Distribution of cardiac output in anaesthetised horses

Pharmacokinetic and pharmacodynamic analysis comparing diverse effects of detomidine, medetomidine, and dexmedetomidine in the horse: a population analysis

The present study characterizes the pharmacokinetic (PK) and pharmacodynamic (PD) relationships of the α2-adrenergic receptor agonists detomidine (DET), medetomidine (MED) and dexmedetomidine (DEX) in parallel groups of horses from in vivo data after single bolus doses. Head height (HH), heart rate (HR), and blood glucose concentrations were measured over 6 h. Compartmental PK and minimal physiologically based PK (mPBPK) models were applied and incorporated into basic and extended indirect response models (IRM). Population PK/PD analysis was conducted using the Monolix software implementing the stochastic approximation expectation maximization algorithm. Marked reductions in HH and HR were found. The drug concentrations required to obtain inhibition at half-maximal effect (IC50 ) were approximately four times larger for DET than MED and DEX for both HH and HR. These effects were not gender dependent. Medetomidine had a greater influence on the increase in glucose concentration than DEX. The developed models demonstrate the use of mechanistic and mPBPK/PD models for the analysis of clinically obtainable in vivo data.

Skeletal muscle blood flow in anaesthetized horses. Part I: measurement techniques

OBJECTIVE

The objective of this review was to describe the methodology and limitations of techniques that have been used to measure skeletal muscle blood flow in anaesthetized horses.

DATABASE USED

Pubmed, personal files.

CONCLUSION

Numerous techniques have been used in horses to study skeletal muscle blood flow during anaesthesia and after the administration of vasoactive agents. Of the available techniques, blood flow measurements are limited to either microvascular flow (radioactive xenon, laser Doppler flowmetry) or total blood flow (radioactive microspheres, electromagnetic flowmetry, Doppler ultrasonography). None of the techniques currently available are able to fully assess the distribution of flow throughout the skeletal muscle. Near-infrared spectroscopy has the potential to assess the adequacy of oxygenation within muscles; however, this technique is not without limitations, and more work is required to assess its suitability. Understanding the limitations of these techniques is an important prerequisite to the critical evaluation of the information currently available on the effects of anaesthesia and vasoactive drugs on skeletal muscle blood flow.

The effects of spontaneous and mechanical ventilation on central cardiovascular function and peripheral perfusion during isoflurane anaesthesia in horses

OBJECTIVE

To compare the effects of spontaneous breathing and mechanical ventilation on haemodynamic variables, including muscle and skin perfusion measured with laser Doppler flowmetery, in horses anaesthetized with isoflurane.

STUDY DESIGN

Prospective controlled study.

ANIMALS

Ten warm-blood trotter horses (five males, five females). Mean mass was 492 kg (range 420-584 kg) and mean age was 5 years (range 4-8 years).

MATERIALS AND METHODS

After pre-anaesthetic medication with detomidine (10 microg kg(-1)) anaesthesia was induced with intravenous (IV) guaifenesin and thiopental (4-5 mg kg(-1) IV) and maintained using isoflurane in oxygen. The horses were positioned in dorsal recumbency. In five animals breathing was initially spontaneous (SB) while the lungs of the other five were ventilated mechanically using intermittent positive pressure ventilation (IPPV). Total anaesthesia time was 4 hours with the ventilatory mode changed after 2 hours. During anaesthesia, heart rate (HR) cardiac output (Qt) stroke volume (SV) systemic arterial blood pressures (sAP), and pulmonary arterial pressure (pAP) were recorded. Peripheral perfusion was measured in the semimembranosus and gluteal muscles and on the tail skin using laser Doppler flowmetry. Arterial (a) and mixed venous (v) blood gases, pH, haemoglobin concentration [Hb], haematocrit (Hct), plasma lactate concentration and muscle temperature were measured. Oxygen content, venous admixture (s/Qt) oxygen delivery (DO(2)) and oxygen consumption (VO(2)) were calculated.

RESULTS

During mechanical ventilation, HR, sAP, pAP, Qt, SV, Qs/Qt and PaCO(2) were lower and PaO(2) was higher compared with spontaneous breathing. There were no differences between the modes of ventilation in the level of perfusion, DO(2), VO(2), [Hb], (Hct), or plasma lactate concentration. After the change from IPPV to SB, left semimembranosus muscle and skin perfusion improved, while muscle perfusion tended to decrease when SB was changed to IPPV. Low-frequency flow motion was seen twice as frequently during IPPV compared with SB.

CONCLUSIONS

Mechanical ventilation impaired cardiovascular function compared with SB in horses during isoflurane anaesthesia. Muscle and skin perfusion changes occurred with ventilation, although further studies are needed to elucidate the underlying mechanisms.

