lung volumes & airway resistance

lung volumes & airway resistance - Dr. Sunil Sharma...

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Unformatted text preview: Dr. Sunil Sharma Senior Resident Dept of Pulmonary Medicine lung volumes measured by spirometry are useful for detecting, characterising & quantifying the severity severity of lung disease Measurements of absolute lung volumes, RV, FRC & TLC are technically more challenging limiting use in clinical practice Precise role of lung volume measurements in the assessment of disease severity, functional disability, course of disease and response to treatment remains to be determined Lung volume are necessary for a correct physiological diagnosis in certain clinical conditions Contrast to the relative simplicity of spirometric volumes volumes variety of disparate techniques have been developed for the measurement of absolute lung volumes Various methodologies of body plethysmography, nitrogen washout, gas dilution, and radiographic imaging methods Eur Respir J 2005; 26: 511–522 ‘‘lung volume’’ usually refers to the volume of th gas within the lungs, as measured by body plethysmography, gas dilution or washout Lung volumes derived from conventional chest radiographs are usually based on the volumes within the outlines of the thoracic cage & include volume of tissue (normal and abnormal) lung gas volume Lung volumes derived from CT scans can also include estimates of abnormal lung tissue volumes There are four volume subdivisions which do not overlap can not be further divided when added together equal total lung capacity Lung capacities are subdivisions of total volume that include two or more of the 4 basic basic lung volumes Tidal Volume Inspiratory Reserve Volume Expiratory Reserve Volume Residual Volume Tidal volume The amount of gas inspired or expired with each breath Inspiratory Reserve Volume Maximum amount of additional air that can be inspired from the end of a normal inspiration inspired Expiratory Reserve Volume The maximum volume of additional air that can be be expired from the end of a normal expiration Residual Volume The volume of air remaining in the lung after a maximal expiration This is the only lung volume which cannot be meas measured with a spirometer Total Lung Capacity Vital Capacity Functional Residual Capacity Inspiratory Capacity Total Lung Capacity volume of air contained in the lungs at the end of a maximal inspiration Sum of all four basic lung volumes TLC = RV + IRV + TV + ERV Vital Capacity The maximum volume of air that can be forcefully expelled from the lungs following a maximal inspiration Largest volume that can be measured with a th spirometer VC = IRV + TV + ERV = TLC - RV Functional Residual Capacity The volume of air remaining in the lung at the end of a normal expiration FRC = RV + ERV Inspiratory Capacity Maximum volume of air that can be inspired from Maximum end expiratory position This capacity is of less clinical significance than the other three IC = TV + IRV Use a spirometer IRV IC VC TV TLC Can Use Spiromenter ERV FRC RV RV Can’t Use a Spirometer Cannot use spirometry Measure FRC, then use: FRC RV = FRC – ERV Residual Volume is determined by one of 3 techniques techniques Gas Dilution Techniques Nitrogen washout Helium dilution Whole Body Plethysmography Radiography Two most commonly used gas dilution methods for measuring lung volume open circuit nitrogen (N2) method closed-circuit helium (He) method Both methods take advantage of physiologically inert gas that is poorly soluble in alveolar blood and lung tissues both both are most often used to measure functional residual capacity In the open-circuit method, all exhaled gas is collected while the subject inhales pure oxygen Initial concentration of nitrogen in the lungs is assumed to be about 0.81 rate of nitrogen elimination from blood and tissues about 30 mL/min measurement of the total amount of nitrogen washed out from the lungs permits the calculation of the volume of nitrogen-containing gas present at the beginning of the manoeuvre Mass Balance: Balance: 0.80 FRC = Vspirometer x FN2 FRC N2 Start N2 Finish V S p iro m e ter • F N ( sp io m eter) 4 0 , 0 0 0 m l • 0 . 