Body Plethysmography By Hossam El-Din Mohamed lecturer of chest diseases Ain Shams University

The word plethysmograph is derived from the Greek plethusmos (enlargement) the fundamental function of a whole-body plethysmograph is the measurement of intrathoracic gas volume (TGV) and volume change. It is also used to measure airway resistance and conductance.

resistance of the respiratory system : lung resistance= resistance of lung tissue + airway resistance (Raw) total respiratory resistance(Rtotal) R total=chest wall +lung tissue +R aw and it is usually measured by IOS.

R aw= flow resistance in the airway between mouth and alveoli. And it is usually measured using body plethysmography. Method for measuring airway resistance: 1-esophageal balloon. 2-IOS. 3-Body plethysmography.

AIRWAY RESISTANCE The relationship between pressure and flow tells us about airway resistance. Pressure Flow Low ResistanceHigh Resistance

ΔPbox Flow Breathe IN Breathe Out Airways Resistance

 Contraindication for use of body box: 1-Mental confusion, muscular incoordination,body cast or any other condition that prevent the patient from entering the box. 2-Claustrophobia. 3-Presence of devices or other condition such as continuous I.V infusion Or any condition that interfere with pressure changes (e.g chest tube, Trans tracheal O2 catheter, or rupture ear drum). 4-Coninous O2 therapy that can not be removed. N.B: The relationship between box pressure and flow at mouth is the resistance.

Indication for R aw measurement: 1-Further evaluation of airflow limitation beyond spirometry. 2-Determining the response to B.D. 3-Determination of bronchial hyperreactivity the commonly used limit for bronchoprovocation is a 15 or 20% decrease in FEV1 relative to control baseline FEV1. The comparable limit for sRtot is 100%, for Rtot 50% increase and for sGtot 40% decrease from baseline, respectively. 4-Diff. between types of obstructive lung disease having similar spirometeric configuration. 5-Following the course of the disease and response to treatment.

Principles of whole-body Plethysmography: The fundamental principle of the variable-pressure plethysmograph is that changes in alveolar pressure (PA) may be inferred from changes in plethysmograph pressure. A shutter mechanism is positioned close to the mouth in the plethysmograph. This shutter may be closed to provide transient airway occlusion. Voluntary respiratory efforts are performed against the closed shutter, during which the change in PA (ΔPA), is estimated by recording the change in mouth pressure (ΔPm). Pm (PA) is plotted against simultaneous plethysmographic pressure changes during respiratory efforts against a closed shutter to measure absolute TGV. The same relationship between alveolar and plethysmographic pressure measured during respiratory efforts against a closed shutter is then extended to dynamic events during free breathing to measure Raw, where airflow is related to PA.

respiratory flow (ΔV') at the patient's mouth, sensed by a pneumotachograph and the thoracic movements, resulting in volume changes (ΔVbox) in the cabin, sensed by a box pressure transducer, are recorded and displayed in form of a xy-plot on the application screen. In a constant volume whole-body plethysmograph, Specific airway resistance (sRaw) is determined from the relationship between variations in respiratory flow and volume shift in the box.

During the measurement, the gas exchange between inspiration of ambient air at ambient temperature and pressure (ATP) and expiration of air at body temperature, pressure and water vapor saturation (BTPS) has to be considered. By means of an automatic, numerical compensation (ASC- "automatic loop compensation") with intelligent software algorithms, the compensation for body temperature, barometric pressure at water vapor saturation (BTPS), gas compression and decompression can be almost completely achieved.

In the first part of the measurement, the determination of Specific airway resistance, the patient should sit upright. He has to hold his head in neutral position or in slight extension, avoiding flexion or rotation. He is asked to breathe normally through the pneumotachograph. Care has to be taken, that the lips are firmly closed around the mouthpiece and the nose is clipped. Test sequence : - Patient should breath spontaneously. - Adaptation phase until patient breathes regularly. - Define slight hyperventilation with a breathing frequency of / min only if specific resistance loops are normal. - Set electronic loop compensation for body temperature and barometric pressure at vapour saturation. Quality control : - Wait for regular, repeatable and best closed loops. - Store the last 5 loops by activating the shutter (determination of ITGV).

