Criteria for a Defibrillator

If someone experiences heart failure, an internal cardioverter-defibrillator (ICD) may save that person's life. Not just anyone can decide to have an ICD implanted. Two important clinical trials, MADIT-I and MADIT-II (Multicenter Automated Defibrillator Implantation Trial), and the resulting data from those studies have led to inclusion criteria for those patients who need a defibrillator as a preventative measure from sudden cardiac death (SCD). Although this article covers the recommendations of those two trials, it is important to understand that medical recommendations do change over time.

Criteria 1: Recent MI

• A patient must have a documented history of a myocardial infarction (MI), or heart attack, greater than 30 days old. Meaning, if the MI occurred 29 days ago or less, the patient is excluded as a candidate.

Criteria 2: EF 20 Percent or Less Verifiable by Echocardiogram

• The heart's ejection fraction (EF), simply put, is the force in which the left ventricle (LV) forces the blood out with each contraction. If the patient has a verified EF of 20 percent or less, the implantation of an ICD can help to combat arrhythmias associated with a weak LV, primarily ventricular tachycardia (VT) and ventricular fibrillation (VF).

Criteria 3: A QRS Complex ≥1,400 ms

• The QRS complex is the electrical signature on an electrocardiogram (ECG) that represents the contraction of the heart muscle. If the QRS is ≥1,400 ms (0.14 seconds), the patient has an increased chance of developing either VT or VF. By definitions, VF is called VF because it is a wide-complex tachycardia. In the past, the number 1,200 ms was bantered about as the QRS width that was needed to be considered for an ICD. However, a normal QRS complex is said to be ≤1,200 ms. The 1,200-ms value became more than just a moot point. Now, to differentiate between a normal QRS and a QRS that is too wide, the standard value for implant criteria is 1,400 ms.

Criteria 4: Verifiable Past Episode of VT or Survival of a VF Episode

• Because a defibrillator is, by invention, the treatment of VT and VF, a patient who has a recorded episode of VT is considered a candidate for implantation. These patients will eventually convert into VF if the VT is untreated. VF is not a viable rhythm that is compatible with life. Early defibrillation is the treatment of choice for these two lethal rhythms.

Criteria 5: Bilateral Ventricular Asynchrony

• As a result of heart disease, the LV can sometimes contract at a slower rate than the right ventricle (RV). When this happens, instead of the two ventricles working together, the heart beats in a lop-sided or asynchronous pattern. In the healthy heart, the RV and LV should beat at the same rate 100 percent of the time. In a heart failure patient, this asynchrony begins to worsen their overall condition, especially in the patient with congestive heart failure (CHF). A special form of defibrillator known as a biventricular device (BiV) paces the heart, helping the RV and the LV to beat in unison. At the same time, it can also defibrillate the heart if the patient experiences an episode of VT or VF.

An ICD in LQTS Patients

• Long QT syndrome (LQTS) is an electrophysiological condition in which the duration of the Q-wave in the QRS complex measured to the terminus of the T-wave after it slopes back from the isoelectric line is longer than the time duration of 4,000 ms, or 0.40 seconds. LQTS is a complicated diagnosis. There are several forms and treatments for those with the condition. Depending on the advice from the diagnosing cardiologist and the consulting electrophysiologist, an ICD may be implanted for precautionary purposes.