Assessment of ventricular mechanical synchronization after left bundle branch pacing using 2-D speckle tracking echocardiography.

BACKGROUND: The left bundle branch pacing (LBBP) makes the ventricular depolarization closer to the physiological state and shortens QRS duration. The purpose of this study is to explore the ventricular systolic mechanical synchronization after LBBP in comparison with traditional right ventricular pacing (RVP) using two-dimensional strain echocardiography (2D-STE). METHODS: Thirty-two patients who received LBBP (n = 16) or RVP (n = 16) from October 2018 to October 2019 and met the inclusion criteria were included in this retrospective study. Electrocardiogram (ECG) characteristics, pacing parameters, pacing sites, and safety events were assessed before and after implantation. Acquisition and analysis of ventricular systolic synchronization were implemented using 2D-STE. RESULTS: In RVP group, ECG showed left bundle branch block patterns. At LBBP, QRS morphology was in the form of right bundle branch block, and QRS durations were significantly shorter than that of the RVP QRS (109.38 ± 12.89 vs 149.38 \± 19.40 ms, P < .001). Both the maximum time differences (TD) and SDs of the 18-segments systolic time to peak systolic strain were significantly shorter under LBBP than under RVP (TD, 66.62 ± 37.2 vs 148.62 ± 43.67 ms, P < .01; SD, 21.80 ± 12.13 vs 52.70 ± 17.72 ms, P < .01), indicating that LBBP could provide better left ventricular mechanical synchronization. Left and right ventricular pre-ejection period difference was significantly longer in RVP group than in LBBP group (10.23 ± 3.07 vs 39.94 ± 14.81 ms, P < .05), indicating left and right ventricular contraction synchronization in LBBP group being better than in RVP group. CONCLUSION: LBBP is able to provide a physiologic ventricular activation pattern, which results in ventricular mechanical contraction synchronization.

Longitudinal Shortening of the Left Ventricle by Cine-CMR for Assessment of Diastolic Function in Patients with Aortic Valve Disease / Encurtamento Longitudinal do Ventrículo Esquerdo por Cine-RMC para Avaliação da Função Diastólica em Pacientes com Doença Valvar Aórtica

Abstract Background: Diastolic dysfunction, commonly evaluated by echocardiography, is an important early finding in many cardiomyopathies. Cardiac magnetic resonance (CMR) often requires specialized sequences that extends the test time. Recently, feature-tracking imaging has been made available, but still requires expensive software and lacks clinical validation. Objective: To assess diastolic function in patients with aortic valve disease (AVD) and compare it with normal controls by evaluating left ventricular (LV) longitudinal displacement by CMR. Methods: We compared 26 AVD patients with 19 normal controls. Diastolic function was evaluated as LV longitudinal displacement in 4-chamber view cine-CMR images using steady state free precession (SSFP) sequence during the entire cardiac cycle with temporal resolution < 50 ms. The resulting plot of atrioventricular junction (AVJ) position versus time generated variables of AVJ motion. Significance level of p < 0.05 was used. Results: Maximum longitudinal displacement (0.12 vs. 0.17 cm), maximum velocity during early diastole (MVED, 0.6 vs. 1.4s-1), slope of the best-fit line of displacement in diastasis (VDS, 0.22 vs. 0.03s-1), and VDS/MVED ratio (0.35 vs. 0.02) were significantly reduced in AVD patients compared with controls, respectively. Aortic regurgitation showed significantly worse longitudinal LV shortening compared with aortic stenosis. Higher LV mass indicated worse diastolic dysfunction. Conclusions: A simple linear measurement detected significant differences on LV diastolic function between AVD patients and controls. LV mass was the only independent predictor of diastolic dysfunction in these patients. This method can help in the evaluation of diastolic dysfunction, improving cardiomyopathy detection by CMR, without prolonging exam time or depending on expensive software.