0 5 FRC = = F N (lu n g ) 0 .8 2 2 An advantage of the open-circuit method is that th permits an assessment of the uniformity of ventilation ventilation of the lungs by analyzing the slope of the change in nitrogen concentration over consecutive exhalations measuring the end-expiratory concentration of measuring nitrogen after 7 minutes of washout by measuring the total ventilation required to reduce end-expiratory nitrogen to less than 2% Am Rev Respir Dis 1980; 121:789-794 The open-circuit method is sensitive to Leaks anywhere in the system – mouthpiece Errors in measurement of nitrogen concentration & exhaled volume If a pneumotachygraph is used attention must be paid to the effects of the change in viscosity of the gas exhaled, because it contains a progressively decreasing concentration of nitrogen Disadvantages Does not measure the volume of gas in poor communication with the airways e.g. lung bullae Assumes that the volume at which the measurement was made corresponds to the end-expiratory point requires a long period of reequilibration with room air before the test can be repeated Measuring spirometric volumes immediately before measuring FRC can eliminate the assumption of a constant constant or reproducible end-expiratory volume Subject rebreathe a gas mixture containing helium in a closed system until equilibriation is achieved Volume and concentration of helium in the gas mixture rebreathed are measured Final equilibrium concentration of helium permits calculation of the volume of gas in the lungs at the start of the manoeuvre Start: known ml of 10% He in Spirometer Rebreath for 10 min (until He evenly distributed) FHe in itia l • V S p ir o m ete r = F H e FVC = ( FHe in itia l fin a l • ( V S p iro m eter + F R C ) − FHe fin a l FHe fin a l ) ⋅ V S p iro m eter Thermal-conductivity meter measures the helium th concentration continuously, permitting return of the sampled gas to the system Because the meter is sensitive to carbon dioxide it is removed from the system by adding carbon dioxide absorber Removal of CO2 & O2 consumption results in a constant fall in the volume of gas in the closed circuit An equivalent amount of oxygen is to be introduced as an initial bolus or as a continuous flow Closed-circuit method is sensitive to errors from leakage of gas and alinearity of the gas analyzer Fails to measure the volume of gas in lung bullae & cannot be repeated at short cannot repeated s ho r t intervals Test results are reproducible Scand J Clin Lab Invest 1973; 32:271-277 Three types of plethysmograph pressure Volume pressure-volume/flow Has a closed chamber with a fixed volume in which the subject breathes Volume changes associated with compression or expansion of gas within the thorax are measured as pressure changes in gas surrounding the subject within the box ithi th Volume exchange between lung and box does not directly cause pressure changes Thermal, humidity, & CO2- O2 exchange differences between inspired and expired gas do cause cause pressure changes Thoracic gas volume and resistance are measured during rapid manoeuvres Small leaks are tolerated or are introduced to vent to slow thermal-pressure drift Best suited for measuring small volume changes because of its high sensitivity & excellent frequency response Measurements are usually brief and are used to study rapid events it need not be leak-free, absolutely rigid, or refrigerated Has constant pressure and variable variable volume When thoracic volume changes, gas is displaced through a hole in the box wall and is measured spirometer or integrating the flow through a pneumotachygraph Suitable for measuring small or large large volume changes To attain good frequency response, the impedance to gas displacement must be very small ll Requires a low-resistance pneumotachygraph sensitive transducer fast, drift-free integrator, or meticulous utilization of special spirometers Difficult to be used for routine studies Combines features of both types As the subject breathes from the room, changes in thoracic gas volume compress or expand the air around the subject in the box and also displace it di it through a hole in the box wall Compression or decompression of gas is measured as a pressure change displacement of gas is measured spirometer connected to the box or integrating airflow through a pneumotachygraph pneumotachygraph in the opening All of the change in thoracic gas volume is accounted for by adding the two components (pressure change and volume displacement) This combined approach has wide range of sensitivities permitting all types of measurements to be made with the same instrument (i.e., thoracic gas volume and airway resistance, spirometry, and flow-volume curves) Box has excellent frequency response and relatively modest modest requirements for the spirometer The integrated-flow version dispenses with waterfilled spirometers and is tolerant of leaks Compressible gas in the thorax, whether or not it th th is in free communication with airways By Boyle's law, pressure times the volume of the gas in the thorax is constant if its temperature remains remains constant (PV = P'V') At end-expiration, alveolar pressure (Palv) equals atmospheric pressure (P) because there is no airflow & V (thoracic gas volume) is unknown Airway is occluded and the subject makes small inspiratory and expiratory efforts against the occluded occluded airway During inspiratory efforts, the thorax enlarge th th (ΔV) and decompresses intrathoracic gas, creating a new thoracic gas volume (V' = V + ΔV) and a new pressure (P' = P + ΔP) A pressure transducer between the subject's mouth and the occluded airway measures the new