P657Importance of force-frequency mechanism for LV elastance recruitment in elderly hypertensives


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Abstract

Purpose: Left ventricular (LV) elastance ("contractility", Elv) must overcome arterial elastance ("afterload", Ea) for ejection of blood to occur. As exercise leads to increased Ea, Elv must be recruited through (1) a shift to smaller LV volumes and/or (2) an increase in heart rate (HR; force-frequency relationship), otherwise LV ejection ceases due to afterload mismatch. We performed computer simulation to study the relative contribution of HR and LV volume change to Elv, using experimental data from supine bicycle stress echocardiography in elderly hypertensives, a group that is at risk of afterload mismatch.Methods and Results: Data obtained at rest and exercise in 21 elderly hypertensives (67±9 years) were entered into computer simulation. Elv was estimated using a single-beat method based on systolic (SBP) and diastolic (DBP) blood pressures, and timings and volumes derived from echocardiography (Chen JACC 2001): ejection fraction (EF), stroke volume (SV), pre-ejection interval and total systolic time (PEI and TST: Q-wave onset to aortic valve opening and closure, respectively). In linear regression, PEI and TST could be approximated reasonably well by HR (PEI=-48.272+0.32 x TST [R2=0.67]; TST=566.8–2.49 × HR [R2=0.64]). A circulatory state simulating physical exercise was modelled by maintaining SBP=160 mmHg, DBP=80 mmHg and EF=0.55, and varying SV and HR. As shown (Figure 1), HR played a larger role for Elv recruitment at small SV.Conclusion: As an ageing, hypertensive LV recruits elastance during exercise by shifting its operating range to smaller volumes, the force-frequency relationship between HR and Elv becomes progressively more important. Whether anti-chronotropic therapy has an adverse effect on afterload mismatch in elderly hypertensives should be explored in vivo.

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