- 30 - THE PERMANENT PACEMAKER SYSTEM FOR THE TREATMENT OF HEART BLOCK IN THE DOG J. N. Lucke - Department of Veterinary Surqery, University of Bristol, Lanqford House, Lanqford, Bristol -- I IGTRODUCT I ON Cardiac conduction defects occur not infrequently in dogs and present with varying clinical signs depending on the severity. Excessive autonomic influence on the sinoatrial pathway is usually Of no clinical significance and can be overcome by parasympathetic blockers such as atropine or just exercise alone. It is usual to classify blocks in atrioventricular conduction into three categories based on the ECG examination:- Eirst deqree (partial atrioventricular block) where there is normal rhythm but the P-R interval is prolonged - greater than 0.13 sec. Second deqree which is similar but periodically a ventricular beat is dropped and this may be associated with either an increasing P-R interval (Wenckebach) or an unchanged P-R interval (Mobitz). In advanced cases the ventricles respond to every 3rd or 4th P wave and there is incomplete atrioventricular dissociation. Third deqree (complete heart block) implies that there is no conduction through the atrioventricular node so that the ventricles beat independantly of higher control at the slow rate of about 40 per min. Many factors contribute to conduction defects including enlargement and fibrosis of the atria, hypoxia and metabolic disturbances, congenital defects and myocardial infections, but often no specific
- 31 - cause can be established. Advanced second degree and third degree heart block will produce clinical signs which include lethargy, reduced exercise tolerance, syncope and overt heart failure. Although mild cases may be treated by correcting predisposing factors and controlling excessive vagal tone with atropine, and congestive heart failure can be treated with diuretics and beta-sympathetic agonists, the only really effective treatment for complete or advanced second degree block is to electrically stimulate ventricular contraction using an electronic pacemaker. CASE REPORTS During 1980 two yellow Labrador dogs were presented to the University of Bristol Veterinary School with heart block and were fitted with pacemakers. Case 1 was 20 months old and had a history of a slight cough for 10 months. The dog, however, had been active, had a good appetite and had grown satisfactorily to 27 kg but it started to lose condition 6 weeks before presentation to the Department of Veterinary Medicine and had developed an ascites which had been treated successfully with frusemide (Lasix). The dog's heart rate was 50 per min, there was no evidence of P waves on the ECG and there was no increase in heart rate after intravenous atropine. Despite the signs of congestive heart failure and a fixed heart rate the dog was still bright and active. Case 2 was 8% years old and had had a history of 'seizures not typical of epilepsy' on four occasions during its life. During the two days before the animal was examined by the veterinary surgeon there had been several attacks with loss of balance and collapse. The heart rate was 58 per min and complete heart block was diagnosed by ECG.
- 32 - Like the previous case, the dog was alert and boisterous and as long as it was kept in the hospital kennel there were no further syncopal attacks. TREATMENT Anaesthesia. On all occasions the dogs were premedicated with 0.03 mg acepromazine and 1.5 mg pethidine per kg body weight intramuscularly 30-40 mins before induction of anaesthesia with methohexitone by slow intravenous injection at a dose rate of approximately 4 mg per kg. After endotracheal intubation the dogs spontaneously breathed nitrous oxide and oxygen (2 : 1) through a 'to and fro' rebreathing system and increments of 25-50 mg methohexitone were given to maintain a light level of anaesthesia. Intermittent positive pressure ventilation was started by hand at the first sign of respiratory depression. Both dogs recovered consciousness within 25 mins of the end of anaesthesia but it was found that further sedation with acepromazine, diazepam and pethidine was necessary for two days after the operation. Implantation of the pacemaker. The left external jugular vein was exposed surgically and the electrode lead was passed into the right atrium and through the atrioventricular valve under fluoroscopic control. Contact was made with the wall of the right ventricle near the apex and the electrode was anchored by winding out the coiled wire spring probe into the myocardium. Pacing was started using an external pacemaker and thresholds were checked while the subcutaneous pocket was created cranial to the presternal notch to take the permanent pacemaker. The function of the internal pacemaker was checked when the electrode lead had been attached and again when it was placed in its pocket, and the wound was closed in the usual way. On the first occasion, the young dog made a good recovery but returned home after just four days where excessive boisterous activity
- 33 - led to detachment of the electrode from the ventricular wall. The dog quickly became lethargic and unwell and there was a variable discharge from the pacemaker. A radiograph showed that the pacemaker had twisted several times in its pocket pulling the electrode back into the right atrium. The dog was treated for two days with frusemide and a new electrode lead was placed under general anaesthesia without difficulty. Care was taken to ensure that there was sufficient slack in the electrode lead in the right atrium and central veins and that the pacemaker was secured in the pocket by layers of subcutaneous sutures. The first case made an uneventful recovery after the second electrode had been placed and is well 14 months later. Unfortunately the excessively active and amorous nature of the young dog has meant that castration was considered necessary. The surgery was straightforward and there were no untoward cardiovascular effects during anaesthesia. The second case made a good recovery but was kept in the hospital for 12 days after placement of the pacemaker. Four months later this older dog has been reported to be well and like a puppy again. DISCUSSION These cases were strong working dogs but their prospects for an active life were poor as one was in congestive heart failure and the other had had several attacks of syncope. Nevertheless, their capability for exercise was surprisingly good and they were, therefore, considered good candidates for a permanent pacemaker. With a fixed idioventricular rate, any compensation for the increased demand for exercise must be through increased force of myocardial contraction and utilisation of diastolic and systolic reserve. It is interesting to note that, in the first case, when the electrode became detached from the ventricular wall the dog deteriorated very quickly suggesting that
- 34 - the myocardium had become more dependant on the increased rate of contraction provided by the pacemaker and was less capable of compensation. The anaesthetic technique was simple and thus suitable for minor surgery in the dim light of a radiography department. Light anaesthesia was maintained with incremental methohexitone supplementing nitrous oxide and in this way it was found that the level of anaesthesia could be adjusted quickly without excessive central depression and without having to resort to high concentrations of volatile inhalational agents which would depress the myocardium. This was thought to be particularly important as compensation by increased heart rate in these cases was not possible. In the event of respiratory depression by the barbiturate the dogs were ventilated manually otherwise they breathed spontaneously. The experience of the first case emphasised the importance of a long period of sedation and control after surgery and ensurance that there was adequate slack in the electrode lead and that the pacemaker was secure in the subcutaneous pocket so that it could not be twisted by scratching. These results show that implantation of a permanent cardiac pacemaker is a feasible treatment for heart block in the dog. It is important to point out, however, that much of the success depends on selection of cases and the support of a clinical team which must include both veterinary and medical physicians as well as technicians, so that the experience gained from the application of pacemaker techniques in man can be extended to animals. I am pleased to acknowledge therefore that the dogs were referred by Catherine McVicar and David Finlay in practice, they were under the care of Dr. Chris Gaskell in the Department of Veterinary Medicine, and Dr. Barbara Wright
- 35 - and Mr. Richard Palmer of Musgrove Park Hospital, Taunton, enthusiastically contributed their considerable experience and expertise.