Michael Lippert

Intracardiac Impedance Monitors Hemodynamic Deterioration in a Chronic Heart Failure Pig Model

Introduction: A large number of heart failure (HF) patients benefit from cardiac resynchronization therapy. Measurements of intrathoracic impedance (ITZ) by implantable devices correlate with intrathoracic fluid content and are used for monitoring lung edema formation in HF patients. However, intrathoracic fluid is only an indirect parameter of cardiac function. We hypothesized that changes in intracardiac impedance correlate with left ventricular (LV) volume changes. Therefore, measurements of intracardiac impedance between a right ventricular lead and a LV lead may be used to monitor long‐term changes of LV function.

Intracardiac impedance monitors stroke volume in resynchronization therapy patients

AIMS Monitoring of haemodynamic parameters or surrogate parameters of the left ventricle is especially important for patients under cardiac resynchronization therapy (CRT). Intracardiac impedance reflects left ventricular (LV) volume changes well in animal models. Since it is unknown whether this also holds in humans with heart failure (HF), we examined the correlation of LV intracardiac impedance with haemodynamic parameters in CRT patients for different positions of the LV lead. METHODS AND RESULTS In 14 HF patients with non-ischaemic cardiomyopathy (four female, age 70 +/- 6 years, NYHA 2.9 +/- 0.3, EF 26 +/- 6%), one or two suitable implantation sites for the LV lead were selected. Following atrial, right ventricular, and LV catheter positioning, a micro-manometer catheter was placed in the ascending aorta. Surface ECG, impedance, and aortic pressure were recorded during graded overdrive bi-ventricular pacing in DDD mode. The correlation between impedance and stroke volume (SV) or pulse pressure (PP) changes was compared for different LV lead positions. In total, 20 overdrive pacing tests were performed at six different LV lead positions. Strong correlations were found between stroke impedance (SZ) and SV (R = 0.82 +/- 0.16) as well as between SZ and PP (R = 0.81 +/- 0.16) without significant influence of LV lead position. CONCLUSION In HF patients, a strong correlation between changes in intracardiac impedance and LV SV was found. Typical LV lead implant positions have been tested and all appear to be suitable for this method of LV volume monitoring.

Resynchronization therapy optimization by intracardiac impedance.

AIMS For successful cardiac resynchronization therapy (CRT), an optimization of left ventricular (LV) lead position and stimulation timing is required. The feasibility of optimizing LV lead position, atrioventricular delay (AVd), and interventricular delay (VVd) in CRT using intracardiac impedance measurement was evaluated. METHODS AND RESULTS Heart failure patients (n = 14, NYHA 13×III, 1×II, ejection fraction: 26 ± 6%, QRS: 165 ± 30 ms) were stimulated by AAI and biventricular (DDD-BiV) pacing in turn. Left ventricular lead site, AVd, and VVd were varied. An external pacemaker measured impedance, and a micromanometer catheter measured LV and aortic pressure. Left ventricular dP/dt(max), pulse pressure (PP), stroke volume (SV), end-systolic impedance (ESZ), and stroke impedance (SZ) were determined. Optimization results achieved by maximum increase in PP, SV, SZ, or ESZ were compared with the reference method (dP/dt(max) increase). Left ventricular lead site variation resulted in a mean optimal dP/dt(max) benefit of 18.2%. Lead site selection by SZ/PP/SV showed benefits of 17.4/17.9/17.2%, respectively. Atrioventricular delay optimization increased the optimal benefit to 22.1%, the methods ESZ/PP/SV achieved 20.1/20.8/19.4%. Interventricular delay optimization resulted in a benefit of 19.1/19.4/19.9% (SZ/PP/SV) with an optimum of 21.8%. The achieved benefit did not differ significantly between impedance, SV, and PP methods. A significant correlation between AVd values selected by dP/dt(max) and by the other methods was observed (r = 0.75/0.67/0.60 for ESZ/PP/SV). CONCLUSION The feasibility of optimizing LV lead site, AVd, and VVd by intracardiac impedance has been demonstrated for CRT patients with a similar performance as using SV and PP. Application of intracardiac impedance for automatic implant-based CRT optimization appears to be within reach.

Assessing acute ventricular volume changes by intracardiac impedance in a chronic heart failure animal model.

Background: Implantable device diagnostics may play an essential role in simplifying the care of heart failure patients by providing fundamental insights into their complex clinical patterns. Early recognition of heart failure progression by a continuous hemodynamic monitoring would allow for timely therapeutic interventions to prevent decompensation and hospitalization. In this study, the feasibility of assessing ventricular volume changes by implant‐based measurements of intracardiac impedance was tested in a heart failure animal model.

Intracardiac impedance as a method for ventricular volume monitoring – Investigation by a finite-element model and clinical data

A method for monitoring left ventricular (LV) volume changes of the human heart by intracardiac impedance measurement was developed. In order to model this method, we simulated the ventricular contraction using a finite-element model (FEM). The myocardium comprised three layers with anatomical fiber orientation. During excitation propagation contraction forces were applied, taking into account the myocardial elastic properties and the blood pressure time course. For a set of 21 contraction stages we calculated the intracardiac impedance Z between the right ventricular (RV) and LV leads for a set of common LV lead positions. The FEM results were compared to clinical data. Impedance and stroke volume were determined during overdrive pacing and end diastolic volume (EDV) at rest in 14 patients. All clinical EDV values were in the range of 147-394ml. Both the clinical data and the FEM in this volume range showed a linear correlation between admittance Y=1/Z and ventricular volume. For a quantitative comparison end diastolic impedance EDZ and the slope dY/dV were calculated. The model results across all LV lead positions were EDZ=0.16-1.2Ω, and dY/dV=3.3-21mS/ml, corresponding to clinical values of EDZ=0.14-1.46Ω and dY/dV=1-64mS/ml. In conclusion, the FEM resembled the clinical measurement data and serves as theoretical basis for ventricular volume monitoring via intracardiac impedance.

Impedance sensor for assessing cardiac hemodynamics

A method for beat-to-beat assessment of left ventricular volume changes by means of intracardiac impedance measurement is presented. Efficient heart failure therapy requires close monitoring of the patientpsilas hemodynamic state. Implantable devices for cardiac resynchronization therapy comprise at least one lead placed in the apex of the right ventricle (RV) and a second one in a coronary vein at the left ventricle (LV). Between the RV and LV bipolar leads transfer impedance Z21 is determined by injecting a sub-threshold current via the RV lead and measuring the voltage at the LV lead. Z21 varies according to LV contraction during heart cycle. Thus LV volume variation can be determined without additional sensors. The potential to assess hemodynamic parameters like stroke volume with this method was demonstrated in an acute animal study. The hemodynamic performance of the heart can be detected by the impedance sensor integrated into an implantable electrotherapeutic device and used for disease monitoring and automatic optimization of therapy parameters.