Determination of Intrathoracic gas volume After successful determination of sRaw, the corresponding Intrathoracic gas volume (ITGV) can be measured. Initiated by a manual keystroke of the assistant, the patient's following inspiration is automatically interrupted by a shutter (at end of expiration and beginning of inspiration respectively). The patient should try to continue normal breathing against the shutter, without arising breathing muscles activities.

Methodology of the volume determination The volume measurement is based on the Boyle-Mariotte's law (1660, 1676) P * V= const. which is applicable to closed systems. The shutter creates a closed lung chambers and prevents further respiration of the patient. The pressure, generated by continuous breathing activities at the airway opening in front of the shutter (ΔPv) is registered by a pressure transducer. Simultaneously, the thoracic movements produce a volume shift (ΔVbox) in the box. The report of these two signals in an xy-plot presents the requested ITGV- loop. The Intrathoracic gas volume is determined, following the Boyle- Mariotte's law.

R aw is most frequently measured while the patient is enclosed in a whole body plethysmography designed to measure pressure changes and flow. There is an inverse relationship between R aw and lung volume. During inspiration, lung volume increase and R aw decrease due to ↑-ve intrapleural pressure.

During expiration, lung volume decrease and R aw increase. i.e. the diameter of airway change during breath cycle lead to change in Raw and lung volume. – G aw ( airway conductance is the reciprocal form (1/ Raw) of airway resistance.) and there is linear relationship between G aw and lung volume. – As Raw and G aw measured by body box lung volume adjusted measurement may made.

 The lung volume adjusted Raw and G aw is called specific airway resistance and conductance (sR aw& sG aw)= Raw X lung volume or G aw / lung volume.

Parameter of airway resistance: 1-sReff ( specific effective airway resistance). And it reflect larger; central airways some what more prominently than sR tot. 2- sRtot(specific total airway resistance). and it reflect smaller; peripheral airways 3- The parameter of sR0.5 reflects the behavior of larger, more proximal airways with mush less sensitivity to peripheral airways.

Definition of abnormal lung function Threshold to abnormality for Rtot and Reff in adults: 0.3 kPa/(L/s) Predicted values for Rtot and Reff in children: normal if below 150% of predicted Standard for RV: abnormal, if above % of predicted Standard for TLC: abnormal, if above % of predicted

Bronchial hyperresponsiveness Povocation: +PD/C 50 in Raw equivalent to -PD/C 20 in FEV1 +PD/C 100 in sRaw equivalent -PD/C 20 in FEV1 -PD/C 40 in sGaw equivalent -PD/C 20 in FEV1 Dilatation: >25 % response to bronchodilator (children aged 2-5) [1]

Reversibility in relation to basic measurement - no reversibility no changes at all - partial reversible improvement, without reaching normal range - complete reversible improvement into the normal range K. G. Nielsen, H. Bisgaard, "Discriminative Capacity of Bronchodilator Response Measured with Three Different Lung Function Techniques in Asthmatic and Healthy Children Aged 2 to 5 Years", Am J Respir Crit Care Med; Vol 164, pp , 2001

Types of plethysmograph The constant-volume or variable-pressure plethysmograph is used to measure small volume changes due to compression and decompression of gas within the lungs. The constant-pressure or volume-displacement plethysmograph is used to measure large changes in lung volume associated with gas flow into and out of the lungs such as the slow or forced vital capacity (FVC). The pressure-corrected variable-volume plethysmograph combines the advantages of both the plethysmographs described above.

