Body Plethysmography

Specimen Collection

Principle

Plethysmography is based on the fundamental principle known as Boyle-Mariotte's law. This law states that under isothermal conditions, the product of a gas's pressure (P) and volume (V)—(P×V)—remains constant and is the basis of the theory. Due to the close contact with capillary blood, the gas in the lungs is isothermal (see Image. Volume-Time Display).[5] One of the notable advantages of this technique is its ability to rapidly obtain multiple measurements of RV and TLC. Because the alveolar gas composition must be restored to the control state before these tests can be repeated—a process that frequently takes 10 to 20 minutes in individuals with COPD—this is not feasible with the washout and dilution procedures. All of the gas in the lung, including that in poorly ventilated places, is measured through plethysmography. Therefore, in patients with COPD, plethysmography typically yields higher and more accurate measurements of functional residual capacity (FRC), RV, and TLC compared to gas dilution methods. TLC values obtained with plethysmography are often 2 to 3 liters higher than those measured by gas methods.[6]

The body plethysmograph was first introduced for clinical use by DuBois et al in 1956.[2][3] The plethysmograph is a closed chamber of about 500 to 800 L capacity. There are 2 types of boxes—the more commonly used constant-volume type and the flow type.

A research participant sits in a sealed, airtight chamber that is designed to measure changes in pressure, flow, or volume during body plethysmography. The body plethysmograph is most frequently used to quantify airway resistance (R) and thoracic gas volume (VTG). The VTG is typically equal to the FRC and measured at the tidal end-expiratory position.The reciprocal of airway resistance (R) is commonly used to calculate airway conductance (G). Specific airway conductance (sG_aw) refers to conductance per unit of lung volume and essentially measures the ease of airflow, independent of lung volume. A higher sG_aw indicates better airflow.[7] The body plethysmograph can also be used to perform spirometry, bronchial challenge, and pulmonary compliance testing.

Procedures

Measurement of Thoracic Gas Volume

Panting method: Respiratory movements producing inspiration and expiration are associated with decompression and compression of the air inside the lungs. Suppose the initial lung volume is V and the airway pressure is P. In that case, inspiration increases ITGV by V, which is associated with a fall in the pressure by P. According to Boyle's law, when temperature is constant, the product of volume and pressure is also constant.[8] 

Thus, PV = (P − ΔP) (V + ΔV) 

Expanding this equation:

PV = PV + PΔV − ΔPV − ΔPΔV

Due to its modest size, ΔPΔV can be disregarded. 

Rearranging the terms gives:

V = (ΔV/ΔP) (P)

The term V is the thoracic gas volume (VTG). The airway pressure is atmospherically corrected for water vapor.

Thus, VTG = (ΔV/ΔP) (P_B − P_H2O) 

In a body plethysmograph, when the participant breathes (pants) against a closed shutter in front of the mouth and the mouth pressure is measured, the fall in pressure during inspiration (P) can be easily estimated. The expansion of the chest produces an increase in the box pressure as it is a closed chamber. The box pressure is calibrated in terms of volume. The increase in volume (V) can be easily measured by recording the change in box pressure. Thus, to measure VTG, measurements of the mouth pressure and the box pressure are required when the participant inspires against a closed shutter (see Image. Body Plethysmography).

Measurement of Airway Resistance

To measure airway resistance (R_aw), the participant breathes through a flow-measuring device (pneumotach). The measured airflow is labeled as V⁰. The change in airflow can be related to the change in pressure, as resistance = pressure/airflow.[9]

Thus, R_aw = ΔP/ΔV²

Multiplying both the numerator and denominator by ΔV,

R_aw = (ΔP/V) (ΔV/ΔV²) 

Although ΔP/ΔV can be measured from the slope of 2 measurements plotted on the X and Y axes, ΔV/ΔV² requires displaying the box volume change against the airflow.

