Specific compliance: is it truly independent of lung volume?

Abstract Specific pulmonary compliance is a widely used metric in neonatal mechanical ventilation, capturing the intrinsic ability of the lung to expand with pressure normalized by the functional residual capacity of the lung. Previous studies have found specific compliance to be similar for both neonates and adults. However, it is unclear what this implies about the structure and growth of the lung. To model the factors affecting specific compliance, a simple model is developed separating respiratory compliance into material-dependent and volume-dependent components. It provides reasonable estimates of compliance for experimental clinical data. The model demonstrates specific compliance is dependent on the alveolar tissue elastic modulus, Poisson’s ratio, septal thickness to alveolar radius ratio, and surface tension effects. The model also indicates specific compliance may not be entirely independent to lung volume as the lung grows, due to surface tension effects varying with alveolar diameter, and the possibility of a varying alveolar wall thickness to diameter ratio. To experimentally evaluate trends in specific compliance, data from 9 infants and 8 adult mechanical ventilation patients was recorded. Median compliance values were 0.51 mL/cmH2O/kg for adult patients and 0.78 mL/cmH2O/kg for neonatal patients (p = 0.098). This result demonstrates mechanically ventilated neonates and adults have statistically similar values of specific compliance, in agreement with prior studies. Treating specific compliance as a constant and using a least-squares fit, the trend of neonatal compliance increasing with body weight can be used to predict typical ranges of adult compliance values, indicating the non-linearity of compliance varying with volume is likely not significant in reality, supporting the use of specific compliance as a measure of the underlying tissue distensibility. However, the variation in values for each cohort suggest that specific compliance, like lung elastance, can vary significantly, and while overall trends hold, central values are not necessarily indicative. Overall, the model provides a unique analysis and insight using a simplified, mechanics relevant model, which is could be generalized to other studies.