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Pacemaker Implanatation In A Boxer.

A very interesting document written by renowned animal cardiologist Nicole Van Israel concerning a pacemaker implantation in a 6 year old male boxer called Murdo. The following is the article which is reproduced here courtesy of Dr Van Israel - Please visit Dr Van Israels site Here, and please view all of her incredible papers Here.

A huge thanks from PWP to Dr Van Israel for taking the time to respond to our emails and granting us permission to direct you to her work. Below is an extract from one of her articles which can be downloaded in full, in PDF format HERE.

PRESENTING HISTORY
Murdo is a six-year-old male neutered Boxer presented for investigation of a bradycardia noticed at a pre-anaesthetic check when the intention was to remove an epulis. The dog was completely asymptomatic and there was no previous history of illness.

CLINICAL EXAMINATION
On presentation Murdo was overweight (36 kg; ideal weight 32 kg) but bright, alert and responsive. On clinical examination his mucous membranes were pink and his capillary refill time was less than 2 seconds. He seemed fully hydrated. Multiple epuli were visible in his mouth. No jugular distension was noticed. A strong and regular apex beat was palpable and no precordial thrill was felt. Abdominal palpation, although difficult because of the dog’s obesity, was unremarkable. All lymph nodes were within normal limits.All his extremities were nicely warm. His pulses were strong and without any deficits.
On auscultation a very regular heart rate was audible.The heart rate was 60 BPM. No murmur was audible. The lungs sounded unremarkable. Rectal temperature was 38.5°C.

PROBLEM LIST AND DIFFERENTIAL DIAGNOSIS
Regular bradycardia Sinus bradycardia
Physiological
•    in very fit animals
•    in animals with increased vagal tone (brachycephalic breeds)
•    sleep
Pathological
•    hypothermia
•    hypothyroidism
•    hyperkalaemia
•    CNS disease, increased intracranial pressure
•    upper airway obstruction
•    gastro-intestinal (obstruction)
•    urinary (obstruction)
Pharmacological
●     tranquilliser, anaesthesia, digoxin, ß-blockers, morphine
Third degree AV-block
•    Infiltrative disease (neoplasia, inflammation)
•    Idiopathic fibrosis
•    Myocardial infarction
•    Hyperkalaemia
•    Lyme disease
•    Bacterial endocarditis
•    Associated with congenital defects (aortic stenosis, ventricular septal defect)

DIAGNOSTIC WORK-UP
The dog was admitted to the hospital for electrocardiography, blood work, thoracic radiography and colour flow Doppler echocardiography.

ECG
A 12-lead ECG confirmed the presence of third degree
AV-block with a ventricular escape rhythm of 60 BPM.
The P-wave rate was 120 per minute (Fig. 1).


Fig. 1: Six-lead ECG showing third degree AV-block with an escape rhythm at 60 BPM (50 mm/s; 1 cm=1 mV).

Atropine response test
An atropine response test (atropine 0.02 mg/kg IV; in
normal animals one can expect a 50-100% increase in
heart rate depending on the initial heart rate) increased the
P-wave rate but not the ventricular escape rate.

Lab-work
a. Haematology (Table 1)
Haematology was unremarkable.


b. Biochemistry (Table 2)
The biochemistry showed the presence of a marginally
elevated cholesterol level.T4 levels were within normal
limits. Mildly increased urea and creatinine were
consistent with a mild prerenal azotaemia




The Vertebral Heart Score was 12.5.There was sign of mild R-sided cardiomegaly.The great vessels (aorta, pulmonary artery and caudal vena cava) and the pulmonary vasculature were within normal limits. The pleural space and the lung fields were unremarkable.

Fig. 2: Right lateral thoracic radiograph (expiratory film):
rounding of the cranial border of the cardiac silhouette.The
lung fields are clear.

Colour flow Doppler echocardiography
All chambers measured within normal limits. Posterior wall and interventricular septal wall thickness were within normal limits in diastole. The fractional shortening (FS) as indicator of myocardial contractility was mildly increased (FS53%).
A hyperechoic area was visualised in the lower part of the interatrial septum (Fig. 3). There was no sign of valvular insufficiency.The velocity across the aorta and pulmonary artery were within normal limits.


Fig. 3: Right parasternal long-axis view showing the
hyperechoic area in the interatrial septum (la: left atrium; ra:
right atrium).

FINAL DIAGNOSIS
Idiopathic third degree AV-block.
 
TREATMENT
The dog was pre-medicated with acepromazine (ACP®, C-Vet; 0.0125 mg/kg) and pethidine (0.5 mg/kg; Martindale) intramuscular. Prophylactic antibiotics (cephalexin, Kefzol®, Lilly; 20 mg/kg IV q 8 hrs) and analgesia (carprofen, Rimadyl®, Pfizer; 4 mg/kg IV) were administered. General anaesthesia was induced with thiopentone IV (Thiovet®, C-Vet; 412 mg) and maintained with isoflurane/O2 (Isoflo®, Mallinckrodt). Under fluoroscopic guidance a bipolar lead (Bipolar Endocardial Lead,Porous tip,Tined;Guidant®, Basingstoke UK; Fig. 4) was implanted in the right ventricle via the left


Fig. 4: Detailed view of the end of a tined pacemaker lead.

jugular vein. The whole procedure was monitored electrocardiographically. The bipolar lead was first connected to an external temporary pacemaker. Once the position of the lead established (pacing threshold less than 1 mA) it was tunnelled subcutaneously and attached to the pacemaker device. The permanent pacing generator (Pacesetter® VVI system) was implanted subcutaneously cranial to the left shoulder.The rate was pre-programmed at 70 BPM. A thoracic radiograph (Fig. 5) was taken for a permanent record of lead placement. The wounds were closed routinely and a protective bandage was applied.


