Relative contributions of the atrial systoleventricular systole interval and of patterns of ventricular activation to ventricular function during electrical pacing of the dog heart

Different lead locations guided by fluoroscopy and ECG parameters and their effect on patients functional status

Background

Right ventricular apical pacing with the resultant left ventricular dyssynchrony often leads to depressed systolic function and heart failure. This study aimed at investigating the relation between various septal locations guided by ECG and fluoroscopy and the intermediate term functional capacity of the patients.

Results

Fifty patients who received a single lead pacemaker with assumed > 90% pacemaker dependency. Patients were randomized according to RV pacing site RV into group 1 “high septum” (n = 15), group 2 “mid septum” (n = 25), and group 3 “low septum” (n = 10) using QRS vector and duration as well as fluoroscopic parameters. Their clinical status was assessed 6 months after device implementation using 6-min walk test (6MWT).

The study showed that paced QRS complex duration itself has no significant difference between the different septal pacing locations (P-value 0.675), although its combination with the paced QRS complex vector can signify the optimal pacing site and 6MWT showed a significant difference among the groups in favor of group 1; group 1 (413.3 ± 148.5), group 2 (359.8 ± 124.6), and group 3 (276.0 ± 98.5) P value 0.04.

Conclusion

There was a significant difference found between the three septal pacing sites concerning the patient functional capacity with superiority of high septal location. By contrast, different septal sites showed no significant difference of the paced QRS complex duration. To optimize the pacing site in the septum, assessment of the paced QRS vector in leads I and III is of a great benefit especially when combined with paced QRS complex duration assessment.

The molecular and functional identities of atrial cardiomyocytes in health and disease

Atrial cardiomyocytes are essential for fluid homeostasis, ventricular filling, and survival, yet their cell biology and physiology are incompletely understood. It has become clear that the cell fate of atrial cardiomyocytes depends significantly on transcription programs that might control thousands of differentially expressed genes. Atrial muscle membranes propagate action potentials and activate myofilament force generation, producing overall faster contractions than ventricular muscles. While atria-specific excitation and contractility depend critically on intracellular Ca(2+) signalling, voltage-dependent L-type Ca(2+) channels and ryanodine receptor Ca(2+) release channels are each expressed at high levels similar to ventricles. However, intracellular Ca(2+) transients in atrial cardiomyocytes are markedly heterogeneous and fundamentally different from ventricular cardiomyocytes. In addition, differential atria-specific K(+) channel expression and trafficking confer unique electrophysiological and metabolic properties. Because diseased atria have the propensity to perpetuate fast arrhythmias, we discuss our understanding about the cell-specific mechanisms that lead to metabolic and/or mitochondrial dysfunction in atrial fibrillation. Interestingly, recent work identified potential atria-specific mechanisms that lead to early contractile dysfunction and metabolic remodelling, suggesting highly interdependent metabolic, electrical, and contractile pathomechanisms. Hence, the objective of this review is to provide an integrated model of atrial cardiomyocytes, from tissue-specific cell properties, intracellular metabolism, and excitation-contraction (EC) coupling to early pathological changes, in particular metabolic dysfunction and tissue remodelling due to atrial fibrillation and aging. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Comparison of ventricular dyssynchrony according to the position of right ventricular pacing electrode: a multi-center prospective echocardiographic study

BACKGROUND

Conventional pacemaker implantation induces left ventricular (LV) dyssynchrony, which might affect the LV function. We sought to evaluate the impact of different right ventricular (RV) pacing sites on the LV dyssynchrony and performance.

METHODS

Comprehensive echocardiographic evaluation including the atrio-ventricular, inter- and intra-ventricular dyssynchrony based on M-mode, conventional Doppler and tissue Doppler imaging (TDI) was done before and immediately after (< 7 days) pacemaker implantation. For the LV performance, LV ejection fraction, longitudinal peak systolic velocity at the mitral annulus (S') annular or mean longitudinal velocity of the 6 basal segments (Sm) were used. These results were compared with those of 15 age matched controls.

RESULTS

A total of 79 patients (48 females, mean age 63 ± 12 years) underwent RV pacing at the apex (n = 45, group I) or the septum (n = 34, group II). After pacemaker implantation, the QRS duration was significantly increased in both groups, but the change was greater in group I (57.1 ± 28.3 versus 32.8 ± 40.5 msec). Both the S' and Sm were lower in pacing groups than those in controls and Sm was significantly higher in group II (4.2 ± 1.0 versus 4.9 ± 1.3 m/sec) than group I despite a similar LV ejection fraction. The aortic pre-ejection time and septal to posterior wall motion delay in patients with pacemaker were longer compared to normal controls, but there were no significant differences. Both the TDI velocity and strain analysis showed no difference of the dyssynchrony indices between the two groups, despite a higher tendency of Doppler strain dyssynchrony indices in the RV apical pacing group compared to those of the control.

CONCLUSION

Despite the marked increase of the QRS duration after pacing, M-mode, Doppler and TDI failed to demonstrate any difference according to the pacing sites. The long-term effect of the longitudinal contraction being less affected and a smaller increase of the QRS duration in the RV septal pacing group needs to be confirmed in a longitudinal follow-up study.

Upgrade to biventricular pacing in patients with pacing-induced heart failure: can resynchronization do the trick?

Dyssynchrony imposed on ventricular function by right ventricular (RV) apical pacing may lead in some cases to worsening or appearance of heart failure (HF) symptoms. This is a result of an altered pattern of activation, leading to several histological and functional adjustments of the left ventricle, including inhomogeneous thickening of the ventricular myocardium and myofibrillar disarray, fibrosis, disturbances in ion-handling protein expression, myocardial perfusion defects, alterations in sympathetic tone and mitral regurgitation. Studies of mid- and long-term effects of RV apical pacing on left ventricular (LV) function have demonstrated a progressive decline in ejection fraction and other indices of LV functional competence. Upgrading RV pacing systems to biventricular resynchronization modalities is a theoretically promising option for paced patients with worsening HF. The potentially favourable effect of upgrading on LV functional indices and patient clinical status has been demonstrated in few, non-randomized trials. Apart from the scantiness of existing clinical data, issues concerning technical aspects of the procedure and selection of eligible patients are raised. Is pacing-induced dyssynchrony equivalent to the indigenous dyssynchrony in unpaced patients with HF? What selection criteria should be applied in order to identify potential responders to cardiac resynchronization therapy in this patient population? Answers to these and more questions are still lacking.

Evolution and Scaling of Atrioventricular Conduction Time in Mammals: Part 1

Scaling can be defined as the adjustment of a structure, a function, or an organ to the size of the mammalian body. An example is the size of the heart in relation to the size of the body. The duration of the PR interval on the electrocardiogram (atrioventricular delay) in relation to the size of the heart is a perplexing example of scaling. During evolution, mammalian species changed their shape, size, and function while adapting to the habitat in which they had to live and survive. This review deals with the problem of the apparent mismatch in scaling of the atrioventricular delay (PR interval) in relation to the size of the mammalian heart from mouse to whale.

Atrioventricular conduction in mammalian species: Hemodynamic and electrical scaling

OBJECTIVES

The purpose of this study was to investigate scaling of the duration of late diastolic left ventricular (LV) filling in relation to AV conduction time (delay) (PR interval on the ECG) in mammals.

BACKGROUND

From mouse to whale, AV delay increases 10-fold, whereas body mass increases one million-fold. The apparent "mismatch" results from scaling of AV delay versus body and heart mass.

METHODS

We measured (1) mitral orifice diameter in 138 postmortem hearts of 48 mammalian species weighing between 17 g and 250 kg and (2) transmitral diastolic flow using magnetic resonance imaging (MRI) recordings of 10 healthy human individuals. (3) We visually inspected early and late diastolic LV filling. (4) We developed two physical models to explain scaling of late diastolic LV filling time.

