Lung resistance (RL) and airway resistance (Raw) were measured in 33 healthy and sick infants. Using strict criteria for quality control, calibrated pressure-flow (P-V) curves were produced with the aid of a computer so that resistance could be analyzed at several points over the breathing cycle. In a subgroup of 11 infants from whom simultaneous measurements were obtained, values of RL and Raw were comparable, suggesting that the tissue component may make only a minimal contribution to lung resistance in infants. For both RL and Raw, the P-V relationship remained linear up to 50% maximum inspiratory flow in all infants. At other points of the respiratory cycle, there were marked changes in resistance, values calculated between points of mid-isovolume or peak pressure being significantly higher than those measured over the linear portion of inspiratory flow. While no single value can accurately reflect the changes in respiratory resistance that occur during the breathing cycle, the authors suggest that for purposes of comparison, RL or Raw should be measured as the slope of the inspiratory loop from the origin up to 50% of maximum inspiratory flow. From a calibrated tidal pressure flow curve values of resistance over other portions of the breathing cycle can easily be determined.

Airway Resistance↗

PubMedDate not supplied

Pressure-flow curves in infancy.

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.

Airway Resistance↗