The mechanism of synchronization in isorhythmic A-V dissociation. II. Clinical studies

Isorhythmic atrioventricular dissociation in an adult with levo-transposition of the great arteries

A 36-year-old woman with levo-transposition of the great arteries presented for a programmed visit. She was asymptomatic and the clinical examination showed no signs of decompensated heart failure. Standard 12-lead ECG showed isorhythmic atrioventricular dissociation. A 24-hour ambulatory recording demonstrated sinus rhythm with intermittent periods of isorhythmic dissociation. This case highlights the association between two rare conditions.

Demonstrating synchronization of isorhythmic dissociation with stress testing

There are a variety of causes of atrial and ventricular dyssynchrony. The mechanism underlying the arrhythmia is usually a guide to further management. This case highlights the key distinguishing features of more benign etiologies.

A Case of Complete Heart Block With Diagnostic Challenge and Therapeutic Dilemma

Permanent pacemaker implantation is a class I indication for all symptomatic patients with complete heart block either congenital or acquired. However, certain portions of patients with congenital complete heart block are asymptomatic. Those patients are often very young, and implanting a permanent pacemaker is not always an easy decision. A therapeutic dilemma arises when a select patient population does not meet certain criteria to gain the maximum benefits out of prophylactic pacemaker therapy. Most asymptomatic patients with congenital complete heart block will eventually become symptomatic and require pacemakers at some point in their life but the definitive answer for the ideal time to initiate pacemaker therapy in such population has not been established. We present a case of asymptomatic congenital complete heart block with junctional escape rhythm, which is capable of incrementing the heart rate with physical activity to result in a challenge in diagnosis as well as the treatment strategy.

The effects of ageing and adrenergic challenge on electrocardiographic phenotypes in a murine model of long QT syndrome type 3

Long QT Syndrome 3 (LQTS3) arises from gain-of-function Na_v1.5 mutations, prolonging action potential repolarisation and electrocardiographic (ECG) QT interval, associated with increased age-dependent risk for major arrhythmic events, and paradoxical responses to β-adrenergic agents. We investigated for independent and interacting effects of age and Scn5a+/ΔKPQ genotype in anaesthetised mice modelling LQTS3 on ECG phenotypes before and following β-agonist challenge, and upon fibrotic change. Prolonged ventricular recovery was independently associated with Scn5a+/ΔKPQ and age. Ventricular activation was prolonged in old Scn5a+/ΔKPQ mice (p = 0.03). We associated Scn5a+/ΔKPQ with increased atrial and ventricular fibrosis (both: p < 0.001). Ventricles also showed increased fibrosis with age (p < 0.001). Age and Scn5a+/ΔKPQ interacted in increasing incidences of repolarisation alternans (p = 0.02). Dobutamine increased ventricular rate (p < 0.001) and reduced both atrioventricular conduction (PR segment-p = 0.02; PR interval-p = 0.02) and incidences of repolarisation alternans (p < 0.001) in all mice. However, in Scn5a+/ΔKPQ mice, dobutamine delayed the changes in ventricular repolarisation following corresponding increases in ventricular rate. The present findings implicate interactions between age and Scn5a+/ΔKPQ in prolonging ventricular activation, correlating them with fibrotic change for the first time, adding activation abnormalities to established recovery abnormalities in LQTS3. These findings, together with dynamic electrophysiological responses to β-adrenergic challenge, have therapeutic implications for ageing LQTS patients.

Assessment of fetal rhythm in complete congenital heart block by magnetocardiography

We report high precision assessment of fetal rhythm in utero in a case of isolated congenital complete heart block using fetal magnetocardiography. The recordings reveal a remarkably strong tendency for the atria and ventricles to synchronize, which is manifested by the continual presence of ventriculophasic sinus arrhythmia and frequent episodes of accrochage and isorhythmic AV dissociation.

Atrial and ventricular fetal heart rate patterns in isolated congenital complete heart block detected by magnetocardiography

Atrial and ventricular fetal heart rate tracings from a patient with isolated congenital complete heart block treated with dexamethasone showed a remarkable degree of correlation and greater reactivity for the ventricular than the atrial fetal heart rate. Ventriculophasic sinus arrhythmia was present continually in the atrial fetal heart rate tracings.

Radiofrequency ablation of a supraventricular tachycardia due to interatrial conduction from the recipient to donor atria in an orthotopic heart transplant recipient

Interatrial conduction of recipient atrial tachycardia to the donor atria of an orthotopic heart transplant recipient resulted in a unique cause of supraventricular tachycardia. An electrophysiologic study was performed, and the recipient atria was found to be in an atrial tachycardia, cycle length of 210 msec, with periods of both 2:1 and 1:1 conduction to the donor atria resulting in a donor atrial cycle length of 420 msec and 210 msec, respectively. The site of interatrial conduction was mapped to the right atrial suture line, along the atrial free wall, and was successfully disrupted with radiofrequency energy. Arrhythmias of a similar mechanism may also be observed in other postsurgical patients.

