Short-Term Hemodynamic Effects of Cardiac Resynchronization Therapy in Patients With Heart Failure, a Narrow QRS Duration, and No Dyssynchrony

Background-Cardiac resynchronization therapy produces both short-term hemodynamic and long-term symptomatic/mortality benefits in symptomatic heart failure patients with a QRS duration > 120 ms. This is conventionally believed to be due principally to relief of dyssynchrony, although we recently showed that relief of external constraint to left ventricular filling may also play a role. In this study, we evaluated the short-term hemodynamic effects in symptomatic patients with a QRS duration < 120 ms and no evidence of dyssynchrony on conventional criteria and assessed the effects on contractility and external constraint. Methods and Results-Thirty heart failure patients (New York Heart Association class III/IV) with a left ventricular ejection fraction <= 35% who were in sinus rhythm underwent pressure-volume studies at the time of pacemaker implantation. External constraint, left ventricular stroke work, dP/dtmax, and the slope of the preload recruitable stroke work relation were measured from the end-diastolic pressure-volume relation before and during delivery of biventricular and left ventricular pacing. The following changes were observed during delivery of cardiac resynchronization therapy: Cardiac output increased by 25 +/- 5% (P < 0.05), absolute left ventricular stroke work increased by 26 +/- 5% (P < 0.05), the slope of the preload recruitable stroke work relation increased by 51 +/- 15% (P < 0.05), and dP/dtmax increased by 9 +/- 2% (P < 0.05). External constraint was present in 15 patients and was completely abolished by both biventricular and left ventricular pacing (P < 0.05). Conclusion-Cardiac resynchronization therapy results in an improvement in short-term hemodynamic variables in patients with a QRS <120 ms related to both contractile improvement and relief of external constraint. These findings provide a potential physiological basis for cardiac resynchronization therapy in this patient population. (Circulation. 2009;120:1687-1694.)