A dataset of clinically recorded radar vital signs with synchronised reference sensor signals


Participants

Ahead of planning the experiments, approval was acquired from the local ethics committee. Overall 14 male and 16 female healthy test subjects were measured with an average age of 30.7 ± 9.9 years and an average BMI of 23.2 ± 3.3 kg/m2. Before cabling, each test person was informed about the measurements that were conducted and each person was assigned a unique ID for pseudonymisation. During this process, any questions about the radar that arose could be answered. A written consent was obtained from all participants that also allows for sharing the pseudonymised data. All test persons had to fill out a questionnaire on epidemiological data, such as age, sex, weight, and history of diseases. An extract of the data is shown in Table 1. In addition, the condition of the subjects was briefly checked by examining blood pressure, heart rate and heart sound. If all criteria were found to be positive, the subjects could be included in the study and the measurement performed.

Table 1 Overview of all test subjects.

Human subjects

The study was approved by the ethics committee of the Friedrich-Alexander-Universität Erlangen-Nürnberg (No. 85_15B). All research was performed in accordance with relevant guidelines and regulations. The informed consent was obtained from all subjects in human trials.

Procedures

All measurements were recorded at the Department of Palliative Medicine at the university hospital Erlangen. At least two persons have carried out the measurements. One person was responsible for the protocol, the other person carried out the interventions. After the test persons filled out the questionnaire and gave their consent, the electrodes for the reference measurement were attached to the upper body. The placement of the electrodes can be seen in Fig. 1c. Before the measurement began, the subjects lay down on the tilt table with their upper body facing the radar, as shown in Fig. 1a. Once the BP cuffs and wiring were in place, the subjects were auscultated and the radar systems was moved so that the laser projection was directed at a point where a strong heart sound signal could be perceived. An image of the laser projection is shown in Fig. 1f. Since the focal point between the receiving and transmitting antenna was designed for a distance of around 40 cm, the distance between the radar and the region of interest (ROI) was chosen accordingly during all measurements. A block diagram of the overall setup can be seen in Fig. 1e. In the following, all components will be described in detail.

Fig. 1

Overview of the measurement setup and the system configuration. (a) Photograph of the measurement setup without reference sensor cables. (b) Photograph of radar system with descriptions of features. (c) Photograph of reference sensors and their locations. (Reprinted under the CC BY license with permission) (d) Photograph of the tilt table. (e) Block diagram of the system configuration. (f) Photograph of the laser projection for positioning.

Radar system

The radar system used is a further development of the system used in11, which was realized with laboratory equipment. It is also based on the Six-Port technology, but has been extended with discrete components into a portable radar system, as can be seen in in Fig. 1a,b,d. In addition, a bistatic antenna design is used to improve signal quality. The inclination angle of the antenna beams is ±10° for transmitting (TX) and receiving (RX) antenna, respectively, with a focal point at 40 cm. The system also has a positioning laser in the middle between the two antennas. This class 1 laser generates a projection on the upper body of the test person, so that the focus point of the system can be easily aligned. The laser in the housing is shown in Fig. 1b and the projection in Fig. 1f. A more detailed view on the system concept of the 24 GHz continuous wave radar is given in15. Therefore, the following will only deal with key points of the system to explain the signal processing and vital sign extraction.

The Six-Port structure, on which the system is based, is completely passive and is known to have a high phase resolution16. A movement in front of the antenna causes a measurable phase change Δφ between TX and RX signal, which can be converted into a displacement change Δx with the known wavelength λ of the TX signal using17:

$$Delta x=frac{Delta varphi }{2{rm{pi }}}cdot frac{{rm{lambda }}}{2}.$$

(1)

The raw signals of the radar, called In-Phase (I) and Quadrature (Q) component, are digitized simultaneously using the 24 bit analog-to-digital converter (ADC) ADS1298 from Texas Instruments at a sampling rate of 2000 Hz. The I and Q signal are used to calculate Δφ by arctangent demodulation after a compensation for nonidealities, called ellipse reconstruction, is made. More detailed descriptions can be found in11. After sampling the data generated from the ADC are stored in the microcontroller XMC4500 from Infineon that is used to control the radar system. Each time after reaching 50 samples per channel, the data is sent in a UDP packet via Ethernet to a PC for storage and further processing.

Besides controlling the radar, the microcontroller is also used to generate a synchronisation sequence. The sequence, which consists of a sequential binary on and off switching of an analogue pin according to the Gold codes, is sampled simultaneously at the ADC of the radar and is also fed to the external input of the reference system. The processing and synchronisation of the sequence is described in the section “Synchronisation betwwen rader and TFM”. In addition, a push-button is connected to the housing of the radar, which is used during the measurements to set interventions. The button signal is transmitted to another external input of the TFM. An overview of the complete system is given in Fig. 1e. In this representation, the two external signals are combined as EXT, but both are sampled separately at the TFM.

Reference system

The sensors of the reference system are explained in the paragraphs below. The reference system used in this study is the Task Force Monitor 3040i from CNSystems Medizintechnik GmbH. The TFM and the corresponding tilt table with the radar mount are illustrated in Fig. 1a. The entire system depicted consists of a monitor, the TFM, a PC, and a printer. In addition, a special tilt table is used which can be tilted up to 90°. In Fig. 1d the table is shown with an angle of about 20°.

In addition to ECG, ICG, oscillometric, and continuous BP the TFM has connections for recording two external inputs. The sensors and external inputs of the system are sampled simultaneously and can be exported from the recording software in a .mat file after the measurement. The included software of the TFM evaluates the raw signals of the sensors during the measurement and also determines different insightful hemodynamic and autonomic parameters of the subject. The sampling frequencies of the individual raw signals described in the following paragraphs refer to the present signals after export from the recording software. Since the signals were processed for synchronisation, other sampling frequencies are provided in the database. The corresponding frequencies are stored additionally in each dataset.