Specific airway resistance (sR_aw) is the ratio of the shift volume, or box pressure, to flow rate, stated in appropriate units. Closed curves are produced by plotting shift volume on the horizontal axis against airflow on the vertical axis. The reciprocal slope of these breathing loops represents a certain amount of airway resistance. In healthy individuals, the curves are approximately straight lines. In contrast, patients with respiratory illnesses display unique patterns, aiding in differential diagnosis and providing insights into the particular condition (see Image. Measurement of Airway Resistance).

The relationship between lung volume and pressure is crucial for understanding lung mechanics. At closing mouth pressure, this relationship is linear, meaning that as intrathoracic pressure changes, lung volume changes proportionally. This principle, based on Boyle's law, is central to plethysmographic calculations.

Calculations

The slopes (measured as the tangent of the angle 0 of the slope) of V⁰/V and P/V obtained with the shutter open and closed, respectively, multiplied by the respective calibration factors, provide the values for these terms. The atmospheric pressure is required to calculate (P_B P_H20).[5][6][7] 

For example, if the calibration (Cal) scale for airflow is 0.5 L/s/cm and for box pressure is 5 mL/cm, then the cal factor for V⁰/V is 100/s. Similarly, for P/V, the cal factor is 1 cm H₂0/mL (taking the mouth pressure cal scale as 5 cm H₂0/cm).

A correction is necessary for the participant's volume if the calibrations are performed with an empty box because the effective volume of the box is reduced by the participant's body volume. Thus, either the box pressure can be calibrated with the participant inside, holding their breath, or a correction may be applied as below:

Corrected ΔV = ΔV × (body volume − body weight)/box volume

This method requires the participant to pant to measure P and also assumes that by breathing against a closed shutter, the mouth pressure reflects the airway pressure. This assumption may not be true in patients with airway obstruction. Panting itself can be challenging, requiring training and cooperation from the participant.

To address these limitations, Krell et al and Agrawal and Kumar described a modification of the body plethysmography that does not require panting (Quiet-breathing method). Using this method, Gaw can be calculated directly without measuring VTG.[10][11]

Measurement of Specific Airway Conductance

Multiplying the equations above,

VTG × R_aw = (ΔV/ΔP) (P_B – P_H2O) × (ΔP/ΔV) (ΔV/ΔV°) = (P_B – P_H2O) (ΔV/ΔV°) 

As measurement of P is no longer required, panting against a closed shutter becomes unnecessary. The product of VTG and R_aw represents airway resistance normalized to lung volume, also known as specific airway resistance (sR_aw).

Thus sR_aw = (P_B – P_H2o) (ΔV/ΔV°)

sG_aw is the inverse of sR_aw. The VTG is expressed in liters (body temperature and pressure saturated) and is the volume of gas in the lung when the mouth shutter is closed. Raw is reported in cm H₂O/L/s. sG_aw is expressed in s/cm H₂O (that is, s·cm H₂O⁻¹) and is the reciprocal of R_aw (1/R_aw) divided by the lung volume at which the resistance measurement is made.

Table 2. Calculations and Definitions of Thoracic Gas Volume, Airway Resistance, and Specific Airway Conductance

Indications

Indications for Body Plethysmography

Determination of VTG, sR_aw, and sG_aw through body plethysmography is indicated in the following situations:

• Diagnosis of restrictive lung disease.
• Measurement of lung volume to differentiate between obstructive and restrictive processes.
• Assessment of obstructive lung conditions that can result in artificially low results when measured by helium dilution or nitrogen washout, such as cystic fibrosis and bullous emphysema.
• Calculation of trapped gas (FRC by Helium Dilution versus FRC by Plethysmography).
• Measurement of lung volume when multiple trials are required and the participant is incapable of performing multibreath tests.
• Evaluation of airflow resistance.
• Evaluation of bronchodilator responsiveness, as indicated by alterations in sR_aw and sG_aw.
• Determining whether methacholine or histamine causes bronchial hyperactivity.
• Monitoring the disease's progression and reaction to therapy.[5][12]

Contraindications to Body Plethysmography

• There are no absolute contraindications to body plethysmography.

