A Portable Respiratory Monitor Using Respiratory Inductive Plethysmography H.T. Ngo1,2, C.V. Nguyen1, T.M.H. Nguyen1, and Toi Van Vo1 1
Biomedical Engineering Department, International University, Quarter 6, Linh Trung, Thu Duc Dist., Ho Chi Minh City, Vietnam 2 Biomedical Engineering Department, Pratt School of Engineering, Duke University, Durham, North Carolina, USA
Abstract –– Monitoring respiratory activity provides important information about patient’s well-being. There are many respiratory activity monitoring techniques introduced so far. This paper presents a portable respiratory monitor using Respiratory Inductive Plethysmography (RIP) technology. RIP sensor’s characteristics are also studied. Experimental results have shown that RIP sensor’s sensitivity and linearity can be improved by designing the RIP sensor properly. In addition, the monitor’s accuracy in term of respiratory rate measurement is verified by experiment on 10 subjects. Experimental results have shown that our monitor can measure respiratory rate with error ±1 bpm. Furthermore, it can transfer data to PC (via Bluetooth) where chest and abdomen movement efforts are plotted. These plots are useful for diagnosing and monitoring several respiratory disorders. Keywords –– Respiratory Monitor, Respiratory Inductive Plethysmography.
I.
INTRODUCTION
From a clinical point of view, 'respiratory activity' includes such descriptors as respiratory rate and depth, as well as quantifiable information about the degree of gas exchange actually taking place. Continuous monitoring of respiratory activity is important for identifying or predicting high-risk situations, making appropriate monitoring techniques potentially life-saving [1]. Many respiratory monitoring devices and methods have been introduced so far. They are classified into three categories which are listed in Table 1. Despite the large number of reported methods and devices for respiratory monitoring, convincing evidence of their clinical usefulness is still lacking. Inherent limitations regarding the detection principles, non-specific response, poor adaptation to clinical conditions and difficulties in clinical evaluation are identified as the primary causes [1]. Therefore, developing a new respiratory monitor is needed.
In this work, Respiratory Inductance Plethysmography (RIP) technology is used. There are hundreds of publications on RIP. Investigators suggest that RIP may be less prone to some of the artifacts caused by cardiac activity, motion of the subject or sensor and changes in external variables such as temperature and sound [2]. This technology is also recommended by American Academy of Sleep Medicine for measuring respiratory effort [3]. Nowadays, RIP is widely used at sleep centers across the world [4]. The principle of RIP technology, its qualities and applications have been introduced in [3], but simply put, the RIP sensor is an inductive coil made of insulated wires woven or sewn in a sine wave or zig-zag pattern on an expandable belt. One belt is placed around the rib cage and the other around the abdomen. During respiration, the length of the belts change and, therefore, self-inductance of the coils change proportionally. By measuring variation in the coils’ self-inductance, we can detect variation of rib cage’s and abdomen’s circumference or cross-sectional area. An important quality of RIP is linearity. In addition, because there is no electrical current passing through the patient and only a weak magnetic field is created, this technology is safe. Some RIP applications include distinguishing central apnea from obstructive apnea during sleep studies, detecting paradoxical breathing or slight phase shifts, and measuring the actual volume of airflow to create a “flow-volume loop” [3]. However, we found no detailed technical description of RIP sensor. The question is that: how pattern of wire woven or sewn along the belt and its parameters affect RIP sensor characteristics? In this work, we investigate effect of one parameter, step size, of sine wave pattern to sensitivity and linearity of RIP sensor. Sine wave pattern is chosen due to its proven effectiveness and ease of construction. In addition, we present a complete design of a respiratory monitor that is accurate, portable, inexpensive, and simple to use. Because RIP sensor is integrated into belt, the term sensor and belt are used interchangeably in this paper.
V. Van Toi et al. (Eds.): 4th International Conference on Biomedical Engineering in Vietnam, IFMBE Proceedings 40, pp. 222–225, 2013. www.springerlink.com
A Portable Respiratory Monitor Using Respiratory Inductive Plethysmography
223
Table 1 Categories of sensing principles for respiratory monitoring devices and methods (table from [1]). Category
II.
Typical measured quantity
Typical sensor position
Movement, volume and tissue Electromyography, abdomen and thoracic circumference, composition detection impedance or blood volume Airflow sensing Respiratory gas flow
Nasal/oral area
Blood gas measurement
Peripheral organ or Nasal/oral area
Arterial gas concentration
EFFECTS OF SINE WAVE’S STEP SIZE TO RIP SENSOR CHARACTERISTICS
To investigate effect of step size of sine wave pattern to characteristics of RIP sensor, we construct three RIP sensors with the same pattern of wire (sine wave), height (2.2 cm), and length (30 cm) but different step sizes as illustrated in Fig. 1 and Fig. 2.
Fig. 1 Three RIP sensors with the same sine wave pattern of wire, height, and length but different step sizes: 1 cm (a), 1.5 cm (b), 3 cm (c).
Abdomen and chest wall
measure of sensitivity of sensor. R-squared value gives a measure of linearity of sensor (values close to 1 indicate excellent linearity). Sensitivity and linearity of sensors with different step sizes are depicted in Fig. 6. The figure clearly shows that the smaller step size, the higher sensitivity. In term of linearity, there is slightly difference between 1 cm and 1.5 cm step size. However, for 3 cm step size, linearity decreases remarkably. Therefore, step size should be kept small to achieve good linearity.
