Endotracheal Tube Position Monitoring Device

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magnetic field sensors to detect an anomaly in magnetic field ... Hall-effect sensor and two magnet attached to the ETT [10]. ... Additional amplifier is added to.
Endotracheal Tube Position Monitoring Device Keith Duran, Byron Hsu, Brandon Pierquet, Warit Wichakool, Rob Sheridan, and Hongshen Ma

Abstract— We have developed an accurate, economical, and portable device that helps to locate the position of an endotracheal tube (ETT). The device uses an grid array of magnetic field sensors to detect an anomaly in magnetic field caused by embedded near the top of an ETT and outputs an intuitive color map of relative magnetic field intensity under the sensor area. The device provides real-time feedback of ETT position to the clinician, so that corrective measures could be taken if the ETT is displaced beyond its normal position with respect to the patient body. The device is also equipped with wireless communications to enable continuous monitoring and automated notification of hospital staff when a potential problem is detected.

I. I NTRODUCTION The endotracheal tube (ETT) is a staple of hospital procedures, used to keep the airway of patients open during anesthesia and many surgical procedures. It is inserted to a specific depth in the trachea through either the mouth or nose, or through an incision in the neck. Properly placing this tube requires a high level of skill and training, and tubes misplaced into the esophagus are responsible for numerous cases of mortality and morbidity. Even a proper insertion can result in later complications, as ETT tubes can become displaced by sudden movements, or the tubes can gradually migrate over time. Improper position of the ETT can cause serious damage to the patient. As a result, there is a need for a reliable method or device for doctors and nurses to monitor and ensure the position of the ETT for a hours or days. Since there are no simple ways to prevent tube migration, the medical staff must take active measures to prevent tubeloss, which may lead to patient mortality or morbidity. The usual approach is regular visual inspections of the ETT’s position. However, due the high pliability of the tube inside the air passages, a problem may not be externally visible. An X-ray examination can determine the tube’s position, but radiography is time consuming, expensive, and exposes the patient to unnecessary radiation. Despite these draw backs, radiography remains the most relied-upon approach for detecting ETT migration. Few methods for monitoring tube position have been investigated to tackle this problem. An acoustic reflectrometry method processes the reflection of the transmitted wave to determine the location of the tube inside the body [1]– [3]. With this method, the signal processing become much more challenging if there is a kink along the tube. A much more complex method uses an ultrasonic wave to detect the location of the ETT [4], [5]. Another technique is to monitor pulmonary compliance and airway pressures and infer the position of the ETT [6]. This method requires a

complex supporting systems and may not be as accurate as other methods. A carbondioxide-based device has also been investigated [7]. This device only assists the intubation procedure but has not been tested for the monitoring purpose. Another ETT position detection includes the use of magnetic field detection scheme. One method detects the change in the mutual inductance of the sensing device and the magnetic material embedded along the ETT [8], [9]. A magnetic sensor device has also be developed using a single Hall-effect sensor and two magnet attached to the ETT [10]. Above devices use only single magnetic field sensor which may be inaccurate for patients with different size. To further improve the accuracy, flexibility, and usability, we have developed another type of magnetic-based ETT position sensor using a two-dimensional array of magnetic sensors with an LCD screen to provide a real-time feedback for the user of the current position of the magnet marker of embedded in the ETT. The location of the sensor is critical in order for the detection algorithm to work consistently. Our device overcomes this problem by giving the clinician a pixelated image of the local magnetic field in two dimentions, which enables detection of the magnetic marker in three dimensions. II. T HEORY OF OPERATION The Magnetic EndoTracheal Tube Imaging Device (METTID) utilizes an array of Giant Magneto-resistance (GMR) sensors to locate the position of a tiny magnet installed permanently into the ETT, near the sternal notch. As the device is moved along the respiratory system, the LCD screen shows the current magnetic field readings form the sensor array. The region where the magnetic field is stronger will turn redder so that the user can intuitively interpret that the magnet is in the direction of the region with the color with warmer tone as shown in Fig. 1. III. S YSTEM DESCRIPTION A. System architecture The detection system consists of three main components: a magnetic sensing analog circuit, a micro-controller unit (MCU), and a display unit. The analog circuit amplifies the sensor signal and provide signal conditioning methods appropriate for further signal processing steps. The MCU samples the signal and processes that data appropriately for the user interface unit. In this case, the output is the LCD screen showing the relative strength of the magnet using an intuitive color scheme. In addition, the system can be configured to transmit data wirelessly to a patient

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area under the sensor head. Additional amplifier is added to increase sensitivity for a signal processing chain. The block diagram for the GMR sensor board is shown in Fig. 3. All sensor outputs are connected to an analog multiplexer to minimize hardware and to allow controllable signal processing by the MCU. D. Processing board Fig. 2: METTID system architecture

monitoring system. This data can be used to warn about the tube migration and to lower the risk of re-intubation or other complication. The system architecture is showed in Fig. 2. B. Magnetic sensor The proposed device uses Giant Magnetoresistive (GMR) to sense a magnetic field created by a small, embedded magnet in the modified ETT. A GMR sensor is more sensitive than a Hall-effect sensor. The module AAH-002 from NVE corporation has a sensitivity of 11mV/V-G (Guass) minimum. With the 3.3V power supply, the minimum sensitivity is approximately 36.3mV/G. The high sensitivity enables the proposed device to detect any small change in magnetic field under the sensing area. As a result, only small magnet is required to be embedded with the ETT. In addition, high sensitivity also allows the device to show the relative depth perception in the color scheme as well. Additional advantage of the GMR sensor is the low-power operation. The sensor output is proportional to the supply voltage. In this case, The system is operational with the supply be dropped to 1 volt. This feature enables the GMR sensor to be used in low-power, portable device. Sensors are arranged in a grid fashion. This layout allows the sensor to take a a reading of a large range under the sensing area. Using this configuration, the device would have multiple readings from all sensors and allow the processor to display the relative intensity under the sensing area. C. Sensor frontend The sensor frontend consists of nine GMR sensors arranged in a three-by-three array. This grid array configuration allows the the device to explore and report the relative wide

