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Fall inairway pressure and location of equal pressure point are unchanged, butbeyond equal pressure point, intrathoracic airways will be compressed toa greater degree by higher pleural pressure. Once maximal airflow isachieved, further increases in pleural pressure produce proportionalincreases in resistance of segment downstream from equal pressure point,so rate of airflow does not change.Figure 14.7 Expiratory Flow–Volume Relationship The expiratory flow–volume curve generatedduring a forced vital capacity maneuver (solid line in graph) is characterized by a peak expiratory flow rate(PEFR) at the top of the curve and a downward slope during the remainder of expiration (A).
The maximalexpiratory flow rate at various lung volumes along this downward slope is effort-independent, due to dynamiccompression of airways (B). When effort is reduced (dotted line in graph), PEFR is lower, but note the overlapin the downward slope of the two lines, consistent with the effort-independence of maximal expiratory flowrate.expiratory volume in the first second of a vital capacity maneuver) and the FEV1-to-FVC ratio is less than the normal valueof 75% (although FEV1 is reduced, FVC is only slightly lowerthan normal).
In contrast, in restrictive diseases such as interstitial fibrosis, thickening of alveolar walls results in decreasedlung compliance. Lung volumes are reduced as a result (Fig.14.9). Although FEV1 is reduced, the FEV1-to-FVC ratio isusually normal or elevated, because FVC is also diminished inrestrictive disease. Various pulmonary function tests andtypical findings in obstructive and restrictive disease are summarized in Figure 14.10.Because of dynamic compression of airways, maximumexpiratory flow is effort-independent.
For any given lungvolume, the associated maximum expiratory flow cannot beexceeded by increasing expiratory effort. The explanation forthis phenomenon is that increased expiratory effort physicallycompresses airways as well as alveoli, increasing resistance andlimiting airflow.The Mechanics of Breathing1757FEV1Residual volume increasedFEV1 and FEF25-75% reduced6%-755Obstruction4FEF25-75%FEV13Residual volumeLung Volume (liters)FEF 252Normal1Residual volume9A87654Time (seconds)321TLC increased largely because ofincreased RV and FRC. VC usuallydecreased but may be normalMaximum Expiratory Flow–Volume CurvesFlow (liters/second)NormalInspiratoryReserveVolume(IRV)6ObstructionTotalLungCapacity(TLC)28TLCBInspiratoryReserveVolume(IRV)VCTidal Vol.
(VT)84007654TLCLung Volume (liters)CExpiratoryReserveVol. (ERV)VitalCapacity(VC)Tidal Vol. (VT)ExpiratoryReserveVol. (ERV)ResidualVol. (RV)3TLCNormalFunctionalResidualCapacity(FRC)ResidualVol. (RV)ObstructionFigure 14.8 Pulmonary Function in Obstructive Lung Disease In emphysema, a chronicobstructive lung disease often associated with smoking, there is inflammatory destruction of elastic tissuesin the lung, resulting in reduced elastic recoil of the lung. Changes in lung volumes (A), flow–volume curves(B), and spirometric measurements (C) associated with emphysema are illustrated.
Notably, FEV1 is reducedin obstructive lung disease, as is the ratio of FEV1 to FVC (A). FEF25–75%, the expiratory flow rate duringthe middle portion of a forced expiration, is also reduced. FEF, forced expiratory flow rate; FEV, forcedexpiratory volume.FRC176Respiratory Physiology7625-75%4FEFFEV13Lung Volume (liters)5Residual volume reducedFEV1 reducedFEF25-75% reduced or normalFEV15-75%2Residual volumeFEF2Normal1RestrictionResidual volume0987654Time (seconds)A3210Maximum Expiratory Flow–Volume Curves8TLC and VC decreasedFlow (liters/second)NormalInspiratoryReserveVolume(IRV)64RestrictionTotalLungCapacity(TLC)276TLCB54TLCLung Volume (liters)3Tidal Vol. (VT)ExpiratoryReserveVol. (ERV)ResidualVol.
(RV)2CNormalVitalCapacity(VC)TLCFunctionalResidualCapacity(FRC)IRVVCVTERVFRCRVRestrictionFigure 14.9 Pulmonary Function in Restrictive Lung Disease Lung compliance is reduced inrestrictive lung diseases such as interstitial fibrosis, resulting in diminished lung volumes. Changes in spirometric measurements (A), flow–volume curves (B), and lung volumes (C) associated with restrictive lungdisease are illustrated.
Because both FEV1 and FVC are reduced in restrictive lung disease (A), the ratio ofFEV1 to FVC is usually normal but may even be increased when FVC is greatly reduced. FEF25–75%, the expiratory flow rate during the middle portion of a forced expiration, is normal or reduced in restrictive disease.FEF, forced expiratory flow rate; FEV, forced expiratory volume.The Mechanics of BreathingSymbolInterpretationObstructionRVTLCSpirometerIRVHe dilutionor bodyplethysmographFRC ⫺ ERVVC ⫹ RV or FRC ⫹ ICMaximal expiratoryflow at 75% of50VC25Vmax 75Vmax 50Vmax 25Lung elasticityStatic recoilpressureStatic compliancePstatCstatSpirometer withsimultaneousrecording of flowand volume andvolume of integratedpneumotachographPleural pressure isrecorded withesophageal balloonwhile airflow isarrested at differentlung volumes;changes in lungvolume recordedwith spirometer orpneumotachographAirway resistanceRawDiffusing capacityDCOBodyplethysmograph todetermine alveolarpressure andpneumotachographto measure airflowLow concentration ofCO inhaled; expired gasanalyzed for COVT2103Time (sec)FEV1/FVC, % > 70FEV3/FVC, % >95FEF25-75%; wide range8FVCFEV3ObstructionVol (L)FEV3FVC4VFE5%-7252103Time (sec)FEV1/FVC, %FEV3/FVC, % DecreasedFEF25-75%54NormalObstructionICTLCERVRVFRCRestrictionFEV1/FVC, %andFEV3/FVC, %usuallynormalFEF25-75%usuallynormalRestriction6Flow4(L)2TLC 75 50 25 RV% VC54VC(L)3mEmphyseNMEFVIRVRVFEF25-75%(FMF)Maximal expiratoryflow–volume curveVCFRCSpirometerForced midexpiratory flowFRCRVNormalFEV1FEV3RestrictionTLCTLCERVICVTERVICVC VTExpiratory flow ratesForced expiratoryvolume in 1 secondin 3 secondsVCVol (L)VTFRCIRVNormalFEV1VCICERV5-75%Lung volumesand capacitiesVital capacityInspiratory capacityExpiratory reservevolumeTidal volumeFunctional residualcapacityResidual volumeTotal lung capacityMethodFEV1FEV2Test177amoralroFib2Static elastic recoil of lung is increased and staticcompliance reduced in diseases such aspulmonary fibrosis.
