Respiratory distress syndrome

Respiratory distress syndrome (RDS), also known as infant RDS and once knownas hyaline membrane disease, is an acute lung disease present at birth, mostfrequently affecting premature babies. Abnormal layers of tissue called hyaline membranes keep oxygen breathed into the lungs from passing into the bloodstream. The lungs are said to be "airless." Without treatment, the infant willdie within a few days after birth, but if oxygen can be provided, and the infant receives modern treatment in a neonatal intensive care unit, complete recovery with no after-effects can be expected.

If a newborn infant is to breathe properly, the small air sacs (alveoli) at the ends of the breathing tubes must remain open so that oxygen can get into the tiny blood vessels that surround the alveoli. Normally, in the last monthsof pregnancy, cells in the alveoli produce a substance called surfactant, which allows these sacs to expand at the moment of birth so the infant can breathe normally. Surfactant is produced starting at about 34 weeks of pregnancyand, by the time the fetal lungs mature at 37 weeks, a normal amount is present.

If an infant is born prematurely, enough surfactant might not have formed, causing the lungs to collapse and making it very difficult for the baby to breathe. Sometimes, a layer of fibrous tissue, called a hyaline membrane, forms in the air sacs, making it even more difficult for oxygen to be absorbed intothe blood vessels.

RDS nearly always occurs following premature births, and the more premature,the greater the chance that the infant will develop RDS. RDS is also seen insome infants whose mothers have diabetes mellitus; however, it is less likelyto occur in the presence of certain conditions which themselves are harmfulto the infant, such as abnormally slow growth of the fetus, high blood pressure in the mother causing a condition caused toxemia, and early rupture of thebirth membranes.

Labored breathing (the "respiratory distress" of RDS) may begin as soon as the infant is born, or within a few hours. Breathing becomes very rapid, the nostrils flare, and the infant grunts with each breath. The ribs, which are very flexible in newborns, move inward each time a breath is taken. Soon, the muscles that move the ribs and diaphragm which draw air into the lungs become fatigued. When the blood oxygen level drops severely, the infant's skin turnsbluish in color. Tiny, very premature infants may not even have signs of trouble breathing because their lungs may be so stiff they cannot even begin to breathe.

There are two major complications of RDS. One is pneumothorax, which means "air in the chest." When the infant's own efforts to breathe, or a breathing machine, applies pressure on the lungs in an attempt to expand them, a lung mayrupture, causing air to leak into the chest cavity. This air causes the lungto collapse further, making breathing even harder and interfering with bloodflow in the lung arteries. The blood pressure can drop suddenly, cutting offthe blood supply to the brain. Pneumothorax is an emergency that must be treated right away. Air may be removed from the chest using a needle and syringe. A tube is then inserted into the lung cavity and suction applied to removethe air. The other complication, intraventricular hemorrhage, is bleeding into the cavities (ventricles) of the brain, which may be fatal.

When a premature infant has obvious trouble breathing, RDS is a definite possibility. If premature birth is expected, or some condition calls for immediate delivery of the baby, the level of surfactant in the amniotic fluid which surrounds the baby in the uterus, will indicate how well the lungs have matured. If little surfactant is found in an amniotic fluid sample in a test calledamniocentesis, which is taken by inserting a needle in the uterus and withdrawing fluid, there is a definite risk of RDS. Often this test is done at regular intervals so that the infant can be delivered as soon as the test indicates the lungs are mature. If the membranes have ruptured, surfactant can easily be measured in a sample of vaginal fluid.

The other major diagnostic test is a chest x ray. Collapsed lung tissue has aspecific appearance; the more collapsed lung tissue, the more severe the RDS. An x ray can also determine if pneumothorax has occurred. Also, the oxygenlevel in the blood can be measured by taking an arterial blood gas analysis,or, more easily, by using a device called an oximeter, which is clipped to anearlobe. Pneumothorax may have occurred if the infant suddenly becomes worsewhile on mechanical ventilation--a machine that takes over the work of the lungs and delivers air under pressure.

If only a mild degree of RDS is present at birth, placing the infant in an oxygen hood may be sufficient. It is important, however, not to administer toomuch oxygen, as this may damage the retina and cause loss of vision. By usingan oximeter to keep track of the blood oxygen level, repeated artery punctures, or pricking the heel to draw blood, RDS can be avoided. In more severe cases, a drug very like natural surfactant (Exosurf Neonatal or Survanta), canbe dripped into the lungs through a fine tube (endotracheal tube) placed in the infant's windpipe (trachea). Typically the infant will be able to breathemore easily within a few days. The drug is continued until the infant startsproducing its own surfactant. However, there is a risk of bleeding into the lungs from surfactant treatment; about 10% of the smallest infants are affected by this.

Infants with severe RDS may require treatment with a ventilator. In tiny infants, ventilation through a tracheal tube is an emergency procedure. Assistedventilation must be closely supervised, as too much pressure can cause a collapsed lung. A gentler way to assist breathing, continuous positive airway pressure, or CPAP, delivers an oxygen mixture through nasal prongs or a tube placed through the nose rather than an endotracheal tube. CPAP may be tried before resorting to a ventilator, or after an infant placed on a ventilator begins to improve. Drugs that stimulate breathing may speed the recovery process.

If an infant born with RDS is not promptly treated, lack of an adequate oxygen supply will damage the body's organs and eventually cause them to stop functioning altogether. Death is the result. The central nervous system in particular--made up of the brain and spinal cord--is very dependent on a steady oxygen supply and is one of the first organ systems to feel the effects of RDS.On the other hand, if the infant's breathing is supported until the lungs mature and make their own surfactant, complete recovery within three to five days is the rule.

The best way of preventing RDS is to delay delivery until the fetal lungs have matured and are producing enough surfactant. If delivery cannot be delayed,the mother may be given a steroid hormone similar to a natural substance produced in the body. This steroid crosses the barrier of the placenta and helpsthe fetal lungs to produce surfactant. The steroid should be given at least24 hours before the expected time of delivery. If the infant does develop RDS, the risk of bleeding into the brain will be much less if the mother has been given a dose of steroid.

If a very premature infant is born without symptoms of RDS, it may still be wise to deliver surfactant to its lungs. This may prevent RDS, or make it lesssevere if it does develop. An alternative is to wait until the first symptoms of RDS appear and then immediately give surfactant. Pneumothorax may be prevented by frequently checking the blood oxygen content, and limiting oxygen treatment under pressure to the minimum needed.

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