The use of atropine to control heart rate responses during detomidine sedation in horses

Physiologic effects of three different protocols of isoflurane anesthesia in captive brown brocket deer (Mazama gouazoubira)

This study evaluates the cardiorespiratory and physiologic effects of three different protocols of chemical restraint using isoflurane in captive brown brocket deer (Mazama gouazoubira). Six adult deer, two males and four females, aged 3-6 yr old and weighing 16.3 +/- 1.5 kg (mean +/- SD), were used. The deer were physically restrained and anesthetized using one of three protocols: protocol 1 (P1), anesthesia was induced directly with isoflurane using a facemask and maintained for 1 hr with isoflurane delivered by endotracheal tube; protocol 2 (P2), oral premedication with midazolam was followed 1 hr later by induction and maintenance of anesthesia with isoflurane, as in P1; and protocol 3 (P3), intravenous anesthesia using a combination of ketamine, xylazine, and atropine was followed by isoflurane anesthesia, as in P1. Cardiorespiratory variables were recorded during physical restraint (TO); 5 min after intubation (T5); and every 10 min during anesthesia (T15-T55). Venous and arterial blood gas analysis was conducted at TO (venous blood only), T15, T35, and T55. Data were analyzed by the appropriate statistical tests. P values < 0.05 were considered significant. Under P1, three deer regurgitated during induction and one of them died because of aspiration pneumonia. The end-tidal isoflurane concentration differed significantly between P1, P2, and P3 (2.6 +/- 0.2, 2.3 +/- 0.5, and 0.9 +/- 0.3%, respectively). Hypotension was observed during anesthesia (T15-T55). Respiratory acidosis occurred at all times and protocols, and pH was significantly lower in P1 compared to P2 and P3. Metabolic acidosis was also observed following physical restraint (T0-T35) in all protocols. The use of isoflurane via facemask for anesthesia induction is not a safe method in deer that are not premedicated. Isoflurane used alone or in combination with midazolam and ketamine-xylazine-atropine combination causes hypotension and dose-dependent respiratory acidosis.

A review of the physiological effects of alpha2-agonists related to the clinical use of medetomidine in small animal practice

Medetomidine is a relatively new sedative analgesic drug that is approved for use in dogs in Canada. It is the most potent alpha2-adrenoreceptor available for clinical use in veterinary medicine and stimulates receptors centrally to produce dose-dependent sedation and analgesia. Significant dose sparing properties occur when medetomidine is combined with other anesthetic agents correlating with the high affinity of this drug to the alpha2-adrenoreceptor. Hypoventilation occurs with medetomidine sedation in dogs; however, respiratory depression becomes most significant when given in combination with other sedative or injectable agents. The typical negative cardiovascular effects produced with other alpha2-agonists (bradycardia, bradyarrhythmias, a reduction in cardiac output, hypertension +/- hypotension) are also produced with medetomidine, warranting precautions when it is used and necessitating appropriate patient selection (young, middle-aged healthy animals). While hypotension may occur, sedative doses of medetomidine typically raise the blood pressure, due to the effect on peripheral alpha2-adrenoreceptors. Anticholinergic premedication has been recommended with alpha2-agonists to prevent bradyarrhythmias and, potentially, the reduction in cardiac output produced by these agents; however, current research does not demonstrate a clear improvement in cardiovascular function. Negatively, the anticholinergic induced increase in heart rate potentiates the alpha2-agonist mediated hypertension and may increase myocardial oxygen tension, demand, and workload. Overall, reversal with the specific antagonist atipamezole is recommended when significant cardiorespiratory complications occur. Other physiological effects of medetomidine sedation include; vomiting, increased urine volumes, changes to endocrine function and uterine activity, decreased intestinal motility, decreased intraocular pressure and potentially hypothermia, muscle twitching, and cyanosis. Decreased doses of medetomidine, compared with the recommended label dose, should be considered in combination with other sedatives to enhance sedation and analgesia and lower the duration and potential severity of the negative cardiovascular side effects. The literature was searched in Pubmed, Medline, Agricola, CAB direct, and Biological Sciences.

Implantation of a dual-chamber, rate-adaptive pacemaker in a horse with suspected sick sinus syndrome

A five-year-old gelding suffered syncope at the end of a period of exercise. A 24-hour electrocardiogram recording revealed intermittent pauses in the sinus rhythm of up to 10 seconds, indicating sinus node disease; the pauses occurred repeatedly, particularly after exercise. A dual-chamber, rate-adaptive pacemaker was successfully implanted, which prevented excessive postexercise bradycardia and syncope, and allowed the horse to return to work.

