meam-445/446 final paper specifications · web viewin summary, there is both medical and market...

DEBUT ChallengeCategory: Diagnostic Devices/Methods May 26, 2012

Department of Mechanical Engineering and Applied Mechanics

School of Engineering and Applied ScienceThe University of Pennsylvania

Philadelphia, Pennsylvania, USA

D1GIT: AUTOMATED, TEMPERATURE-CALIBRATED MEASUREMENT OF CAPILLARY REFILL TIME

Anat Bordoley1 Rikki Irwin1 Viraj Kalyani1

Craig McDonald1 Dorsey Standish1

Vinay Nadkarni MD 2

sponsorKatherine J.

Kuchenbecker PhD 1,faculty advisor

Robert L. Jeffcoat PhD1

instructor

1 Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, and 2 Children’s Hospital of the University of Pennsylvania, Philadelphia PA, USA.

ABSTRACTThe team has designed and prototyped an

automated device to measure capillary refill time. The capillary refill test, used medically to test for shock and/or dehydration, is currently conducted manually: the practitioner depresses the patient’s finger for an arbitrary time with an arbitrary amount of force, relying on his/her own judgment to quantify the rate of returning blood flow. To address the need for a standardized capillary refill test, our device, the D1GIT, performs three main functions: standardizing the force applied to the patient’s finger, sensing the blood flow returning to the finger, and outputting a digital reading of temperature-adjusted capillary refill time.

Prior art in this area has been limited to the development of sensing techniques that output a digital reading of refill time but still require unregulated manual application of force to the patient’s finger independent of the sensing setup. Therefore, the D1GIT is a novel device in that it is a compact solution for standardizing the capillary refill time test, incorporating and facilitating all of the actions of a human doctor.

The D1GIT uses spatial calibration to transfer the same force (determined by doctor-patient testing) to each patient’s finger. An LED-phototransistor system measures the blood flow leaving and returning to the finger. Finally, a zener diode measures the temperature of the patient’s finger and adjusts the capillary refill time (CRT) accordingly. The D1GIT uses a 7-segment display to output both the nominal CRT and the temperature-adjusted CRT. All of these components are incorporated into a sleek outer casing, which is 3D-printed ABS plastic. This design has been fully realized in prototype form; the following figure depicts the fifth and final iteration.


1. INTRODUCTION AND BACKGROUNDThe capillary refill test measures the rate of

blood flow to empty capillaries and yields a capillary refill time (CRT). The test is performed by holding the patient’s hand above the heart, depressing the finger or other soft tissue until the area turns white, and measuring the time it takes for the area to return to full color. The capillary refill test is frequently used in emergency care to test the patient’s levels of dehydration and peripheral perfusion. The test is performed by doctors, nurses, EMTs, and other medical personnel, but there is no standard for the magnitude or the duration of the force applied, and human error introduces variability in judging the elapsed time before refill [1]. Therefore, standardization of the test through an automated device would greatly benefit the medical world, especially in the area of emergency care.

Figure 1 - Capillary refill test procedure

Figure 1 shows the capillary refill test process, although the D1GIT performs the test with the patient’s palm facing up. The capillary refill test can be performed in many places: the fingers, the toes, the top of the foot, and even the chest [1]. The D1GIT measures capillary refill time from a simple and universal test point, the middle finger.

Not only do CRT observations differ among medical personnel, but also the test can be variable among different patients. However, the healthy capillary refill time is generally under two seconds, and over three seconds is considered symptomatic [2]. The capillary refill test is often used in combination with other diagnostic factors (e.g. respiration rate, blood pressure, heart rate) to make a definitive diagnosis or

recommendation for care [3]. While some existing medical devices are

capable of measuring CRT, the test is generally a secondary function, and the technology to perform the primary tasks can be complex and therefore expensive. Furthermore, while several devices are capable of sensing and displaying capillary refill time, most still require separate, unregulated manual finger compression, which can be variable in pressure and time.

