Quantitative assessment of increased sensitivity of chronic laminitic horses to hoof tester evoked pain.

REASONS FOR PERFORMING STUDY: To evaluate quantitative sensory testing (QST) of the feet of laminitic horses using a power-assisted hoof tester. HYPOTHESIS: Hoof Compression Thresholds (HCTs) can be measured reliably and are consistently lower in horses with chronic laminitis than in normal horses. METHODS: HCTs of chronic laminitic (n=7) and normal horses (n=7) were repeatedly measured using a hydraulically powered and feedback controlled hoof tester. Data from 2 tests, at 3 sites in both forefeet, during 3 sessions were collected and statistically analysed using linear mixed models. RESULTS: The mean±s.e. HCT for the laminitic horses was 29.6±3.5 kg/cm2 and for horses in the normal group was 59.8±4.3 kg/cm2. Residual variance was the largest of the error components and was greater (P<0.001) for the normal horses; none of the other components significantly differed between the 2 groups. Averaging of HCTs from each foot could produce a test with intraclass correlation coefficients of 0.83 for the normal group and 0.87 for the laminitic group, with an estimated sensitivity of 0.94 and a specificity of 0.93. This test would permit detection with 80% power and 95% confidence of a reduction of over 40% in the difference in mean HCTs between laminitic and normal horses following effective treatment provided that the experimental groups are of 9 or more horses. CONCLUSIONS: HCTs can be safely and reliably measured experimentally using this hoof tester. The level of variability found indicates that, under these conditions, treatments may need to produce at least a 40% improvement to be detected. Simplification of the hoof tester, training of the horse and repeated testing may permit the method to be used clinically to detect changes in the HCTs of individual laminitic horses but these potential improvements will require further investigation. POTENTIAL RELEVANCE: Measurement of HCTs can provide an additional means for assessing the effectiveness of treatments for alleviation of chronic equine laminitis.

Histological and ultrastructural evidence that recurrent laryngeal neuropathy is a bilateral mononeuropathy limited to recurrent laryngeal nerves.

REASONS FOR PERFORMING STUDY: Recurrent laryngeal neuropathy (RLN) is a common and debilitating peripheral nerve disease of horses, but it remains unclear if this disease is a mono- or polyneuropathy. An understanding of the distribution of the neuropathological lesions in RLN affected horses is fundamental to studying the aetiology of this very significant disease of tall horses. OBJECTIVE: To determine whether RLN should be classified as a mono- or polyneuropathy. METHODS: Multiple long peripheral nerves and their innervated muscles were examined systematically in 3 clinically affected RLN horses RESULTS: Severe lesions were evident in the left as well as right recurrent laryngeal nerves in all horses, both distally and, in one case, also proximally. No primary axonal lesions were evident in other nerves nor were changes found in their innervated muscles. CONCLUSIONS: RLN is not a polyneuropathy but should be classified as a bilateral mononeuropathy. POTENTIAL RELEVANCE: Genetic and local factors specifically affecting the recurrent laryngeal nerves in RLN-affected horses should now be investigated further.

Opioid-mediated changes in nociceptive threshold during pregnancy and parturition in the sow.

