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, andportable device that helps to locate the position of an en-dotracheal tube (ETT). The device uses an grid array ofmagnetic field sensors to detect an anomaly in magnetic fieldcaused by embedded near the top of an ETT and outputs anintuitive color map of relative magnetic field intensity underthe sensor area. The device provides real-time feedback ofETT position to the clinician, so that corrective measures couldbe taken if the ETT is displaced beyond its normal positionwith respect to the patient body. The device is also equippedwith wireless communications to enable continuous monitoringand automated notification of hospital staff when a potentialproblem is detected.

I. INTRODUCTION

The endotracheal tube (ETT) is a staple of hospital pro-cedures, used to keep the airway of patients open duringanesthesia and many surgical procedures. It is inserted toa specific depth in the trachea through either the mouth ornose, or through an incision in the neck. Properly placingthis tube requires a high level of skill and training, and tubesmisplaced into the esophagus are responsible for numerouscases of mortality and morbidity. Even a proper insertioncan result in later complications, as ETT tubes can becomedisplaced by sudden movements, or the tubes can graduallymigrate over time. Improper position of the ETT can causeserious damage to the patient. As a result, there is a need fora reliable method or device for doctors and nurses to monitorand 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 tube-loss, which may lead to patient mortality or morbidity. Theusual approach is regular visual inspections of the ETT’sposition. However, due the high pliability of the tube insidethe 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 exposesthe patient to unnecessary radiation. Despite these drawbacks, radiography remains the most relied-upon approachfor detecting ETT migration.

Few methods for monitoring tube position have been in-vestigated to tackle this problem. An acoustic reflectrometrymethod processes the reflection of the transmitted wave todetermine the location of the tube inside the body [1]–[3]. With this method, the signal processing become muchmore challenging if there is a kink along the tube. A muchmore complex method uses an ultrasonic wave to detectthe location of the ETT [4], [5]. Another technique is tomonitor pulmonary compliance and airway pressures andinfer the position of the ETT [6]. This method requires a

Fig. 1: METTID system architecture

complex supporting systems and may not be as accurateas other methods. A carbondioxide-based device has alsobeen investigated [7]. This device only assists the intubationprocedure but has not been tested for the monitoring purpose.

Another ETT position detection includes the use of mag-netic field detection scheme. One method detects the changein the mutual inductance of the sensing device and themagnetic material embedded along the ETT [8], [9]. Amagnetic sensor device has also be developed using a singleHall-effect sensor and two magnet attached to the ETT[10]. All the magnetically based devices show that they areaccurate enough to sense the position of the marker fromoutside the body.

To further improve the accuracy, flexibility, and usability,we have developed another type of magnetic-based ETTposition sensor using a two-dimensional array of magneticsensors with an LCD screen to provide a real-time feedbackfor the user of the current position of the magnet marker ofembedded in the ETT. Information form the two-dimensionalsensor array provides additional information relating to rel-ative magnetic field strength under the sensing area. Thisadditional information can be used to benefit the navigationand continuous monitoring of the tube migration. [Hong saysit’s not necessary].

II. THEORY OF OPERATION

The Magnetic EndoTracheal Tube Imaging Device (MET-TID) utilizes an array of Giant Magneto-resistance (GMR)sensors to locate the position of a tiny magnet installedpermanently into the ETT, near the sternal notch. The systemarchitecture is shown in Fig. 1. The detection system con-sists of three main components: a magnetic sensing analogcircuit, a micro-controller unit (MCU), and a display unit.The analog circuit amplifies the sensor signal and providesignal conditioning methods appropriate for further signalprocessing steps. The MCU samples the signal and processesthat data appropriately for the user interface unit. In this case,the output is the LCD showing the relative strength of the

magnet using an intuitive color scheme. As the device ismoved along the respiratory system, the LCD would showthe current magnetic field readings form the sensor array. Theregion where the magnetic field is stronger will turn redderso that the user can intuitively interpret that the magnet is inthe direction of the region with the color with warmer tone.In addition, the system can be configured to transmit datawirelessly to a patient monitoring system. This data can beused to warn about the tube migration and to lower the riskof re-intubation or other complication.

