Techniques & Measurements

This page lists techniques and measurements employed to capture information on pulmonary function in conscious subjects using plethysmographs.


Air volume changes created by a spontaneously breathing, conscious subject within a body box (or plethysmograph) are the basis of the plethysmography technique. These changes are captured using pneumotachographs or pressure transducers, translating into an airflow or pressure signal which is examined and analyzed to gather information on the subject’s pulmonary function. Variations of the technique exist whether the subject is free to move or restrained within the body chamber. These are referred to as unrestrained whole body plethysmography (WBP), double chamber plethysmography (DCP), or head-out plethysmography (HOP).


Unrestrained whole body plethysmography

In unrestrained whole body plethysmography (WBP), the subject is free to move within a small, closed plethysmograph chamber. As it breathes spontaneously, the airflow in and out of the body box or the changes in pressure are recorded. The flow signal generated reflects the box pressure changes and is not a direct respiratory flow measurement. Analysis of the WBP waveform follows the subject’s breathing pattern and analyzers within the software (iox by emka TECHNOLOGIES) provide endpoints related to the breathing pattern in general (e.g. respiratory rate, estimates of tidal volume, minute ventilation) or to specific aspects of it (e.g. inspiratory / expiratory time, estimates of peak inspiratory or expiratory flows). The controversial dimensionless quantity known as enhanced pause (Penh) can also be calculated.

Double chamber & head-out plethysmography

In double chamber (DCP) and head-out plethysmography (HOP), the measurements are performed in conscious, spontaneously breathing subjects positioned within a restraint. As with WBP, the HOP and DCP techniques provide parameters describing the breathing pattern or characteristics of it. The major difference with these techniques and WBP is that the air displaced by the chest wall movements can be captured to permit true respiratory flow and tidal volume measurements. In the DCP technique, the set-up includes a head chamber to separately record nasal and thoracic flows. Specific airway resistance (sRaw), and its reciprocal specific airway conductance (sGaw), can therefore be calculated from the time shift (dT) between these flow signals.


Abbreviation Parameter Description Whole body plethysmograph (WBP) Head-out double chamber plethysmograph (HOP) Double chamber plethysmograph (DCP)
AV Accumulated volume Sum of tidal volumes. Updated during storage and restored at each storage start.
dT Time delay The time delay between the thoracic and the nasal flow signals.
EEP End-expiratory pause The pause that follows expiration.
EF50* Mid-expiratory flow The flow rate when half the volume has been expired.
EIP End-inspiratory pause The pause that follows inspiration.
EV* Expired volume The integral of the expiratory (positive) time.
f Breathing frequency The breath-by-breath rate of breathing computed on PEF to PEF or on Ti+Te.
MV Minute volume The total volume breathed during one minute, computed on a breath-by-breath basis (TV*f).
n Number of beats The number of beats averaged in one data line.
N Count Number of events detected during the log/storage time period.
Ni Total accumulated count Total number of events detected since the start of experiment.
Np Period accumulated count Number of events detected during the current storage period. This number resets to zero at each new storage period.
NEV Nasal expired volume The integral of the expiratory (positive) time for the nasal chamber.
NTV Nasal tidal volume The integral of the inspiratory (negative) time for the nasal chamber
Pau Pause An indicator of bronchoconstriction, computed as: (Te/ RT – 1).
PEF* Peak expiratory flow The maximum positive flow during one breath.
Penh* Enhanced Pause
Dimensionless indicator of broncho-constriction, computed as: (TE/RT – 1) * (PEF/PIF)
PIF* Peak inspiratory flow The maximum negative flow during one breath.
RT Relaxation time The time to expire a defined percentage of tidal volume.
sGaw Specific airway conductance The inverse of the airway resistance times the thoracic gas volume.
Sr Success rate The ratio number of valid beats against detected beats in one data line. Expressed in percentage. Reported in the data table for each new value.
sRaw Specific airway resistance The airway resistance times the thoracic gas volume.
Te Expiratory time The time from start of expiration to beginning of next inspiration.
Ti Inspiratory time The time from start of inspiration to end of inspiration.
TV* Tidal volume The integral of the inspiratory (negative) time.

*Estimated in WBP

Learn about the advantages and limitations of sRaw and more here