Conversely, in emphysema,static lung compliance is increased and elasticrecoil is reduced.sis10765Raw4(cm H2O/ 3L/sec)3210102030Pressure (cm H2O)40In obstructive lung disease, airway resistance isincreased. If obstruction involves only smallairways (<2 mm diameter), only minimal changesRestrictionin overall resistance may result. In restrictiveNormal disorders, resistance is often reduced because ofrangeincreased traction on intrathoracic airway walls.2 3 4 5 6 7Lung vol. (L)Obstruction1Diffusing capacity is reduced when alveolar walls are destroyed and pulmonary capillaries are obliteratedby emphysema and when alveolar-capillary membrane is thickened by edema, consolidation, or fibrosis.Figure 14.10A Pulmonary Function Tests Tests of pulmonary function are defined and illustrated,with comparisons between values observed in normal lungs and in restrictive and obstructive disease.178Respiratory PhysiologyTestSymbolMethodInterpretationSmall NormalairwaydiseaseIVIII30Tests for smallairway diseaseClosing volumeCVClosing capacityCCFollowing a fullinspiration of O2the expired lungvolume from TLCto RV is plottedagainst the N2concentration% N2inexpiredair0IICVCCITLC 54V iso V˙RV 10AirHe-O28Air4Spirometer orpneumotachographto record flow andvolumeSmall airway diseaseFlow 12(L/sec)⌬ V˙ max 50Flow 8(L/sec)⌬V˙ max 5032Lung vol.
(L)NormalHe-O212Maximal expiratoryflow–volume curvebreathing 80% Heand 20% O2Airways in the lower lung zones close at lowlung volumes and only those alveoli at top oflungs continue to empty. Because concentrationof N2 in alveoli of upper zones is higher, theslope of the curve abruptly increases (phase IV).Phase IV begins at larger lung volumes inindividuals with even minor degrees of airwayobstruction, increasing both CV and CC.˙ max 50⌬V4˙V iso VV iso V˙10050Vol (% VC)010050Vol (% VC)0During a maximal expiratory maneuver, resistance to airflow is normally due to turbulence and convectiveacceleration. Breathing He, which is less dense than air, lowers resistance and increases flow at all but thelowest volumes.
In small airway disease, resistance to laminar flow makes up a larger portion of total resistancand airflow is relatively independent of gas density. Increase in expiratory flow at 50% of VC while breathing˙ max 50) will be less, and volume at which flows while breathing He-O2 and while breathing air areHe-O2 (⌬V˙ will be higher in patients with small airway disease than in normal individuals.identical (V iso V)Gas exchangePartial pressure ofO2 in arterial bloodPartial pressure ofCO2 in arterial bloodArterial blood pHAlveolar-arterialO2 differencePO2PCO2Arterial blood iscollectedanaerobically inheparinized syringeNormal valuesAbnormalities60 to 100 mm HgHypoxemia indicative of ventilation/perfusion abnormalities, shunts,breathing room air at sea diffusion defect, alveolar hypoventilationlevel; falls slightly with air36 to 44 mm Hg7.35 to 7.45 pHpHA-aDO2A-aPO2Dead space/tidalvolume ratioV˙ D/V˙TDetermined fromarterial and mixedexpired PCO2Shunt fraction˙ S/Q˙TQDetermined fromPO2 after a period ofbreathing 100% O2PCO2 proportional to metabolic rate (CO2 production) and inversely relatedto volume of alveolar ventilationAcidosis (pH ⬍7.35)Respiratory (inadequate alveolar ventilation)Metabolic (gain of acid and/or loss of base)Alkalosis (pH ⬎7.45)Respiratory (excessive alveolar ventilation)Metabolic (gain of base or loss of acid)⬍10 mm Hg breathingroom airPrimarily reflects mismatching of ventilation and perfusion and/or shunts;may also be affected by diffusion defects⬍0.3Elevated ratio indicates wasted ventilation; i.e., that volume of gas whichdoes not take part in gas exchange⬍5%Elevation indicates increased amount of mixed venous blood enteringsystemic circulation without coming into contact with alveolar air, eitherbecause of shunting of blood past lungs to left side of heart or perfusion ofregions of lung which are not ventilatedFigure 14.10BPulmonary Function Tests (cont’d)179CHAPTER15Oxygen and Carbon DioxideTransport and Controlof RespirationWhile Fick’s law describes the diffusion of gases across thealveolar-capillary membrane, many other factors are important in determining the actual content of gases in blood andin the process of gas transport.
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