Pressure-volume relationships in the right atrium were examined before and after the creation of acute experimental
tricuspid regurgitation in pigs. A 1.3 kHz multielectrode impedance
catheter with a measuring current of 4 mA was used to determine instantaneous right atrial pressure and relative blood volume; right atrial dimension was assessed simultaneously with ultrasonic crystals attached to the atrial walls. Impedance volume waveforms and ultrasonic crystal dimensions closely paralleled each other at baseline and after the induction of
tricuspid regurgitation. The normal right atrial pressure-volume plot exhibited a figure-of-eight configuration, with an "a-loop" and a "v-loop" corresponding to the a-wave and v-wave of the right atrial pressure tracing. With severe
tricuspid regurgitation, atrial pump function was abolished, and the pressure-volume plot exhibited a single clockwise loop, consistent with complete ventricularization of the right atrium. Intermediate degrees of
tricuspid regurgitation preserved the figure-of-eight loop, but the size of both the a-loop and the v-loop were increased, consistent with a Starling-type load imposed on the atrium by the regurgitant blood volume. Increased right ventricular afterload mediated by constriction of the pulmonary artery and infusion of
methoxamine reversibly converted the right atrial pressure-volume loop from that of mild to that of severe
tricuspid regurgitation. Alternatively, constriction of the inferior vena cava and infusion of
nitroprusside changed the right atrial pressure-volume loop from that of a severe pattern of
tricuspid regurgitation to a less severe type of pattern. Infusion of
dobutamine increased the size of the a-loop relative to the v-loop both at baseline and after induction of
tricuspid regurgitation. We conclude that
tricuspid regurgitation induces changes in right atrial mechanics that can be detected and quantified with an impedance
catheter.