Optimal pulmonary response to AS (two 12 mg doses of betamethasone [BMZ]) is expected to occur with dosing 24 hours prior to delivery and delivery within 7 days. Recent animal data showed an improvement in lung function after a 15 hour exposure to BMZ (Am J Obstet Gyn 1996;174:1408 -13). We have demonstrated that AS given at least 24 hours before but within 7 days of delivery significantly increases FRC and passive compliance (Crs) in treated vs. controlled infants (Pediatr Res 1995;37:224). We hypothesized fetal exposure to AS for <24 hours would improve FRC and Crs. To evaluate the effect of partial AS therapy on lung function, we measured FRC and Crs in 10 infants 25-34 weeks gestation (mean BW=1106g; GA=29.3 wks; 80% female; 80% Caucasian [Cau]) exposed to AS for a minimum of 6 hours, but less than 24 hours. 10 matched infants who received no AS (mean BW=1310g; GA=29.4 wks; 60% female; 90% Cau) served as our controls and 10 matched infants who received a full course of AS (mean BW=1309g; GA=29.8 wks; 70% female, 90% Cau) were our comparison group. FRC was meaasured with the nitrogen washout technique within 24 hours of age and prior to surfactant therapy, if needed. A minimum of 2 measurements were done with the neonate supine and quiet. Only consistent tracings started at end expiration and without evidence of a leak were accepted. A study was acceptable if the coefficient of variation was <10%. Crs was measured using a single breath occlusion (SensorMedics 2600). Values are mean ± SEM. Table
Optimal improvement in FRC and Crs is seen with at least 24 hours of fetal exposure to AS. Exposure to AS for <24 hours (average of 12) significantly improves FRC when compared to controls. These results support initiation of AS therapy in preterm labor even if potential successful tocolysis occurs for<24 hours.
