Assessment of right ventricular function in swine using sonomicrometry and conductance

Assessment of right ventricular (RV) pressure-volume relations has been hampered by difficulty measuring instantaneous, absolute RV volume. Accordingly, several methods were tested for their ability to reflect relative RV volume and to determine changes in RV contractile state.

Swine (46 to 54 kg; n = 7) were anesthetized and instrumented to measure instantaneous RV pressure, septal-to-RV free wall diameter (SFWD), RV free wall segment length (FWSL), RV volume via conductance (CV), and pulmonary artery flow, the integral of which was used as the standard for stroke volume. Flow-derived stroke volume was correlated with the systolic change in CV, FWSL, and SFWD in the steady state after incremental volume loading and on a beat-to-beat basis during transient inferior vena caval occlusion. Contractility was altered by calcium and pentobarbital and assessed by preload recruitable stroke work (PRSW).

Mean (+/- standard error of the mean) correlations (r) versus stroke volume during steady state conditions were 0.85 +/- 0.04 for FWSL, 0.83 +/- 0.04 for CV, and -0.04 +/- 0.24 for SFWD. Mean r values versus stroke volume during caval occlusions were 0.83 +/- 0.03 for FWSL, 0.85 +/- 0.04 for CV, and -0.03 +/- 0.31 for SFWD. Calcium increased mean PRSW slope compared with control using CV (20.3 +/- 2.6 versus 16.1 +/- 1.9 mm Hg; p < 0.05), and pentobarbital decreased mean PRSW slope compared with control using both CV and FWSL (11.3 +/- 1.0 versus 16.1 +/- 1.9 mm Hg, p < 0.05; and 11.9 +/- 2.1 versus 26.1 +/- 4.0 mm Hg, p < 0.05, respectively). There were no changes in PRSW slope with either calcium or pentobarbital using SFWD. The PRSW function was linear with both FWSL and CV but not with SFWD.

In the normal heart, both FWSL and CV, but not SFWD, accurately reflect relative instantaneous RV volume and are thus useful for determining RV contractility by pressure-volume (pressure-dimension) indices.