1625915643-5d53d156c9525bd62bd0d3434ecdc231 (843955), страница 42
Текст из файла (страница 42)
Aftermeasuring the final concentration of helium, FRC can bedetermined from the formula:C1VS = C2 × (VS + FRC)where C1 is the initial, measured concentration of helium inthe spirometer, VS is the known volume of the spirometrysystem, and C2 is the measured concentration of heliumduring breathing (after equilibrium is reached).Pulmonary Ventilation and Perfusion and Diffusion of GasesWaterFigure 13.9 The Spirometer Pulmonary function testing involves multiple instruments and techniques. One basic instrument is the spirometer. The subject breathes in and out of a tube, resulting in verticaldisplacement of the inner canister, which floats upside down in water.
The recorded tracing is analyzed toobtain several basic lung volumes and capacities (see Fig. 13.10).6Maximal inspiratory level5Inspiratoryreservevolume(IRV)Inspiratorycapacity(IC)Volume (L)4Vitalcapacity(VC)Tidalvolume (VT)3Resting end-expiratory levelExpiratoryreservevolume(ERV)2Functionalresidualcapacity(FRC)*Maximal expiratory level1Residualvolume(RV)**Not determinedby spirometryFigure 13.10 Measurement of Lung Volumes and Capacities by Spirometry In this spirogram, beginning on the left, the subject breathed quietly for a few breaths, exhaled maximally, breathedquietly for a few breaths, inhaled maximally, and then breathed normally again.
Note that residual volume,functional residual capacity, and total lung capacity cannot be measured by spirometry alone. FRC is measured by another technique (often helium dilution); when FRC is known, TLC and RV can be calculated fromthe spirometry tracing. The volumes represented in the tracing are those of a typical healthy adult.Totallungcapacity(TLC)*155156Respiratory PhysiologyOnce FRC is known, TLC and RV can be readily calculatedfrom spirometry measurements (see Fig.
13.10):TLC = FRC + ICRV = TLC − VCWhole-body plethysmography is based on Boyle’s law, whichstates that the product of pressure and volume for a gas isconstant:P1V1 = P2V2To employ whole-body plethysmography to measure FRC,the subject is put into an airtight box equipped with a mouthpiece through which he breathes outside air. During normal,quiet breathing, the subject is asked to stop and relax after anormal, quiet exhalation.
At that point, the mouthpiece isclosed, and the subject attempts to inhale through the closedmouthpiece. This attempt causes the air in the lungs (FRC) toexpand due to the negative pressures created by the expansionof the chest wall, and results in an equal reduction of thevolume of the air in the box outside the patient’s body. Thepressure of the air in the box increases due to this reductionin volume. Changes in pressure in the box are used to calculate the change in volume in the box using Boyle’s law.
Thischange in volume is the same as the change in volume of thelungs from the resting value (FRC). Based on the change inpressure in the respiratory system (measured at the mouthpiece during the attempt to inhale) and the change in volumeof the lungs, the initial volume (FRC) can be calculated usingBoyle’s law.VENTILATION AND ALVEOLARGAS COMPOSITIONVentilation is the movement of air in and out of the respiratory system. During normal, quiet breathing, the volume ofair inhaled and exhaled with each breath is approximately500 mL.
This is known as the resting tidal volume. Thenormal, resting respiratory rate is 12 to 20 breaths per minute(min). Taking an averagerate of 15 breaths per minute, the.minute ventilation (V E, the volume exhaled per minute) iscalculated by the formula:.VE = R × VTwhere R is the respiratory rate and VT is. the tidal volume.Thus, 15 breaths/min × 500 mL yields a VE of 7500 mL/min(7.5 L/min)..Minuteventilation (VE) is greater than alveolar ventilation.(VA, ventilation of the respiratory zone of the lungs), becausea portion of the tidal volume remains in the anatomical deadspace (the conducting zone) and is not involved in gasexchange. Because anatomical dead .space is approximately150 mL, this dead space ventilation (VD) is roughly 150 mL ×15/min, or 2250 mL/min.
Alveolar ventilation is calculated bythe formula:The symbols used in respiratory physiology can beconfusing. The uppercase V is used to designate.volume, whereas airflow rates are designated. by the symbol V.Uppercase A used as a subscript, as in VA, .specifies that theparameter refers to the alveolar space. Thus, VA is alveolar ventilation. Lowercase a and v are used to specify arterial andvenous measurements, respectively, as in PaCO2or PvCO2(partialpressure of carbon dioxide in arterial and venous blood, respectively). PACO2refers to partial pressure of carbon dioxide inalveolar gas..VA = R(VT − VD)where R is respiratory rate, VT is tidal volume, and VD is deadspace volume.
Stated another way,...VA = VE − VDOf the 500 mL tidal volume, only 350 mL is entering thealveoli with each breath, and of the 7500 mL/min minuteventilation, only the alveolar ventilation of 5250 mL/min(7500 mL/min − 2250 ml/min) is available for gas exchange.Composition of Alveolar AirThe composition of alveolar air is dependent on severalfactors, including the composition of inspired air, alveolarventilation, and the concentration of dissolved gases inmixed venous blood. Our atmosphere is composed of 21%oxygen, 79% nitrogen, and less than 1% other gases, including carbon dioxide, with a total atmospheric pressure of760 mm Hg at sea level. Gas concentrations may also beexpressed as fractional concentrations in an inspired gasmixture; in our atmosphere, FiO2 = 0.21 and FiN2 = 0.79.According to Dalton’s law, the sum of the partial pressures ofgases in a mixture is equal to the total pressure. Thus, for dryair at sea level:PO2 = 0.21 × 760 mm Hg = 160 mm HgPN2 = 0.79 × 760 mm Hg = 600 mm HgPtot = 760 mm HgAs air is inspired, it warms rapidly to body temperature andbecomes saturated with water vapor.
