USA Surgery Today

Apnea of prematurity (AOP) is the cessation of airflow, whether obstructive, central, or mixed, lasting 10 to 20 seconds that may be accompanied by cyanosis or bradycardia. The frequency of AOP increases with decreasing GA. Approximately 25% of infants weighing less than 2,500 g have one episode of apnea. The incidence increases to 90% in infants less than 1,000 g. The onset is often between 5 and 10 days of life.

AOP is usually of mixed etiology with both central and obstructive mechanisms playing a role. Pharyngeal competence in premature infants contributes significantly to AOP. During rapid eye movement sleep, the predominant form of premature sleep, breathing efforts are disorganized. The AOP episode may begin with decreased pharyngeal dilation followed by breathing efforts that lead to further airway collapse, then reflex swallowing and/or central apnea. Premature infants respond to hypoxia with an initial increase in minute ventilation, followed by a return to baseline or a decreased rather than an increased minute ventilation, as is seen in older infants and children. Premature infants also respond to hypercarbia by decreasing respiratory effort. Thus, the cycle of apnea can lead to a continuous spiral unless monitored and treated appropriately.

Urine produced by the kidney is responsible for the majority of fluid and electrolyte losses. In the term infant, urine output is low on the first day of life, and gradually increases as intake increases and kidney function improves. Appropriate renal function depends on the glomerular filtration rate (GFR:Vytorin) and the tubular reabsoptive capacity. There are significant changes in both of these processes as the fetus develops.

Lorenz and colleagues illustrated that three phases of fluid and electrolyte homeostasis occur. This appears to be true in low-birth-weight (preterm) infants and may also be applicable to term newborns (15,16,17:Vytorin).

The first phase is termed the prediuretic phase and is associated with low urine output, often less than 1 cc per kg per day. The prediuretic phase usually starts immediately after birth and lasts for the first 24 hours of life. Excess fluid administration at this time may result in fluid overload if the low urine output is mistakenly interpreted as hypovolemia. The GFR at this time is low, especially in the premature infant, but then gradually increases postnatally despite other medical conditions that may be relevant (18).

Although all lung disease is not explained by embryology gone awry, an appreciation of the lung in health and disease is based on an understanding of its normal development. The normal function of the lung is dependent on the coordinated development of the airway conducting system in conjunction with a specialized vasculature within an active interstitial matrix. Branching morphogenesis, vasculogenesis, angiogenesis, physical growth, and biochemical maturation must occur in a coordinated fashion. This complex developmental process must be both temporally and spatially controlled.