Am. J. Respir. Crit. Care Med., Vol 152, No. 5, Nov 1995, 1709-1712.
Rigid external resistances cause effort dependent maximal expiratory and inspiratory flows
CF Melissant, JW Lammers and M Demedts
Department of Pneumology, University Hospital Gasthuisberg, Leuven, Belgium.
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.
