on the y axis. If the depolarization wave (summation of all cardiac cell potentials) is moving from the
negative electrode towards the positive electrode, the record will produce a positive or upward ECG
deflection. When the wave of depolarization moves towards the reference electrode, the recording
will be a negative ECG wave deflection. If the wave of depolarization moves perpendicular to the
positive and negative electrodes it is called isoelectric (no net positive or negative ECG deflection).
The normal ECG has a series of distinct waves called deflection waves (P wave, QRS complex and T
wave). Each part of the ECG represents a specific electrical event in the ventricle. The first wave
deflection generally observed on the ECG is the small P wave and is produced when SA node causes
the right and then the left atria to depolarize. The QRS complex represents ventricular
depolarization and the T-wave represents the electrical signature of ventricular repolarization.
The QRS complex consists of three deflections. The first negative ventricular deflection is called
the Q-wave. The first positive deflection from the ventricle is called the R-wave. The first negative
deflection after the R-wave is called the S-wave. If there is no observed Q-wave you may observe a
RS only. If there is no positive R-wave, the negative deflection is called a QS-wave. The QRS complex
marks arrival of the wave of depolarization into the septum and ventricular walls. Larger wave sizes
(i.e., large voltage amplitude change) are caused by either a larger muscle mass of the ventricles or
the fact that the wave of depolarization is moving directly towards the electrode. The QRS complex
is typically about 0.08 seconds in duration. IT IS VERY IMPORTANT to remember that depolarization
(QRS) comes BEFORE myocardial contraction. IT IS ALSO very important to remember that just
because the heart depolarizes, blood need not be ejected into the pulmonary artery or aorta (ECG
rate does not always equal the pulse rate!)
The T wave is the final noteworthy
deflection on the trace. This wave marks
the change in voltage created by
ventricular repolarization.
An “interval” includes both a wave and
the baseline period between waves (e.g.,
PR interval, QRS interval, ST interval).
The time it takes for the impulses to
travel from the SA node and enter the
septum is represented by the PR-
segment (typically about 0.16-0.18 sec).
The R-R interval is one of the most
important, if your heart rate is 60
beats/minute the R-R interval is 1 second. If the average RR interval of five cardiac cycles was 0.5
seconds, the heart rate would be 120 beats/minute (1 beat/ 0.5 sec X 60 seconds/minute = 120
beats/minute or tachycardia).
A “segment” is a period of time when no waves occur on an ECG trace from the end of one wave to
the beginning of the next wave or complex The PR segment tells the clinical how long it takes for the
wave of depolarization to pass through the AV node. The ST segment represents the amount of time
that the ventricle is depolarized and contracting.
You might assume the atria do not go through repolarization based on the above description, but that
would be the wrong assumption. The atria do repolarize, however the mass of the atria is very small
relative to the mass of the ventricles and this electrical repolarization event is obscured (hidden) by
the depolarization of the ventricles and the QRS complex.