Rhythm

Normal cardiac rhythm arises from the SA node (sinus rhythm) but pacemaker impulses can come from ectopic foci in the atria, the AV junction, and the ventricles under abnormal conditions. When an ectopic impulse occurs singly, it generates a beat; when the beat repeats itself, it becomes a rhythm. In addition, ectopic impulses can arise through an escape mechanism or through prematurely. Each of these terms is explained in the sections that follow.

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Sinus Rhythm

Sinus rhythm implies that the SA node is the pacemaker and normal sinus rhythm (NSR) is simply sinus rhythm with heart rate in the normal range of 60 V 100 beats/min. The P waves in sinus rhythm have normal axis and are positive in lead II and negative in lead aVR. The QRS width in sinus rhythm is normal because the ventricles are activated rapidly by impulses conducted down the His bundle and bundle branches.

Sinus rhythm is regular with the exception of a phenomenon called sinus arrhythmia during which there is a minimal increase in heart rate during inspiration and a minimal decrease in heart rate duringexpiration. Although arrhythmia means abnormal cardiac rhythm, sinus arrhythmia is truthfully not an abnormal rhythm.

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Sinus Pause or Arrest

In disease (e.g. sick sinus syndrome) the SA node can fail in its pacing function. If failure is brief and recovery is prompt, the result is only a missed beat (sinus pause). If recovery is delayed and no other focus assumes pacing function, cardiac arrest follows.

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Escape rhythms

An escape beat is a heart beat arising from an ectopic focus in the atria, the AV junction, or the ventricles when the sinus node fails in its role as a pacemaker or when the sinus impulse fails to be conducted to the ventricles as in complete heart block (see section on Heart Blocks below). The ectopic impulse in this instance is always late, appearing only after the next anticipated sinus beat fails to materialize. If the sinus node failure or heart block is only brief, the ectopic focus may generate only a single escape beat; if the sinus node failure or heart block is prolonged, the ectopic focus produces a rhythm of escape beats to assume full pacing function. This escape mechanism offers protection against total cardiac standstill in the event of sinus node failure or complete heart block.

 

    

Atrial Escape

Atria escape, either in escape beat or escape rhythm, produces a P wave that has abnormal axis and looks different from the P wave produced by the sinus beat. However, depolarization spreads to the ventricles normally down the AV junction, the His bundle, and bundle branches. Therefore the QRS complex of the atrial escape beats looks exactly like the QRS complex of the sinus beat. The inherent rate of atrial escape rhythm is between 60 and 80 beats/min.

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Junctional Escape

In junctional (AV junctional) beat or rhythm the atrial depolarization current points cephalad and to the right, away from lead II and toward lead aVR. Therefore the P wave, if seen, would be negative in lead II and positive in lead aVR. However this P wave is usually buried by the QRS complex and not visible. On less common occasions when the P wave is visible, it may be either immediately before or immediately after the QRS complex. Since the impulse is conducted to the ventricles via the His bundle and bundle branches, the QRS complex of junctional beats is narrow and looks exactly like the QRS complex of the sinus beat. The inherent rate of junctional escape rhythm is 40 V 60 beats/min.

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Conceptually the visibility and position of the P wave in junctional beat or rhythm can be explained as follows:

Back to contents If the ectopic junctional focus is in the center of the node, the depolarization impulse has to travel an equal distance up and down the node to depolarize the atria and the ventricles. Hence activation of atria and ventricles is simultaneous (conduction down the His bundle and bundle branch is very fast) and the P wave is buried within the QRS complex.
Back to contents If the ectopic focus is high up in the AV node, the depolarization wave reaches the atria before the ventricles and atrial activation precedes ventricular activation. As a result, the P wave is in front of the QRS complex.
Back to contents If the ectopic focus is low down in the AV node, ventricular activation precedes atrial activation and the P wave follows the QRS complex.

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Ventricular Escape

In ventricular escape beat or rhythm, the depolarization wave spreads slowly via abnormal pathway in the ventricular myocardium and not via the His bundle and bundle branches. Therefore, the QRS complex is wide (>120 ms) and has a shape different from that of the sinus beat.

If the ventricular escape rhythm is the result of sinus node failure, no P wave of atrial contraction is seen as in the tracing above. If the ventricular escape rhythm is the result of 3rd degree (complete) heart block, the sinus node paces the atria independently and regular P waves unrelated to the ventricular escape beats can be seen. The inherent rate of ventricular escape rhythm is between 20 and 40 beats/min.

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Premature Beats

A premature beat also arises from an ectopic pacemaker: in the atria, the AV junction, or the ventricles. The non-sinus impulse is early, initiating a heart beat before the next anticipated sinus beat as its name implies. The reason the ectopic focus discharges a pacing impulse early in this instance is because the ectopic focus is irritable and competes with the sinus node.

