Ischaemia induced development of functional coronary collateral circulation in ponies.

The response of coronary collaterals in nine ponies subjected to repeated reversible occlusions (2 min duration, 30 min interval) of the left anterior descending coronary artery was studied at rest. Each pony was instrumented with a Doppler flowmeter and hydraulic cuff occluder around the left anterior descending coronary artery, left ventricular subendocardial sonomicrometers, and a left ventricular micromanometer. Initial occlusions increased end diastolic myocardial segment length by 3% and decreased segment systolic shortening, stroke work, and velocity of shortening by 103%, 95%, and 79% respectively in the left ventricular apex. Left ventricular systolic and end diastolic pressure, peak positive dP/dt, and heart rate were not significantly changed by occlusion. After 421(70) (mean(SEM)) occlusions no sustained alterations in myocardial segment function occurred in response to occlusion. Thus the presence of a subendocardial plexus did not protect against a severe loss of myocardial segment function when the ponies were initially subjected to occlusions of the left anterior descending coronary artery. However, repeated reversible occlusions enhanced coronary collateral blood flow such that it was adequate to maintain left ventricular function in the absence of left anterior descending coronary artery flow. It is concluded that the pony is highly suitable for use in studies of coronary collateral circulation because of its coronary anatomical similarity to man and its capacity to develop functional collateralisation.

Cardiac cycle length variability in ponies at rest and during exercise.

We evaluated cardiac cycle length variability in ponies at rest and during strenuous exercise with and without premedication with atropine. In the absence of premedication, cardiac cycle length at rest was 1,112 +/- 53 ms, the individual cardiac cycle length standard deviation (SDCL) was 75 +/- 23 ms, and the individual cycle length coefficient of variation (CVCL) was 6.32 +/- 1.62. Exercise significantly decreased (P < 0.05) all three indexes (290 +/- 9 ms, 5 +/- 1 ms, and 1.65 +/- 0.20, respectively). Atropine premedication significantly reduced resting cardiac cycle length (685 +/- 46 ms), SDCL (10 +/- 2 ms), and CVCL (1.45 +/- 0.19) compared with nonpremedicated values. Cardiac cycle length was significantly decreased by exercise after atropine premedication, but no statistically significant changes occurred in SDCL or CVCL. Thus, although considerable cardiac cycle length variability exists in nonpremedicated ponies at rest, it is nearly completely abolished by strenuous exercise. The absence of significant differences between the indexes of variability during exercise without premedication, at rest after atropine, and during exercise after atropine indicates that cardiac cycle length variability in the pony is mediated primarily through activity of the parasympathetic system.

Plasma endotoxin levels in horses subjected to carbohydrate induced laminitis.

Thirteen (65 per cent) of 20 horses subjected to carbohydrate overload developed Obel Grade 3 lameness within 56 h. Increases in plasma endotoxin from control levels of less than 0.1 ng/litre to values ranging from 2.4 to 81.53 ng/litre were measured in 11 (85 per cent) of 13 horses during the onset of Obel Grade 3 lameness. Obel Grade 3 lameness was associated with rises in plasma Gram-negative endotoxin levels in 11 (92 per cent) of 12 horses. Two peak increases separated by 16 h were verified in five (45 per cent) of 11 horses that exhibited both endotoxaemia and Obel Grade 3 lameness. The first peak occurred, on average, at 32 h and the second peak at 48 h post overload when only one peak was measured, this occurred in four horses at the average time of 24 h whereas in another three horses only a 48 h or 56 h peak was detected after carbohydrate overload.

Arterial oxygen and carbon dioxide tensions in conscious laterally recumbent ponies.

Six adult ponies were trained calmly to assume and maintain left lateral recumbency without the use of sedative or immobilising agents. During a 30 min recumbent period, pHa, arterial oxygen and carbon dioxide tensions (PaO2 and PaCO2) and heart and respiratory rates were monitored at regular intervals to evaluate ventilatory response. Overall, there were no statistically significant differences found between mean control and recumbent or final standing values. When lightweight ponies were compared to heavyweight ponies, only mean PaO2 at 10 mins recumbency was different. This information supports the hypothesis that ventilation impairment during the first 30 mins of equine general anesthesia is primarily drug-mediated, rather than initiated by recumbency per se.

Haematological changes during development of acute laminitis hypertension.

