Final answer:
Bifascicular block on an ECG is identified by a right bundle branch block combined with either left anterior or posterior fascicular block, indicated by specific QRS patterns and axis deviations. Third-degree block shows complete disassociation between P waves and QRS complexes. ECGs also reveal other cardiac abnormalities but have limitations in showing certain infarctions and pumping effectiveness.
Step-by-step explanation:
The ECG patterns that identify a bifascicular block typically include a combination of right bundle branch block (RBBB) with either left anterior fascicular block (LAFB) or left posterior fascicular block (LPFB). The RBBB is identified on the ECG by a widened QRS complex that is greater than 120 ms, a secondary R wave (r') in the right-sided ECG leads (V1 and V2), and a broad, slurred S wave in the left-sided leads (V5 and V6). When paired with LAFB, the ECG will show a left axis deviation, and when paired with LPFB, there will be a right axis deviation. These specific features on an ECG indicate a block in the conduction pathway that affects two of the three fascicles of the heart's electrical conduction system.
In cases of a third-degree block, which is a complete disassociation between atrial and ventricular activity, the ECG shows no correlation between the P waves and the QRS complexes. An ECG can also reveal other cardiac abnormalities like infarcts, enlargement of heart chambers, and signs of myocardial ischemia, such as ST segment changes. Importantly, limitations of ECG include its inability to show certain areas of myocardial infarction and the effectiveness of the heart's pumping action, needing additional tests like echocardiograms for more comprehensive assessment.