Understanding ECGs: Minding your Ps and Qs

In the second article in our series Dominic Cox and Hamish Dougall discuss what you need to learn and what you need to understand In this series of articles on electrocardiograms (ECGs) we hope to convey the basic concepts of reading and using this simple clinical tool. We re-emphasise that a grasp of only a few rudimentary concepts will allow all of us, not just the cardiologists, to understand the majority of clinically important ECGs. In the first article we explained how the heart works as an engine and outlined the power supply (coronary arteries), control (electrical supply), and how that in an ECG, the electrical supply is recorded as it moves across the heart by a series of detectors placed strategically around the body. An ECG is a two dimensional recording of a three dimensional process. A cardiac electrical impulse does not travel in a single direction down a straight line with an arrow on the end. It in fact spreads out in all directions across the heart. The ECG leads allow us to look at this depolarisation wave from different views--that is, in the vertical and horizontal planes. When the wave is heading towards a specific lead we will get the largest positive deflection in that lead. When it is heading directly away from it we will get the opposite: the largest negative response. Leads looking at right angles to the wave front will see smaller biphasic responses as the wave passes them. In this article we will go through the different parts that make up the ECG recording (see figure 1) in turn. We appreciate that the material in this, and the previous, article may seem a little dry. It is, however, worth trying to grasp the basic concepts without getting bogged down too much in the …