Resumo Fundamentos: A disfunção diastólica, comumente avaliada por ecocardiografia, é um importante achado precoce na maioria das cardiomiopatias. A ressonância magnética cardíaca (RMC) frequentemente requer sequências específicas que prolongam o tempo de exame. Recentemente, métodos de imagens com monitoramento de dados (feature-tracking) foram desenvolvidos, mas ainda requerem softwares caros e carecem de validação clínica. Objetivos: Avaliar a função diastólica em pacientes com doença valvar aórtica (DVA) e compará-la a controles normais pela medida do deslocamento longitudinal do ventrículo esquerdo (VE) por RMC. Métodos: Nós comparamos 26 pacientes com DVA com 19 controles normais. A função diastólica foi avaliada como uma medida do deslocamento longitudinal do VE nas imagens de cine-RMC no plano quatro câmaras usando a sequência steady state free precession (SSFP) durante todo o ciclo cardíaco com resolução temporal < 50 ms. O gráfico resultante da posição da junção atrioventricular versus tempo gerou variáveis de movimento da junção atrioventricular. Utilizamos nível de significância de p < 0,005. Resultados: Deslocamento longitudinal máximo (0,12 vs. 0,17 cm), velocidade máxima em início de diástole (0,6 vs. 1,4s-1), velocidade máxima na diástase (0,22 vs. 0,03s-1) e a razão entre a velocidade máxima na diástase e a velocidade máxima em diástole inicial (0,35 vs. 0,02) foram significativamente menores nos pacientes com DVA em comparação aos controles normais, respectivamente. Pacientes com insuficiência aórtica apresentaram medidas de encurtamento longitudinal do VE significativamente piores em comparação aqueles com estenose aórtica. O aumento da massa ventricular esquerda indicou pior disfunção diastólica. Conclusões: Esta simples medida linear detectou diferenças significativas na função diastólica do VE entre pacientes com DVA e controles normais. A massa ventricular esquerda foi o único preditor independente de disfunção diastólica nesses pacientes. Este método pode auxiliar na avaliação da disfunção diastólica, melhorando a detecção de cardiomiopatias por RMC sem prolongar o tempo de exame ou depender de caros softwares.

Longitudinal Shortening of the Left Ventricle by Cine-CMR for Assessment of Diastolic Function in Patients with Aortic Valve Disease.

BACKGROUND: Diastolic dysfunction, commonly evaluated by echocardiography, is an important early finding in many cardiomyopathies. Cardiac magnetic resonance (CMR) often requires specialized sequences that extends the test time. Recently, feature-tracking imaging has been made available, but still requires expensive software and lacks clinical validation. OBJECTIVE: To assess diastolic function in patients with aortic valve disease (AVD) and compare it with normal controls by evaluating left ventricular (LV) longitudinal displacement by CMR. METHODS: We compared 26 AVD patients with 19 normal controls. Diastolic function was evaluated as LV longitudinal displacement in 4-chamber view cine-CMR images using steady state free precession (SSFP) sequence during the entire cardiac cycle with temporal resolution < 50 ms. The resulting plot of atrioventricular junction (AVJ) position versus time generated variables of AVJ motion. Significance level of p < 0.05 was used. RESULTS: Maximum longitudinal displacement (0.12 vs. 0.17 cm), maximum velocity during early diastole (MVED, 0.6 vs. 1.4s-1), slope of the best-fit line of displacement in diastasis (VDS, 0.22 vs. 0.03s-1), and VDS/MVED ratio (0.35 vs. 0.02) were significantly reduced in AVD patients compared with controls, respectively. Aortic regurgitation showed significantly worse longitudinal LV shortening compared with aortic stenosis. Higher LV mass indicated worse diastolic dysfunction. CONCLUSIONS: A simple linear measurement detected significant differences on LV diastolic function between AVD patients and controls. LV mass was the only independent predictor of diastolic dysfunction in these patients. This method can help in the evaluation of diastolic dysfunction, improving cardiomyopathy detection by CMR, without prolonging exam time or depending on expensive software.

Novel Method for Assessment of His Bundle Pacing Morphology Using Near Field and Far Field Device Electrograms.

