Respiratory Distress Syndrome Workup

Updated: Jan 16, 2015
  • Author: Arun K Pramanik, MD, MBBS; Chief Editor: Ted Rosenkrantz, MD  more...
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Workup

Approach Considerations

Several diagnoses may coexist with and complicate the course of respiratory distress syndrome, including the following:

  • Pneumonia - Usually secondary to group B beta-hemolytic streptococci and often coexists with respiratory distress syndrome
  • Metabolic problems - Eg, hypothermia, hypoglycemia
  • Hematologic problems - Eg, anemia, polycythemia, jaundice
  • Transient tachypnea of the newborn - Usually occurs in term or near-term neonates, often after cesarean delivery; the chest radiograph of an infant with transient tachypnea shows good lung expansion and, often, fluid in the horizontal fissure
  • Aspiration syndromes - May result from aspiration of amniotic fluid, blood, or meconium; aspiration syndrome is observed in more mature infants and is differentiated by obtaining a history and by viewing the chest radiographs.
  • Pulmonary air leaks - Eg, pneumothorax, interstitial emphysema, pneumomediastinum, pneumopericardium; in premature infants, these complications may be due to excessive positive-pressure ventilation (in rare cases, spontaneous pneumothorax may occur in large infants)
  • Congenital anomalies of the lungs - Eg, diaphragmatic hernia, chylothorax, congenital cystic adenomatoid malformation of the lung, lobar emphysema, bronchogenic cyst, pulmonary sequestration
  • Congenital anomalies of the heart

Congenital anomalies of the lungs and heart are uncommon in premature infants; these entities can be diagnosed on the basis of chest radiographic or echocardiographic findings. They coexist only rarely with respiratory distress syndrome.

Fetal lung maturity tests

Prediction of fetal lung maturity is derived by estimating the lecithin-to-sphingomyelin ratio and/or by testing for the presence of phosphatidylglycerol in the amniotic fluid obtained with amniocentesis.

Antenatal diagnosis of SP-B deficiency, a rare genetic disease, can also be antenatally diagnosed by analyzing the amniotic fluid; this diagnostic testing should be undertaken in previously affected siblings.

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Procedures

Vascular access procedures

Vascular access procedures used in infants with respiratory distress syndrome include:

  • Intravenous (IV) line placement
  • Umbilical arterial catheterization
  • Umbilical artery cut down
  • Peripheral artery cannulation
  • Umbilical venous catheterization

Other procedures

The following procedures may also be employed in infants with respiratory distress syndrome:

  • Sedation, analgesia, or anesthesia whenever feasible
  • Arterial puncture, venous puncture, and capillary blood sampling
  • Tracheal intubation or tracheostomy
  • Bronchoscopy
  • Placement of thoracotomy tubes
  • Placement of pericardial tubes
  • Placement of gastric tubes
  • Transfusion of blood, blood products, and exchange transfusion
  • Lumbar puncture
  • Suprapubic bladder aspiration and bladder catheterization
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Blood Gases

Blood gases are usually obtained in respiratory distress syndrome, as clinically indicated, from an indwelling peripheral or central (umbilical) arterial catheter or by means of arterial puncture. In a multicenter study by Billman and colleagues, an in-line, ex-vivo, point-of-care monitor was shown to be reliable in critically ill neonates and infants. [12] It can be reliably used without adverse consequences associated with serial phlebotomy.

Blood gases show respiratory and metabolic acidosis along with hypoxia. Respiratory acidosis occurs because of alveolar atelectasis and/or overdistension of terminal airways. Metabolic acidosis is primarily lactic acidosis, which results from poor tissue perfusion and anaerobic metabolism.

Hypoxia occurs from right-to-left shunting of blood through the pulmonary vessels, patent ductus arteriosus (PDA), and/or patent foramen ovale.

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Pulse Oximetry

Pulse oximetry is used as a noninvasive tool to monitor oxygen saturation, which should be maintained at 90-95%. However, it is unreliable for determining hyperoxia because of the flat-top portion of the S -shaped oxygen-hemoglobin dissociation curve. In the past, continuous, in-line arterial PaO2 monitoring and transcutaneous monitoring were used. Transcutaneous CO2 monitors should be used in infants with ongoing respiratory distress to monitor ventilation if it correlates with PaCO2.

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Chest Radiography and Echocardiography

Chest radiography

Chest radiographs of a newborn infant with respiratory distress syndrome reveal bilateral, diffuse, reticular granular or ground-glass appearances; air bronchograms; and poor lung expansion. The prominent air bronchograms represent aerated bronchioles superimposed on a background of collapsed alveoli.

The cardiac silhouette may be normal or enlarged. Cardiomegaly may be the result of prenatal asphyxia, maternal diabetes, patent ductus arteriosus (PDA), an associated congenital heart anomaly, or simply poor lung expansion. These findings may be altered with early surfactant therapy and adequate mechanical ventilation. (See the image below.)

Chest radiographs in a premature infant with respi Chest radiographs in a premature infant with respiratory distress syndrome before and after surfactant treatment. Left: Initial radiograph shows poor lung expansion, air bronchogram, and reticular granular appearance. Right: Repeat chest radiograph obtained when the neonate is aged 3 hours and after surfactant therapy demonstrates marked improvement.

The radiologic findings of respiratory distress syndrome cannot be reliably differentiated from those of pneumonia, which is most commonly caused by group B beta-hemolytic streptococci. If the radiograph shows streaky opacities, the diagnosis of Ureaplasma or Mycoplasma pneumonia should be considered and confirmed by means of tracheal aspirate cultures grown in the appropriate medium.

Echocardiography

Echocardiographic evaluation is performed in selected infants to assist in diagnosing PDA and in determining the direction and degree of shunting on Doppler study. It is also useful in diagnosing pulmonary hypertension, assessing cardiac function, and excluding structural heart disease.

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Pulmonary Mechanics Testing

Although pulmonary mechanics testing (PMT) has primarily been used 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 premature newborn infants with respiratory distress syndrome.

Constant PMT may be helpful in preventing volutrauma due to alveolar and airway overdistension. Monitoring may also facilitate weaning the infant from the ventilator after surfactant therapy or in determining if the infant can be extubated. However, clinical studies of PMT to date have not proven its long-term outcome benefits in neonates with respiratory distress syndrome.

Infants with respiratory distress syndrome have substantially decreased lung compliance, with a range of 0.0005-0.0001 L/cm water. Therefore, for the same pressure gradient, the delivered tidal volume is reduced in premature infants with respiratory distress syndrome compared with healthy newborn infants.

Pulmonary compliance may considerably improve after surfactant administration. Hence, the patient's lung compliance and end-expiratory tidal volume should be monitored closely after surfactant therapy, and the peak inspiratory pressure should be adjusted accordingly.

The resistance (airway and tissues) may be normal or increased. The time constant and the corresponding pressure and volume equilibration are shortened. The anatomic dead space and the functional residual capacity are increased.

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