Design and development of a novel Invasive Blood Pressure simulator for patient's monitor testing

sensors Article Design and Development of a Novel Invasive Blood Pressure Simulator for Patient’s Monitor Testing Daniele Bibbo 1,* , Jan Kijonka 2, Petr Kudrna 3, Marek Penhaker 2, Petr Vavra 4and Pavel Zonca 4 1Department of Engineering, University of Roma Tre, Via Vito Volterra, 62, 00146 Rome, Italy 2Department of Cybernetics and Biomedical Engineering, Faculty of Electrical Engineering and Computer Science, VSB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava–Poruba, Czech Republic; [email protected] (J.K.); [email protected] (M.P.) 3Department of Biomedical Technology, Faculty of Biomedical Engineering, Czech Technical University in Prague, nam. Sitna 3105, 272 01 Kladno, Czech Republic; petr[email protected] 4 Department of Internal Medicine, University Hospital in Ostrava, 17. Listopadu 1790, 70852 Ostrava, Czech Republic; petr[email protected] (P.V.); [email protected] (P.Z.) *Correspondence: [email protected]; Tel.: +39-06-5733-7298 Received: 18 November 2019; Accepted: 30 December 2019; Published: 1 January 2020   Abstract: This paper presents a newly-designed and realized Invasive Blood Pressure (IBP) device for the simulation on patient’s monitors. This device shows improvements and presents extended features with respect to a first prototype presented by the authors and similar systems available in the state-of-the-art. A peculiarity of the presented device is that all implemented features can be customized from the developer and from the point of view of the end user. The realized device has been tested, and its performances in terms of accuracy and of the back-loop measurement of the output for the blood pressure regulation utilization have been described. In particular, an accuracy of ± 1 mmHg at 25 ◦ C, on a range from − 30 to 300 mmHg, was evaluated under different test conditions. The designed device is an ideal tool for testing IBP modules, for zero setting, and for calibrations. The implemented extended features, like the generation of custom waveforms and the Universal Serial Bus (USB) connectivity, allow use of this device in a wide range of applications, from research to equipment maintenance in clinical environments to educational purposes. Moreover, the presented device represents an innovation, both in terms of technology and methodologies: It allows quick and efficient tests to verify the proper functioning of IBP module of patients’ monitors. With this innovative device, tests can be performed directly in the field and faster procedures can be implemented by the clinical maintenance personnel. This device is an open source project and all materials, hardware, and software are fully available for interested developers or researchers. Keywords: invasive blood pressure; medical devices; exciter voltage; waveform simulation 1. Introduction Direct Blood Pressure (BP) measurement, achieved by invasive techniques, is used in a specific context when a continuous and reliable monitoring is needed. This method of blood pressure measurement can provide detailed results, and it is mainly used for patients that need to be continuously monitored (e.g., hospitalization in intensive care unit). The most common technique to obtain the BP measurement is based on the use of a special catheter inserted into an artery, on which a pressure transducer is fixed, and then the signal is transmitted by a cable inserted into the catheter itself. These systems are generally made by a primary mechanical transducer to which one or more strain gauges are fixed, in order to transduce the mechanical deformation into an electrical quantity. Usually, Sensors 2020,20, 259; doi:10.3390/s20010259 www.mdpi.com/journal/sensors Sensors 2020,20, 259 2 of 19 these transducers are connected to a patient’s monitor, in order to provide invasive blood pressure information together with a plethora of vital parameters. A patient’s monitor, in order to be compliant with certified standards for clinical environment, has to be tested before its installation, and it has to undergo periodic maintenance to validate the correct functioning of all available features. To this aim, simulators able to easily reply physiological signals, such as electrocardiography or body temperature, are available for most monitoring systems. Concerning Invasive Blood Pressure (IBP) measurement, to the best of our knowledge, very few attempts have been realized for simulating this signal and the corresponding sensor, as demonstrated by a low number of papers in the corresponding literature. This aspect shows the need for such a device to improve the knowledge in this field. Anyway, the simulation of IBP may be useful for testing the functionality of the patient’s monitor IBP module, its zero and calibration function, and function of alarms. Moreover, the possibility of simulating many types of waveforms allows the use of this device in a wide range of applications, from research to equipment maintenance in a clinical environment to educational purposes, which is a growing strategical sector for the users of this kind of instruments [1–4]. The simulation of IBP on a patient’s monitor can be performed in two ways: The first is to use a standard IBP transducer connected to some hydraulic or pneumatic system, simulating blood pressure. In a simple case, this system may consist of a manual pump [ 5 ]. The second way is to use an electronic circuit instead of a pressure transducer that allows generation of an output signal similar to the IBP transducer’s output. This method does not require any mechanical parts, and both the static and dynamic pressures can be simulated with the required bandwidth and high accuracy can be achieved [6,7]. In this framework, the IBP simulator presented in this paper represents an innovation, both in terms of technology and methodologies: currently, it is very difficult to realize a quick and efficient test to verify the proper