RESPIRATORY DISTRESS SYNDROME LAB

Lab Studies:
• Blood gases are usually obtained as clinically indicated from either an indwelling arterial (umbilical) catheter or an arterial puncture. Blood gases exhibit respiratory and metabolic acidosis along with hypoxia.
o Respiratory acidosis occurs because of alveolar atelectasis and/or overdistension of terminal airways.
o Metabolic acidosis is primarily lactic acidosis, which results from poor tissue perfusion and anaerobic metabolism.
o Hypoxia occurs from right-to-left shunting of blood through the pulmonary vessels, PDA, and/or foramen ovale. Pulse oximetry is used as a noninvasive tool to monitor oxygen saturation, which should be maintained at 90-95%.

Imaging Studies:
• Chest radiographs of an infant with RDS exhibit bilateral diffuse reticular granular or ground glass appearance, air bronchograms, and poor lung expansion.
o The prominent air bronchograms represent aerated bronchioles superimposed on a background of collapsed alveoli.
o The cardiac silhouette may be normal or enlarged. Cardiomegaly may be the result of prenatal asphyxia, maternal diabetes, PDA, an associated congenital heart anomaly, or simply poor lung expansion.
o These findings may be altered with either early surfactant therapy or indomethacin treatment with mechanical ventilation.
o The radiologic findings of RDS cannot be differentiated reliably from those of pneumonia, which is caused most commonly by GBBS.

• Echocardiographic evaluation is performed in selected infants to assist the clinician in diagnosing PDA and determine the direction and degree of shunting. It is also useful in making the diagnosis of pulmonary hypertension and excluding structural heart disease.

Other Tests:
• Pulmonary mechanics testing
o Although pulmonary mechanics testing (PMT) has been used primarily as a research tool in the past, newer ventilators are equipped with PMT capabilities to assist the neonatologist in adequately managing the changing pulmonary course of RDS.
o Constant PMT monitoring may be helpful in preventing volutrauma from alveolar and airway overdistension. Monitoring may also facilitate weaning the infant from the ventilator after surfactant therapy or determining if the infant can be extubated.
o Infants with RDS have significant decrease in lung compliance with a range of 0.0005-0.0001 L/cm H2O. Therefore, for the same pressure gradient (compared to healthy lungs), the delivered tidal volume is reduced in infants with RDS. The resistance (airway and tissues) may be normal or increased. The time constant and the corresponding pressure and volume equilibration are shorter. The anatomic dead space and the functional residual capacity are increased.

Procedures:
• Sedation, analgesia, or anesthesia whenever feasible
• Arterial puncture, venous puncture, and capillary blood sampling
• Vascular access
o Intravenous line placement
o Umbilical arterial catheterization
o Umbilical artery cut down
o Peripheral artery cannulation
o Umbilical venous catheterization
• Tracheal intubation or tracheostomy
• Bronchoscopy
• Thoracotomy tubes
• Pericardial tubes
• Gastric tubes
• Transfusion of blood, blood products, and exchange transfusion
• Lumbar puncture
• Suprapubic bladder aspiration and bladder catheterization