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Respiratory Monitoring: Advantages of Inductive Plethysmography over Impedance Pneumography

Chris Landon M.D. FAAP, FCCP Pediatric Pulmonologist/Director of Pediatrics Ventura County Medical Center

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Respiratory Monitoring: Advantages of Inductive Plethysmography over Impedance Pneumography ABSTRACT Inductive plethysmography and impedance pneumography are two non-invasive technologies for measuring respiratory function. The following report describes the advantages of inductive plethysmography, which include greater accuracy and sensitivity; the ability to assess thoracoabdominal coordination; and the minimization of motion artifact. Together, these properties make inductive plethysmography a superior technique for identifying and differentiating obstructive, central and mixed sleep apneas, and for detecting hypopneas. INTRODUCTION The ability to accurately monitor respiratory function is critical to the study of such disorders as sleep apnea and chronic obstructive pulmonary disease, where respiratory dysfunction can be dangerous and even fatal. In addition, certain medications can affect respiration and monitoring these effects can be useful to the clinical researcher. Respiratory monitoring can also provide insight into such conditions as anxiety disorder, where respiratory instability has been documented even independent of panic attacks.1 Impedance pneumography and inductive plethysmography are two noninvasive technologies available for measuring respiratory function. Impedance pneumography has been available for decades and is commonly used in clinical research. However, the technology has limitations that can affect the accuracy and application of this method. Inductive plethysmography is a newer alternative to impedance pneumography that is associated with a higher degree of accuracy and has additional practical benefits to the clinical researcher. IMPEDANCE PNEUMOGRAPHY Impedance pneumography employs low amplitude, high frequency (50 to 500 kHz) alternating current (AC) between two surface electrodes to record thoracic movements or volume changes at the rib cage (RC) during a respiratory cycle. Based on Ohm's

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Law, the voltage drop across the electrodes is computed as impedance, which increases during inspiration and decreases during expiration. Several limitations inherent in impedance pneumography technology can lead to errors in respiratory measurement (Table 1). First, the electrical resistance of RC tissues is less than air; therefore AC passing across the thoracic cavity reflects mainly tissue impedance. Thus, while the technique can provide a qualitative indication of chest-wall movement, there is no direct relationship to thoracic volume. Further, the electrodes attached to the skin record impedance of all tissue types through which the electrical current travels, including muscle. The technology is therefore more prone to motion artifact. Cardiogenic artifact is another source of recording error inherent in impedance pneumography. Further, the RC signal in impedance pneumography is dependent on posture, making tidal volume difficult to estimate. The RC signal also is difficult to calibrate and the polarity of the signal is prone to changing suddenly and erratically. Measurement errors also can result from internal impedance of the device (e.g., components, wires and cables). Finally, because impedance pneumography is unable to assess thoracoabdominal coordination, it cannot be used to distinguish central or mixed apnea from obstructive apnea during sleep studies.

Table 1. Limitations of impedance pneumography include: · Provides only a qualitative indication of chest-wall movement; there is no direct relationship to the volume of air within the chest. · Susceptible to motion and cardiogenic artifact. · Prone to signal degradation with changes in body position. · Difficult to calibrate RC signal and achieve stable signal polarity · Cannot clearly differentiate obstructive apneic events from central or mixed apneic events in the absence of airflow measures. Adapted from2

INDUCTIVE PLETHYSMOGRAPHY Inductive plethysmography employs sensors to measure changes in a crosssectional area of the RC and abdominal (AB) compartments during a respiratory and cardiac cycle. The sensors consist of arrays of sinusoidally arranged copper wires excited by a low-current, high-frequency (300 kHz) electrical oscillator circuit. Movement of the RC or AB compartments causes the sensors to generate magnetic fields, which are measured