This study aimed to investigate if pregnancy-induced hypoalgesia occurs in the sow, and to examine the role of endogenous opioids which are known to be released in response to nociception. Sixteen Large White x Landrace multiparous sows were tested in straw bedded pens (2.5 x 2.5 m) during weeks 4, 8 and 12 of pregnancy and over the farrowing period. Testing involved thermal stimulation of eight areas on the rear-quarters of the sows with a CO2 infra-red laser until a physical response was seen (tail flick, leg move or muscle twitch) or for a maximum of 16 s. Over the farrowing period testing was more frequent, and at 3.75 h after the birth of the first piglet, half the sows received an injection (i.m.) of an opioid antagonist naloxone (N) (1 mg kg(-1) body weight) with the remainder receiving a control dose of saline (S). Responses were recorded 15 and 30 min post-injection. There was no significant difference between response times over weeks 4, 8 and 12 of pregnancy (P = 0.152), however a significant rise was seen from week 12 to 5 days before parturition (P = 0.002). Response times continued to rise until the birth of the first piglet by which time the majority of sows had stopped responding within 16 s (P < 0.001). Response times fell over days 1, 2 and 7 post-partum. After administration of naloxone response times fell compared to control animals at 15 min (P < 0.001) and 30 min (P < 0.01) post-injection. These results suggest that nociceptive threshold increases during late pregnancy in the sow, perhaps as an endogenous defence against labour pain, and that during parturition this change in nociceptive threshold is, at least in part, opioid-mediated. Oxytocin is known to be inhibited by endogenous opioids at parturition, thus future research should consider the potential role of increased nociception at birth as a negative feedback to oxytocin release.

The influence of opioid receptor subtypes on the processing of nociceptive inputs in the spinal dorsal horn of the cat.

Extracellular recordings were made of the cutaneous sensory responses of spinocervical tract (SCT) neurones in the lumbar dorsal horn of anaesthetised and paralysed cats. All of the neurones studied were multireceptive, showing excitatory responses to both innocuous and noxious (thermal and when tested, mechanical) stimuli applied to their cutaneous receptive fields on the ipsilateral hindlimb. The effects of iontophoretically applied opioids were studied on a regular cycle of responses to these cutaneous stimuli and also to D.L-homocysteic acid (DLH). In the first series of experiments, drugs were applied in the vicinity of the SCT neurones. The kappa-receptor agonists dynorphin A(1-13) and U50488H, but not dynorphin A(2-13), the mu-agonist DAGO, or the delta-agonist DADL, caused a selective reduction of the nociceptive responses of the neurones. The corresponding responses to innocuous stimuli or to DLH, and spontaneous activity were unaffected. In the second series of experiments, drugs were applied from a second electrode placed in the region of the substantia gelatinosa directly dorsal to the tip of the recording electrode. Under these conditions, the mu-receptor agonist DAGO, but not the kappa-agonist dynorphin A(1-13) or the delta-agonists DADL, DSLET or DLPEN, showed a selective antinociceptive effect. In both series, the antinociceptive effects of the opioids were readily reversed by iontophoretically applied naloxone. The effect of dynorphin A(1-13) applied close to SCT neurones, but not that of DAGO applied in the region of the substantia gelatinosa, was reversed by the alpha 2-adrenoceptor antagonist, idazoxan. The results indicate that both mu- and kappa-opioid receptors (at anatomically distinct sites) can participate in the selective antinociceptive influence that opioids can exert over somatosensory information ascending to supraspinal levels. The antagonism of kappa-receptor-mediated antinociception by idazoxan is consistent with an interaction of opioid and noradrenaline influences at the level of the dorsal horn.

An alpha 2 receptor mediates the selective inhibition by noradrenaline of nociceptive responses of identified dorsal horn neurones.

Extracellular recordings were made of 59 neurones with long, ascending projections (spinocervical tract (SCT) and dorsal column postsynaptic (DCPS) neurones) in the lumbar dorsal horn of anaesthetized and paralyzed cats. All showed prominent excitatory responses to innocuous stimuli, applied to their cutaneous receptive fields on the ipsilateral hindlimb. The majority of the population investigated (83%) was multireceptive, being activated by noxious as well as innocuous cutaneous stimuli. Drug effects were examined on a regular cycle of responses to these cutaneous stimuli and also to DL-homocysteic acid (DLH). In 49 multireceptive SCT and DCPS neurones, ionophoretically-applied L-noradrenaline (NA) produced a potent selective inhibition of the nociceptive responses (to heat or pinch) in 40 out of 44 SCT and 3 out of 5 DCPS neurones, with no statistically significant change in the responses to innocuous brush or DLH, or in spontaneous activity. NA had no effect on the majority of cells (8 out of 11) that responded only to innocuous stimuli. In 19 SCT neurones that showed NA-selectivity, the alpha 2-selective agonists clonidine (in 12 out of 15) and metaraminol (in 2 out of 3) mimicked this selective effect, whereas, the alpha 1 agonist, phenylephrine and the beta agonist, isoprenaline did not. Furthermore, the alpha 2 antagonists, yohimbine and idazoxan (RX781094), either reversed or reduced the potency of the NA-elicited inhibition of nociceptive responses in all 7 SCT neurones tested. These results are discussed in relation to other evidence for spinal antinociceptive effects of noradrenergic systems acting at a spinal level and the possible involvement of an alpha 2 receptor in such effects.