pressure (P’) Assumed - Pmouth = Palv during compressional changes while there is no airflow at the mouth pressure changes are equal throughout a static fl fluid system (Pascal's principle) (P Boyle –Mariotte’s Law : P x V = constant under isothermal conditions PA x TGV = (PA - Δ PA)(TGV + Δ V) Expanding and rearranging equation TGV =(Δ V / Δ PA)(PA - Δ PA) Since Δ PA is very small compared to PA (<2%) it is usually omitted in the differential term TGV ~ (Δ V / Δ PA) x PA with PA = Pbar - PH2O,sat TGV ~ (Δ V / Δ PA) x (Pbar - PH2O,sat) The measured TGV additionally includes any apparatus dead spaces (Vd,app) as well as any volume inspired above resting end-expiratory lung volume at the moment of occlusion (Vt,occ) FRCpleth can be derived from TGV by subtraction of these two volume components of FRCpleth = TGV - Vd,app - Vt,occ The thoracic gas volume usually measured is th slightly larger than FRC unless the shutter is closed precisely after a normal tidal volume is exhaled Connecting the mouth-piece assembly to a valve and spirometer (or pneumotachygraph and integrator) using a pressure-volume plethysmograph makes it possible to measure TLC and all its subdivisions in conjunction with the measurement of thoracic gas volume Problems Effects of Heat, Humidity, and Respiratory Gas Exchange Ratio Changes in Outside Pressure Cooling Underestimation of Mouth Pressure Compression Volume In uncooperative subjects radiographic lung volumes may be more feasible than physiological measurements The definition of the position of lung inflation at the time of image acquisition is clearly essential Volumes measured carry their own assumptions and limitations, and cannot be directly and compared with volumes measured by the other techniques techniques The principle is to outline the lungs in both A-P & lateral chest radiographs, and determine the outlined areas assuming a given geometry or using using planimeters in order to derive the confined volume Adjustments are made for magnification factors volumes of the heart intrathoracic tissue and blood infradiaphragmatic spaces In the determination of TLC, 6–25% of subjects differed by >10% from plethysmographic measurements in adult subjects Academic Academic Press Inc., New York, 1982; pp. 155–163 New 1982; 155 In addition to thoracic cage volumes, CTs can provide estimates of lung tissue and air volumes volume of lung occupied by Increased density (e.g. In patchy infiltrates) or Decreased density (e.g. in emphysema or bullae) In a study of children, comparable correlations were observed for CT and radiographic measurements as compared with plethysmographic TLC Am J Respir Crit Care Med 1997; 155: 1649– 1656 155 1649 Disadvantage high radiation dose MRI offers the advantage of a large number of images within a short period of time, so that volumes volumes can be measured within a single breath Potential for scanning specific regions of the lung, as well as the ability to adjust for lung water and tissue water despite the advantages of an absence of radiation exposure its use for measuring thoracic gas volume is limited by its considerable cost Resistive Forces Inertia of the respiratory system (negligible) Friction lung & chest wall tissue surfaces gliding past each other lung tissue past itself during expansion frictional resistance to flow of air through the airways (80%) Airflow in the Airways Exists in Three Patterns Laminar Turbulent Transitional [distributed laminar] Reynolds number = ρ X Ve X D η ρ= density Ve= linear velocity of fluid D = diameter of tube η = viscosity of fluid Turbulent flow tends to take place when gas density, linear velocity & tube radius are large velocity Linear velocity (cm/sec) of gas in the tube is calculated by di dividing the flow rate (L/sec) by tube area (cm2) th fl (L/ Tube area refers to total cross sectional area of the airways of a given generation Airflow is transitional throughout most of tracheobronchial tree Energy required to produce this flow is thi fl intermediate between laminar and turbulent Many bifurcations in tracheobronchial tree, flow becomes laminar at very low tree flow Reynolds number in small airways distal to the terminal bronchioles Flow is turbulent only in the trachea where the radius is large and linear velocities reach high values [during exercise, during a cough] Airway resistance is easy to measure repeatedly & is always related to the lung volume at which it is measured • Measurements of RAW useful in differential diagnosis of type of airflow obstruction localization of the major site of obstruction • Measured during airflow & represents the ratio of the the driving pressure and instantaneous airflow RAW is determined by measuring the slope (β) of a curve of plethysmograph pressure (x-axis) displayed against airflow (y-axis) on an oscilloscope during rapid, shallow breathing ap through a pneumotachygraph within the plethysmograph Shutter is closed across the