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What is plethysmography?
A plethysmograph is a virtually airtight body box that allows the subject to sit inside. Plethysmography measures static lung volumes, i.e.: those volumes that include residual volume (RV) – the gas volume that remains inside the lung at the end of a full expiration, i.e. Functional residual capacity (FRC), the lung volume at the end of a tidal expiration, and total lung capacity (TLC) the total volume of gas in the lung after a full inspiration.
In addition, plethysmography is a unique technique that provides an estimate of alveolar pressure, and therefore has the potential to measure specific airway resistance (sRaw) and calculate airway resistance (Raw) which are most valuable for asthma diagnosis (Goldman, 2005; Miller et al., 2005).
Lung volume measurement
Plethysmography was first described by Dubois in 1956 (Dubois et al., 1956) as a method to measure static lung volume. The principle is based on Boyle’s law that states that, under isothermal conditions, the volume of a gas enclosed in a rigid container decreases when pressure increases, and vice versa. Hence the product of volume and pressure is constant (Dubois et al., 1956) and may be written: P.V = nRT [1].
(P = pressure of the gas, V = volume of the gas, n is Avogadro number, R is gas constant, T is gas temperature). When temperature is constant, the equation can then be rewritten as: P1.V1 = P2.V2; or P.V = (P + ΔP) . (V + ΔV) [2].
ΔP and, ΔV are respectively the change in pressure and volume during compression – decompression. Equation [2] can be expanded: P.V = P.V + P.ΔV + ΔP.V + ΔP.ΔV [3].
Specific airway resistance and airway resistance measurement
Plethysmography also allows the measurement of airway resistance (Raw) and the specific airway resistance (sRaw) which are both important tools to diagnose asthma in children. Raw represents pressure losses due to flow in the airways. In a model where flow (V’) regimen is laminar, Raw may be determined according to Poiseuille’s law: Palv = Raw. V’ [9].
Where Palv is alveolar pressure referenced to the pressure at the airway opening. When a subject breathes inside a plethysmograph, the pressure in the box reflects Palv and the volume changes are related to gas compression and decompression (Vbox). The latter also depends on Cg, i.e. the larger the Cg the larger the Vbox. Therefore: ΔVbox = Palv . Cg [10]. Using eq 6 and 9, eq 10 may be altered: ΔVbox = Raw. V’.TGV/(PB – 47) [11].
And the important relationship expressing sRaw (i.e., the product of Raw by TGV) is obtained as (16): sRaw = (ΔVbox / V’).(PB – 47) [12].
sRaw expresses the resistance of the airways per unit of lung volume. As Raw decreases when lung volume increases, sRaw is constant in a given individual (Dubois et al., 1956). Once sRaw and TGV have been measured, Raw may be easily calculated.
The theory however does not take into account the fact that ΔVbox has a large component related to the change in temperature and humidity of the respired gas. The artifact has long been recognized and may be eliminated most simply during panting. In young children however where panting may be difficult to achieve, a numerical algorithm has been proposed to achieve the correction during tidal breathing, in replacement for the BTPS conditioning of the respired gas. During my stay in Nancy, I have been involved in studies showing that in those children able to perform the panting maneuver as reference measurement, the numerical correction lead to a significant overestimation of sRaw (Coutier et al., 2014a; Coutier et al., 2014b). These studies have been very helpful to me to become familiar with measurements in children and will be presented further in this document.
Equipment and study set up
Two Jaeger MedGraphics 1085 plethysmographs were customized in the department of physiology, ULFM. Body boxes were equipped with similar transducers, electronics, filtering, acquisition procedures and mathematical handling that have been previously described (Peslin et al., 1987). One equipment was set in the lung function department of HELFD, and the other plethysmograph was shipped to the department of physiology, HMU where it was serviced on site by the research engineer (Bruno DEMOULIN) that developed both measuring apparatus. The HMU body box was subjected to a mid-study on site visit, and remote on demand technical assistance was available via an internet connection. Daily quality control procedures included adjusting the gain of the pressure transducer against a water manometer, calibrating the pneumotachograph by the integral method and the plethysmograph signal using the built-in 50 mL reciprocating pump.
Ethnicity and airway resistance
The difference in Raw between the two ethnicities is original and was not previously identified in the literature. The finding is particularly valuable in view of the particular protocol that involved similar equipment in two laboratories in different countries. Even though efforts have been made to standardize protocols in the two laboratories, slight differences in measuring conditions were observed. The larger Raw in Vietnamese could be explained by the higher panting frequency in Vietnamese subjects. In these conditions, higher, and therefore more turbulent flows are likely to occur. As a result, the computed airway resistance will be larger, according to Rohrer’s model described in figure 11. In laminar conditions observed at low flow, the slope of the relationship between pressure loss and flow is smaller than during turbulent conditions because of non linearities.
Lung volumes
Reference values are important to consider normality of measured lung volumes in an individual. Ideally, they should be obtained in the relevant particular ethnical population. This study provides the first description of lung volumes in Vietnamese young healthy adults.
TLC depends on airspace volume, rib cage dimensions and the ability to expand the lungs and it was found significantly higher in Caucasians than Vietnamese. The statistical model to predict TLC was based on ethnicity, gender, age, standing height, sitting height, and weight. Figure 5 indicates that TLC is significantly related to ethnicity, standing height and gender. The model could explain 71.3 % of the variation in TLC. Surprisingly, sitting height was not a significant predictive variable of any lung volume, as it would have been expected a closer proxy to chest wall dimensions than standing height. A possible explanation may relate to the larger variability in the measurement of sitting than standing height. It must also be emphazised that the age and stature ranges were rather small, and the current findings would perhaps not apply in a population including children and adults.
Studies on TLC from various countries are presented in figure 13. TLC of Vietnamese young adults appear to be close to those reported in Chinese, both being also lower than in subjects of Spanish, Latin Brazilian, French or German origin. The trend for TLC to be smaller in female than male also appeared in figure 13.
Table of contents :
1. INTRODUCTION
1.1. Asthma has urgent requirement for better diagnosis in Viet Nam
1.2. Lack of awareness of lung function tests in Viet Nam
2. MOTIVATION
3. AIMS OF THE STUDY
4. THE PRINCIPLE OF PLETHYSMOGRAPHY
4.1. What is plethysmography?
4.2. Lung volume measurement
4.3. Specific airway resistance and airway resistance measurement .
5. MATERIALS AND MEASUREMENT
5.1. Subjects
5.2. Equipment and study set up
5.3. Measurements
5.4. Ethical aspects
5.5. Statistics
6. RESULTS
6.1. Comparison between two ethnicities
6.2. Raw and sRaw
6.3. Lung volumes
6.4. Predictors of lung volumes
7. DISCUSSION
7.1. Ethnicity and airway resistance
7.2. Lung volumes
8. PROSPECTIVES
9. SUMMARY OF CONCLUSION
10. REFERENCES
11. LIST OF PUBLICATIONS
