The Physics of ECGs

Cardiac Activity as Electrical Currents

Depolarization of myocardium is caused by an influx of Na+ cations. Therefore, depolarized myocardium will be relatively positively charged, compared to the resting membrane potential. The electrochemical gradient between depolarized and resting myocardium causes a flow of positive charge away from the depolarized myocardium and towards the resting myocardium. This flow of positive charge is known as a current.

Conversely, repolarization of myocardium is caused by an efflux of K+ ions from the cell. Therefore, repolarized myocardium will be relatively negatively charged, compared to depolarized myocardium. Similarly, because of the electrochemical gradient between depolarized and repolarized myocardium, the current of positive charges moves away from the depolarized myocardium and towards the resting myocardium.

So what does the ECG measure? 

The ECG measures the voltage of the net cardiac dipole. While not exactly the same thing, an easier way of thinking about it is that the ECG measures the net direction and amplitude of electrical activity in the heart. 

On the left, notice the currents formed by the movement of positive charge along the advancing depolarization wavefront. The ECG measures the cumulative effect of all the cardiac events in the heart, and not the individual currents themselves, as shown on the left.

The voltage of the cardiac dipole, which correlates to the magnitude and direction of overall electrical activity in the heart, is measured by attaching electrodes to the surface of the skin in strategic spots. This is possible because the cardiac dipole generates an electric field that propagates to the surface of the skin.

To measure voltage, we need to designate a positive electrode and negative electrode. The combination of a positive and negative electrode forms a lead, which is a vector going from the negative electrode to the positive electrode.

Positive charges moving toward a lead, such as all the currents shown in purple above, will contribute a positive voltage.

Positive charges moving away from a lead, as shown above, will contribute a negative voltage. 

Putting it all together:

Deflections

We look at ECG leads to visualize the voltages formed by the electrical activity in the heart, in order to give us an idea of which way positive charges are moving relative to our lead. The ECG represents this voltage as a deflection (change from baseline) of a voltage vs time graph.

When there is no deflection, this is referred to as the “isoelectric line” or the “baseline”. It represents no net electrical activity being measured by the leads.

Currents moving towards a lead will generate positive voltages, which will manifest as positive deflections on the ECG. The amplitude of the deflection depends on the amplitude of the net current as well as its degree of alignment with the lead.

Currents moving away from a lead will generate negative deflections. Once again, the amplitude depends on both the amplitude and direction of the net current.

A biphasic deflection (shown above) consists of adjacent positive and negative deflections. It represents a current that is changing directions. It is important to note that the current moves predominantly in the direction of the component of the biphasic deflection with the greater amplitude.

An equiphasic deflection (above) is a biphasic deflection with equal positive and negative components. It represents a current generally moving perpendicular to the lead.

Faster currents tend to produce narrower, taller, more symmetric deflections. Slower currents tend to produce wider, shorter, and less symmetric deflections. This is because, with slower currents, myocardial electrical activity is spread out over a longer period of time.

Practice

Below are some practice examples to help solidify your understanding of deflections. The answers to each question will be on the following slide.