Many of us will have heard of humans patients undergoing procedures to fit pacemakers, place stents across narrowed coronary arteries to prevent heart attacks or close holes in the heart, all of which thesedays are performed routinely without need for open heart surgery thereby improving recovery times and patient wellbeing. Fewer of you will be aware that this knowledge has been transferred into veterinary medicine and we have several referral centres in the South East where pets may undergo such life-saving treatment and consequently enjoy a better quality of life.
Pacemakers are used where the normal rhythm of the heart has been disrupted so that the heart rate is abnormally slow (bradycardia or bradyarrhythmias), which may or may not result in clinical signs such as syncope (fainting), weakness and exercise intolerance.
Some patients with bradyarrhythmias will not be severely enough affected to require treatment. These patients will be free of symptoms despite have lowered heart rates and may well be able to lead acceptable lives. The risk of pacemaker implantation or its complications are not justified in such individuals.
Once the heart rate falls to a certain level, signs will be more frequent/severe and generally unresponsive in the long term to drug therapy to increase the heart rate. Additionally, the excessively slow heart rate may fail to push blood forward and congestive heart failure may occur with time. These patients will be candidates for pacemaker implantation.
The pacemaker is comprised of a pulse generator which supplies an electrical current to a lead which passes into the heart via the major blood vessels in the neck and chest. The generator sits in a pocket created under the skin in the neck and can be programmed to deliver pulses of electricity to the heart at a chosen rate. The tip of the lead is buried into the muscle of the heart chamber to be paced. When the pulse reaches the heart muscle, contraction is stimulated in the same way that it would be with electrical input from nerves in normal animals. In this way, the heart rate can be increased from the abnormally low level associated with the bradyarrhythmia to the normal resting heart rate for the species and breed of patient. The generator can be reprogrammed at a later date if necessary but placing the programmer over the skin covering the pulse generator which will temporarily allow the ability to manipulate the frequency and strength of the pulse generated.
Different centres have different protocols to fit pacemakers and only a general overview will be given. Pacemakers require fluoroscopy in order to be fitted, which gives a continuous x-ray appearance of the heart and great blood vessels – the picture shows a chest x-ray with the metal lead of the pacemaker running into the chest and heart from the left, whilst the pacemaker itself is the square shape at the upper left of the image. Scrupulous sterility is essential as any infection on the system inside the heart or blood vessels will almost certainly require removal of the pacemaker.
Many dogs in the UK have now benefited from pacemaker implantation, continuing to live normal lives for many years afterwards. The biggest complication is disruption of the lead tip so that it no longer sits in the correct part of the heart muscle or even floats free in the chamber. This will mean that it cannot stimulate effective cardiac contraction. One important point to remember is not to collect blood samples from the jugular vein on the side of the pacemaker in order to prevent damaging or dislodging the lead inside the vein.
Whilst this topic may not be directly relevant to pet-sitters, it is hopefully a matter for general interest and, one day, you may well come across a pet with a pacemaker implanted.
