Sequential Effects of Acute Meconium Obstruction on Pulmonary Function

Abstract

The relationship between pulmonary function and the migration of meconium to distal airways was determined in 10 rabbits (mean weight 2.6 kg) after insufflation of a meconium-saline mixture (1--2 ml/kg). Animals were anesthetized, cannulated, intubated, and mechanically ventilated with 100% oxygen. Lung mechanical dysfunction was most severe during the early phase of meconium migration, 15 min postinsufflation. Substantial increases in inspiratory lung resistance (RI) and expiratory lung resistance (RE) suggest that the site of obstruction at 15 min was the large airways. A decrease in dynamic lung compliance with unchanged static compliance characterizes the obstruction as partial. At 15 min and throughout the migration process, RE was greater than RI, demonstrating a check-valve effect. This phenomenon was substantiated by an increased functional residual capacity (FRC) in all rabbits, presumably due to gas trapping. Secondary to these changes, marked hypoxemia, hypercapnea, and acidosis developed in spite of assisted ventilation with 100% oxygen. At 60 and 120 min postinsufflation, both RI and RE decreased as compared to 15 min. This suggests that the predominant site of obstruction shifted to medium and small airways concomitant with the migration of meconium. Widespread and uneven distribution of meconium still produced significant frequency dependence of lung compliance. Static compliance remained unchanged, indicating that meconium does not affect surface-active or tissue properties of the lung within 120 min postinsufflation. These data suggest that effective respiratory management after meconium aspiration is dependent on the degree of meconium migration, as reflected by pulmonary mechanics.

Pentoxifylline Reduces Regional Inflammatory and Ventilatory Disturbances in Meconium-Exposed Piglet Lungs

“…Initial deterioration in neonatal lung mechanics after aspiration of meconium is supposed to be mainly obstructive, caused by thick or particulate meconium blocking the airways partially or completely (3,6). Experimental investigations have shown that dynamic lung compliance drops rapidly during the first hour after meconium insufflation and remains low for the first days after the insult (3,6).…”

“…Perinatal aspiration of meconium frequently produces respiratory failure with hypoxemia, hypercapnia, and acidosis in the newborn infant and may be complicated by air leaks, pulmonary hypertension, hypoxic organ damage, and long-term pulmonary sequelae (1,2). The pathophysiology of the meconium aspiration syndrome (MAS) is complex and includes initial mechanical obstruction of the airways, ventilation/perfusion mismatch, surfactant inactivation, and progressively developing inflammatory lung injury (1)(2)(3)(4)(5)(6). Aspirated meconium may directly damage the alveolar-capillary membrane and may also augment pulmonary injury through activation of alveolar macrophages, inflammatory cytokine production, and accumulation of polymorphonuclear neutrophils (PMNs) (1,2,6,7).…”

Resuscitation of the Meconium-Stained Infant and Prevention of Meconium Aspiration Syndrome

“…In a similar model, it has been shown that mechanical dysfunction in lungs is most severe during the early phase of MAS, with diminished dynamic and specific lung compliance, but unchanged static lung compliance, suggestive of partial random obstruction of large airways. 32 This study, however, is limited in its duration, and effects occurring more than 2 hours following meconium aspiration were not evaluated. Similar findings were observed by Yeh and colleagues 33 who evaluated pulmonary function tests in the first 3 days of life in neonates with mild MAS not requiring mechanical ventilation.…”

Constrictor Action of Substances in the Pulmonary Artery in Newborn with the Amnional Fluid Aspiration Syndrome (MAS)

“…It is supposed that in the early stages presence of the amniotic fluid can be associated with an increase of pulmonary resistance, dysfunction of surfactant, decrease of dynamic compliance of the lungs, hypoxemia with hypercapnia [14,15].…”