Infant Respiratory Distress Syndrome; Hyaline membrane disease.
The respiratory distress syndrome is one of the major clinical problems affecting premature babies.
Incidence - RDS occurs primarily in the immature lung.
(1) 60 % of cases occur in infants born at less than 28 weeks’ gestation
(2) 5 % of cases occur in infants born at less than 37 weeks’ gestation.
Although the exact pathogenesis of the syndrome is not clear, the evidence indicates that it is related to or caused by immaturity of the lung and inadequate release or storage of surfactant by type II pneumocytes.
At term the fetal lungs are anatomically mature and functionally prepared to adapt for the transition from an intrauterine to extrauterine environment.
The alveoli are fully formed by the 25th week, and by the 31st week the type II pneumocytes already produce adequate amounts of surfactant, which still differs from that of the term newborn.
The bronchial and pulmonary circulations are characterized in utero by high pressure and low flow.
This situation leads to a high pulmonary resistance, which exceeds the systemic resistance.
Most of the blood circulates through the placenta, and only 5% to 10% perfuses the lungs.
Neuromuscular control of respiration is effectively established long before birth.
External compression of the thorax during passage through the vaginal canal ejects some of the amniotic fluid from the lungs and also leads to subsequent recoil of the chest wall.
These actions facilitate the first active inspiratory movement and the entry of air into the lungs, which inflates the alveoli.
The amniotic fluid is exhaled and resorbed, leaving the alveoli coated with surfactant.
The sequence of events decreases the external compression of the capillaries in the alveolar walls, and the pulmonary blood flow suddenly rises.
Increased alveolar oxygen tension further expands the precapillary vascular space through dilatation of arteries and arterioles.
Decreased vascular pressure in the entire pulmonary circulation, accompanied by an enormous influx of blood from the right ventricle, further facilitates the absorption of alveolar fluid and results in increased lymph flow and increased flow of oxygenated blood into the left atrium.
These events, followed in early infancy by the closure of the ductus arteriosus, lead to the transformation of the fetal high-resistance, low-flow pulmonary circulation into the adult low-resistance, high-flow circulation.
Disturbances in lung maturation and in the various transitions from fetal to adult pulmonary circulation, mechanical factors, and disturbances in the neural control of respiration all cause respiratory distress in newborn.
Among the identifiable causes are incidents affecting the respiratory center in the central nervous system.
For example, oversedation of the mother during the delivery may affect the fetal brain.
Traumatic brain injury of the fetus at birth, with bleeding into or ischemic necrosis of the respiratory centers, prevents normal respiration.
Asphyxia may be due to mechanical factors, birth trauma, or umbilical cord strangulation.
Metabolic disorders in the mother- for instance, diabetes mellitus-may impair respiration.
Blockage of the air passages due to particulate matter, blood clots, or aspirated meconium from the amniotic fluid can also cause respiratory distress.
However, more common than all other forms of respiratory distress is the idiopathic syndrome, presumed to be due to functional and anatomical immaturity of the fetal lungs.
The idiopathic respiratory distress syndrome of neonates is usually a disease of preterm, appropriate for gestational age (AGA) babies.
Most infants appear normal and have a good Apgar score.
However, some were born in protracted labour, showed intrapartum asphyxia, and needed resuscitation.
Typically, increased respiratory effort is the first symptom and is easily recognizable 30 to 60 minutes after birth.
The infants show forceful intercostals retraction and use accessory neck muscles.
A loud expiratory grunt is produced by the forceful passage of air through the partially closed glottis.
Partial closure of the glottis represents a Valsalva maneuver to maintain the positive end-respiratory pressure and keep the alveoli open.
However, as the compliance of the lungs diminishes, this maneuver becomes less efficient and the baby cannot compensate for the respiratory insufficiency by closing the glottis.
To meet for the demand for oxygen, the respiratory center increases the number of respirations to more than 100 per minute, and the accessory thoracic respiratory muscles are used to maintain the terminal airways open by forcing the expansion of lungs through negative intrathoracic pressure.