At 37°C, the vapor pres.At the same minute ventilation (VE), deep,. slow breathing yields greater alveolar ventilation (VA) than rapid,shallow breathing. Comparing alveolar ventilation when respiratory rate is 15/min and tidal volume is 500 mL to alveolarventilation at a respiratory rate of 30/min and tidal volume of250 mL:.V. A = 15/min (500 mL − 150 mL) = 5250 mL/minVA = 30/min (250 mL − 150 mL) = 3000 mL/minThus, slower, deeper ventilation produces greater alveolarventilation than more rapid ventilation at proportionally lesstidal volume.Pulmonary Ventilation and Perfusion and Diffusion of Gases157sure of water is 47 mm Hg and must be accounted for whendetermining the gas composition of inspired air.
In inspiredair:A × D ( P1 − P2 )V gas =TPH2O = 47 mm HgPO2 = 0.21 × (760 − 47) mm Hg = 150 mm HgPN2 = 0.79 × (760 − 47) mm Hg = 563 mm HgPtot = (150 + 563 + 47) mm Hg = 760 mm Hgwhere A is the area of a membrane separating two compartments, T is the thickness of the membrane, D is the diffusionconstant, and P1 and P2 are the gas concentrations in the twocompartments. Thus, diffusion of a gas is:This composition of inspired air is constant throughout theconducting zone of the lung, where no gas exchange occurs.Within the respiratory zone, oxygen diffuses from alveolar airto blood, while carbon dioxide diffuses from blood to alveolarair, resulting in alveolar air composition that is differentfrom composition of inspired air.
The relationship betweenpartial pressure of oxygen and carbon dioxide in alveolar airis described by the important alveolar gas equation:PAO2 = PIO2 − PACO2/Rwhere PAO2and PIO2are the partial pressures of oxygen in alveolar air and inspired air, respectively; PACO2 is partial pressureof carbon dioxide in alveolar air; and R is the respiratoryquotient, which usually has a value of 0.8. This equation canbe used to predict the partial pressure of oxygen in alveolarair based on measurement of carbon dioxide in systemic arterial blood in a healthy person, because PCO2 is normally fullyequilibrated between blood and alveolar air in the alveolarcapillaries. For example, at 760 mm Hg atmospheric pressure(sea level), when PaO2in an arterial blood gas determination(ABG) is 40 mm Hg:■■■■Directly related to the surface area for diffusion.Directly related to the difference in partial pressure ofthe gas on each side of the membrane.Directly related to the diffusion constant of the gas.Inversely related to the thickness of the membrane.The diffusion constant of a gas is directly related to solubilityand inversely related to the square root of its molecularweight.Perfusion-Limited Gas TransportDIFFUSION OF GASES.Diffusion of gas (Vgas) between alveolar gas and capillary bloodfollows Fick’s law,The concentrations of dissolved gases in blood equilibratewith the gas concentrations of alveolar air as blood flowsthrough pulmonary capillaries (Fig.
13.12). Under restingconditions, the transit time for blood through the alveolarcapillary is only approximately 0.75 seconds (sec). In normal,healthy lungs at rest, oxygen and carbon dioxide are equilibrated between blood and alveolar gas by the time blood haspassed one third of the way through an alveolar capillary.Transport of these gases is perfusion-limited under theseconditions, because the only way to increase gas transfer isto increase perfusion (blood flow, rather than diffusion, isthe limiting factor for exchange). As a consequence, cardiacoutput can be increased substantially without compromisingoxygenation of blood despite reduced transit time throughthe alveolar capillaries, for example during exercise. Nitrousoxide (laughing gas; N2O) is a textbook example of perfusionlimitation.
N2O, unlike many other gases including O2 andCO2, is not bound by blood components but exists only asdissolved gas in blood. As a result, in a subject breathing agas mixture containing N2O, blood and alveolar PN2O equilibrate rapidly in blood flowing through the alveolar capillary,reaching this equilibrium one fifth of the way through thecapillary.The respiratory quotient (R) is the ratio of carbondioxide production to oxygen consumption and isdependent on metabolism. For pure carbohydrate metabolism,R has a value of 1.0, because oxidative metabolism of one moleof glucose requires six moles of oxygen and produces six molesof carbon dioxide. During pure lipid metabolism, the approximate value of R would be 0.7, whereas the respiratory quotientfor amino acids varies.
Under metabolic conditions involvingoxidation of a typical mixture of substrates, the respiratoryquotient is approximately 0.8 (0.8 moles of carbon dioxide areproduced for each mole of oxygen consumed).Transit time is the time required for blood or a formedelement to pass through a portion of the circulation.Because blood volume in the entire pulmonary circulation isapproximately 500 mL and cardiac output at rest is about5000 L/min (83 ml/sec), transit time through the entire pulmonary circulation is approximately 6 seconds. Of this, about 0.75second is transit time through the alveolar capillaries, duringwhich all exchange of gases takes place.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