Alpha 2 adrenoceptor agonists in the horse--a review

In recent years the usefulness of the alpha 2 adrenoceptor agonist drugs has been recognized in equine practice. Several agents have become available and are now licensed for use in a number of countries. The principle actions of all alpha 2 adrenoceptor agonists are similar, in that they produce a reduction in heart rate and alteration of heart rhythm, an initial hypertension followed by a prolonged hypotension, a decrease in the cardiac output and respiratory depression. For clinical purposes, these agents produce sedation and analgesia, they are useful for premedication and markedly potentiate the effects of other sedative/analgesic agents. Differences in receptor specificity between the alpha 2 adrenoceptor agonists results in the distinguishing characteristics of the individual agents, particularly with respect to their duration of action, sedative effect and analgesic properties; their cardiopulmonary effects are however similar, when equipotent sedative doses are administered. When used in combination with other agents, the alpha 2 adrenoceptor agonists all appear to act in a similar manner, with the greatest difference being related to their duration of action.

Cardiopulmonary effects of combinations of medetomidine hydrochloride and atropine sulphate in dogs

Medetomidine and xylazine are alpha 2 adrenoceptor agonists which are used as sedatives and premedicants in small animals. However, bradycardia is a side effect and the use of atropine sulphate has been recommended to counteract it. This study investigated the effects of combining medetomidine (40 micrograms/kg) and atropine (30 micrograms/kg) on the cardiopulmonary function of six dogs. Medetomidine administered alone caused severe bradycardia, but hypertension was mild and transient. Medetomidine and atropine administered together caused an initial bradycardia, but within 15 minutes there was tachycardia accompanied by a mean arterial blood pressure of 210 mm Hg. When atropine was administered 30 minutes before medetomidine, tachycardia and hypertension were observed within five minutes of the medetomidine injection. Thus, although atropine will counteract medetomidine-induced bradycardia, its use results in prolonged and severe hypertension, in association with the tachycardia. Although atropine may be life-saving when bradycardia is profound, its indiscriminate use in combination with alpha 2 adrenoceptor agonists may be disadvantageous.

Effects of sedation with detomidine hydrochloride on echocardiographic measurements of cardiac dimensions and indices of cardiac function in horses

Twenty-six horses were sedated with detomidine hydrochloride (Domosedan, SmithKline Beecham Animal Health, Tadworth, Surrey, UK) at a dose of 10 micrograms/kg bwt, administered i.v. Echocardiograms were recorded before and after sedation, measurements of cardiac dimensions were made and functional indices were calculated. The pre- and post sedation values were compared. No significant change was detected following sedation in the ejection time (ET), left ventricular dimension, left atrial diameter at the sinotubular junction at end-systole or end-diastole or at the sinus of Valsalva at end-diastole. Sedation was found to result in a statistically significant increase in end-systolic left ventricular diameter and internal area and aortic diameter measured at the sinus of Valsalva. Left ventricular free-wall thickness and interventricular septal thickness at end-systole, fractional shortening, fractional area change and heart rate were significantly reduced following sedation. A significant increase in pre-ejection period (PEP), duration of electromechanical systole (EMS) and PEP/ET (ejection time) was detected.

Clinical effects of detomidine with or without atropine used for arthrocentesis in horses

The effectiveness of detomidine with or without atropine sulfate premedication in producing sedation and analgesia for arthrocentesis was studied in 12 horses. The effects were evaluated by monitoring heart and respiratory rates, borborygmi, distance from the lower lip to the floor, systolic blood pressure, and response to needle insertion. Either atropine or saline (as a placebo) was administered immediately prior to detomidine. All drugs were administered intravenously. Measurements were taken prior to drug injection and at 1, 5, 10, 15, 20, 25, 30, 40, 50, 60, 120, 180 and 240 minutes postinjection. Detomidine with atropine resulted in significantly higher heart rates than detomidine without atropine for the three hours of observation. Borborygmi were significantly decreased for four hours following detomidine with atropine and for three hours following detomidine without atropine, when compared to preinjection levels. Systolic blood pressure was significantly increased for 15 minutes following detomidine and atropine compared to the preinjection level. The head was markedly lowered for 60 minutes with either treatment. Atropine prevented the bradyarrhythmia and bradycardia induced by detomidine, but it induced a tachycardia. A satisfactory response for needle insertion and adequate synovial fluid aspiration was achieved in 95% of the trials with detomidine, with or without atropine sulfate premedication. The results suggest that, although atropine prevents bradyarrhythmia and bradycardia following detomidine, administering detomidine without atropine is satisfactory for arthrocentesis in untrained horses.