mouth-piece, and the slope (α) of plethysmographic pressure (xaxis) axis) displayed against mouth pressure (y-axis) is measured during panting under static conditions Because Pmouth equals Palv in a static system it serves two purposes serves Relates changes in plethysmographic pressure to changes in Palv in each subject Relates RAW to a particular thoracic gas volume Physiologic factors affecting plethysmographic measurement of RAW Airflow RAW pertains to a particular flow rate during continuous pressure-flow curves, so the slope may be read at any desired airflow rate RAW is measured at low flows, at which transmural compressive pressures across the airways are small and the relation to Palv is linear Airway dynamics measured during forced respiratory maneuvers is associated with large transmural compressive pressures across the airways maximal dynamic airway compression limiting airflow rates and possible alterations in airway smooth muscle tone under such circumstances, RAW may be increased markedly be Volume Near TLC, resistance is small, but near RV, TLC RV resistance is large Lung volume may be changed voluntarily to evaluate RAW at larger or smaller volumes in health and disease As a first approximation, airway conductance (GAW), the reciprocal of RAW, is proportional to lung lung volume Transpulmonary Transpulmonary Pressure RAW is related more directly to lung elastic recoil pressure th than to lung volume Subjects with increased lung elastic recoil have a higher GAW at a given lung volume because of increased tissue tension pulling outward on airway walls Loss of elastic recoil results in loss of tissue tension and decreased traction on airway walls, so GAW is decreased This relationship may be used to analyze the mechanism of airflow limitation in various obstructive ventilatory defects (e.g., bullous lung disease) Airway Airway Smooth Muscle Tone. Airways affected markedly by smooth muscle tone, depending on the state of inflation and volume hi history Relationships are relevant to diseases in which elat smooth muscle tone is increased (e.g., asthma) low lung volumes are encountered (e.g., during cough, when pneumothorax is present) when Bronchoconstriction is not demonstrable temporarily after after a deep breath or at TLC in healthy subjects RAW in healthy subjects may be greater when a given lung volume is reached from RV than from TLC Panting Panting Panting minimizes changes in the plethysmograph caused by thermal, water saturation, and carbon dioxide-oxygen exchange exchange differences during inspiration and expiration Improves the signal-to-drift ratio, because each respiratory cycle cycle is completed in a fraction of a second gradual thermal changes and small leaks in the box become insignificant compared with volume changes attributable to compression and decompression of alveolar gas Glottis stays open, rather than partly closing and varying position, as it does during tidal breathing DuBois and colleagues described an oscillatory method to measure the mechanical properties of the the lung and thorax Eur Respir J 1996; 9:1747-1750 Use an external loudspeaker or similar device to generate and impose flow oscillations on spontaneous breathing Impulse oscillometry measures RAW and lung compliance independently of respiratory muscle strength and patient cooperation Sound waves at various frequencies (3 - 20 Hz) (3 20 are applied to the entire respiratory system piston pump can be used to apply pressure waves around the body in a whole-body respirator Slow frequency changes in pressure, flow, and volume generated by the respiratory muscles during normal breathing are subtracted from the Raw Raw data permitting analysis of the pressure-flow-volume relationships imposed by the oscillation device The elastic forces of the lungs and chest wall oppose the volume changes induced by the applied pressure & decrease as the frequency of oscillation increases The total force or pressure that opposes the driving pressure applied by the loudspeaker can be measured as peak-to-peak pressure difference divided by peakto-peak flow combination of the resistance and reactance This resistance is proportional to the RAW in healthy subjects and patients, although it does include a small component of lung tissue and chest wall resistance resistance as well as the resistance of the airways High frequency oscillating air flow is applied to fl the airways Resultant pressure & airflow changes are measured Applying a/c theory Raw can be measured contineously J Appl Physol 1970; 28: 113-16 Measures total respiratory resistance through out the vital capacity – displaying resistance as function function of lung volume ...
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This note was uploaded on 12/03/2011 for the course MEDICINE 350 taught by Professor Dr.aslam during the Winter '07 term at Medical College.

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