Drbeen Test Taking Tip: Normal Electrocardiogram (ECG or EKG) is a concept driven topic and USMLE and FSMB will probably not test normal ECG directly. However, it is imperative that you start with knowing the components of a normal ECG wave in order to identify the deviated and abnormal ECGs (covered in part 2). These skills will be inevitable in diagnosing conditions like Arrhythmias, Myocardial infarction, Hypertrophy of various cardiac chambers and Electrolyte disturbances. Here, we cover the high yield components on the ECG wave as these are the most commonly tested topics on Board exams.
ECG refers to the the process of recording potential fluctuations during the cardiac cycle. As a result of sequential spread of excitation in the atria, the interventricular septum and the ventricular walls and finally repolarization of the myocardium, a series of positive and negative waves designated as P, Q, R, S, and T are recorded during cardiac cycle. Depolarization moving towards an active electrode in a volume conductor produces a positive deflection, whereas depolarization moving in the opposite direction produces a negative deflection.
WAVES OF ECG
- P WAVE
+P wave is a positive (upright rounded) deflection caused by the depolarization of atrial musculature also known as the atrial complex.
+Duration of P wave is not more than 0.1 second and Amplitude of P wave is from 0.1 to 0.12 mV.
+Clinical significance of P wave is the extrapolation in diagnosing conditions related to the functional activity of atria.
- QRS COMPLEX
+QRS complex consists of three consecutive waves. Q wave is a small negative wave which may be normally absent quite often. It is continued by a tall R wave followed by a small negative S wave. QRS complex is a result of ventricular depolarization.
+Duration of QRS complex is normally less than 0.08 second, it is a measure of interventricular conduction time.
+Amplitude of Q wave is 0.1 to 0.2mV, R wave is 1mV and S wave is 0.4mV (Total being 1.5-1.6mV)
+Clinical significance of QRS complex from precordial leads are more important than limb leads.
- T WAVE
+T wave is the last, positive, dome shaped deflection. Normally, it is in the same direction as that of QRS complex, because ventricular repolarization follows a path opposite to that of depolarization. T wave represents ventricular repolarization.
+Duration of T wave is approximately 0.27 seconds and Amplitude is about 0.3mV.
+Clinical significance of T wave involves in diagnosing the following;
- U WAVE
+U wave is a small and positive round wave representing slow repolarization of papillary muscle.
+The duration of U wave (when present) is 0.08 seconds and the amplitude is about 0.2mV.
Note: Since atrial repolarization coincides ventricular depolarization, so it is merged with QRS complex and thus not recorded as a separate wave.
INTERVALS AND SEGMENTS OF ECG
- PR INTERVAL
+It is measured from the onset of P wave to the onset of the QRS complex. Functionally, it is the PQ interval.
+This interval is the measure of AV conduction time, including the AV nodal delay.
+The duration varies from 0.12 to 0.21 second depending on the heart rate.
+A prolonged PR interval is helpful in diagnosing AV conduction block. First degree AV block is produced when PR interval is between 0.2-0.3 second and the second degree block is produced when the PR interval is increased to 0.3-0.45 second.
- J point
+Point on ECG that coincides with the end of depolarization and start of repolarization of ventricles, i.e. it occurs after the end of QRS complex.
+Since all parts of the ventricle at this point are depolarized, no current is flowing around the heart. Thus, J point is a ZERO VOLTAGE point. It is the time from the start of the QRS complex to the end of T wave.This indicates total systolic time of ventricles, i.e. ventricular depolarization and repolarization.
+The duration of QT interval is about 0.4 second.
+An iso-electric period between the end of QRS complex and the beginning of T wave.
+The duration is about 0.32 second
+ST segment corresponds to the period of ventricular repolarization phase of cardiac cycle.