Body plethesmography

Respiratory medicine post graduate

1. DR ANUSHA CM BODY PLETHESMOGRAPHY

2. INTRODUCTION Body plethysmography is a well-established technique of lung function determination 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.

3. Spirometry is considered the gold standard in lung function. It can, however, not provide information on, e.g., lung residual volume (RV) and total lung capacity (TLC),

4. WHAT WE CAN MEASURE

5. INDEX USEFULLNESS Vital Capacity Useful marker for the effect of disease and assessing outcomes from exercise (6MWT) Expiratory Reserve Volume Effects of obesity on lung volumes, particularly where BMI > 35 kg.m-2 Inspiratory Capacity Marker of Bronchodilator response where FEV1 shows no significant change – effects of BD on hyperinflation

6. WHAT WE CANNOT MEASURE Index Usefulness Total Lung Capacity Marker of effects of obstructive airways disease and key index to confirm the presence of a restrictive ventilatory defect Functional Residual Capacity Marker of hyperinflation and reflects changes in PV relationships of chest wall and/or lungs. FRC/TLC ratio reflects the degree of hyperinflation Residual Volume Marker of “gas trapping”. Reflects the effects of obstructive or restrictive disease on lung volumes. RV/TLC ratio reflects poor gas mixing and hence gas trapping

7. WAYS TO MEASURE FRC Multi-breath He dilution measurement Nitrogen washout Body plethysmography - BEST METHOD

8. HISTORY 1790 Menzies - Dissertation on Respiration Plunged a man into water in a hogshead up to his chin and measured the rise and fall of the level in the cylinder round the chin. With this method of body plethysmography he determined the tidal volume

9. MODERN DAY PLETHESMOGRAPHY Dubois et al 1956 Forms the basis of constant-volume Plethysmography in use today for lung volume and airway resistance measurements

10. BODY PLETHESMOGRAPHY TODAY

11. TYPES OF BODY PLETHESMOGRAPH PRESSURE PLETHESMOGRAPH- in which pressure during breathing varies and volume remains constant VOLUME PLETHESMOGRAPH- in which volume during breathing varies and pressure remains constant PRESSURE FLOW PLETHESMOGRAPH- couples pressure plethesmograph fidelity of response to high speed events with the volume plethesmography ability to follow large changes in volume

12. PRINCIIPLE OF BODY PLETHESMOGRAPH 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 ).

13. 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

14. Based on BOYLE LAW Boyle’s Law: for fixed mass of gas at constant temperature : P1V1 = P2V2 Brief occlusion at airway opening to seal a fixed mass of gas in the lungs (V1) - i.e. the FRC to be measured Pressure within lungs at end expiration (P1) ~ atmospheric pressure. P2 and V2 represent the pressure and volume in the lungs after a respiratory effort against the occlusion .

15. Thus - PV = (P + deltaP ).(V - deltaV ) = V(P - delta P ) + (P - delta P ) delta V = PV - V delta P + (P - delta P ) delta V Re-arranging - Delta PV = (P - delta P ) delta V VL = (P - delta P )( deltaV / delta P ) Delta P is such a small fraction of P (barometric pressure) that it can be omitted without loss of accuracy VL = P(delta V/ deltaP )

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

17. 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( impulse oscillometry ) 3. Body plethysmography

18. 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. Continous O2 therapy that can not be removed.

19. INDICATION OF RAW 1. Further evaluation of airflow limitation beyond spirometry . 2. Determining the response to B.D. 3. Determination of bronchial hyperreactivity 4. Diff . between types of obstructive lung disease having similar spirometeric configuration . 5. Following the course of the disease and response to treatment.

20. 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.

21. 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 20-25 / 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).

22. SUMMARY OF THE PROCEDURE

23. DETERMINATION OF INTRATHORASIC 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.

24. METHODOLOGY OF 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.

25. 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.

26. 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.

27. 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.

28. SHIFT VOLUME This is the change in volume within the lungs in relation to the change in box pressure used as a surrogate marker of changes in volume. As the subjects breathes against the shutter, the lung volume changes, so the box pressure changes . By calibrating the box pressure for volume change, the actual change in volume – the shift volume can be estimated The shift volume is useful in assessing the effects of disease on resistance

29. 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

30. INTERPRETATION OF THE RESULTS • In patients with obstructive diseases – airway closure occurs at an abnormally high lung volume Increased FRC (functional residual capacity) Increased RV (residual volume ) • Patients with reduced lung compliance (e.g., diffuse interstitial fibrosis) – stiffness of the lungs + recoil of the lungs to a smaller resting volume Decreased FRC Decreased RV

31. CLINICAL APPLICATION AND INTERPRET ATION INCREASED FRC Gas trapping due to intrathoracic airway obstruction Cystic lung disease DECREASED FRC Abnormal alveolar development Reduced recoil of chest-wall Decreased lung compliance Atelectasis

32. 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 20 -35 %. 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

33. 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

34. VOLUME Air trapping Restrictive pattern Hyperinflation RV/TLC% Increase Normal Increase TLC Normal Decrease Increase RV Increase Decrease Increase FRC Increase Decrease Increase VC Decrease Decrease Normal

35. OBSTRUCTIVE LUNG DISEASE