III. SYSTEM COMPONENTS

A. Magnetic sensor

The proposed device uses Giant Magnetoresistive (GMR)to sense a magnetic field created by a small, embeddedmagnet in the modified ETT. A GMR sensor is more sensitivethan a Hall-effect sensor. The module AAH-002 from NVEcorporation has a sensitivity of 11mV/V-G (Guass) mini-mum. With the 3.3V power supply, the minimum sensitivityis approximately 36.3mV/G. The high sensitivity enables theproposed device to detect any small change in magnetic fieldunder the sensing area. As a result, only small magnet isrequired to be embedded with the ETT. In addition, highsensitivity also allows the device to show the relative depthperception in the color scheme as well.

Additional advantage of the GMR sensor is the low-poweroperation. The sensor output is proportional to the supplyvoltage. In this case, The system is operational with thesupply be dropped to 1 volt. This feature enables the GMRsensor to be used in low-power, portable device.

Sensors are arranged in a grid fashion. This layout allowsthe sensor to take a a reading of a large range under thesensing area. Using this configuration, the device would havemultiple readings from all sensors and allow the processorto display the relative intensity under the sensing area.

B. Sensor frontend

The sensor frontend consists of nine GMR sensors ar-ranged in a three-by-three array. This grid array configurationallows the the device to explore and report the relative widearea under the sensor head. Additional amplifier is added toincrease sensitivity for a signal processing chain.

The block diagram for the GMR sensor board is shownin Fig. 2. All sensor outputs are connected to an analogmultiplexer to minimize hardware and to allow controllablesignal processing by the MCU.

C. Processing board

The processing element of the METTID digitizes the GMRsensor data and computes the X, Y, and Z axis positionof the magnet. It then relays the processed information tothe LED driver for display. An Atmel Atmega324p is usedfor acquisition and processing of the GMR sensor data.The onboard 10-bit analog-to-digital-converter (ADC) of theAtmega324p is used to sample the sensor data as receivedfrom the sensor board. The USB port is also used forcharging the onboard lithium-polymer battery. A Maxstream

Vdd Vdd Vdd

Vdd Vdd Vdd

Vdd Vdd Vdd

−

+

ADCMultiplexer

A3 A2 A1 A0

G = 20 LPF

GMR Sensor Array

Fig. 2: Sensor array circuit

High

Low(a)

Magnet

Magnetic sensor

(b)

Fig. 3: LCD color map output example. Hong suggests wemove this figure to Theory of operations

Zigbee module is also installed on the processing board tofacilitate communications wirelessly with a PC, allowingdata acquisition for development, and also allowing thepossibility of a continuous monitoring system if the devicewere affixed to the patient.

D. User interface

A 132x132 pixel color LCD is installed directly on topof the sensor board. The LCD is controlled by the MCUthe displayed pattern is only limited by display resolutionand processor speed. In one configuration, the display wasdivided into nine equal squares. Each square would changecolor progressively from green to red as the square’s asso-ciated GMR sensor measured a larger magnetic field. Thisintuitive color scheme enable the user to navigate the devicearound to search for the actual location of the embeddedmagnet and the ETT. In the case of constant monitoring, ifthe device were to be affixed to the patient, the color wouldchange as the tube migrate.

E. Physical features

A case to hold the electronic components was designedusing SolidWorks solid modeling software. Tabs were de-signed into both the upper and lower pieces of the case tolocate the internal components. These features constrain thecircuit boards and battery in all directions; no screws arerequired to fasten the internal components. Figure 4 showsthe solid model of the case.

