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Despite the multicenter and prospective design, the study demonstrates high success rates and low complication rates, which are comparable to prior single-center retrospective studies. These results may serve as the current best benchmark for expected results in the pediatric population, aged 0 to 16 years, both in terms of acute success rates and the occurrence of complications.

The strategic use of remote pacemaker interrogation follow-up detects actionable events that are potentially important more quickly and more frequently than transtelephonic rhythm strip recordings. The use of transtelephonic rhythm strips for pacemaker follow-up is of little value except for battery status determinations. (PREFER [Pacemaker Remote Follow-up Evaluation and Review]; NCT00294645).

Recurrence after initially successful ablation occurs commonly in children. It is least common after AVNRT ablation and most common following ablation of right-sided pathways. These results serve as a benchmark for the time course of recurrence following initially successful ablation of supraventricular tachycardia in children.

The bidirectional Glenn operation may be particularly useful as an intermediate procedure before Fontan correction in high-risk patients. From October 1989 through February 1992, 50 patients 1 to 60 months old (median 12) have undergone a bidirectional Glenn operation. Diagnoses included hypoplastic left heart syndrome in 21 patients, pulmonary atresia with intact ventricular septum in 10, tricuspid valve atresia in 9, other complex univentricular heart defects in 9, and Ebstein's anomaly in 1. Mean pulmonary vascular resistance was 2.2 +/- 0.2 Wood U (range 0.5 to 7.3) and mean pulmonary artery area Nakata index was 318 +/- mm2/m2 (range 80 to 821). Additional procedures were performed in 17 patients, including pulmonary artery reconstruction in 15 (29%) and bilateral caval anastomoses in 5 (10%). There were 4 hospital deaths (8%). Two deaths resulted from myocardial infarction in patients with pulmonary atresia with intact ventricular septum and sinusoids and 1 from severe pulmonary vascular disease in a patient with hypoplastic left heart syndrome. There was 1 late death from pneumonia. Actuarial survival is 92 +/- 4% at 1 month and beyond, with a mean follow-up of 13.4 +/- 1 months. Risk factor analysis showed that pulmonary vascular resistance > 3 Wood U and pulmonary artery distortion were associated with increased mortality. Twelve patients have undergone a Fontan procedure at a mean duration after bidirectional Glenn of 18 months with 1 death (8%). The bidirectional Glenn procedure provides excellent palliation in high-risk patients and appears useful as a staging procedure before Fontan correction.

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Simultaneous extracellular and intracellular recordings of normal action potentials, action potentials initiated at a time when the membrane was partially depolarized (by premature beats or elevated extracellular potassium), and action potentials at reduced temperature were made for Purkinje strands from the left ventricle of the dog with a 50fi tungsten extracellular electrode and a special guarded intracellular microelectrode. The peak-to-peak amplitude of the extracellular wave form was proportional to the maximum rate of rise of the intracellular action potential, and the duration of the extracellular wave form was proportional to the duration of the upstroke of the intracellular potential. Wave forms of extracellular potentials were computed from the recorded intracellular potentials with an equation which included the effects of membrane currents away from the point of observation. The computed wave forms accurately reproduced the recorded extracellular wave forms in all cases, and the wave forms were not directly proportional to the second spatial derivative or the second temporal derivative of the intracellular potential. Extracellular potentials are shown to be directly related to the spatial distribution of the intracellular potential and as such are a sensitive index of propagation and a source of information of the kind previously thought to be obtainable only with an intracellular electrode. KEY WORDS extracellular-intracellular voltage relations excitation waves temperature effects on excitation waves effects of premature beats on excitation wavesPurkinje strands intracellular action potentials computed extracellular wave forms• Arguments about the mechanism of cardiac arrhythmias center around questions of propagation: are arrhythmias due to reentry or repetitive firing? A necessary condition for reentry is slow propagation (1), whereas repetitive firing does not demand this. The hallmark of each mechanism is its unique excitation sequence. To identify such sequences requires a detailed knowledge of the spatial distribution of membrane activity, information which has come to be considered obtainable only from intracellular recordings. With the intracellular method, however, it is not feasible to record simultaneously from a sufficiently large number of sites because of 505 by guest on

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