Figure 2. Electrocardiograph printout (25 mm/s) obtained by continuous Holter monitoring. For the sake of clarity, only one out of three leads is shown (CM3). Top panel, Onset of first degree heart block. The arrow indicates the first beat with a PR interval >or= to 0.21 ms (corresponds to point C in Figure 1). Middle panel, First degree heart block progresses to third degree heart block (corresponds to point D in Figure 1). Bottom panel, First degree heart block with intermittent second degree heart block (corresponds to point I in Figure 1).