Electrocardiographic Anomalies and Their Underrated Role in Sudden Cardiac Events

Abstract

In 2008, Professor Breijo-Márquez described an electrocardiographic pattern consisting of the presence of a short PR (or PQ) interval together with a short QT interval in the same individual. It was published under the headline "Decreased cardiac electrical systole" in the International Journal of Cardiology (IJC) [1].

From then until today, this electrocardiographic pattern is increasingly studied and diagnosed by various cardiologists, both in isolation and as part of other well-known entities in cardiology (to be described below).

As is well known, the PR interval on the ECG tracing represents the distance from the onset of atrial depolarization (P wave) to the onset of ventricular depolarization (QRS complex).  Normalized values, considered within normal ranges, range from 0.120 milliseconds to 0.200 milliseconds (below the 0.120 millisecond digit is considered "short"; above the 0.200 millisecond value is considered "atrioventricular block")[2].

Similarly, it is also well known that the QT interval includes both ventricular depolarization (QRS complex) and ventricular repolarization; it spans from the beginning of the Q wave (if present) or the beginning of the R wave to the end of the descending branch of the T wave, when this branch reaches the isoelectric line of the electrocardiographic (ECG) tracing.

However, at present there are still many discrepancies as to which values should be considered as standard when it comes to the QT interval. 

The standard values for the length of this interval are not precisely uniform for all authors.  For most authors, including us, the values range from 0.360 milliseconds to 0.450 milliseconds (for some authors in women they would be in ranges up to 0.460 milliseconds).

The shortening of the PR interval by less than 0.120 milliseconds makes the myocardium more unstable and more prone to cause cardiac electrical disturbances, which can lead to serious arrhythmias that can be life-threatening; the most frequent arrhythmias are tachyarrhythmias in their different types and etiologies [2].

Short QT interval (equal to or less than 0.350 milliseconds) appears to be a rare form of channelopathy with a high risk of sudden cardiac death, but it is still not well and completely defined, and information on long-term follow-up remains very scarce at the moment [3]. 

A short QT interval is the major component, but because of a strange relationship between the QT interval and the RR interval in patients with a short QT interval, the shortened QT interval in such patients is often only apparent at a heart rate near 60 bpm.

In fact, a large percentage of physicians consider electrocardiographic tracings carrying the aforementioned pattern to be within normality, when in fact this is not the case.

 Because routine ECGs are usually taken at a higher than normal heart rate, many patients with cardiac electrical involvement may go undetected [3].

Many mutations are responsible for the short QT interval, but in published families, only one in four were found, which have been genetically tested [4, 5]. 

Thus, most diagnoses are based on the patient's own clinical presentation with a relative who has died suddenly, and the latter is also difficult to prove. The treatment of choice is an implantable defibrillator [4,5, 6].