Filling characteristics of left ventricle in newborn lambs

An integral component of increased cardiac performance during the immediate newborn period is an increased rate of left ventricular (LV) filling. To determine the factors that facilitate increased LV filling in newborns, we compared filling characteristics in chronically instrumented awake newborn (7 +/- 2 days old) and older lambs (55 +/- 5 days old). The studies were performed 4 +/- 2 days after surgery, during which pressure transducers, dimension crystals, a flow transducer, and vascular occluders were placed. Newborn lambs had an increased cardiac index (247 +/- 40 vs. 127 +/- 48 ml.kg-1.min-1; P < 0.01) due to an increased heart rate (223 +/- 24 vs. 120 +/- 9 beats/min; P < 0.01) despite a similar stroke volume index (1.1 +/- 0.1 vs. 1.1 +/- 0.5 ml/kg; not significant). In newborn lambs, indexed mean LV filling rates were more than twofold higher (10.5 +/- 2.5 vs. 4.6 +/- 1.7 ml.kg-1.s-1; P < 0.01), and a much greater percentage of filling occurred in the first 35 ms after mitral valve opening (52 +/- 7 vs. 23 +/- 8%; P < 0.01). In newborn lambs, early diastolic filling was facilitated by more rapid LV relaxation (tau: 17.2 +/- 2.2 vs. 23.3 +/- 1.2 ms; P < 0.01) and a higher left atrial (LA) pressure at the time of mitral valve opening (11.6 +/- 2.4 vs. 7.2 +/- 3.2 mmHg; P < 0.05). LV filling in newborn lambs also occurred at the expense of a higher mean LA pressure (8.4 +/- 2.4 vs. 5.1 +/- 2.6 mmHg; P < 0.05). These findings improve our understanding of LV diastolic function in newborns.