BACKGROUND: The 12-lead ECG is considered the gold standard to differentiate between selective (S), nonselective (NS) His bundle pacing (HBP), and right ventricular septal capture in routine clinical practice. We sought to assess the utility of device EGM recordings as a tool to identify the type of HBP morphology. METHODS: One hundred forty-eight consecutive patients underwent HBP with a 3830 Select Secure lead (Medtronic, Inc) at 3 centers between October 2016 and October 2017. The near field V-EGM morphology (NF EGM), near field V-EGM time to peak (NFTime to peak), and far-field EGM QRS duration (QRSd) were recorded while pacing the His lead with simultaneous 12-lead ECG rhythm strips. RESULTS: Indications for HBP were sinus node dysfunction, atrioventricular conduction disease, and cardiac resynchronization therapy in 68 (46%), 56 (38%), and 24 (16%) patients, respectively. Baseline QRSd was 108±38 ms with QRSd >120 ms in 57 (39%) patients (27 right bundle branch block, 18 left bundle branch block, and 12 intraventricular conduction delay). S-HBP was noted in 54 (36%) patients. A positive NFEGM and NFTime to peak >40 ms were highly sensitive (94% and 93%, respectively) and specific (90% and 94%) for S-HBP irrespective of baseline QRSd. All 3 parameters (+NFEGM, NFTime to peak >40 ms, and far-field EGM QRSd <120 ms) had high negative predictive value (97%, 95%, and 92%). A novel device-based algorithm for S-HBP was proposed. EGM transitions correlated with ECG transitions during threshold testing and can help accurately differentiate between S-HBP, NS-HBP, and right ventricular septal pacing with a cumulative positive predictive value of 91% (positive predictive value =100% in patients with baseline QRSd <120 ms). CONCLUSIONS: We propose a novel and simple criteria for accurate differentiation between S-HBP, NS-HBP, and right ventricular septal capture morphologies by careful analysis of device EGMs alone. This study paves the way for future studies to assess autocapture algorithms for devices with HBP.

Simultaneous His Bundle and Left Ventricular Pacing for Optimal Cardiac Resynchronization Therapy Delivery: Acute Hemodynamic Assessment by Pressure-Volume Loops.

BACKGROUND: Previous studies have investigated the role of intrinsic conduction in optimizing cardiac resynchronization therapy. We investigated the role of fusing pacing-induced activation and intrinsic conduction in cardiac resynchronization therapy by evaluating the acute hemodynamic effects of simultaneous His-bundle (HIS) and left ventricular (LV) pacing. METHODS AND RESULTS: We studied 11 patients with systolic heart failure and left bundle-branch block scheduled for cardiac resynchronization therapy implantation. On implantation, LV pressure-volume data were determined via conductance catheter. Standard leads were placed in the right atrium, at the right ventricular apex, and in a coronary vein. An additional electrode was temporarily positioned in the HIS. The following pacing configurations were systematically assessed: standard biventricular (right ventricular apex+LV), LV-only, HIS, simultaneous HIS and LV (HIS+LV). Each configuration was compared with the AAI mode at multiple atrioventricular delays (AVD). In comparison with the AAI, right ventricular apex+LV and LV-only pacing resulted in improved stroke volume (85±32 mL and 86±33 mL versus 58±23 mL; P<0.001), stroke work, maximum pressure derivative, and systolic dyssynchrony at individually optimized AVD. The optimal AVD was close to the P-H interval in the majority of patients. By contrast, HIS-LV pacing improved hemodynamic indexes at all AVD (stroke volume >76 mL at all fixed intervals and 88±31 mL at optimal interval; all P<0.001). CONCLUSIONS: Standard right ventricular apex+LV and LV-only pacing enhanced systolic function and LV synchrony at individually optimized AVD close to the measured intrinsic P-H interval. By contrast, HIS+LV pacing yielded improvements, regardless of AVD setting. These findings support the hypothesis of the crucial role of intrinsic right ventricular conduction in optimal cardiac resynchronization therapy delivery.

Hyperthyroidism and complete atrioventricular block--a report of 2 cases with electrophysiologic assessment.

Although cardiovascular manifestations in thyroid disorders are frequently encountered in clinical practice, atrioventricular (AV) conduction disorders, especially in hyperthyroidism, are rare. There are some proposed mechanisms for AV blocks in hyperthyroidism but the exact mechanism is still unknown. The authors report 2 cases with thyroid function disorders and complete AV block, and the electrophysiologic characteristics of these 2 patients, and they review and speculate on similar reported cases.

Clinical in vivo calibration of pulse wave tissue Doppler velocities in the assessment of ventricular wall motion. A comparison study with M-mode echocardiography.

