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Measurement of Lung Volumes by Plethysmography

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Measurement of Lung Volumes by Plethysmography Eur Respir J, 1997, 10, 1415–1427 Copyright ERS Journals Ltd 1997 DOI: 10.1183/09031936.97.10061415 European Respiratory Journal Printed in UK - all rights reserved ISSN 0903 - 1936 ERS/ATS WORKSHOP REPORT SERIES Measurement of lung volumes by plethysmography A.L. Coates*, R. Peslin**, D. Rodenstein+, J. Stocks++ CONTENTS Theory Thermal drift Assumptions................................................................ 1416 Frequency response................................................... 1422 Types of plethysmograph Measurement techniques........................................... 1423 Variable pressure plethysmograph............................. 1417 Quality control.......................................................... 1424 Volume displacement plethysmograph...................... 1419 Plethysmograph in infants Flow plethysmographs............................................... 1420 Measurement techniques........................................... 1425 Variable pressure-flow plethysmographs................... 1421 Conclusions.................................................................... 1425 Material and methods References...................................................................... 1425 Calibration.................................................................. 1421 There are various ways of measuring absolute lung plethysmography (FRCpleth) by subtracting or adding volume. These range from measurements derived from the appropriate volume correction. In normal children chest radiographs to the more common laboratory mea- and adults, there should be no difference in FRC bet- surements employing either gas dilution or plethysmo- ween gas dilution techniques and plethysmography. How- graphy. The principal difference between the latter two ever, in patients with lung disease associated with methods is that gas dilution techniques measure gas gas-trapping and in normal infants [3], FRCpleth gen- that is in free communication with the airway opening, erally exceeds FRC measured by gas dilution. whilst plethysmography measures all intrathoracic gas. Once the absolute lung volume is known, the other lung volumes can be measured from the change in volume Theory during specific respiratory manoeuvres. Work on this article was undertaken as part of a joint In 1956, DUBOIS et al. [4] described a technique for European Respiratory Society/American Thoracic Society measuring the volume of gas in the thorax by using the effort to develop a joint "Statement" on the measurement measurement of a volume change when a gas is com- of lung volumes. It will review the principles, practice pressed, as assessed by Boyle's Law. Boyle's Law states and limitations of plethysmography, and recommend that, under isothermal conditions, when gas in a closed standards for the plethysmographic measure of lung vol- container is compressed, its volume decreases while the umes. These standards will include equipment specifi- pressure inside the container increases, such that the cations and measurement techniques over the age range product of volume and pressure, at any given moment, from infancy to adulthood. is constant. Isothermal conditions imply that, during For the purpose of this review, functional residual ca- either compression or rarefaction, heat is exchanged pacity (FRC) is defined as the absolute volume of gas across the walls of the container so that the tempera- in the lung at the end of a normal expiration. Thora- ture of the gas remains the same. If no heat is lost, cic gas volume (TGV) is defined as the volume of intra- changes in gas temperature occur and conditions are thoracic gas at the time the airway is occluded for the considered to be adiabatic and follow the Ideal Gas plethysmographic measurement; while this is usually at Law. This states that the product of pressure (P) and FRC, in special circumstances it may not be [1, 2]. volume (V) divided by temperature remains constant. Irrespective of where in the volume cycle TGV is mea- Conditions in between these two extremes are poly- sured, it should be adjusted to the FRC derived from tropic. For plethysmography, Boyle's Law is: *Dept of Pediatrics, McGill University, Faculty of Medicine, Montréal, Québec, Canada. **INSERM U14, Vandoeuvre les Nancy, France. +Université Catholique de Louvin, Clinical Universitaires Saint-Luc, Bruxelles, Belgium. ++Portex Anaesthesia, Intensive Care and Respiratory Medicine Unit, Institute of Child Health, London, UK. Correspondence: A.L. Coates, The Montreal Children's Hospital, Division of Respiratory Medicine, Room D-380, 230 Tupper Street, Montréal, Québec, H3H 1P3, Canada Keywords: Infant pulmonary function test, lung volumes, plethysmography Received: June 24 1996; Accepted after revision August 19 1996 This publication evolved from a workshop on "measurement of lung volumes" convened by the European Respiratory Society and the American Thoracic Society, with additional support from the National Heart, Lung and Blood Institute (Grant No. R13 HL48384). 