Effects of an external resistance on maximum flow in chronic obstructive lung disease: implications for recognition of coincident upper airway obstruction.

Abstract

To determine how the presence of generalised airflow limitation due to chronic obstructive lung disease affects the recognition of simulated upper airway obstruction, a study was carried out in 12 patients (mean (SD) age 57 (7) years) with chronic obstructive lung disease (FEV, % predicted 53 (22), range 21-70) and 12 matched control subjects. Patients and control subjects performed maximal inspiratory and expiratory flow-volume curves in a variable volume plethysmograph with and without upper airway obstruction simulated at the mouth with a series of polythene washers of internal diameter 4, 6, 8, 10, and 12 mm. In patients, as in normal subjects, peak expiratory flow (PEF) and maximum inspiratory flow at 50% of vital capacity (Vimax5O) were more sensitive to upper airway obstruction than were FEV, or maximum expiratory flow at 50% VC (VEmax5o); but the reductions in all indices caused by simulated upper airway obstruction were smaller in the patients than in the controls. The fall in PEF (whether expressed in absolute units or as a percentages) consequent on severe (4 mm) upper airway obstruction became smaller with increasing severity ofchronic obstructive lung disease. The subjects also produced flow-volume curves with and without 6 mm upper airway obstruction while breathing helium and oxygen (heliox). In both groups the effects of heliox on PEF and Vimax_0 were increased when upper airway obstruction was simulated. It was confirmed that the functional recognition of upper airway obstruction is more difficult in patients with chronic obstructive lung disease than in normal subjects and this difficulty increases with severity of disease; an unusually large increase in PEF or Vimax5o while the patient is breathing heliox should raise the suspicion ofcoexisting upper airway obstruction, but such a pattern is not specific.

Changes in lung function and tidal airflow patterns after increasing extrathoracic airway resistance

“…It is known how dynamic lung function, as reflected by maximal flow volume loops, changes with fixed or variable narrowing of extrathoracic airways. [13][14][15][16][17] However, the effect of increased extrathoracic resistance on the indices of tidal flow/time patterns has not yet been described. In this observational study we have added a single, fixed, large resistance to the expiratory line.…”

“…Obstruction during expiration alone is rare but does occur in vocal cord dysfunction 12 and in pursed lip breathing. It is known how dynamic lung function, as reflected by maximal flow volume loops, changes with fixed or variable narrowing of extrathoracic airways 13–17 . However, the effect of increased extrathoracic resistance on the indices of tidal flow/time patterns has not yet been described.…”

Effect of breathing circuit resistance on the measurement of ventilatory function

“…Robertson et al 4 studied the effects of an external resistance on ventilatory function in normal controls and patients with COPD. They also found that PEF was the most sensitive index to the presence of an external resistance when the internal diameter of an resistor was decreased to 12 mm in the control group and 10 mm in the COPD group, with significant reductions whether expressed in absolute units or as a percentage change.…”

“…Zwi et al 6 found that the maximum voluntary ventilation fell when artificial flow restrictors were inserted into the breathing circuit, and Robertson et al 4 reported similar reductions in peak expiratory flow (PEF) and mid expiratory flow (MEF 50% ), whereas the forced expiratory volume in one second (FEV 1 ) was little affected unless the added restrictor had an orifice of less than about 8 mm.…”