Effects ofanexternal resistance on maximumflow in chronic obstructive lungdisease: implications for recognition ofcoincident upperairway obstruction

To determine howthepresenceofgeneralised airflow limitation duetochronic obstructive lungdisease affects therecognition ofsimulated upperairway obstruction, astudy was carried outin12patients (mean(SD) age57(7) years) withchronic obstructive lung disease (FEV, % predicted 53(22), range21-70) and12matched control subjects. Patients andcontrol subjects performed maximal inspiratory andexpiratory flow-volume curvesina variable volumeplethys- mographwithandwithout upperairway obstruction simulated atthemouthwitha series of polythene washers ofinternal diameter 4,6,8,10, and12mm. Inpatients, asinnormal subjects, peak expiratory flow(PEF) andmaximuminspiratory flow at50%ofvital capacity (Vimax5O) weremore sensitive toupperairway obstruction than wereFEV,ormaximumexpiratory flowat50%VC (VEmax5o); butthereductions inallindices caused bysimulated upperairway obstruction were smaller inthepatients thaninthecontrols. Thefall inPEF(whether expressed inabsolute units orasa percentages) consequent on severe(4mm)upperairway obstruction became smaller withincreasing severity ofchronic obstructive lung disease. Thesubjects also produced flow-volume curveswithand without 6mm upperairway obstruction while breathing helium andoxygen(heliox). Inboth groups theeffects ofheliox on PEFandVimax_0 were increased whenupperairway obstruction was simulated. Itwas confirmed thatthefunctional recognition ofupperairway obstruction ismore difficult inpatients withchronic obstructive lung disease thaninnormalsubjects andthis difficulty increases withseverity ofdisease; anunusually large increase inPEForVimax5o while thepatient is breathing heliox should raise thesuspicion ofcoexisting upperairway obstruction, butsuch apattern