The ECG can be split into different segments and intervals which relate directly to phases of cardiac conduction. Limits can then be set on these from which to diagnose deviations from normality.
The main propagation characteristics are:
- the PR interval,
- the PR segment,
- the QRS complex,
- the QT interval,
- the ST segment,
- and the RR interval.
The PR Interval
The PR interval begins at the onset of the P wave and ends at the onset of the QRS complex. This interval represents the time the impulse takes to reach the ventricles from the sinus node. It is termed the PR interval because the Q wave is frequently absent. Normal values lie between 0.12 and 0.20 seconds.
Where ventricular preexcitation is due to an accessory pathway which bypasses the atrioventricular node the duration of the PR interval will shorten. A value of less than 0.12 seconds is termed a short PR interval. The PR interval may also decrease in duration with ectopic beats, because the impulse does not have as far to travel.
First degree atrioventricular block is diagnosed if the PR interval is greater than 0.20 seconds.
The PR interval will gradually increase with the Wenckebach phenomenon.
The PR Segment
The PR segment begins at the endpoint of the P wave and ends at the onset of the QRS complex. It represents the duration of the conduction from the atrioventricular node, down the bundle of His and through the bundle branches to the muscle.
The PR segment may be considered isoelectric, representing atrial systole and ventricular diastole, but may deviate in the presence of atrial injury.
The QRS Complex
The QRS complex begins at the onset of the Q wave and ends at the endpoint of the S wave. It represents the duration of ventricular depolarisation.
Normally all QRS complexes look alike. They are still termed QRS complexes even if all three waves are not visible. Bizarre morphology and duration prolongation may be symptomatic of aberrant conduction, ventricular ectopic or ventricular escape beats, ventricular hypertrophy, or molecular problems, such as electrolyte imbalance or drug toxicity.
The QT Interval
The QT interval represents the duration from the depolarisation to the repolarisation of the ventricles. It begins at the onset of the QRS complex and ends at the endpoint of the T wave.
The duration of the QT interval varies with heart rate, gender, and age.
Short QT intervals are seen with hypercalcaemia, hyperthyroidism, hyperkalaemia, digitalis toxicity and class IB drugs.
Long QT intervals are seen with slow heart rates, myocarditis, hypokalaemia, hypocalcaemia, myocardial disease, anorexia, congenital heart disease, coronary heart failure and class IA and III drugs.
Long QT intervals increase the risk of ventricular ectopic beats. Pauses are a potential arrhythmogenic mechanism with prolonged QT intervals for ventricular tachycardia, and in particular, torsade de pointes (a polymorphic ventricular tachycardia) which may lead to sudden cardiac death.
The ST Segment
The ST segment begins at the endpoint of the S wave and ends at the onset of the T wave.
During the ST segment, the atrial cells are relaxed and the ventricles are contracted so electrical activity is not visible. the ST segment is normally isoelectric.
ST segment depression may occur when the ventricles are being starved of oxygen (normally due to blocked arteries). This is termed myocardial ischaemia.
ST segment elevation occurs with recent cardiac injury, ventricular aneurysms, Prinzmetals angina, or pericarditis.
The RR Interval
The RR interval is the time measurement between the R wave of one heartbeat and the R wave of the preceding heartbeat.
RR intervals are normally regular, but may be irregular with sinus node disease and supraventricular arrhythmias.