Left bundle branch block increases left ventricular diastolic pressure during tachycardia due to incomplete relaxation

Abstract

Background: We investigated whether tachycardia in left bundle branch block (LBBB) decreases left ventricular (LV) diastolic distensibility and increases diastolic pressures due to incomplete relaxation, and if cardiac resynchronization therapy (CRT) modifies this response. Methods: Thirteen canines were studied at baseline heart rate (120 bpm) and atrial paced tachycardia (180 bpm) before and after induction of LBBB and during CRT. LV and left atrial (LAP) pressures were measured by micromanometers and dimensions by sonomicrometry. The time constant tau of exponential pressure decay and degree of incomplete relaxation at mitral valve opening (MVO) and end diastole (ED) based on extrapolation of the exponential decay were assessed. Changes in LV diastolic distensibility were investigated using the LV transmural pressure-volume (PV) relation. Results: LBBB caused prolongation of tau (p<0.03) and increased the degree of incomplete relaxation during tachycardia at MVO (P<0.001) and ED (P=0.16) compared to normal electrical activation. This was associated with decreased diastolic distensibility seen as upward shift of the PV-relation at MVO by 18.4±7.0 vs. 12.0±5.0 mmHg, at ED by 9.8±2.3 vs. 4.7±2.3 mmHg and increased mean LAP to 11.4±2.7 vs. 8.5±2.6 mmHg, all P<0.006. CRT shifted the LV diastolic PV-relation downwards during tachycardia, reducing LAP and LV diastolic pressures (P<0.03). Conclusions: Tachycardia in LBBB reduced LV diastolic distensibility and increased LV diastolic pressures due to incomplete relaxation, while CRT normalized these effects. Clinical studies are needed to determine if a similar mechanism contributes to dyspnea and exercise intolerance in LBBB and if effects of CRT are heart rate dependent.