functioning of a patient’s devices features related to IBP measures, and the common solution is to send these devices to maintenance technical services. Using the device proposed in this paper, these tests can be performed directly in the field and faster procedures can be implemented by clinical maintenance personnel. A simple approach to the realization of these devices’ has been exploited in previous works [ 8 – 11 ], where the basic design of an electronic IBP simulator was discussed. Moreover, some tests with a patient’s monitor and a procedure for the calibration of the controlling circuits of such a kind of device was presented in [12]. In this paper, some improvements and extended features of a newly-designed device are reported, including all the new capabilities that guarantee better accuracy. Moreover, this new prototype allows regulation of its output on the basis of both the output and the input exciter voltage measurements, together with an auto-calibration feature implemented after power-on. To validate this, the accuracy assessment of the device under different conditions was performed and results are described in this paper. This is a crucial aspect since the evaluation of the accuracy is necessary when dealing with systems used to obtain information on human performances or on the health state of a patient [13]. Finally, it is worth highlighting that the possibility of sharing information with a community of developers is an important feature that can improve the knowledge in the development of a system or a device. This allows us to overcome problems and find smarter solutions. To this aim, this device was designed as an open source project, in order to share materials with interested researchers and developers. 2. Design and Implementation The IBP Simulator (IBPS) can simulate a bridge pressure transducer, which consists of a two-port network with input voltage VIN =(+IN)−(−IN) and output voltage VOUT =(+OUT)−(−OUT) . An example is reported in Figure 1, where a schematic diagram for a commercial pressure transducer (i.e., NovaSensor NPC-100 series @Amphenol) is reported [14]. Sensors 2020,20, 259 3 of 19 Sensors 2020, 20, x 3 of 19 Figure 1. NovaSensor NPC-100 pressure transducer schematic diagram. The device has been designed in order to simulate a wide range of IBP modules; this is possible because the analog interface is driven by a digital system that can control a wide range of parameters. The general scheme of the designed IBPS is reported in Figure 2. The quality of the performed design is guaranteed by the adoption of well-established procedures for embedded applications [15,16] in different fields, such as electronic circuits [17], transmission of data [18], and consumption optimization [19]. Figure 2. Invasive Blood Pressure Simulator (IBPS) scheme of functioning. ADC = analog to digital converter; LCD = liquid crystal display; MCU = microcontroller unit. Three controlling stages have been designed in order to drive the output for the simulation of the different IBP modules. This can be obtained by controlling the range and the positive or negative offset, as reported in the following. In general, the input exciter voltage 𝑉 can vary for different IBP modules but should be in the range 1–10 VDC; a typical value for a wide range of IBP module is 𝑉 =5 VDC. The output voltage 𝑉 depends linearly on 𝑉 . Considering as an example an Figure 1. NovaSensor NPC-100 pressure transducer schematic diagram. The device has been designed in order to simulate a wide range of IBP modules; this is possible because the analog interface is driven by a digital system that can control a wide range of parameters. The general scheme of the designed IBPS is reported in Figure 2. The quality of the performed design is guaranteed by the adoption of well-established procedures for embedded applications [15,16] in different fields, such as electronic circuits [ 17 ], transmission of data [ 18 ], and consumption optimization [19]. Sensors 2020, 20, x 3 of 19 Figure 1. NovaSensor NPC-100 pressure transducer schematic diagram. The device has been designed in order to simulate a wide range of IBP modules; this is possible because the analog interface is driven by a digital system that can control a wide range of parameters. The general scheme of the designed IBPS is reported in Figure 2. The quality of the performed design is guaranteed by the adoption of well-established procedures for embedded applications [15,16] in different fields, such as electronic circuits [17], transmission of data [18], and consumption optimization [19]. Figure 2. Invasive Blood Pressure Simulator (IBPS) scheme of functioning. ADC = analog to digital converter; LCD = liquid crystal display; MCU = microcontroller unit. Three controlling stages have been designed in order to drive the output for the simulation of the different IBP modules. This can be obtained by controlling the range and the positive or negative offset, as reported in the following. In general, the input exciter voltage 𝑉 can vary for different IBP modules but should be in the range 1–10 VDC; a typical value for a wide range of IBP module is 𝑉 =5 VDC. The output voltage 𝑉 depends linearly on 𝑉 . Considering as an example an Figure 2. Invasive Blood Pressure Simulator (IBPS) scheme of functioning. ADC =analog to digital converter; LCD =liquid crystal display; MCU =microcontroller unit. Three controlling stages have been designed in order to drive the output for the simulation of the different IBP modules. This can be obtained by controlling the range and the positive or negative offset, as reported in the following. In general, the input exciter voltage VIN can vary for different IBP modules but should be in the range 1–10 VDC; a typical value for a wide range of IBP module is VIN = 5 VDC . The output voltage VOUT depends linearly on VIN . Considering as an example Sensors 2020,20, 259 4 of 19 an exciter voltage VIN = 1 VDC , a transducer sensitivity k= 5 µV/V/mmHg , and a