Pressure-flow curves in infancy

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Abstract
Calibrated pressure‐flow (P‐V) curves were plotted by computer using data from both the plethysmographic method for measuring airway resistance and the esophageal balloon technique for measuring lung resistance. P‐V curves from 100 sick, healthy, and convalescent infants (age range 2 days to 19 months, weight range 0.9–10.4 kg) were classified into five distinctive types according to shape and direction of looping. Two of these patterns, one with a virtually closed, the other with a narrow figure‐of‐eight loop, reflected the normal physiologic changes in airway caliber that may occur during tidal breathing. The remaining three patterns, with far more marked changes in resistance, were associated with particular pathophysiologic mechanisms of airway obstruction. A wide figure‐of‐eight configuration, in which the expiratory loop rotated clockwise with marked flow limitation toward end expiration, was found for infants with chronic lung disease. By contrast, a rise in initial expiratory resistance due to dynamic glottic narrowing, with an anticlockwise rotated expiratory P‐V loop, occurred in infants with reduced or unstable lung volumes. A clockwise inspiratory loop was observed only for infants intubated during the neonatal period, many of whom had clinical evidence of extrathoracic airway obstruction. Inspection and analysis of P‐V curves provides more information about the state of the airways than does a single numerical expression of resistance. However, since normal patterns of P‐V curves are not restricted to infants with healthy airways, a combined qualitative and quantitative approach to these measurements is recommended.