Chapter 8. Cohort Studies

• Studies in which the investigator passively observes events without determining assignment of exposure are referred to as observational.
•  Cohort studies are observational studies in which the investigator determines the exposure status of subjects and then follows them for subsequent outcomes.
• A prospective cohort study is one in which the exposure and subsequent outcome status of each subject is determined after the onset of the investigation.
• A retrospective cohort study is one which utilizes historical information on exposure status and subsequent outcome.
• The risk ratio or relative risk is a measure of association between the exposure and disease and is defined as the risk among the exposed divided by the risk among the unexposed.
• A risk ratio > 1 implies that exposure increases the risk of disease, whereas a risk ratio < 1 implies that exposure decreases the risk of disease.
• The attributable risk percentage is a measure of the proportion of the total risk among exposed persons that is related to their exposure.

A pediatrician was called to the hospital to attend the delivery of a newborn. The mother, a 28-year-old primigravida, had experienced elevated blood pressure during an otherwise uncomplicated pregnancy. The labor was induced because the pregnancy had continued 2 weeks past the expected date of delivery. During labor, evidence of fetal distress was seen. When the membranes ruptured, the obstetrician noted thick greenish fluid containing meconium. At the time of delivery, the male newborn was limp and cyanotic, and had no spontaneous respiratory effort and a heart rate of only 50 beats/minute. When meconium was suctioned from his mouth and nose, the baby did not grimace, cough, or sneeze.

Vigorous efforts at resuscitation were initiated, including bag-and-mask ventilation with 100% oxygen and chest compressions, but the Apgar score at 1 minute of life was 1. The Apgar score (Table 8–1), an index of neonatal asphyxia, can range from 0 (very asphyxiated) to 10 (no asphyxia). Despite continuing resuscitation, the 5-minute Apgar score improved only to 2, with a heart rate of 110 beats/minute. The 10-minute Apgar score remained depressed at 3, and the neonate was transferred to the newborn intensive care unit. With aggressive medical management, the 3100-g neonate continued to improve without evidence of acute neurologic complications. He was discharged from the hospital on the twelfth day of life.

Perinatal asphyxia can be defined ...