pressure applied to the transducer of 1 mmHg, an output voltage of 5 µV is obtained. The transducer sensitivity k can vary for different pressure transducers, even if the most common values are 5 µV/V/mmHg or 40 µV/V/mmHg. The simulator output circuit (Figure 3) is obtained by means of a Wheatstone bridge driven by three independent controlling stages. Since the Wheatstone bridge output is usually characterized by very low voltage values, in each stage a controllable current source is used (Figure 4). The output current from these type of sources is linearly dependent on the input exciter voltage VIN from the IBP module. Moreover, two measuring stages have been designed to control VIN and VOUT : the measurement of these two voltage levels is necessary to improve the accuracy of the device. In fact, the regulation of the output and its fine tuning are performed through a feedback loop control. Sensors 2020, 20, x 4 of 19 exciter voltage 𝑉 =1 VDC, a transducer sensitivity 𝑘 = 5 µV/V/mmHg, and a pressure applied to the transducer of 1 mmHg, an output voltage of 5 µV is obtained. The transducer sensitivity 𝑘 can vary for different pressure transducers, even if the most common values are 5 µV/V/mmHg or 40 µV/V/mmHg. The simulator output circuit (Figure 3) is obtained by means of a Wheatstone bridge driven by three independent controlling stages. Since the Wheatstone bridge output is usually characterized by very low voltage values, in each stage a controllable current source is used (Figure 4). The output current from these type of sources is linearly dependent on the input exciter voltage 𝑉 from the IBP module. Moreover, two measuring stages have been designed to control 𝑉 and 𝑉: the measurement of these two voltage levels is necessary to improve the accuracy of the device. In fact, the regulation of the output and its fine tuning are performed through a feedback loop control. V IN +V OUT -V OUT R R R R BA D C R IN R IN R OUT R OUT Figure 3. IBPS output circuit. Figure 4. Controllable current source circuit. 2.1. Controlling Stage 1 This stage is essential for the generation of dynamic BP waveforms; it consists of a range setting block, a buffer, a modulation block, and a current source (Figure 4). The blood pressure range can be adjusted from 0 to 330 mmHg with 10-bit resolution. The waveform modulation is performed using a 10-bitcontroller that is used for any pressure range setting. The current source calculation for a blood pressure of: 𝐵𝑃 ∈ 〈0,330 mmHg〉 considering V  = 1 VDC,𝑘 = 5µV/V/mmHg It is obtained as: |V|= k × V  × |BP|= 5 × 1 × 330 = 1650 µV, (1) so the maximum output current is: Figure 3. IBPS output circuit. Sensors 2020, 20, x 4 of 19 exciter voltage 𝑉 =1 VDC, a transducer sensitivity 𝑘 = 5 µV/V/mmHg, and a pressure applied to the transducer of 1 mmHg, an output voltage of 5 µV is obtained. The transducer sensitivity 𝑘 can vary for different pressure transducers, even if the most common values are 5 µV/V/mmHg or 40 µV/V/mmHg. The simulator output circuit (Figure 3) is obtained by means of a Wheatstone bridge driven by three independent controlling stages. Since the Wheatstone bridge output is usually characterized by very low voltage values, in each stage a controllable current source is used (Figure 4). The output current from these type of sources is linearly dependent on the input exciter voltage 𝑉 from the IBP module. Moreover, two measuring stages have been designed to control 𝑉 and 𝑉: the measurement of these two voltage levels is necessary to improve the accuracy of the device. In fact, the regulation of the output and its fine tuning are performed through a feedback loop control. V IN +V OUT -V OUT R R R R BA D C R IN R IN R OUT R OUT Figure 3. IBPS output circuit. Figure 4. Controllable current source circuit. 2.1. Controlling Stage 1 This stage is essential for the generation of dynamic BP waveforms; it consists of a range setting block, a buffer, a modulation block, and a current source (Figure 4). The blood pressure range can be adjusted from 0 to 330 mmHg with 10-bit resolution. The waveform modulation is performed using a 10-bitcontroller that is used for any pressure range setting. The current source calculation for a blood pressure of: 𝐵𝑃 ∈ 〈0,330 mmHg〉 considering V  = 1 VDC,𝑘 = 5µV/V/mmHg It is obtained as: |V|= k × V  × |BP|= 5 × 1 × 330 = 1650 µV, (1) so the maximum output current is: Figure 4. Controllable current source circuit. 2.1. Controlling Stage 1 This stage is essential for the generation of dynamic BP waveforms; it consists of a range setting block, a buffer, a modulation block, and a current source (Figure 4). The blood pressure range can be adjusted from 0 to 330 mmHg with 10-bit resolution. The waveform modulation is performed using a 10-bitcontroller that is used for any pressure range setting. The current source calculation for a blood pressure of: BP ∈D0, 330 mmHgconsidering VIN =1 VDC, k=5uV/V/mmHg It is obtained as: |VOUT|=k×VIN ×|BPMAX|=5×1×330 =1650 µV, (1) Sensors 2020,20, 259 5 of 19 so the maximum output current is: |IOUT|=2|VOUT| R=2×1650 ×10−6 100 =33 µA, (2) where Ris the Wheatstone bridge resistor. The maximum reference voltage VREF for the selected R1= 6.8 kΩ , where R1 is a current source resistor, is given by: VREF =|IOUT|×R1=33 ×10−6×6.8 ×103=0.2244 V. (3) After these values are computed, it is possible to obtain the theoretical R3 value when the digital potentiometer is set with R2=20 kΩ: R3TH =VIN −VREF VREF ×R2=1−0.2244 0.2244 ×20 ×10369 kΩ. (4) This value is not available among commercial precision resistors. For this reason, the closest available one R= 68 kΩ , was selected. Using this value, the maximum output pressure for R3≡ R3real =68 kΩ(E12 series) is re-calculated considering the obtained parameters: VREF, MAX =0.2272 V, (5)   IOUT, MAX  =VREF, MAX R1 =33.4 uA, (6)   VOUT, MAX  =  IOUT, MAX  ×R 2=1670.5 uV(334 mmHg), (7) which gives a maximum blood pressure correspondent to the maximum digital potentiometer output VREF −LSB (where LSB is the Least Significant Bit, and its value is LSB =0.326 mmHg) BPMAX =333.7 mmHg. (8) The current source is connected to the B node of the Wheatstone bridge for the positive output (Figures 2and 3). VOUT is linearly dependent on any exciter voltage VIN according to the relation: VOUT =VIN R×R2 2R1(R2+R3). (9) The range setting block controls the Reference Voltage in the range from 0 V to VREF (V REF =0.2244 V given by Equation (3)). 