The effect of xylazine on intracranial pressure in anesthetized and standing horses

OBJECTIVE

To determine the effect of xylazine on intracranial pressure (ICP) in standing compared to isoflurane-anesthetized horses.

DESIGN

Prospective, crossover study design.

SETTING

University Teaching Hospital.

ANIMALS

Six adult horses donated to the University. Horses were determined to be healthy via physical examination, complete blood count, and neurological evaluation.

INTERVENTIONS

Horses were anesthetized, maintained on isoflurane in oxygen in left lateral recumbency, and ventilated to normocapnia. Horses were instrumented for intraparenchymal measurement of ICP, invasive blood pressure, pulse oximetry, and end tidal gas analyzer. Xylazine 1 mg/kg was administered IV and ICP, systolic arterial pressure, mean arterial pressure (MAP), diastolic arterial pressure, and heart rate were recorded and cerebral perfusion pressure (CPP) was calculated for the following 15 minutes. Twenty-four to 36 hours following anesthetic recovery, xylazine 1 mg/kg was administered IV and ICP, heart rate, and Doppler blood pressure (BPdop) on the tail were monitored for 15 minutes.

MEASUREMENTS AND MAIN RESULTS

There was a decrease in ICP following administration of xylazine in anesthetized horses (P < 0.003) but not standing horses (P = 0.227). There was an increase in systolic arterial pressure, MAP, diastolic arterial pressure (P < 0.001), and BPdop (P = 0.001) following administration of xylazine. As a result, CPP increased in anesthetized horses (P < 0.03). There was a negative association between ICP and MAP in anesthetized horses (P = 0.007) but not ICP and BPdop conscious horses (P = 0.379).

CONCLUSIONS

Administration of xylazine to anesthetized horses resulted in an increased CPP due to decreased ICP with concurrent increased MAP. Administration of xylazine to standing horses did not result in a change in ICP. However, with the increase in BPdop found in awake horses, it is likely that CPP would also increase in awake horses following xylazine administration.

Transcranial bioimpedance measurement in horses: a pilot study

OBJECTIVE

This pilot study aimed to evaluate the feasibility of transcranial bioimpedance (TCBI) measurement and variability of TCBI values in healthy conscious horses and to study effects of body position and time on TCBI in anaesthetized horses.

STUDY DESIGN

Prospective, observational study.

ANIMALS

A total of four research horses and 16 client-owned horses presented for surgery.

METHODS

After establishing optimal electrode position using computed tomography scans of cadaver heads, TCBI [described using impedance at zero frequency, R_0, (Ω)] was measured in four conscious, resting horses to investigate the feasibility and changes in TCBI over time (80 minutes). Data were compared using a paired t test. TCBI was then measured throughout anaesthesia (duration 92 ± 28 minutes) in 16 horses in dorsal and lateral recumbency. Data were analysed using a general linear model; gamma regression was chosen as a model of characteristic impedance [Z_c; (Ω)] against time. Data are presented as mean ± standard deviation.

RESULTS

No change in R₀ was seen in conscious horses (age = 15.3 ± 7.3 years, body mass = 512 ± 38 kg) over 80 minutes. The technique was well tolerated and caused no apparent adverse effects. In 16 horses (age = 7.4 ± 4.7 years; body mass = 479 ± 134 kg) anaesthetized for 92 ± 28 minutes, Z_c fell during anaesthesia, decreasing more in horses in lateral recumbency than in horses in dorsal recumbency (p = 0.008). There was no relationship between Z_c and body mass or age.

CONCLUSIONS AND CLINICAL RELEVANCE

TCBI is readily measured in horses. TCBI did not change with time in conscious horses, but decreased with time in anaesthetized horses; this change was greater in horses in lateral recumbency, indicating that TCBI changes in anaesthetized horses may be related to the effects of recumbency, general anaesthesia, surgery or a combination of these factors.

Intracranial pressure monitoring in normal dogs using subdural and intraparenchymal miniature strain-gauge transducers

Background

Monitoring of intracranial pressure (ICP) is a critical component in the management of intracranial hypertension. Safety, efficacy, and optimal location of microsensor devices have not been defined in dogs.

