Respiratory inductance plethysmography

Respiratory inductance plethysmography (RIP) is a method of evaluating pulmonary ventilation by measuring the movement of the chest and abdominal wall. 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. Konno and Mead extensively evaluated a two-degrees-of-freedom model of chest wall motion, whereby ventilation could be derived from measurements of rib cage and abdomen displacements. With this model, tidal volume (Vt) was calculated as the sum of the anteroposterior dimensions of the rib cage and abdomen, and could be measured to within 10% of actual Vt as long as a given posture was maintained. Changes in volume of the thoracic cavity can also be inferred from displacements of the rib cage and diaphragm. Motion of the rib cage can be directly assessed, whereas the motion of the diaphragm is indirectly assessed as the outward movement of the anterolateral abdominal wall. However, accuracy issues arise when trying to assess accurate respiratory volumes from a single respiration band placed either at the thorax, abdomen or midline. Due to differences in posture and thoraco-abdominal respiratory synchronization it is not possible to obtain accurate respiratory volumes with a single band. Furthermore, the shape of the acquired waveform tends to be non-linear due to the non-exact co-ordination of the two respiratory compartments. This further limits quantification of many useful respiratory indices and limits utility to only respiration rates and other basic timing indices. Therefore, to accurately perform volumetric respiratory measurements, a dual band respiratory sensor system must be required.