Home sleep testing requires coordination between the clinical team and the patient that goes far beyond the norm in a clinical diagnostic setting. The patient is required to set up the study and in many cases initiate that study, independent of clinical participation. The system I have the most experience with is the Nox T3 by Carefusion which the AASM qualifies as a level 3 portable monitor. One of my favorite features of the Nox T3 by Carefusion is the redundancy in flow signal measurement. It is possible with this device to register flow with an oronasal thermal sensor (thermistor) and a/or a nasal pressure transducer. This device also registers flow through the use of RIP or (respiratory inductance plethysmography). It is common for patients to incorrectly set up the nasal cannula, or pull the thermistor off in the night. With most HST devices this results in a lost study, with all the costs that go with it. Nox T3 by Carefusion allows the clinician to re-reference the study to the RIP belts and use the RIP Flow channel, for the study which saves wasted time, cost of disposables, increases patient satisfaction and increases clinical yield.
RIP or (respiratory inductance plethysmography) is a critical part of a home sleep testing product in my opinion. I hope to set out in this article a clear rational for using RIP for home sleep testing. First HST guidelines set out by the AASM precisely set out the indications, methodology and technology required for home sleep testing. In the Clinical Practice Guideline cited above section 2.3 states that Ideally the sensor for identification of respiratory effort is “either calibrated or uncalibrated inductance plethysmography”. The guidelines state that “at minimum HST must record airflow, respiratory effort and blood oxygenation”. For this article we will concentrate on RIP.
What is Respiratory Inductance Plethysmography:
RIP relies on the principle that electric current applied through a loop of wire generates a magnetic field normal to the orientation of the loop (Faraday’s Law) and that a change in the area enclosed by the loop creates an opposing current within the loop directly proportional to the change in the area (Lenz’s Law). 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).
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.
With RIP, no electrical current passes through the body (a weak magnetic field is present that does not affect the patient or any surrounding equipment). The signal produced is linear and is a fairly accurate representation of the change in cross-sectional area. In addition, RIP does not rely on belt tension, so is not affected by belt trapping.
The picture below shows why it is possible to calculate a patient’s flow and volume by accurately measuring the cross-section area of the rib cage and abdomen. The reason lies within the interdependency of the lungs, chest, sternum, rib cage and diaphragm. Without abdomen and thorax movements – there is no flow.
The guidelines don’t make a distinction between uncalibrated and calibrated RIP. In my opinion calibrated RIP is much better because it is a way to increase study accuracy and clinical yield.
A calibrated RIP signal is necessary to measure a quantitative change in volume and thereby determining an accurate flow signal. This ensures that a change in volume of the abdomen or the thorax result in the same signal amplitude resulting from the abdomen or thorax RIP belt, respectively. Calibration of these signals is performed by determining the correct weight between the signal coming from the abdomen on one hand, and the thorax on the other hand. This is necessary so that the contribution from both abdomen and thorax can be correctly calculated to deliver accurate flow and volume measurements.
I cannot tell you how many times I have re referenced a study to my RIP bands because the cannula fell off or the patient decided on their own that the thermistor was uncomfortable. In my opinion it is critical to HST that two effort bands be used and that they be either calibrated or uncalibrated RIP in order to fit AASM Clinical Guidelines for the Use of Unattended Portable Monitors in the Diagnosis of Obstructive Sleep Apnea in Adult Patients.