Electrocardiographic evaluation of two anesthetic combinations in dogs

Avaliação eletrocardiográfica de duas associações anestésicas em cães

Electrocardiographic evaluation of two anesthetic combinations in dogs

[Avaliação eletrocardiográfica de duas associações anestésicas em cães]

K.M. Tárraga^1, H.S. Spinosa^2, A.A. Camacho^3*

1Faculdade de Medicina Veterinária - UNG

²Faculdade de Medicina Veterinária e Zootecnia da USP

³Faculdade de Ciências Agrárias e Veterinárias – UNESP

Rodovia Carlos Tonanni, km 5

14870-000 – Jaboticabal, SP

Recebido para publicação em 2 de fevereiro de 1999.

Trabalho parcialmente financiado pela Fundação Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES

(*)Autor para correspondência

E-mail: [e-mail]

ABSTRACT

This study aimed to investigate electrocardiographic changes in dogs aged 5 years or more submitted to two anesthetic combinations: atropine, levomeprazine, thiopental and halothane (ALTH), and atropine, tiletamine and zolazepam (ATZ). Forty dogs (24 males/16 females) weighing 5-24kg, were used. Dogs had no cardiac problems and were submitted to tartarectomy. All animals were submitted to two electrocardiograms (ECG), one before anesthesia and other immediately before surgery. The dogs were divided into two groups: group 1 received ALTH and group 2 received ATZ. Alterations in the ST segment, T wave, cardiac rhythm and a significant reduction of vagal tonus index were observed in both groups, but in group 2 a significant reduction of the PR and QT intervals and an increase in heart rate were also observed. These data suggest that the ALTH combination caused fewer changes in the ECG than the ATZ combination.

Keywords: Dog, electrocardiogram, anaesthesia, tartarectomy

RESUMO

Este trabalho teve o objetivo de avaliar as possíveis alterações eletrocardiográficas de cães submetidos a duas condutas anestésicas, indicando aquela que produz menores alterações no eletrocardiograma (ECG). Foram utilizados 40 cães com idade igual ou superior a cinco anos, 24 machos e 16 fêmeas, com peso entre 5 e 42kg, oriundos de clínicas particulares da cidade de São Paulo e do canil da Polícia Militar do Estado de São Paulo, sem sintomas cardiovasculares e triados para tartarectomia. Todos os cães foram submetidos a dois ECG: um antes da anestesia e outro imediatamente antes de se iniciar o procedimento cirúrgico. As características eletrocardiográficas avaliadas foram: ritmo, duração da onda P, do complexo QRS e dos intervalos PR e QT, amplitude das ondas P e R e alterações do segmento ST e da onda T; avaliou-se também o índice de tono vagal (ITV). Os cães foram divididos em: grupo 1, que recebeu atropina, levomepromazina, tiopental e halotano (ALTH) e grupo 2, que recebeu atropina, tiletamina e zolazepam (ATZ). Os resultados mostraram que a associação ALTH produziu algumas alterações discretas no ECG dos cães avaliados, caracterizadas por modificações qualitativas no segmento ST, na onda T e no ritmo cardíaco, reduzindo significativamente o ITV, com tendência à bradicardia e hipóxia. Já a associação ATZ, além das alterações já citadas, produziu diminuição da duração dos intervalos PR e QT, bem como aumentou a freqüência cardíaca, com tendência à taquicardia e hipóxia.

Palavras-Chave: Cão, eletrocardiograma, anestesia, tartarectomia

INTRODUCTION

Electrocardiography (ECG) has been increasingly used as a complementary examination in veterinary diagnosis. Over the last 20 years it has become the most used examination in both human and veterinary cardiology, not only for the diagnosis of arrhythmia but also for the preoperative evaluation of patients (Miller & Tilley, 1988; Tilley, 1992).

The preoperative evaluation of cardiac activity is very important in patients with signs and symptoms of fatigue, coughing and dyspnea, suggesting congestive cardiac insufficiency. Thus, the ECG is important for the definition of the appropriate preanesthetic and/or anesthetic to be used in these cases. In addition, cardiac monitoring during surgery offers rapid and safe information about the alterations that may appear during the surgical procedure, such as arrhythmias, ischemia and electrolytic disturbances that can affect cardiac function (Edwards, 1987; Mason & Hubbell, 1988; Haskins, 1992).

