Components of the ECG Strip
A cardiac cycle, or one heartbeat, is represented on the ECG as one PQRST complex. Between cardiac cycles, the ECG recorder returns to the isoelectric line, the flat line on the ECG strip where electrical activity is absent. Any waveform deflecting above the isoelectric line is considered positive and below the line is considered negative. A waveform that deflects both above and below the isoelectric line is called biphasic.
The first deflection in the cycle is the P wave. The first part of the P wave represents right atrial depolarization and the second part of the P wave represents left atrial depolarization. As both atria depolarize at roughly the same time, we see a single waveform on the ECG.
Normal P waves are smooth and rounded, upright in lead II, between 0.5 and 2.5 mm high, and 0.12 sec or less in width. There is one P wave to every QRS complex.
Not all P waves are created equal. Abnormal P waves can occur when the impulse has to travel through an enlarged atria causing abnormal depolarization of the atria (see examples). P waves may also originate from a location other than the SA node - atria or AV junction. These P waves are called ectopic and may be positive, negative, small, pointed, flat, wavy, or saw tooth in appearance. P waves originating from the AV junction are always negative (inverted) and may precede, follow, or be hidden in the QRS complex.
PR Interval (PRI)
The PRI is the time from the beginning of atrial depolarization to the beginning of ventricular depolarization. It represents the spread of the impulse from the SA node through the atria, AV node, bundle of His, bundle branches and Purkinje fibres. It includes the P wave and the short isoelectric line that follows it. The PRI varies with heart rate – as heart rate increases the PRI shortens and vice versa.
A normal PRI is 0.12 to 0.20 seconds in adults. An abnormal PRI, greater than 0.20 sec, can result with delays in conduction through the atria, AV node, or bundle of His. A shorter than normal PRI, less than 0.12 sec, may occur if the impulse originates in an ectopic pacemaker close to the AV node or bundle of His, or travels down an abnormal conduction pathway (accessory pathway) that bypasses the AV node and depolarizes the ventricles earlier than usual. Wolff-Parkinson-White is an example of such a condition.
The QRS complex represents depolarization of the left and right ventricles and is made up of the Q wave, R wave and S wave. Ventricular depolarization normally triggers contraction of the ventricles.
The QRS complex is much larger than the P wave as ventricular depolarization involves a greater muscle mass than that of the atria. Remember, the QRS represents depolarization of both ventricles, however, because the left ventricle has a greater muscle mass than the right ventricle, we generally don’t see the deflection from the right ventricle as it is hidden by the larger left ventricle.
A QRS complex normally follows each P wave. Many variations exist in the configuration of the QRS complex and not every QRS complex contains a Q, R and S wave. The shape of the QRS indicates what path the electrical signal took through the ventricles. Whatever the presenting variation, it's called the QRS complex. The Q wave, if present, is always a negative deflection. The large triangular upright waveform is the R wave and is the first positive deflection following the P wave. The S wave is the negative waveform following the R wave.
Abnormal QRS complexes may be a result of an impulse originating in an ectopic pacemaker. For example, if the impulse originates in the Purkinje fibres or the ventricular myocardium, the QRS is greater than 0.11 sec (often 0.16 sec or more). Right ventricular enlargement produces an abnormally tall R wave and left ventricular enlargement produces an abnormally deep S wave. Other variations in QRS morphology may be caused by a block in the bundle branches, premature beats, conduction along an accessory pathway, or from an ectopic site in the ventricles.
The QRS is measured from the point where the complex begins to move from the baseline and ends where the last wave of the complex begins to level out, or distinctly changes direction. The normal duration of the QRS complex is less than 0.11 sec or 3 small boxes.
The ST segment represents early ventricular repolarization and is normally at the isoelectric line on the ECG. It begins where the QRS complex and the ST segment meet. This point is referred to as the J point. The ST segment may be above (elevated) or below (depressed) the isoelectric line. To determine if elevation or depression exists, look at the ST segment 0.06 sec (1.5 small boxes) after the J point. Elevation or depression of 1 mm (one small box) or more is considered abnormal.
The T wave represents ventricular repolarization. The normal T wave begins as the deflection slopes upward from the ST segment and ends when the waveform returns to baseline. Normal T waves are rounded and slightly asymmetrical, positive in lead II, and less than 5 mm high. The T wave always follows the QRS complex. Abnormal T waves can be tall, peaked, flattened, low, biphasic or inverted. Common causes include MI, ischemia, pericarditis, hyperkalemia, bundle branch block and ventricular enlargement.
The QT interval is the period from the beginning of the QRS complex to the end of the T wave and represents total ventricular activity (depolarization to repolarization). The length of the QT varies according to age, gender, and heart rate. A normal QT should be half the R-R interval when the rhythm is regular. A prolonged QT indicates a delay in ventricular repolarization which can allow more time for an ectopic focus to take control and put the ventricles at risk for ventricular dysrhythmias. Common causes include electrolyte imbalances, medications, MI, long QT syndrome.
The U wave is a small waveform that follows the T wave and represents repolarization of the Purkinje fibres. It may or may not be present. Normal U waves are small, round, asymmetric and about 10% of the height of the T wave.
R-R and P-P Intervals
The distance from one R wave to the next R wave (and P wave to P wave) is used to determine rhythm rate and regularity. To determine ventricular regularity, measure the distance from one R wave to the next consecutive R wave and compare this to the other R-R intervals. Regular rhythms will have R-R intervals that measure the same. To determine atrial regularity, do the same thing with the P-P interval.