Isorhythmic atrioventricular dissociation following pancuronium

We report a case of isorhythmic atrio-ventricular dissociation caused by pancuronium. The possible mechanism of the rhythm disorder is discussed and the haemodynamic consequences are described. It will be useful to realise that the circulatory effects of the arrhythmia may be pronounced in patients with decreased cardiovascular reserve.

Carotid sinus massage. Its diagnostic and therapeutic value in arrhythmias

Carotid sinus massage is a simple bedside maneuver that helps to clarify the type and sometimes also the mechanism of different rhythm disturbances. The major indication for carotid sinus massage is the diagnosis of tachyarrhythmias in which the atrial activity is either absent or intermittently present. Carotid sinus massage is also useful in some patients with normal heart rates; increased vagal tone may normalize a bundle branch block or localize the site of type I second-degree atrioventricular block and can be used for evaluation of the sensing function of permanent pacemakers. Carotid sinus massage is also an important diagnostic procedure in patients with suspected hypersensitivity of the carotid sinus. Massage of the carotid sinus is contraindicated in patients with diseased carotid arteries because of the risk of cerebrovascular accident. In rare instances, carotid sinus massage may initiate ventricular tachycardia.

Unusual atrial potentials in a cardiac transplant recipient. Possible synchronization between donor and recipient atria

It is usual to record independent activity from both the innervated recipient and the denervated donor atria in cardiac transplant recipients except for occasional, short-lived periods of entrainment that may occur during exercise. In this report a case is described in which, following orthotopic cardiac transplantation, the recipient and donor atria remained synchronized during a variety of physiological and non-physiological situations. Under no circumstances did the two sets of atria beat independently. The mechanisms that might be involved in this unique situation are discussed.

Three cases of hypotension and syncope with ventricular pacing: possible role of atrial reflexes

Hypotension with lightheadedness and near syncope occurred in three patients during effective ventricular pacing. Hemodynamic studies demonstrated a decrease in cardiac output ranging from almost no decrease to 15 percent, presumably related to the loss of effective atrial contraction. The decrease in output was too small to explain by itself the reduced blood pressure, which resulted from paradoxic reduction of total peripheral resistance in one patient and from failure of resistance to increase in two. Baroceptor reflexes (Valsalva response) were normal in all three patients; hence it is suggested that the failure of compensatory increases in total peripheral resistance may be due to a reflex from the sudden atrial distension that occurs during atrioventricular (A-V) dissociation. The fluctuations in arterial pressure during ventricular pacing were synchronous with the appearance of cannon waves in the right atrial pressure tracing. Arterial pressure during A-V dissociation thus appears to be balanced by two opposite reflexes: the baroceptor reflex, which attempts to compensate for reduction in output, and atrial distension, which reduces peripheral resistance.

Isorhythmic Dissociation--a "physiological" arrhythmia

The behavior of the sino-atrial mechanism in isorhythmic dissociation (IRD) was studied in 21 patients, nine with spontaneous IRD and 12 with artificially pacemaker-induced IRD following electrode placement for heart block. Successive P-P, R-R and P-R intervals and blood pressure (BP) fluctuations were determined and graphically interrelated at control and during IRD. Several features were observed: a. IRD was present only when the independent ventricular rate was close to the atrial; b. P rate oscillations closely followed the P-R interval-dependent BP fluctuations (mean difference 30 mmHg) during IRD. In cardiogenic shock and in severe hypertension IRD could not be achieved easily; c. While during complete dissociation or during 1:1 A-V conduction the sinus rate was remarkably constant (2-4 beats/min variations), it showed marked oscillations (differences of 6-19, mean 13, beats/min) during IRD. All the data and calculations support the theory that in most instances of IRD, the arrhythmia is sustained by the normal physiologically active baroreceptor reflex arc.

Mechanism of escape, extrasystolic, and parasystolic arrhythmias. Study on an electrical analogue

A simple analogue of the heart consisting of a system of neon relaxation oscillators is presented. The analogue may display rhythm patterns similar to sinus rhythm, escape rhythm, isorrhythmic dissociation with synchronization, atrial extrasystoles, ventricular extrasystoles, and parasystole. The strict rules followed by these arrhythmias, as well as the deviations from the rules commonly followed by the equivalent heart arrhythmias, may be easily reproduced on the analogue. Such features are the Treppe phenomenon and captured beats in escape rhythm, fixed coupling intervals in extrasystoles, partial or complete atrioventricular block in very premature atrial extrasystoles, prolongation of the period following an atrial extrasystole, interpolated premature beats, complete compensatory pause and the rule of bigeminy in ventricular extrasystoles, slight instability of the parasystolic period, multiple length parasystolic periods slightly different from the exact multiples of the parasystolic idioperiod, preference of the parasystoles for certain phase in the sinus cycle, synchronization at a phase difference and fluctuation repeatedly and without interruption from a parasystolic to an extrasystolic rhythm and synchronization in escape rhythm with isorrhythmic dissociation. The mechanisms involved in these phenomena are discussed in detail. The striking similarity between the properties of the cardiac pacemakers and those of the relaxation oscillators on the one hand and betwen the rhythm patterns of the heart and those of the analogue on the other may permit the hypothesis that the mechanisms operating in the analogue may be used in analyzing and understanding heart arrhythmias.