The Waves

All ECG waves reflect a change in myocardial excitation. They are always looked at with reference to the so-called isoelectric line, or baseline. An isoelectric line is seen when NOTHING is "happening", that is when the cells are either in their resting state or completely depolarized. This is called "asystole". If the myocardial cells remain in this condition for more than a few seconds, you should check whether the patient is still alive, because it means that the heart is not pumping!

The ECG waves indicate that some degree of excitation is spreading or receding. A positive deflection is directed upwards from the isoelectric line. A negative deflection is directed downwards.

Whether a deflection is negative or positive depends on two things. First, it depends on the spatial orientation, or the vector of the current excitation. Second, it depends on which lead you are looking at. If depolarization moves towards the lead of interest, or at an angle of less than 90 degrees to it, the deflection will be positive. If depolarization moves away from the lead of interest, the deflection will be negative.

This diagram shows a normal excitation sequence. The sequence begins with atrial depolarization. This is represented on the ECG by what is termed the P-wave.

This is followed by a phase in which the excitation travels through the AV node. The AV node contains so few cells that their depolarization causes no deflection from the isoelectric line. This isoelectric line is called PR-interval. Ventricular excitation then occurs. This is represented on the ECG by the so-called QRS complex. The so-called "J point" indicates the transition to the ST segment, which is usually isoelectric. Finally, ventricular repolarization occurs, which is represented on the ECG by the T-wave.

In the following chapters, we will discuss each of the ECG waves in more detail.