Lung Function Changes and Exercise-Induced Ventilatory Responses to External Resistive Loads in Normal Subjects

Abstract

The aim of this study was (1) to assess the value of common lung function variables in the follow-up of patients with obstructive lesions of the larynx and trachea and (2) to study the respiratory response to progressive upper airway stenosis at rest and during exercise. Lung function tests, including vital capacity (VC), residual volume (RV), total body plethysmographic resistance (RT), specific body plethysmographic resistance (Rs), total body plethysmographic resistance at low inspiratory/expiratory flows (Rlo), forced expiratory volume in 1 s (FEV1), peak expiratory flow rate (PEF), peak inspiratory flow rate (PIF), maximum expiratory flow at 50% VC(MEF50) and maximum inspiratory flow at 50% VC(MIF50) were performed in 14 normal subjects with added external resistive loads (inner diameter of circular orifice: 15, 10, 8, 6 mm; equivalent to surface areas of 177, 79, 50 and 28 mm2, respectively), applied in random order. The relative sensitivity of each variable to detect a stepwise decrease in orifice size by evoking 'relevant' obstructive responses was calculated. In addition, ventilatory and gaz exchange responses to loaded breathing (10-, 8- and 6-mm orifices, randomly inserted) during steady-state bicycle exercise (80 and 160 W) were assessed in a subgroup of 10 subjects. The gradual increase of external load did not influence static lung volumes (VC, RV). MEF50/MIF50 (1.63 +/- 1.03 at baseline) remained essentially unchanged, consistent with the rigid nature of the obstructive device. PEF was superior in reproducing the transition from unloaded over mild (15 mm) and moderate (10 mm) to high grade (8 mm) central obstruction. Except for mild loads, PIF was equal in sensitivity to PEF. At high grade (8 mm) and severe loads (6 mm), no difference in sensitivity could be established between RT, Rs, FEV1, PIF and PEF. However, they were all superior to Rlo. Compared to unloaded breathing, ventilation across the 10- and 8-mm orifices at both rest and exercise produced a continuous significant increase in oxygen uptake (VO2). Respiratory rate (RR) fell, and a simultaneous enlargement of tidal volume (VT) was observed to maintain or even enhance minute ventilation (VE). However, ventilation across the 6 mm orifice was associated with a substantial decline in VO2, VCO2 and VE relative to the preceding load (8 mm) It is suggested that the experimental use of inspiratory and expiratory extrinsic loads can mimic rigid obstructive lesions of the larynx and trachea. Among all conventional lung function values, PEF and, to a certain degree, PIF, seem to be the best suitable follow-up parameters to assess airway mechanics before and after surgical/endoscopic procedures. Upper airway stenosis involving surface areas of no more than 50 mm2 can be overcome using adequate respiratory compensation. But any additional narrowing below this limit will result in hypoventilation, inappropriate oxygen uptake and retention of CO2. Thus, experimental evidence indicates that laryngotracheal obstruction within a critical range below 50 mm2 surface area (diameter of circular orifice < 8 mm) compromises respiratory efforts enough to be of clinical importance.