Ask The Doctors: December 2012
Q. I have read a lot in Heart Advisor about atrial fibrillation, but one of my good friends was recently diagnosed with ventricular fibrillation, and may be at risk of sudden cardiac arrest. Is ventricular fibrillation more dangerous that atrial fibrillation?
Ventricular tachycardia (VT) and ventricular fibrillation (VF) are dangerous abnormal heart rhythms, and the most common causes of sudden cardiac arrest, so they always require the urgent attention of a cardiologist. Normally, the heart’s electrical activity is referred to as “sinus rhythm,” because the pace is being set by the sinus node in the right upper chamber, the right atrium. Most people experience the occasional heartbeat which originates from the ventricles, known as a premature ventricular contraction, or PVC. Such PVC’s may produce the sensation of skipped beats, but are generally benign and self-resolving. However, when a string of fast ventricular beats occurs—referred to as VT—blood pressure can drop quickly, causing lightheadedness or loss of consciousness. VF is a chaotic electrical heart rhythm in which the ventricles quiver uncontrollably. Since the heart cannot pump blood in this situation, persons with VF almost immediately lose consciousness, and if the VF is not stopped by an electrical shock (defibrillation), death will ensue.
The blockage of a coronary artery, as happens at the beginning of a heart attack, cuts off the blood and oxygen supply to the myocardium very quickly. This acute stress greatly increases the chance of VT or VF, and it is estimated that approximately half of all persons who develop a completely blocked coronary artery experience sudden cardiac death (SCD) due to VT or VF. Apart from coronary disease, VT and VF can also result from an enlarged and weakened heart, such as in heart failure, as well as from other structural problems such as hypertrophic cardiomyopathy. In response to your query, VF is certainly more dangerous than atrial fibrillation, and your friend would be well-advised to follow the advice of his cardiologist, which may include recommending an implantable cardioverter defibrillator (ICD).
Q. The spot where I had a coronary stent put in several years ago is narrowing again. My doctor said he may put a new stent in the same location. How is it possible? It sounds risky.
While coronary stents are a remarkable invention and treatment for coronary artery disease, they have their limitations. The type of renarrowing you describe is referred to as “in-stent restenosis,” or ISR. Individual persons vary in their likelihood to develop ISR, which is a different process than the cholesterol plaque formation and subsequent inflammation which led to the initial blockage. Instead, the body is reacting to the presence of a foreign object – the metal stent – and trying to grow scar tissue over it. When the first type of stents were observed in some people to reblock within a short period of time, drug-eluting stents (DES) were developed. The newer stents release medications which reduce ingrowth of scar tissue, and tend to stay open longer. Even so, DES can still fall victim to ISR. One treatment of this problem is to inflate a balloon with sharp metal edges inside the narrowed stent, to chisel the scar tissue out of the way. Another accepted option, as mentioned by your cardiologist, is to place another stent inside of the first. Sometimes using a different type of DES will reduce the rate of restenosis. It is possible to place more than one stent in the same place by inflating a balloon to high pressure inside the newer stent, pushing it up tightly inside the older one. The major concern in this procedure is insufficient expansion of the new stent, which can lead to gaps between the two layers of metal. Such gaps can increase the risk of blood clots forming and causing a heart attack. A tiny ultrasound probe called intravascular ultrasound (IVUS) is often used to ensure the two nested stents fit flush against each other.