeletal muscle blood flow was observed (P less than 0.01). Decreased equine skeletal muscle perfusion during isoflurane anesthesia also may be accompanied by accentuated O2 loss from the arterial blood via the countercurrent O2 exchange between large arterioles and venules.     

Arterial-venous difference in atrial natriuretic peptide concentration during exercise in horses.

Six nontrained mares were subjected to steady-state, submaximal treadmill exercise to examine the effect of exercise on the plasma concentration of atrial natriuretic peptide (ANP) in arterial, compared with mixed venous, blood. Horses ran on a treadmill up a 6 degree grade for 20 minutes at a speed calculated to require a power equivalent to 80% of maximal oxygen uptake (VO2MAX). Arterial and mixed venous blood samples were collected simultaneously from the carotid and pulmonary arteries of horses at rest and at 10 and 20 minutes of exercise. Plasma was stored at -80 C and was later thawed; ANP was extracted, and its concentration was determined by radioimmunoassay. Exercise caused significant (P < 0.05) increases in arterial and venous plasma ANP concentrations. Mean +/- SEM arterial ANP concentration increased from 25.2 +/- 4.4 pg/ml at rest to 52.7 +/- 5.2 pg/ml at 10 minutes of exercise and 62.5 +/- 5.2 pg/ml at 20 minutes of exercise. Mean venous ANP concentration increased from 24.8 +/- 4.3 pg/ml at rest to 67.2 +/- 14.5 pg/ml at 10 minutes of exercise and 65.3 +/- 13.5 pg/ml at 20 minutes of exercise. Significant differences were not evident between arterial or mixed venous ANP concentration at rest or during exercise, indicating that ANP either is not metabolized in the lungs or is released from the left atrium at a rate matching that of pulmonary metabolism.     

Cardiovascular demands of competition on low-goal (non-elite) polo ponies.

Knowledge of the competitive demands of different sports or activities is important for designing appropriate training programmes to ensure that animals reach a sufficient level of fitness to reduce the risk of overexertion and injury or illness and to achieve the best possible performance in relation to an individual's genetic potential. Whilst the physiological demands of many equestrian sports have been described, to the best of our knowledge the cardiovascular demands of polo have not. The aims of the present study were therefore to record heart rate during and after competitive polo games in a group of low-goal (non-elite) polo ponies in order to describe the absolute heart rates during play, the relationship of these heart rates to maximal heart rate and the characteristics of a typical chukka in terms of effort. Six low-goal polo ponies were studied during a total of 59 chukkas. Heart rate was monitored continuously before, during and after competition using a commercial heart rate monitor. Maximal heart rate was determined with field and treadmill incremental exercise tests and used to express work intensity in terms of time during play that each ponies heart rate was less than 75% HRmax, between 75 and 90% HRmax and greater than 90% HRmax. Mean maximum heart rate was not different during play or during field and treadmill exercise tests; 215+/-7 (mean +/- s.d.), 211+/-7 and 213+/-2 beats/min, respectively (P>0.05). Mean heart rate for all ponies over all chukkas was 166+/-6 beats/min with a mean chukka duration of 611+/-18 s. Of this time, 44+/-7% of the time was spent below 75% HRmax, 39+/-8% between 75 and 90% HRmax and 17+/-8% of time above 90% HRmax. When only one chukka had been played, there was a good correlation between mean heart rate during play and 3 min recovery heart rate (r = 0.63, P<0.001). Based on these observations, it is proposed that low-goal polo places moderate to high stress on the cardiovascular system.     

Plasma theophylline concentration and lung function in ponies with recurrent obstructive lung disease

Six ponies with recurrent obstructive lung disease were studied during two separate 60 min periods while receiving constant equal volume infusions of either aminophylline or sterile water. Dynamic lung compliance, pulmonary resistance, respiratory rate, tidal volume, blood gas tensions and heart rate were measured. Plasma samples were obtained for determination of theophylline concentrations before, and at 10 min intervals during, the infusion period. Excitability was assessed subjectively at these same time periods. The plasma theophylline concentrations in ponies were well predicted by a previously published model of theophylline pharmacokinetics in the horse. Sterile water had no effect on lung function. Aminophylline produced significant changes in lung function compared to baseline values, including a decrease in resistance at 30 min when the mean plasma theophylline concentration was 59 +/- 14 mumol/litre and an increase in compliance at 60 mins at a mean plasma theophylline concentration of 102 +/- 23 mumol/litre. Excitement was noted between 40 and 50 mins in all ponies (mean plasma theophylline concentration 74 +/- 20 and 84 +/- 24 mumol/litre, respectively). Heart rate increased at 50 mins. The therapeutic range for intravenous (iv) theophylline concentration in 'heavey' ponies therefore appears to be between 59 and 84 mumol/litre when aminophylline is administered iv. Below 59 mumol/litre there was no consistent bronchodilator activity and above 84 mumol/litre excitement and tachycardia limited the usefulness of the drug.     

Neuromuscular and cardiovascular effects of atracurium in ponies anesthetized with halothane

Atracurium besylate, a recently developed, intermediate-duration acting, neuromuscular-blocking agent, was given to 15 halothane-anesthetized ponies to produce surgical relaxation (95% to 99% reduction of hoof twitch). All 15 ponies were given 3 injections; 8 of the 15 ponies were given 2 additional injections. Initial dosage of 0.11 +/- 0.01 mg/kg (mean +/- SD) and all subsequent injections of 0.052 mg/kg produced desired relaxation. Paralysis phase (maximum twitch reduction to 10% twitch recovery) lasted 24 +/- 5 minutes for the initial injection. Paralysis from subsequent injections lasted for a slightly shorter time. Recovery phase (10% to 75% twitch recovery) was similar for all injections (initial and repeated) and lasted approximately 11 minutes. Cardiovascular side effects were not seen. Reversal of effects of atracurium with administration of 0.5 mg of edrophonium/kg was achieved when the evoked digital extensor tension (twitch height) had returned to 95% of base line after the last atracurium injection. Edrophonium caused systolic blood pressure to increase 121% +/- 7% of base-line pressure, which was 133 +/- 18 mm of Hg. Heart rate changed to 93% +/- 9% of base line after edrophonium was given, which was 49 +/- 7 beat/min, but this change did not occur until after the blood pressure increased. Recovery to standing was smooth and strong. Five ponies stood on their first attempt to rise, 5 on the 2nd attempt, 2 on the 3rd, and 1 on the 4th. Seven ponies stood within 30 minutes after transportation to the recovery stall, 7 within an hour.(ABSTRACT TRUNCATED AT 250 WORDS)