The involvement of neurokinin receptor subtypes in somatosensory processing in the superficial dorsal horn of the cat.

As well as substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) have recently been found in the superficial dorsal horn of the spinal cord; NKA originating mainly in fine primary afferents. We have investigated the effects of these tachykinins and a range of analogues on somatosensory responses of single identified dorsal horn neurons, when applied ionophoretically to the region of the substantia gelatinosa. Behavioural reflex tests of thermal nociception were carried out in parallel. The role of NK-1, NK-2 and NK-3 receptors was addressed. NK-1-selective agonists attenuated the non-nociceptive responses of identified multireceptive spinocervical tract (SCT) neurons. Of the endogenous tachykinins, both SP and NKB (a weak NK-1 agonist) showed this effect. No role for NK-3 receptors was identified in our experiments. NK-2-selective agonists (including NKA) caused a unique and selective facilitation of thermal nociceptive responses. NKA also reduced reflex response latency in tail-flick and hot plate tests. NKA as a primary afferent transmitter may thus be involved in mediating or facilitating the expression of thermal nociceptive inputs in the substantia gelatinosa. NKA and SP could be considered as acting in concert in the superficial dorsal horn in an effectively pro-nociceptive modulatory role. Evidence from receptor-selective antagonists supports that obtained with agonists for the roles of particular NK receptors in somatosensory processing. NK-2, but not NK-1 or NK-3 antagonists attenuated endogenous thermal nociceptive responses, supporting the hypothesis that an NK-2 agonist (such as NKA) may normally participate in expression of thermal nociception in the superficial dorsal horn. Behavioural experiments showing increased response latencies with a putative NK-2 selective antagonist further supported the involvement of NK-2 receptors in thermal nociception.

Behavioural responses of lambs of three ages in the first three hours after three methods of castration and tail docking.

The behavioural responses of groups of seven lambs were compared with control groups after castration and tail docking by rubber rings, application of a Burdizzo clamp in addition to a rubber ring and after surgical castration at five, 21 and 42 days. All methods at all ages produced changes in behaviour which were interpreted as indicative of considerable pain. The rubber ring groups showed most changes in behaviour at all ages. The rubber ring with Burdizzo groups showed least changes and some lambs in these groups showed much less response than others. The surgical groups showed some behavioural responses which were different, both qualitatively and quantitatively to those in other groups. It is concluded that indices used for recognition and assessment of acute pain received conditional support, that modification of the rubber ring with Burdizzo may provide the least painful method without local anaesthesia and that age had little effect on the responses.

Changes in plasma cortisol concentration in lambs of three ages after three methods of castration and tail docking.

