Variation in Tau, the time constant for isovolumic relaxation, along the left ventricular base-to-apex axis

Abstract

Tau (τ), the time constant for isovolumic relaxation, is often used as a measure of cardiac diastolic function. However, several methods of calculating τ have been published which may produce different results and, thereby, different conclusions. The purpose of this study was to determine if the method of τ calculation effects the results when left ventricular pressure (LVP) is measured at different positions along the base-to-apex axis. In 16 dogs, we measured LVP at 6 positions along the base-to-apex axis. We calculated τ using three different methods: 1) a monoexponential model (P(_t)=[P₀–P_asym]e^At+P_asym, where t=time, P₀=LVP at t=0, P_asym is asymptotic pressure as t→∞, A is –1/τ) with a zero asymptote 2) a monoexponential model with a variable asymptote in which the monoexponential decay equation is differentiated with respect to time and substituted into the original equation so that dP/dt vs. LVP is a (–/τ), and 3) a monoexponential decay model with variable asymptote in which P_asym and A are varied until the best fit line is reached by minimizing the residual sum of squares. When τ is calculated using method 1, τ measured at the LV base is 98.01%±8.85% of the τ at the apex. If calculated using method 2, τ measured at the LV base was 75.46±39.4% of τ measured at the apex. When method 3 is used for τ calculations, base τ increases to 117.76±4.91% of the apical τ. We conclude: 1) the method used to calculate τ will effect the results and, thus, conclusions drawn from τ data. 2) When using Method 3, which appears to be the best method for τ calculation, τ increases at the LV base compared to the apex.