2.2. Controlling Stage 2 This stage is designed to obtain a positive static pressure and to adjust the diastolic pressure. It consists of a positive offset block and a current source. The positive static blood pressure can be adjusted in the range from 0 to 330 mmHg with 10-bit resolution. The calculation of the current source and the realization of this stage is the same as the one for controlling stage 1. 2.3. Controlling Stage 3 Similarly to controlling stage 2, a further stage 3 is included to set a negative output. In this case, the current source is connected to the A node of the Wheatstone bridge (Figures 2and 3). The negative static blood pressure can be adjusted in the range from − 60 mmHg to 0 mmHg with 10-bit resolution. Sensors 2020,20, 259 6 of 19 2.4. Measuring Stage 1 and 2 The first measuring stage (i.e., measuring stage 1) was been designed and implemented to continuously monitor the output voltage VOUT of the device. VOUT is amplified by an accurate instrumentation amplifier INA101HP (Texas Instruments, USA) with gain G =201. After amplification, an offset voltage of 1 V is added in order to fit the signal into the A/D converter range, avoiding possible saturation problems. This signal is digitalized by CH1 channel of the 16-bit analog to digital converter (ADC) with the common reference ADCREF =2.5 V. A second measuring stage (i.e., measuring stage 2) has been included to monitor the input exciter voltage VIN that differently from VOUT is a constant value. The blood pressure can be calculated from VIN,VOUT and kas: BP =VOUT k×VIN . (10) VIN is divided by a factor of 2.5 before its digitalization by CH0 channel of the ADC. 2.5. Digital Control The management of the device is performed by a central microcontroller unit (MCU, Atmel ATMEGA644A). It drives the digital potentiometers in the controlling stages and manages the acquisitions of VIN and VOUT . Both potentiometer sets and ADC are connected to the MCU via Serial Peripheral Interface (SPI), which allows easy and reliable communication among the different blocks of the device. Moreover, the device is equipped with a Universal Serial Bus (USB) port that is used for multipurpose tasks: to store custom waveforms, to perform calibration data storage, and to control the device remotely. Finally, a flash memory and the necessary control circuits were embedded on the device to store data. All these features can be managed directly onboard by means of four navigation buttons, while commands and data are displayed on an embedded liquid crystal display (LCD) screen. 3. Features As mentioned above, the IBPS (Figure 5) is equipped with a user interface implemented using four control buttons and a 4-row, 20-character LCD screen (no. 1 in Figure 5). The four control buttons are divided in one navigation button (no. 2 in Figure 5) used to control the cursor in different navigation menu, two setting buttons (no. 3 in Figure 5) for setting parameters, and one enter button (n.4 in Figure 5) used to set the selected function. Sensors 2020, 20, x 6 of 19 A second measuring stage (i.e., measuring stage 2) has been included to monitor the input exciter voltage 𝑉 that differently from 𝑉 is a constant value. The blood pressure can be calculated from 𝑉, 𝑉 and 𝑘 as: 𝐵𝑃 =   ×  . (10) 𝑉 is divided by a factor of 2.5 before its digitalization by CH0 channel of the ADC. 2.5. Digital Control The management of the device is performed by a central microcontroller unit (MCU, Atmel ATMEGA644A). It drives the digital potentiometers in the controlling stages and manages the acquisitions of 𝑉 and 𝑉. Both potentiometer sets and ADC are connected to the MCU via Serial Peripheral Interface (SPI), which allows easy and reliable communication among the different blocks of the device. Moreover, the device is equipped with a Universal Serial Bus (USB) port that is used for multipurpose tasks: to store custom waveforms, to perform calibration data storage, and to control the device remotely. Finally, a flash memory and the necessary control circuits were embedded on the device to store data. All these features can be managed directly onboard by means of four navigation buttons, while commands and data are displayed on an embedded liquid crystal display (LCD) screen. 3. Features As mentioned above, the IBPS (Figure 5) is equipped with a user interface implemented using four control buttons and a 4-row, 20-character LCD screen (no. 1 in Figure 5). The four control buttons are divided in one navigation button (no. 2 in Figure 5) used to control the cursor in different navigation menu, two setting buttons (no. 3 in Figure 5) for setting parameters, and one enter button (n.4 in Figure 5) used to set the selected function. Figure 5. IBPS user interface. On the top are reported (1) an LCD display (2) a navigation button, (3) two setting buttons, and (4) an enter button. On the front side of the IBPS there are two connectors (Figure 6). The 4-pin output connector (no. 2 in Figure 6) is used for the connection to the IBP module input of the patient monitor. The amplified output signal is also available on a BNC connector (no. 1 in Figure 6). Figure 5. IBPS user interface. On the top are reported ( 1 ) an LCD display ( 2 ) a navigation button, (3) two setting buttons, and (4) an enter button. Sensors 2020,20, 259 7 of 19 On the front side of the IBPS there are two connectors (Figure 6). The 4-pin output connector (no. 2 in Figure 6) is used for the connection to the IBP module input of the patient monitor. The amplified output signal is also available on a BNC connector (no. 1 in Figure 6). Sensors 2020, 20, x 7 of 19 Figure 6. (1) Scope connector for signal monitoring and (2) 4-pin connector for IBP monitor. On the backside of the IBPS, an embedded USB Mini-B connector is provided (no. 3 in Figure 7), and this is used to connect the device to a computer to exploit all software-driven features (e.g., to upstream data of different waveforms), together with the power switch and a connector (no. 1,3 in Figure 7) to supply the device with a standard 230V alternating current (AC). Figure 7. (1) Power switch, (2) alternating current (AC) power connector, and (3) Universal Serial Bus (USB) mini-B connector. 