Final answer:
In a third-degree heart block, the impulses from the SA node occasionally fail to reach the AV node, and the P waves on an EKG are not followed by QRS complexes. This leads to a slower and irregular heart rate due to the independent, slower pacing of the AV node. The normal cardiac cycle requires the coordinated contraction of atria and ventricles for efficient blood pumping.
Step-by-step explanation:
In a third-degree heart block, the electrical impulses from the sinoatrial (SA) node intermittently fail to be conducted through the atrioventricular (AV) node. This disconnect results in P waves not being followed by QRS complexes, disrupting the normal cardiac cycle. Consequently, atrial contraction is not properly coordinated with ventricular contraction, leading to an irregular and often slower heart rate. Furthermore, because the AV node is capable of initiating its impulse, albeit, at a slower rate than the SA node, the ventricles may contract independently, leading to a pulse rate that is typically slower than the atrial rate observed in the electrocardiogram (EKG) P waves. In this condition, the ventricles rely on the lower, innate pacemaking ability of the AV node or even the Purkinje fibers, resulting in a significantly decreased cardiac output which can be life-threatening