Respiratory Inductance Plethysmography An Introduction

Transcription

1 Respiratory Inductance Plethysmography An Introduction Gandis G. Mazeika, MD Rick Swanson, RPSGT, CRTT Committed to Excellence Pro-Tech Services, Inc Harbour Pointe Blvd. S.W. Mukilteo, WA (425) (800) Fax: (425) Copyright 2007 by Pro-Tech Services, Inc. All rights reserved

2 Background 3 Measuring Respiratory Effort 3 Elastomeric plethysmography 4 Impedance plethysmography 4 Respiratory Inductance Plethysmography (RIP) 5 Historical Considerations 5 Technical Considerations 5 Applications of Respiratory Inductance Plethysmography (RIP) 6 Recording RIP Effort Signals 7 Comparing RIP to Piezo Crystal Effort Belts 8 Poor Signals 8 Waveform Samples 10 2

3 Respiratory Inductance Plethysmography An Introduction Background In a polysomnographic study, a variety of physiologic parameters must be measured. One of the most important assessments, breathing, is obtained by measurement of nasal and/or oral airflow in tandem with measurements of chest and abdominal wall movement. An important task in scoring and interpreting a polysomnogram is to assess whether apnea is present and to distinguish between obstructive and central apnea. Obstructive apnea is defined as an absence of airflow in the presence of continued effort to breathe. While this is a fairly straightforward definition, physiological assessment of obstructive apnea can be challenging. The essential task is to demonstrate effort to breathe in the absence of significant airflow. Measuring Respiratory Effort Respiratory effort is directly measured by esophageal manometry. Esophageal pressure (Pes) is measured by having the patient swallow a pressure catheter which then resides in the esophagus throughout the sleep study. Rhythmic fluctuations in thoracic pressure in the absence of significant nasal and oral airflow are the best proof of the presence of obstructive apnea. In clinical practice however, esophageal pressure is bothersome to most patients and is therefore not used routinely. A reasonable surrogate measure of respiratory effort can be obtained by measuring changes in chest and/or abdominal volume, also known as plethysmography. Changes in lung volume are most accurately measured using spirometry equipment, in which lung volumes (Tidal Volume) and flow rates (Flow Loop) are determined by having the patient breathe through a closed tube. Spirometry typically requires use of a nasal air seal and conscious effort, therefore it is impractical for use in clinical polysomnography. 3

4 There are three primary methods of non-invasive chest and abdominal plethysmography (measurement of change in volume) in current use: measurement of changes in elastic belt tension, measurement of changes in electrical impedance, and measurement of changes in electrical inductance. Elastomeric plethysmography An elastic belt fastened around the chest or abdomen will exhibit a change in tension as the chest or abdomen expands or contracts. This change in tension can be easily measured and converted to a voltage by a variety of methods. The most common method in current use is a piezo-electric sensor, i.e., a crystal that directly generates a voltage when compressed or stretched. This method, while simple and inexpensive, is subject to trapping artifact: it is fairly easy to imagine how a portion of elastic belt may become trapped as a person turns from one side to another, resulting in variable tension along the belt circumference. Thus this method can both significantly under and/or overestimate the actual degree of chest or abdominal movement in addition to creating a false signal when belt tension suddenly changes with a change in body position. Impedance plethysmography The human body is a fairly poor conductor of electricity. In other words, it presents a fairly high impedance to electrical current flowing through it. This impedance changes as the cross-section of the body expands and contracts, allowing qualitative measurement of thoracic and abdominal movement during breathing. Two (or sometimes four or more) electrodes are attached to the skin. A weak alternating electrical current is passed through these electrodes, allowing the impedance to be measured. This method yields a non-linear signal, thus is useful only as a qualitative measure of chest or abdominal movement. Given that an electrical current must be passed through the body, care must be taken to choose a frequency range that would not interfere with other monitoring equipment or with implanted equipment such as pacemakers or defibrillators. 4