RESULTS

(1) Diameter of the mitral orifice proportionally relates to heart length (third root of heart mass). (2) Atrial contraction starts at a fixed instant (+/- 80%) of the (normalized) cardiac cycle and contributes 31% +/- 5% to LV filling. (3) MRI shows that during diastole, the left atrium (LA) and LV form a single space. (4) The physical models relate the duration of late diastolic LV filling directly to heart length, the third root of heart mass.

CONCLUSIONS

(1) Late diastolic (LV) filling time scales with heart length (third root of heart mass). (2) No "mismatch" exists between AV delay and heart size. (3) Knowledge of the actual starting time of atrial contraction may contribute to better treatment of patients with heart failure. (4) The findings suggest that in evolution of mammalian species, hemodynamics commands electrical behavior of the heart.

Window to the heart: the value of a native and paced QRS duration. Current perspective and review

Given the technological advances and reliance upon expensive testing for guiding therapy, it is surprising how an inexpensive, low tech electrocardiogram can provide a wealth of information pertaining to the underlying cardiovascular status of a patient. In this article we review the changes in hemodynamics, prognosis and guidance of therapeutic options associated with a prolonged QRS duration.

Atrial contraction and mitral annular dynamics during acute left atrial and ventricular ischemia in sheep

In six sheep, radiopaque markers were placed on the left ventricle (LV), the mitral annulus, the left atrium (LA), and the central edge of both mitral leaflets to investigate the effects of acute LV ischemia on atrial contraction, mitral annular area (MAA), and mitral regurgitation (MR). Animals were studied with biplane videofluoroscopy and transesophageal echocardiography before and during balloon occlusion of the left anterior descending (LAD), distal circumflex (dLCX), and proximal circumflex (pLCX) coronary arteries. MAA and LA area were calculated from the corresponding markers. LAD occlusion did not alter LA area reduction or presystolic MAA reduction, whereas dLCX occlusion resulted in a mild decrease in the former with no change in the latter. Neither occlusion resulted in MR. pLCX occlusion, however, significantly decreased LA area and presystolic MAA reduction and resulted in increased end-diastolic MAA, delayed valve closure from end diastole, and MR. Decreased atrial contractile function, as observed during acute posterolateral ischemia, is linked to diminished presystolic mitral annular reduction, a larger mitral annular size at end diastole, and MR.

Functional abnormalities in patients with permanent right ventricular pacing: the effect of sites of electrical stimulation

OBJECTIVES

We sought to evaluate the long-term effects of alternative right ventricular pacing sites on myocardial function and perfusion.

BACKGROUND

Previous studies have demonstrated that asynchronous ventricular activation due to right ventricular apical (RVA) pacing alters regional myocardial perfusion and functions.

METHODS

We randomized 24 patients with complete atrioventricular block to undergo permanent ventricular stimulation either at the RVA (n = 12) or right ventricular outflow (RVOT) (n = 12). All patients underwent dipyridamole thallium myocardial scintigraphy and radionuclide ventriculography at 6 and 18 months after pacemaker implantation.

RESULTS

After pacing, the mean QRS duration was significantly longer during RVA pacing than during RVOT pacing (151 +/- 6 vs. 134 +/- 4 ms, p = 0.03). At six months, the incidence of myocardial perfusion defects (50% vs. 25%) and regional wall motion abnormalities (42% vs. 25%) and the left ventricular ejection fraction (LVEF) (55 +/- 3% vs. 55 +/- 1%) were similar during RVA pacing and RVOT pacing (p > 0.05). However, at 18 months, the incidence of myocardial perfusion defects (83% vs. 33%) and regional wall motion abnormalities (75% vs. 33%) were higher and LVEF (47 +/- 3 vs. 56 +/- 1%) was lower during RVA pacing than during RVOT pacing (all p < 0.05). Patients with RVA pacing had a significant increase in the incidence of myocardial perfusion defects (p < 0.05) and a decrease in LVEF (p < 0.01) between 6 and 18 months, but patients with RVOT pacing did not (p > 0.05).

CONCLUSIONS

This study demonstrates that preserved synchronous ventricular activation with RVOT pacing prevents the long-term deleterious effects of RVA pacing on myocardial perfusion and function in patients implanted with a permanent pacemaker.

Cardiac resynchronization therapy restores optimal atrioventricular mechanical timing in heart failure patients with ventricular conduction delay

We characterized the relationship between systolic ventricular function and left ventricular (LV) end-diastolic pressure (LVEDP) in patients with heart failure (HF) and baseline asynchrony during ventricular stimulation. The role of preload in the systolic performance improvement that can be obtained in HF patients with LV stimulation is uncertain.We measured the maximum rate of increase of LV pressure, LVEDP, aortic pulse pressure (PP) and the atrioventricular mechanical latency (AVL) between left atrial systole and LV pressure onset in 39 patients with HF. Two subgroups were identified: "responder" if PP improved, or "nonresponder."Maximum hemodynamic improvement occurred at an atrioventricular (AV) delay that did not decrease LVEDP. Left ventricular and biventricular (BV) stimulation increased systolic hemodynamics significantly, despite no significant increase in LVEDP. All parameters decreased when the LVEDP was decreased by shorter AV delay. Left ventricular and BV stimulation provided better hemodynamics than right ventricular (RV) stimulation. For the nonresponder subgroup, systolic hemodynamics only worsened during AV delay shortening. For the responder subgroup, optimum PP was achieved when AVL was near zero. Restoration of optimal left atrial-ventricular mechanical timing partly contributes to the hemodynamic improvements observed in this patient subgroup. However, preload alone cannot explain the differences seen between RV and BV stimulation and the contradictory PP decreases even at maximal preload in the nonresponder subgroup. These results may be explained by a site-dependent mechanism such as the degree of ventricular synchrony. Caution should be taken in these patients when optimizing AV delays using echocardiography techniques that focus on LV inflow.

Mitral annular dynamics during rapid atrial pacing

INTRODUCTION

Ovine mitral annular area (MAA) reduction predominantly occurs before ventricular systole. We used the myocardial marker methods to investigate left atrial and MAA dynamics during rapid atrial pacing.

METHODS

Seven sheep underwent implantation of 21 myocardial markers around the mitral annulus, the left ventricle and left atrium. After 7 to 10 days, animals were studied with biplane videofluoroscopy to determine 3-dimensional marker coordinates unpaced and during rapid atrial pacing at 140 minutes(-1). Left ventricle volume, left atrial volume (LAV), and MAA were calculated from marker coordinates. End diastole (ED) was defined at peak of the electrocardiogram R wave; times of minimum MAA and minimum LAV were expressed relative to ED (t = 0). Percent reduction in MAA and LAV were calculated from maximum and minimum values between diastole and early systole.

RESULTS

The time of minimum MAA occurred earlier relative to ED during rapid pacing compared with control (-48 +/- 21 vs 19 +/- 14 msec; P <.001), as did the time of minimum LAV (-47 +/- 18 vs 4 +/- 16 msec; P <.001). Minimum MAA and LAV were significantly smaller with rapid pacing (6. 8 +/- 0.6 vs 6.5 +/- 0.5 cm(2); P <.05, respectively; and 15.4 +/- 2. 4 vs 16.5 +/- 2.3 mL; P <.01, respectively), and a relatively greater fractional reduction in MAA and LAV was observed during presystole.

CONCLUSIONS

Rapid atrial pacing resulted in greater MAA and LAV reduction, both of which occurred entirely during diastole. This study supports the notion that MAA reduction is closely linked to LA dynamics.

Mapping of regional myocardial strain and work during ventricular pacing: experimental study using magnetic resonance imaging tagging

OBJECTIVES

The purpose of this study was to determine the spatial distribution of myocardial function (myofiber shortening and work) within the left ventricular (LV) wall during ventricular pacing.

