CLASSIFICATION OF ACUTE RESPIRATORY DISORDERS: METHODS
We prospectively studied all infants born at Cook County Hospital, a tertiary perinatal center serving a low-income population, over a 12-month period. About 50-60 % of deliveries had significant perinatal risk factors such as preterm labor, preeclampsia or other significant medical or social problems (E. Swift, MD, personal communication). The birth population was 58% African American and 31% Latino. The rate of low birth weight (<2500 grams) for 1995 was 16.8 % overall, with the black rate over twice as high as that for Hispanic infants (21.9% versus 9.7%). All live born infants weighing at least 500 grams and assessed to be at least 22 weeks gestation who displayed signs of respiratory disturbance within 24 hours of birth were evaluated from November 1994 to November 1995. Neonates who were judged to be pre-viable and who were not resuscitated, were excluded. Previable was defined by being less than 500grams and 22 weeks gestation.
After initial stabilization in the delivery room, all infants were transferred to the transitional nursery and observed for respiratory symptoms. In the symptomatic infants the nurses recorded the signs of respiratory distress hourly in the first 24 hours and at varied intervals afterwards until asymptomatic. Signs of respiratory diseases evaluated were (1) tachypnea of more than 60 breaths per minute, (2) retractions, (3) grunting, and (4) nasal flaring. Cyanosis, the fifth classic sign, could not be determined in our population because our nursery protocols include liberal use of supplemental oxygen in most term infants with any respiratory signs. All signs were recorded on a special ‘RDS score’ flowsheet by the nursing staff as part of routine care. canadian antibiotics
If the infant’s condition improved rapidly so that he was in room air by six hours and his overall condition remained well, then no further action was taken. In the presence of other risk factors, such as prolonged rupture of membranes, if the symptoms persisted for six hours or more, or the infant’s condition got worse, then basic diagnostic work up was done. This included a chest film, complete blood count, blood culture, and urine latex agglutination for group В Streptococcal (GBS) antigen. Blood gases were done as indicated. Only endotracheal cultures done within 24 hours of first intubation were used for analysis. Infants were intubated for apnea, severe retractions, abnormal blood gases such as pH less than 7.20, C02 more than 55 or inspired oxygen requirement over 60 to 70% to maintain normal oxygenation.
Symptomatic infants were followed until they became asymptomatic or for five days, whichever came earlier. Various parameters such as perinatal risk factors (maternal GBS, meconium staining, prolonged rupture of membranes, maternal fever, bleeding, drugs and anesthesia), type of delivery (vaginal, cesarean, forceps or vacuum assisted), resuscitation effort (oxygen, bag and mask ventilation, intubation, drugs), Apgar scores, weight, gestation, age of onset and duration of symptoms and survival status at five days were recorded.
Blood gases obtained immediately before and within six hours after administration of surfactac-nt were used for analysis of response to this therapy. Both pre- and post-surfactant arterial/alveolar (a/A) ratios were calculated in infants receiving surfactant, using the modified gas equation.
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Pao2a/A ratio = -Pl02 . PaC02/R
Where Pa02 is the partial pressure of oxygen in the arterial blood, РЮ2 is the partial pressure of inspired oxygen, PaC02 is the partial pressure of carbon dioxide in the arterial blood and R is the respiratory quotient, 0.8. PI02 is calculated from the atmospheric pressure (-760 mmHg) minus the partial pressure of water vapor at physiologic temperatures (-47 mmHg) times the fraction of inspired oxygen:
PIq2 =FIo2 (760-47).
A single radiologist (V.J.H.), who was unaware of the clinical diagnosis, reviewed the radiographs.
The first technically adequate chest film done within 24 hours of onset was used for review. Twenty-six infants could not be classified because x-rays were not available.
The criteria used to classify infants are given in Table 1. In this study we intentionally used inclusive criteria for classification. Given the fluid and somewhat arbitrary nature of disease definition, we sought at least one plausible diagnosis for each infant, realizing that such an approach would inevitably lead to some diagnoses which would not satisfy more stringent criteria. For example, a symptomatic infant with a reticulogranular pattern on chest film was always diagnosed as having RDS, even if he also met criteria for GBS pneumonia, which is known to mimic the radiographic picture of RDS. We did not require such a patient to also demonstrate a response to surfactant in order to be classified as RDS. Obviously, some patients will meet diagnostic criteria for more than one disorder using this approach. In this example, the infant would be classified as RDS and pneumonia.
We incorporated surfactant response as a criterion for the diagnosis of RDS, using an improvement in the a/A ratio as an objective measure of surfactant response in oxygenation. We selected a threshold value based on the findings of several large controlled trials, which demonstrated mean improvements in the a/A ratio of approximately 0.2 after administration of surfactant. We arbitrarily took an improvement of about one-half of this average response, a rise of at least 0.1 in a/A ratio after surfactant, as a minimal diagnostic criterion for RDS.
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We modified the established textbook diagnostic criteria so they would be able to classify all the symptomatic infants in our population. To do this we expanded the definition of TTN and proposed and additional category. This last additional category, “transient respiratory insufficiency of the newborn” (TRIN), is made up of newborns with transient respiratory failure who fail to meet diagnostic criteria for any of the major disorders in Table 1. We then compared distributions of symptomatic cases under the standard and modified classification schemes.
