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Summary of lecture
The Normal ECG (EKG) waves, intervals, durations and rhythm
ECG interpretation requires firm knowledge of the characteristics of the normal ECG. The waves, intervals, durations and assessment of rhythm is fundamental to ECG interpretation. Below follows a brief summary of the ECG waves, their appearance, terminology, physiological background and interpretation.
Overview of the normal electrocardiogram (ECG / EKG)
Figure 1. The classical ECG curve with its most common waveforms. Important intervals and points of measurement are depicted. ECG interpretation requires knowledge of these waves and intervals.
The P-wave, PR interval and PR segment
ECG interpretation starts with assessment of the P-wave and PR interval.The P-waveis generated by depolarization (activation, contraction) of the atria.The PR interval is the intervalbetween the startof the P-wave andthe startof the QRS complex. The PR interval determines whether impulse transmissionfrom atria to ventricles is normal. The isoelectric (flat)line between the end of the P-wave and the startof the QRS complex is called the PR segment.The PR segment isthe baseline (also referred to as reference line or isoelectric line) of the ECG curve. Thus, when measuring the amplitude of awave on the ECG, the PR segment is the baseline.Refer to Figure 1.
The QRS complex
The QRS complexreflects the depolarization (activation, contraction) of the ventricles. Although it may not always include a Q-wave, R-wave and S-wave, it is still referred to as a QRS complex. In other words, if ventricular depolarization only generates a Q-wave and an R-wave, that complex may still be referred to as a QRS complex. However, one may also be more explicit and refer to such a complex as a QR complex. Because the left ventricle is usually considerably larger than the right ventricle, the QRS complex is actually a reflection of the electrical potentials generated by the left ventricle.
QRS duration
QRS duration is the time durationfrom the startto the end of the QRS complex. A short QRS complex impliesthat the ventricles are depolarized rapidly, which in turn implies that the electrical conduction system functions normally. Wide (also referred to as broad) QRS complexes indicate that ventricular depolarization is slow, which may be a consequence of disturbances in the electrical conduction system.
The J point and the ST segment
The ST segment must always be studied carefullysince its appearance may be changed in a wide range of seriousconditions. The ST segment is of specialinterest in the setting of acute myocardial ischemia. This is explained by the fact that ongoing ischemia causes deviation (shift) of the ST segment (ST segment deviation). ST segment depression implies that the ST segment is depressedbelow the level of the PR segment. ST segment elevation implies that the ST segment is elevatedabove the level of the PR segment. The magnitude of any ST segment deviationis measured as the height difference (millimeters) between the J point and the PR segment. The J point is where the ST segment starts.
The T-wave
The T-wave reflects the rapid repolarization (recovery) of the myocardiumand T-wave changes occur in numerousconditions. T-wave changes are frequently misunderstood. The transition from the ST segment to the T-wave should be smooth. The normal T-wave is somewhatasymmetric, with a steeper downward slope.
The U-wave
The U-wave, which is a positive wave after the T-wave,appearsoccasionally on the ECG. Its height (amplitude) is approximatelyone fourth of the amplitude of the T-wave. The U-wave is most oftenseen in leads V2, V3 and V4. Individuals with prominent T-waves display U-waves more often. Moreover, the U-wave is clearer during slow heart rates (bradycardia). The physiological processthat generates the U-waveremain elusive.
QT duration and QTc duration (corrected QT duratation)
QT duration reflects the total duration of ventricular depolarization (activation) and repolarization (recovery). It is measured from the startof the QRS complex to the end of the T-wave. The QT interval increases at slower heart rates and vice versa (i.e it decreases at higher heart rates). Therefore, to judgewhether the QT interval is normal it is necessary to take the heart rate into account.The heart rate adjusted QT interval is the corrected QT interval, or simply the QTc interval. A long QTc interval causes electrical instability in the ventricles and this may cause lethal ventricular arrhythmias.