Shavit et al. from the University of Calgary in Canada have employed an advanced imaging technique to automate the CRT timing process [3]. They use manual finger compression together with image processing software to automatically calculate and display the refill time on a digital screen[3].

The sensors used in pulse oximeters can also be useful in measuring CRT. Pulse oximeters produce a graphical display of blood volume changes, or a photoplethysmograph (PPG) [4]. The PPG measures signal quality and perfusion levels to confirm the validity of the pulse oximetry test, but digital signal processing methods may allow the extraction of other clinical parameters such as CRT [4].

Karlen et al. of the British Columbia Children’s Hospital have developed a mobile phone application, iRefill, that measures CRT with the attachment of a PPG sensor to an iPhone or iPod Touch [5]. However, this system still requires unregulated manual compression of the patient’s finger.

No device currently exists which measures the temperature of the patient’s extremity and calibrates the CRT reading accordingly. Cold extremities, most frequently resulting from cold ambient conditions, have been shown to significantly slow CRT [6]. In fact, Gorelick et. al has developed an algorithm which correlates patient temperature with CRT. However, no device has yet incorporated temperature adjustment in measuring and reporting CRT.

The D1GIT standardizes the test process by performing an integrated capillary refill test—stabilizing the patient’s fingers while the user depresses the distal extremity, recording the compressive force trajectory applied by the device, and displaying both the nominal and the


temperature-adjusted CRT. Because of the compact design requirements and the incorporation of temperature-based CRT adjustment, our design constraints differ markedly from the prior art in the area of capillary refill testing.

2. DESIGN DESCRIPTIONThe final D1GIT prototype is a result of

several rounds of brainstorming, planning, and iteration. The system can be thought of as an integration of three functionalities: force application, sensing, and temperature measurement. Given these components, the D1GIT design can then be organized according to mechanical, electrical, and software components, as follows.

2.1 Mechanical DesignThe D1GIT must accurately and precisely

apply a controlled force to the patient’s finger while compactly housing all of the integral components.

2.1.1 Initial TestingIn the early design phases, we considered

depressing the patient’s finger through both electronic actuation (solenoid) and manual actuation. However, running 63 transmission tests on six different subjects with different force magnitudes and different compression times showed that capillary refill time is sensitive to the magnitude of force applied but not it’ (Figure 2).

Figure 2 - CRT Compression Variables

Therefore, electronic actuators were rejected because not only were they larger and more expensive, they would also fail to improve

the accuracy of the device. We then decided to regulate the force applied to the patient through the use of a linear compression spring in series with the device pushbutton and the patient’s finger. The setup, shown in Figure 3, is designed to prevent the doctor from pressing farther than the mechanical stop.

Figure 3 - Mechanical force limiter

This entire system, referred to as the force applicator, lies flush with the patients finger inside the device. In the final D1GIT prototype (Figure 4), a linear spring compresses 0.1in in order to apply a 5.9N force to each patient’s finger.

Figure 4 - Force applicator layout and integration

2.1.2 Component Integration

Mechanically, our design required the incorporation of many electronic components while still adhering to practical size and aesthetic requirements. These requirements included making the manual force applicator easily accessible, accommodating for various finger sizes, and allowing access to all of the electronics, while remaining compact enough for easy transportation.

Given these stringent requirements, we decided on a design of two hinged components


held in clamping torsion. Figure 5 shows the hinged design clamped onto a finger; the top and bottom are held together by a torsional spring at the rear of the device. The 3.1 in-lbs 180° torsional spring holds the patient fingers in place, calibrating the force applicator to lie flush with the patients finger and therefore apply a consistent 5.9N force.

Figure 5 - Final D1GIT prototype

The entire model was designed in Solidworks and fabricated through 3D printing and laser cutting with ABS plastic and acrylic, respectively. The electronics are held in place using adhesive and press fits. To easily open and close the device, the top and bottom each have a removable cover attached with screws.

