Figure 17.

Third-degree (complete) block. There are P waves but the PR intervals appear inconsistent; no pattern is repeated. If impulses were being conducted into the ventricles, the R-R intervals would be irregular and the QRS complexes would be narrow. Neither is the case, however; the R-R intervals are regular and the complexes are slightly widened. (They get wider and wider according to the location of the ventricular pacemaker. In this case, the pacer is probably in the bundle of His, because the complex is relatively narrow.) On closer analysis, one can detect that intervals between P waves (P-P intervals) are consistent and that R-R intervals are consistent. The only explanation is that the SA node is pacing the atria but impulses are not reaching the ventricles. Therefore, the ventricles have developed their own pacemaker and we have a complete (third-degree) heart block.