. 2019 Dec 19;7(1):1. doi: 10.3390/bioengineering7010001

Table 1.

Summary of variables and meanings.

Variable	Physiological Meaning (Unit)
$v_{c}$	Velocity of the contractile element (s⁻¹)
$ε_{c}$	Strain of the contractile element
$τ_{c}$	Active tension of the contractile element (mmHg)
$k_{c}$	Stiffness of the contractile element (mmHg)
$σ_{P}$	Passive stress (mmHg)
$u$	Chemical activity (s⁻¹)
$p$	Slow electric variable
$q$	Fast electric variable
$P_{V}$	Left ventricular pressure (mmHg)
$V$	Left ventricular volume (mL)
$P_{R}$	Atrial pressure (mmHg)
$P_{S}$	Arterial pressure (mmHg)
$m$	Aortic pressure (mmHg)
$m_{0}$	Arterial pressure parameter for the basic model (mmHg)
$F_{a}$	Aortic (total) flow (mL/s)
$n$	Pump flow (mL/s, mL/min)
$δ_{p}$	Pump parameter (1 for pump, 0 for no pump)

Note that the parameter values for the contractile components ( $σ_{o}, k_{c}, k_{1}, k_{2}$ ) are based on [21,22,23], while those for resistance, compliance, inertance, pump, and are taken from [14,15]. The advantage of this basic model is that solutions are not too sensitive to initial conditions due to the nonlinearity of Equations (1)–(10) for mechanical and electric activity and the fixed value of $P_{R}$ and $m_{0}$ in the circulation model in Equations (11) and (12), allowing us to find (almost) unique solutions after initial transients disappear.