3.1. Static Blood Pressure Simulation This feature can be used for zero setting and for calibrating the patient’s monitors IBP modules. The output pressure can be regulated from a minimum negative value of −30 mmHg to a maximum positive value of 300 mmHg, with increments of 1 mmHg. The output pressure accuracy of the device is less than ±1 mmHg at 25 °C on the fixed value. The measured output voltage and input excitation voltage is also displayed on the LCD to allow the users to easily manage these parameters. The four push buttons are used for the menu navigation, blood pressure adjustment, and confirmation of the selected value. 3.2. Dynamic Blood Pressure Simulation The device can simulate dynamic BP waveforms; this feature is suitable for many physiological and pathological BP waveforms simulation (e.g., arterial, left and right ventricles, pulmonary artery, right atrium). To extend the possibilities of the simulation, there is a USB upstream port, allowing us to store up to 15 user-defined blood pressure waveforms. Moreover, it is possible to simulate respiration and other physiological parameters. To implement these features, the user can select one of the waveforms displayed in a list on the LCD screen. Thus, the waveform details are reported, and the user can accept them or set custom waveform properties. The systolic, diastolic, and mean pressures can be independently set with a resolution of 1 mmHg for each step. In addition, the simulation of the heart beats per minute can be regulated with a resolution of 1 BPM. The required BP value and the actual BP value are both visible on the LCD. Figure 6. (1) Scope connector for signal monitoring and (2) 4-pin connector for IBP monitor. On the backside of the IBPS, an embedded USB Mini-B connector is provided (no. 3 in Figure 7), and this is used to connect the device to a computer to exploit all software-driven features (e.g., to upstream data of different waveforms), together with the power switch and a connector (no. 1,3 in Figure 7) to supply the device with a standard 230V alternating current (AC). Sensors 2020, 20, x 7 of 19 Figure 6. (1) Scope connector for signal monitoring and (2) 4-pin connector for IBP monitor. On the backside of the IBPS, an embedded USB Mini-B connector is provided (no. 3 in Figure 7), and this is used to connect the device to a computer to exploit all software-driven features (e.g., to upstream data of different waveforms), together with the power switch and a connector (no. 1,3 in Figure 7) to supply the device with a standard 230V alternating current (AC). Figure 7. (1) Power switch, (2) alternating current (AC) power connector, and (3) Universal Serial Bus (USB) mini-B connector. 3.1. Static Blood Pressure Simulation This feature can be used for zero setting and for calibrating the patient’s monitors IBP modules. The output pressure can be regulated from a minimum negative value of −30 mmHg to a maximum positive value of 300 mmHg, with increments of 1 mmHg. The output pressure accuracy of the device is less than ±1 mmHg at 25 °C on the fixed value. The measured output voltage and input excitation voltage is also displayed on the LCD to allow the users to easily manage these parameters. The four push buttons are used for the menu navigation, blood pressure adjustment, and confirmation of the selected value. 3.2. Dynamic Blood Pressure Simulation The device can simulate dynamic BP waveforms; this feature is suitable for many physiological and pathological BP waveforms simulation (e.g., arterial, left and right ventricles, pulmonary artery, right atrium). To extend the possibilities of the simulation, there is a USB upstream port, allowing us to store up to 15 user-defined blood pressure waveforms. Moreover, it is possible to simulate respiration and other physiological parameters. To implement these features, the user can select one of the waveforms displayed in a list on the LCD screen. Thus, the waveform details are reported, and the user can accept them or set custom waveform properties. The systolic, diastolic, and mean pressures can be independently set with a resolution of 1 mmHg for each step. In addition, the simulation of the heart beats per minute can be regulated with a resolution of 1 BPM. The required BP value and the actual BP value are both visible on the LCD. Figure 7. ( 1 ) Power switch, ( 2 ) alternating current (AC) power connector, and ( 3 ) Universal Serial Bus (USB) mini-B connector. 3.1. Static Blood Pressure Simulation This feature can be used for zero setting and for calibrating the patient’s monitors IBP modules. The output pressure can be regulated from a minimum negative value of − 30 mmHg to a maximum positive value of 300 mmHg, with increments of 1 mmHg. The output pressure accuracy of the device is less than ± 1 mmHg at 25 ◦ C on the fixed value. The measured output voltage and input excitation voltage is also displayed on the LCD to allow the users to easily manage these parameters. The four push buttons are used for the menu navigation, blood pressure adjustment, and confirmation of the selected value. 