Optimizing electrode placement for hemodynamic benefit in cardiac resynchronization therapy

Low Intensity Epicardial Pacing During the Absolute Refractory Period Augments Left Ventricular Function Mediated by Local Catecholamine Release

BACKGROUND

Biventricular epicardial (Epi) pacing can augment left ventricular (LV) function in heart failure. We postulated that these effects might involve catecholamine release from local autonomic nerve activation. To evaluate this hypothesis we applied low intensity Epi electrical stimuli during the absolute refractory period (ARP), thus avoiding altered activation sequence.

METHODS

Anesthetized pigs (n = 6) were instrumented with an LV pressure (LVP) transducer, left atrial (LA) and LV Epi pacing electrodes, and sonomicrometer segment length (SL) gauges placed proximal and remote to the LV stimulation site. A catheter was placed into the great cardiac vein adjacent to the LV pacing site for norepinephrine (NE) analysis. During LA pacing at constant rate, 3 pulses (0.8 milliseconds, 2-3x threshold) were applied to the LV Epi electrodes during the ARP. An experimental run consisted of baseline, stimulation (10 minutes), and recovery (5 minutes), repeated 3 times before and after β1 - receptor blockade (BB, metoprolol).

RESULTS

ARP stimulation produced significant increases in cardiac function reflected by elevated LVP, LV, dP/dtmax , and reduced time to LV dP/dtmax . This was accompanied by increased coronary NE levels and increases in LVP versus SL loop area in the remote myocardial segment. In contrast, the proximal segment exhibited early shortening and decreased loop area. BB abolished the changes in SL and LV function despite continued NE release.

CONCLUSION

These results demonstrate that ARP EPI stimulation induces NE release mediating augmented global LV function. This effect may contribute to the beneficial effect of biventricular Epi pacing in heart failure in some patients.

QRS frequency characteristics help predict response to cardiac resynchronization in left bundle branch block less than 150 milliseconds

BACKGROUND

Baseline QRS duration (QRSd) ≥150 ms is a recognized predictor of clinical improvement by cardiac resynchronization therapy (CRT), particularly for those with left bundle branch (LBBB). Patients with QRSd <150 ms are considered less likely to respond.

OBJECTIVE

The purpose of this study was to test our theory that left ventricular dyssynchrony, although usually associated with wider QRSd, also exhibits lower QRS frequency characteristics and that low-frequency content predicts CRT response in LBBB patients.

METHODS

We retrospectively examined the QRS frequency content of 170 heart failure patients with LBBB and QRSd ≥120 ms using Fourier transformation. Ninety-four responders to CRT (defined as reduction in left ventricular end-systolic volume by ≥15% from baseline) were compared to 76 nonresponders (<15% reduction). Analysis of 3 standard ECG leads (I, aVF, and V3) representing the 3 dimensions of depolarization was performed, and V3 provided the best predictive value.

RESULTS

The QRSd of responders (160.3 ± 17.8 ms) and nonresponders (161.8 ± 21.1 ms, P = .604) were similar. We found that the percentage of total QRS frequency power below 10 Hz that exceeded 52% was most predictive of CRT response compared to other cutoff values. However, the percentage of patients with total QRS power >52% below 10 Hz was especially predictive of response in those with QRSd <150 ms. In these patients, this power threshold was highly predictive of CRT response (positive predictive value 85.7%, negative predictive value 71.4%).

CONCLUSION

In this group of CRT recipients with LBBB, retrospective analysis of QRS frequency content below 10 Hz had greater predictive value for CRT response than baseline QRSd, particularly in those with QRSd <150 ms.

Toward the virtual stomach: progress in multiscale modeling of gastric electrophysiology and motility

Experimental progress in investigating normal and disordered gastric motility is increasingly being complimented by sophisticated multiscale modeling studies. Mathematical modeling has become a valuable tool in this effort, as there is an ever-increasing need to gain an integrative and quantitative understanding of how physiological mechanisms achieve coordinated functions across multiple biophysical scales. These interdisciplinary efforts have been particularly notable in the area of gastric electrophysiology, where they are beginning to yield a comprehensive and integrated in silico organ modeling framework, or 'virtual stomach'. At the cellular level, a number of biophysically based mathematical cell models have been developed, and these are now being applied in areas including investigations of gastric electrical pacemaker mechanisms, smooth muscle electrophysiology, and electromechanical coupling. At the tissue level, micro-structural models are being creatively developed and employed to investigate clinically significant questions, such as the functional effects of ICC degradation on gastrointestinal (GI) electrical activation. At the organ level, high-resolution electrical mapping and modeling studies are combined to provide improved insights into normal and dysrhythmic gastric electrical activation. These efforts are also enabling detailed forward and inverse modeling studies at the 'whole body' level, with implications for diagnostic techniques for gastric dysrhythmias. These recent advances, together with several others highlighted in this review, collectively demonstrate a powerful trend toward applying mathematical models to effectively investigate structure-function relationships and overcome multiscale challenges in basic and clinical GI research.