Hypothesis/Objective

Assessment of ICP using a microsensor transducer is feasible in anesthetized and conscious animals and is independent of transducer location. Intraparenchymal transducer placement is associated with more adverse effects.

Animals

Seven adult, bred‐for‐research dogs.

Methods

In a prospective investigational study, microsensor ICP transducers were inserted into subdural and intraparenchymal locations at defined rostral or caudal locations within the rostrotentorial compartment under general anesthesia. Mean arterial pressure and ICP were measured continuously during physiological maneuvers, and for 20 hours after anesthesia.

Results

Baseline mean ± SD values for ICP and cerebral perfusion pressure were 7.2 ± 2.3 and 78.9 ± 7.6 mm Hg, respectively. Catheter position did not have a significant effect on ICP measurements. There was significant variation from baseline ICP accompanying physiological maneuvers (P < .001) and with normal activities, especially with changes in head position (P < .001). Pathological sequelae were more evident after intraparenchymal versus subdural placement.

Conclusions and Clinical Importance

Use of a microsensor ICP transducer was technically straightforward and provided ICP measurements within previously reported reference ranges. Results support the use of an accessible dorsal location and subdural positioning. Transient fluctuations in ICP are normal events in conscious dogs and large variations associated with head position should be accounted for when evaluating animals with intracranial hypertension.

Equine anaesthesia-associated mortality: where are we now?

OBJECTIVES

To review the literature concerning mortality associated with general anaesthesia in horses and to assess whether there is evidence for a reduction in mortality over the 20 years since the Confidential Enquiry into Perioperative Equine Fatalities (CEPEF).

DATABASES USED

PubMed, Scopus, Google Scholar. Search terms used: horse; pony; equine; anaesthesia; anesthesia; recovery; morbidity, and mortality.

CONCLUSIONS

The most recent studies, in which isoflurane and sevoflurane have been more commonly used for anaesthesia maintenance, report fewer intraoperative cardiac arrests than older studies in which halothane was favoured. Catastrophic fractures, however, have become the greatest cause of recovery-associated mortality.

Inhaled anesthetics in horses

Inhaled agents represent an important and useful class of drugs for equine anesthesia. This article reviews the ether-type anesthetics in contemporary use, their uptake and elimination, their mechanisms of action, and their desirable and undesirable effects in horses.

The effects of prolonged endurance exercise on the neurological system in horses

Horses compete routinely in endurance-type activities. Many of the various pathophysiological mechanisms which arise during endurance exercise have implications for the health and function of the neurological system. The development of centrally-mediated fatigue is a normal homeostatic physiological event with several possible mechanisms. Development of pathophysiological phenomena such as cerebral oedema may be near-terminal events during or after endurance exhaustion. Cellular damage resulting in cytotoxic cerebral oedema may result from decreases in circulating blood volume (dehydration), blood pressure, oxygen, and glucose, or increases in brain temperature. Vasogenic cerebral oedema arises from changes in cerebral vascular perfusion, tone, and permeability. Increased vascular permeability results from increased brain temperature, poor vascular integrity due to severe dehydration, disseminated intravascular coagulation due to hemoconcentration or endotoxemia, and iatrogenic overhydration during therapy. Clinical signs of intracranial disease after endurance exercise include staggering, shaking, ataxia, paresis, poor tongue tone, facial twitching, collapse, recumbency, seizures, and death. Treatment should include active and aggressive cooling, intravenous polyionic fluids, acid-base imbalance correction, intravenous glucose and calcium supplementation, non-steroidal anti-inflammatory agents once the patient is better hydrated, intra-nasal oxygen therapy if practicable, and achievement of a non-dependent head posture to prevent jugular venous hypertension and further increases in intracranial pressure. The prognosis for central fatigue is good with appropriate supportive care, but the prognosis for successful treatment of cerebral oedema must be considered guarded at best. Prevention is critical and must be through incorporation of mandatory rest stops with sufficient length and veterinary monitoring to allow prevention and detection of exhaustion, excessive dehydration, and neurological signs. Management flexibility in shortening or postponing rides in hot and humid conditions, mandated use of aggressive cooling techniques, and more restrictive entry criteria for upper level Fédération Equestre Internationale races should all be considered as viable options for optimising the safety of endurance horses.