Currently available literature data indicate that preanesthetic and/or anesthetic drugs may cause alterations in cardiac function. However, there is a lack of information about the effects of the combinations of these agents on the electrocardiographic tracing, particularly in elderly animals.

Information obtained from private veterinary clinic which request the electrocardiographic services of PROVET (Instituto Brasileiro de Diagnóstico e Especialidades Veterinárias) for the evaluation of dogs to be submitted to anesthesia indicates that the two combinations most used by veterinarians are atropine, levomeprazine, thiopental and halothane (ALTH) and atropine, tiletamine and zolazepam (ATZ). Thus, the objective of the present study was to evaluate the effect of administration of these two anesthetic combinations on the ECG.

MATERIAL AND METHODS

The subjects of this study were healthy male and female dogs from private veterinary clinics in the city of São Paulo and from the São Paulo State Police. All animals had a clinical indication for the extraction of tartar from their teeth (tartarectomy) and were clinically evaluated in order to exclude those with symptoms such as coughing, fatigue or dyspnea, and those taking any drug that could alter the ECG or interact with the anesthesia. The selected dogs were divided into two groups: group 1 received atropine, levomepromazine, thiopental and halotane (ALTH) and group 2 received atropine, tiletamine and zolazepam (ATZ). Group 1 consisted of 20 dogs of different breeds (5 mongrels, 4 miniature Poodles, 3 miniature Dachsunds, 2 Basset Hounds, 2 miniature Pinschers, 2 English Cocker Spaniels, 1 miniature Schnauzer, and 1 German Shepherd) all from private practice clinics - 12 males and 8 females aged 5 to 15 years (mean 10.0 years) and weighing 5 to 25kg (mean ± SD = 10.3± 6.4kg). Group 2 consisted of 20 animals of different breeds (12 German Shepherds, 3 English Cocker Spaniels, 2 Labrador Retrievers, 1 Springer Spaniel, and 1 Irish Setter) from private practice clinics and from the State Police - 12 males and 8 females aged 6 to 14 years (mean = 7.0 years), and weighing 12 to 42kg (mean ± SD = 28.8± 10.4kg). The animals were deprived of food and water for 12 hours before anesthesia.

The following drugs were used: atropine sulfate (Apsen); levomepromazine (Neozine^â Rhodia Farma Ltda), thiopental (Thionembutal^â , Abbott Labs.), halothane (Fluothane^â , Wellcome-Zeneca Labs.), and tiletamine + zolazepam (Zoletil^â , Virbac).

After screening for tartarectomy by their own veterinarians, the animals were referred to the electrocardiographic service, where they were submitted to specific evaluation of their cardiovascular system, with a preanesthetic ECG and other immediately before the beginning of tartarectomy. In both cases the dogs were placed on a surface that did not conduct electricity (a rubber mat) in right lateral decubitus. The electrodes were placed directly on the skin of the animal, one on each foreleg close to the distal region of the olecranon and on each hindleg close to the cranial region of the patella. In order to obtain better electric conductivity, 96% alcohol was applied to the skin where the electrode was placed.

The electrocardiographic parameters considered were: rhythm, heart rate (HR), duration of the P wave, of the PR and QT intervals and of the QRS complex, amplitude of the P and R waves, and evaluation of the ST segment and of the T wave. The classification of rhythm 9 was carried out establishing the criteria for the systematic analysis of the electrocardiographic tracings of the dogs, where the normal rhythm is denoted normal sinusal or sinus arrhythmia and altered when there are arrhythmias (bradyrrhythmia and tachyrrhythmia) related to the origin and conductivity of the stimuli in the heart (supraventricular and ventricular arrhythmia and atrioventricular blocks). HR was calculated in the D II lead, counting the number of QRS complexes in three seconds at the speed of 50 mm/s, and multiplying by 20 in order to obtain the number of beats per minutes (bpm) (Cohen & Tilley, 1979; Edwards, 1987; Tilley, 1992). All the wave and interval measurements were also obtained from the D II lead, recorded at 50mm/s, in sensitivity where 0.1mV is equivalent to 1 mm. The ST segment was evaluated considering that supra or infra-disleveled in 0.1mV is normal (Cohen & Tilley, 1979; Edwards, 1987; Tilley, 1992). The T wave was classified according to its polarity, being considered positive, negative or isoelectric and smaller than 25% of QRS. The presence or absence of polarity inversion between the first and the second ECG was also recorded. Alterations of the T wave associated with alterations in the ST segment were considered to be indicators of the presence of myocardial hypoxia (Edwards, 1987; Tilley, 1992).