Lambs were handled only or castrated and tail docked at five, 21 and 42 days of age by either surgery, rubber ring or rubber ring and Burdizzo. Plasma cortisol was measured in blood samples taken before and at 12, 24, 36, 48, 60, 72, 84, 96, 138 and 180 minutes after castration and docking. Pre-treatment and peak cortisol values were highest in five-day-old lambs. The peak cortisol values, at each age, were similar for surgery and rubber ring groups. However, the peak occurred earlier after surgery and rubber ring Burdizzo than after rubber ring only treatment. The cortisol peak was 28 nmol litre-1 lower after rubber ring Burdizzo than surgery or rubber ring only. Plasma cortisol returned to pretreatment values within 84 minutes after rubber ring Burdizzo, 96 to 138 minutes after rubber ring only but not within 180 minutes after surgery. The changes in plasma cortisol together with the changes in behaviour suggest that the rubber ring Burdizzo method of castration and docking of lambs at all ages, was probably the least painful of the methods tested.

Effect of different methods of castration on behaviour and plasma cortisol in calves of three ages.

Calves of six, 21 and 42 days were either handled (controls) or castrated by Burdizzo, by surgical or by rubber ring techniques and their behaviour and plasma cortisol monitored for three hours after treatment. The rubber ring caused significant increases in active behaviour and abnormal postures for two hours compared with handled or castrated by Burdizzo and surgical techniques, and surgical techniques resulted in a significant increase in abnormal standing, particularly in the first 30 minutes. Abnormal behaviour and posture were recorded less frequently in six-day-old calves. The mean cortisol peaks for six-, 21- and 42-day-old calves were, respectively, 36, 31 and 23 nmol litre-1 in the handled groups; 60, 43 and 48 nmol litre-1 in the rubber ring groups; 77, 50 and 62 nmol litre-1 in the Burdizzo groups; and 106, 63 and 107 nmol litre-1 in the surgical groups. The greatest cortisol response occurred in 42-day-old surgically treated calves and the shortest response after Burdizzo castration. These responses suggest that irrespective of age, all methods of castration studied caused acute pain. Burdizzo castration appeared to produce the least pain, particularly in younger calves.

Effects of castration on behaviour and plasma cortisol concentrations in young lambs, kids and calves.

Behavioural and cortisol responses to the husbandry practice of castration with tight rubber rings were investigated in lambs and kids one day after birth and in hand reared calves aged one to seven days. There were three treatments: control handling and blood sampling, castration and, in lambs and kids only, intravenous adrenocorticotrophin injection (ACTH). The integrated cortisol responses (area under the cortisol curve) in lambs and kids were least in control, intermediate in castrated and greatest in ACTH animals. No cortisol responses were detected in control or castrated calves. The incidences of behaviour used to assess the intensity of distress apparently experienced in the different species corresponded generally with the magnitudes of the cortisol responses. Behavioural and cortisol responses together suggested that the distress caused by castration was greatest in lambs, intermediate in kids and least but not necessarily absent in hand reared calves.

Respiratory responses of the domestic fowl to low level carbon dioxide exposure.

Respiratory function in the restrained, conscious domestic fowl was measured using a non-invasive technique and computer aided analysis to examine respiratory flow, and in-dwelling arterial catheters to monitor blood carbon dioxide levels. The effects of two low ranges of inspiratory carbon dioxide (0.2 to 1.0 per cent and 0.25 to 2.25 per cent) were studied, simulating levels of carbon dioxide that may occur in commercial poultry units and representing a mild environmental stress for the birds. A linear increase in minute volume with inspiratory carbon dioxide was observed, due primarily to increases in tidal volume. Arterial carbon dioxide (PaCO2) tension also rose as the inspiratory carbon dioxide concentration was raised, but higher inspiratory carbon dioxide levels were required to affect significantly blood carbon dioxide concentration than to modify respiratory parameters. Variation was observed in the individual bird's response to carbon dioxide (bird X carbon dioxide interaction), suggesting that resting values of the respiratory parameters measured were important in determining the bird's ventilatory response to carbon dioxide.

Variability in respiratory function of the domestic fowl.