Muscle Metabolic Changes Associated with Long-term Inhalation Anaesthesia in the Horse Analysed by Muscle Biopsy and Microdialysis Techniques

During anaesthesia in the horse, muscle blood flow has been found to be reduced, possibly leading to hypoxia or ischaemia in the muscle. The aim of this study was to use the muscle biopsy and microdialysis techniques to determine whether long-term inhalation anaesthesia in laterally recumbent horses induces metabolic changes in gluteal muscle indicative of anaerobic metabolism. Muscle biopsies and plasma samples were taken from seven horses at the start and end of halothane anaesthesia. In six isoflurane-anaesthetised horses, given three pharmacological provocations (dobutamine, detomidine, acepromazine), repeated blood samples and microdialysis was performed during anaesthesia and muscle biopsies were taken before and at the end of anaesthesia. Adenosine triphosphate (ATP), adenosine diphosphate, adenosine monophosphate, inosine monophosphate (IMP) creatine phosphate and lactate concentrations did not differ between dependent and non-dependent muscles at either sampling time. Creatine phosphate decreased in both the halothane (-38%) and isoflurane (-28%) group. In the halothane group, ATP was decreased (-15%) at the end of anaesthesia, while IMP was increased (+32%). Lactate in muscle and plasma increased in both groups. Lactate in dialysate increased after induction and remained elevated above plasma concentrations. These results show that long-term inhalation anaesthesia in horses is associated with an anaerobic metabolic response within the muscle and that microdialysis can be used to detect metabolic changes within the muscle during equine anaesthesia.

Life-threatening hemorrhage from enterotomies and anastomoses in 7 horses

OBJECTIVE

To report our experience with horses that presumptively had severe intraluminal hemorrhage from enterotomy or anastomosis.

STUDY DESIGN

Clinical study.

ANIMALS

Six adult horses and 1 adult donkey.

METHODS

A retrospective study was conducted at the University of Illinois (April 1994 to December 2001) to determine the clinical course and outcome of horses with melena and/or anemia and evidence of life-threatening hemorrhage from intestinal incisions. Medical records of all horses that had colic surgery were reviewed to determine the proportion of horses with this complication. In addition, horses that fit the same criteria identified in 3 other veterinary clinics were included.

RESULTS

Three horses (1.3%) of those that had enterotomy or anastomosis at the University of Illinois and 4 horses from other clinics had complications presumptively related to severe hemorrhage from these intestinal procedures. Melena became evident within 72 hours of surgery and lasted 12 to 96 hours. Six horses had an acute and severe drop in packed cell volume (PCV), increased heart rates, and other signs of acute hemorrhage, and 1 horse had signs of colic postoperatively. Horses were administered intravenous formalin (3 horses) and whole blood transfusions (4 horses). Repeat celiotomy was performed on 2 horses. In 1 of these horses, a bleeding artery was ligated in the edge of the original enterotomy, and, in the other, a 25-cm-diameter intraluminal blood clot was found occluding the pelvic flexure. A horse that had jejunocolostomy for cecal impaction was not treated for hemorrhagic shock but was euthanatized and necropsied. Necropsy revealed blood-filled bowel from the jejunocolostomy to the anus. One of the remaining 6 horses died of enterocolitis and 5 survived to discharge.

CONCLUSIONS

Hemorrhage from incisional edges, particularly in the large intestine, should be considered a rare but possibly fatal complication of enterotomy or anastomosis in horses.

CLINICAL RELEVANCE

To prevent fatal hemorrhage from incisional edges during enterotomy or anastomosis, large vessels should be ligated at the original surgery, and hemostatic effects of different closure techniques should be considered. No intraoperative or postoperative findings were useful to predict this complication, and response to supportive medical therapy was favorable.

Ascorbic acid disposition kinetics in the plasma and tissues of calves

Kinetic plasma disposition parameters and tissue distribution of ascorbic acid (AA) and dihydroascorbic acid (DHA) were determined in newborn calves. After a radiolabeled AA intravenous administration, the plasma clearance (Cl) was low (40.8 +/- 9.5 ml.kg-1.h-1), the steady-state volume of distribution (Vss) was very high (8.9 +/- 2.2 l/kg), and the AA mean residence time (MRT) was long (230 +/- 85 h). After administration of a 3-g dose of AA, the Cl was high (450 +/- 146 ml.kg-1.h-1), the Vss was low (0.658 +/- 0.236 l/kg), and the MRT was short (1.49 +/- 0.41 h), indicating a strong nonlinearity of AA disposition in calves and the impossibility of preventing scurvy with the use of a loading AA dose. Nonlinearity was explained by the saturation of both kidney reabsorption and tissue uptake. The estimated AA body pool size was 23.1 +/- 6.8 mg/kg. On the basis of a compartmental analysis and actual tissue concentration measurements, it is suggested that the lung (19% of the pool) constitutes a low-capacity but rapidly mobilized pool able to cover an acute need for AA, whereas muscle and liver (40 and 33% of the pool, respectively) are high-capacity AA pools, but slowly mobilized and involved in covering the calf's long-term AA requirements. The average daily AA entry rate over the first 7 days of life was 3.43 +/- 1.16 mg/kg, and it is suggested that the calf is able to synthesize AA at an early stage.

