Fig. 22.8A , is not sufficient, and additional energy is needed for exhalation. With time or disease progression, these respiratory muscles can fatigue and result in respiratory failure. The work of breathing is also increased when deeper breaths are taken (an increase in tidal volume requires more elastic work to overcome) and when the respiratory rate increases (an increase in minute •Fig. 22.8 Mechanicalworkdoneduringarespiratorycycleinanormallung(A),alungwithreducedcompliance(B),andalungwithincreasedairwayresistance(C).Breakdownofthetrapezoidalareasenablesonetoappreciatetheindividualaspectsofthemechanicalworkload,whichincludethefollowing:OABCD,worknecessarytoovercomeelasticresistance;AECF,worknecessarytoovercomenonelasticresistance;AECB,worknecessarytoovercomenonelasticresistanceduringinspiration;ABCF,worknecessarytoovercomenonelasticresistanceduringexhalation(representsstoredelasticenergyfrominspiration).
ventilation requires more flow resistance force to overcome) (
Fig. 22.9 ). Normal individuals and individuals with lung disease adopt respiratory patterns that minimize the work of breathing. For this reason, individuals with pulmonary fibrosis (increased elastic work) breathe more shallowly and rapidly, and those with obstructive lung disease (normal elastic work but increased resistive work) breathe more slowly and deeply.
60 50 40 Total work Elastic work Nonelastic work 20 10 30 10 20 30 40 Mechanical work (kg-m/min) Respiratory rate (no./min)
Chronic obstructive pulmonary disease (COPD)isageneraltermthatincludesdiseasessuchasemphysemaandchronicbronchitis.COPDmostcommonlyoccursinindividualswhosmoke,inwhompathologicchangesinthelungconsistentwithbothemphysemaandchronicbronchitiscancoexist.ForindividualswithCOPDinwhomemphysemaisamajorcomponent,theelastictissueinthealveolarandcapillarywallsisprogressivelydestroyed,whichresultsinincreasedlungcomplianceanddecreasedelasticrecoil.Thedecreaseinelasticrecoilresultsinmovementoftheequalpressurepointtowardthealveolusandprematureairwayclosure.ThisproducesairtrappingandincreasesinRV,FRC,andTLC.Airwayresistanceisalsoincreased.Theseincreasesinlungvolumesincreasetheworkofbreathingbystretchingtherespiratorymusclesanddecreasingtheirefficiency.
Inchronicbronchitis,accumulationofmucusandairwayinflammationcausetheequalpressurepointtomovetowardthealveolus,whichleadstoprematureairwayclosureandincreasesinRV,FRC,andTLC.Airwayresistanceandtheworkofbreathingareincreased,butlungcomplianceisnormal.
Inrestrictivelungdiseasessuchaspulmonaryfibrosis,lungcomplianceisdecreased.Lungvolumesaredecreased,butflowratesarereasonablynormal.Someofthechangesinpulmonaryfunctionvaluesinobstructiveandrestrictivepulmonarydiseasesareshownin
Table22.1
Inthethirdtrimesterofpregnancy,theenlargeduterusincreasesintraabdominalpressureandrestrictsmovementofthediaphragm.TheFRC,asaresult,decreases.Thischangeinlungvolumeresultsindecreasedlungcomplianceandincreasedairwayresistanceinotherwisehealthywomen.
•Fig. 22.9 Effectoftherespiratoryrateontheelastic,nonelastic,andtotalmechanicalworkofbreathingatagivenlevelofalveolarventilation.Individualstendtoadopttherespiratoryrateatwhichthetotalworkofbreathingisminimal(arrow) forthosewithoutlungdisease.
1.