Mechanics of the Respiratory System during Passive Exhalation in Preterm Lambs

Abstract

ABSTRACT. Previous studies have shown large inhomogeneities in the distributions of ventilation and perfusion of newborn infants with hyaline membrane disease. The purpose of this study was to show that measurements of lung mechanics also show evidence of lung inhomogeneities and that a multiple compartment analysis of mechanics gives a more accurate representation of passive exhalation flow and volume than single valued mechanics. We studied 10 sedated preterm lambs (130 d gestation) weighing 2.2 f 0.3 kg at 4 h postnatal age. Passive exhalation lung mechanics of the respiratory system were measured by obstructing gas flow near end inhalation then, after pressures within the lung reached equilibrium, allowing the animals to exhale to the atmosphere. Airway pressure and flow signals were monitored by a computer then analyzed using single and multiple compartment analyses. Single compartment analysis of time constant (7) in s, respiratory system resistance (R) in cm H20/L/s and quasistatic compliance (C) in mL/cm H 2 0 yielded 7 = 0.16 f 0.07, R = 92 f 17, and C = 1.8 f 1.1 (mean f SD). Multiple compartment analysis yielded "fast compartment" 7, = 0.10 f 0.04, R1 = 90 + 28, and C1 = 1.1 f 0.5 and "slow compartment" 7 2 = 0.25 f 0.12, R2 = 503 + 288, and C2 = 0.7 + 0.6. All of the animals studied exhibited nonlinearity in their flow-volume plots. Calculated flow-volume plots were much more accurately portrayed by the twocompartment analysis than by single valued mechanics. Multiple compartment analysis of lung mechanics may provide useful insight into the pathophysiology of the preterm lab with hyaline membrane disease. (Pediafr Res 26: 425-428,1989) Abbreviations C, quasistatic compliance of the respiratory system WT, fraction of tidal volume ventilating each lung compartment HMD, hyaline membrane disease R, respiratory system resistance 7, passive exhalation time constant V, volume of gas in the lungs above functional residual capacity subscript 1 and 2 refer to the "fast" and "slow" compartments, respectively. PIP, peak inspiratory pressure PEEP, positive end expiratory pressure Several studies on infants with HMD suggest that gross inhomogeneities exist in their distributions of ventilation and perfusion. The elevations of nitrogen (1, 2), oxygen (1-3), and carbon dioxide (4, 5) gradients between alveolar gas and arterial blood suggest that ventilation-perfusion mismatching exists and that there are respiratory units with both lower and higher than normal ventilation-perfusion ratios. Additional studies using nitrogen washout suggest that these infants also have an uneven distribution of ventilation (6). Multiple compartment analyses of the results from these studies provide valuable information that influences the interpretations of the pulmonary pathophysiology of infants with HMD.After considering the strong evidence of lung inhomogeneities in infants with HMD, we speculated that lung mechanics measurements would also require multiple compartment analysis for accurate representation of flow and volume d...

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