The processing element of the METTID digitizes the GMR sensor data and computes the X, Y, and Z axis position of the magnet. It then relays the processed information to the LED driver for display. An Atmel Atmega324p is used for acquisition and processing of the GMR sensor data. The onboard 10-bit analog-to-digital-converter (ADC) of the Atmega324p is used to sample the sensor data as received from the sensor board. The USB port is also used for charging the onboard lithium-polymer battery. A Maxstream Zigbee module is also installed on the processing board to facilitate communications wirelessly with a PC, allowing data acquisition for development, and also allowing the possibility of a continuous monitoring system if the device were affixed to the patient. E. User interface A 132x132 pixel color LCD is installed directly on top of the sensor board. The LCD is controlled by the MCU the displayed pattern is only limited by display resolution and processor speed. In one configuration, the display was divided into nine equal squares. Each square would change color progressively from green to red as the square’s associated GMR sensor measured a larger magnetic field. This intuitive color scheme enable the user to navigate the device around to search for the actual location of the embedded magnet and the ETT. In the case of constant monitoring, if the device were to be affixed to the patient, the color would change as the tube migrate. F. Physical features A case to hold the electronic components was designed using SolidWorks solid modeling software. Tabs were designed into both the upper and lower pieces of the case to locate the internal components. These features constrain the circuit boards and battery in all directions; no screws are

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required to fasten the internal components. Figure 4 shows the solid model of the case. The METTID is designed to be portable; it should fit comfortably into the hand or a pocket. The sensor array is positioned in the head of the device for ease of use, and the LCD display is directly on top of sensor array to intuitively convey the magnet’s position. The handle of the device houses the processing circuit as well as the battery and the Zigbee radio. Figure 5 shows photographs of the circuit boards and the top part of the case. The bottom photograph shows how the boards stack and fit into the case. IV. I NITIAL EXPERIMENTS To demonstrate the feasibility of the proposed device, experiment has been perform to model the actual usage of the device. In order to observe the behavior of the magnetic sensor, experiments were perform to measure the sensor response and sensitivity. Experimental setup is shown in Fig. 6 and the result is shown in Fig. 7. In the controlled experiment, the magnet is positioned about 15mm from the sensor plane. The result shows that the sensor output corresponds to the magnetic field strength. The external magnetic probe confirms that the magnetic field stays within the linear region of the GMR. The maximum Hall probe

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field is approximately 6G in this experiment. According to the result, the sensor array can detect the progression of the magnetic field strength as each individual sensor moved pass the magnet. Furthermore, the result also shows that the sensor frontend can actually detect the magnetic field at a very low level, i.e. 1-2 G. For example, if the distance is increased to 20mm, the filed will be maximum field will be approximately at 2.5 G or only 40% of the 15mm case. The result in the experiment confirm that the sensor is still sensitive enough to read the low magnetic field. The final report will include additional test results and sensitivity data from the device in various settings. V. C ONCLUSION We have developed a hand-held, battery operated device to determine the position of a small magnet affixed to an ETT. The device has a great sensitivity and the 2-dimensional sensor array configuration allows the unit to display an intuitive, user-friendly color map on the LCD. The color scheme enable the depth perception of the device that allows the detection of intubation of the ETT in the esophagus. This device will allow doctors and hospital staffs to perform an intubation with increased confidence that the tube is properly located and can be configured to continuously monitor and alarm hospital staff of any potential tube migration using its wireless capability. R EFERENCES [1] J. P. Mansfield, R. P. Lyle, W. D. Voorhees, and G. R. Wodicka, “An acoustical guidance and position monistoring system for endotracheal tubes,” IEEE Trans. Biomed. Eng., vol. 40, no. 12, pp. 1330–1335, 1993. [2] E. J. Juan, J. P. Mansfield, and G. R. Wodika, “In-line acoustic system to position and monitor infant-size endotracheal tubes,” in Proc. of the 22nd Annual EMBS International Conference, Chicago, IL, Jul 2000, pp. 2571–2574. [3] D. T. Raphael, “Determining endotracheal tube placement using acostic reflectometry,” WO Patent Application, 2003. [4] R. B. Lipscher and J. G. Mottley, “Signal generating endotracheal tube apparatus,” US Patent, 1998. [5] M. Miller and C. T. Hovland, “Ultrasonic placement and monitoring of an endotracheal tube,” US Patent, 2006.

[6] A. Mahajan, N. Hoftman, A. Hsu, R. Schroeder, and S. Wald, “Continuous monitoring of dynamic pulmonary compliance enables detection of endobronchial intubation in infants and children,” Anestesis & Analgesia, vol. 105, no. 1, pp. 51–56, 2007. [7] G. Depotis, “Endotracheal intubation device,” US Patent, 1988. [8] D. J. Cullen, R. S. Newbower, and M. Gemer, “A new method for positioning endotracheal tubes,” Anesthesiology, vol. 43, no. 5, pp. 596–599, Nov 1975. [9] C. Ashley-Rollman, M. C. O’Donnell, and W. McCormick, “Device for accurately detecting the position of a ferromagnetic material inside biological tissue,” US Patent, 1990. [10] W. Pan, J. Lou, Y. Zhang, and X. Jin, “A new magnetic device for the identification of endotracheal tube position,” in Proc. of the 23nd Annual EMBS International Conference, Istanbul, Turkey, Oct 2001, pp. 3273–3276.