ECG

A three channel ECG is used to record electrical activity of the heart during the measurements. The four color coded leads are attached according to clinical standard: red at the right arm, yellow on the left arm, green on the left leg, and black on the right leg. Exemplary applied electrodes are shown in Fig. 1c. A new set of gel electrodes was used for each subject. The TFM recorded the raw data of leads 1 and 2 according to Einthoven’s triangle18. In the supplied software lead 3 and the augmented limb leads are calculated from the two raw channels, but not exported to the .mat file. The ECG channels are digitized at the TFM with a sampling rate of 1000 Hz and a precision of ±5 μV.

ICG

ICG provides insight into the impedance change of the thorax by applying an alternating small current between two electrodes on the body. Based on Ohm’s law, the measured voltage is proportional to the impedance. Four electrodes are attached to the upper body for the measurement as seen in Fig. 1c: one band electrode on the nape of the neck, two band electrodes lateral on the thorax at the height of the xiphoid process, and one neutral gel spot electrode at the left leg19. After export, the ICG raw signal is available with a sampling frequency of 500 Hz, whereas the impedance signal is specified with a sampling frequency of 50 Hz.

Blood pressure

The TFM is able to measure a continuous BP signal non-invasively. This technology is called Continuous Noninvasive Arterial Pressure (CNAP) and is done by combining the measurement of an oscillometric BP cuff and a cuff at the fingers measuring the vascular unloading20. In Fig. 1c the placement of the BP cuffs is shown. Using this method changes in BP can be monitored during the different interventions. The signal has a sampling frequency of 100 Hz.

External input

To enable synchronisation, the external input of the TFM is used. Here the system offers two inputs which are sampled with 1000 Hz. Thus the synchronisation sequence is digitized on one of the inputs and the push-button signal for setting interventions on the other one.

Parameters

In addition to the synchronous recording of the various sensors, the TFM software evaluates the individual signals and calculates various parameters of the subject’s hemodynamics and autonomic functions. A list of the parameters with their short and full names is given in Table 2. The values are determined on the basis of each individual heartbeat and are therefore not sampled continuously but with different time intervals beat-to-beat. Because of this, a dedicated time vector is provided for the parameters.

Table 2 Overview of aggregated parameters from the reference system. Short names are used in the database.

Measurement protocol

The measurements were carried out under the supervision of at least two persons according to an established protocol. Before each scenario, the condition of the test person was queried and the measurements could be stopped by the test person at any time. For some subjects, not all scenarios could be carried out. An Excel table, called additional_data.xlsx, with corresponding information is available in the database. The different scenarios are explained in the sections below. After lying down and completing the setup, the subjects were asked to relax for at least 10 min before the measurements were started. During the scenarios the subjects were told to breath calm and avoid large movements.

Resting

After the relaxation phase the first scenario was started. During the resting scenario the participants continued to lie relaxed with calm breathing. The measurement duration of the scenario is at least 10 min. An exemplary extract of all raw signals from a resting measurement is shown in Fig. 2a. There you can see in the synchronous signals that the test person breathes calmly and it can be derived that the resting heart rate is at about 54 BPM. How to extract the displayed radar vital signs from the raw displacement signal is described in10,11,15. Furthermore, in this scenario no marker is set with the push-button as shown in Fig. 3.

Fig. 2
figure2

Exemplary signals from radar and TFM during different scenarios: (a) Resting, (b) Valsalva, and (c) Apnea.

Fig. 3
figure3

Overview and description of the intervention setting protocol in the different scenarios.

Valsalva

In the next scenario the Valsalva manoeuvre is performed three times with pauses in between. The manoeuvre is understood to be the forceful expiration against the closed glottis for 20 s21. After the VM the test person breathes out and then continues breathing calmly. The sequence of the scenario consists of three runs of 20 s VM with 5 min of recovery in each case afterwards.

The effects of the VM are hemodynamic changes of the circulatory system which are also visible in the parameters measured by the TFM, as can be seen in Fig. 2b. To mark the manoeuvre in the measured data the start and the end is highlighted with the push-button. In Fig. 3 the intervention setting for this scenario is shown as a visual example.

Apnea

During this scenario the subjects hold their breath in two defined states as long as possible. In the first state, the subject breathes in completely before apnea, in the second state, the subject breathes out completely before apnea. In Fig. 2c the measured raw signals at the transition from normal respiration to exhaled apnea are illustrated.

The intervention in this scenario is set by the subjects themselves. The test person presses the button as long as the breath is held. This results in the intervention signal seen in Fig. 3.

Tilt up

Throughout the tilt up scenario the tilt table is raised to trigger the ANS of the subject. This test leads to a strong reaction of the ANS, among other things, BP and heart rate change significantly. The measurement is started before the table is slowly raised to 70°. Afterwards the measurement is continued for 10 min. While measuring, the start and end of the table movement is marked with the button, as illustrated in Fig. 3.

Tilt down

This scenario is the opposite of tilt up. The upright table is moved back to the starting position. Thus, after the start of the measurement, the table is lowered back to 0° and the recording continued for 10. Again reactions of the autonomic nervous system will occur during the procedure. As in the previous scenario, the interventions are set at the start and end of the table movement.

Exemplary signals of three different scenarios can be seen in Fig. 2. In Fig. 2a,c synchronised raw signals of both systems and in Fig. 2b only TFM aggregated parameters are shown. Looking at the raw signal plots you can see the radar displacement signal, radar breathing, impedance, radar pulse, ICG, BP, both ECG signals, and radar heart sound. Apart from the radar vital parameters only the ECG signals were bandpass filtered in the range of 1 Hz to 20 Hz.



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