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as voltage changes over time (i.e., waveforms). No electricity passes through the monitored individual. By virtue of its design, inductive plethysmography offers advantages over impedance pneumography for accurately measuring patients' respiratory function (Table 2). For example, the technology is associated with less signal interference and distortion. When contrasted against spirometry (the "gold standard" for determining lung volumes), inductive plethysmography is associated with less error (±10%) compared to impedance pneumography (> 15%).3,4 In addition, inductive plethysmography includes bands placed over the abdomen in addition to the rib cage, allowing measurement of the phase relationship between the two bands. Therefore, unlike impedance pneumography, inductive plethysmography can help determine central apnea from obstructive apnea during sleep studies. Several studies have compared the efficacy of impedance pneumography and inductive plethysmography to measure respiratory events in sleep. A study published in the Journal of Pediatrics found that thoracic impedance monitors, when compared to inductive plethysmography monitors, may be less sensitive to monitoring respiratory events related to obstructive sleep apnea in infants.5 Specifically, the study reported that impedance monitors "may fail to detect obstructive apnea, may falsely alarm when the infant is breathing, and may confuse cardiac artifact with respiratory impedance." The results in this study were directly attributed to the fact that impedance pneumography did not allow for the measurement of thoracoabdominal coordination. A study by the American Academy of Sleep Medicine Task Force evaluated hypopnea detection with several non-invasive methods to measure breathing patterns during sleep.6 The results showed that inductive plethysmography was the best non-invasive tool in the assessment of sleep related breathing disorders. Similarly, a study designed to evaluate the efficacy of inductive plethysmography in the assessment of Upper Airway Resistance Syndrome (UARS) showed that the ratio of peak inspiratory flow to mean flow (PIF/MF) measured by inductive plethysmography resulted in the most accurate identification of UARS patients when breaths were selected for analysis immediately prior to arousals.7

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Table 2. Comparison Between Inductance And Impedance Technologies

Feature Detects obstructive and mixed apneas Detects central apneas Measures changes in tidal volume Detects hypopneas Means to detect a breath from a calibrated waveform thereby not counting smaller deflections from motion artifacts as breaths Provides accurate breath rates Displays breath waveforms that have equivalent shapes to waveforms from spirometers and pneumotachographs connected to airway Can be calibrated to volume equivalency from spirometer, pneumotachograph or fixed volume chamber Used with heart rate for time series respiratory sinus arrhythmia measure Accurate timing of breath waveforms Assesses thoracoabdominal coordination Wakefulness and sleep staging capabilities Detects all elements of the sequence of respiratory muscle fatigue and dysfunction Digital data stream output Breath amplitude not susceptible to postural alterations Random, unexplained variability of breath waveforms in terms of polarity shape and amplitude do not occur Cardiogenic artifacts do not distort respiratory waveforms Electric current not passed through body Analog signal outputs Inductance Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Impedance No Yes No No No No No No No No No No No No No No No No Yes

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SUMMARY AND DISCUSSION Inductive plethysmography and impedance pneumography are two non-invasive technologies for measuring respiratory function. Although both methods can provide similar information, inductive plethysmography is more accurate and allows for a higher degree of specificity. While impedance plethysmography has been available for decades and is commonly used, it is a less sensitive tool, requires application of an electrical current directly to the patient, and is more prone to cardiogenic and motion artifact. By virtue of its design, inductive plethysmography eliminates the signal interference and distortion that is often found with impedance pneumography, enabling clinicians to obtain a more accurate measurement of patients' respiratory and cardiac functions. Specifically, the physician can detect obstructive apnea, a partial or complete blockage of the airway caused during sleep. The detection of this condition can be useful when studying sleep disorders or sudden infant death syndrome (SIDS).8,9 In the latest innovation in inductive plethysmography, the technology has successfully been incorporated into a continuous ambulatory monitoring system, the central component of which is a comfortable, lightweight, washable garment that collects around-the-clock information on a customizable range of cardiopulmonary parameters. As an ambulatory monitoring device, the garment is designed to capture physiologic and other data continuously in patients' real-world setting. In addition to respiratory monitoring, the system can monitor blood pressure, heart rate, movements in posture and other parameters of health and activity. Further, the system includes a digital patient diary for recording qualitative information such as patient mood, providing the ability to crosscorrelate physiological and psychological symptoms. The device also addresses limitations of traditional inductive plethysmography technology, such as band slippage, since the fitted shirt ensures correct placement of leads and bands. In general, clinical researchers should consider the advantages of inductive plethysmography over impedance pneumography when conducting respiratory monitoring studies. The ambulatory monitoring system, which as recently been cleared by the Food and Drug Administration, provides additional advantages for real-world, ambulatory patient monitoring that include the capability to capture patient self-reports of activity and mood. This ability to cross-correlate physiological parameters with other real-

