Respiratory inductance plethysmography

Respiratory inductance plethysmography (RIP) is a method of evaluating pulmonary ventilation by measuring the movement of the chest and abdominal wall.

Accurate measurement of pulmonary ventilation or breathing often requires the use of devices such as masks or mouthpieces coupled to the airway opening. These devices are often both encumbering and invasive, and thus ill suited for continuous or ambulatory measurements. As an alternative RIP devices that sense respiratory excursions at the body surface can be used to measure pulmonary ventilation.

According to a paper by Konno and Mead “the chest can be looked upon as a system of two compartments with only one degree of freedom each”. Therefore, any volume change of the abdomen must be equal and opposite to that of the rib cage. The paper suggests that the volume change is close to being linearly related to changes in antero-posterior (front to back of body) diameter. When a known air volume is inhaled and measured with a spirometer, a volume-motion relationship can be established as the sum of the abdominal and rib cage displacements. Therefore, according to this theory, only changes in the antero-posterior diameter of the abdomen and the rib cage are needed to estimate changes in lung volume.

Several sensor methodologies based on this theory have been developed. RIP is the most frequently used, established and accurate plethysmography method to estimate lung volume from respiratory movements.

RIP has been used in many clinical and academic research studies in a variety of domains including polysomnographic (sleep), psychophysiology, psychiatric research, anxiety and stress research, anesthesia, cardiology and pulmonary research (asthma, COPD, dyspnea).

A respiratory inductance plethysmograph consists of two sinusoid wire coils insulated and placed within two 2.5 cm (about 1 inch) wide, lightweight elastic and adhesive bands. The transducer bands are placed around the rib cage under the armpits and around the abdomen at the level of the umbilicus (belly button). They are connected to an oscillator and subsequent frequency demodulation electronics to obtain digital waveforms. During inspiration the cross-sectional area of the rib cage and abdomen increases altering the self-inductance of the coils and the frequency of their oscillation, with the increase in cross-sectional area proportional to lung volumes. The electronics convert this change in frequency to a digital respiration waveform where the amplitude of the waveform is proportional to the inspired breath volume.

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