E C G Basics

Overview

During the spread of depolarisation through the heart when the chambers contract, the extracellular fluid around the myocardium becomes more negatively charged. The polarised and repolarised regions of the heart remain relatively more positive. This creates a potential difference between the different regions, which ECG electrodes detect. The cardiac dipole is a vector which has both a direction (from the most negative to most positive regions of the heart) as well as an amplitude (voltage). Several electrodes are placed on the body to “look” at the cardiac dipole from different points of view.

The cardiac dipole and its wanderings

During depolarisation of the heart (as the cardiac action potential spreads from its origin in the atria to the farthest corners of the ventricles), the extracellular fluid surrounding the myocardium becomes more negative, because positively charged ions (Na⁺ and Ca²⁺) enter cardiac myocytes. Because the spread of the wave of depolarisation isn’t instantaneous and even, and because the walls of the myocardium aren’t equal in mass, where the least and most depolarised mass of myocardium changes over time. It’s a bit like a rotating battery with a positive and negative terminal spinning in three dimensions as the wave of depolarisation spreads through the chambers of the heart.

Figure 1: Changes in the direction and the magnitude of the cardiac dipole as the wave of depolarisation spreads through the ventricles of the heart (corresponding to the QRS complex on the ECG). We are only considering the movements of the cardiac dipole in the frontal plane; it also moves in the horizontal place. During depolarisation, the extracellular fluid surrounding the myocardium becomes more negative because cations (Na⁺ and Ca²⁺) enter cells during the action potential). The dipole points from the most negative (depolarised) to the most positive (polarised) regions of the heart. The mass of muscle depolarised is an important factor.

The positive and negative terminals represent the cardiac dipole, and this is what an ECG records. Since the human body is essentially a bag of highly conductive, salty water, you can pick up the cardiac dipole by placing a pair of electrodes anywhere on the skin. As the cardiac dipole spins around the heart, its movements generate a current that moves towards or away from electrodes connected to the skin. Whether the dipole causes an upward or downward deflection (or none at all) depends on which direction the dipole is pointing at any particular time with respect to a pair of electrodes. A full, 12-lead ECG shows the movement of the cardiac dipole from multiple, cleverly arranged points of view.

• The dipole exists because there is a difference in charge between different areas of the myocardium.
• An ECG reports the orientation and magnitude of the cardiac dipole from several points of view.
• When the heart is completely depolarised or repolarised, there is no dipole and the ECG is flat (isoelectric).

A dipole is a vector. That is, it has a both a direction and a magnitude, like the wind. You can think of the cardiac dipole as an arrow pointing from the most depolarised region of the heart to the most polarised region. Where each end of the arrow is located depends on how the wave of depolarisation spreads through the heart, and how much mass of myocardium is depolarised or repolarised. The dipole points from the biggest mass of depolarised myocardium to the biggest mass of repolarised myocardium at any particular instant. How this appears on the ECG depends on where electrodes are placed to record the changes.

A handy analogy for the cardiac dipole – although it’s not perfect – is a weather vane showing the direction of the wind. The defect with this analogy is that a vector like the cardiac dipole has both a direction and a magnitude, whereas a weathervane simply shows merely the direction of the wind. However, the analogy describes well what happens when you watch a vector change direction from a fixed viewpoint (Figure 2).

Figure 2: (A) A weather vane points into the direction that wind is coming from because the tail of the pointer presents the most resistance to wind flow and is blown parallel to it. (B) Imagine you are looking a weathervane from the east towards the west. If the wind is blowing from the north or south, then the weathervane will appear as a straight, long arrow. (C) If the wind blows more westerly or easterly – and you don’t change your position – then the length of the weathervane will appear to shorten as it rotates. From a fixed position, you can’t tell if the wind has blown more easterly or westerly. (D) If the wind blows directly east or west, the weathervane will effectively vanish. If you had two friends in different positions you could tell more or less where the weather vane was pointing. This is why an ECG has multiple leads.