• Relative contraindications to body plethysmography are as follows:
  • Body casts, mental disarray, poor motor coordination, or other issues that make it difficult for the patient to enter the body box or carry out the necessary movements, such as breathing against a closed shutter.
  • Claustrophobia can be exacerbated by entering the body box.
  • Existence of equipment or other conditions that could interfere with pressure changes, such as a transtracheal oxygen catheter, a chest tube, or a ruptured eardrum, or that cannot fit into the plethysmograph, such as continuous intravenous infusions with pumps or other equipment.
  • Oxygen treatment is ongoing and cannot be stopped momentarily.[5][12][13]

Potential Diagnosis

Table 3. Changes Observed in Lung Volumes and Resistance on Body Plethysmography in Major Pulmonary Pathologies [14][15]

Abbreviations: FRC, functional residual capacity; RV, residual volume; TLC, total lung capacity; R_aw, airway resistance; sR_aw, specific airway conductance.

Normal and Critical Findings

Reporting Findings of Body Plethysmography

The reported VTG should adhere to the following guidelines:

• Should be averaged from a minimum of 3 to 5 separate, acceptable panting maneuvers.
• Should be calculated using values that agree within 5% of the mean. Widely varying values should be averaged and reported as a variable.
• Should indicate whether the thoracic volume was at FRC or some other level.
• Should be compared with other lung volume measurements, such as helium dilution or nitrogen washout, if these tests are performed.
• Should be corrected for patient weight for some systems.[16][17]

During the same testing session, lung volumes, including the slow vital capacity (VC) maneuver, its subdivisions, inspiratory capacity (IC), and expiratory reserve volume (ERV), should be measured. Before disconnecting from the measuring system, VC, IC, and ERV should be measured with each VTG trial.

• The largest volume of VC or FVC obtained should be used to calculate derived lung volumes, such as TLC, RV, and RV/TLC.
• The mean values should be reported for IC and ERV from acceptable VTG maneuvers.
• Various methods exist to calculate TLC; the most common is TLC = mean FRC + mean IC.
• RV = TLC − largest VC.

The reported R_aw and sG_aw should adhere to the following guidelines:

• Should be calculated from the ratio of closed and open shutter tangents for each maneuver.
• Should be averaged from 3 to 5 separate, acceptable maneuvers; reproducibility should be based on sG_aw, and the suggested limit for variance is within 10% of the mean, such as if the measured results are ≤0.17, accept ±0.01, or if the measured results are ≥0.20, use ±0.02.
• The open-shutter tangent (V/P_box) should be measured between flows of +0.5 and −0.5 L/s. For loops that display hysteresis, the inspiratory limbs may be used.
• sG_aw should be calculated using the VTG at which the shutter was closed for each maneuver.

The normal range of predicted values of lung volumes is as follows:

• RV and TLC normal range: 80% to 120% of predicted.
• RV/TLC and IC/TLC normal range: 80% to 120% of predicted.
• R_aw: 130% predicted.
• G_aw: 80% predicted.[18]

Interfering Factors

Acceptability Criteria

The validity of the results depends on both the proper functioning of the equipment and the individual's ability to perform acceptable and reproducible maneuvers.[5] 

VTG maneuvers are acceptable in these situations:

1. The displayed tracing demonstrates proper panting technique, with the loop generated against a closed shutter appearing closed or nearly closed. To avoid pressure changes brought on by the lips, the patient should use their hands to support their cheeks without raising the shoulders or supporting the elbows.
2. All recorded pressure changes must remain within the calibrated range of each transducer, and the complete trace should be visible. Pressure fluctuations that are too large or too small may result in inaccurate measurements.
3. Tracings should remain stable on the screen or recording device, with thermal equilibrium clearly achieved, typically after 1 to 2 minutes.
4. The frequency of panting is roughly 1 Hz. The plethysmograph system may be specifically made to conduct non-panting motions.

