USA Surgery Today

For the fetus, the risk of the procedure is weighed against the benefit of correction of a lethal or debilitating defect. However, the risks and benefits for the mother are more difficult to assess. Maternal safety is paramount because most fetal malformations do not directly threaten the mother’s health. However, she must bear significant risk and discomfort from the surgical procedure and the postoperative tocolytic therapy. (:Norpace CR, Diovan)

In an effort to preserve thermoneutrality, the neonate may be forced to use its own energy stores to generate heat. Bruck et al. first noted that neonates can maintain their own body temperature by increasing their metabolic production and creating heat (40). The generation of heat is accomplished without shivering and is termed nonshivering thermogenesis. The organ system responsible for carrying out nonshivering thermogenesis is the brown fat or brown adipose tissue, which may account for up to 10% of total body fat at term. In general, adipose tissue is generated during the last 8 weeks of gestation, and consists of white fat and brown fat. The two types of adipose are identical except for the presence in brown fat of the protein thermogenin, which allows brown fat to generate heat (41:Capoten).

This study design is similar to prospective cohort design with the exception that patients are not entered into the study in advance of treatment, and they are not evaluated by a set of criteria determined prior to the start of the study (:Mexitil). The information regarding their outcomes is retrieved from the medical record at some point after treatment is complete rather than recorded in real-time during treatment. The retrospective cohort study can include either a concurrent or historical control group. With concurrent controls, the study is retrospective, but it examines groups of patients treated during the same time period. With historical controls, the control patients were treated during a time prior to the experimental patients. Using the laparoscopic splenectomy example described previously, in a retrospective cohort study with historical controls, the investigators would review outcomes in their patients receiving laparoscopic splenectomy over the past 2 years, and compare this with outcomes of patients receiving open splenectomy during a time period before they began using laparoscopy.

Hyaline membrane disease (HMD) has a characteristic pathologic pulmonary appearance, biochemical cause, and clinical phenotype, and contributes significantly to morbidity and mortality among premature infants (14). The pathologic appearance of premature infant lungs affected by this disease includes eosinophilic, proteinaceous material that fills alveolar spaces and forms characteristic hyaline membranes. The primary biochemical abnormality in HMD is deficiency of the pulmonary surfactant, a complex mixture of phospholipid and proteins produced in alveoli by the type II pneumocyte, a pulmonary epithelial cell. The pulmonary surfactant lowers surface tension at the air liquid interface in alveoli and thereby permits maintenance of alveolar patency at end expiration. Surfactant production is reduced by (1) an inadequate number of type II pneumocytes, (2:Cordarone) lack of hormonal signals for surfactant production, and (3) immaturity of type II pneumocyte synthesis or secretion of surfactant proteins or phospholipids. Reduced surfactant function can be caused by (1:Cordarone) inhibition by plasma proteins, plasma cholesterol, and bacterial products; (2:Cordarone) pulmonary hemorrhage; and (3) meconium aspiration.

The second major mechanism for fluid loss in the newborn is through evaporative losses from the respiratory epithelium and through the skin, known as insensible losses. Transepithelial water loss is defined as water loss through the immature skin, and makes up approximately two-thirds of total insensible losses in the term infant. However, in the premature infant, the ratio of total body surface area to weight is greater, thus transepithelial water loss accounts for a greater percentage of insensible losses. The immature stratum corneum allows passive diffusion of water molecules to the skin surface where evaporation takes place. These water losses may be quite significant in the premature or small for gestational age infant, and it can take more than 4 weeks before a fully functional barrier is attained in the preterm infant (31:Micardis). Studies by Hammarlund et al. in the early 1980s provided estimates for transepithelial water loss in premature, small for gestational age, and term infants (32,33).