2.2 Electronics Design 2.1.1 Initial Testing

After considering temperature, color, and light-based sensing methods, we chose to use an IR LED and a phototransistor to detect the changes in light transmitted through the finger as blood volume changes during the capillary refill test.

To verify the validity of our CRT test setup, we ran 63 transmission tests conducted on patients of varying skin color, with various amounts of applied force and compression time, having some “unhealthy” subjects soak their fingers in ice first. This process helped to calibrate our device to recognize “healthy” and “unhealthy” patients. Our analysis has shown (in agreement with doctors and medical literature) that healthy subjects have a CRT of less than two seconds, while the CRT of unhealthy patients lasted up to greater than four seconds. The clear division among our measured capillary

refill times between healthy and unhealthy patients agrees with the medical standard, thereby validating our sensing method and the resulting CRTs.

2.1.2 Electronics DescriptionIn addition to the IR LED and

phototransistor, other electronic components include an On button, LEDs to guide the user, a temperature sensor, a force sensing resistor to detect force application from the doctor, a display to output the results, a microcontroller to control the process, and batteries to power the device.

The electronics are incorporated on a printed circuit board for easy mass production (Figure 8). It uses an M2 microcontroller to direct the electronics. Power consumption is limited by a feature that shuts off the device after five minutes of inactivity.

Figure 8 - D1GIT PCB and internal wiring

2.3 Software design To meet operational requirements, we

decided that the device software would be best abstracted as a finite state machine. This coding structure was implemented in C, the preferred language for programming embedded systems. The work was also duplicated in Matlab to confirm the functionality of the algorithms.

The D1GIT’s software must accurately and precisely compute capillary refill time. Figure 9 illustrates the calculations performed onboard the D1GIT every time CRT is read. The top graph shows the characteristic voltage signal read from the phototransistor during CRT, sampled at 100 Hz. The bottom graph shows the derivative of that signal, scaled by an arbitrary constant to avoid rounding errors. As it is annotated in the figure, the device begins


examining the signal when pressure is released from the finger. This action translates into a steep drop off in phototransistor voltage. Looking at the bottom graph, we differentiate the voltage signal in real time until this differentiated signal is approximately zero for a sufficient amount of time. We interpret the difference between the time when the differentiated signal reaches zero and the time when the recording began as CRT.

Figure 9 - Calculation of CRT

Figure 10 shows the differnet elements of the user interface at various stages of measuring CRT, as it is programmed by the M2 microcontroller.

Figure 10 - D1GIT sequence of functions

3.4 Temperature CorrectionVariations in both ambient temperature and

extremity surface temperature significantly affect capillary refill time. These observations have been validated by research conducted by Gorelick, Shaw and Baker and are shown in Figure 11. Most importantly, decreases in ambient temperature within a range found in typical office/emergency department settings may cause significant prolongation of CRT in children with normal circulatory status [6].

Figure 11 - CRT vs. finger surface temperature

This prior research was translated into an equation for correcting CRT based on finger temperature: ,

((Eq. 1)

Thus, this step adjusts raw CRT measurements to represent the CRT that the patient would have if his or her finger were at room temperature (25°C). A button on the device toggles between displaying regular CRT and temperature-corrected CRT.

5. EVALUATION AND TESTTesting was done both to validate the

device’s uniform application of force and its consistent overall measurement of capillary refill time. We also sought qualitative feedback from doctors at the Children’s Hospital of Philadelphia.

6.1 Validating the force applied In order to validate the consistency of the

applied force both intra- and inter-patient, we conducted tests to measure the force exerted by the D1GIT. To perform this experiment, we used a force-sensing resistor to measure the force applied to six “patients” by three “doctors”. These tests were performed on an old model of the device which used a different spring, so the magnitude of the force received is not as


important the device’s precision. 24 tests were performed on 24 tests on six subjects with a variety of finger sizes, skin colors, and genders. The force applied by the D1GIT ranges from 4 N to 4.6 N, with an average force of 4.50 N and a standard deviation of .14 N.