BACKGROUND

Asynchronous electrical activation, as induced by ventricular pacing, causes various abnormalities in LV function, perfusion and structure. These derangements may be caused by abnormalities in regional contraction patterns. However, insight into these patterns during pacing is as yet limited.

METHODS

In seven anesthetized dogs, high spatial and temporal resolution magnetic resonance-tagged images were acquired in three orthogonal planes. Three-dimensional deformation data and LV cavity pressure and volume were used to determine midwall circumferential strain and external and total mechanical work at 192 sites around the left ventricle.

RESULTS

During ventricular pacing, systolic fiber strain and external work were approximately zero in regions near the pacing site, and gradually increased to more than twice the normal value in the most remote regions. Total mechanical work, normalized to the value during right atrial pacing, was 38 +/- 13% (right ventricular apex [RVapex] pacing) and 61 +/- 23% (left ventricular base [LVbase] pacing) close to the pacing site, and 125 +/- 48% and 171 +/- 60% in remote regions, respectively (p < 0.05 between RVapex and LVbase pacing). The number of regions with reduced work was significantly larger during RVapex than during LVbase pacing. This was associated with a reduction of global LV pump function during RVapex pacing.

CONCLUSIONS

Ventricular pacing causes a threefold difference in myofiber work within the LV wall. This difference appears large enough to regard local myocardial function as an important determinant for abnormalities in perfusion, metabolism, structure and pump function during asynchronous electrical activation. Pacing at sites that cause more synchronous activation may limit the occurrence of such derangements.

ECG of the "newborn" mouse (Mus domesticus) with specific reference to comparative AV transmission

INTRODUCTION

The objective of this study was to record the ECG of the smallest living mammal to extend the domain of data for comparative AV nodal electrophysiologic purposes. These data are needed to establish the relationship between the PR interval and heart size in mammalian species of all sizes.

METHODS AND RESULTS

In recently born mice (age 1.5 to 8 weeks) weighing between 2.5 and 10 g and with estimated heart weights between 15 and 60 mg, ECGs, using bipolar limb leads, were recorded during general anesthesia. The PR interval, representing AV transmission time was about 40 msec, which is quite long for hearts of this size. On the basis of detailed analysis of the data, we postulate the presence of a fixed delay or discontinuous propagation in the AV node not only in newborn mice, but in mammals of all sizes.

CONCLUSION

AV transmission times obtained in mammals (including humans) cannot be explained on the basis of generally accepted, classic AV conduction theories. The acceptance of the presence of a fixed delay in the AV node may ultimately be of value to better understand AV node function during sinus rhythm and supraventricular arrhythmias.

Influence of right ventricular stimulation site on left ventricular function in atrial synchronous ventricular pacing

OBJECTIVES

The study investigates the correlation between left ventricular function and QRS duration obtained by alternate right ventricular pacing sites.

BACKGROUND

1. Right ventricular apical pacing is associated with alterations of left ventricular contraction sequence. 2. A stimulation producing narrow QRS complexes is supposed to provide for better left ventricular contraction patterns.

METHODS

Fourteen patients with third degree AV block received one ventricular pacing lead in apical position. The alternate lead was attached to that site on the septum that produced the smallest QRS complex as measured from the earliest to the last deflection in any of the orthogonal Frank leads (xyz). During atrial synchronous ventricular pacing, the AV delay was optimized individually and for each stimulation site using mitral valve doppler or impedance cardiography. By radionuclide ventriculography, the phase distribution histogram of left ventricular contraction was evaluated as area under the curve (AuC); systolic function was determined as ejection fraction (EF) and as absolute ejected counts (EC) in random order. The difference (delta) in QRS duration between apical and septal stimulation (deltaxyz) was correlated with the difference in phase distribution (deltaAuC) and ejection parameters (deltaEF, deltaEC).

RESULTS

QRS duration was shorter with septal than with apical pacing in 9 out of 14 patients (64%); it was longer in 4 (29%), and no difference was seen in 1 patient. There was a significant positive correlation between the change in QRS duration (deltaxvz) and phase distribution (deltaAuC: r = 0.66393, p = 0.010) and a significant negative correlation to systolic function (deltaEF: r = 0.70931, p = 0.004; deltaEC: r = 0.74368, p = 0.002).

CONCLUSIONS

In atrial synchronous right ventricular pacing, if the AV delay is adapted individually, decreased QRS duration obtained by alternate pacing sites is significantly correlated with homogenization of left ventricular contraction and with increased systolic function in acute tests.

Optimization of ventricular function by improving the activation sequence during ventricular pacing

Abnormal electrical activation occurring during ventricular pacing reduces left ventricular (LV) pump function. Two strategies were compared to optimize LV function using ventricular pacing, minimal asynchrony and optimal sequence of electrical activation. ECG and hemodynamics aortic flowprobe, thermodilution cardiac output, LV pressure and its maximal rates of rise (LVdP/dtpos) and fall (LVdP/dtneg) were measured in anesthetized open-chest dogs (n = 7) with healthy hearts. The QRS duration (a measure of asynchrony of activation) was 47 +/- 5 ms during sinus rhythm and increased to 110 +/- 12 ms during DDD pacing at the right ventricular (RV) apex with a short AV interval. During pacing at the LV apex and LV base, the QRS duration was 8% +/- 7% and 15% +/- 7% (P < 0.05) longer than during RV apex pacing, respectively. Stroke volumes, LVdP/dtpos and LVdP/dtneg, however, were higher during LV apex (15% +/- 16%, 10% +/- 12% [P < 0.05], and 15% +/- 10%, respectively) and LV base pacing (11% +/- 12% [P < 0.05], 3% +/- 12%, and 3% +/- 11%, respectively) than during RV apex pacing. Systolic LV pressure was not influenced significantly by the site of pacing. Biventricular pacing (RV apex together with one or two LV sites) decreased the QRS duration by approximately 20% as compared with RV apex pacing, however, it did not improve stroke volumes, LVdP/dtpos and LVdP/dtneg beyond those during pacing at the LV apex alone. In conclusion, the sequence of electrical activation is a stronger determinant of ventricular function than the synchrony of activation. For optimal LV function the selection of an optimal single pacing site, like the LV apex, is more important than pacing from multiple sites.

Asynchronous electrical activation induces asymmetrical hypertrophy of the left ventricular wall

BACKGROUND

Asynchronous electrical activation, induced by ventricular pacing, causes regional differences in workload, which is lower in early- than in late-activated regions. Because the myocardium usually adapts its mass and structure to altered workload, we investigated whether ventricular pacing leads to inhomogeneous hypertrophy and whether such adaptation, if any, affects global left ventricular (LV) pump function.

METHODS AND RESULTS

Eight dogs were paced at physiological heart rate for 6 months (AV sequential, AV interval 25 ms, ventricular electrode at the base of the LV free wall). Five dogs were sham operated and served as controls. Ventricular pacing increased QRS duration from 47.2+/-10.6 to 113+/-16.5 ms acutely and to 133.8+/-25.2 ms after 6 months. Two-dimensional echocardiographic measurements showed that LV cavity and wall volume increased significantly by 27+/-15% and 15+/-17%, respectively. The early-activated LV free wall became significantly (17+/-17%) thinner, whereas the late-activated septum thickened significantly (23+/-12%). Calculated sector volume did not change in the LV free wall but increased significantly in the septum by 39+/-13%. In paced animals, cardiomyocyte diameter was significantly (18+/-7%) larger in septum than in LV free wall, whereas myocardial collagen fraction was unchanged in both areas. LV pressure-volume analysis showed that ventricular pacing reduced LV function to a similar extent after 15 minutes and 6 months of pacing.

CONCLUSIONS

Asynchronous activation induces asymmetrical hypertrophy and LV dilatation. Cardiac pump function is not affected by the adaptational processes. These data indicate that local cardiac load regulates local cardiac mass of both myocytes and collagen.