Experimental carbohydrate overload of 15 horses resulted in consistent haematological changes during development of laminitis hypertension. Significant alterations in packed cell volume, leucocyte differential count, serum glucose levels and protein values occurred before onset of Obel grade 3 lameness and hyperkinetic circulatory state. Blood platelets were significantly decreased 8 h after the onset of severe lameness. Findings in this study were indicative of haemoconcentration due to compartmental fluid shifts and leucocytic stress response consistent with increased circulating adrenoglucocorticoids and/or catecholamines.

Ultrasonic measurement of arterial blood pressure in conditioned thoroughbreds.

Indirect systolic and diastolic arterial pressure measurements of 456 Thoroughbred horses of various ages and sex stabled at 9 different race tracks were measured with an ultrasonic-Doppler device interfaced with a sphygmomanometer cuff. The mean systolic pressure was 111.8 +/- 13.3 mmHg and the mean diastolic pressure was 69.6 +/- 13.8 mmHg in this population.

Prevention of endotoxin-induced arterial hypoxaemia and lactic acidosis with flunixin meglumine in the conscious pony.

Bacterial endotoxin injected intravenously into conscious ponies produced alterations in cardiopulmonary and gastrointestinal function. Specifically, tachypnoea, dyspnoea, hypoxaemia, colic, lactic acidosis and diarrhoea resulted from administration of 10 micrograms/kg Escherichia coli endotoxin. Pretreatment of the ponies with a potent prostaglandin synthetase inhibitor, flunixin meglumine, prevented these ill effects of endotoxin.

Recovery from anaesthesia in ponies: a comparative study of the effects of isoflurane, enflurane, methoxyflurane and halothane.

The duration and quality of recovery after separate 2 hour anaesthetic periods with equipotent alveolar concentrations of isoflurane, enflurane, halothane and methoxyflurane were evaluated in experimental ponies. Recovery was shortest after isoflurane anaesthesia, followed by enflurane, halothane and finally methoxyflurane, although standing was achieved more rapidly after enflurane than after isoflurane. This sequence of recovery times was compatible with the respective solubilities of the 4 agents. The smoothest recovery was obtained after isoflurane anaesthesia, followed by methoxyflurane, halothane and then enflurane. The isoflurane recovery was characterized by very quiet and prolonged sternal recumbency with excellent coordination upon standing. The chief difficulties with halothane recovery were shivering and delayed coordination upon standing. Enflurane anaesthesia was followed by a very brief period of sternal recumbency but there was considerable shivering and incoordination upon standing. This may have been due to central nervous stimulation during anaesthesia producing a residual excitatory effect. On the basis of this investigation, isoflurane was considered to be the most satisfactory volatile anaesthetic for the horse.

Left ventricular function and haemodynamics in ponies during exercise and recovery.

Myocardial and haemodynamic responses to strenuous treadmill exercise were monitored with chronically implanted instrumentation in seven physically untrained ponies. In two other ponies, haemodynamics were monitored using a conventional catheter technique. During exercise (mean +/- sem heart rate = 203 +/- 3 beats/min), left ventricular peak systolic ahd end-diastolic blood pressure significantly increased from 125 +/- 2 to 208 +/- 6 mmHg and from 29 +/- 1 to 58 +/- 3 mmHg, respectively (P less than 0.05). Peak positive first derivative of left ventricular pressure (dP/dt) was also increased significantly, (1397 +/- 144 to 6016 +/- 325 mmHg/sec), as were myocardial segment velocity of shortening during systole (dL/dtsys) and stroke work (193 and 109 per cent, respectively). Paradoxically, segment systolic shortening was reduced significantly (P less than 0.05) with respect to resting control values for 10 mins after cessation of exercise, with the maximal reduction of 22 per cent occurring at 2 mins post exercise. In addition, left ventricular segment stroke work decreased significantly to 67 per cent of resting control value at 2 mins post exercise. This study demonstrates that analysis of regional myocardial performance in conjunction with haemodynamic data permits a detailed description of cardiac events during and immediately following exercise, facilitating definition of changes which are not readily reflected by haemodynamics. Based on these findings, the authors conclude that untrained ponies may readily complete short bouts of near-maximal exercise, yet experience significantly diminished left ventricular function in the immediate post exercise period.

Changes in the caecal flora associated with the onset of laminitis.