The Neperian logarithm (nl) of the variance of a minimum of 10 RR intervals was used to determine the vagal tone index (VTI). This was done in order to quantify HR variability, which describes small oscillations between each heart beat (Latson & O'Flaherty, 1993). The calculation was done for each animal before and after anesthesia, counting 10 RR intervals in seconds (s) and calculating the mean, the variance and the nl.

All dogs where submitted to electrocardiographic evaluation followed by anesthesia, which in group 1 allowed to establish general anesthesia and evaluate the classic anesthetic Guedel plans, while in group 2 the anesthetic combination was of the dissociative type.

Group 1 received atropine sulfate (0.044 mg/kg, SC) and after 15 minutes levomeprazine (1 mg/kg, IV). Fifteen minutes later, thiopental (10 mg/kg) was slowly administered IV until the interdigital and palpebral reflexes disappeared (Guedel‘s plan 2 of Stage III). The animal’s trachea was then intubated and anesthesia was maintained with 2% halothane.

The anesthetic protocol for group 2 was as follows: atropine sulfate (0.044 mg/kg, SC), followed by tiletamine + zolazepam (6 mg/kg of each active principle, IM) 15 minutes later.

A new ECG was obtained for both groups after anesthesia was established and before the procedure and compared to the first

Data were analyzed statistically using the GraphPad Instat SoftwareÔ 2.01 (1990 – 1993). Analysis of variance followed by the Tukey-Kramer multiple comparisons test was used to analyze the parametric data. The level of significance was set at P<0.05 for all statistical analyses.

RESULTS

The ALTH anesthetic combination had no effect on the different parameters of the electrocardiographic tracings (Table 1), whereas the ATZ combination significantly reduced the duration of the PR and QT intervals and the VTI, with a consequent increase in HR.

The groups did not differ significantly regarding the morphology of the ST segment, the T wave and the cardiac rhythm (Table 2). Both anesthetic combinations significantly reduced the mean VTI value (85% of the animals in group 1 and 90% in group 2).

The ATZ combination apparently caused more evident alterations in the qualitative tracings of the dogs evaluated, particularly the T wave and the cardiac rhythm.

DISCUSSION

Subject homogeneity within and between groups was not ideal due to the varied origins of the animals. On the other hand, this condition is more similar to that encountered by the professional in his daily routine. The dogs used were screened by age, with all animals being five years old or older. This age group was chosen because it is the age when a preventive ECG for cardiac disease in dogs is recommended, as well as for the cardiologic evaluation of dogs that are going to be submitted to anesthesia (Edwards, 1987). Another criterion used to select the dogs was the surgical indication for tartarectomy, which is carried out in dogs at this age as a preventive measure against bacterial endocarditis (R.L. Hamlin, personal communication).

The ALTH combination did not cause significant alterations in the duration of the P wave, the PR and QT intervals and the QRS complex or in the amplitude of the P and R waves of the dogs. With this combination, the use of atropine is recommended in order to reduce the arrhythmogenic effects of thiopental (Bertram, 1980) and halothane (Nishikawa et al., 1989). It is known that atropine, used as pre-anesthetic medication (Cohen & Tilley, 1979), may cause sinus tachycardia (Muir, 1977) due to the reduction of the vagal activity on the heart with a consequent increase of the tonus of the sympathetic autonomic nervous system leading to arrhythmias (Muir, 1978). These characteristic effects of atropine were not observed in group 1, probably due to the concomitant use of levomepromazine which induces low blood pressure and bradycardia blocking the alpha-adrenergic receptors (Cohen & Tilley, 1979; Bertram, 1980; Muir & Hubbell, 1989). These effects of levomepromazine, allied to those already mentioned for atropine, may have been responsible for the absence of alterations in the different electrocardiographic parameters observed in this study.