Tissue perfusion in anaesthetised sheep

1. Tissue perfusion was measured in seven sheep by the radioactive microsphere method. 2. The sheep were anaesthetised and ventilated to a near normal arterial carbon dioxide tension. 3. Cardiac output was measured utilizing the Fick principle as applied to thermodilution and at the end of the investigation the sheep were killed by an overdose of anaesthetic. 4. The results were compared with previously published values for conscious sheep and anaesthetised horses and found to be similar when known variations in body composition and physiology were considered.

A study of the effect of isoflurane anaesthesia on equine skeletal muscle perfusion

The effects of 1.1, 1.5 and 1.8 MAC (minimum alveolar concentration) isoflurane-02 anaesthesia on skeletal muscle blood flow, 02 delivery and vascular resistance were studied in the non-dependent region of seven healthy normothermic, isocapnoeic ponies. Muscle blood flow was determined with 15 microns diameter radionuclide labelled microspheres that were injected into the left ventricle. Muscle blood flow during anaesthesia was compared to unanaesthetised (control) measurements. Isoflurane administration caused a dose dependent decrease in mean aortic pressure, and skeletal muscle (temporalis, triceps brachii, longissimus dorsi, gluteus medius, biceps femoris, vastus lateralis) blood flow. However, in the masseter, triceps brachii at 1.1 and 1.5 MAC, and longissimus dorsi lumborum at 1.1 MAC anaesthesia, blood flow values were not different from the control value. Vascular resistance did not change significantly in any of the muscles with any concentration of isoflurane. With the exception of the masseter muscle, isoflurane anaesthesia also decreased skeletal muscle 02 delivery. There were no significant differences in the muscle parameters studied between the three concentrations of isoflurane. Because skeletal muscle vascular resistance was unchanged throughout the study, it is possible that there may be an interference of 'autoregulation' in the skeletal muscle vascular beds of isocapnoeic ponies during isoflurane-02 anaesthesia.

Comparison of echocardiographic and autopsy measurements of cardiac dimensions in the horse

This study was initiated to determine the accuracy of M-mode echocardiography in measuring left ventricular dimensions and estimating heart weights in horses. Left ventricular free wall and interventricular septal thickness and left ventricular external and internal diameters were measured and heart weights estimated from the echocardiograms of 47 horses. Autopsy measurements of the same parameters were then recorded. Statistical comparison of the data demonstrated: (1) Systolic measurements of wall thickness more closely resembled the heart in death than the diastolic measurements; (2) good correlations existed between parameters measured echocardiographically and at autopsy, especially wall thicknesses and left ventricular external diameter (maximum r = 0.82); (3) heart weight was readily predicted from echocardiographic wall thickness regressions (maximum R-squared = 68 per cent). M-mode echocardiography demonstrated the potential for direct and accurate measurements of cardiac mass and some ventricular dimensions in the horse. The data suggested that intense rigor and exsanguination may render the autopsied heart unsatisfactory for comparative measurements when assessing techniques such as echocardiography.

A "standard horse" for use in physiologically based mathematical modelling

Standard data for the horse which can be used in physiologically based mathematical computer modelling are presented. The data includes figures for tissue mass, density and perfusion, obtained by measurement mainly from horses weighing 200 to 300 kg. Other related parameters such as mean transit times and tissue blood volume have been calculated and included in the actual values listed for a 250 kg horse.

Muscle perfusion in the horse

Muscle perfusion was studied in conscious and anaesthetised horses by monitoring the clearance of a locally injected bolus of saline containing radioactive xenon (133Xe). The mean of all the measurements made from the brachiocephalicus and semimembranosus muscles in conscious subjects was 1.29 ml/min/100 g while in anaesthetised subjects it was 0.40 ml/min/100 g. The significance of these results is discussed in relation to the clinical occurrence of post anaesthetic myositis.

Sir Frederick Smith Memorial Lecture. A superb transport system--the circulation

This paper discusses certain aspects of circulatory physiology, including foetal circulation and the role of the thorax, and gives parameters for total blood volume, cardiac output, stroke volume and the distribution of blood between the pulmonary and peripheral circulation within the horse. The reader is taken on a journey around the circulatory system, beginning in the left atrium, into which richly oxygenated blood runs from the pulmonary veins. From here the blood is squeezed via the mitral valve into the left ventricular pump, where it is forced under high pressure through the peripheral circulation, the arterioles and capillaries, to collect, deoxygenated and under much lower pressure, into the venules and veins. The pumping action of the muscles and the negative intrathoracic pressure help to draw the blood back up to the right atrium, itself exerting a suction effect on the great veins. The blood is then expelled from the right ventricle into the low resistance, high capacitance pulmonary circuit, where it picks up oxygen, releases carbon dioxide and returns to the left atrium.