Several respiratory parameters were measured in five groups of hens to estimate the variation in respiratory function of the general hen population. A non-invasive technique was used to measure respiratory flow, with computer-aided analysis of the data, and the results were examined statistically. Using different groups of hens, additional information was obtained regarding possible sources of variation between individual hens, such as egg laying condition, age and health. Significant differences between individuals within the groups were observed, but there were also changes in breath-to-breath variation for individual hens. While some changes in the variation between birds were observed from one group to another, variation within individual birds made interpretation of the results difficult. Repeated measurements made on one group revealed significant changes in minute volume with time, but the changes were not related to learning. Significant interactions between hens and time of measurement occurred for certain parameters, showing that the individuals' responses changed with time in different ways. The groups of hens were compared to observe whether they represented the same or distinct populations, and were found to overlap to varying extents for different respiratory parameters. Single respiratory variables and weighted combinations of certain variables were also used to separate the groupings, as a possible diagnostic method for partitioning groups of hens.

Effects of local anesthesia and intravenous naloxone on the changes in behaviour and plasma concentrations of cortisol produced by castration and tail docking with tight rubber rings in young lambs.

The reliability of some behavioral and physiological indices used for the recognition and assessment of acute pain in lambs after castration and tail docking has been examined. Changes in the indices were measured after blocking neural activity with local anaesthetic (lignocaine) and after an opioid antagonist (naloxone) was administered. Six lambs, aged less than one week, were allocated randomly to each of six treatments. (i) control handling and blood sampling; (ii) castration plus tail docking with tight rubber rings; (iii) local anaesthesia; (iv) local anaesthesia followed by castration and tail docking; (v) intravenous naloxone only (0.2 mg kg-1); and (vi) intravenous naloxone followed by castration and tail docking. Local anaesthesia eliminated the behavioural and plasma cortisol changes which usually follow castration and tail docking. Naloxone had a limited effect on the increase in cortisol but altered the behaviour. The results support the view that such indices are useful for assessment of the response to acute pain and that, although endogenous opioids do reduce pain in young lambs after castration and tail docking, the effect is small.

Effects of four analgesic treatments on the behavioural and cortisol responses of 3-week-old lambs to tail docking.

The behavioural and cortisol responses of groups of seven or eight lambs were used to determine which of three methods of tail docking (rubber ring, Burdizzo and rubber ring combined, or heated docking iron) produced the least signs of pain in the first 3 h after use and which of four analgesic treatments (1.0 ml bupivacaine subcutaneously, 0.5 ml bupivacaine epidurally, a topical cold analgesic spray or diclofenac 1.5 mg kg-1) was most effective in reducing these signs. Amputation with a heated docking iron produced levels of behaviour and cortisol responses which did not differ markedly from those of handled controls. The rubber ring method produced the greatest increase in all parameters (total active behaviour 110 +/- 91 counts; 51 +/- 23 min spent in abnormal postures; peak cortisol 93 +/- 51 nmol l-1). Subcutaneous bupivacaine, administered immediately prior to application of the ring, appeared to be the analgesic treatment most effective at reducing these responses (23 +/- 15 counts; 24 +/- 22 min.; 44 +/- 20 nmol l-1).

Comparison of methods for the reduction of acute pain produced by rubber ring castration or tail docking of week-old lambs.

