Figure 18.18(a) Normal sinus rhythm.(c) Second-degree heart block.Some P waves are not conductedthrough the AV node; hence more P than QRS waves are seen. In this tracing, the ratio of P waves to QRS waves is mostly 2:1.(d) Ventricular fibrillation. Thesechaotic, grossly irregular ECGdeflections are seen in acuteheart attack and electrical shock.(b) Junctional rhythm. The SA node is nonfunctional, P waves are absent, and heart is paced by the AV node at 40 - 60 beats/min.