R_aw and sG_aw maneuvers may be considered acceptable in these situations:

1. They satisfy the aforementioned requirements (points 1-3).
2. A comparatively closed loop is demonstrated by the open-shutter panting maneuver, especially between +0.5 and −0.5 L/s.
3. To facilitate interpretation, the panting frequency during serial measurements in a particular patient is maintained constant (a range of 90-150 cycles/min, 1.5-2.5 Hz). The frequency should also be maintained constant for serial testing and within-testing session comparisons, such as pre- and post-bronchodilator testing.

Complications

Possible Complications

• Syncopal attacks secondary to raised intrathoracic pressures due to improper panting against a closed shutter while measuring VTG and R_aw. 
• Hypercapnia or hypoxia secondary to prolonged confinement in the body box chamber, if not opened periodically.
• Infections can spread through unclean equipment, such as mouthpieces, or inadvertent transmission of bodily fluids or droplet nuclei from patient to patient or patient to technician.[5]

Patient Safety and Education

Quality Control

Body plethysmography is technically demanding, requiring careful attention to various details. Quality control must be maintained.[14] Otherwise, the following errors can happen:

• Overestimation of VTG in patients with severe obstruction or induced bronchospasm unless a slow panting speed (approximately 1 cycle/s) is maintained.
• Erroneous measurement of VTG, R_aw, or G_aw due to improper panting technique. Excessive pressure fluctuations or signal drift during panting may invalidate VTG, R_aw, or sG_aw.
• VTG or R_aw measurements may be invalidated by non-panting maneuvers in plethysmographs with integrated thermal leakage.
• Slopes of VTG or R_aw tangents, computer-determined, may be inaccurate. In many systems, the slopes are determined using a best-fit regression analysis. If unnecessary data points are collected because of poor panting or severe signal drift, this technique may yield results that differ greatly.
• Every slope must be visually examined and modified following a recognized laboratory protocol.
• The measured VTG may be elevated due to compression effects caused by excessive abdominal gas or the use of accessory muscles during panting.
• In many systems, the slopes are determined using a best-fit regression analysis. If unnecessary data points are collected because of poor panting or severe signal drift, this technique may yield results that differ greatly. Every slope must be visually examined and modified following a recognized laboratory protocol.
• Several transducers should be carefully calibrated, with attention given to leakage, thermal stability, and frequency response.
• When the equipment is moved to a different place or calibrated at prescribed frequencies, accuracy is questionable.[5][12][19]

Precautions

The following precautions are recommended to ensure accurate and reliable body plethysmography measurements:

• The mouth pressure, box, and volume should be checked every day.
• In addition to using the autocalibration system daily, systems should be manually calibrated at least once every 3 months.
• The linearity of the flow-sensing device should be checked at least once every week.
• A known resistor to measure airway resistance should be used, and the findings should be computed at least once every quarter.
• Volume should be checked with an isothermal bottle at least annually.
• Tests on standard participants (biological controls, or bio-QC) should be performed at least monthly and at any time accuracy is suspected.
• To establish statistical variation for comparison, standard participants should be tested more frequently at first.

Clinical Significance

Body plethysmography is a valuable tool for diagnosing, assessing, and treating pulmonary diseases. The accurate quantification of the lung volumes, airway resistance, and thoracic gas volume allows the distinction between obstructive and restrictive lung diseases. The capacity of body plethysmography to identify gas trapping and small airway dysfunction makes it particularly useful for identifying diseases such as COPD and asthma, even in their early stages. Body plethysmography is also valuable for assessing treatment response, monitoring disease progression, and guiding individualized therapeutic interventions in pulmonary disorders.

Table 4. Clinical Interpretation of Derived Indices

Abbreviations: TLC, total lung capacity; RV, residual volume; R_aw, airway resistance; sG_aw, Specific airway conductance; COPD, chronic obstructive pulmonary disease.

Body plethysmography is an essential tool for diagnosing and monitoring respiratory diseases. The ability of body plethysmography to measure lung volumes beyond conventional spirometry makes it indispensable in pulmonology. Integrating plethysmographic data with clinical assessment enhances disease diagnosis, management, and treatment optimization.