The effects of ventricular pacing on left ventricular geometry, function, myocardial oxygen consumption, and efficiency of contraction in conscious dogs

The effects of ventricular pacing on left ventricular (LV) dynamic geometry, function, and myocardial oxygen consumption (MVO2) were measured in 12 conscious dogs using sonomicrometry, micromanometry, ultrasonic flow probes, and oximetry catheters during right atrial (A-) and right ventricular (V-) pacing at 150 beats/min. Systolic function was quantified using slopes (MW) and volume-intercepts (VW) of linear relationships between end-diastolic volume (EDV) and stroke work (SW) for data obtained during vena caval occlusion. V-pacing shifted SW-EDV relationships downward (MW decreased from 97 +/- 21 to 81 +/- 21 Kerg/mL) and to the right (VW increased from 14 +/- 11 to 20 +/- 12 mL) in comparison with A-pacing (P < 0.02). These functional changes correlated with altered contractile geometry manifest as early shortening in the septal free wall relative to anterior-posterior dimension (increased minor axis mid-wall eccentricity at end-diastole and begin-ejection). Steady-state LV power output decreased from 802 +/- 213 mW during A-pacing to 514 +/- 170 mW during V-pacing (P < 0.05), while MVO2 remained relatively unchanged during V-pacing (10 +/- 3 mL O2/min vs 11 +/- 3 mL O2/min during A-pacing, P = NS). As a result, overall LV efficiency decreased from 0.24 +/- 0.08 during A-pacing to 0.16 +/- 0.06 during V-pacing (P < 0.05). These data illustrate the impact of V-pacing on dynamic LV geometry and function, including impaired LV work output at all physiological levels of preload. Most importantly, the relationship between LV work output and MVO2 is depressed during V-pacing, emphasizing the interaction between LV mechanics and pump efficiency in intact subjects. As a result, measures taken to restore normal contractile geometry might improve LV efficiency and performance when V-pacing is necessary.

Hemodynamic benefits of right ventricular outflow tract pacing: comparison with right ventricular apex pacing

To assess optimal hemodynamics in relation to stimulation site during right ventricular pacing, 17 consecutive patients who underwent cardiac catheterization were studied. In all patients, right ventricular apex and right ventricular outflow tract stimulation was performed at 85, 100, and 120 beats/min. Cardiac index at both pacing sites was compared using the left ventricular outflow tract continuous wave Doppler technique. Comparison of the two stimulation sites demonstrated that right ventricular outflow tract pacing resulted in a higher cardiac index at 85 beats/min (2.42 +/- 1.2 vs 2.04 +/- 1.0 L/min per m2, P < 0.002) at 100 beats/min (2.78 +/- 1.4 vs 2.35 +/- 1.1 L/min per m2, P < 0.001) and 120 beats/min (3.00 +/- 1.5 vs 2.61 +/- 0.9 L/min per m2, P < 0.001). From a total of 51 paired observations, 45 showed an increase in cardiac index during outflow tract pacing as compared to apex pacing. Right ventricular outflow tract pacing at 120 beats/min resulted in a lower cardiac index than right ventricular apex pacing in patients with significant coronary artery disease and/or impaired left ventricular function (ejection fraction < or = 50%), whereas right ventricular outflow tract pacing produced higher cardiac indices in the absence of these abnormalities. Right ventricular outflow tract pacing resulted in higher cardiac indices as compared to apex pacing in all other subgroups at all other pacing sites tested. It is concluded that stimulation of the right ventricular outflow tract offers a significant hemodynamic benefit during single chamber pacing as compared to conventional apex pacing, particularly in the absence of significant coronary artery disease and/or left ventricular dysfunction.

Hemodynamic importance of preserving the normal sequence of ventricular activation in permanent cardiac pacing

Pacing the right ventricle in the apex profoundly modifies the sequence of activation and thus the sequence of contraction and relaxation of the left ventricle. To evaluate the relative importance of preserving normal ventricular activation sequence and optimal atrioventricular (AV) synchrony in permanent pacing, we compared the effects of three pacing modes: AAI, preserving both normal AV synchrony and normal activation sequence; DDD, with complete ventricular capture that preserves only AV synchrony; and VVI, disrupting both, at rest and during exercise. Hemodynamic and radionuclide studies were performed in 11 patients who had normal intrinsic conduction and who were implanted on a long-term basis with a DDDR pacemaker for isolated sinus node dysfunction. AAI versus DDD and VVI significantly increased cardiac output at rest (6.6 +/- 1.3 L/min vs 6 +/- 0.9 L/min vs 5 +/- 1 L/min; p < 0.01) and during exercise (13.5 +/- 2 L/min vs 12.1 +/- 2.2 L/min vs 14.4 +/- 2.1 L/min; p < 0.01). Pulmonary capillary wedge pressure was lowest with AAI (15.4 +/- 4.5 mm Hg), with an average reduction of 17% compared with DDD (19.6 +/- 5 mm Hg; p < 0.01) and of 30% compared with VVI (25.8 +/- 7 mm Hg; p < 0.01) during exercise. Identical benefits were observed for all other hemodynamic parameters: right atrial pressure, pulmonary artery pressure, left ventricular (LV) stroke work index, and systemic vascular resistances. LV ejection fraction was significantly higher in AAI than in DDD at rest (61% vs 58%, respectively; p < 0.05) and during exercise (65% vs 60%, respectively; p < 0.05). This improvement in LV systolic function resulted principally from the increase in septal ejection fraction. LV filling also was improved in AAI as demonstrated by a significant increase in peak filling rate at rest and during exercise. These data show the importance of preserving, whenever possible, not only normal AV synchrony but also normal ventricular activation sequence in permanent cardiac pacing.

Pacing for sinus node disease: a therapeutic rationale

Symptomatic sinus node disease is a common indication for permanent pacemaker implantation. Single-chamber ventricular (VVI) pacing, single-chamber atrial (AAI) stimulation, and dual-chamber (DDD or DDI) systems are used to a varying extent at different implanting centers. Hemodynamic and clinical studies relevant to the choice of pacing mode in these patients are reviewed. The data currently available strongly support the use of pacing systems providing atrial stimulation. The choice between single-chamber atrial or dual-chamber pacing can be based on the relative importance assigned to a number of factors: Hemodynamic aspects, the risk of ventricular lead problems, cost, and complexity aspects favor AAI pacing, whereas patients with a substantial risk of developing atrioventricular block should receive a DDD or DDI unit.

Measurements of cardiac output by impedance cardiography in pacemaker patients at rest: effects of various atrioventricular delays

OBJECTIVES

The purpose of this study was to evaluate the ability of impedance cardiography to determine the change in cardiac output caused by modifications in the atrioventricular (AV) delay in DDD (dual-chamber) pacing mode while pacing the atrium and ventricle at different programmed rates.

BACKGROUND

Impedance cardiography permits continuous noninvasive monitoring of hemodynamic variables on a beat to beat basis.

METHODS

Eleven patients with a DDD pacemaker were evaluated by impedance cardiography. Stroke volume, cardiac output and total peripheral resistance were assessed in the supine rest position during both DDD and ventricular (VVI) pacing. Hemodynamic variables were measured during DDD pacing at rates ranging from 60 to 110 beats/min in 10-beats/min increments with programmed AV delay varying from 50 to 250 ms in 50-ms increments. When the pacemaker was reprogrammed to the VVI pacing mode, these measurements were repeated at the same pacing rates.

RESULTS

Cardiac output measurements during programmed conditions were found to be highly reproducible. The mean coefficient of variation was 3% during DDD pacing; it was 6% in the VVI pacing mode. A large decrease in cardiac output (approximately 30%) was found when a pacemaker was reprogrammed from the DDD to the VVI pacing mode. At DDD pacing rates between 70 to 110 beats/min, the highest cardiac output occurred at an average AV delay of < 120 ms from atrial stimulus to ventricular stimulus. At an average AV delay of > or = 200 ms, the cardiac output in the DDD and VVI pacing modes was similar.

