Robert A. Miller

Demonstration of dual atrioventricular nodal pathways in man

Abstract Electrophysiologic studies in a patient manifesting two P-R intervals revealed two ranges of atrioventricular (A-V) nodal conduction time (A-H intervals) and two A-V nodal effective and functional refractory periods. Similar demonstrations in patients with paroxysmal supraventricular tachycardia would strongly support the presence of longitudinal A-V nodal dissociation with reentry as a causative mechanism.

Sites of Congenital and Surgical Heart Block as Defined by His Bundle Electrocardiography

Catheter recordings of His bundle electrograms were obtained in seven patients with congenital heart block (CHB) and in two with surgical heart block (SHB). In the latter two patients block occurred following total correction of tetralogy of Fallot. In six patients with CHB block occurred proximal to H. In one CHB patient block occurred in the His bundle with “split” H potentials. Intraventricular conduction was normal in five of the patients with CHB who had narrow QRS and H-V intervals ranging from 35 to 45 msec. H-V intervals were short in two CHB patients (25 and 30 msec), one of whom had QRS widening with initial slowing. In the latter patient a functioning infranodal bypass (Mahaim tract) inserting into the right ventricular septum could explain the findings. In the two patients with SHB block was distal to H with P-H intervals of 125 msec and 160 msec, respectively.The degree of bradycardia and the occurrence of symptoms partially correlated with a location of block in or distal to the His bundle. It is suggested that recording of H potentials is useful in the evaluation of children with complete atrioventricular block.

Surgical bifascicular block.

Electrophysiological studies were performed in 18 patients who developed bifascicular block after repair of ventricular septal defect (VSD) or tetralogy of Fallot (TF). Two had transient complete heart block (CHB) in the immediate postoperative period. The P-A intervals were normal in all. A-H and H-V intervals were prolonged in three and four cases, respectively. Atrial pacing at progressively increasing heart rates was performed in 15 patients; two developed type II block distal to the His bundle (H). The effective and functional refractory periods (ERP and FRP) of the atrium (11 cases) were normal in all. The ERP of the A-V node (seven cases) was prolonged in four and the FRP was increased in three. The ERP of the ventricular specialized conduction system was measured in two cases and was prolonged in one. In all, seven cases had abnormalities indicating disease of the A-V node and/or His-Purkinje system. Recording of intervals, atrial pacing, and determination of refractory periods (RPs) was necessary to reveal all conduction abnormalities. One patient died of unrelated causes. The others are alive and in sinus rhythm with intact conduction 3 to 16.5 years following surgery (mean follow-up of 8.3 ± 0.95 years). The clinical course in patients with normal and abnormal findings was equally benign. Prophylactic insertion of demand pacemakers does not appear indicated in these patients.

Results Following Physiological Repair for Tricuspid Atresia

Fourteen patients underwent a physiological operation for tricuspid atresia. Seven patients survived up to 3 years after operation. Six patients underwent hemodynamic studies 2 to 34 months postoperatively (mean, 15 months). Average age at the time of operation was 14 years (range, 5 to 25 years). Postoperative studies showed 4 patients to be in sinus rhythm and 2 in junctional rhythm. Right atrial pressure was elevated an average of 17 mm Hg (range, 10 to 34 mm Hg). All patients showed good atrial transport function regardless of their rhythm. Two had a right atrium-left pulmonary artery gradient of 1.5 to 10 mm Hg across the conduit. Average arterial saturation was 92% (range, 87 to 97%), on improvement of 13% over preoperative values. Residual hypoexmia was due to pulmonary vein desaturation and to atrial right-to-left shunting early postoperatively and later, to atrial right-to-left shunting alone.

Long-Term Hemodynamic Studies in Postoperative Patients with Transposition of the Great Vessels Results of the Venous Switching Operation

ONE OF THE early operations which permitted long-term survival of a large number of children with transposition of the great vessels was the procedure devised by Baffes in 1956, consisting of surgical transplant of the right pulmonary veins into the right atrium and of the inferior vena cava to the left atrium via a graft.1 2 This palliative operation has given many of these children time to await further developments in the surgical approach to this serious cardiac malformation. The success of recent techniques for complete surgical repair of transposition of the great vessels3 has prompted us to review the present clinical and hemodynamic status of a group of children who had a Baffes operation. These studies also helped to delineate some of the physiological problems of partially corrected transposition of the great vessels.

TOWARDS AN IN VIVO INDEX OF VASOMOTOR TONE INDEPENDENT OF FLOW MEASUREMENT

When the heart stops, arterial pressure decays asymptotically to a value different from the venous pressure. Our study was performed to test the hypothesis that this decay is influenced by vasomotor tone and contains information that could lead to the development of an index of vasomotor tone. Twelve anesthetized dogs were subjected to brief periods of cardiac arrest by stimulating the distal ends of the cut vagi. This was done approximately 9 times in each dog at varying levels of vasomotor tone produced by IV nitroprusside and methoxamine and decay of central aortic pressure was recorded. A mathematical model of the lumped arterial bed was evolved in which each term had recognizable physiologic meaning. Using multiple nonlinear regression analysis, the pressure-time data were fitted to this model and, in most cases, fitted and observed decays were hardly distinguishable to the eye (r2 values were greater than 9). It was predicted that asymptotic pressure, calculated using the model, would rise or fall as vasomotor tone rose or fell. The average asymptotic pressures under the influence of methoxamine were significantly greater (P<.05) than control values and average asymptotic pressures under the influence of nitroprusside were significantly less (P<.05) than average control values. It is suggested that these asymptotes may form the basis for developing an index of vasomotor tone independent of flow measurement.