Anatomy of an ICD

An implantable cardiac defibrillator (ICD) is a small electronic device installed inside the chest to prevent sudden death from cardiac arrest due to life threatening abnormally fast heart rhythms (tachycardias).

The ICD is capable of monitoring the heart rhythm. When the heart is beating normally, the device remains inactive.

If the heart develops a life-threatening tachycardia, the ICD delivers an electrical "shock(s)" to the heart to terminate the abnormal rhythm and return the heart rhythm to normal.

An ICD consists of one or more leads (conducting wires insulated with silicone or polyurethane) and a defibrillator unit. The defibrillator unit is a small titanium case containing a microchip computer, a capacitor, and a battery.

The leads carry electrical signals between the heart and the defibrillator unit. One end of a lead is placed on the inner wall of the heart while the other end is attached to the defibrillator unit. The leads help the defibrillator unit monitor the natural heart rhythm. The leads also deliver electrical shock(s) from the defibrillator unit to the heart when tachycardias occur.

The microchip computer runs the defibrillator, monitors the natural heart rhythm, instructs the capacitor to send electrical shock(s) when tachycardias occur, determines the strength of the shock(s) sent, and also keeps a record of the heart rhythms as well as the shock(s) sent by the defibrillator.

Modern ICDs have programmable features that allow the doctor to change the cutoff heart rate for activating the defibrillator. Tachycardias with rates higher than the cutoff heart rate activate the firing of shocks by the defibrillator. The doctor can also adjust the strength (amount of energy delivered) of each shock, and the number of shocks delivered with each tachycardia episode.

Implantation of an ICD is similar to that of a permanent pacemaker. The procedure, which lasts 2-3 hours, is considered minor in that it does not involve major heart surgery. Patients are typically sedated during the procedure. A local anesthetic is injected under the skin over the area where the ICD will be placed, usually in the right or left upper chest near the collarbone. The lead is then inserted into a vein located in the upper chest near the collarbone. The tip of the lead is placed on the inner wall of the heart with the visual guidance of x-rays. If there is more than one lead, the process is repeated. The other end of the lead (or leads) is connected to the defibrillator unit, which is then inserted under the skin at the incision site. Because there are no nerve endings inside the blood vessels and the heart, the patient usually does not feel the placement of the lead(s).

Heavy sedation is used during the procedure when the defibrillator is tested for proper functioning. Testing an ICD involves inducing a rapid heart rhythm and allowing the defibrillator to detect the abnormal rhythm and then terminate it with a shock (just as the device would be expected to operate in a real-life tachycardia episode).

While in the hospital, the patient's heart rhythms, pulse, and blood pressures are routinely monitored. The doctor may check or adjust the settings on the defibrillator (done from outside the body). The nurses also periodically examine the incision over the implantation site for bleeding, redness, or other signs of infection. It is normal to feel some pain over the incision for 1-2 weeks. Medications are usually given to alleviate pain.

Patients are typically discharged from the hospital in 1- 2 days after the procedure. Once home, the patient can usually return to most activities. Instructions are given to avoid raising the arm over the shoulder on the side of the ICD implantation for several weeks. This precaution is to avoid dislodging the leads before they become secure inside the veins and the heart. Patients are also asked to avoid contact sports, vigorous exercises, and heavy lifting for several weeks.

In a week, the sutures over the incision are removed in the doctor's office. This is a good opportunity to discuss the following issues with the doctor:

  1. Level of physical activity;
  2. Return to work;
  3. Permission to drive automobiles;
  4. How frequently should the ICD and battery level be checked?
  5. What are the signs of device failure?
  6. When to replace the ICD (most ICD batteries last 3-7 years)?
  7. Precautions regarding interference with the device by outside power sources; and
  8. What to do when tachycardias occur.

Call the doctor if there is bleeding from the incision site, increasing pain over the incision site, fever, heat, swelling, or fluid draining from the incision site. Also call if the arm becomes swollen on the side of the implantation or if there is twitching of chest muscles, persistent hiccups, dizziness, fainting, chest pain, or shortness of breath.

Common complications include pain, swelling, and minor bleeding at the implantation site. More serious complications are uncommon and typically occur less than 2% of the time. Serious complications include major bleeding requiring blood transfusions, introduction of air into the space between the lung and chest wall (pneumothorax) requiring tube drainage, perforation of the heart muscle by the leads, activation of an intractably fast heart rhythm, stroke, heart attack, need for emergency heart surgery, and death. Although there are no official guidelines, ICDs should be implanted by or in conjunction with a cardiologist specially trained in clinical cardiac electrophysiology (electrical diseases of the heart).