CONCLUSIONS

1) Impedance cardiography allows highly reproducible noninvasive assessments of cardiac output in pacemaker patients; 2) inappropriate programming of the AV interval in patients with atrial and ventricular pacing can decrease cardiac output significantly, and the extent of the decrease is similar to or less than that observed in ventricular pacing; 3) hemodynamic measurements obtained with impedance cardiography can facilitate optimal programming of pacemaker variables.

Electrocardiogram of the humpback whale (Megaptera novaeangliae), with specific reference to atrioventricular transmission and ventricular excitation

OBJECTIVES

The objective of the study was to record the electrocardiogram (ECG) of a large whale to obtain crucial data for comparative electrophysiologic analysis.

BACKGROUND

The data were needed to establish the mismatch between heart size and PR interval and QRS duration in mammals.

METHODS

In the waters off the coast of Newfoundland, in two humpback whales (Megaptera novaeangliae) with an estimated weight of 30,000 kg a 1-lead ECG was recorded, enabling reliable assessment of P waves and QRS complexes.

RESULTS

It was found that both the PR interval (atrioventricular [AV] transmission time) and QRS duration (ventricular excitation) are extremely short for animals of this size. These findings are difficult, if not impossible, to explain on the basis of currently accepted electrophysiologic theories. However, the narrow QRS complex may be due to a very dense His-Purkinje network in the ventricular wall of whales. Alternative mechanisms that can explain the function of the mammalian AV node need to be considered and explored.

CONCLUSIONS

The results of the study may be of value for the understanding of the ECG in humans.

Effects of increasing heart rate induced by efferent sympathetic neuronal stimulation, isoproterenol or cardiac pacing on myocardial function and oxygen utilization

The effects of increasing heart rate by six different methods on cardiac function were investigated in 17 open-chest anesthetized dogs. Heart rate was increased approximately 30% by (1) right interganglionic nerve stimulation, (2) atrial pacing, (3) ventricular pacing, (4) atrioventricular sequential pacing, (5) right stellate ganglion stimulation, and (6) isoproterenol administration. During heart rate increases induced by atrial pacing left ventricular intramyocardial pressure, coronary blood flow, oxygen delivery per unit of myocardial oxygen consumption, and myocardial efficiency were unchanged. Ventricular pacing reduced left ventricular cavity and septal intramyocardial pressure, while circumflex coronary flow increased, resulting in reduced oxygen delivery relative to myocardial oxygen consumption. Similarly, atrioventricular sequential pacing increased circumflex coronary artery flow and myocardial oxygen consumption, and decreased septal intramyocardial pressure and oxygen delivery per unit of myocardial oxygen consumption. Right stellate ganglion stimulation and isoproterenol increased left anterior descending and circumflex coronary artery blood flow, intramyocardial pressure, and myocardial oxygen consumption. Estimated myocardial efficiency (left ventricle) was decreased by ventricular pacing and isoproterenol, and was unchanged by atrial pacing and right interganglionic nerve stimulation. Increases in heart rate induced by right interganglionic nerve stimulation did not alter myocardial oxygen consumption, or the index of cardiac efficiency. It is concluded that augmentation of heart rate by either ventricular or atrioventricular pacing impairs myocardial function so that there is a decrease of left ventricular efficiency and isoproterenol augments chronotropism and myocardial force relative to cardiac external work so there is a reduction in cardiac efficiency. In contrast, atrial pacing or right interganglionic nerve stimulation augments chronotropism such that myocardial oxygen consumption and efficiency are unchanged.

Contribution of atrioventricular synchrony to left ventricular systolic function in a closed-chest canine model of complete heart block: implications for single-chamber rate-variable cardiac pacing

This study assessed the impact of atrioventricular (AV) synchrony on characteristics of left ventricular (LV) systolic function during ventricular pacing over a wide heart rate range in a conscious closed-chest canine model of complete AV block. Ten healthy adult dogs underwent thoracotomy during which complete AV block was created by formaldehyde injection, and paired ultrasonic sonomicrometers were positioned on the LV anterior-posterior minor axis. Following recovery from surgery, peak and end-diastolic LV transmural pressure, maximum dP/dt, stroke work, end-diastolic minor axis dimension, and maximum velocity of shortening, were quantitated at heart rates of 80, 100, 120, 140, and 160 beats per minute (bpm) during both ventricular pacing alone and AV sequential pacing with increasing AV intervals (0, 50, 100, 150, 200, 250, and 300 ms). Over the heart rate range tested, parameters of LV systolic function did not differ significantly during ventricular pacing with or without AV synchrony. For example, during ventricular pacing alone maximum LV dP/dt varied from 2110 +/- 70 mmHg/s to 2463 +/- 567 mmHg/s, a range essentially identical to that observed in the presence of AV synchrony. On the other hand, although the impact on LV performance of varying AV interval from 0 to 300 ms was small, differences tended to become more pronounced at higher pacing rates. At 80 bpm, neither stroke work nor maximum LV dP/dt were affected by change in AV interval, while at heart rates greater than or equal to 120 bpm both stroke work and LV dP/dt tended to maximize at AV intervals of 50 and 100 ms and thereafter declined.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation by pulsed Doppler echocardiography of the atrial contribution to left ventricular filling in patients with DDD pacemakers

To evaluate the significance of the left atrial (LA) contribution to left ventricular (LV) filling in cardiac pacing, LV inflow velocity was recorded with pulsed Doppler echocardiography in 20 patients with a DDD pacemaker. The pacemaker was programmed to atrioventricular (AV) sequential pacing with AV intervals of 50, 100, 150, 200 and 250 ms, and then to VVI pacing at a fixed rate of 70 beats/min. To evaluate the relative changes of LV filling volume in individual patients, the percent change in time-velocity integral of LV inflow velocity in each pacing mode was calculated as the ratio to that of AV sequential pacing with an AV interval of 150 ms. To estimate the degree of LA contribution to LV filling, the ratio of time-velocity integral during LA ejection phase to that during total LV filling phase was measured at the optimal AV interval. The percent LV inflow volume in AV sequential pacing was 74% for an AV interval of 50 ms, 87% for 100 ms, 98% for 200 ms and 90% for 250 ms. The percent LV inflow volume in VVI pacing was 72%. The percent LV inflow volume at AV intervals of 150 ms was significantly greater than that at an AV interval of 50, 100 and 250 ms, and in VVI pacing (p less than 0.05). The degree of LA contribution to LV filling showed a positive correlation with the percent increase of LV inflow volume with mode conversion from VVI to AV sequential pacing (p less than 0.005) and also with age (p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamic changes in left ventricular regional wall thickness during premature ventricular contraction in conscious dogs

The contractile pattern of the regional left ventricular wall during premature ventricular contraction was analyzed in conscious dogs instrumented with an ultrasonic dimension gauge across the anterior and posterior left ventricular walls. Aortic flow was measured with an electromagnetic flow probe. A single premature ventricular contraction was induced by stimulating either the anterior or posterior wall with varied coupling intervals from 380 to 650 msec. Stroke volume of premature ventricular contraction was significantly smaller than that of premature atrial contraction with identical coupling intervals. In premature contractions, stroke volume was linearly related to coupling intervals. Though there was no isovolumic wall thickening in premature atrial contraction, the wall started to thicken during isovolumic ventricular systole in premature ventricular contraction. There was a clear inverse correlation between the ratio of the isovolumic wall thickening to the total wall thickening and coupling intervals. In premature ventricular contractions with identical coupling intervals, the deformation of thickening characteristics was more pronounced in regions with closer proximity to the ectopic focus. Thus it is concluded that the pump function is depressed in premature ventricular contraction, in part due to the increased ratio of wall thickening during isovolumic systole before the opening of the aortic valve. Isovolumic wall thickening increases along with the shorter coupling intervals and closer proximity to the ectopic focus. These alterations in left ventricular mechanical function due to ectopic contraction might induce serious sequelae, depending upon the ectopic focus in the presence of already depressed regional function.

