Fig. 18.1
Lead vector for the three standard limb leads, the three augmented limb leads (left), and the six unipolar precordial leads
Technique
The patient is initially asked to lie supine and relax as much as possible. The chest is exposed so that there is no interaction with lead placement, but recognizing that patient’s privacy should be maintained at all times. Figure 18.2 reveals the anatomic landmarks for lead placement. These areas should be identified and cleaned to ensure appropriate electrode placement and contact with the skin. ECG recorders have specific manufacturer’s instructions. Nonetheless, beforehand, it is crucial to determine if there are any possible issues that may interfere with the accurate recording, i.e., electrical calibration, signal interference, wandering baseline, among others.
Fig. 18.2
ECG lead placement
Data Interpretation
Most ECGs are recorded on standard settings at 25 mm/Section (X Axis) and 10 mm/mV (Y axis). Different settings can be obtained depending on the heart rate, or voltage. Before attempting to interpret an ECG, the reader should pay close attention to the standardization. For ease, this can generally be found at either the left or right sides of the tracing.
The normal ECG is composed of multiple waves and intervals, which represent the different portions of the cardiac cycle (Fig. 18.3). The P wave, representing atrial activation is best appreciated in lead II or V1. The PR segment is usually an isoelectric segment that begins at the end of the P wave and ends with the QRS. Depression of the P wave is associated with pericardial disease. Ventricular activation ensues with a number of complex interactions, which are recorded as a set of deflections that give rise to the QRS. The T wave represents ventricular repolarization [3]. The QT interval is measured from the last portion of the QRS to the end of the T Wave, and represents a measure how fast the ventricles repolarize. A prolonged QT constitutes a marker for potential ventricular tachyarrhythmias. Since the QT measurements vary with heart rates, a standardized measurement, the QTc, is preferred at times. Different formulas have been derived. The Bazett equation calculates the QTc by dividing the QT by the square root of the RR interval. The Firderecia method uses the cube root. Other more complex methods, including regression analysis exist but are beyond the scope of this book.