The METTID is designed to be portable; it should fitcomfortably into the hand or a pocket. The sensor array

(a) Prototype case (b) Exploded view

Fig. 4: Solid model of a prototype case

(a) Prototyped circuits (b) Device assembly

Fig. 5: Circuit boards and prototype assembly

is positioned in the head of the device for ease of use,and the LCD display is directly on top of sensor array tointuitively convey the magnet’s position. The handle of thedevice houses the processing circuit as well as the battery andthe Zigbee radio. Figure 5 shows photographs of the circuitboards and the top part of the case. The bottom photographshows how the boards stack and fit into the case.

IV. INITIAL EXPERIMENTS

To demonstrate the feasibility of the proposed device,experiment has been perform to model the actual usage ofthe device. In order to observe the behavior of the magneticsensor, experiments were perform to measure the sensorresponse and sensitivity. Experimental setup is shown in Fig.6 and the result is shown in Fig. 7.

In the controlled experiment, the magnet is positionedabout 15mm from the sensor plane. The result shows thatthe sensor output corresponds to the magnetic field strength.The external magnetic probe confirms that the magnetic fieldstays within the linear region of the GMR. The maximumfield is approximately 6G in this experiment. According tothe result, the sensor array can actually “see” the progressionof the magnetic field strength as each individual sensorpasses the magnet. Furthermore, the result also shows that

magnetHall probe

Fig. 6: Experimental setup

0 0.2 0.4 0.6 0.8 1−0.2

0

0.2

0.4

0.6

Time (s)

Vou

t (V)

0 0.2 0.4 0.6 0.8 1−2

0

2

4

6

8

Time (s)

Mag

netic

fiel

d (G)

#1#2#3

Hall probe

Fig. 7: Sensor response and corresponding Hall probe outputfor the 15mm case

the sensor frontend can actually detect the magnetic field ata very low level, i.e. 1-2 G. For example, if the distanceis increased to 20mm, the filed will be maximum field willbe approximately at 2.5 G or only 40% of the 15mm case.The result in the experiment confirm that the sensor is stillsensitive enough to read the low magnetic field.

Because actual distance between the embedded magnetand the sensor can vary for different patients, the final reportwill include the test for various distances between the magnetand the sensor plane. Furthermore, the sensitivity along thesensor plane will be measure the resolution and precision ofthe device’s ability to locate the position of the embeddedmagnet along the trachea length.

V. CONCLUSION

We have developed a hand-held, battery operated deviceto determine the position of a small magnet affixed to anETT. The device has a great sensitivity and the 2-dimensionalsensor array configuration allows the unit to display anintuitive, user-friendly color map on the LCD. The colorscheme enable the depth perception of the device that allowsthe detection of intubation of the ETT in the esophagus. Thisdevice will allow doctors and hospital staffs to perform anintubation with increased confidence that the tube is properlylocated and can be configured to continuously monitor andalarm hospital staff of any potential tube migration using itswireless capability.

REFERENCES

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[2] E. J. Juan, J. P. Mansfield, and G. R. Wodika, “In-line acoustic systemto position and monitor infant-size endotracheal tubes,” in Proc. of the22nd Annual EMBS International Conference, Chicago, IL, Jul 2000,pp. 2571–2574.

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[4] R. B. Lipscher and J. G. Mottley, “Signal generating endotracheal tubeapparatus,” US Patent, 1998.

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[6] A. Mahajan, N. Hoftman, A. Hsu, R. Schroeder, and S. Wald, “Contin-uous monitoring of dynamic pulmonary compliance enables detectionof 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, “Devicefor accurately detecting the position of a ferromagnetic material insidebiological tissue,” US Patent, 1990.

[10] W. Pan, J. Lou, Y. Zhang, and X. Jin, “A new magnetic device forthe identification of endotracheal tube position,” in Proc. of the 23ndAnnual EMBS International Conference, Istanbul, Turkey, Oct 2001,pp. 3273–3276.