 

    

Atrial Premature Beat

Atrial premature beat (APB) arises from an irritable focus in one of the atria. It depolarizes the atria prematurely (premature to the next timely sinus beat) and produces a P wave that looks different from a sinus-node generated P wave because the direction in which the atria depolarize is abnormal (abnormal P wave axis). Since the premature atrial impulse is conducted in a normal fashion via the AV node, the His bundle, and the bundle branches to depolarize the ventricles, the QRS complex associated with an APB has normal QRS duration and the same morphology as that of the sinus beat.

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Junctional Premature Beat

Junctional premature beat (JPB) arises from an irritable focus at the AV junction. The P wave associated with atrial depolarization in this instance is usually buried inside the QRS complex and not visible (see Junctional Escape above).

However, the P wave may appear on occasions either immediately before or immediately after the QRS complex. When it is visible, the P wave is negative in lead II and positive in lead aVR because of retrograde atrial depolarization.

Since the premature junctional impulse is conducted in a normal fashion down the His bundle and bundle branches to depolarize the ventricles, the QRS associated with JPB has normal duration and the same morphology as that of the sinus-node generated beat.

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Ventricular Premature Beat

Ventricular premature beat arises from an irritable focus in the ventricles. Ventricular premature impulse is not transmitted to the rest of the ventricles along the His bundle and bundle branches. It is conducted along abnormal pathway in the ventricular myocardium. This slow process produces an abnormally wide QRS and bizarre looking T wave. Being a ventricle-generated beat, there is no P wave activity before the QRS complex.

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Tachycardias

If an ectopic focus discharges a premature impulse only occasionally, the result is premature beats superimposed on the basic rhythm; if the irritable focus generates 3 premature beat repeatedly in a continuous sequence, the result is ectopic tachycardia. The run is called non-sustained if it lasts up to 30 seconds and sustained if longer than 30 seconds.

Tachycardias, other than sinus tachycardia, can be classified into supraventricular tachycardia (SVT) or ventricular tachycardia (VT), depending on their site of origin.

    

Supraventricular Tachycardia

Tachycardias arising from an ectopic focus in the atria or AV junction are called supraventricular tachycardias (SVT). Heart rate is faster than 150 per minute and commonly around 180 per minute. At this very fast heart rate, the P waves of atrial contraction are buried within the waves of the beats before irrespective of whether the tachycardia is of atrial or junctional origin. Differentiation of the two is not possible on the surface ECG and they are simply called paroxysmal supraventricular tachycardia (PSVT) because of their paroxysmal (sudden) onset. Since PSVT impulses depolarize the ventricles by passing down the His bundle and bundle branches, the accompanying QRS complexes are of normal width and have the same morphology as that of sinus beats.

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Atrial Flutter

In atrial flutter an atrial focus activates the atria at a rate of around 300 times per minute. The baseline of the ECG becomes all P waves, giving it a saw tooth appearance in one or more leads. Since it is unusual for the AV node to conduct impulses at a rate faster than 200 per minute, AV block occurs: commonly at a 2 to 1, 3 to 1, or 4 to 1 ratio, yielding a ventricular response rate of 150, 100, or 75 per minute respectively. (NB: When the ratio of P waves to QRS complex is 2:1, 3:1, or 4:1 it would be more correct to use the term 2:1, 3:1, or 4:1 conduction rather than block. To avoid confusion, some authors simply use the term 2:1, 3:1, or 4:1 flutter.) Since the atrial flutter impulses depolarize the ventricles by passing down the His bundle and bundle branches, the accompanying QRS complexes are normal in width and have the same morphology as that of sinus beats.

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Atrial Fibrillation

Atrial fibrillation is one of the most common arrhythmias in which multiple foci in the atria depolarize rapidly and erratically at a combined rate of 400 times/min or more. Instead of generating well recognized P waves, the atria just quiver and produce fine f waves on the ECG baseline seen in one or more leads. The AV node is constantly bombarded by depolarization impulses but only some of these impulses manage to get through. The ventricular response is totally irregular without discernible pattern (irregularly irregular) generally at a rate between 110 and 180 beats/min. Since impulses that manage to pass through the AV node are conducted down the His Bundle and bundle branches, the ventricles are activated normally and their QRS complexes are normal in width and have the same morphology as that of sinus beats.

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Monomorphic Ventricular Tachycardia

Ventricular tachycardia (VT) arises from an irritable ventricular focus that discharges premature impulses for 3 or more beats without interruption. The rate of depolarization is 150/min or faster. Since these impulses are conducted to the rest of the ventricles via abnormal pathway in the ventricular myocardium and not via the His Bundle and bundle branches, the QRS complexes are broader than normal and without distinguishable T waves. In monomorphic VT, consecutive QRS complexes have the same appearance.

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Polymorphic Ventricular Tachycardia

In polymorphic ventricular tachycardia, there is beat-to-beat variation in the QRS morphology. A common example is Torsades de Pointes (twisting of the points) shown below:

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Ventricular Fibrillation

Ventricular fibrillation occurs when multiple ventricular foci discharge rapidly and chaotically. The ventricles twitch asynchronously and are not effective as pumps. No organized QRS complexes are seenXjust disorganized oscillatory waves which can be coarse (as shown) or fine in appearance.

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