Final answer:
The QRS complex duration is 80-120 milliseconds, extended in bundle branch block. Third-degree heart block results in dissociation of atrial and ventricular activities, causing a slower heart rate (pulse).
Step-by-step explanation:
The QRS complex on an electrocardiogram (ECG) typically lasts 80-120 milliseconds and represents the depolarization of the ventricles, preceding their contraction. When a bundle branch block occurs, this depolarization process is disrupted, resulting in an elongation of the QRS duration; it often exceeds 120 milliseconds because the electrical signal must travel through the heart muscle at a slower pace than normal.
In the case of a third-degree or complete heart block, there is a lack of synchronization between the atrial and ventricular contractions, as indicated by the absence of correlation between the P waves and QRS complexes on the ECG. This type of block can lead to a decreased heart rate, as the atrioventricular node (AV node) or other pacemaker cells distal to the block assume the role of initiating contractions at a slower inherent rate (normally 40-60 beats per minute), which can still maintain consciousness.
Briefly summarizing the effects of bundle branch block and heart blocks on the QRS complex: Bundle branch block lengthens the QRS duration, while a third-degree heart block dissociates the P waves from the QRS complexes, leading to a slower pulse.