Learning from mistakes: the case of clinical electrophysiology: a perspective on evidence-based rhythmology

[Primary prophylaxis with the implantable cardioverter-defibrillator in heart failure: a clinical point of view]

The implantable cardioverter-defibrillator (ICD) is one of the great inventions of modern cardiology. Its use for the prevention of sudden death in patients with left ventricular dysfunction has meant that clinical cardiologists are now fully involved in decision-making on the implantation of these devices. The majority of clinical trials, which have used low ejection fraction as the only or main criterion for patient recruitment, have shown that ICD use leads to a significant improvement in survival. Three trials, two of which were carried out soon after myocardial infarction and one of which was performed at the same time as surgical revascularization, were exceptions. However, it is important to be aware that the improvements observed in the most recent trials have not been as large as those seen in the initial studies. Reduced efficacy has meant that in the Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT), for instance, 25 ICDs had to be implanted to save one life over a 2-year period. The most likely explanation for this observation is that the better prognosis achieved by present-day pharmacologic treatment of heart failure has reduced the margin of benefit associated with ICD use. Another consequence is that depressed left ventricular ejection fraction has lost some of its specificity in predicting sudden death. New predictive variables are needed to improve risk stratification in this population. Without these variables, the use of ICDs in the primary prevention of sudden death in patients with left ventricular dysfunction will not seem a very attractive option from the point of view of good clinical practice.

The long QT interval is not only inherited but is also linked to cardiac hypertrophy

This review focuses on the molecular determinants of the duration of the QT interval as measured on by electrocardiography in normal subjects and during cardiac hypertrophy and failure. (a) In control conditions, on a single cell, the shape and duration of the action potential is the result of a balance between different ion currents which in turn were determined by the number of functional channels. On multicellular preparations the QT duration also represents the repolarization time; nevertheless it is modified by the transmural gradients. On body-surface electrocardiography the duration of the QT interval depends also of an additional factor: the spatial three-dimensional projection of the electrical waves vectors, which makes any determination of the epicardial dispersion by measuring QT interval dispersion questionable. (b) The enhanced action potential duration is well documented in cardiac hypertrophy and heart failure and is usually caused by a reduction in outward current densities in most of the species except mice. Among these currents I(tO) is the most frequently altered, especially in humans. Such an altered current density is caused by a diminished expression of the genes encoding either the ion channel subunits or regulatory proteins, such as KChIP2. In addition, hypertrophy modifies or even reverses the transmural gradient. In human and rats hypertensive cardiopathy is associated with a prolongation of the QT interval duration. The reduction in I(tO) is likely to be adaptive; it participates in the slowing of the cardiac cycle and reflects the fetal genetic reprogramming. Recent data also suggest that a reduction in the transient outward K(+) current density triggers protein synthesis through an activation of the calcineurin pathways. Thus a prolongation of the QT interval is not only inherited or drug-induced; it is also an essential component of the adaptive process in chronic mechanical overload. It is fundamentally incorrect to measure QT dispersion on a surface electrocardiography, but the mean QT interval may provide information concerning the progression of the disease, just as, and with the same restrictions, in the case of the quantification of V(max).