Fig. 3 Inductance vs. stretched length of RIP sensor with step size 1 cm.
Fig. 2 Three RIP sensors depicted in Fig. 1 are constructed. We then measure inductance of each sensor versus stretched length of the sensor. The Agilent 4285A Precision LCR Meter is used. The measured data are plot in Fig. 3, Fig. 4, and Fig. 5. In these figures, in addition to plotting measured data scatterly, trend-line based on linear regression is also plotted. Slope of the trend-line gives a
Fig. 4 Inductance vs. stretched length of RIP sensor with step size 1.5 cm.
IFMBE Proceedings Vol. 40
224
H.T. Ngo et al.
Fig. 5 Inductance vs. stretched length of RIP sensor with step size 3 cm.
breathing rate, plot chest’s and abdomen’s movement, and Konno-Mead loop (Lissajous). The monitor is powered by a lithium-ion cell phone battery which can be recharged. The monitor is hence portable. We decided to use this type of battery due to its commercial availability, rechargeable ability, lightweight, and high capacity. Total cost of the monitor (Fig. 8) is less than one-hundred USD. To test the monitor’s accuracy in term of respiratory rate measurement, we do experiment with 10 subjects (7 male, 3 female, age from 18 to 21), each subject do experiment two times (Fig. 9). We measure their respiratory rate by two approaches simultaneously; one using our monitor and one asking subjects to count their own respiratory rate. Each experiment lasts one minute. During the experiments subjects seat comfortably without changing body’s posture. They were instructed to focus on counting their respiratory rate and not allowed to talk. They were also not allowed to look at the monitor’s LCD display and computer’s screen. Student T-test shows that there is no statistically significant difference between the two approaches with level of significance 5%. In addition, maximum error between two approaches is ±1 respiratory beat per minute. Chest belt
Fig. 6 Sensitivity and linearity of RIP sensors with different step sizes. III.
DESIGN OF A PORTABLE RESPIRATORY MONITOR
A portable respiratory monitor has been built. Two RIP belts are worn around chest and abdomen of subject. The belts are connected to Colpitts oscillators (Fig. 7). Variation in chest’s and abdomen’s cross-sectional area results in variation of inductance of the belts and therefore of frequency of the oscillators’ sinusoidal output. An FM demodulation circuit using Phase-locked loop is used to convert the variation in frequency into voltage. The voltage signal is then filtered and amplified before being fed to a microcontroller with built in 10-bit ADC. Here, the signal is digitized and is transmitted to computer wirelessly via Bluetooth module. In addition, an algorithm for the microcontroller has been developed to calculate breathing rate and display on a 16 characters x 2 lines LCD. Whenever breathing rate drops lower than a predefined threshold, an alarm buzzer is turned on. On the computer side, we have developed a LabVIEW program to display
Abdomen belt
Colpitts oscillator
Digital trimmer
Amplifier & Demodulator
Filter & Amplifier
Amplifier & Demodulator
Filter & Amplifier
Colpitts oscillator
Digital trimmer
LCD
10-bit ADC
MCU
Calib. button
Fig. 7 Portable respiratory monitor’s block diagram.
Fig. 8 Portable respiratory monitor with two RIP belts.
IFMBE Proceedings Vol. 40
Buzzer
Bluetooth module
Batt. & Regulator
A Portable Respiratory Monitor Using Respiratory Inductive Plethysmography 2.
3. 4.
Kevin P Cohen, Dorin Panescu, John H Booske, John G Webster and Willis J Tompkins, Design of an inductive plethysmograph for ventilation measurement, Physiol. Meas. 15, 217229, 1994. J. Scott Cardozo, New AASM Recommendations for Sensors: A Simple Guide for the Sleep Technologist, Sleep Diagnosis and Therapy, Vol 3 No 5, September-October, 2008. Jahan Naghshin, Patrick J. Strollo, Respiratory Monitoring Equipment and Detection of Respiratory Events, Sleep Med Clin 4, 353–360, 2009. Author: Institute: Street: City: Country: Email:
Fig. 9 Our monitor can transmit data to laptop wirelessly via Bluetooth. On laptop side, chest & abdomen movement efforts and Konno-Mead loop are plotted. Breathing rate is displayed on both monitor and laptop.
IV. CONCLUSIONS Two parts of our work are presented. In the first part, we investigate the effect of step size of sine wave pattern to RIP sensor characteristic. Three RIP sensors with different step sizes are built and their inductance are measured. Experimental results have shown that the smaller step size, the better sensitivity and linearity. In the second part, we present a complete design of a portable respiratory monitor. We then verify the monitor’s performance in term of respiratory rate measurement by doing experiment on 10 subjects. Experimental results have shown that the presented monitor can measure respiratory rate with high accuracy. Furthermore, the monitor can transfer data to PC (via Bluetooth) where chest & abdomen movement efforts and Konno-Mead loop are plotted. These plots are useful for diagnosing and monitoring several respiratory disorders.
ACKNOWLEDGMENT The authors are grateful to Associate Professor M.D./Ph.D. Le Thi Tuyet Lan, the University of Medicine Hospital. This study was funded by the International University, VNU-HCM.
REFERENCES 1.
225
M. Folke, L. Cernerud, M. Ekstrom, B. Hok, Critical review of non-invasive respiratory monitoring in medical care, Medical & Biological Engineering & Computing, Vol. 41, 377-383, 2003.
IFMBE Proceedings Vol. 40
H. T. Ngo Duke University Durham, NC 27708 USA
[email protected]