Behavioural and plasma cortisol changes were recorded for groups of eight Suffolk x Greyface lambs subjected to castration or tail docking using rubber rings with and without local anaesthetic treatment. Immediately after application of the rubber ring, local anaesthetic (2 x 0.2 ml 2% lignocaine) was administered either by needle and syringe or by high-pressure needleless injection into each side of the neck of the scrotum or tail at the site of the ring, or by high pressure needleless injection into the testes before ring application. In other groups, the innervation to the scrotum or tail was disabled by crushing with a powered bloodless castrator just proximal to the ring. Measurements were recorded in groups of control (handled) lambs, with and without local anaesthetic treatment. Application of local anaesthetic by high pressure needleless injection had little effect on either plasma cortisol values or behaviour of control lambs. For castration, application of the bloodless castrator and/or local anaesthetic at the ring site reduced the peak plasma cortisol concentration by 50% (P < or = 0.01), the incidence of active behavioural responses by 80 and 64% (P < or = 0.01) respectively, and the time spent in abnormal postures by 68 and 59% (P < or = 0.01) respectively. Both methods were effective in reducing pain for tail docking [active behavioural responses reduced by more than 80% (P < or = 0.01) and abnormal postures by 56% (P < or = 0.01)], although local anaesthetic was more effective, reducing the peak cortisol by 60% (P < or = 0.01) [crushing by 44% (P > 0.05)]. Injection of local anaesthetic into the testes was less effective than injection into the neck of the scrotum at the site of the ring [reduction in abnormal lying postures (P < or = 0.05), 45 vs 71%, respectively]. The rapid action, effectiveness, and ease of application of these experimental methods may provide the basis for commercially viable methods for reducing the acute pain produced by rubber ring castration and tail docking of lambs.

Effects of acute pain reduction methods on the chronic inflammatory lesions and behaviour of lambs castrated and tail docked with rubber rings at less than two days of age.

Lesions were produced by castration and tail docking of two-day-old Dorset-cross lambs with elastrator rings with (RRla) and without (RR) local anaesthetic or after destruction of the innervation by crushing close to the ring (Brr). The lesions were monitored twice weekly for six weeks and the behaviour of handled controls (H), RR and RRla lambs was recorded for two 3 h periods 10, 20, 31 and 41 days after treatment. There was no significant effect of castration and tail docking, with or without pain reduction methods, on daily liveweight gain. In the Brr lambs, the dead tails were cast 10 days earlier than from RR and RRla lambs. The time taken for the scrotal lesion to reach maximum severity was halved in Brr and RRla lambs, although the maximum severity of the lesion was unaffected by the methods of pain reduction. During the four 6 h behavioural observation periods, RR lambs showed a significant increase in the mean (+/- sd) frequency of foot stamping (RR13 +/- 13; H2 +/- 2.5), tail wagging (61 +/- 26; 15 +/- 6), head turning to the scrotum and inside hind-leg (12 +/- 10; 1 +/- 1). Less abnormal behaviour was found after RRla than after RR treatment. This unexpected finding may be evidence of long-lasting increases in pain sensitivity after an episode of intense acute pain in young animals.

Reduction in pain suffered by lambs at castration.

The acute pain produced by bloodless castrators was studied by comparing the behavioural and plasma cortisol changes in groups (n=8) of 3-week-old Dorset cross lambs after castration with a 22 cm (9") Burdizzo, a new power assisted castrator and by a combined method using a Burdizzo and elastrator ring. The time spent in abnormal postures (52-58 min) and the peak cortisol response (110-120 mmol l(-1)) were similar for the three methods, although the powered castrator produced a more sustained response. The Burdizzo method halved the incidence of active behaviours compared with the powered castrator and combined methods (16 versus 30, 32 counts). Intratesticular local anaesthetic administered 2 min before the Burdizzo castrator and combined method, or intramuscular injection of the non-steroidal anti-inflammatory drug diclofenac, 20 min before the application of a Burdizzo, significantly reduced the peak plasma cortisol response to 80 nmol l(-1). Diclofenac also significantly reduced the time spent trembling or in abnormal postures.

Haemodynamic effects of a sixty minute infusion of dopamine hydrochloride in horses anaesthetised with halothane.