 
Fig. 5: Right lateral thoracic radiograph showing correct placement of the pacemaker lead in the right ventricle.The lead has an S-bend to allow movement of the neck without putting strain on the lead.

The dog recovered well from anaesthesia, stayed in the Intensive Care Unit for 48 hours and was hospitalised for another three days. After a last technical check at the human hospital, he was discharged with a one-week course of antibiotics (20 mg/kg cephalexin BID PO, Ceporex®;Virbac). 

OUTCOME
Three years post-intervention Murdo is still doing very well and the pacemaker seems to be pacing at all times (Fig. 6).


DISCUSSION
Third degree AV-block is an uncommon arrhythmia in the dog. It has been associated with several other conditions (Table 4). In this case no underlying cause could be determined. Dogs with third degree AV-block are usually middle-aged suggesting a degenerative change in the conduction system. Degeneration of the conduction system is common in many breeds. It has been stated that German Shepherd Dogs and Cockers Spaniels are over represented and there is one report where Dobermanns are the most common breed.

 

The importance of the hyperechoic area in the inter-atrial septum remains unclear. Hyperechogenicity is seen with fatty infiltration and collagen changes. Fibrosis and infiltrative disease are known aetiologies for third degree AV-block. Only a histopathological section might give an explanation.
Although no acetylcholine-esterase receptor antibodies were determined, myasthenia gravis was excluded as a possible aetiology on the basis of the complete absence of clinical signs. Borrelia burgdorfi antibody titres were ignored because Lyme disease is not endemic in the area where the animal lives.
Full bloodwork excluded electrolyte imbalances as a possible reason for the third degree AV-block. The azotaemia was, despite the lack of urinalysis (urine specific gravity), thought to be prerenal secondary to a decreased cardiac output due to the bradycardia.The renal perfusion and renal function parameters (urea, creatinine) should return to normal after pacing but no follow-up data are available in this case regarding this aspect. The mildly increased cholesterol levels could have been suggestive of hypothyroidism but T4 levels in the higher range of normal made this very unlikely.
Most animals with third degree AV-block have exercise intolerance as the main presenting sign. Dogs may also be presented because of syncope or congestive heart failure. In this case the dog was completely asymptomatic. It should be appreciated that third degree AV-block may not cause clinical signs unless there are periods with inadequate escape activity. Most healthy dogs can maintain a normal arterial blood pressure at rest with a heart rate as low as 40¬60 BPM.The ECG confirmed the presence of a relatively stable escape rhythm at 60 BPM. Considering the rate, the escape rhythm was thought to be originating from the subsidiary pacemakers in the bundle of His and the AV-node was the most likely site of the block.
On clinical examination no heart sounds (4th heart sounds) consistent with atrial contraction were noticed, partly due to the broad-chested nature of the dog and his obesity.The bouncy nature of the dog also made prolonged auscultation very difficult.The slow regular rhythm and the absence of a sinus arrhythmia were suggestive of an abnormal rhythm in this dog.The typical cannon A-waves, generated when atrial contraction occurs when the mitral valve is closed, were not observed in this animal’s jugular veins.
The radiographic changes were consistent with breed variation. Boxers have an increased VHS compared to other dogs (normal for Boxers 10.3-12.6v). As well as that bradycardia causes prolonged filling and can cause apparent cardiomegaly (diastolic frame). The echocardiographic changes (increased FS) in this case were typical of a hyperdynamic ventricle (Frank Starling mechanism).
Since the atropine response test was negative (atropine will usually increase the atrial rate without changing the ventricular rate because the ventricles are supplied mainly by sympathetic fibres and have few parasympathetic fibres) and since there was no increase of the heart rate after theophylline treatment a pacemaker implantation was the only alternative.



Fig. 7: Different types of pacemakers available with nomenclature.

Fig. 7 explains the three or five letter abbreviations used for describing the capabilities of the pulse generator.The first letter stands for which chamber is paced (V for ventricle, A for atrium,...), the second letter for which chamber is sensed (ditto), the third letter indicates the mode of response (I for inhibitory, T for triggered,...), the fourth letter shows that the device has multiple parameters that can be programmed and the final letter denotes if an anti¬tachydysrhythmia function is present.

The procedure and the post-operative period passed uneventfully in this case. However, one has to be aware that major (Table 5) and minor complications (Table 6) can occur. Survival analysis in the largest retrospective canine pacemaker study published revealed 1-, 2-, 3- year survival rates of 70, 57 and 45 % respectively.



REFERENCES AND FURTHER READING
TILLEY, L. P. (1992) Third degree AV-block. In: Essentials of canine and feline
electrocardiography, 3d Ed, Lea & Febiger, Philadelphia. Pp 175-178.
KITTLESON, M. D. (1998) Diagnosis and treatment of arrhythmias. In:Small
Animal Cardiovascular Medicine. Eds Kittleson & Kienle, Mosby, Inc, St-Louis.
Pp 489-492
BONAGURA, B. D. (1999) Cardiac pacing. BSAVA Congress synopses,
specialist session.
FOX, SISSON, MOISE (2000) Textbook of canine and feline cardiomyopathy
(W. B. Saunders).
OYAMA, SISSON, LEHMKUHL (2001) Practices and outcome of artificial
cardiac pacing in 154 dogs
(CANPACERS Study). JVIM; 15: 229-239.
© Illustrations Nicole Van Israël.