Comparative aspects of the dual role of the human atrioventricular node

As well as transmitting the impulse from the atria to the ventricles the atrioventricular node has two other important functions namely: synchronisation of atrial and ventricular contractions by a varying delay; and protection of the ventricles from rapid atrial arrhythmias. The relative importance of these two functions appears to differ in large and small mammalian hearts. In small mammals synchronisation of atrial and ventricular contractions is the major function of the atrioventricular node, whereas protection from rapid atrial arrhythmias may be its most important function in large mammals. Atrioventricular conduction time in sinus rhythm is ten times longer in the whale (500 ms) than in the rat (50 ms). A whale heart, however, is about 100 000 times heavier than a rat heart. During atrial fibrillation the ventricular rate in a dog heart is only three times faster than in a horse, whereas a horse heart may be 40 times as heavy as that of a dog. Hence there is a considerable discrepancy between the size of the mammalian heart and the functions of its atrioventricular node. Analysis of several anatomical and functional aspects of atrioventricular conduction systems in mammals of all sizes showed that the importance of the delaying role of the atrioventricular conduction system diminishes as the size of the mammal increases, whereas the protective role of the atrioventricular node probably increases. The function of the human atrioventricular node seems to be intermediate between that of of the small and large mammals.

Effect of alteration of left ventricular activation sequence on the left ventricular end-systolic pressure-volume relation in closed-chest dogs

We investigated the effect of pacing from the atrium and various ventricular sites on the left ventricular end-systolic pressure-volume relation following autonomic blockade in a total of 10 dogs chronically instrumented to measure left ventricular pressure and determine left ventricular volume from three ultrasonic endocardial dimensions. During ventricular pacing, left ventricular end-diastolic volume, stroke volume, and end-systolic pressure were decreased, while the end-systolic volume was relatively unchanged. Left ventricular end-systolic pressure-volume relations were generated by vena caval occlusions during pacing at a constant rate from the left atria, and the epicardium of the right ventricular free wall, right ventricular apex, and left ventricular free wall. The left ventricular end-systolic pressure-volume relations were described by straight lines for each site (r greater than 0.96 and SEE less than 2.9 mm Hg in all but one instance). Compared to atrial pacing, the left ventricular end-systolic pressure-volume relations were shifted (P less than 0.001) to the right during pacing from ventricular sites. During atrial pacing, the volume intercept of the left ventricular end-systolic pressure-volume relation was 16.0 +/- 7.2 ml (mean +/- SD), and increased to 18.7 +/- 7.8 ml (P less than 0.05) during pacing from the right ventricular free wall, to 19.6 +/- 7.7 ml (P less than 0.05) during pacing from the right ventricular apex, and to 20.0 +/- 7.5 ml (P less than 0.05) during pacing from the left ventricular free wall. These volume intercepts correlated roughly with the extent of dyssynchronous activation as estimated by the QRS duration (r = 0.59 to 0.93) and the time for left ventricular endocardial activation (r = 0.92 and 0.95). During ventricular pacing, the slope of the left ventricular end-systolic pressure-volume relation changed only slightly. Similar results were obtained during pacing from right ventricular endocardial sites. We conclude that alterations of the normal activation sequence produced by ventricular pacing depress left ventricular pumping function independent of loading conditions, as indicated by a rightward shift of the left ventricular end-systolic pressure-volume relation. The extent of this shift appears to be in proportion to the degree of dyssynchronous activation. The decreased stroke volume during ventricular pacing is due both to a decreased end-diastolic volume (decreased preload) and the rightward shift of the end-systolic pressure-volume relation (decreased pump function).

Role of atrial fibrillation and atrioventricular conduction (including Wolff-Parkinson-White syndrome) in sudden death

A short refractory period of the accessory pathway is considered a major threat for sudden death in patients with Wolff-Parkinson-White syndrome and atrial fibrillation. RR interval and QRS signal analysis together with signal analysis of a bipolar high right atrial electrogram were obtained in six patients with Wolff-Parkinson-White syndrome and either induced or spontaneous atrial fibrillation. A record of a sufficiently long episode of atrioventricular (AV) conduction by way of the bypass tract that could be used for satisfactory RR interval sequence and QRS analysis was obtained from only one patient. The results were compared with those of a representative patient with atrial fibrillation and normal AV nodal-His conduction. In a patient with Wolff-Parkinson-White syndrome, atrial fibrillation and AV conduction by way of the bypass tract may exhibit high ventricular rates (median RR intervals of about 300 ms) and long/short RR interval ratios of just over 1 (RR intervals not exceeding 400 ms). The right atrial electrogram showed a noiselike excitation pattern. This study suggests that rather than a short refractory period of the bypass tract, it is lack of concealed conduction, responsible for the presence of long RR intervals, that allows the ventricles to reach very high ventricular rates and at times to fibrillate. The normal AV nodal-His system seems to protect the heart against high ventricular rates and ventricular fibrillation during atrial fibrillation by its relatively long refractory period and capacity to induce long RR intervals by means of concealed conduction.

Effects of altered site of electrical activation on myocardial performance during inotropic stimulation

The effects of altering the site of electrical activation on responses to isoproterenol (ISO) and treadmill exercise were examined in mongrel dogs instrumented for long-term measurement of left ventricular pressure, left ventricular dP/dt, coronary blood flow, cardiac output, left ventricular diameters, and mean arterial pressure and O2 content in the coronary sinus and aorta. During spontaneous rhythm, 0.2 micrograms/kg/min ISO increased heart rate by 90 +/- 7 beats/min, left ventricular dP/dt by 2479 +/- 301 mm Hg/sec, cardiac output by 3.5 +/- 0.9 liters/min, coronary blood flow by 30.4 +/- 3.9 ml/min, and myocardial oxygen consumption (MVO2) by 3.91 +/- 0.84 ml/min. During right atrial pacing at 193 +/- 7 beats/min, the effects of ISO were not different from the effects during spontaneous rhythm, with the exception of a lesser increase in coronary blood flow and lesser reductions in coronary resistance and left ventricular end-diastolic diameter and pressure. During right ventricular pacing at an identical rate, ISO increased left ventricular dP/dt (1140 +/- 158 mm Hg/sec) and cardiac output (2.2 +/- 0.5 liters/min) significantly less (p less than .025) than during either sinus rhythm or right atrial pacing, while MVO2 rose to a higher value. During right ventricular pacing the changes in mean arterial pressure and left ventricular end-diastolic diameters with ISO were not significantly different from those during right atrial pacing. Treadmill exercise induced significantly smaller (p less than .025) increases in left ventricular dP/dt during right ventricular pacing as compared with during either right atrial pacing or sinus rhythm, while MVO2 rose to a higher value.(ABSTRACT TRUNCATED AT 250 WORDS)

The control of atrial contraction by ventriculo-atrial pacing in the dog with AV block

Atrial contraction in dogs with atrioventricular (AV) block was controlled by multiple atrial stimuli delivered during ventricular diastole. Acute hemodynamic changes were assessed. At a ventricular rate of 60 bpm, the spontaneous atrial rate averaged 83, and atrial cannon waves were frequent. When the atria were given two stimuli at an interval of 500 ms during ventricular diastole, the cannon waves were eliminated completely, and a fall in mean central venous pressure and a rise in systemic blood pressure were found. At a pacing rate of 90 bpm, similar changes in the cannon waves, mean central venous pressure, and systemic blood pressure were found when two atrial stimuli followed ventricular stimulation. Ventriculo-atrial multiple pacing may be useful in both clinical and experimental AV block.

