Right atrial pressure as measure of ventricular constraint in newborn lambs

Although the lungs and pericardium constrain the heart and limit cardiac ou tput, no method exists to assess this constraint in sick newborns. We hypot hesize that a useful estimate of ventricular constraint may be obtained by measuring right atrial pressure (P-RA) in the newborn. To test this hypothe sis, we measured P-RA, thoracic inferior vena caval pressure (P-IVC; saline -filled catheters), and ventricular constraint (pericardial pressure, P-PER ; liquid-containing balloon) in 4-wk-old (neonatal, n = 12) and 3-day-old ( newborn, n = 6) anesthetized lambs. The measurements were made while LV fil ling pressure was altered (0-20 mmHg) and while positive end-expiratory pre ssure (PEEP) was maintained at 2.5 or 15 cmH(2)O. In all of the lambs, a st rong linear relationship (r) existed between P-RA and P-PER (P-RA = 1.19 P- PER + 0.0, r = 0.99) and between P-IVC and P-PER (P-IVC = 1.24 P-PER + 0.1, r = 0.99; PEEP of 2.5 cmH2O). Similar relationships were also observed wit h increased PEEP (P-RA = 1.29 P-PER-1.2, r = 0.98 and P-IVC = 1.32 P-PER-1. 2, r = 0.97). Because PRA provides an accurate measure of ventricular const raint in the normal lamb, it may be a useful measure of ventricular constra int in the sick newborn.