RESUMO
We developed a device for monitoring instantaneous left ventricular (LV) volume using an alternating-current excitation two-electrode conductance catheter. Instantaneous conductance between a pair of electrodes was amplified by a non-inverting circuit. The level of conductance was linearly related to changes in blood volume from 0.8 to 2.0 ml in a latex balloon (r2 = 0.95), and to changes in blood volume from 0.4 to 2.2 ml in a post-mortem rabbit left ventricle (r2 = 0.99). The difference between the maximal and minimal conductance of the LV in situ during a cardiac cycle was closely correlated with changes in stroke volume, measured by an electromagnetic flow probe (r2 = 0.97). The endsystolic pressure-conductance relation (ESPCR) was highly linear (r2 = 0.92). Changes of the slope (Ees) of the ESPCR correlated directionally with changes of the time derivative of LV pressure (LVdP/dt) during intravenous infusions of dobutamine and propranolol. Accordingly, the two-electrode conductance catheter was useful in vivo in rabbits for continuously assessing changes in the LV volume.
