A Flexible 12-Lead/Holter Device with Compression Capabilities for Low-Bandwidth Mobile-ECG Telemedicine Applications

Abstract

In recent years, a number of proposals for electrocardiogram (ECG) monitoring based on mobile systems have been delivered. We propose here an STM32F-microcontroller-based ECG mobile system providing both long-term (several weeks) Holter monitoring and 12-lead ECG recording, according to the clinical standard requirements for these kinds of recordings, which in addition can yield further digital compression at stages close to the acquisition. The system can be especially useful in rural areas of developing countries, where the lack of specialized medical personnel justifies the introduction of telecardiology services, and the limitations of coverage and bandwidth of cellular networks require the use of efficient signal compression systems. The prototype was implemented using a small architecture, with a 16-bits-per-sample resolution. We also used a low-noise instrumentation amplifier TI ADS1198, which has a multiplexer and an analog-to-digital converter (16 bits and 8 channels) connected to the STM32F processor, the architecture of which incorporates a digital signal processing unit and a floating-point unit. On the one hand, the system portability allows the user to take the prototype in her/his pocket and to perform an ECG examination, either in 12-lead controlled conditions or in Holter monitoring, according to the required clinical scenario. An app in the smartphone is responsible for giving the users a friendly interface to set up the system. On the other hand, electronic health recording of the patients are registered in a web application, which in turn allows them to connect to the Internet from their cellphones, and the ECG signals are then sent though a web server for subsequent and ubiquitous analysis by doctors at any convenient terminal device. In order to determine the quality of the received signals, system testing was performed in the three following scenarios: (1) The prototype was connected to the patient and the signals were subsequently stored; (2) the prototype was connected to the patient and the data were subsequently transferred to the cellphone; (3) the prototype was connected to the patient, and the data were transferred to the cellphone and to the web via the Internet. An additional benchmarking test with expert clinicians showed the clinical quality provided by the system. The proposed ECG system is the first step and paves the way toward mobile cardiac monitors in terms of compatibility with the electrocardiographic practice, including the long-term monitoring, the usability with 12 leads, and the possibility of incorporating signal compression at the early stages of the ECG acquisition.

Keywords: Android™; ECG; Holter; STM32F microcontroller; rural areas; signal compression low-bandwidth; telemedicine.

Figure 1

General diagram of the flexible 12-lead/Holter prototype hardware architecture, where large city and small city refer to urban and rural areas.

Figure 2

Block diagram of the acquisition module.

Figure 3

Acquisition module implementation: (a) Electronic circuit diagram. (b) Upper layer. (c) Lower layer. (d) Bluetooth module and BITalino™ power module. (e) Acquisition module mounted in a box. The electronic card was designed by considering the dimensions of the microSD memory support (15 mm × 17 mm) and the power module (20 mm × 30 mm). The dimensions of the shown electronic card which was designed for the acquisition module were 62 mm × 70 mm.

Figure 3

Acquisition module implementation: (a) Electronic circuit diagram. (b) Upper layer. (c) Lower layer. (d) Bluetooth module and BITalino™ power module. (e) Acquisition module mounted in a box. The electronic card was designed by considering the dimensions of the microSD memory support (15 mm × 17 mm) and the power module (20 mm × 30 mm). The dimensions of the shown electronic card which was designed for the acquisition module were 62 mm × 70 mm.

Figure 4

Hardware architecture: (a) Cellphone module. (b) Web server.

Figure 5

Application flowchart of the acquisition module.

Figure 6

Smartphone software: (a) Patient data base field. (b) Home screen. (c) Patient registration screen. (d) Stored-record screen. (e) 12-lead ECG signal screen. (f) Single ECG signal screen.

Figure 7

Web server application: (a) 12-lead ECG signals screen; (b) online display screen.

Figure 8

Scenario of Experiment 1 for the battery charge-duration test.

Figure 9

Results of Experiment 1: (a) Energy autonomy tests. (b) Storage capacity tests.

Figure 10

Experiment 3 for validation of signal compression and decompression: (a) ECG signal quality tests with compression and decompression processes. (b) ECG signal after the compression and decompression process. (c) The original uncompressed signal and the example signal when compressed and decompressed by the prototype.

Figure 11

Experiment 4 for prototype calibration with respect to commercial electrocardiograph: (a) Calibration scenario of the 12-lead/Holter ECG prototype. (b) D2 lead reference signal of commercial electrocardiograph. (c) D2 lead calibrated signal of 12-lead/Holter ECG prototype (right). (d) 12-lead ECG signals obtained with the prototype. (e) 12-lead ECG signals obtained with the commercial electrocardiograph.

Figure 12

Experiments 4 and 5: (a) Test scenario for clinical validation with real patients and with ECG simulator. The ECG signals presented to physicians specialized in cardiology are also shown—in signals from the simulator with AB1 pathology (left) and from patients (right). (b,e) ECG signals obtained with the prototype from simulator and the patient, and without compression. (c,f) The same obtained by the prototype with compression. (d,g) ECG signals obtained by the commercial electrocardiograph.