The P Wave:
The P wave represents the spread of electrical activity over the atrium. The normal depolarization begins at the sinoatrial (SA) node near the top of the atrium. Because of the top-to-bottom, right-to-left path of the P wave, it’s normally largest in lead II. The normal P wave is upright in all leads except R.The P wave normally lasts less than 0.11 seconds (just less than three small boxes). An abnormally long P wave occurs whenever it takes extra time for the electrical wave to reach the entire atrium. This occurs in left atrial enlargement.
The height of the P wave is normally less than 2.5 small boxes (less than 0.25 millivolts). An abnormally tall P wave is seen when larger amounts of electricity are moving over the atrium. This usually indicates hypertrophy of the right atrium. The P wave may be decreased in height by hyperkalemia.
The PR Interval:
Following the P wave is the PR segment. (NOTE: the PR segment and the PR interval are NOT the same thing.) The PR segment is not routinely measured, but may be commented on if it is depressed or elevated. During the PR segment, the electrical wave moves slowly through the atrioventricular (AV) node. This activity is not seen on the ECG.
The PR interval is the time from the beginning of the P wave until the beginning of the QRS complex. It is normally between 0.12 and 0.2 seconds (three to five small boxes) in length.The PR interval may be prolonged when conduction of the electrical wave through the AV node is slow. This may be seen with degenerative disease of the node, or with digoxin, hyperkalemia, hypercalcemia, or hypothermia.
The PR interval may be unusually short when conduction is rapid. A mildly short PR interval may be seen with hypokalemia or hypocalcemia. An artificially-short PR interval occurs when the QRS complex begins early, as happens with an extra conducting bundle — Wolff-Parkinson-White Syndrome (WPW).
The QRS Complex:
The QRS complex represents activation of the ventricle. Special conducting bundles spread the wave of depolarization rapidly over the ventricle.
The QRS complex is normally less than 0.10 seconds in length — two and a half boxes. Lengthening of the QRS indicates some blockage of the electrical action in the conducting system. This may be due to ischemia, necrosis of the conducting tissue, electrolyte abnormality, or hypothermia.
If the first deflection of the QRS is downward, it’s called a Q wave. The Q wave represents activation of the ventricular septum. The electricity spreads from right to left through the septum.
Q waves may be normal. For example in lead I, a Q less than 1/4 of the R height, and less than one box wide, is considered normal. This is the early activation of the septum. This activation goes left — away from lead I — and is therefore negative on the ECG. “Septal Qs” are normal in I, F, V5 and V6. Qs are also generally innocent in lead III and lead V1 if no other abnormality is seen.
Q waves are “significant” if they are greater than 1 box in width (longer than 0.04 msec) OR are larger than 1/4 of the R wave. Significant Q waves indicate either myocardial infarction or obstructive septal hypertrophy (IHSS).
The first upward deflection of the QRS is called the R wave. Most of the ventricle is activated during the R wave. The R wave may be prolonged if the ventricle is enlarged, and may be abnormally high (indicating strong voltage) if the ventricular muscle tissue is hypertrophied.
The S wave is any downward deflection following the R wave. Like the R wave, an abnormally large S wave may indicate hypertrophy of the ventricle.
If a second upward deflection is seen, it’s called an R-prime wave. R-prime waves are never normal, but indicate a problem in the ventricular conduction system.
QRS complexes may be described by naming the waves that form them. For example, a complex with an R, an S, and an R’ is called an RSR’ complex.
The ST Segment:
The ST segment is the portion of the tracing falling between the QRS complex and the T wave. During this time, the ventricle is contracting, but no electricity is flowing. The ST segment is therefore usually even with the baseline.
The length of the ST segment shortens with increasing heart rate. Abnormality of electrolytes may also affect the ST segment length, however measurement of the length of the ST segment alone is usually not of any clinical use.
Upward or downward shifts in the ST segment are extremely important. Deviation of the ST segment from baseline can indicate infarction or ischemia, pericarditis, electrolyte abnormality, or ventricular strain. ST segment elevation or depression is generally measured at a point two boxes beyond the QRS complex.
The T wave:
The T wave represents the wave of repolarization, as the ventricle prepares to fire again. The T wave is normally upright in leads I, II, and V3-V6. It is normally inverted in lead R. Ts are variable in the other leads (III, L, F, and V1-V2).
T wave abnormalities may be seen with, or without ST segment abnormality. Tall T waves may be seen in hyperkalemia or very early myocardial infarction. Flat T waves occur in many conditions. Inverted T waves may be seen in both ischemia and infarction, late in pericarditis, ventricular hypertrophy, bundle branch block, and cerebral disease.
In young children, T waves may be inverted in the right precordial leads (V1 to V3). Occasionally, these T inversions persist in young adults.
The U Wave:
A second wave following the T wave is called a U wave. Large U waves may be seen in electrolyte abnormality (such as hypokalemia), or with drug effects.
The QT Interval:
The QT interval is the time from the beginning of the QRS complex until the end of the T wave. The “normal” QT length varies with heart rate. Very fast rates shorten the QT length.
At normal heart rates, QT length is abnormal if it’s greater than 0.40 sec (10 boxes) for males and 0.44 sec (11 boxes) for females. Extreme QT prolongations (greater than 0.60 sec — 15 small boxes) predispose the patient to arrhythmias.The QT interval may be prolonged with electrolyte abnormality, such as hypokalemia, hypocalcemia, or hypomagnesemia. Myocardial ischemia may also prolong the QT interval
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