Caecal fluid samples collected 8 and 24 hours after carbohydrate overload were quantitatively compared to control samples in terms of aerobic and anaerobic bacteria. Concomitant increases in lactic acid-producing bacteria and decreases in Gram negative bacteria were substantiated during the onset of acute laminitis. Progressive decreases in caecal fluid pH were also quantitated. Although endotoxin assays of caecal fluid and blood were not done, the caecal flora changes suggest its presence during the onset of acute laminitis.

Reduced blood flow to peripheral and core body tissues in sheep and cattle induced by endophyte-infected tall fescue.

Four wethers and 14 steers (environmentally heat stressed, 32 degrees C, 60% relative humidity) were evaluated for changes in blood flow induced by endophyte-infected tall fescue. Concentration of the ergopeptide ergovaline was used as an indicator of diet toxicity due to the endophytic fungus Acremonium coenophialum. Blood flow to specific tissues was measured using radiolabeled microspheres. Wethers received one of two dietary treatments for 30 d before determination of tissue blood flow: 1) a low-endophyte diet (less than .05 ppm ergovaline) or 2) a high-endophyte (1.18 ppm ergovaline) diet. Blood flows to the adrenal glands and skin covering the inner hind leg were less (P less than .10) in wethers consuming the high-endophyte diet than in those consuming the low-endophyte diet. Tissue blood flows in steers were determined on two occasions: 1) after steers had received a low- (less than .01 ppm ergovaline) or high-endophyte (.52 ppm ergovaline) fescue diet for 14 d and 2) 8 d after steers had been switched to a common, fescue-free diet. Blood flows to skin covering the ribs, cerebellum of the brain, duodenum, and colon were less (P less than .10) in steers consuming the high-endophyte diet. However, 8 d after consuming fescue-free diets, steers that had previously consumed the high-endophyte diet had greater (P = .08) blood flow to the coronary bands of the front hooves than steers that had consumed the low-endophyte diet. Blood flows to all other tissues were similar between treatments. We inferred from these experiments that the toxin(s) associated with endophyte-infected tall fescue caused decreased blood flow to peripheral and core body tissues and that this effect was abated within 8 d of removing the toxin(s).

Plasma volume, electrolyte, and endocrine changes during onset of laminitis hypertension in horses.

Acute laminitis-hypertension was produced by carbohydrate overloading of the gastrointestinal tract in 12 adult horses. Obel grade 3 (OG3) lameness developed 40 hours (+/- 3.5, SEM) after overfeeding. At OG3 lameness, mean plasma volume was significantly decreased (P less than 0.005) when compared with base-line values. Before OG3 lameness, transient decreases in serum phosphorus and calcium were recorded. Mild hyponatremia also developed before OG3 lameness and persisted. After establishment of OG3 lameness, persistent hypokalemia and increased plasma aldosterone concentration occurred coincidently. Transient increase in plasma hydrocortisone (cortisol) and renin activity and transient hypochloremia were also recorded during the syndromal phase. Changes in plasma volume and serum electrolytes are discussed and related to the pathogenesis of acute equine laminitis. The alterations in plasma renin activity and aldosterone concentration were interpreted as homeostatic adjustments to fluid and electrolyte imbalances. Differences between the horse and pony during onset of experimental alimentary laminitis are also discussed.

Radioimmunoassay for the detection of antigen-specific IgM, IgG, and IgA in equine sera.

A radioimmunoassay was developed to discriminate immunoglobulin (Ig) classes specific for the J-5 mutant of Escherichia coli (serotype O:111-B4). Adult horses were periodically inoculated IM with a nonviable suspension of the J-5 mutant emulsified in Freund's incomplete adjuvant. Before and after the horses were inoculated, sera were collected sequentially and examined by radioimmunoassay. Rabbit anti-(horse) Ig and [125I]protein A served as the indicator system. Antigen-specific IgM, IgG, and IgA were observed to follow a classic immune response. The radioimmunoassay offers a valuable tool for the discrimination of circulating, antigen-specific Ig in the horse.

Ultrastructural study of the equine cecum during onset of laminitis.

The morphologic and pathologic changes which occurred within the cecal mucosa of 4 horses during the onset of laminitis were determined from cecal biopsy materials obtained via a cecal fistula; the laminitis was induced with carbohydrate overload. The cecal epithelial mucosa specimens were obtained at 0 (base line), 24, 32, 48, and 72 hours after horses were given carbohydrate overload, and these were fixed and subsequently photographed. Changes in the cecal epithelium were examined by transmission electron and scanning electron microscopies. These histopathologic changes indicated that the mucosal barrier was substantially damaged by the carbohydrate overload.

