Impedance cardiography: Pulsatile blood flow and the biophysical and electrodynamic basis for the stroke volume equations

Abstract

Impedance cardiography (ICG) is a branch of bioimpedance primarily concerned with the determination of left ventricular stroke volume (SV). As implemented, using the transthoracic approach, the technique involves applying a current field longitudinally across a segment of thorax by means of a constant magnitude, high frequency, low amplitude alternating current (AC). By Ohm’s Law, the voltage difference measured within the current field is proportional to the electrical impedance Z (Ω). Without ventilatory or cardiac activity, Z is known as the transthoracic, static base impedance Z 0 . Upon ventricular ejection, a characteristic time dependent cardiac-synchronous pulsatile impedance change is obtained, ΔZ(t), which, when placed electrically in parallel with Z 0 , constitutes the time-variable total transthoracic impedance Z(t). ΔZ(t) represents a dual-element composite waveform, which comprises both the radially-oriented volumetric expansion of and axially-directed forward blood flow within both great thoracic arteries. In its majority, however, ΔZ(t) is known to primarily emanate from the ascending aorta. Conceptually, commonly implemented methods assume a volumetric origin for the peak systolic upslope of ΔZ(t), (i.e. dZ/dt max ), with the presumed units of Ω·s –1 . A recently introduced method assumes the rapid ejection of forward flowing blood in earliest systole causes significant changes in the velocity-induced blood resistivity variation (Δρ b (t), Ωcm·s –1 ), and it is the peak rate of change of the blood resistivity variation dρ b (t)/dt max (Ωcm·s –2 ) that is the origin of dZ/dt max . As a consequence of dZ/dt max peaking in the time domain of peak aortic blood acceleration, d v /dt max (cm·s –2 ), it is suggested that dZ/dt max is an ohmic mean acceleration analog (Ω·s –2 ) and not a mean flow or velocity surrogate as generally assumed. As conceptualized, the normalized value, dZ/dt max /Z 0 , is a dimensionless ohmic mean acceleration equivalent (s –2 ), and more precisely, the electrodynamic equivalent of peak aortic reduced average blood acceleration (PARABA, d /dt max /R, s –2 ). As necessary for stroke volume calculation, dZ/dt max /Z 0 must undergo square root transformation to yield an ohmic mean flow velocity equivalent. To compute SV, the square root of the dimensionless ohmic mean acceleration equivalent ([dZ/dt max /Z 0 ] 0.5 , s –1 ) is multiplied by a volume of electrically participating thoracic tissue (V EPT , mL) and left ventricular ejection time (T LVE , s). To find the bulk volume of the thoracic contents (i.e. V EPT ), established methods implement exponential functions of measured thoracic length (L(cm) n ) or height-based thoracic length equivalents (0.01×%H(cm) n ). The new method conceptualizes VEPT as the intrathoracic blood volume (ITBV, mL), which is approximated through allometric equivalents of body mass (aM b ). In contrast to the classical two-element parallel conduction model, the new method comprises a three-compartment model, which incorporates excess extra-vascular lung water (EVLW) as a component of both Z 0 and V EPT . To fully appreciate the evolution and analytical justification for impedance-derived SV equations, a review of the basics of pulsatile blood flow is in order.