BACKGROUND: Pulsed Wave Tissue Doppler (PWTD) recording of myocardial velocities has been widely used for assessing ventricular function but the output trace has finite thickness that leads to potential ambiguity in determining velocity and timing. OBJECTIVE: To determine optimal method of measurement of PWTD traces by comparing them with those obtained from digitised M-mode recorded from the atrioventricular (AV) valve ring (septal, LV and RV free wall). METHODS: We studied 100 subjects, 49 normal and 51 with coronary artery disease (15 patients with reduced left ventricular wall motion, mean systolic amplitude of LV free wall 0.8+/-0.3 cm), mean age 53+/-15 years. We recorded AV ring motion using PWTD and M-mode echo techniques. PWTD velocity signals were measured separately at: outer, inner and mid-points of the envelope and compared with peak velocities obtained from digitised M-mode long axis. RESULTS: Peak systolic (S), early diastolic (E) and late diastolic (A) PWTD velocities at outer, inner and middle envelope correlated closely with the corresponding M-mode measurements at left, septal and right ventricular free wall. However, only the midpoint S and E wave PWTD signal velocities agreed numerically with those obtained by digitised M-mode velocities; S (left 6.56+/-1.80 vs. 6.54+/-1.91 cm/s N.S.); E (left 8.50+/-3.25 vs. 7.65+/-3.30 cm/s N.S.). Agreement was somewhat less satisfactory for A wave; left 7.40+/-2.13 vs. 6.23+/-2.09 cm/s p<0.05. CONCLUSION: Atrioventricular valve ring echo provides an excellent in vivo calibration model for validating tissue Doppler velocity estimates. Since the mid-point of the envelope of the tissue Doppler signal is the most closely related value to that of the digitised M-mode, it may be recommended as a convention for routine practice.

Quantitative assessment of the recovery property of atriofascicular/atrioventricular-type Mahaim fiber.

INTRODUCTION: One of the characteristics of the Mahaim fiber is that it possesses a decremental property related to the slow rate of recovery of its excitability similar to that of AV node. The aim of this study was to evaluate the recovery property of the atriofascicular/atrioventricular-type Mahaim fiber and compare it with that of the AV node. METHODS AND RESULTS: Nine patients with a Mahaim fiber were studied; 8 of the patients had atriofascicular/atrioventricular-type fiber. Different models were used to analyze the relationship between conduction time for the Mahaim fiber and the corresponding coupling intervals. The simplest model with the best fit was found to be the linear regression of the natural log of conduction time on corrected coupling intervals. The individual R2 values ranged between 0.43 and 0.98 for the Mahaim fiber and between 0.79 and 0.98 for AV node. The final model chosen for the log transformed data for the Mahaim fiber and for the AV node was the line with parameter estimates defined as a weighted average, over patients, of the corresponding individual line parameters. The weight used for each parameter was the inverse of its variance. The slopes of the lines of the transformed data were not significantly different between the Mahaim fiber and the AV node. Thus, the best fitting curve for the recovery property of the AF-type Mahaim fiber is a simple exponential curve similar to that of the AV node. CONCLUSION: The atriofascicular/atrioventricular-type Mahaim fiber has a quantitative recovery property very similar to that of the AV node.

Long-term left ventricular pacing: assessment and comparison with biventricular pacing in patients with severe congestive heart failure.

OBJECTIVE: The purpose of this study is to report prospectively the results of six-month follow-up of permanent left ventricular (LV) based pacing in patients with severe congestive heart failure (CHF) and left bundle branch block (LBBB). BACKGROUND: Left ventricular pacing alone has been demonstrated to result in identical improvement compared to biventricular pacing (BiV) during acute hemodynamic evaluation in patients with advanced CHF and LBBB. However, to our knowledge, the clinical outcome during permanent LV pacing alone versus BiV pacing mode has not been evaluated. METHODS: Pacing configuration (LV or BiV) was selected according to the physician's preference. Patient evaluation was performed at baseline and at six months. RESULTS: Thirty-three patients with advanced CHF and LBBB were included. Baseline characteristics of LV (18 patients) and BiV (15 patients) pacing groups were similar. During the six-month follow-up period, seven patients died three BiV and four LV). In the surviving patients at 6 months, 8 of 14 patients in the LV group and 9 of 12 in the BiV group were in New York Heart Association class I or II (p = 0.39). No significant difference was observed between the two groups in terms of objective parameters except for LV end-diastolic diameter decrease (-4.4 mm in BiV group vs. -0.7 mm in LV group; p = 0.04). CONCLUSION: At six-month follow-up, a trend toward improvement was observed in objective parameters in patients with severe CHF and LBBB following LV-based pacing. The two pacing modes (LV and BiV) were associated with almost equivalent improvement of subjective and objective parameters.