3.2. Dynamic Blood Pressure Simulation The device can simulate dynamic BP waveforms; this feature is suitable for many physiological and pathological BP waveforms simulation (e.g., arterial, left and right ventricles, pulmonary artery, right atrium). To extend the possibilities of the simulation, there is a USB upstream port, allowing us to store up to 15 user-defined blood pressure waveforms. Moreover, it is possible to simulate respiration and other physiological parameters. Sensors 2020,20, 259 8 of 19 To implement these features, the user can select one of the waveforms displayed in a list on the LCD screen. Thus, the waveform details are reported, and the user can accept them or set custom waveform properties. The systolic, diastolic, and mean pressures can be independently set with a resolution of 1 mmHg for each step. In addition, the simulation of the heart beats per minute can be regulated with a resolution of 1 BPM. The required BP value and the actual BP value are both visible on the LCD. 3.3. Custom Waveforms and Usage of Built-in Memory Custom waveforms can be uploaded to the device via USB port, considering one period of the desired signal. The IBPS update tool, shown in Figure 8, allows to program a waveform in one of the device’s empty positions in the memory. Each waveform is characterized by a specific length (as number of samples) and sampling period. The parameters of the uploaded waveform, such as name, BPM, and systolic, diastolic, and mean pressure, must be specified by the user. These values are included at the beginning of the data waveform file as initialization bytes. The waveform data can be imported from data file format (e.g., CSV, TXT, MAT). Each data waveform is automatically converted to 10-bit resolution and normalized in the range from 0 to 1023. The update tool also provides to program raw data in a specified memory location of the device. This feature can be useful for future memory usage, such as for calibration data. Backup memory to a data file and erase operations are also possible. Sensors 2020, 20, x 8 of 19 3.3. Custom Waveforms and Usage of Built-in Memory Custom waveforms can be uploaded to the device via USB port, considering one period of the desired signal. The IBPS update tool, shown in Figure 8, allows to program a waveform in one of the device’s empty positions in the memory. Each waveform is characterized by a specific length (as number of samples) and sampling period. The parameters of the uploaded waveform, such as name, BPM, and systolic, diastolic, and mean pressure, must be specified by the user. These values are included at the beginning of the data waveform file as initialization bytes. The waveform data can be imported from data file format (e.g., CSV, TXT, MAT). Each data waveform is automatically converted to 10-bit resolution and normalized in the range from 0 to 1023. The update tool also provides to program raw data in a specified memory location of the device. This feature can be useful for future memory usage, such as for calibration data. Backup memory to a data file and erase operations are also possible. (a) (b) Figure 8. (a) The IBPS update tool, showing IBP waveform parameters (for one period) such as name, beats per minute (BPM), and systolic, diastolic, and mean pressure. (b) An example of the obtained waveform. 3.4. Remote-Control The device can be controlled remotely by a PC through the USB port. Even if the device has been designed and realized for a stand-alone use, an additional remote-control option has been implemented to allow the device integration in a computer-controlled environmental. The remote control allows the same operation as in stand-alone mode. Moreover, waveforms of any length can be simulated in real-time via serial data communication. 3.5. Power-on Self-Test and Auto Calibration After connecting the IBP module to the device, the device provides the necessary input excitation voltage V to generate an appropriate output voltage V according to the Equation (10). When the device is powered on, V and V are measured automatically by the measuring stages 1 and 2 (Figure 2) to allow different settings of the control stages. The following parameters are evaluated by the auto calibration process: • BP offset (controlling stages powered down); • BP range (controlling stage 3 at its maximum); • BP positive range (controlling stage 2 at its maximum); • BP range offset (controlling stage 1 range at its maximum, modulation at its minimum); • BP modulation minimum range (controlling stage 1 range at its minimum, modulation at its maximum); • BP modulation maximum range (controlling stage 1 range at its maximum, modulation at its maximum); • BP modulation half range (controlling stage 1 range at the half, modulation at its maximum). Figure 8. ( a ) The IBPS update tool, showing IBP waveform parameters (for one period) such as name, beats per minute (BPM), and systolic, diastolic, and mean pressure. ( b ) An example of the obtained waveform. 3.4. Remote-Control The device can be controlled remotely by a PC through the USB port. Even if the device has been designed and realized for a stand-alone use, an additional remote-control option has been implemented to allow the device integration in a computer-controlled environmental. The remote control allows the same operation as in stand-alone mode. Moreover, waveforms of any length can be simulated in real-time via serial data communication. 3.5. Power-on Self-Test and Auto Calibration After connecting the IBP module to the device, the device provides the necessary input excitation voltage VIN to generate an appropriate output voltage VOUT according to the Equation (10). When the device is powered on, VIN and VOUT are measured automatically by the measuring stages 1 and 2 (Figure 2) to allow different settings of the control stages. The following parameters are evaluated by the auto calibration process: •BP offset (controlling stages powered down); Sensors 2020,20, 259 9 of 19 •BP range (controlling stage 3 at its maximum); •BP positive range (controlling stage 2 at its maximum); •BP range offset (controlling stage 1 range at its maximum, modulation at its minimum); • BP modulation minimum range (controlling stage 1 range at its minimum, modulation at its maximum); • BP modulation maximum range (controlling stage 1 range at its maximum, modulation at its maximum); •BP modulation half range (controlling stage 1 range at the half, modulation at its maximum). All these parameters are evaluated and displayed on LCD at power-on. If one or more of these are not in the desired interval, an error symbol indicates that the device did not pass the power-on self-test. These parameters, which represent the input values for calculation of the measured BP, are used to remove the BP offset caused by the measurement stages. 