- Artifical pacing (AP) provides an artificial electrical stimulus that elicits cardiac contraction. AP is used to ensure adequate heart rate in animals with bradyarrhythmias.
Principles of AP
- There are two main components of a pacemaker, the generator and the lead .
- The pacing generator contains a battery and sophicated circuitry that allows customized programing of the pacemaker's function.
- The pacing lead, delivers the electrical stimulus from the generator to the myocardial tissue.
- Pacemakers serve multiple functions. Not only do they deliver the electrical impulse (pace) but they also detect native cardiac depolarizations (sense). Based on the activity the pacemaker senses, various responses can be programmed.
- For instance, the pacemaker listens to see if a spontaneous impulse is generated by the myocardium - if so, this is sensed by the pacemaker and the next pacemaker signal is delayed by an appropriate interval.
- Pacemaker batteries have a lifespan of approximately 4-10 years.
Reducing the number of pacemaker impulses generated increases the lifespan of the battery - although battery life is not normally an issue in animals.
- The nomenclature surrounding pacemakers appears complex but is actually very straightforward.
- Generators are either unipolar or bipolar. These need to be matched to corresponding pacemaker leads.
- A code comprising 5 letters is used to describe the generator:
- The 1st letter describes the cardiac chamber paced by the pacemaker either atrium (A), ventricle (V) or dual (D).
- The 2nd letter describes the cardiac chamber sensed by the pacemaker either atrium (A), ventricle (V), or dual (D).
- The 3rd describes the response to a sensed signal either triggered (T), inhibited (I) or dual (D).
- The 4th denotes rate-responsiveness (R) - ability to vary impulse rate with activity.
- The 5th describes programmability (P).
- Most human pacemakers are DDD, whereas most pacemakers used in pets are VVI which means that the lead is implanted in the ventricle and if an impulse is sensed from the ventricle the pacemaker pulse is inhibited.
- The ability to program the pacemaker to respond to patient physical activity and to increase the heart rate of (rate responsiveness) is available in most pacemakers.
- Leads are either unipolar or bipolar. Unipolar systems use the generator as part of the electrical circuit (one pole); bipolar systems have both the positive and negative poles within the tip of the lead.
- Leads are classified as epicardial or endocardial:
- Epicardial leads: implanted into the ventricle from outside the heart.
- Endocardial leads: implanted in the right ventricle via a transcutaneous route (usually through the jugular vein).
- The end of the endocardial lead which attaches to the myocardium has either a corkscrew end or a number of plastic hooks (tines) which hook onto trabeculae in the ventricle and push the electrode tip against the endocardium.
Always make sure that the lead has the correct attachment for connection to the generator as the 2 components are not always compatible. (It is sometimes possible to use a bipolar lead with a unipolar generator - the system functions as unipolar system.)
- Programing units can be used to reset pacemakers in situ but these are not normally available to veterinary practitioners and most pacemakers for animals are programed to a pre-set rate before delivery. Most veterinary cardiologists either have programing units, or have access to them as necessary.
- Pacemaker rates can be set at a predetermined rate which in animals is usually selected to be 80-100 BPM for dogs and >120 BPM for cats. Rate-responsive pacemakers are often programed to provide a range of heart rates from 70-150 BPM in dogs.
- Pacemaker implantation is indicated for management of bradycardia usually caused by:
- Sinus node dysfunction, ie sick sinus syndrome.
- Symptomatic second degree AV block.
- Third degree AV block.
- They are used where animals are symptomatic with syncope or exercise intolerance as a result of bradycardia that cannot be medically controlled.
Method of implantation
- Originally all pacemakers in animals were placed using a transdiaphragmatic approach.
- The unit is located in an abdominal pouch and the lead passes through the diaphragm and is attached to the epicardial surface of the heart.
- This method is still used for implantation in cats.
- In the dog endocardial leads are now usually implanted via the jugular vein.
- The generator unit is located in a pouch in the neck.
Technique for implantation
- Pre-operative preparation includes heavy sedation.
- A transvenous temporary pacemaker and lead are placed through one of the jugular veins to ensure heart rate control during induction of anesthesia. The patient is then anesthetized and the heart is paced via the temporary (external) unit.
- The contralateral jugular vein is clipped from the jaw to the ventral midline and aseptically prepared,
- The jugular vein is exteriorized and the lead inserted into the vein and guided down to the heart.
- Fluoroscopic guidance is necessary for assisting placement of the lead in the correct position in the right ventricle. Corkscrew leads are screwed into the myocardium, tined leads are tested to make sure they are positioned.
- The lead is then connected to the generator and tested to ensure successful pacing.
- A subcutaneous pouch is excavated in the neck to accommodate the generator and residual lead.
- The generator is secured in the subcutaneous pocket and wounds closed routinely.
There is some high risk of lead dislodgement for 24-48 h after surgery and animals should be cage rested and sedated if necessary.
Thoracic radiographs should be taken 24 h after implantation to check lead placement. These may be monitored on a regular basis.
ECG recordings will show adequate capture of myocardial contraction by the pacemaker and will need to be repeated to confirm continued function.
Fine tuning of generator settings after 4-8 weeks, and then annually using the programs will ensure optimum battery life and safety.
Complications associated with implantation
Complications are not uncommon but in most cases these are not serious.
- Lead dislodgement may occur and is probably more common following epicardial lead placement.
- Lead failure is rare due to a fracture in the wire.
- Muscle twitching occurs at generator site due to conduction of impulse from generator into surrounding muscle tissue. This either resolves spontaneously, or after reprogramming the generator to a lower output.
- Seroma formation Seroma is not uncommon where the generator unit is sited.
Never drain a seroma, because of risk of infection. Hot packing and light pressure bandaging is adequate to resolve these.
- Infections of the generator or lead. This is a very serious complication, necessitating replacement of the system.
- Fibrosi sor exit block in the myocardium where the lead is implanted prevents conduction of electrical pulse. This can usually be overcome by adjusting the output of the unit.
- Generator/battery failure usually results in slower pacemaker discharges. The generator unit can be replaced without recourse to repositioning the wire provided the wire/generator connection is compatible.
- Units will pace in 'end-of-life' mode when the battery is failing for up to 6 months. Thus, owner monitoring and routine annual pacemaker rechecks can prevent complete loss of pacing and allow timely replacement of generators.