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).
Restrained Versus Freely Moving
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.