Load dependence of cardiac output in biventricular pacing: left ventricular volume overload in pigs

[Resynchronization Therapy for Chronic Heart Failure: Diagnostic and Therapeutic Approaches]

Chronic heart failure (CHF) remains one of the most important problems of modern cardiology. One of the effective treatment methods is resynchronization therapy (RT). The article presents an analysis of literature data on the effectiveness of RT in improving the quality of life, reducing the number of hospitalizations and mortality in patients with heart failure with severe left ventricular systolic dysfunction and expanding QRS complex, and also discusses key methods for optimizing RT.

Wall Thickness, Pulmonary Hypertension, and Diastolic Filling Abnormalities Predict Response to Postoperative Biventricular Pacing

OBJECTIVE

Post-cardiopulmonary bypass biventricular pacing improves hemodynamics but without clearly defined predictors of response. Based on preclinical studies and prior observations, it was suspected that diastolic dysfunction or pulmonary hypertension is predictive of hemodynamic benefit.

DESIGN

Randomized controlled study of temporary biventricular pacing after cardiopulmonary bypass.

SETTING

Single-center study at university-affiliated tertiary care hospital.

INTERVENTIONS

Patients who underwent bypass with preoperative ejection fraction ≤40% and QRS duration ≥100 ms or double-valve surgery were enrolled. At 3 time points between separation from bypass and postoperative day 1, pacing delays were varied to optimize hemodynamics.

PARTICIPANTS

Data from 43 patients were analyzed.

MEASUREMENTS AND MAIN RESULTS

Cardiac output and arterial pressure were measured under no pacing, atrial pacing, and biventricular pacing. Preoperative echocardiograms and pulmonary artery catheterizations were reviewed, and measures of both systolic and diastolic function were compared to hemodynamic response. Early after separation, improvement in cardiac output was positively correlated with pulmonary vascular resistance (R(2) = 0.97, p<0.001), ventricle wall thickness (R(2) = 0.72, p = 0.002)), and E/e', a measure of abnormal diastolic ventricular filling velocity (R(2) = 0.56, p = 0.04). Similar trends were seen with mean arterial pressure. QRS duration and ejection fraction did not correlate significantly with improvements in hemodynamics.

CONCLUSIONS

There may be an effect of biventricular pacing related to amelioration of abnormal diastolic filling patterns rather than electrical resynchronization in the postoperative state.

Regional and global strain changes during biventricular pacing in a porcine model of acute left ventricular volume overload

OBJECTIVES

Biventricular pacing may ameliorate symptoms of acute heart failure. Speckle-tracking echocardiography can assess cardiac function to elucidate mechanisms of benefit. Accordingly, radial and circumferential strain and radial and circumferential strain synchrony were measured with speckle-tracking echocardiography during biventricular pacing in a model of left ventricular (LV) volume overload.

METHODS

Heart block was established in 4 open-chest anesthetized pigs. Left ventricular volume overload was induced with an ascending aorta-LV apex conduit. Measurements included cardiac output by an aortic flow probe, the maximum derivative of LV pressure versus time (dP/dtmax), and transseptal pressure synchrony. Biventricular pacing was performed for combinations of 3 interventricular delays and 3 LV pacing sites. Speckle-tracking echocardiographic analysis was applied to short-axis images at the midpapillary LV for 9 pacing combinations. Strain and synchrony parameters were correlated with hemodynamics.

RESULTS

Increased cardiac output correlated with improved global circumferential strain (P = .002) but not changes in global radial strain or radial strain synchrony. Increased LV dP/dtmax was associated with improved circumferential strain in the septum (P < .001) and radial strain in the lateral wall (P = .046). Improved transseptal pressure synchrony was associated with improved global circumferential strain, but primarily in the septum (P < .001). Aortic valve closure occurred before peak radial strain in 62% of beats and before peak circumferential strain in 6%.

CONCLUSIONS

During acute LV volume overload, hemodynamic improvement with biventricular pacing was associated with improved circumferential strain primarily in the septum. Radial strain and radial strain synchrony did not correlate with improvement, possibly due to delayed systolic contraction. An increase in circumferential strain in the septum associated with optimum transseptal pressure synchrony suggested improvement by interventricular assist from the right ventricle.

Hemodynamic stability during biventricular pacing after cardiopulmonary bypass

OBJECTIVE

To assess the stability of cardiac output, mean arterial pressure, and systemic vascular resistance during biventricular pacing (BiVP) optimization.