6.2 Validating the CRT readings The final device works and reads out

reasonable CRTs, but the readings are not always consistent. When we performed repeatability tests with the final D1GIT prototype, we experienced variable readings, especially when the device was used continuously. A string of reasonably precise values would be interrupted by a reading of 5.0 (the maximum) or 0.0 (the minimum). In the next iteration of the D1GIT, software improvements such as additional filtering and doctor calibration should correct these repeatability and reliability issues. Regardless, an error message (‘Er’) will be displayed upon determining the minimum or maximum value so as not to confuse the user.

6.3 Doctor feedback Besides the quantative testing described in

the previous sections, we also received qualitative feedback from the doctors of the Children’s Hospital of Philadelphia. Not only were they impressed with the sophisticated nature of our final device, but they were also eager to begin testing the D1GIT on actual sick patients with slower capillary refill times. If we choose to pursue this project further, we would seek IRB approval to conduct a human subjects test with our device. Their critique included the issues we mentioned before: the doctors would prefer a more robust device (i.e., something industrially manufactured) that took more repeatable readings. However, the device incited animated discussions among the doctors and holds great promise for widespread medical use.

7. CONCLUSION The D1GIT project is a classic example of a

successful engineering endeavor: medical professionals needed an automated way to read capillary refill time, so our team investigated

various technologies, chose the most reliable and inexpensive options, and then iterated designs to incorporate these components into a compact, ergonomic package. The D1GIT design is refined enough to consider marketing it as a medical product.

As Dr. Vinay Nadkarni of CHOP expressed excitedly, the device’s incorporation of temperature-calibrated CRT measurement “truly pushes the field forward.” The only shortcoming of the current D1GIT device is its lack of consistency in reading capillary refill time. Especially when the device was used outside continuously during design demonstrations, the device overcorrected for outside temperature rather than finger temperature. However, our team believes that this problem could be addressed and overcome by additional testing and iteration, especially in an industry setting.

In this vein, there are several “next steps” to be taken with this product. First, additional doctor opinions should be gathered, and the D1GIT should be calibrated to agree with average doctor assessments of CRT. Once the device has been medically calibrated, the prototype should undergo repeatability testing, in order to ensure that the D1GIT can be used repeatedly and consistently in a medical setting. Finally, obtaining IRB approval for a human subjects study would allow the use of the D1GIT on actual sick patients with slow or “unhealthy” capillary refill times.

In summary, there is both medical and market promise for this device. The prototype, which smoothly incorporates mechanical, electrical, and software design, would benefit from additional testing and refinement before use in a clinical setting.

9. REFERENCES1. Brabrand, M., et al. Interobserver variability in grading capillary refill time by nurses and nurses assistants. Odense University Hospital, Odense, Denmark. European Journal of Emergency Medicine February 2011: Vol. 18, Issue 1, p 46–49. 2. Bumke, Kristin, and Ian Machonochie. Paediatric capillary refill times. Trauma 2001: Vol. 3, 217-220.


3. Shavit, Itai, et. al. A Novel Imaging Technique to Measure Capillary Refill Time: Improving Diagnostic Accuracy for Dehydration in Young Children With Gastroenteritis. Pediatrics 2006: Vol. 118, 2402-2408.4. Mobile Phone Pulse Oximeter. Compendium of new and emerging technologies that address global health concerns 2011. World Health Organization. <http://www.who.int/>.

5. Karlen, W., et al. Capillary Refill Time Assessment Using a Mobile Phone Application (iRefill). ASA abstracts 2010. ASA, 2010. A575.6. Gorelick, M., et al. Effect of Ambient Temperature on Capillary Refill Time in Healthy Children. Pediatrics November 1993: Vol. 3, Issue 5, p 699-702.


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