Larger QRS but better hemodynamic function after high output His bundle pacing in patients with heart failure and narrow QRS. A paradox or a new concept?

Abstract Background QRS duration is an important factor in determining Cardiac Resynchronisation Therapy (CRT) response and a useful surrogate for electromechanical dyssynchrony. However, we don’t have studies confirming this concept in conduction system pacing. Purpose We performed His Bundle Pacing (HBP) with high output (3.5V/1msec) in HF patients with dilated or ischemic cardiomyopathy and narrow QRS to evaluate hemodynamic benefit independent of the QRS duration or AV delay shortening. Methods 14 patients with dilated (7 patients) or ischemic (7 patients) cardiomyopathy (EF<35%) and narrow QRS (<120ms) were referred for implantation of a defibrillator. We implanted an ICD- HBP system, and we collected clinical, echocardiographic, and electrocardiographic data. Results We obtained HBP in all 14 patients successfully. PQ after implantation was shorter from 172±48 to 143±16 (P=0.048) so as to pace the ventricle. Baseline QRS duration increased after pacing from 110±15 to 123±16ms (P=0.008) as measured by ECG analyzer. Despite this increase in QRS duration basal mean EF increased from 30±4% to 46±10% (P<0.001) after a median follow-up of 17 months. Left ventricular end-diastolic diameter and left ventricular end-diastolic volume decreased from 62±5 to 57±5mm (P=0.013) and from 195±69 to 126±27ml (P<0.001), respectively. Left ventricular end-systolic volume decreased from 139±58 to 69±27 (P<0.001). TAPSE increased from 19±4 to 27±4. NYHA class decreased by one class in every patient after only one month. HBP threshold was 1.05V±0.7/1msec and raised to 1.33±0.93/1msec (due mainly to 2 patients with very high threshold). The impedance was 588±109 and decreased to 484±69 Ohms. R-wave at implant was 3.2±1.8mV. In 2 out of 14 patients, threshold increased but was still less than 3.5V/1msec. Conclusions HBP in HF patients with dilated or ischemic cardiomyopathy and narrow QRS improves hemodynamic function in right and left ventricle and decreases NYHA class. This benefit appears to be produced by a higher stimulation output despite a larger QRS after pacing. We exclude that a very short AV delay could be favorable, by measuring acute EF changes only after increasing the pacing output without AV delay modifications. To our knowledge these are the first cases of beneficial HBP in HF patients with narrow QRS. Interestingly, larger QRS corresponds to higher EF in patients with high output HIs Bundle Pacing.