MECHANISMS FOR EPISODES OF HYPOXEMIA IN PRETERM INFANTS UNDERGOING MECHANICAL VENTILATION

Objective: To ascertain possible mechanisms implicated in the developm ent of transient episodes of hypoxemia (oxygen saturation <85%) freque ntly observed in preterm infants undergoing mechanical ventilation, ev en after the acute phase of respiratory failure has passed. Study desi gn: Tidal flow, airway and esophageal pressure, and oxygen saturation were continuously recorded in 10 infants (mean +/- SD, birth weight 73 3 +/- 149 gm, gestational age 25.5 +/- 2.2 weeks, age 26.3 +/- 11.9 da ys) who had repeated episodes of hypoxemia without any evident cause. Measurements of minute ventilation (V-E), inspiratory compliance (C-i) , and inspiratory resistance (R(i)) were compared before and during ep isodes of hypoxemia. Results: All episodes of hypoxemia were preceded by an active exhalation that produced a mean decrease in end-expirator y lung volume of 6.4 +/- 2.8 ml/kg. The reduction in lung volume was i mmediately followed by a sudden decrease in tidal flow and volume, des pite continuation of mechanical ventilation at the same rate and peak pressure. The resulting hypoventilation was associated with a drop in Ci to approximately one half and an increase in R(i) to more than doub le the baseline values. Approximately 30 seconds after the beginning o f hypoventilation, the arterial oxygen saturation reached a hypoxemic level (oxygen saturation <85%). Conclusion: Most hypoxemic episodes we re triggered by an expiratory effort that produced a large decrease in lung volume. This reduction in lung volume probably leads to closure of small airways and the development of intrapulmonary shunts, which w ould explain the rapid development of hypoxemia.