Rigid external resistances cause effort dependent maximal expiratory and inspiratory flows.

A fixed orifice or a fixed upper airway obstruction (UAO) causes an expiratory and inspiratory plateau-shaped limitation on maximal flow-volume (MEFV, MIFV) curves and, according to the classic concept, a MEF50/MIF50 ratio of 0.9-1.1. However, since maximal expiratory static transrespiratory pressures (PEmax,stat) are clearly greater than the inspiratory ones (PImax,stat), the pressures applied during forced expiration also must be expected to be greater than inspiratory pressures; therefore, the MEF should be larger than the MIF because orifice flow is effort-dependent. We investigated this hypothesis in seven healthy, nonsmoking male volunteers (mean age +/- 1 SD: 34 +/- 10 yr, FVC: 5.9 +/- 1.0 L, PEmax,stat: 168 +/- 16 cm H2O, PImax,stat: 107 +/- 33 cm H2O). They performed MEFV curves and MIFV curves through four different added resistances placed in between the pneumotachograph and the mouth (the orifice diameters ranged between 7.8 mm and 2.8 mm). During these maneuvers dynamic mouth pressures were also measured (PE and PI). We found that the MEF50/MIF50 ratios were significantly increased (p < 0.05) from a control value of 1.1 +/- 0.4 up to 1.5 +/- 0.3 with the resistances. For each added resistance the PE/MEF ratios and (-)PI/MIF ratios were situated on a single line corresponding with the pressure-flow (P/V) characteristics of the resistance. We concluded that external resistances cause a MEF50/MIF50 ratio of clearly more than 1 and that this is determined by the PE/(-)PI ratio, which in healthy subjects is markedly larger than 1.