A characteristic seesaw respiration reflects the diminished compliance of the lungs , which forces the protrusion of the diaphragm into the abdominal cavity and, thus, the bulging of the abdomen with each inspiration.
Finally, the sternum and the anterior portion of the ribs collapse under the negative intrathoracic pressure.
The skin is pale because of peripheral vasoconstriction, but the internal organs are congested with unoxygenated blood.
Edema, most prominent on the face, palms, and soles, becomes generalized (anasarca).
The chest radiograph shows a characteristic “ground glass” granularity, with prominent bronchi extending into pulmonary periphery.
In terminal stages the fluid-filled alveoli contribute to the complete “whiteout” of the chest.
The infant becomes progressively obtunded and flaccid and experiences long periods of apnea interspersed with periods of irregular breathing.
Many infants are saved by intensive care and assisted ventilation, but the mortality is still high.
Type II pneumocytes need 3 to 4 days to become fully functional.
If the child survives these first few days, it often recovers. Recovery, unless there are major complications, is usually quite satisfactory.
After protracted respiratory distress and prolonged treatment with a respirator, a few babies develop chronic pulmonary disease.
A basic defect causing the idiopathic respiratory distress syndrome in preterm babies is immaturity of the lungs, particularly type II penumocytes.
Qualitatively and quantitatively, fetal surfactant is less efficient than adult surfactant in lowering the alveolar surface tension and keeping the alveoli open.
Because lung compliance is low, the critical negative pressure needed to allow influx of air into the lungs cannot be attained.
The collapse of alveoli (atelectasis) not adequately coated with surfactant reduces the pulmonary surface, allowing exchange of gases only through the walls of alveolar ducts and terminal bronchioles- structures that are not suitable for that purpose.
Anoxia and hypercapnia cause acidosis, leading to peripheral vasodilatation and pulmonary vasoconstriction.
This situation, in turn, leads to the reestablishment of a partial fetal circulatory pattern.
Right-to-left shunting of unoxygenated blood through the ductus arteriosus and foramen ovale further contributes to hypoperfusion of the lungs, jeopardizing the respiratory oxygen supply even more.
Hypoxia adversely affects pulmonary cells, and necrosis of endothelial, alveolar, and bronchial cells takes place.
Vascular disruption causes transudation of plasma into the alveolar spaces and layering of fibrin (”hyaline membranes”) along the surface of alveolar ducts and respiratory bronchioles partially denuded of their normal cell lining.
This in turn further impedes the passage of oxygen from the alveolar spaces across the respiratory surface into the pulmonary vasculature.
Moreover, extravasation of blood into the respiratory passages, combined with the collapse of the alveoli, further contributes to the consolidation of the lungs.
In terminal stages air is found only in bronchi and dilated bronchioles, and the rest of the lung is consolidated and airless.
Within these spaces cellular debris, proteinaceous edema fluid and some red blood cells accumulate.
The lining of the alveolar ducts is covered with fibrin-rich hyaline membranes. Image Link
The walls of the collapsed alveoli are thick, the capillaries are congested, and the lymphatics are filled with proteinaceous material.
Outcome the patient:
The outcome of the idiopathic respiratory distress syndrome of the neonate depends on the severity of the disease, the gestational age of the infant at birth, and the presence of complications or aggravating conditions.
The overall mortality is still about 30%, and in infants born before 30 weeks of pregnancy it is over 50%.
Aggravating factors: Factors such as maternal diabetes, anoxia during delivery, or extensive blood loss, also adversely influence the outcome.
Rapid development of profound respiratory insufficiency unresponsive to standard treatment is a bad sign and is associated with high mortality.
Oxygen therapy may alleviate the symptoms in mild cases, but in some serious cases respiratory distress persists, cyanosis increases and the infant becomes flaccid, unresponsive and apnoetic. Such cases require intensive care to correct acidosis, sustain the failing circulation, and assist the respiration.
Surfactant replacement is often administered while ventilatory assistance is provided.
In uncomplicated cases, recovery begins in 3 to 4 days.