This is how the spread of depolarisation looks to a pair of electrodes oriented in a particular direction across the heart as the wave moves and the dipole changes its direction. Some of the time, some leads of the ECG are flat, when there is no movement of charge, because the myocardium is neither depolarising nor repolarising between a given set of electrodes. Some waves are upright in some leads, but upside down in others, depending on which way the leads are “looking” at the heart as the dipole moves around it.

ECG waveforms and intervals

Consider the waveforms on the ECG below (running at the standard 25 mm/sec.) There are 5 waveforms, arbitrarily designated P, Q, R, and T. This traces is representative of what you’d typically see in lead II, which is in line with the cardiac dipole at its biggest.

Figure 3:A typical Lead II ECG recording. Note the time scales when the paper is running at the usual 25 mm/sec. The basic PQRST waveforms are present on this representation, but aren’t always apparent on every lead and in every patient. Q waves, in particular are small or absent in most leads

What each waveform represents:

P wave
This is atrial depolarisation. The single P wave represents two overlapping waves of depolarisation – one for each atrium. When conduction between the atria is delayed – as in atrial enlargement – the P wave may appear notched because the waves are spread slightly apart due to the extra time it takes for the passage of depolarisation from the right atrium to the left.. Because the atria have relatively little mass, the slower and less well synchronised process of repolarisation doesn’t appear on the ECG.

QRS complex
This is ventricular depolarisation. The Q wave represents depolarisation of the septum from the left to the right. It appears in those ECG leads that look at the heart from the left (I, II, aVL, V3 and V6). When Q waves are large and appear in other leads they are deemed “pathological Q waves”. The shape of the R and S waves varies from lead to lead, depending on how each provides a different view of the heart and the movement of the cardiac dipole.

T wave
The T wave represents ventricular repolarisation. As in the atria, the spread of repolarisation is slower and less synchronised than the depolarisation following the sharp upstroke of the cardiac action potential. However, the mass of the ventricles is such that it is plainly visible on the ECG as a broad T wave. The real mystery here is why is the T wave upright, if it represents repolarisation, not depolarisation? Shouldn’t the deflection be downwards if the opposite electrical event is occurring?

The myocytes towards the surface of the heart have a shorter action potential than those deeper down in the myocardium. This is because myocytes near the surface of the heart (subepicardial myocytes) express more potassium channels, leading to more rapid repolarisation of the action potential. Because of this, repolarisation of the ventricles begins in the outer subepicardium, whereas you might expect it to start in the septum and spread outwards, as is the case for depolarisation. The consequence of this is that the distribution of positive and negative charges is similar during depolarisation and repolarisation of the ventricles, so both appear as deflections in same direction on the ECG (Figure 4).

Figure 4: The pattern of spreading of depolarisation and repolarisation explains why the T wave is upright in lead II of the ECG. The extracellular fluid is more negative in the parts of the ventricle that are depolarised. Depolarisation (seen as the QRS complex) spreads from the bundles of His to the outer surface of the heart. However, the action potentials in the myocytes of the subepicardium are shorter than in deeper parts of the heart, so this region repolarises first. For this reason, the relative distribution of charges during depolarisation and repolarisation are the same, so both appear as upward deflections on the ECG. (Adapted from Levick’s An introduction to cardiovascular physiology, 5th Ed.)

Rate and rhythm are read from the RR interval

You can work out the heart rate from an ECG by counting the number of big boxes between the obvious and sharply defined R waves. If you divide 300 by the number of boxes, the result is heart rate in bpm. You can quickly learn to “guesstimate” heart rate by remembering the rates associated with the distances between multiple big boxes:

Figure 4: : Estimating heart rate the fast way. Five big boxes between R waves is 60 bpm, the borderline for bradycardia. Three boxes represents 100 bmp, the borderline for tachycardia.

By examining the RR interval for a couple of cycles, you can determine whether the rhythm is steady, or irregular.