3.6. Programming the Device The firmware can be upgraded using an In-System Programming (ISP) feature by a firmware upload connector available inside the device. The program is written in AVR GCC language for ATMEL MCU’s and consists of several files. 3.7. Technical and Simulation Parameters The technical parameters of the IBPS, obtained on the basis of the performed tests on the device, are summarized in Table 1. Table 1. IBPS Technical Parameters. Technical Parameter Value Unit Description Input impedance 3700 Ω Output impedance 300 Ω Exciter Input voltage VIN range 1 to 10 VDC Output Transducer Sensitivity 5 µV/V/mmHg Constant parameter Output pressure range −30 to 300 mmHg Output Offset Range −25 to 25 mmHg Output voltage VOUT range (VIN ={1; 5; 10}VDC) −150 to 1500 µVEquivalent to −30 to 300 mmHg −750 to 7500 µV −1500 to 15,000 µV Output voltage offset range at VIN =5 VDC −625 to 625 µVEquivalent to −25 to 25 mmHg Maximum difference between the desired and actual BP 0.4 (0.1 for static BP) mmHg Displayed BP (BP errors not included) Accuracy for ambient temperature 125 ◦C0.4 mmHg Test at the normal ambient temperature Maximum accuracy for ambient temperature 1from 0 ◦C to 50 ◦C1.4 mmHg Maximum value in the complete range of temperature BP offset for ambient temperature 125 ◦C0.2 mmHg Compensated by calibration BP offset for ambient temperature 1from 0◦C to 50 ◦C12 mmHg Maximum value in the complete range of temperature 1. For the full input voltage range (Vin={1; 5; 10}VDC) and all modes of blood pressure generating. In particular, output and input impedance, excitation voltage VIN , transducer sensitivity and the corresponding output voltage VOUT match the standard IBP transducer (as reported in [12]). The maximum difference between the required and measured values of BP is given by the controlling stages resolution and the noise (see Tables 3, 4 and 5). The accuracy of the device was evaluated through experimental tests that are described in the following. More specifically, tests Sensors 2020,20, 259 16 of 19 Table 8. VIN measurement errors. VIN ∆VIN at 25 ◦C∆VIN at 50 ◦C 1 V 0 mV 0.6 mV 5 V 0.02 mV 0.1 mV 10 V 0 mV 0.4 mV 5. Discussion The results described above show that the IBPS presented in this paper is a valid and reliable instrument for testing pressure monitors. The calculation and measurement of the accuracy show that there are potential improvements for the measuring stages, even if the actual performances allow the use of the instrument in most practical cases. Better accuracy could be achieved by replacing the components that cause the largest errors with higher quality components: this operation of re-designing can be easily achieved, but in this phase, it is has not been realized because the principal aim was to demonstrate how useful can be such kind of instruments, while testing BP monitoring devices. The minimum accuracy of the device is achieved for the lowest input exciter voltage VIN and for the lowest sensitivity. The sensitivity obtained at this stage of the device development is a constant parameter, but the simulation of 40 µ V/V/mmHg is necessary only for a minority of the commercial IBP transducers. The switchable sensitivity feature in the range 5/40 µ V/V/mmHg could be added by modifying the current sources (shown in Figure 4) by means of switchable R1 or R3 resistors. Moreover, the adjustable gain of the measuring stage 1 could improve the accuracy for lower VIN values. As explained above, the temperature drift can cause a maximum BP calculated offset of about 0.5 mmHg/ ◦ C: this is mainly due to the contribution of drift on the output voltage measurement (i.e., 0.43 mmHg/ ◦ C, as results from Table 5data show) and to the contribution of drift on the input voltage measurement (i.e., 0.015 mmHg/ ◦ C, as results from Table 7data show). According to this dimensioning data, this drift contribution could be compensated by the integration of a temperature sensor that can allow the implementation of an automatic calibration feature to compensate the drift. From the measurements performed during tests, a maximum drift value of 1.32 mmHg was obtained when considering a variation of the temperature from 25 to 50 ◦ C: this corresponds to a maximum BP offset of 0.053 mmHg/ ◦ C due to the temperature drift that is mainly caused by the instrumentation amplifier selected for the measurement stage 1. The drift value calculated according to the parameters given in the components datasheets is higher than the measured one. It is important to highlight that measured values of the instrument accuracy have been obtained as average of repeated tests on different specimens of IBPS. The use of a higher quality model of Instrumentation Amplifier (i.e., with built-in gain resistor once the fixed gain is determined), the total calculated drift can be theoretically reduced from 0.5 mmHg/ ◦ C to 0.05 mmHg/ ◦ C, thus reducing the instrument’s sensitivity to temperature changes and increasing its accuracy for the range 0–50 ◦C. The results reported in Figures 11–13 and summarized in Table 6show a very high accuracy: the maximum value is about ± 0,8 mmHg that corresponds to the 0.2% of setting when the V in =1 V for tests conducted with a temperature of 50 ◦ C. This value increases in normal operative conditions (i.e., less than 0.1% with a temperature of 25 ◦C) and the accuracy increases for higher values of Vin. The device presented in this work