5 Respiratory Inductance Plethysmography (RIP) RIP relies on the principle that a 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). An elastic belt into which a zigzagging (coiled) wire is sewn (to allow for expansion and contraction) is worn around the chest or abdomen. An alternating current (AC) is passed through the belt, generating a magnetic field. The frequency of the alternating current is set to be more than twice the typical respiratory rate in order to achieve adequate sampling of the respiratory effort waveform. The act of breathing changes the cross-sectional area of the patient s body, and thus changes the shape of the magnetic field generated by the belt, inducing an opposing current that can be measured, most easily as a change in the frequency of the applied current. 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, thus is not affected by belt trapping. Historical Considerations Inductance plethysmography was initially developed as a tool to non-invasively measure respiratory volumes and was used primarily in pediatric and veterinary asthma research. It was adapted for human polysomnography in the 1990 s, and its use has rapidly increased as patents on the technology have expired and associated costs have come down. Technical Considerations RIP equipment consists of the following: 1. Effort belt, consisting of an elastic material with a zigzagging (coiled) wire sewn into the belt. 2. Connecting wire sets. 3. Driver module consisting of a frequency generator, signal processor and analog/digital converter. 5

6 Applications of Respiratory Inductance Plethysmography (RIP) RIP can be in tandem with nasal/oral airflow to produce a flow-volume hysteresis loop. This technique can be employed to assess inspiratory and expiratory flow limitation as a function of body position, sleep stage, etc. The chest and abdominal signals can be represented independently, or they can be mathematically summed. Mathematical summing of the signals is particularly useful as a screen for paradoxical breathing. Because there are differences in the amplitudes of chest and abdominal output signals, these values are typically normalized prior to summation. 6

7 The summing channel can also be thought of as an indicator of the phase relationship of the chest and abdominal belts. The more out of phase the signals are becoming (moving toward paradox), the smaller the sum channel will be. When expansion and contraction of chest and abdomen are completely out of phase, the sum channel will be flat. However, due to the method in which the sum channel is created, a completely flat sum signal is rare due to the delay in the summing and normalization of the channels in the summing process. The sum signal does provide a very useful function by presenting a definite decrease in the signal amplitude during events which include paradoxical breathing signals. Recording RIP Effort Signals When recording through an AC channel the following settings would be used: Low Frequency Filter (High Pass Filter) (Time Constant) 0.1 Hz or lower ( Hz) 0.1 Hz or lower ( Hz) 1.0 Sec or longer (3-5 Sec) Using a low frequency setting of 0.05 to 0.01 Hz can allow visualization of possible flattening in the thorax belt which sometimes occurs along with a flattening of the observed signal from a pressure transducer. High Frequency Filter (Low Pass Filter) 35 Hz 35 Hz An average resting respiratory rate is 12 breaths per minute. This is equal to approximately 0.2 Hz a relatively low setting of the High Frequency Filter (Low Pass Filter) should not influence this signal. Sampling Rate 10 Hz or higher 7

8 This figure shows how the RIP belts can parallel the output of a pressure transducer used for flow. Comparing RIP to Piezo Crystal Effort Belts The sensing element, a zigzagging wire, on a RIP effort belt runs the entire length of the belt. When placed on the patient the sensing element covers their entire circumference. As such, all changes in breathing are detected regardless of the position the patient is in. The sensing element, a piezo crystal, on a piezo effort belt is located only on a very small section of the belt s length. As such, there are situations, for example when a patient is lying on top of the piezo crystal, where the effort signal can be dampened, not detected, producing erroneous readings or unexplained changes in polarity that look like paradoxical effect. For the reasons listed above, RIP effort sensors are superior to that of piezo. Poor Signals RIP technology has been shown to be very accurate in determining the effort of breathing. However, there are conditions that can decrease the accuracy of the device. If the belts are placed too tight causing the actual cross sectional change of the chest or abdomen to be 8

9 restricted, it will not reflect the patient s true breathing efforts. If the belts are placed too loosely, the belts will have a tendency to move and may overlap one another. Another consideration is belt placement. For example, if a RIP effort belt is placed around the hips, there will be little to no change in the cross sectional areas during diaphragmatic excursions. To ensure quality signals, RIP belts should be placed at the standard locations: near the nipple line (or mid-chest) and just above the belly button. 9

10 Waveform Samples Note the flattening of the Chest signal that occurs simultaneously with the Snore signal 10

11 Cardio-ballistic artifact in Pressure/flow signal 11

12 If this was UARS, there would be a shift in the phase between the chest and abdominal signals. Check the close up below. 12

13 Abdomen and Chest inputs connected with reversed polarity. Make certain the same inputs are used for Grid 1 and Grid 2. Reference and signal inputs must be connected with the same polarity. 13