Volume 2, Issue 1

 

Atrial Dissociation

January 25, 2011

Introduction

Atrial dissociation is a rare form of atrial arrhythmia characterized by an independent ectopic impulse limited to one area in the atrium. The ectopic focus does not affect, nor is it influenced by the underlying rhythm (usually sinus), thus being truly dissociated from its surroundings. It is seen in patients with serious heart diseases such as intractable congestive heart failure and most commonly presents as a pre-morbid event. This is a case report of atrial dissociation seen in a patient with a sudden neurologic event.

Case History

JL is a 75-year old man who suddenly collapsed and became unresponsive while talking to his wife. He has a past history of systemic hypertension and pulmonary emphysema but no history of coronary heart disease or congestive heart failure.

Vital signs on admission revealed a blood pressure of 158/100 mmHg and a heart rate of 100 bpm. The lungs were clear to auscultation. Cardiac examination revealed normal heart sounds without murmurs or gallops. There was no jugular venous distention or pedal edema. Neurological examination demonstrated right-sided hemiparesis.

The initial laboratory data were within normal limits, including a potassium level of 4.0 mmol/l. Computerized tomography of the head demonstrated a left caudate and thalamic bleed with intraventricular extension. The electrocardiogram (ECG) on presentation is shown (Figure 1).

Figure 1. The underlying rhythm is sinus. Note that the ectopic P’ waves (arrows) are bizarrely-shaped and not conducted to the ventricles. There is also finite variation between the inter-ectopic (P’-P’) intervals. In addition to the rhythm disturbance, the presence of a right axis deviation, prominent P wave in lead II, as well as a qS pattern throughout the precordial leads suggest right ventricular hypertrophy associated with a pseudoinfarction pattern, which may be seen in patients with chronic lung disease.

The patient was managed with intravenous mannitol, diuretics, and mechanical intubation with subsequent improvement in this mental status. A repeat ECG showed spontaneous conversion to sinus rhythm (Figure 2). He was transferred to another institution for intraventricular drainage and decompression, and was eventually discharged home.

Figure 2. This rhythm strip demonstrating sinus rhythm was obtained 15 hours after the first ECG.

Discussion

Atrial dissociation is characterized by the presence of two coexisting sets of P waves. The electrocardiographic findings involve two distinct sets of P waves occurring independently of each other. One set of P waves may “capture” the ventricle, i.e. it is followed by the QRS-T complexes. Varying degrees of AV block may also accompany this phenomenon, including complete AV block.

Functionally, these two P waves represent activation of the right and left atria. Each atrium beats independently under the control of its own focus. For this to occur, an intra-atrial block is a prerequisite.

In the patient described, atrial dissociation is seen in the setting of an intracranial bleed. There was no prior cardiac history. A previous ECG confirmed the presence of sinus rhythm (Figure 3).

Figure 3. This ECG showing sinus rhythm was taken two years before admission.

Atrial dissociation is a rare arrhythmia, first described in man in 1906 by Wenckebach. Since then, it has been reproduced surgically (via ligation of a coronary artery), using pharmacologic intervention, and following insertion of a cardiac pacemaker. The arrhythmia has been seen in various settings including rheumatic heart disease, cardiomyopathy, uremia, hypertension, “scarlatina”, glomerulonephritis, pneumonia, myocardial infarction involving either the atrium and ventricle, diphtheria, and congenital heart disease (1). Combinations of rhythm disturbances may be possible in association with the underlying rhythm, which is usually sinus. These include slow atrial rhythm, atrial tachycardia, atrial fibrillation, atrial flutter, and coronary sinus pacemaker rhythm.

The possible mechanism of atrial dissociation is based on several observations. Of note is the recognition that myocardial cells other than the sinus node have the capability of spontaneous phase 4 depolarization. If this increase in automaticity is the cause, the ectopic focus resides within specialized conducting fibers rather than in contracting myocardial cells, similar to other parasystolic rhythms (2). A secondary characteristic of this ectopic focus is it appears to be protected from the dominant cardiac impulses. One possible reason for this is that cells with enhanced automaticity also have the ability to impair propagation of impulses into and out of its surrounding location, thus creating a physiologic entrance and exit block (3). A final hypothesis postulates the presence of a complete intra-atrial block, preventing subsequent atrial or ventricular activation (4), enabling the formation of independent atrial impulses.

Compared to other supraventricular arrhythmias such as atrial parasystole, the inter-ectopic intervals of atrial dissociation demonstrate greater variability. The morphology of the ectopic P’ wave is generally bizarre-appearing and small. Occasionally, P’ waves may appear to have increased amplitude, especially when they are superimposed upon the P wave of the basic rhythm. Again, these ectopic impulses never conduct to the ventricles, even when they occur during the non-refractory period of the action potential.

An important differential to consider is the postcardiac transplantation ECG, which may have findings similar to those of atrial dissociation. Nonconducted atrial premature contractions occurring in a regular fashion may also be confused with atrial dissociation; however, a fixed coupling interval and other features may be helpful in the differential diagnosis. Other conditions that may be mimic atrial dissociation include rhythmic movements of the musculature (e.g. muscular twitches, hiccough, parkinsonism), electrical interference, or rhythm from another person in proximity to the electrodes. A curious artifact may arise in the setting of ambulatory monitoring, when an incompletely-erased magnetic tape is reused in a different patient. In this situation, “ectopic” P waves may be seen superimposed upon an underlying rhythm (personal observation).

Summary

Atrial dissociation is commonly viewed in pre-morbid conditions, generally occurring hours before death. This is a case report of transient atrial dissociation. The patient described survived his neurological insult.

References

  1. Chung EK. Principles of cardiac arrhythmias, 2nd ed. Baltimore: Williams & Wilkins, 1977:458-65.
  2. Watanabe Y. Reassessment of parasystole. Am Heart J 1971;81:451-66.
  3. Hoffman BF. The electrophysiology of the heart muscle and the genesis of arrhythmias. In: Dreifus LS, Likoff W, eds. Mechanisms and therapy of cardiac arrhythmias. 14th Hahnemann Symposium. New York: Grune and Stratton, 1966:27.
  4. Schamroth L. The disorders of cardiac rhythm, 2nd ed. Oxford: Blackwell Scientific Publications, 1980:232.3.