Table 2.

Invasive haemodynamic measurements of left ventricular (LV) stiffness and LV relaxation

Invasive haemodynamic measure	Physiological relevance	Definition(s)	Calculation
LV stiffness constant (mL−1/mL) (denoted as β; also referred to as passive LV stiffness constant)	Stiffness is the change in ventricular pressure relative to a change in volume of the ventricular chamber (dP/dV). The relationship is non-linear	Multiple pressure–volume curves generated during preload reduction generates the EDPVR (Fig. 4). EDPVR is exponential, therefore an exponential or power-function curve-fit equation is used	P = αeβ V or P = αVβ P = pressure; V = volume; both α and β are curve fitting constants
LV chamber stiffness (mmHg/mL) (denoted as b or dP/dV; also referred to as LV operating stiffness, denoted by KLV)	Stiffness is the change in ventricular pressure relative to a change in volume of the ventricular chamber (dP/dV)	The slope of LV pressure change relative to LV volume change. LV pressure change defined as difference between LV minimal pressure and LVEDP	b = dP/dV
Tau (ms) (denoted as τ; also referred to as time constant of LV relaxation)	A measure of ventricular relaxation during a defined isovolumetric period. The most commonly used isovolumetric relaxation period is defined as the time from dP/dtmin to ‘Tau end-point’, defined as the time when LV pressure falls to 5 mmHg above LVEDP of next cardiac cycle (to ensure it occurs before MV opening) [97]	Weiss et al. [98] (zero-asymptote)	P(t) = P0 × e−t/τ P(t) = LVP as a function of time; P0 = pressure at dP/dtmin when t = 0 ms
		Raff et al. [97] (non-zero asymptote)	P(t) = (P0-P∞) × e−t/τ + P∞ P(t) = LVP as a function of time; P0 = pressure at dP/dtmin when t = 0 ms; P∞ = baseline (asymptotic) pressure
		Half-pressure method (Tau ½): time required for LV pressure to decline to half the value recorded at dP/dtmin [9]

EDPVR end-diastolic pressure-volume relationship; LVP left ventricular pressure; MV mitral valve