Intracecal endotoxin and lactate during the onset of equine laminitis: a preliminary report.

Cecal fluid from two adult horses was assayed by the limulus amebocyte lysate system for endotoxin before and after carbohydrate overload of the gastrointestinal tract. There were increases in cecal fluid endotoxin concentrations at the 3-, 6-, and 12-hour samplings when compared with base-line values. Concomitant cecal fluid lactate concentrations and pH values increased and decreased, respectively. Both horses subsequently developed clinical signs of acute laminitis.

Lactic acidosis and arterial hypoxemia during sublethal endotoxemia in conscious ponies.

Effects of a sublethal IV dose (10 micrograms/kg of body weight) of Escherichia coli endotoxin were studied in 7 conscious ponies. Arterial blood gases and plasma lactic acid were determined periodically throughout the 180 minutes of the study. Arterial hypoxemia occurred within 5 minutes despite intense hyperventilation. Colic and diarrhea occurred in all ponies within 30 minutes after endotoxin administration. Metabolic acidosis developed within 30 minutes and persisted throughout the study, indicating widespread reduced tissue perfusion.

Effect of intraluminal oxygen in intestinal strangulation obstruction in ponies.

This study examined the effect of intraluminal oxygen administration on mucosal morphology following intestinal strangulation obstruction (ISO) in anesthetized ponies. The ISO was produced by ligation of the intestinal vasculature in 5 ponies for 50 minutes and 2 ponies for 90 minutes. Two ponies served as controls. Light and scanning electron microscopic examination of intestinal biopsy specimens revealed progressive mucosal degeneration following ISO in nontreated intestines, whereas high magnification scanning electron microscopic examination documented subtle evidence of microvilli disruption 120 minutes following ISO in oxygen-treated intestines.

Equine laminitis of alimentary origin: an experimental model.

Acute alimentary form of laminitis was uniformly induced in 11 of 12 horses by administration of a starch and wood flour gruel and could be graded by previously established (Obel) and presently defined criteria. The experimentally induced laminitis was similar to naturally occurring laminitis, as determined on the basis of lameness severity and vital signs. Packed cell volume, leukocyte count, and total protein were significantly increased (P smaller than 0.05) at 24 and 40 hours after administration of gruel. Arterial systolic and diastolic pressures increased, central venous pressure decreased, heart rate increased, and rectal temperature increased consistently within the 56-hour experimental period. Of the 11 affected horses, 7 horses had Obel grade 3 lameness (horse moved most reluctantly and vigorously resisted attempts to lift a forefoot) at 40 hours after gruel was placed in the alimentary tract, 2 horses had Obel grade 3 lameness at 32 hours, and 2 horses had Obel grade 3 lameness at 48 hours.

Renal function of the pony and the horse.

Simultaneous renal clearances of inulin (CIN), p-aminohippurate (CPAH), and creatinine (CCR) were measured in hydrated mares (6 ponies and 2 horses). The CIN and CPAH were determined during steady-state infusion at 3 different infusion rates. A 6-fold change in plasma IN concentration did not produce alteration in CIN, nor was there a difference between the ponies and horses (P greater than 0.2). The overall average (mean +/- SEM) was 190.6 +/- 5.89 ml . min-1 . 100 kg of body weight-1. There was no difference noted between simultaneous CIN and CPAH. Clearance of PAH remained essentially constant during the change in plasma PAH from 0.33 mg/dl to 5.27 mg/dl. The extraction ratio of PAH for the nonanesthetized pony was 0.966. Effective renal plasma flow (CPAH) of the pony exceeded that of the horse.

Hemodynamic effects of prostacyclin (prostaglandin I2) in conscious ponies.

The infusion (IV) of prostacyclin (PGI2) into conscious ponies resulted in systemic arterial hypotension and tachycardia. Mean systemic arterial pressure decreased from 103.3 mm of Hg to 88.5 mm of Hg when 125 ng of PGI2/kg/minute was infused, and heart rate increased from 55.0 to 102.0 beats/minute. When 208 ng of PGI2/kg/minute was infused, mean systemic arterial pressure decreased from 103.3 to 78.0 mm of Hg and heart rate increased from 55.0 to 109.3 beats/minute. There were no significant alterations in pulmonary arterial pressure, lactate, PCV, total WBC count, platelet count, serum lactic dehydrogenase, and acid phosphatase activities, or rectal temperature at either dosage. The ponies became anxious and difficult to manage during the infusion.