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An understanding of neonatal circulatory pathology is intrinsically linked to an understanding of circulatory adaptation to extrauterine life (see Chapter 43). In the fetal circulation, better oxygenated blood is returned from the placenta to the fetus via the umbilical vein. This blood is streamed by the ductus venosus across the right atrium, through the foramen ovale, and into the left atrium, facilitating delivery of the best oxygenated fetal blood to the brain and upper body. Blood returning via the vena cavae is streamed through the right side of the heart and into the descending aorta via the ductus arteriosus. This blood preferentially streams through the ductus because of arteriolar constriction and the high vascular resistance in the fetal lungs. The right-to-left ductal blood flow supplies the lower part of the body and also returns blood to the placenta via the umbilical arteries, which arise from the iliac arteries.

At birth, this blood flow pattern changes quickly. The lungs expand with the first breaths, the pulmonary arterioles dilate, right heart pressures fall, and blood pours into the pulmonary circulation to collect oxygen. The removal of the low-resistance placenta from the systemic circulation increases resistance and pressure on the left side of the circulation, while the pulmonary blood flow increases the left heart preload. The result is a dramatic increase in the workload of the left heart. The muscle in the wall of the ductus arteriosus constricts in response to rising oxygen levels, closing functionally within the first 24 hours after birth and structurally after several days to become a fibrous band.

During the last trimester, much of the fetal cardiopulmonary development is in preparation for the changes that have to occur at birth. Babies born prematurely have exquisite circulatory vulnerability during this period of the transitional circulation. More mature babies are also vulnerable if born in a compromised condition or if they become unwell shortly after birth.

Poor color, increased heart rate, prolonged capillary refill, and low urinary output suggest circulatory compromise. Blood measurements such as low pH and rising lactate can supplement the clinical assessments. These indicators are useful in identifying the baby with severe circulatory compromise, but they have limited accuracy for babies with lesser degrees of compromise.2

Blood Pressure

Blood pressure can be accurately measured and continuously monitored if there is intra-arterial vascular access. Because blood pressure is relatively easy to monitor, it has traditionally been the primary indicator of neonatal circulatory status, and much conventional circulatory support has focused on increasing the low blood pressure.3 Strong data exists for the importance of a normal blood pressure range4 (Fig. 56-1), but controversy arises over what constitutes an adequate blood pressure that is, the blood pressure below which organ injury can result.5 Further, an emerging body of evidence questions the accuracy of blood pressure as a gold standard for circulatory well-being.6-8 There ...

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