Critical pressures required for generation of forced expiratory wheezes

Flow limitation (FL) has recently been shown to be a necessary condition for the generation of forced expiratory wheezes (FEW) in normal subjects. The present study was designed to investigate whether it is also a sufficient condition. To do so we studied the effects of varying expiratory effort on generation of FEW. Six normal subjects exhaled with varying force into an orifice in line with a high-impedance suction pump. Esophageal (Pes), airway opening, and transpulmonary (Ptp) pressures were measured alongside flow rate, lung volume, and tracheal lung sounds. In each subject a certain critical degree of effort had to be attained before FEW were generated. This effort, measured as Pes at the onset of wheezes, varied among the subjects (range -11 to 45 cmH2O). Similarly, a minimal Ptp had to be reached for FEW to evolve (mean +/- SD -34 +/- 12 cmH2O, range -18 to -50 cmH2O). These critical Pes and Ptp values were significantly higher than those required for FL. It was concluded that, in addition to the requirement for FL, sufficient levels of effort and negative Ptp must exist before FEW can be generated. By analogy to experimental and theoretical results from studies on flow-induced oscillations in self-supporting collapsible tubes, it was further concluded that these pressures are required to induce flattening of the intrathoracic airways downstream from the choke point. It is this configurational change that causes air speed to become equal to or exceed the critical gas velocity needed to induce oscillations in soft-walled tubes.