The pulse on pacemakers - DVM
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The pulse on pacemakers
More use signals special-care considerations for small animals


DVM360 MAGAZINE


In some cases, a 24-hour ambulatory ECG (Holter monitor) may be required to confirm the diagnosis. These data are used to rule out other causes of the arrhythmia that may obviate the need for cardiac pacing and indicate prognosis.

In the face of severe dilated cardiomyopathy, for example, the long-term benefits of pacemaker therapy are diminished. Venipuncture should not be performed using either jugular vein prior to referring a patient for pacemaker implantation. A hematoma around the jugular may preclude using that vein for the pacing lead. The lateral saphenous vein is preferred for blood collection so the cephalic veins can be preserved for intravenous catheter placement.

Patients with signs of congestive heart failure should be stabilized appropriately with medications such as furosemide, nitroglycerin and possibly an ACE inhibitor with or without temporary pacing.

In some cases, a constant-rate infusion of isoproterenol or dopamine may be administered to increase the heart rate of escape rhythms and augment cardiac contractility. Lidocaine, digoxin, beta-blockers and/or calcium channel-blockers are contraindicated because these drugs can worsen the bradycardia or suppress the ventricular escape beats that are sustaining the patient.

Patients with severe bradycardia or escape activity that changes rate erratically may require temporary transvenous pacing with a special catheter to reduce the risk of sudden death from anesthesia.

A combined anesthetic protocol of heavy intravenous sedation, neuromuscular blockade, intubation and ventilation, and local anesthesia can be used to gain vascular access and place a pacing lead for this function. An external temporary pulse generator is then attached, using sterile connectors. This allows control of the heart rate during the procedure and allows safe administration of inhalation anesthesia.

An alternative to temporary catheter pacing is transthoracic pacing, in which two wide electrodes are adhered to each side of the shaved thorax of the patient, through which just enough current is applied to depolarize the heart with an external pacing system.

Post-operative care

Transvenous endocardial pacing is the primary method of pacing today. The surgical site is small and post-operative pain is less than with epicardial pacing. Post-implantation care involves analgesia, sedation and continuous electrocardiographic monitoring. Antibiotics are given peri-operatively and post-operatively. Opiate sedation/analgesia also keep the patient quiet after surgery to avoid dislodging the lead, thereby necessitating surgical repositioning. A sterile bandage is placed around the neck to protect the surgical site and reduce seroma formation. Radiographs are taken in the right lateral and dorsoventral views the next day to confirm lead placement.

Avoid positioning the patient on its back during the first two weeks after implantation because this may dislodge the lead. Neck leads or choke-chain collars should never be used on a dog with a transvenous pacemaker system; they should always wear a harness.

An ECG is used to verify pacemaker function and rate. Pulse generator parameters are programmed to a base setting and the patient is released from the hospital one to three days after implantation.

At discharge, the owner is instructed how to take the patient's heart rate and asked to check it daily until the next visit. If the heart rate goes below the pacing rate, the owner is instructed to call the hospital immediately.

Owners are asked to return for suture removal 10 days to 14 days from discharge. At this visit an ECG and thoracic radiographs are performed. The pulse generator also is evaluated with the programmer. As the endocardium develops a reaction around the pacemaker lead tip where it contacts the heart, increased resistance to the pacing impulse can develop.

The minimum current for the pulse generator needed to capture the ventricle, causing it to depolarize and contract is determined. To verify capture, a continuous ECG is performed while programming the pacemaker, with the current gradually reduced until capture is lost. When this threshold is determined, the pacemaker is programmed at a sufficiently high output to assure capture in the event that resistance increases. Typically, an output setting of two times the threshold is programmed to allow a sufficient margin of safety.

A similar follow-up visit is scheduled about one month after implantation and again at three months, at which time radiographs and pulse-generator evaluation are performed. Unless complications arise, the owner is asked to see a local veterinarian for biannual radiographs and to visit the cardiologist annually for pacemaker system evaluation. At all follow-up visits, an ECG, echocardiogram and pacemaker programming are repeated. Other tests may be added as indicated.


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Source: DVM360 MAGAZINE,
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