To describe the haemodynamic effects of a 60 min infusion of dopamine 4 microg/kg bwt/min during halothane anaesthesia, 7 mature Thoroughbred horses were studied. The infusion began 1 h after induction of anaesthesia by romifidine (100 microg/kg) and ketamine (2.2 mg/kg bwt). Throughout the period of dopamine infusion and for 30 min after its discontinuation, the horses were ventilated by intermittent positive pressure to maintain PaCO2 between 4.6-5.4 KPa. Inspired halothane concentration was adjusted to maintain an end tidal halothane concentration of 0.9%. Haemodynamic variables were measured using intracardiac strain gauge transducers sited in the left and right ventricle, aorta, and pulmonary artery. Left ventricular pressure was differentiated to obtain maximal rate of increase of intraventricular pressure (LVdp/dtmax). Transoesophageal Doppler echocardiography was performed to measure maximum aortic blood flow velocity (vmax) and acceleration (dv/dtmax), left ventricular velocity time integral (vTI) and cardiac output (CO), and left ventricular pre-ejection period (PEP) and ejection time (ET). Measurements were made during the 60 min infusion, and for 30 min after the infusion was discontinued. Infusion of dopamine 4 microg/kg/min significantly decreased mean aortic pressure, while left and right ventricular end-diastolic pressure and mean pulmonary artery pressure remained unchanged. There was a small, but significant, increase in heart rate during dopamine infusion. Maximum acceleration of aortic blood flow, CO and vTI were also significantly increased by dopamine infusion. Maximal rate of increase of intraventricular pressure (LVdp/dtmax) was significantly decreased 10 min after commencing infusion, but then returned to baseline for the remainder of the study. Left ventricular pre-ejection period (PEP) decreased during dopamine infusion whilst ejection time (ET) significantly increased. All measured variables except LVET had returned to baseline values within 30 min of discontinuing the infusion. This study demonstrated beneficial effects of dopamine infusion upon left ventricular systolic function. However, the therapeutic value of the drug is likely to be limited in clinical anaesthesia due to the simultaneous falls in arterial blood pressure which accompany its administration.

Assessment of acute pain in farm animals using behavioral and physiological measurements.

In this paper various aspects of animal pain and methods for its assessment are considered. The responses of lambs and calves to castration and of lambs to tail docking are used to illustrate quantitative approaches to the recognition and assessment of acute pain in farm animals. the validation of physiological and behavioral measurements for assessment of pain is examined by relating measurements made from young lambs, after a range of treatments, to an independent ranking of the order of severity of the treatments.

Temporal effects of an infusion of dopexamine hydrochloride in horses anesthetized with halothane.

OBJECTIVE: To evaluate the hemodynamic effects of a 60-minute infusion of dopexamine in horses anesthetized with halothane. ANIMALS: 7 adult Thoroughbreds. PROCEDURE: Measurements of left ventricular function obtained by transesophageal Doppler echocardiography and cardiac catheterization. RESULTS: Infusion of dopexamine (4 micrograms/kg of body weight/min) significantly increased heart rate, cardiac output, maximal rates of increase and decrease of left ventricular pressure, and maximal acceleration and maximal velocity of aortic blood flow. Left ventricular ejection time significantly increased, and pre-ejection period decreased during the infusion. Cardiac output, maximal rate of increase of left ventricular pressure, and maximal acceleration continued to increase as the infusion progressed. Right ventricular end-diastolic pressure was significantly decreased after 20 minutes of infusion and decreased progressively throughout the remaining time of infusion. Many hemodynamic variables, including right ventricular end-diastolic pressure, had not returned to control values 30 minutes after the infusion was discontinued. A number of undesirable adverse effects were observed in horses receiving dopexamine infusion; during administration, profuse sweating occurred in every horse. In 6 horses, recovery from anesthesia was associated with excitement and violent shivering. Colic developed in 2 horses within 3 hours of recovery. CONCLUSIONS: Dopexamine (4 micrograms/kg/min) does not achieve a peak effect on many hemodynamic variables within a short period of commencing administration, and the effects of infusion may persist for extended periods after drug administration is discontinued. CLINICAL RELEVANCE: Dopexamine has a hemodynamic profile suited to treatment of low cardiac output in anesthetized horses; however, at the dosage rate studied (4 micrograms/kg/min), its administration was associated with a number of undesirable adverse effects which could preclude its clinical use.