Final answer:
A second-degree AV block shows some P waves not followed by a QRS complex on an ECG, indicating that not all heartbeats are conducted, whereas a third-degree AV block shows a complete disconnection between the P waves and QRS complexes, often resulting in the need for a pacemaker. The heart rate typically decreases significantly in a third-degree block.
Step-by-step explanation:
Difference Between 2nd Degree & 3rd Degree AV Block
The difference between a second-degree AV block and a third-degree AV block lies in the degree of interruption in the heart's electrical conduction system. In a second-degree AV block, some electrical impulses from the SA node are unable to reach the AV node and are blocked, which leads to an inconsistent transmission of signals to the ventricles. This is visible on an ECG as some P waves not being followed by a QRS complex, indicating that not all heartbeats are conducted.
In contrast, a third-degree AV block, which is a complete block, there is no transmission of electrical impulses from the atria to the ventricles. As a result, the atrial activity (P wave) is not correlated with the ventricular activity (QRS complex), and the ventricles will beat at their own inherent rate set by a secondary pacemaker. This condition is serious and often necessitates immediate medical attention and potentially the placement of a pacemaker to maintain an adequate heart rate.
In terms of heart rate, during a third-degree AV block, you would expect a significant decrease in the heart rate (pulse), since the AV node assumes the role of pacemaker at a lower rate (40-60 contractions per minute) than the normal sinus rhythm initiated by the SA node.