The McGill Physiology Virtual Lab

Cardiovascular Laboratory

ECG> Basics
  In brief, you need to understand and remember:
  • the depolarization and repolarization sequence in the heart
  • the fact that when a wavefront of depolarization travels towards the + electrode and away from the electrode attached to the - terminal, a positive-going deflection will result.
  • the voltage recorded along a particular lead axis  at a particular time is obtained by taking a projection onto that axis of the vector representing the magnitude and direction of depolarization at that time.
  • Einthoven's law: a complex in lead II is equal to the sum of the corresponding complexes in leads I and III.
   

Activation of the Heart and the ECG

 

The drawings to the right in the table below show the main stages of activation of the heart, as well as the ECG recorded in lead II at those stages. 

 

The electrical activity of the heart originates in the sino-atrial node.  The impulse then rapidly spreads through the right atrium to the atrioventricular node.  It also spreads through the atrial muscle directly from the right atrium to the left atrium. The P-wave is generated by activation of the muscle of both atria.

 
The impulse travels very slowly through the AV node, then very quickly through the bundle of His,  then the bundle branches, the Purkinje network, and finally the ventricular muscle.
The first area of the ventricular muscle to be activated is the interventricular septum, which activates from left to right.  This generates the Q-wave.
Next, the left and right ventricular free walls, which form the bulk of the muscle of both ventricles, gets activated, with the endocardial surface being activated before the epicardial surface.   This generates the R-wave.
A few small areas of the ventricles are activated at a rather late stage.  This generates the S-wave.
Finally, the ventricular muscle repolarizes. This generates the T-wave.

 

To understand the morphology of the ECG waveforms one needs to appreciate only one biophysical fact: if a wavefront of depolarization travels towards the electrode attached to the + input terminal of the ECG amplifier and away from the electrode attached to the - terminal, a positive-going deflection will result.  If the waveform travels away from the + electrode towards the - electrode, a negative going deflection will be seen. 
If the waveform is travelling in a direction perpendicular to the line joining the sites where the two electrodes are placed, no deflection or a biphasic deflection will be produced.

One can thus see that the voltage recorded along a particular lead axis (the vector joining the - to the + electrode) at a particular time is obtained by taking a projection onto that axis of the vector representing the magnitude and direction of depolarization at that time.  Thus, when the lead axis in the figure above points from left to right, parallel to the direction of movement of depolarization, a positive-going complex results.  When the two directions are anti-parallel, a negative-going complex is produced.
From the principles outlined above, one can determine how the ECG waveforms arise at each point in time.  For example, since the direction of atrial depolarization is almost exactly parallel to the axis of lead II (which is from RA to LL), a positive-going deflection (P wave) would result in that lead.
Since the ventricular muscle is much thicker in the left than in the right ventricle, the summated depolarization of the two ventricles is downwards and toward the left leg:   this produces again a positive-going deflection (R-wave) in lead II, since the depolarization vector is in the same direction as the lead II axis.
As septal depolarization moves from left to right, the depolarization vector is directed towards the - electrode of lead II (RA), and therefore a negative-going deflection (Q-wave) is produced.
Cardiac axis
The cardiac axis refers to the mean direction of the wave of ventricular depolarization in the frontal plane, measured from a zero reference point. The mean QRS axis is obtained from measurements of the heights of the QRS waves in the 3 leads.

In the example to the right, notice that there are tall R waves in leads I and II, and that in lead III, the R and the S waves are of equal size and opposite direction.
 

Let us now calculate the direction of depolarization of the ventricular muscle. We have to arrive at a vector such that the projections of this vector onto the three lead axes is consistent with the height of the QRS complexes in the three leads.
 

COMMON MISCONCEPTIONS ABOUT THE ECG
  1. The PR interval is NOT in general measured from the P wave to the R wave. It is rather defined to be the time from the beginning of the P-wave to the beginning of the QRS complex. Thus the PR interval is measured from the beginning of the P-wave to the beginning of the R-wave only if the first deflection in the QRS complex happens to be an R-wave (i.e. no Q-wave present).
     
  2. Similarly, the QT interval is NOT in general measured from the Q-wave to the T-wave. It is rather defined as the time from the beginning of the QRS complex to the end of the T-wave.
     
  3. The P-wave (QRS complex) is NOT generated by the contraction of the atria (ventricles). It is generated by electrical activity (more specifically depolarization or activation) of the muscle.
     
  4. Purkinje fibre cells are NOT nerve cells. Rather, they are specialized cardiac muscle cells. The sinoatrial node, atrioventricular node, bundle of His, and bundle branches are also made up of specialized cardiac muscle cells.


The following statements are true:

  1. One does NOT see any deflection on the ECG during the time that the sinoatrial node is being depolarized. The depolarization of the atrioventricular node and the His-Purkinje system also does not generate any electrical activity that is detectable in the ECG.
     
  2. One does not necessarily see a Q-wave or an R-wave or an S-wave in each lead that one examines. Indeed, in some individuals with perfectly normal hearts, there is no Q-wave present in any of the three leads I-III. Other normal individuals have no S-waves in any of the three leads.

For a fun, interactive ECG exercise click here

To continue with the next section: ECG Experiments, click here