A preliminary study on the effects of atropine sulphate on bradycardia and heart blocks during romifidine sedation in the horse

Romifidine (STH 2130-Cl or Sedivet) is an alpha 2-agonistic imino-imidazol sedative for intravenous use in horses recently developed by Boehringer Ingelheim, Vetmedica GmbH. An exploratory study was done in nine warm-blood horses, randomly divided into three groups, which received different dosages of romifidine (0.04, 0.08 and 0.12 mg/kg of body weight (BWT) intravenously (i.v.)) with at least one week's interval between tests. Romifidine induced a marked bradycardia accompanied by second degree atrioventricular (AV) block and some sinus blocks at all tested dosages. A placebo (NaCl 0.9% i.v.) given 5 min before and after romifidine did not affect the cardiac disturbances induced by romifidine. A low dose of atropine sulphate (0.005 mg/kg of BWT i.v.) given 5 min before romifidine counteracted the bradycardia and caused a normal to increased heart rhythm at all romifidine dosages. A higher dose of atropine sulphate (0.01 mg/kg of BWT i.v.) administered 5 min before sedation induced a tachycardia (average 70 beats/min) at all romifidine dosages and completely prevented the bradycardia and the heart blocks. The positive chronotrope effects of atropine sulphate were attenuated by increasing doses of romifidine. The effects of atropine sulphate (low or high doses) given 5 min after romifidine only appeared after 5 min. Both dosages counteracted the bradycardia and suppressed the heart blocks. No atropine-dependent side effects were observed in non-fasted horses. The degree of the romifidine induced sedation was not affected by the use of atropine sulphate given before or after romifidine.

Haemodynamic changes during sedation in ponies

The cardiovascular changes induced by several sedatives were investigated in five ponies with a subcutaneously transposed carotid artery by means of cardiac output determinations (thermodilution technique), systemic and pulmonary artery pressure measurements (direct intravascular method) and arterial blood analysis (blood gases and packed cell volume). The cardiovascular depression (decrease in systemic blood pressure and cardiac output) was long lasting (greater than 90 min) after administration of propionylpromazine (0.08 mg/kg intravenous (i.v.)) together with promethazine (0.08 mg/kg i.v.). The phenothiazine-induced sedation was not optimal. alpha 2-Agonists (xylazine (0.60 mg/kg i.v.) and detomidine (20 micrograms/kg i.v.)) induced initial but transient cardiovascular effects with an increase in systemic blood pressure and a decrease in cardiac output for about 15 min. Second degree atrioventricular blocks and bradycardia were seen during this period. The cardiovascular depression was more pronounced during detomidine sedation. Atropine (0.01 mg/kg i.v.) induced a tachycardia with a decrease in stroke volume but did not alter the cardiac output or other cardiovascular parameters. It prevented the occurrence of the bradycardia and heart blocks normally induced by xylazine or detomidine. Atropine potentiated the initial hypertension induced by the alpha 2-agonistic sedatives (especially detomidine). The decrease in cardiac output induced by xylazine, and to a lesser extent by detomidine, was partially counteracted when atropine was given in advance. The atropine-xylazine combination seemed the best premedication protocol before general anaesthesia as it only resulted in minor and transient cardiovascular changes.

Cardiovascular effects of detomidine, a new alpha 2-adrenoceptor agonist, in the conscious pony

The cardiovascular effects of detomidine and xylazine were compared in six chronically instrumented, conscious ponies. Ponies were instrumented with a micromanometer in the left ventricular chamber, a Doppler flow probe on a coronary artery and sonomicrometer crystals in the left ventricular free wall. Heart rate, ventricular systolic pressure, stroke work, dP/dtmax, minute work and coronary blood flow were measured for 4 h following intravenous injection of detomidine at several doses or xylazine at 1.1 mg/kg. Both drugs caused a profound hypertensive response at 15 s post-injection. The magnitude of the pressure change did not increase with detomidine doses greater than 20 micrograms/kg. There was a dose-dependent effect on the duration of the hypertension. Bradycardia and A-V blockade of similar magnitude followed the hypertension at all drug doses. Both drugs caused a negative inotropic effect on the heart at all doses. Minute work, a mechanical index of myocardial O2 demand, and coronary flow decreased to a similar extent following all drug treatments. With the exception of a greater hypertensive response, detomidine at the dosages studied, produced cardiovascular effects that were very similar to those of the recommended dosage of xylazine.