Schematic representation of specific resistance loops in a) a normal subject, b) a subject with increased large airway resistance, c) a subject with chronic airflow obstruction d) and a subject with upper airway obstruction. Mouth flow (V') is plotted on the vertical axis, with inspiration positive and expiration negative

Lung volume  Lung volume tell us about: 1-disease severity. 2-recovery from exacerbation. 3-limited by hyperinflation.  Aim : 1- to measure TLC. 2- to measure RV. 3- to measure FRCpleth.=ITGV

 The most significant volume for evaluating the effect of pulmonary disorder are VC,FRC, RV, TLC.  A useful tool in evaluating lung volume studies is the RV/TLC %.  Normal value of RV/TLC % in normal young adult %.  Increase value of RV/TLC % indicate air trapping, hyperinflation of the lung is demonstrated when in addition to the increase RV/ TLC %. The TLC is significantly greater than normal

 Two general pattern of lung volume changes can results from pulmonary disorder. Restrictive pattern which demonstrate reduction in all lung volumes. And Obstructive pattern which tend to demonstrate increase in only some volume.  The exception to the general pattern of increase lung volume for obstructive disorder is that either no or even decrease in VC.

HyperinflationAir trapping Restrictive pattern VOLUME Increase NormalRV/TLC% IncreaseNormalDecreaseTLC Increase DecreaseRV Increase DecreaseFRC NormalDecrease VC

 Two obstructive pattern are possible one where there is increase RV results in proportional reduction in VC where TLC remain normal ( air trapping )  The 2 nd, RV increase with little or no changes in VC this cause an increase in TLC in direct proportion with RV (hyperinflation)  An abnormally increase in RV/TLC% will demonstrated in both pattern

Severity of lung volume disorder ObstructiveRestrictiveDegree % %Mild % %Moderate > 150 %< 60 %Severe TLC ( N %of predicted)

VC ( N. > 90% of predicted ) ObstructiveRestrictiveDegree % Mild % Moderate < 50 % Severe

FRC ( N % of predicted) ObstructiveRestrictiveDegree % %Mild % %Moderate > 200 %< 45 %Severe

RV ( N. 65 – 135 % of predicted) ObstructiveRestrictiveDegree % %Mild %45 – 55 %Moderate > 250 %< 45 %Severe

How to perform the test?  The body plethysmography program allows an analysis of the complete breathing mechanics. It principally includes the measurement of the airway resistance and intrathoracic gas volume. In addition, all important parameters of the slow spirometery (ERV,VC,……)and the forced spirometery ( FEV1, FVC, MEF50, PEF ….)

 The program is called up from the main group. Click the “ Body plethysmography” icon.  Ask the patient to take a seat in the box ( but not to put the mouthpiece into his mouth yet ).  Shut the box door. The intercom system is automatically activated.  After approximately one minute for temperature compensation ask the patient to approach the mouthpiece and close his nose with nasal clip.

 The measurement is started automatically.  A real- time display of the specific resistance loops and the spirogram appears in the window.  Once you are satisfied with the display of the resistance curves, start the ITGV measurement.  Click the “ ITGV “ icon.  At the beginning of the next breath the shutter is closed.

 Ask the patient to continue breathing normally, i.e. without increasing effort even after the shutter is closed ( he / she should not suck ).  It is useful to perform several shutter maneuver (> 2).  A Spirometery / Flow- volume measurement should be performed immediately after having opened the shutter.  Finally, save the measurement and exit the program with “F12”.

Interpretation Flow Chart FEV 1 /FVC VC Spirometry Normal DLCO Normal PV Disorder TLC Restriction Obstruction Obstructive & Restrictive Defects DLCO Chest wall or Neuromuscular Disorders ILD or Pneumonitis DLCO Asthma or Chronic Bronchitis Emphysema Chest wall, Neuromuscular Disorders, Asthma, or Chronic Bronchitis ILD, Pneumonitis, or Emphysema Normal or high Low Normal Lo w Normal or high Low Normal or high Normal Low Normal or high Normal or high Normal or high Interpretation Flow Chart - Paul L. Enright MD

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