Energy Costs of Singular and Concomitant Pressure and Volume Overload Lesions

Abstract

Severity assessment in concomitant hypertension (HT) and heart valve disease or multiple heart valve disease is vague, and there exists no established severity index. The objective of this study is to propose a theoretical framework to utilize the ventricular power overhead rates of HT, aortic regurgitation (AR), aortic stenosis (AS), and mitral regurgitation (MR) as a new means to assess severity, particularly under concomitant conditions. A lumped parameter model was utilized to evaluate the ventricular energy budget under normal, singular, and concomitant combinations of HT, AS, AR, and MR; and calculate energy overhead rate defined as the % increase in ventricular power output. Disease severity for each lesion was modeled at mild, moderate, and severe levels per AHA/ACC guidelines. The overhead rate for HT and AS were 15% (mild), 25% (moderate), and 45% (severe); AR and MR corresponded to 40% (moderate) and 100% (severe). The overhead rate as a function of regurgitant fraction was shown to be highly nonlinear. The overhead rate for concomitant lesions were 39% (mild HT+mild AS), 51% (mild HT+mild AR), 46% (mild HT+mild MR), 51% (mild AS+mild AR), 37% (mild AS+mild MR), and 44% (mild AR+mild MR). Power overhead under volume overload increases nonlinearly while that for pressure overload is linear. Concomitant lesions involving pressure (and volume?) overload produce a net overhead rate greater than the sum of individual lesions. Synergy in overhead rates is most with the presence of uncontrolled HT.

Appendix: Estimating Power Overhead For AR and MR

Power Overhead for MR

Baseline: Let P be the mean ventricular pressure, and SV the stroke volume. Work done in one heart beat equals P*SV.

MR: Now, let RF be the mitral regurgitant fraction. If the net cardiac output is the same as baseline, then the ventricular stroke volume equals SV/(1 − RF). Work done in one heart beat equals P*SV/(1 − RF)

$$ \begin{gathered} {\text{Overhead rate }} = \, \left( {P*SV/\left( {1 - RF} \right) \, - P*SV} \right)/ \, \left( {P*SV} \right) \hfill \\ = \, 1/\left( {1 - RF} \right) \, {-} \, 1 \, = {\text{ RF}}/\left( {1 - {\text{RF}}} \right). \hfill \\ \end{gathered} $$