Noncontact Vital Sign Monitoring System for Isolation Unit (Casualty Care System)

Military Medicine, Jul 2006 by Matsui, Takemi, Gotoh, Shinji, Arai, Ikuo, Hattori, Hidemi, Et al

For measuring the vital signs of casualties inside an isolation unit, we developed a noncontact vital sign monitoring system using a microwave radar. The system was tested on eight healthy volunteers ranging in age from 30 to 48 years. The heart and respiratory rates derived by the microwave radar correlated with the heart and respiratory rates determined by electrocardiogram and respiratory sensor (r = 0.98, P

Introduction

For measuring the vital signs of casualties inside an isolation unit (Casualty Care System [CCS], Centex Corporation, Carbondale, Pennsylvania), we have developed a noncontact vital sign monitoring system using a microwave radar to simultaneously measure the heart and respiratory rates of casualties under biochemical hazard conditions from outside the CCS without touching the patient. The CCS is an isolation unit used to protect casualties from contamination or cross-contamination during or after a catastrophic event and to transport patients who are exposed to toxic chemicals or infectious organisms. The CCS provides protection from contamination by both chemical and biological materials for 24 hours using multiple reactive fabric laminates. The laminates can pass oxygen and carbon dioxide, but prevent infiltration by viruses and chemicals and reduce the carbon dioxide concentration inside the CCS. The CCS is equipped with a medical interface bulkhead for two intravenous lines and a respiratory line. The pressure inside the isolator can be maintained below or above the outside atmospheric pressure, according to conditions: the pressure inside the isolator is maintained below atmospheric pressure when the casualty is contaminated and above atmospheric pressure when the atmosphere outside the isolator is contaminated. The CCS may be useful for nuclear casualties, because it prevents the secondary exposure of the casualty to airborne radioactivity, when the pressure inside the isolator is maintained above the atmospheric pressure.

We have previously reported the noncontact method used to monitor the heart and respiratory rates of experimental animals exposed to toxic materials or under a hypovolemic state1,2 to determine the pathophysiologic condition of the subjects, such as exposure to toxins or shock induced by hemorrhage.

The purpose of the present study was to demonstrate the usefulness of the proposed noncontact monitoring system in measuring the vital signs of humans inside the CCS. In this study, the concentration of exhaled CO and CO2 of humans inside the CCS was also measured in real time using an exhaled gas analyzer. Trauma injury increases blood carboxyhemoglobin (CO-Hb), which correlates with exhaled CO.3,4 Exhaled CO2 correlates with arterial partial pressure of carbon dioxide (PaCO^sub 2^), which is a respiratory factor that determines arterial pH.5

System Design and Testing of Noncontact Vital Sign Monitoring System for the CCS

The noncontact monitoring system consists of a microwave radar (LDR-1, Tau Giken Corporation, Yokohama, Japan) and an analyzing recorder (AR1199A, Yokogawa Corporation, Tokyo, Japan) that is illustrated in Figure 1. The microwave radar generates a very stable microwave at 1.215 MHz, with a maximum output power of 70 mW. The microwave radar cancels microwaves reflected from stationary objects around the antennas, and consequently the radar system monitors only the small waves reflected from the subject's body, which are modulated by the subject's body motion.6-9

In this study, in addition to noncontact vital sign monitoring, exhaled CO and CO2 were measured in real time using an exhaled gas analyzer (Carbolyzer mBA-2000. Taiyo Corporation, Osaka, Japan). The gas exhaled by healthy volunteers during spontaneous breathing was collected using a respiratory mask and a breathing tube. The sampling rate of exhaled gas was measured to be 200 mL/minute. The sampled CO and CO2 gas concentrations were measured with the exhaled gas analyzer using the controlled potential electrolysis method and the nondispersive infrared method, respectively. The measured CO and CO2 concentrations were recorded by computer with a sampling interval of 1 second. The temperature of the exhaled gas at the vent of the respiratory mask was measured using a thermocouple. Simultaneously, this thermocouple was used as a respiratory sensor to count respiratory rate. The body temperature was measured by thermometer in the axilla.

Healthy volunteers were placed in the CCS. The system was tested on eight healthy male volunteers ranging in age from 30 to 48 years (35 � 14 years old). During the examinations, the pressure inside the isolator was maintained above atmospheric pressure. A microwave antenna box (37 � 15 � 5 cm) with two microstripe antennas (radiating and receiving antennas, 70 mm in diameter) was positioned 30 cm away from the surface of the CCS, as shown in Figure 1. To detect cardiac signal clearly, the distance from the microwave antenna to the subject body should be shorter than 50 cm. The analogue output of the microwave radar was transferred to an analyzing recorder. A fast Fourier transform (FFT) of the microwave radar analogue output was conducted using an analyzing recorder with a Manning window to derive the respiratory and cardiac peaks. FFT separates a waveform into peaks of different frequencies and distinguishes different frequency peaks and their respective amplitudes. A high-pass filter was used to derive the cardiac peaks (heart rate), but was not used for calculating the respiratory peaks. For comparison with the noncontact-derived heart and respiratory rates, electrocardiograms of the healthy volunteers were measured by a polygraph system (Fukuda Denshi, DS520) and the respiratory rate was calculated from the respiratory fluctuation of the electrical impedance between the electrodes for electrocardiogram measurement.