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232 The Bradyarrhythmias
David D. Spragg / Gordon F. Tomaselli

	Figure 232-1  Action potential profiles recorded in cells isolated from sinoatrial or atrioventricular nodal tissue compared with those of cells from atrial or ventricular myocardium. Nodal cell action potentials exhibit more depolarized resting membrane potentials, slower phase 0 upstrokes,...
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	Figure 232-2  Schematics of nodal action potentials and the currents that contribute to phase 4 depolarization. Relative increases in depolarizing L- (ICa-L) and T- (ICa-T) type calcium and pacemaker currents (If) along with a reduction in repolarizing inward...
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	Figure 232-3  Sinus slowing and pauses on the ECG. The ECG is recorded during sleep in a young patient without heart disease. The heart rate before the pause is slow, and the PR interval is prolonged, consistent with an increase in vagal tone. The P waves have a morphology consistent with...
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	Figure 232-4  Mobitz type I SA nodal exit block. A theoretical SA node electrogram (SAN EG) is shown. Note that there is grouped beating producing a regularly irregular heart rhythm. The SA node EG rate is constant with progressive delay in exit from the node and activation of the atria,...
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	Figure 232-5  First-degree AV block with slowing of conduction in the AV node as indicated by the prolonged atrial-to-His bundle electrogram (AH) interval, in this case 157 ms. The His bundle-to-earliest ventricular activation on the surface ECG (HV) interval is normal. The normal HV interval...
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	Figure 232-6  Mobitz type I second-degree AV block. The PR interval prolongs before the pause, as shown in the ladder diagram. The ECG pattern results from slowing of conduction in the AV node.
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	Figure 232-7  Paroxysmal AV block. Multiple nonconducted P waves after a period of sinus bradycardia with a normal PR interval. This implies significant conduction system disease, requiring permanent pacemaker implantation.
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	Figure 232-8  High-grade AV block. A. Multiple nonconducted P waves with a regular narrow complex QRS escape rhythm probably emanating from the AV junction. B. A wide complex QRS escape...
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	Figure 232-9  High-grade AV block below the His. The AH interval is normal and is not changing before the block. Atrial and His bundle electrograms are recorded consistent with block below the distal AV junction. I, II, III, and V1 are surface ECG leads. HISp, HISd, and RVA are the proximal...
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