DESIGN

Substudy analysis of data collected as part of a randomized controlled study examining the effects of optimized temporary BiVP after cardiopulmonary bypass (CPB).

SETTING

A single-center study at a university-affiliated tertiary care hospital.

PARTICIPANTS

Cardiac surgery patients at risk of left ventricular failure after CPB.

INTERVENTIONS

BiVP was optimized immediately after CPB. Atrioventricular delay (7 unique settings) was optimized first, followed by the left ventricular pacing site (3 unique settings) and then the interventricular delay (9 unique settings). Each setting was tested twice for 10 seconds each time. Vasoactive medication and fluid infusion rates were held constant.

MEASUREMENTS AND MAIN RESULTS

Aortic flow velocity and radial artery pressure were digitized, recorded, and averaged over single respiratory cycles. Least squares and linear regression/Wilcoxon analyses were applied to the first 7 patients studied. Subsequently, curvilinear analysis was applied to 15 patients. Changes in mean arterial pressure and systemic vascular resistance were statistically insignificant or too small to be meaningful by least squares analysis. During interventricular synchrony optimization, cardiac output and mean arterial pressure decreased (mean changes -5.7% and -2.5%, respectively; with standard errors 2.3% and 1.5%, respectively), whereas SVR increased (mean change 3.1% with standard error 3.4%). Only the change in cardiac output was statistically significant (p = 0.043). Curvilinear fits to data for 15 patients demonstrated progressive hemodynamic stability over the total testing period.

CONCLUSION

BiVP optimization may be done safely in patients after CPB. With continuous monitoring of mean arterial pressure and cardiac output, the procedure results in no harmful hemodynamic perturbation.

Optimized temporary biventricular pacing acutely improves intraoperative cardiac output after weaning from cardiopulmonary bypass: a substudy of a randomized clinical trial

OBJECTIVE

Permanent biventricular pacing benefits patients with heart failure and interventricular conduction delay, but the importance of pacing with and without optimization in patients at risk of low cardiac output after cardiac surgery is unknown. We hypothesized that pacing parameters independently affect cardiac output. Accordingly, we analyzed aortic flow measured with an electromagnetic flowmeter in patients at risk of low cardiac output during an ongoing randomized clinical trial of biventricular pacing (n = 11) versus standard of care (n = 9).

METHODS

A substudy was conducted in all 20 patients in both groups with stable pacing after coronary artery bypass grafting, valve surgery, or both. Ejection fraction averaged 33% ± 15%, and QRS duration was 116 ± 19 ms. Effects were measured within 1 hour of the conclusion of cardiopulmonary bypass. Atrioventricular delay (7 settings) and interventricular delay (9 settings) were optimized in random sequence.

RESULTS

Optimization of atrioventricular delay (171 ± 8 ms) at an interventricular delay of 0 ms increased flow by 14% versus the worst setting (111 ± 11 ms, P < .001) and 7% versus nominal atrioventricular delay (120 ms, P < .001). Interventricular delay optimization increased flow 10% versus the worst setting (P < .001) and 5% versus nominal interventricular delay (0 ms, P < .001). Optimized pacing increased cardiac output 13% versus atrial pacing at matched heart rate (5.5 ± 0.5 vs 4.9 ± 0.6 L/min, P = .003) and 10% versus sinus rhythm (5.0 ± 0.6 L/min, P = .019).

CONCLUSIONS

Temporary biventricular pacing increases intraoperative cardiac output in patients with left ventricular dysfunction undergoing cardiac surgery. Atrioventricular and interventricular delay optimization maximizes this benefit.

Simultaneous variation of ventricular pacing site and timing with biventricular pacing in acute ventricular failure improves function by interventricular assist

The goal of this work was to investigate the hemodynamic effects of simultaneous left ventricular (LV) pacing site (LVPS) and interventricular pacing delay (VVD) variation with biventricular pacing (BiVP) during acute LV failure. Simultaneously varying LVPS and VVD with BiVP has been shown to improve hemodynamics during acute right ventricular (RV) failure. However, effects during acute LV failure have not been reported. In six open-chest pigs, acute LV volume overload was induced by regurgitant flow via an aortic-LV conduit. Epicardial BiVP was implemented with right atrial and ventricular leads and a custom LV pacing array. Fifty-four LVPS-VVD combinations were tested in random order. Cardiac output was evaluated by aortic flow probe, ventricular systolic function by maximum rate of ventricular pressure change, and mechanical interventricular synchrony by normalized RV-LV pressure diagram area. Simultaneous LVPS-VVD variation improved all measures of cardiac function. The observed effect was different for each functional index, with evidence of LVPS-VVD interaction. Compared with effects of LVPS-VVD variation in a model of acute RV failure, hemodynamic changes were markedly different. However, in both models, maximum rate of ventricular pressure change of the failing ventricle was improved with synchronous interventricular contraction, suggesting that, in acute ventricular failure, BiVP can recruit the unstressed ventricle to support systolic function of the failing one. Thus simultaneously varying LVPS and VVD with BiVP during acute ventricular failure can improve cardiac function by "interventricular assist", with hemodynamic effects dependent on the type of failure. This supports the potential utility of temporary BiVP for the treatment of acute ventricular failure commonly seen after cardiac surgery.