Assessment of pacing maneuvers used to validate anterograde accessory pathway potentials.

Four pacing maneuvers have been proposed to validate an anterograde accessory pathway potential (APP): (1) atrial pacing to induce complete block between the atrial electrogram and the APP; (2) ventricular pacing to advance the APP without altering the timing of the atrial electrogram; (3) atrial pacing to induce complete block between the APP and the ventricular electrogram; and (4) ventricular pacing to advance the ventricular electrogram without altering the timing of the APP. The purpose of this study was to assess these validation techniques by applying them to electrograms that simulated APPs but which were known to be atrial in origin. In 32 patients undergoing an electrophysiology procedure, a split atrial electrogram containing two components separated by at least 30 msec (mean 54 +/- 15 msec) was recorded. Using an atrial extrastimulus technique, complete block between the two components of the atrial electrogram (criterion 1) could never be induced, but complete block between the second component of the atrial electrogram and the ventricular electrogram (criterion 3) consistently was induced. Using a ventricular extrastimulus technique, the second component of the atrial electrogram consistently could be advanced by 10 to 40 msec without altering the timing of the first component (criterion 2). In addition, with ventricular pacing, the ventricular electrogram consistently was advanced without altering the timing of the two components of the atrial electrogram (criterion 4). In conclusion, among the four pacing maneuvers used to validate an anterograde APP, the only one that may be specific for an APP is the ability to induce complete block between the atrial electrogram and the APP.

Comparison of ramp and stepwise incremental pacing in assessment of antegrade and retrograde conduction.

Conventional assessment of antegrade (AV) and retrograde (VA) conduction involves stepwise increments in pacing rates until block in conduction is observed. This study was designed to establish the comparative characteristics of ramp pacing, in which the rate is continuously and smoothly incremented until block occurs. Two hundred and ten patients participated in portions of this study. Stepwise pacing was performed in 10 beat/minute steps, with the rate held for at least 15 seconds at each step; if marked prolongation or variability in conduction was observed, the rate was held constant for up to 60 seconds to allow for accommodation. With ramp pacing, the rate was gradually increased at a steady 2-4 beats/minute/second. Whenever possible, both stepwise and ramp pacing were performed for assessment of both antegrade and retrograde conduction. All patients had conducted sinus rhythm as their baseline mechanism. Antegrade conduction was similar using incremental stepwise and ramp pacing. The AH interval at a cycle length (CL) of 500 ms, the maximum AH increment, the cycle length at AV block were all remarkably similar (p = NS). Assessment of retrograde conduction produced similar results, with insignificant differences between maximum conducted VA intervals, and cycle length at VA block using the two pacing techniques. Ramp pacing provides a useful and rapid alternative to conventional stepwise incremental pacing in the assessment of antegrade and retrograde conduction in patients using both normal and accessory pathways. Ramp pacing was better tolerated, and some correlations between antegrade and retrograde conduction were stronger with the ramp pacing technique.

Limitation of the atrial extrastimulus technique for assessment of the His-Purkinje system in patients with right bundle branch block.

A prospective study of the usefulness of the atrial extrastimulus technique for assessment of the His-Purkinje system was undertaken in 19 consecutive patients with complete RBBB and an abnormal or indeterminate frontal QRS axis. A total of 59 basic cycle lengths (BCLs) could be analyzed in detail. The most common patterns of His-Purkinje conduction seen were simple linear (V1V2 vs H1H2, N = 49) or flat (H2V2 vs H1H2, N = 44) curves. This finding is related to the limitation imposed by interposition of the slowly conducting AV node between the site of the extrastimulus and the His-Purkinje system. One or more refractory periods could be measured in only 27% of BCLs (10 Effective Refractory Periods of the remaining intact His-Purkinje system plus 16 Functional Refractory Periods and 19 Relative Refractory Periods). No refractory periods of the His-Purkinje system could be determined in 8 of 19 patients (42%), the major limitation being slowed AV node conduction. The results suggest limited likelihood that additional clinically useful information about His-Purkinje system conduction can be obtained by the atrial extrastimulus technique, as used in this study, although it may be helpful in assessing other parts of the conduction system.

Value and limitations of clinical electrophysiologic study in assessment of patients with unexplained syncope.

