I was recently asked a question regarding the amplitude of a deflection on an ECG - more specifically, the question was about *how to determine the amplitude of a deflection*. When we say amplitude, we really mean *magnitude*.

First of all, it is very important to understand that every deflection on the ECG is a *vector* and also that every vector has *direction* and *magnitude*. Our measurements of a vector's direction also occur at one instance in time. Vectors can change directions - up, down, right, left. They can even travel in circles. But at each instance in time they are traveling in one specific direction.

The ECG itself is a graph of these vectors with time represented on the horizontal axis and voltage on the vertical axis. We are trained to view voltage as the measurement of the amplitude of a vector. However, the *amplitude of a deflection* (again, *all deflections are vectors*) is actually the ** area under the curve using the baseline to serve as the third side thus enclosing the area**. Any attempt to use the height or depth of a deflection as the amplitude of that deflection is only going to be a very rough approximation of the amplitude... somewhat analogous to determining the volume of a box by considering only how tall the box is. What if the box is short but very, very wide? How would that compare with a box that was just slightly taller but very, very narrow?

Unless you are a genius at determining the formula for the slope of that deflection and can then determine the area under the curve using integral calculus... ** using the height or depth as measured on the vertical axis is just about the best we can do**. In the case of a QRS interval that may contain both positive and negative deflections,

In the "old" days, Braunwald, Marriott and others would try to count the squares contained within a deflection for a better approximation of the amplitude (they even estimated *partial *squares!). This is obviously not feasible in today's busy practices. *The best we can do is measure the height or depth of the deflection, often considering its width as well*. This is why you sometimes see the ECG machine giving a mean axis in the frontal plane that is based on a QRS that does not appear to be the tallest of all the limb leads.

Today, the amplitude of a deflection (i.e., magnitude of a vector) is used to determine the frontal plane axis of that deflection, the origin of a P wave, the acuity of a subendocardial and/or epicardial ischemia and the size of a heart chamber.

Just remember that using the height or depth of a deflection to determine amplitude (magnitude) - which is the best we can do - is like estimating the volume of a box using only its height. Always be aware of the inherent vagaries of determining amplitude by this method.