offers similar performances to more complex and expensive systems used for the same purpose. There are few available device on the market available for IBP simulation used to test BP monitors; among these, FLUKE Prosim 3 is one of the most complete and performant system to simulate a wide range of biomedical signals, such as electrocardiogram (ECG), respiration, temperature, cardiac output, etc., and IBP. This is a very complex and expensive device, also considering the maintenance aspects, so the IBPS presented in this work can be a valid alternative to such type of multi-simulators when simulations other than IBP are not needed. Moreover, the performances of the two devices are very similar, in terms of sensitivity, pressure range simulation, pressure accuracy, and input/output impedance, as reported in Table 9. The FLUKE Prosim presents Sensors 2020,20, 259 17 of 19 four different and independent channels, but only prefixed constant pressure values or pre-loaded pressure waveforms can be simulated. The IBPS presented in this paper has only one channel, but an extended version to four or more channels can be easily implemented. As described above, the tuning of the static level of pressure simulation of the presented device is finer than FLUKE Prosim 3, and the possibility of simulating custom waveforms gives rise to a wider range of applications; in fact, by using the USB connection, the presented IBPS can be programmed to simulate unlimited and custom waveforms. The IBPS presented in this work can simulate a transducer sensitivity of 5 µ V/V/mmHg, but in future developments, it will be easy implementable a second option for the value of 40 µ V/V/mmHg (it is HW reconfigurable ready). Finally, the pressure accuracy presented by the IBPS described in this paper is considerably higher than the commercial one, also considering that it can increase in normal operative conditions or with higher settings of Vin. Table 9. Comparison between FLUKE Prosim 3 and the developed IBPS performances. Fluke Prosim 3 IBPS Channels 4 1 Input impedance 300 Ω3700 Ω Output impedance 300 Ω300 Ω Exciter input range 2.0 to 16.0 VDC 1.0 to 10.0 VDC Exciter-input frequency range DC to 5000 Hz DC to 10,000 Hz Transducer sensitivity 5 or 40 µV/V/mmHg 5µV/V/mmHg (40 µV/V/mmHg HW reconfigurable) Pressure accuracy ±2% of setting +2 mmHg (valid for dc excitation only) ±0.2% of setting (maximum with Vin =1 V @50 ◦C) Static Levels 16 possible combining four channels −30 to 300 mmHg continuous Dynamic waveform 10 predefined combining four channels User programmable via USB Respiration Artifact BP delta changes from 3 to 16 mmHg User programmable via USB 6. Conclusions The designed and developed IBPS presented in this paper is a low-cost single purpose device compliant with the requirements for patient monitor IBP modules testing, zero setting, and calibration. It is possible to simulate a wide range of situations by setting different options for static and dynamic blood pressure waveform simulation. The device allows setting a constant BP and to generate user-defined BP that can be adjusted by setting the parameters of the waveforms. Both modes have an accuracy in simulating BP data that is better than ± 1 mmHg at 25 ◦ C, in which temperature represents the most common condition for the environment in which this instrument is used. The controlling stages are sufficiently precise, allowing to set the desired BP waveform in time and amplitude, with high sample rates and 10-bit resolution for all the modulation ranges. All described features can be easily implemented by a simplified user interface consisting in a four-button navigation panel and a display. The USB connectivity increases the capabilities of the device: It is possible to implement the remote control and to use the built-in memory to perform operations, like uploading custom waveforms or generating real-time ones. The USB connection can be used to upgrade the firmware in order to make the device hardware controllable. Moreover, a MATLAB graphical user interface (GUI) is available for custom waveforms upstream in order to make easy the use of the device for research and educational users. The device has been fully tested and compared with one existing device, and it has proven to have comparable and competitive features and performances; it can be used in research and clinical environments and, after a proper certification process, it could be used as a medical device. Finally, it is important to highlight that this device is an open source project, fully available for interested developers and researchers. All the materials (i.e., schematics firmware, software, and user manual) can be requested to the authors. In the future, it will be available on a web page, in order to create a forum of developers where they can share information and contributions. Sensors 2020,20, 259 18 of 19 Author Contributions: Conceptualization, D.B. and J.K.; Methodology, J.K.; Software, J.K.; Validation, D.B. and M.P.; Formal Analysis, P.K.; Investigation, P.V.; Data Curation, D.B., J.K. and P.Z.; Writing-Original Draft Preparation, D.B., J.K. and P.K.; Visualization, D.B.; Supervision, P.K. and M.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The work and the contributions were supported by the project SV450994/2101 Biomedical Engineering Systems XV’. This study was also supported by the research project The Czech Science Foundation (TACR) ETA No. TL01000302 Medical devices development as an effective investment for public and private entities. Conflicts of Interest: The authors declare no conflict of interest. References 1. Eason, M.P. Simulation devices in cardiothoracic and vascular anesthesia. Semin. Cardiothorac. Vasc. Anesth. 2005,9, 309–323. 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In Proceedings of the 38th International Conference on Telecommunications and Signal Processing (TSP), Prague, Czech, 9–11 July 2015; pp. 444–447. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).