We assessed the value of clinical electrophysiologic study using intracardiac recording and programed electrical stimulation in 34 patients who had unexplained syncope and/or presyncope. All patients had normal electrocardiograms, and no abnormality was detected by clinical examination, ambulatory electrocardiographic recording, or treadmill testing. The electrophysiologic results were diagnostic in four patients (11.8 percent) and led to appropriate therapy that totally relieved symptoms. The results were abnormal but not diagnostic in two patients (5.8 percent) and normal in the remaining 28 patients (82.4 percent). The patients were followed for a mean period of 15 months (range two to 44) after electrophysiologic testing. Sixteen patients (47 percent) had no further episodes in the absence of any intervention. In four patients (11.8 percent), a definitive diagnosis was made during follow-up. In seven patients, permanent pacing was instituted empirically with relief of syncope. Two patients continued to have syncopal spells. We conclude that the diagnostic yield of electrophysiologic testing is low in a patient population that has no electrocardiographic abnormality or clinical evidence of cardiac disease. Empirical permanent pacing in patients with symptoms continuing after our study appeared to be beneficial, but this result is difficult to evaluate because of the high incidence of spontaneous remission in this group. Persistent attempts to document electrocardiographic abnormalities during a typical episode of symptoms appears to be the only definitive way to confirm or exclude an arrhythmic cause of the symptoms.

Left ventricular relaxation, mitral valve prolapse, and intracardiac conduction in myotonia atrophica: assessment by digitized echocardiography and noninvasive His bundle recording.

Myotonia atrophica, a neuromuscular disease marked by autosomal dominant transmission and delayed relaxation of skeletal muscle, has been associated with cardiac failure, conduction abnormality and mitral prolapse (MVP). In order to determine the relaxation rate of cardiac muscle, left ventricular (LV) size and function, and the presence of MVP, 30 patients with myotonia atrophica were studied using digitized M-mode echocardiography (MME). Intracardiac conduction intervals were determined by noninvasive His bundle recording (HBR) from surface electrodes using a high-resolution, R-wave triggered, signal averaging computer. Neurologically unaffected first-degree relatives of the patients with myotonia atrophica were also studied to determine if cardiac abnormalities may be present in the absence of neurologic manifestations of the disease. Peak normalized diastolic endocardial velocity in patients with myotonia atrophica (3.7 +/- 0.8 sec-1) did not differ from unaffected first-degree relatives (3.8 +/- 0.8 sec-1) or normal subjects (3.6 +/- 0.8 sec-1). Systolic LV function and LV dimensions on MME were normal in both groups. However, MVP was present in 7 of 24 (29%) of patients who could be evaluated, but not in unaffected first-degree relatives. Despite normal LV systolic and diastolic function, infranodal intracardiac conduction was prolonged in patients with myotonia atrophica (average HV interval 50 +/- 5 SD msec) but not in neurologically unaffected relatives (average HV interval 40 +/- 5 msec). Delay in proximal intracardiac conduction was also found in patients with myotonia atrophica (average PH interval 140 +/- 20 msec) but not in neurologically unaffected relatives (average PH interval 115 +/- 6 msec). Hence cardiac findings in myotonia atrophica include proximal and distal conduction delay by external HBR even in the absence of abnormality of the standard 12-lead ECG. There may also be an increased frequency of MVP; however, early diastolic relaxation of the LV is unimpaired, and cardiac manifestations of myotonia are not transmitted independently of neurologic abnormality.

Assessment of refractoriness of the human Purkinje system.

In 35 unselected patients premature right ventricular stimulation during a constant right ventricular drive rhythm was performed at the rates of 80, 100, and 120 per min. Several surface ECG leads and intracardiac electrograms were simultaneously recorded. To assess the beginning of decreased conductivity of the His-Purkinje system for retrograde conduction the premature test pulse interval was measured when the retrograde intraventricular conduction to the bundle of His prolonged for the first time. This interval was defined as the beginning of relative refractoriness of the Purkinje system. Additionally, the effective refractory period of the right ventricular muscle was determined. A rate dependancy between the beginning of relative refractoriness of the Purkinje system and the effective refractory period of the right ventricular muscle, as it is known from animal experiments, could be established with significantly different (P less than 0.001) mean values at the three different heart rates. The method described may be the only means by which to obtain approximate information about normal and abnormal conductivity of the human Purkinje system. It is inferred that critical prolongation of refractoriness of the Purkinje system relative to the refractoriness of the myocardium may be relevant to the occurrence of ventricular arrhythmias.