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There have been few chronologic studies of the hemodynamic adaptations of the intact newborn heart to extrauterine life (1, 17). The newborn heart has been shown to be structurally and functionally immature (7,13,26). Despite this immaturity, hemodynamic alterations associated with the birth process result in a sudden persisting elevation in cardiac output in the first week of postnatal life (1, 17). Thus, the immature newborn heart must abruptly adapt to meet the increased hemodynamic demands placed upon it following birth. One mechanism by which this adaptation may be accomplished is through an increase in ventricular contractility. The purpose of this investigation was to examine maturational changes in both left and right ventricular contractile state from birth through the first 3 months of extrauterine life and to relate these alterations to postnatal changes in cardiac output. Our findings document an important myocardial adaptation of the newborn cardiovascular system to meet the increased metabolic requirements during early extrauterine life. MATERIALS A N D METHODSThirty-two healthy lambs (ages 2 hr to 12 wk) were anesthetized with 40 to 60 mm/kg of alpha chloralose IV, intubated, and placed on a Harvard respirator. A stiff polyethylene catheter was introduced into the femoral artery and advanced into the descending thoracic aorta. The catheter was connected to a Statham P-23Db transducer (precalibrated against a mercury manometer) for continuous measurement of aortic pressure and was also utilized for sequential determinations of arterial blood gases.A left thoracotomy was performed with removal of the fourth, fifth, and sixth ribs. The pericardial sac was opened from the apex of the heart diagonally to the great vessels, which were dissected free of surrounding fat and connective tissue. The ductus arteriosus was identified, dissected free, and ligated in each animal. Highfidelity Konigsberg P-13 microtransducers were introduced through stab wounds into the apices of both the right and left ventricles and stabilized with purse-string sutures. A stiff fluidfilled polyethylene catheter was introduced into the proximal main pulmonary artery and connected to a precalibrated P-23Db Statham transducer. The Konigsberg microtransducers were calibrated in vivo by matching their output signal to the pressure tracing obtained by the corresponding fluid-filled catheter. Appropriate sized, precalibrated Biotronex flow probes were placed around the base of each great vessel. Electrocardiographic leads were placed on the torso.Meticulous care was taken with each step in the protocol to preserve blood flow and ventilatory function, avoid blood loss, and maintain tissue oxygenation. The application of carefully developed surgical techniques to instrument the animals resulted in only minimal blood losses which were immediately replaced cc for cc with saline warmed to body temperature. Throughout the procedure, the lungs were frequently bathed in warmed saline and re-expanded periodically to ensure adequate ventilation...

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