With the escalating costs of hospital visits, clinicians are opting to use at-home monitoring devices to diagnose patients.  Current ECG Holter monitoring devices typically have 24-48 hour memory and battery capacity ^[1] .  With many patients experiencing intermittent heart problems that can occur once every week or month, the Holter monitor is not a good solution; an event recorder or loop recorder is required ^[2] .  However, each of these recorders can save only up to a few minutes of ECG recordings.  This constraint leads to the loss of most of the data, which could be very important in alerting the user to the onset of future episodes.  Therefore, we have developed a Holter monitor prototype with the goal of battery and memory capacity of two weeks.  Figure 1 shows a block diagram of the system.

We based the long-term monitor prototype around a Texas Instruments MSP430 low-power microcontroller that enables high computing power with very low power consumption.  The prototype monitor is mounted on standard 3M 2560 Red Dot electrodes and fabricated on a flexible PCB substrate.  Mounting the PCB directly on the electrodes improves the SNR by an estimated 40 dB compared to using wired leads ^[3] .  The monitor is “L”-shaped with rounded corners and placed on the patient’s chest (Figure 2).  The “L” shape enables several different ECG vectors to be recorded, depending on what the cardiologist wants to observe.  The monitor has 1 GBit of FLASH memory, which is enough to store 6 days of data sampled at 250 Hz continuously without compression.  Total power consumption of the system is approximately 2 mW.

1. D. Jabaudon, J. Sztajzel, K. Sievert, T. Landis, and R. Sztajzel, “Usefulness of ambulatory 7-day ECG monitoring for the detection of atrial fibrillation and flutter after acute stroke and transient ischemic attack,” Stroke, J. Amer. Heart Assoc., vol. 35, pp. 1647–1651, May 2004.
2. M. A. Rockx, J. S. Hoch, G. J. Klein, R. Yee, A. C. Skanes, L. J. Gula, and A. D. Krahn, “Is ambulatory monitoring for “Community-acquired” syncope economically attractive? A cost-effective analysis of a randomized trial of external loop recorders versus Holter monitoring,” AHJ 150(5), pp. e1.1065 – e5.1075, Nov. 2005.
3. A. Searle and L. Kirkup, “A direct comparison of wet, dry and insulating bioelectric recording electrodes,” Physiol. Meas., vol. 21, pp. 271-283, 2000.

Vital signs such as heart rate, blood pressure, blood oxygenation, cardiac output, and respiratory rate are necessary in determining the overall health of a patient.  Continuous monitoring of these vital signs can help assess the wearer’s overall state of health and identify risks for cardiovascular diseases ^[1] .

We propose the site behind the ear as a location for an integrated wearable vital signs monitor ^[2] . This location offers physiological signals such as the electrocardiogram (ECG), the photoplethysmogram (PPG), and the head ballistocardiogram (hBCG). The ECG measures the electrical activity from the heart and offers information such as continuous heart rate, blood pressure (when coupled with PPG or hBCG), and respiratory rate. The PPG measures the blood volume and color under the skin using optical illumination. The PPG offers information such as continuous heart rate and blood oxygenation. The hBCG measures the head’s mechanical reaction to the blood expelled by the heart and offers information about continuous heart rate, cardiac output, and respiratory rate.

A simultaneous measurement of ECG, PPG, and hBCG is shown in Figure 1. Using the peak timing data from ECG, PPG, and hBCG, blood pressure can be estimated. Figure 2 compares the estimated blood pressure with a commercial blood pressure measurement during a Valsalva breath-hold maneuver.

To make the monitor wearable, the electrodes must be small, comfortable, and gel-less to avoid skin irritation. We use 1-cm² capacitive and dry electrodes made of wearable fabric materials. The device is designed to use the ear as a discreet and a natural anchor that reduces device visibility and the need for skin adhesives.

1. S. D. Pierdomenico, M. Di Nicola, A. L. Esposito, R. Di Mascio, E. Ballone, D. Lapenna, F. Cuccurullo, “Prognostic value of different indices of blood pressure variability in hypertensive patients,” American Journal of Hypertension, vol. 22(8), pp. 842-847, June 2009.
2. D. He, E. S. Winokur, T. Heldt, C. G. Sodini, “The ear as a location for wearable vital signs monitoring,” Proc. of the IEEE Engineering in Medicine and Biology Conference, Sept. 2010, pp. 6389-6392.