Cardiovascular System

Preterm infants can experience a variety of cardiovascular disorders, ranging from major morphological defects to dysfunctional autoregulation of blood vessels (hypotension). By embryonic day 20, the cells that will form the heart begin to differentiate (Maschoff and Baldwin, 2005; Schultheiss et al., 1995). The primitive heart beats by 4 weeks of gestation and is fully formed at the end of the 6th week. Because gas exchange occurs in the placenta, most of the fetal blood flow bypasses the lungs through the ductus arteriosus.

The ductus arteriosus normally closes after birth, when the lungs expand; air enters the lungs; and blood is redirected from the right side of the heart, through the lungs, back to the left side of the heart, and out to the body. In preterm infants, the duct may not close properly, which results in a patent (open) ductus arteriosus, which can lead to heart failure and reduced blood flow to vital body organs (e.g., the kidney and the GI tract). Heart murmur, active precordium, and bounding pulses are clinical signs; and an echocardiography performed at the bedside can confirm the presence of a patent ductus arteriosus and an otherwise normal anatomy. A patent ductus arteriosus can be asymptomatic and may close spontaneously in the first week of life, or it complicate a preterm infant's clinical course and increase the risks of intraventricular hemorrhage (IVH), NEC, BPD/CLD, and death (Shah and Ohlsson, 2006).

Approximately 5 percent of infants with birth weights of less than 1,500 grams are treated for patent ductus arteriosus (Lee et al., 2000). Medication and surgery are equally effective at closing a patent ductus arteriosus, and each has significant side effects and outcomes (Malviya et al., 2006). The most common medication used, indomethacin, has significant side effects because of the decreasing blood flow to the lower body (which results in decreased urine output and gastrointestinal perforation). Ibuprofen is effective and may have fewer side effects, but it has not been as well studied (Shah and Ohlsson, 2006). The value of indomethacin for the prevention of patent ductus arteriosus or the treatment of a patent with asymptomatic ductus arteriosus remains controversial (Cooke et al., 2003; Fowlie and Davis, 2002). Although the focus has been on closing the patent ductus arteriosus, lower rates of mortality or morbidity (BPD/CLD, NEC, or neurodevelopmental disability) have not been demonstrated (Fowlie, 2005).

Hypotension is a frequent concern in preterm infants, but there is no consensus as to what the blood pressure readings should be in preterm infants with gestational ages of less than 26 or 27 weeks. The administration of boluses of normal saline and pressors is used to support blood pressure. Although preterm infants with severe refractory hypotension are often treated with physiological doses of hydrocortisone, its safety or efficacy has not been established.

Apnea and bradycardia are common in premature infants and are manifestations of immature cardiorespiratory control (Veerappan et al., 2000). However, preterm infants and, indeed, some term infants can have bradycardia during feeding, despite the absence of other cardiorespiratory symptoms and a lack of clinical reflux. The nature of the autonomic nervous system's con tribution to these symptoms is not well understood (bradycardia could be due to increases in reflex parasympathetic autonomic nervous system activity).

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