Sinus rates less than 60 beats per minute (bpm) are defined as bradycardia in adults. , Lower rates are common in well-trained athletes and occasionally in the young. During sleep the sinus rate may fall to 35 bpm with pauses up to 3 seconds and this is not considered abnormal.
Sinus arrhythmia usually refers to sinus cycle phasic variation that is related to the respiratory cycle and is normal.
Sinus arrest (sinus pauses) manifests itself as pauses with the PP interval containing the pause not being equal or multiple of the basic PP interval.
In sinoatrial exit block (first, second, or third degree) the atrium is not depolarized despite the sinus stimulus and the duration of the pause is a multiple of the basic PP interval, unless there is Mobitz I block out of the sinus node.
Wandering atrial pacemaker refers to shift of the dominant pacemaker from the sinus node to other atrial focus, usually lower at the crista terminalis.
Sinus node disease or sick sinus syndrome refers to any of the following conditions that may coexist:
Persistent spontaneous bradycardia less than 50 bpm not caused by drugs and with chronotropic incompetence—that is, inability to achieve 85% of the age-predicted maximum heart rate on the treadmill. ,
Sinus pauses greater than 3 seconds while awake or greater than 5 seconds during sleep, either because of sinus arrest or exit block.
Alternating episodes of bradycardia usually after tachycardia (mostly atrial fibrillation).
First-degree atrioventricular (AV) block is characterized by a PR interval greater than 200 ms. A narrow QRS complex indicates that the block is most probably at the AV node, whereas wide QRS can be seen with a block either at the node or His-Purkinje system.
Second-degree AV block is characterized by P waves not conducted to the ventricles.
Type I second-degree AV block (Mobitz I or Wenckebach) is progressively increasing PR intervals until a P wave fails to be conducted to the ventricles.
Type II second-degree AV block (Mobitz II) is consecutive , nonconducted P waves without visible changes in the PR interval (i.e., AV conduction time) before and after the blocked impulse, provided there is normal sinus rhythm. The diagnosis of type II block cannot be established if the first postblock P wave is followed by a shortened PR interval or is not discernible.
Third-degree (complete) AV block is when no P wave is conducted to the ventricles and ventricular contraction is maintained by a nodal or infranodal escape rhythm. Unless the escape rhythm is lower than the sinus rate, complete AV block cannot be differentiated from AV dissociation.
AV dissociation is independent beating of the atria and ventricles as a result of the following:
Slowing of the sinus rate that allows escape of a subsidiary or latent pacemaker.
Acceleration of a latent pacemaker, as happens in nonparoxysmal AV junctional tachycardia or ventricular tachycardia.
Complete heart block with junctional or ventricular escape rhythm.
Left bundle branch block (LBBB) and right bundle block (RBBB) lead to interventricular dyssynchrony.
RBBB may display normal axis or 180 degrees (common type or Wilson block). Left or extreme right axis deviation suggests coexistent hemiblock. Pure RBBB and bifascicular blocks are associated with S waves in leads I and aVL. An electrocardiography (ECG) pattern of RBBB in lead V1 with absent S wave in leads I and aVL indicates concomitant LBB delay and bilateral block.
In mirror-image dextrocardia or dextroversion, relocating the V 1 through V 6 leads (with the V 1 –V 6 leads placed in the V 6R –V 3R , V 1 , and V 2 positions) is necessary for ECG interpretation ( Fig. 9.1 ). Thus an apparent RBBB may actually be a LBBB in this setting.
LBBB may display normal or more often left axis deviation, which implies a worse prognosis. Right axis deviation may be seen in dilated cardiomyopathy. Current criteria for LBBB include a QRS duration 120 ms or longer, and this threshold is also used for CRT recommendations. However, certain patients may not have true complete LBBB but likely have a combination of left ventricular hypertrophy and left anterior fascicular block, and stricter criteria such as a QRS duration 140 ms or longer for men and 130 ms or longer for women, along with mid-QRS notching or slurring in two or more contiguous leads, have been proposed. Septal conduction during LBBB is heterogenous. There may be complete block, usually high at the left-sided His fibers (left intra-Hisian block, and most amenable to corrective His bundle pacing) and, less commonly, more distally at the left bundle level, or conduction slowing distally to Purkinje fibers. The presence of an initial r wave of 1 mm or greater in lead V 1 usually indicates intact left-to-right ventricular septal activation, unless the r wave is due to a large septal scar. In this case and ventricular activation occurs without right-to-left septal activation resulting in initial positivity in lead V 1 during LBBB because of unopposed endoepicardial activation of the right ventricular wall.
Bifascicular block refers to RBBB plus a hemiblock.
Trifascicular block refers to block of both the left and right bundles or to first-degree AV block with additional bifascicular block.
Atypical intraventricular block refers to wide QRS without any typical ECG pattern and usually occurs in patients with ischemic or nonischemic heart failure.
Electrophysiologic testing for the investigation of bradycardias
In patients investigated for bradycardias either because of sinus node disease or AV conduction disturbances, the role of the electrophysiology study (EPS) is not well defined. Most of the time the clinical findings are sufficient to determine whether or not pacemaker implantation is indicated. However, there are certain occasions in which an EPS is helpful either for the establishment of diagnosis or for appropriate implementation of prophylactic pacing.
Early studies on limited patient cohorts suggested that the corrected sinus node recovery time (C-SNRT) may be useful in predicting the development of syncope and the need of permanent pacing in patients with bradycardia. A marked prolongation of the CSRT and an absent or blunted response to atropine and exercise suggest impaired sinus node function. However, a wide range of “normal” C-SNRT has been published, and it seems that only a very prolonged c-SNRT (>800 ms) has reasonable predictive ability. When considering conventional upper limits, such as 500 to 550 ms, the sensitivity of the test in asymptomatic patients with dizziness or no symptoms who will need a pacemaker in the future is only 50% to 65%. The sinoatrial conduction time (SACT) is an insensitive indicator, being prolonged in only 40% of patients with clinical findings of sinus node dysfunction.
Atrioventricular conduction disturbances
Atrial premature beats may produce prolonged PR intervals caused by refractory fast pathway of the AV node, and nonconducted atrial premature beats can mimic AV block. The ECG appearance of first- or second-degree block may be due to junctional (His bundle) extrasystoles that are concealed (not conducted to the atria or ventricles) but render a portion of the conduction system refractory to propagation of a sinus beat. The presence of junctional premature depolarizations on the surface ECG suggests that concealed His bundle extrasystoles are responsible for the apparent AV block, but a His-bundle recording is the only method for positive identification ( Fig. 9.4 ).