Effect of electrical activation site on left ventricular performance in ventricular tachycardia patients with coronary heart disease

Some patients with ventricular tachycardia (VT) remain virtually asymptomatic, whereas others have syncope despite similar VT rates. The role of ventricular activation site on left ventricular (LV) function was examined in 10 patients undergoing electrophysiologic evaluation for recurrent ventricular arrhythmias. Radionuclide ventriculograms were acquired to calculate LV volumes and aortic or LV pressure was measured during right atrial and right ventricular (RV) pacing. RV apical pacing resulted in end-diastolic volumes similar to those achieved with right atrial pacing (171 +/- 27 vs 175 +/- 19 ml), whereas RV outflow tract pacing showed a trend toward lower volumes (168 +/- 32 vs 177 +/- 33 ml). Comparison between RV apical and RV outflow tract pacing showed that apical activation resulted in higher end-diastolic volumes (216 +/- 26 vs 194 +/- 22 ml, p = 0.020), end-systolic volumes (175 +/- 25 vs 158 +/- 20 ml, p = 0.041), stroke volumes (42 +/- 4 vs 36 +/- 6 ml, p = 0.046), peak rates of LV ejection (309 +/- 57 vs 245 +/- 40 ml/s, p = 0.034) and peak rates normalized for differences in end-diastolic volume (1.5 +/- 0.3 vs 1.3 +/- 0.3; p = 0.047) without a significant increase in peak pressures (131 +/- 12 vs 127 +/- 14 mm Hg, p greater than 0.30) or ejection fractions (24 +/- 5 vs 22 +/- 4%, p = 0.187). These changes were accompanied by an increase in LV/RV stroke count ratios during RV apical vs RV outflow tract pacing (1.6 +/- 0.2 vs 1.2 +/- 0.2, p = 0.030), suggesting the development of mitral regurgitation.(ABSTRACT TRUNCATED AT 250 WORDS)

Pacemaker-mediated tachycardias: an unresolved problem

The advent of dual chamber pacing systems that restore atrioventricular synchrony and allow rate variability in response to physiologic stress has provided a major advance in pacing therapy. Undesirable pacemaker-mediated tachycardias may arise when the atrial sensing amplifier of a VDD or DDD device detects a retrogradely conducted P-wave, ectopic atrial tachycardia, myopectoral contractions, or environmental electromagnetic interference. The rate of the resultant tachycardia is determined by the programmed upper tracking rate of the pacemaker. Although seldom life-threatening, troublesome myocardial ischemia may ensue in patients with coronary artery disease. In most instances, the incidence and rate of pacemaker-mediated tachycardias may be minimized by judicious programming.

Studies on left ventricular function during sustained ventricular tachycardia

The acute effects of rapid ventricular pacing and sustained ventricular tachycardia on left ventricular function were examined in patients with recurrent sustained ventricular tachycardia. Programmed electrical stimulation and left ventricular hemodynamic measurements were performed in 20 patients (19 men and 1 woman), with an age range of 49 to 79 years (mean 63 +/- 9). Indexes of left ventricular function that were analyzed included left ventricular peak systolic pressure, end-diastolic pressure, first derivative of peak left ventricular pressure (dP/dt) and negative left ventricular dP/dt. Measurements were obtained during sinus rhythm, after paced premature ventricular depolarizations, during rapid ventricular pacing (cycle lengths 600 to 250 ms) and immediately after induction of sustained ventricular tachycardia. Mean left ventricular peak systolic blood pressure was 123 +/- 19 mm Hg during sinus rhythm, decreased to 77 +/- 23 mm Hg (p less than 0.05) at the induction of ventricular tachycardia and remained decreased during arrhythmia (p less than 0.01). Mean left ventricular end-diastolic pressure was 22 +/- 5 mm Hg during sinus rhythm, did not change after arrhythmia induction (22 +/- 9 mm Hg, p greater than 0.2) and remained unchanged during sustained ventricular tachycardia (p greater than 0.2). Mean peak left ventricular dP/dt was 1,400 +/- 620 mm Hg/s in sinus rhythm, decreased to 810 +/- 580 mm Hg/s (p less than 0.05) at ventricular tachycardia induction and remained decreased during sustained ventricular tachycardia (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Two-dimensional echocardiographic contrast assessment of pacing-induced mitral regurgitation: relation to altered regional left ventricular function

Two-dimensional echocardiography during agitated saline contrast injections into the left ventricle was applied in eight closed chest dogs to examine the degree of mitral valve regurgitation encountered with pacing from two sites: 1) at the right ventricular apex and 2) within the coronary sinus at the base of the left ventricle. Pacing was at a rate of 10 beats/min above the sinus rate, and ranged from 60 to 120 beats/min. Hemodynamic variables were monitored, and data on global and regional left ventricular function were derived from a series of short- and long-axis cross-sectional echographic images. The degree of valvular regurgitation was assessed independently by two observers, and systolic appearance of echo contrast in the left atrium was graded as 0 to +4. Although no mitral regurgitation was noted in sinus rhythm, regurgitation was severe with right ventricular apical pacing (3.2 +/- 0.7, mean +/- standard deviation) and relatively mild (0.9 +/- 0.7) with basal pacing (p less than 0.01 and 0.05, respectively). Relative to sinus rhythm, thermodilution stroke volume was significantly (p less than 0.05) depressed by both apical and basal pacing (from 32.6 +/- 14.6 to 25.0 +/- 7.9 and 26.0 +/- 7.6 cc, respectively), but there was no significant difference between the two pacing sites. Mapping of regional function at six levels of the left ventricle revealed significant heterogeneities, with maximal dysfunction noted in the vicinity of the pacing site. It is concluded that significant differences in mitral regurgitation exist depending on the site of pacing, with apical pacing causing severe regurgitation and abnormal regional contraction near the pacing site.

Two-dimensional echocardiographic studies during sustained ventricular tachycardia

We evaluated left ventricular function in patients with recurrent sustained ventricular tachycardia (VT) using two-dimensional echocardiography (2DE). Thirteen patients, 11 men and 2 women, age range 42-77 (mean 62 +/- 12) years were studied in sinus rhythm (SR) and immediately after VT induction. 2DE parameters analyzed included wall motion, mitral valve leaflet motion, and ejection fraction (EF). In SR, 21 segments/walls in 12 patients showed wall motion abnormalities (WMA) ranging from hypokinesis to dyskinesis and one patient had generalized LV hypokinesis. In VT, new WMA were noted in 2 patients. Thirteen segments/walls in 8 patients showed further worsening of pre-existing WMA. In 1 patient there was worsening of generalized LV hypokinesis. Three patients showed apparent improvement in pre-existing WMA during VT. In 2 patients large apical aneurysms showed a reduction of dyskinesis in VT. Mitral valve opening was intermittent in patients with shorter VT cycle lengths and was maximal when atrial systole preceded or coincided with ventricular depolarization. Doppler echocardiography in 1 patient confirmed the pattern of intermittent mitral flow, with greatest flow occurring when mitral valve opening occurred well before the QRS peak. In 5 patients, 2DE permitted EF measurements. EF in SR ranged from 24-56% (mean 36 +/- 13), decreased to 6-33% (mean 21 +/- 11) within the first ten beats of VT and 6-25% (mean 19 +/- 8) after twenty beats of VT. EF decreased more in patients with shorter VT cycles as compared to those with longer VT cycle lengths.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of left ventricular performance by left bundle branch block simulated with atrioventricular sequential pacing

