This can occur with bundle branch blocks or whenever a ventricular foci (abnormal pacemaker site) becomes the pacemaker driving the ventricle. If the QRS complex is prolonged (> 0.10 sec), conduction is impaired within the ventricles. This relatively short duration shows that ventricular depolarization normally occurs rapidly despite the large mass of the ventricles. The duration of the QRS complex is normally 0.06 to 0.10 seconds. Click here to see how the ventricular rate is calculated. The QRS complex represents ventricular depolarization. Ventricular rate can be calculated by determining the time interval between QRS complexes. If the PR interval is >0.20 sec, there is an AV conduction block, which is called a first-degree heart block if each impulse from the atria can still be conducted into the ventricles. The period of time from the onset of the P wave to the beginning of the QRS complex is termed the PR interval, which normally ranges from 0.12 to 0.20 seconds in duration. This interval represents the time between the onset of atrial depolarization and the onset of ventricular depolarization. Click here to see how the atrial rate is calculated. The brief isoelectric (zero voltage) period after the P wave represents the time in which the impulse is traveling within the AV node (where the conduction velocity is retarded) and the bundle of His. Atrial rate can be calculated by determining the time interval between P waves.
The P wave represents the wave of depolarization that spreads from the SA node throughout the atria and is usually 0.08 to 0.10 seconds (80-100 ms) in duration.
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