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world data offers the potential to advance the field of respiratory monitoring and improve the quality of data collected in clinical trials and other research studies. We have initiated studies to develop the technology for quantitation of the ventilatory and sleep abnormalities associated with sleep-disordered breathing in children. The current gold-standard, polysomnography, can be performed satisfactorily in children of any age, providing that appropriate equipment and trained staff is used and scoring and interpretation utilize age-appropriate criteria. However, there is currently a shortage of facilities that perform pediatric polysomnography.(10) Abbreviated or screening techniques, such as videotaping, nocturnal pulse oximetry, and daytime nap polysomnography tend to be helpful if results are positive but have a poor predictive value if results are negative. (10) We look forward to evaluating the ambulatory monitoring system described above against measured polysomnographic parameters, correlating its output with adverse outcomes in children with obstructive sleep apnea syndrome (OSAS), and using it to establish criteria for differentiation between primary snoring and OSAS.

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Roth WT, Wilhelm FH, Trabert W. Voluntary breath holding in panic and generalized anxiety disorders. Psychosom Med. 1998;60:671-679. 2 Blunt, JY. Impedance pneumography. In: Spacelabs Biophysical Measurement Series, Respiration. Redmond, Washington: Spacelabs Inc; 1992, 107-126. 3 Ellis WS, Jones RT. Using LabVIEW to facilitate calibration and verification for respiratory impedance plethysmography. Computer Methods & Programs in Biomedicine. 1991;36:169-175. 4 Tobin MJ, Chadha TS, Jenouri G, Birch SJ, Gazeroglu HB, Sackner MA. Breathing patterns. 1. Normal subjects. Chest. 1983;84:202-205. 5 Brouillette RT, Morrow AS, Weese-Mayer DE, Hunt CE. Comparison of respiratory inductive plethysmography and thoracic impedance for apnea monitoring. Journal of Pediatrics. 1987;111:377-383. 6 Flemons WW, Buysee D. Sleep-related breathing disorders in adults: recommendations for syndrome definition and measurement techniques in clinical research. The Report of an American Academy of Sleep Medicine Task Force. Sleep. 1999;22:667-689. 7 Loube DI, Andrada T, Howard RS. Accuracy of respiratory inductive plethysmography for the diagnosis of upper airway resistance syndrome. Chest. 1999;115:1333-1337. 8 Miyasaka K, Kondo Y, Suzuki T, Sakai H, Takata M. Toward better home respiratory monitoring: a comparison of impedance and inductance pneumography. Acta Paediatrica Japonica. 1994;36:307-310. 9 Ramanathan R, Corwin MJ, Hunt CE, Lister G, Tinsley LR, Baird T, et al. The Collaborative Home Infant Monitoring Evaluation Study. Cardiorespiratory events recorded on home monitors: Comparison of healthy infants with those at increased risk for SIDS. JAMA. 2001;285:2199-2207. 10 Section on Pediatric Pulmonology, Subcommittee on Obstructive Sleep Apnea Clinical Practice Guideline: Diagnosis and Management of Childhood Obstructive Sleep Apnea Syndrome PEDIATRICS Vol. 109 No. 4 April 2002, pp. 704-712

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