1416 A.L. COATESETAL. P1 × V1 = P2 × V2 (1) generally assumed that the discrepancy between PA1 and PB during a panting manoeuvre is small so that the where the smaller script "1" and "2" denote the first solution for TGV1 is reduced to: and second condition of the gas. Applied to the deter- mination of TGV, the subject is placed in an enclosed TGV = -(∆V/∆P) × PB (5) space, the plethysmograph, and is instructed to pant against an obstruction at the airway opening at a fre- The term ∆V/∆P is the slope of the pressure volume quency of approximately 1 Hz [5–7], resulting in the relationship and is always negative. While technically rarefaction and compression of the gas in the thorax. a hyperbola, it is so close to being a straight line over During these manoeuvres, changes in thoracic volume the range of interest, that few errors are introduced when (measured from the plethysmograph) and pressure mea- it is treated as such. This "simplified" equation is the sured at the airway opening (Pao) are recorded. Assum- one used in most automated plethysmographs. ing that Pao is representative of alveolar pressure (PA), Because of the problems experienced with the pant- TGV can be calculated using the following application ing technique, particularly the difficulty in obtaining of Boyle's Law: even gentle pants at a frequency of 1 Hz, a single inspi- ratory effort has been proposed as an alternative [8]. × × (PA1-PH2O) TGV1 = (PA2-PH2O) TGV2 (2) However, this method of determining TGV may give rise to changes in pressure that are of sufficient mag- where the smaller script denotes differing values of nitude (i.e. up to 5 kPa) that the ∆P ×∆V should not pressure and volume during the respiratory manoeuvre. be ignored [9]. An additional consideration when cal- PA1 and PA2 are expressed as absolute pressures, not culating TGV is that where PA1 differs from PB, such the differences between barometric pressure (PB) and as will occur if the occlusion is not performed at FRC, Pao. PH O represents water vapour pressure of saturated 2 a correction factor (PA1/PB) may be needed to convert gas at 37°C (6.3 kPa). Water vapour pressure is sub- TGV to body temperature and pressure saturated (BTPS). tracted from all pressures because, in saturated gas, its Hence, Equation (3c) can be rewritten as: magnitude relates only to temperature, passing in and out of the vapour state as the total pressure changes. TGV = -(∆V/∆P) × PA2 × (PA1/PB) (6) Hence, under fully saturated conditions, water vapour does not behave as a compressible gas. For the sake of This is the "complete" equation and it contains both PH O clarity in the development of this theoretical basis, 2 the ∆P ×∆V term and volume correction necessary will not appear in the subsequent equations, but must should PA1 be different from PB. As mentioned pre- be subtracted from all measurements of PA or PB. viously, PH O should be subtracted from PA1, PA2 and A more common way to write Equation (2) is: 2 PB. With today's modern computing facilities, there is no reason to use the simplified version of Boyle's Law, PA1 × TGV1 = (PA1 + ∆P) × (TGV1 + ∆V) (3) Equation (5), rather than the complete version, Equa- which, when all term are multiplied becomes: tion (6). The errors introduced by the simplified version during panting are small (in the order of 3%) and cen- PA1×TGV1=PA1×TGV1+∆V×PA1+∆P×TGV1+∆P×∆V tred around zero. For the single inspiratory manoeuvre, ∆ × ∆ ∆ they are in the order of 5%, systematically greater than or - P TGV1= V(PA1+ P) (3a) zero and related to the magnitude of the pressure gen- erated during the manoeuvre [9]. Furthermore, such sys- where PA1 is the alveolar pressure at the start of the tematic errors are unnecessary [9]. manoeuvre, and ∆P is the change in the alveolar pres- sure measured at the airway opening under conditions of no flow during the panting manoeuvre, so that ∆P Assumptions = PA2 - PA1. ∆V is the resulting change in volume of the thorax, and is equal to TGV2 - TGV1. The plethysmographic method of determining FRCpleth Solving Equation (3) for TGV1: is based on a number of assumptions, which are as fol- TGV1 = -(∆V/∆P) × (PA1 + ∆P) (3b) lows: Since ∆P=(PA2-PA1), this can be further simplified to: 1. There is no gas flow, and hence no flow-resistive losses of pressure in the airways, during the respira- TGV1 = -(∆V/∆P) × PA2 (3c) tory efforts against an occluded airway. Hence, Pao is equal to PA. When performing a panting manoeuvre, it is frequ- This assumption has been challenged [10, 11]. In ently assumed that the
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