Left ventricular pacing site and timing optimization during biventricular pacing using a multielectrode patch in pigs

OBJECTIVES

Biventricular pacing is important therapy for congestive heart failure, reversing left ventricular dysfunction in dilated cardiomyopathy. Although left ventricular lead location and right ventricular-left ventricular delay are believed to be critical in biventricular pacing, there is no established technique for optimizing pacing site and timing.

METHODS

After median sternotomy in 8 anesthetized pigs, an ultrasonic flow probe was placed on the ascending aorta to measure cardiac output, and pressure catheters were inserted into both ventricles. Temporary bipolar epicardial pacing leads were attached to the right atrium and anterior right ventricle. A patch with 5 bipolar electrodes was placed behind the left ventricle. A temporary bipolar lead was also placed on the left ventricular apex. Complete heart block was established by ethanol ablation. Right ventricular pressure overload was induced by snaring the pulmonary artery until right ventricular systolic pressure doubled. Dual-chamber mode biventricular pacing was instituted at 9 right ventricular-left ventricular delays, +80 ms to -80 ms in 20 ms increments, and 6 left ventricular sites. Data from the 54 combinations of these variables were acquired in a randomized fashion. Mixed model technology was used for statistical analysis.

RESULTS

Qualitatively, two unique site/timing pairs were optimal. Statistically, pacing the obtuse margin at a right ventricular-left ventricular delay of +60 ms (mean cardiac output = 1.80 L/min) and the inferolateral wall at a right ventricular-left ventricular delay of -20 ms (mean cardiac output = 1.79 L/min) was superior to all other site/timing combinations (mean cardiac output = 1.71 L/min; P = .006).

CONCLUSIONS

Left ventricular pacing site and right ventricular-left ventricular delay can be optimized with a multielectrode patch and randomized data collection. This technique can be used further in clinical studies.

Effects of sequential biventricular pacing during acute right ventricular pressure overload

Temporary sequential biventricular pacing (BiVP) is a promising treatment for postoperative cardiac dysfunction, but the mechanism for improvement in right ventricular (RV) dysfunction is not understood. In the present study, cardiac output (CO) was optimized by sequential BiVP in six anesthetized, open-chest pigs during control and acute RV pressure overload (RVPO). Ventricular contractility was assessed by the maximum rate of increase of ventricular pressure (dP/d tmax). Mechanical interventricular synchrony was measured by the area of the normalized RV-left ventricular (LV) pressure diagram ( APP). Positive APP indicates RV pressure preceding LV pressure, whereas zero indicates complete synchrony. In the control state, CO was maximized with nearly simultaneous stimulation of the RV and LV, which increased RV ( P = 0.006) and LV dP/d tmax ( P = 0.002). During RVPO, CO was maximized with RV-first pacing, which increased RV dP/d tmax ( P = 0.007), but did not affect LV dP/d tmax, and decreased the left-to-right, end-diastolic pressure gradient ( P = 0.023). Percent increase of RV dP/d tmax was greater than LV dP/d tmax ( P = 0.014). There were no increases in end-diastolic pressure to account for increases in dP/d tmax. In control and RVPO, RV dP/dtmax was linearly related to APP ( r = 0.779, P < 0.001). The relation of CO to APP was curvilinear, with a peak in CO with positive APP in the control state ( P = 0.004) and with APP approaching zero during RVPO ( P = 0.001). These observations imply that, in our model, BiVP optimization improves CO by augmenting RV contractility. This is mediated by changes in mechanical interventricular synchrony. Afterload increases during RVPO exaggerate this effect, making CO critically dependent on simultaneous pressure generation in the RV and LV, with support of RV contractility by transmission of LV pressure across the interventricular septum.