Flow‐pressure looping during plethysmography in wheezy infants

Marked looping of the expiratory portion of the flow‐pressure relationship was noted in some infants during measurements of airway resistance in a whole‐body plethysmograph while the respired air was maintained at body temperature and humidity. An investigation of 13 infants who had varying degrees of airway obstruction showed that there was a negative correlation (r = 0.72) between the severity of the looping and specific airway conductance (SG_aw). An even stronger correlation (r = 0.85) was found between the tangent of the angle of phase lag between flow and pressure (θ) and the forced expiratory time constant (t) obtained from the partial forced expiratory flow‐volume curve. Such a relationship would be predicted from a model in which the lung behaved as a simple electrical resistance‐capacitance network during expiration. It is suggested that the looping is the result of small airway closure during expiration in wheezy infants, with a consequent rise in resistance and prolongation of the time constant of the lung.