Heart cells not only have the mechanical role of contracting to enable the heart to squeeze and hence pump blood, they are also part of the electrical conduction system that coordinates these contractions in a precise manner.
Q: My boyfriend had a big heart attack a couple of months ago. They said his echo last week showed his heart is not squeezing hard enough so he needs an internal defibrillator. We are now pretty confused. Help!
A: Heart cells not only have the mechanical role of contracting to enable the heart to squeeze and hence pump blood, they are also part of the electrical conduction system that coordinates these contractions in a precise manner.
Normally, about 55 percent to 75 percent of the blood in the left ventricle is pumped out of the heart to the body (the left ventricular ejection fraction or LVEF), and this is one measure of how effectively the heart is working. If the mechanical function of the heart is severely compromised after a heart attack (the LVEF is significantly reduced) then many heart cells were lost, increasing the risk that the electrical conduction system is also compromised.
To prevent an injury to the sinoatrial node (where the electrical impulse normally originates) from stopping the heart and killing the patient, essentially, every heart cell can initiate the heart's electrical activity. The downside of this is that some heart cells may inappropriately initiate the flow of electricity, causing a premature electrical impulse. If this only happens sporadically it is a benign condition.
However, sustained haphazard electrical signals initiating from the ventricles may cause the ventricles to quiver (called ventricular fibrillation or Vfib) so no blood is effectively pumped from the heart, leading to sudden death. An abnormal heart rhythm is thought to be the most common cause of the 200,000 to 400,000 cases of sudden death each year in the U.S.
If Vfib occurs, applying electrical energy appropriately can "reset" the heart's conduction system, allowing the normal flow of electricity to resume. We have all seen this done on countless TV shows.
Rather than relying on the availability of a defibrillator machine, people at very high risk of Vfib may have an automated internal cardiac defibrillator implanted. An AICD monitors the patient's heart beat and if Vfib occurs, it delivers electrical energy to reset the heart's conduction system. An AICD consists of a battery and electronics, and is attached to wires (leads) that are placed into the heart to carry the monitoring and resetting signals.
Although some people are born with certain heart electrical abnormalities (such as prolonged QT syndrome or Brugada syndrome) that may benefit from an AICD, the most common reason for a high risk of Vfib is heart damage from a heart attack or some other medical condition.
The National Institute for Health and Clinical Excellence recommends an AICD for patients that:
- Survived a cardiac arrest from Vfib (or from the similar condition of pulse-less ventricular tachycardia).
- Have spontaneous VT that causes fainting or hemodynamic instability.
- Have sustained VT and LVEF less than 35 percent.
- Had a heart attack at least four weeks prior to testing and show LVEF less than 35 percent and inducible VT, or LVEF less than 30 percent with a prolonged QT.
The 15 to 30 minute procedure to implant an AICD (often done using just a sedative and local anesthesia) requires creation of a pocket under the skin beneath the collar bone for the battery/electronic device (about half the size of a cell phone) and the leads to be placed from the device, through a vein and into the heart muscle. Complications occur in less than 1 in 100 placements and may include infection, incorrect lead placement, lead migration, bleeding, perforation of the lung, perforation of the heart (uncommon) and even death (very uncommon).
An AICD should be monitored every several months to verify it is working correctly and that the battery is still good (the battery usually lasts five to 10 years). Due to advances in technology, this monitoring can be done over a phone or Internet connection.
The AICD should also be monitored after a shock has been delivered to verify the shock was for an appropriate reason.
Patients at high risk of sudden death from Vfib should talk with their cardiologist about the potential benefits of an AICD. People with an AICD should avoid exposure to electromagnetic energy that can interfere with it, such as certain electronic devices and certain medical procedures/tests (MRI, for example).
Massachusetts-based Jeff Hersh, Ph.D., M.D., F.A.A.P., F.A.C.P., F.A.A.E.P., can be reached at DrHersh@juno.com.