ELECTRICAL RESYNCHRONISATION BY BIVENTRICULAR PACING: PRELIMINARY INSIGHTS FROM VECTORCARDIOGRAPHY

Niraj Varma MRCP, Bruce Stambler MD, Jerome Liebman MD.
University Hospitals and Case Western Reserve University,
Cleveland, Ohio, USA.

Abstract: In patients with heart failure and conduction system disorders, electrical resynchronisation by biventricular pacing (BiV) may be beneficial. However, abbreviation of QRS duration by BiV has not been a consistent index of successful resynchronisation. We postulated that effective electrical resynchronisation should be accompanied by significant cancellation of forces, and hence a reduction in the magnitude of the spatial vector. We tested this in a group of 14 heart failure patients treated with BiV. We found that BiV reduced the maximum spatial vector in 10/12 patients compared to baseline. This was unaccompanied by consistent effects on the QRS duration.  The most striking clinical effects were observed in 3 patients with the most marked (>30%) reduction in the maximum spatial vector. In contrast, in all patients, single-site ventricular pacing increased the maximum spatial vector when compared to baseline and to BiV. Conclusion A reduction in the magnitude of the maximum spatial vector may be a useful index of successful electrical resynchronisation therapy.

INTRODUCTION

Electrical resynchronization by biventricular pacing (BiV) in patients with heart failure and conduction system defects (especially LBBB), may improve hemodynamics and thus exercise tolerance and quality of life [1], and reduce their incidence of ventricular arrhythmias [2]. However, some patients demonstrate marked clinical benefit, balanced by others with very little. Reduction of QRS complex duration has been postulated to be a marker of successful electrical resynchronisation therapy [3], but this effect is inconsistently observed. Clinical improvement may occur without QRS narrowing, and other patients may derive no benefit from BiV despite successful QRS abbreviation [4].

OBJECTIVE AND METHODS

Since vectorcardiography (VCG) may more accurately depict the timed sum of ventricular activation during the QRS than the standard ECG (ECG), we hypothesised that VCGs may facilitate understanding of electrical resynchronisation occurring during BiV. Normal synchronous ventricular activation results in cancellation of the much of the forces, but in LBBB this effect is reduced. Hence, we postulated that successful electrical resynchronization by BiV would be accompanied by resumption of cancellation, thus reducing the magnitude of the maximum spatial vector. In heart failure patients receiving BiV (n=14, 11 males), ECGS and VCGs were both recorded during native rhythm (where possible), single site ventricular pacing (SiV), and BiV.

RESULTS

Baseline QRS duration was 155 ± 7.6 ms, and maximal spatial vector 1.09 ± 0.09 mV (n=12). SiV increased both QRS duration (to 206 ± 7.3 ms, p<0.01), and the magnitude of the maximal spatial vector (to 1.30 ± 0.08 mV, p<0.01) in all patients. BiV  i) Overall, QRS duration was not reduced by BiV (baseline vs BiV = 155 ± 7.6 vs 160 ± 7.3 ms, NS). However, BiV did reduce QRS duration in 5/14 individual patients when compared to baseline, and in all patients when compared to SiV (BiV vs SiV = 160 ± 7.3 vs 206 ± 7.3 ms, p<0.01). BiV significantly changed the direction of the VCG, usually shifting it to the right posterior quadrant in the horizontal plane. ii) BiV significantly abbreviated vector magnitude in 10/12 patients when compared to baseline (Baseline vs BiV = 1.09 ±  0.1 vs 1.04 ± 0.1 mV, p< 0.01), and in all patients when compared to SiV (p<0.001). This effect was observed even in patients in whom QRS duration was increased by BiV. iii) Three patients demonstrated dramatic acute clinical effects. Strikingly, these were the patients with the most prominent (~30%) reduction in the magnitude of the spatial vector. One patient, previously ventilator- and pressor-dependent, was weaned within 24h. 2 patients with previously incesssant ventricular arrhythmia, were rendered arrhythmia-free (fig 1 depicts horizontal plane VCGs from one such patient). In these 2 patients, BiV actually increased QRS duration. iv) In 2 patients, BiV increased the magnitude of the maximum spatial vector. Perhaps of importance, these were patients whose QRS was <130 ms at baseline, and they did not derive clinical benefit from BiV.

					Biventricular pacing (BiV)
	Native

 

Fig 1.Horizontal plane vectorcardiograms recorded from a patient in whom BiV resulted in a ~30% diminution of the maximum spatial vector, with an accompanying beneficial clinical effect.

 

DISCUSSION

VCGs reveal important characteristics of ventricular activation during BiV, not readily apparent from the ECG. In contrast to SiV, BiV significantly reduced the magnitude of the maximum QRS vector. This effect was more consistently observed than QRS duration abbreviation. In this small series, the most dramatic clinical effects were observed in those with the most pronounced reduction in magnitude of the vector. We speculate that effective electrical resynchronisation therapy is characterised by increased cancellation of forces, (though depolarisation may still proceed slowly), irrespective of QRS duration reduction.

REFERENCES

[1]. Cazeau S, Leclercq C, et al. Effects of multisite biventricular pacing in patients with heart failure and intraventricular conduction delay. New Engl J Med, vol.344, pp. 873-80, 2001

[2]. Higgins SL, Young P, et al. Biventricular pacing diminishes the need for implantable cardioverter defibrillator therapy. J Am Coll Cardiol, vol 36, pp. 824-827, 2000

[3]. Alonso C, Leclerq C, et al. Electrocardiographic predictive factors of long-term clinical improvement with multisite biventricular pacing in advanced heart failure. Am J Cardiol, vol 84, pp. 1417-1421

[4]. Reuter S, Garrigue S, et al. Intermediate term results of biventricular pacing in heart failure. PACE vol 23, pp.1713-1717, 2000