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Figure 1. When the alveoli decrease in size during expiration, pressure within the airways increases. Surfactant, which prevents alveolar collapse, is deficient in the preterm lung. Continuous positive airway pressure (CPAP) increases basal airway pressure, holding the alveoli open during expiration and preventing alveolar collapse.

...that improved ventilatory methods have markedly increased survival of preterm infants? 

Supplemental oxygen delivered by mechanical ventilation is often required for premature infants because the preterm lung exhibits thick saccular walls that limit diffusion of oxygen and carbon dioxide.  Current methods for maintaining adequate respiration in infants stem from early methods of resuscitation.  In the mid-1700s, the Royal Humane Society recommended mouth-to-mouth resuscitation for infants following the demonstration of this technique by William Smellie [1, 5].  However, the obstetrician William Hunter considered this to be a method used by the vulgar, resulting in the recommendation of bellows [5]. In 1827, d’Etiolles showed that air blown into the lung through a canula caused pneumothoraces, leading to the abandonment of positive pressure ventilation for a number of years [2, 5].   Thirty years later, Wollitz presented data to the Academie de Medicine suggesting that air should be at atmospheric pressure when it enters the lung in order to avoid pneumothoraces [2].  He then demonstrated the damaging effects of high pressure on both isolated lungs and animals using early positive pressure ventilators.  These developments led to the use of more gentle positive pressure devices that were modified by Braun for use in newborns in 1889 [5, 9, 10].

In addition to changes in techniques used for resuscitation and ventilation, the supportive treatment of preterm infants was also evolving.  In 1914, Julius Hess created a heating bed to keep infants warm, and this bed was used in the first Neonatal Intensive Care Unit (NICU) [8].   The heating bed was later modified by Hess to allow for the introduction of oxygen, but Haldane cautioned physicians to “make every effort to avert the effects of want of oxygen,” recognizing the benefits of keeping inspired O2 as low as possible while supplying sufficient O2 for gas exchange [8].  In 1928, Drinker and Shaw successfully ventilated an adult patient using alternating positive and negative pressures, but this method was ineffective in maintaining optimal blood gases in infants [2, 3, 4, 6, 10]. As more gentle methods of ventilation were introduced, infant survival improved. However, with increased survival came altered pathological features in the preterm lung.

While improved short-term survival of infants with respiratory distress came with the changes in treatment strategies, many neonates who survived for the first few days after birth later died of respiratory complications.  A glass-like opacity seen in the histological analysis of lungs of preterm infants was termed “hyaline membranes” by Miller and Hamilton, and they proposed that this was a result of injury to the lung [7].   Clinical evidence showed damage related to the use of all forms of mechanical ventilation, but this support was required for infant survival.  One complication in the preterm infant was alveolar collapse during expiration due to the lack of adequate surfactant production.  This problem of alveolar collapse was addressed in 1971, when a clinical trial demonstrated an increase in arterial oxygen tension when infants with respiratory distress were ventilated with continuous positive airway pressure (Figure 1) [12].   The success of these ventilation techniques in combination with other supportive therapies led to survival of infants who were born at even earlier gestational ages [11].   Today, infants who have respiratory distress at birth and require mechanical ventilation have decreased lung injury and increased survival due to advances made in ventilatory methods.  


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Author: Ashley DeCoux
Chief editor: Donna Cioffi, Ph.D., Nov., 2011


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