An electrocardiogram - an ECG - or the dutch and german version of the word - elektrokardiogram or EKG, is a tool used to visualize “gram” the electricity “electro” that flows through the heart “cardio”.
An ECG tracing specifically shows how the depolarization wave moves during each heartbeat - which is a wave of positive charge - looks from the perspective of different sets of electrodes.
This particular set of electrodes is called lead II, with one electrode on the right arm and the other on the left leg, so essentially when the wave’s moving toward the left leg electrode, you get a positive deflection, like this big positive deflection correspond to the wave moving down the septum.
There are lots of things to look for when reading an ECG, one of them includes figuring out if part of the heart has undergone hypertrophy or enlargement.
Hypertrophy means that a heart’s muscular wall has increased in thickness while dilation refers to an increase in the volume of the chamber.
The term enlargement is generally used when both hypertrophy and dilation occur together, and this is what typically happens in the atria.
In contrast, the ventricles often undergo hypertrophy without dilation. An ECG can show evidence of hypertrophy and enlargement in all of the heart’s four chambers - so let’s go through them one by one.
Normally, atrial depolarization produces a pretty normal looking - symmetric P wave. In right atrial enlargement, all of that extra right atrial muscle tissue results in a large P wave in leads V1 and V2, often over 1.5 mm, as well as in the inferior leads - leads II, III, and aVF, often over 2.5 mm.
One reason why right atrial enlargement develops is that there can be a stenotic or narrowed tricuspid valve that makes it more difficult for the atria to eject blood into the ventricles, and in response, the right atrium enlarges.
In left atrial enlargement, the left atrium has extra muscle tissue and that results in a P wave with two peaks in lead II, with the entire thing stretching out over 110 ms, and a gap of over 40 ms separating the two peaks.
In lead V1 the P wave is biphasic, meaning that it looks like a hill with a valley alongside it. The negative portion is usually 1 mm deep and lasts for more than 40 ms.
Left atrial enlargement also develops from a stenotic valve, but this time it’s the mitral valve that causes the left atrium to get bigger.
Normally, the QRS complex is mostly negative in lead V1 because the large left ventricle, which carries the greatest amount of muscle tissue, is oriented down and away from this electrode.