The effects of atrioventricular (AV) sequential pacing-induced left bundle branch block (LBBB) on left ventricular (LV) performance were evaluated during cardiac catheterization in 9 randomly selected patients being investigated for chest pain. All patients were in normal sinus rhythm with a normal P-R interval and QRS duration. LV performance was assessed by both hemodynamic and angiographic measurements. The maximal rate of LV pressure increase (dP/dt), rate of maximal LV pressure decrease (-dP/dt), LV end-diastolic pressure (LVEDP), end-diastolic volume (LVEDV), end-systolic volume (LVESV), stroke volume and percent ejection (EF) were measured during right atrial and AV sequential pacing at a constant pacing rate. The average pacing rate was 97 +/- 3 beats/min (mean +/- standard error of the mean). In each patient, both dP/dt and -dP/dt decreased significantly (p less than 0.001) during AV sequential pacing compared with atrial pacing at the same rate, from 1,541 +/- 68 to 1,319 +/- 56 mm Hg/s for dP/dt and from 1,506 +/- 86 to 1,276 +/- 92 for -dP/dt. LVEDP did not change significantly when atrial (17 +/- 3 mm Hg) and AV sequential pacing (16 +/- 2 mm Hg) were compared. Mean LVEDV did not change during atrial (135 +/- 13 ml) or AV sequential pacing (137 +/- 14 ml). In contrast, the LVESV during AV sequential pacing was higher by 15 ml (23%) (from 48 +/- 10 to 63 +/- 12 ml) (p less than 0.001); as a result, the stroke volume was lower by 13 ml (15%) and the EF decreased by 10%, from 66 to 56% (-15%).(ABSTRACT TRUNCATED AT 250 WORDS)

Non-invasive evaluation of ventricular function and volumes during atrioventricular sequential and ventricular pacing

Thirteen patients who all had previously inserted temporary or permanent pacemakers (6, VVI; 7, A-V sequential) were studied by two-dimensional echocardiography and radionuclide gated blood pool ventriculography (RVG) for non-invasive evaluation of cardiac performance. Patients were paced in both the VVI mode and during sinus/atrial or A-V sequential pacing. Although there was no objective change of the ejection fraction during V-pacing and atrial/A-V sequential pacing or sinus rhythm, as has been previously reported, A-V sequential pacing did result in significant improvement in overall cardiac function and output as judged by radionuclide ventriculography and blood pressure response in most of our patients. An appropriately timed atrial contribution to ventricular systole resulted in improved ventricular function in those individuals with pre-existing systolic or diastolic myocardial dysfunction and/or sick sinus syndrome in whom pacemaker therapy was indicated. Radionuclide ventriculography appears to be a reliable, accurate, non-invasive method that can be used to evaluate patients before implantation of a permanent ventricular or atrioventricular pacemaker in order to decide which pacing mode is best for that particular individual.

Effect of ventricular pacing on left ventricular function assessed by radionuclide angiography

Radionuclide angiography was used to evaluate left ventricular contraction on and off ventricular pacing in 35 patients. Twenty patients had permanent rate-adjustable ventricular demand pacemakers whose rate could be programmed to below an underlying sinus rhythm. In these patients there was no difference in left ventricular ejection fraction on and off pacing whether or not heart failure was present (n = 7) or whether or not ventriculoatrial conduction (n = 10) developed during pacing. Eight of the 20 patients also underwent supine bicycle exercise, and rest and exercise ejection fraction values were not significantly different at a similar heart rate and workload when on and off pacing were compared. During pacing, there appeared to be abnormal motion in septal, apical, and inferior walls. In addition, dysynchrony developed, consisting of early right ventricular and anterobasal and posterobasal left ventricular motion with subsequent contraction of the apex. Conversion to atrioventricular pacing produced a clinical improvement in all seven patients with heart failure and reduced ejection fraction and also in one of the remaining 13 patients without heart failure. Fifteen other patients were studied early after cardiac surgery. Temporary ventricular epicardial pacing resulted in a significant increase in right and left atrial pressure and a significant reduction in stroke volume, end-diastolic volume, and end-systolic volume with no significant change in ejection fraction. Conversion from sinus rhythm to ventricular pacing produces a deterioration in cardiac performance and severe regional left ventricular wall motion abnormalities, but no significant change in ejection fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual AV nodal pathways--vagal influences on AV nodal conduction

This 52-year-old male presented with syncope and demonstrated two distinct PR intervals on the electrocardiogram. Electrophysiologic studies showed dual AV nodal pathways. Right-sided carotid sinus massage induced prolonged periods of sinus arrest with no change in AH interval. Left-sided carotid sinus massage produced long AH intervals (slow pathway conduction) with some slowing of sinus rate. Whenever sinus rhythm with slow pathway conduction was observed (long AH) a 20-30 mmHg drop systolic pressure was seen. Following implantation of an AV sequential pacemaker, the patient has been asymptomatic.

Endocardial pacing site affects left ventricular end-diastolic volume and performance in the intact anesthetized dog

We investigated how pacing from different endocardial sites affects the left ventricular three-dimensional contraction pattern and performance in intact anesthetized dogs. We used data from the motion of radiopaque markers implanted in the left ventricular endocardium in an analysis based on the polar decomposition theorem to determine the left ventricle's three-dimensional principal directions and magnitudes of deformation, and its axis and angle of rotation during the cardiac cycle. This paper also derives a new procedure that permits statistical comparison of different left ventricular cavity deformation patterns. During normal sinus rhythm and pacing from the right atrium, left ventricular septum, left ventricular apex, and right ventricular apex, the principal directions of left ventricular deformation remained relatively fixed with respect to the left ventricle's anatomy, independent of heart rate and pacing site. These directions were oriented in septum-free wall, anterior-posterior, and apex-base directions. End-systolic pressure and volume did not vary significantly among pacing sites. End-diastolic volume varied significantly among pacing sites, with right ventricular apical pacing producing the smallest end-diastolic and stroke volume. These results reveal that beats produced by right ventricular apical pacing eject less blood compared with beats produced by right atrial, left ventricular septal, or left ventricular apical pacing.

Influence of atrial systole on the Frank-Starling relation and the end-diastolic pressure-diameter relation of the left ventricle

The influence of atrial systole on the left ventricular function curve (stroke volume vs end-diastolic pressure or end-diastolic diameter) and on the left ventricular end-diastolic pressure diameter relation was studied in nine anesthetized, open-chest dogs whose atrioventricular (AV) node had been completely blocked. Measurements were made during volume loading with the pericardium closed and opened and during alternate AV sequential pacing (to permit atrial contribution to ventricular filling) and AV simultaneous pacing (to prevent atrial contribution). When the pericardium was closed, withdrawal of the atrial contribution shifted the stroke-volume end-diastolic pressure relation downward, but did not shift the stroke volume-end-diastolic diameter relation, i.e., it reduced stroke volume for a given end-diastolic pressure according to a reduction in end-diastolic volume. The downward shift of the stroke volume-end-diastolic pressure relation was caused by an upward shift of the end-diastolic pressure-diameter relation, which, for a given end-diastolic pressure, resulted in a smaller end-diastolic diameter and, thus, in a smaller stroke volume. The reason for the upward shift in the end-diastolic pressure-diameter relation was that the atrium remained full and thus increased pericardial pressure by increasing pericardial volume. Opening the pericardium shifted the end-diastolic pressure-diameter relation downward and to the right, shifted the stroke volume-end-diastolic diameter relation upward and abolished the effect of withdrawal of the atrial contribution on these curves. We conclude that in the presence of an intact pericardium, atrial systole shifts the stroke volume-end-diastolic pressure relation because it shifts the end-diastolic pressure-diameter relation and it improves left ventricular performance by increasing preload.

AV pacing and LV performance

Five patients with impaired left ventricular function (LV) and implanted AV sequential pacemakers underwent serial radionuclide angiograms. The goal was a non-invasive evaluation of the rapid changes in left ventricular performance elicited by rate, pacing mode and AV interval manipulation. End diastolic volume, end systolic volume, stroke volume and cardiac output were increased by AV sequential pacing in comparison with ventricular pacing at 70 beats per minute. No significant change in ejection fraction and blood pressure were noted with changing AV sequential pacing rates at usual pacing rates. Our data suggest that a short AV interval (150 ms) improved LV performance more than a long AV interval (250 ms). A non-invasive technique to optimize left ventricular performance on an acute basis by varying heart rate, AV interval and pacing mode with the implanted AV sequential pacemaker is feasible and may be useful in selective clinical situations.