The animals in group 1 presented a significant reduction of the VTI, which, however, was not sufficient to induce tachycardia (that is, predominance of the tonus of the sympathetic autonomic nervous system). However, in group 2, the reduction in VTI showed a decrease of the activity of the parasympathetic autonomic nervous system, and led to tachycardia (Malliani et al., 1994) as a result of the predominance of the sympathetic over the parasympathetic autonomic nervous system, probably due to the use of atropine or tiletamine (Stepien, 1995).

Analysis of the ST segment and of the T wave of the ECG may suggest the occurrence of alterations of ventricular repolarization due to myocardial hypoxia (Tilley, 1992). This may have occurred in group 1, where alteration of the ST segment and of the T wave was observed in 25% and 70% of the dogs, respectively, after administration of the ALTH anesthetic combination.

In group 2 atropine was used as pre-anesthetic medication because its anticholinergic effect reduces the excessive saliva secretion, which occurs when using dissociative anesthetic agents, and zolazepam was used to reduce the muscular rigidity caused by tiletamine (Muir & Hubbell, 1989); it should also be pointed out that benzodiazepines are practically free of cardiovascular effects (Hellyer, 1992; Stepien, 1995). The tiletamine + zolazepam combination is recommended for several surgical procedures, particularly those of short duration, such as dental interventions (Forest, 1991). In this group the administration of the anesthetic combination did not cause significant alterations in the duration of the P wave and the QRS complex, or in the amplitude of the P and R waves. On the other hand, there was a significant reduction in the duration of the PR and QT intervals probably due to an increase in HR caused by atropine and/or tiletamine (Muir & Hubbell, 1989; Stepien, 1995). The reduction of these intervals reflects an increase of the conductivity of the cardiac stimuli between the sinus knot and the atrioventricular knot (PR interval) as well as in the ventricles (QT interval) (Forest, 1991; Tilley, 1992). A significant decrease in VTI and a significant increase in HR were also observed.

In group 2, 25% of the dogs presented an alteration of the ST segment and 40% an alteration of the T wave, suggesting the occurrence of myocardial hypoxia. This could be attributed to the increase in HR, which reduces the diastolic period and coronary irrigation, consequently reducing the oxygen supplied to the myocardium (Smith Jr. & Hadlock, 1995).

When the electrocardiographic parameters of the animals in groups 1 and 2 were compared before and after the administration of the anesthetic combinations, significant alterations were observed in the duration of the PR and QT intervals, in HR and VTI.

After the administration of the anesthetic combination, HR was lower in group 1 than in group 2. This fact suggests a higher influence of the activity of the parasympathetic autonomic nervous system in group 1, where, despite the use of atropine, the effects of levopremazine, thiopental, and halothane predominated on cardiac activity. In group 2 there was a greater influence of the activity of the sympathetic autonomic nervous system, with predominance of the effects of atropine and tiletamine on cardiac activity. The VTI results confirm this hypothesis, since VTI was higher in group 1 than in group 2 after administration of the anesthetic combination.

These data, taken together, show that the ALTH anesthetic combination promotes fewer alterations of the electrocardiographic tracings of five-year-old-dogs of age or older than the ATZ anesthetic combination. However, when choosing one of the combinations the clinical condition of the patient must be taken into account.

CONCLUSION

The ALTH anesthetic combination caused discrete alterations of the electrocardiographic tracing five-year-old dogs or older, characterized by qualitative modifications of the ST segment, T wave and cardiac rhythm, and by a significant reduction of the VTI, with a tendency to myocardial hypoxia and bradycardia. The ATZ combination caused alterations of the ST segment, T wave, and cardiac rhythm and of the VTI. It also caused a shorter duration of the PR and QT intervals and an increase in heart rate, with a tendency to myocardial hypoxia and tachycardia.

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Publication Dates

• Publication in this collection
  14 Aug 2000
• Date of issue
  Apr 2000

History

• Received
  02 Feb 1999