A Virtual Instrumentation (VI) based system to acquire physiological parameters during preclinical animal trials of a prosthetic heart valve device
Company:
Sree Chitra Tirunal Institute for Medical Sciences & Technology (SCTIMST) Biomedical Technology Wing, Thiruvananthapuram 695012, India
Authors: Sujesh S, Scientist C; Rajeev A, Technical Assistant; Muraleedharan CV, Scientist F Category: Research, Biomedical Engineering. Author for Contact: Sujesh S, Engineer C, BMT Wing, SCTIMST. Poojappura, Thiruvananthapuram – 695012. Email:
[email protected] Ph: 0471 2520269. Virtual Instrumentation A typical virtual instrumentation (VI) system provides all functionalities of an instrumented test system effectively reducing the cost of realization. Most of the components of a VI are versatile and reusable providing the system designer the flexibility in the use of the same components for a variety of applications, there by reducing the over all cost per deployment. The look and feel of any real instrumented test and measuring equipment can be created using the software, and an extent of standardization is also possible taking the ergonomics and human factors engineering requirements into consideration. In the present solution under discussion, all the steps involved in recording and processing of the physiological signals, from setting the gain of signal conditioning amplifiers to digitizing, displaying, recording and post processing the data, were implemented using Virtual Instruments (VIs) on a Microsoft Windows XP platform. National Instrument products used: PXI-8186 PCI compatibly system controller PXI-1042 Chassis with universal power supply PXI-6220 DAQ Module SCXI-1000 SCXI Chassis SCXI-1520 Signal Conditioning Module SCXI-1314 Terminal Block Lab VIEW 7.1 Express. Problem Preclinical animal trial is an important phase in the evaluation for any medical device, especially for an implantable life saving device like an artificial heart valve. The data generated during these trials form a critical component of the preclinical data for establishing the safety and efficacy of the product. An instrumented test system is to be designed and developed for use during the preclinical evaluation of an artificial tilting disc heart valve being developed at SCTIMST. Monitoring and recording physiological
parameters such as blood pressure, ECG and heart valve sounds provide inputs on the condition of the animal during and subsequent to the implantation of the device and provides information on the safety and performance of the device under evaluation. The challenge was to develop reliable instrumentation that can be quickly set up and used in a typical surgical theatre environment. Solution The instrumentation system was developed using LabVIEW software platform and data acquisition and signal conditioning using hardware from National Instruments, USA. The hardware of the system was designed around the PXI modular data acquisition chassis. A set of transducers were identified, validated and incorporated into the system for converting the physical signals to analog electrical signals. The signal conditioning module SCXI 1520 was handy to provide excitation to the transducers as well as conditioning of the signals from them prior to being fed to analog to digital converters. The system could acquire and record data in real time with good signal quality, and also make it available later for offline processing using VIs developed for the purpose. Introduction Artificial heart valves are devices used for replacing the diseased or damaged natural valves of the heart. In vivo implantation studies using suitable animal models is mandatory during the development of such devices. Primarily, for obtaining regulatory approvals, a series of animal experiments need to be conducted as part of establishing the preclinical safety and efficacy of the device. International standards like ISO 5840: Cardiovascular implants – cardiac valves [1] provide the details on the selection of animal models, planning and conduct of the experiments and data collection requirements. Another important reason for conducting animal experiments is to observe and analyze valve function in vivo [2]. The influence of the presence of an artificial device on the heart function and the
Figure 1: Instrumentation in Operating Theater Environment
integration of the device with its surrounding can only be established through chronic implantation studies lasting at least 20-24 weeks. At the end of the designated period, the animals are electively sacrificed to study the incorporation of the device as well as to assess any adverse effects due to the implantation of the device. During the animal trial of the prosthetic heart valve, a number of physiological parameters need to be monitored and recorded. This includes the monitoring of the electrocardiograms (ECG), dynamic pressure levels in the heart chambers and major blood vessels, sound generated by the prosthetic heart valve, flow through the valve or the cardiac output. These parameters along with other biochemical and hematological investigations provide information for the management of the animal during the subsequent to the valve implantation. They also provide critical inputs for the assessment of the device performance. Based on a need assessment arterial pressure (ABP), left ventricular pressure (LVP), left atrial pressure (LAP), electrocardiogram (ECG) and heart valve sound (HVS) were identified as parameters that need to be monitored and recorded during the experiment. The system is to be employed during the valve implantation as well as the valve explantation procedures. System Description The instrumentation has been developed using the data acquisition modules of National Instrument (NI) which interfaces the transducers and custom developed instrumentation modules as described in Fig. 2. Three physiological semiconductor pressure transducers (PT1 to PT3) with 5µV/V/mmHg sensitivity (M/s. Spectramed, Singapore) are employed for converting the pressure signals. These transducers employ Wheatstone bridge configuration and the excitation signals are generated using the SCXI 1520 signal conditioning module [3, 5]. A conventional bedside monitor was used along with ECG electrodes was employed for interfacing the electrocardiogram signals. The heart valve sound was sensed using electret microphones with specially designed audio coupling which were mounted on the side port of the ventilator connector. A special circuit was assembled for biasing and interfacing the microphone to the system. The channels are configured as shown in Table 1.
LabVIEW ® On PXI 8186 / IBM PC With Windows XP
PXI 6220 DAQ
SCXI 1520 Signal Conditioning Module
SCXI 1314 Terminal Block
PT 1 (ABP) PT 2 (LVP) PT 3 (LAP) ECG Monitor (ECG) Microphone (HVS) Figure 2: Schematic depicting the interconnection of various hardware subsystems employed in the implementation
The data is captured and processed using customized software developed on the LabVIEW platform running on a PC or the PXI 8186 controller [6]. The software performs a number of post processing steps like applying suitable filters and gain to the acquired data and finally writes it to a file on the PC. This file can later be viewed offline and further processed to study the various physiological parameters. Software Implementation The software has been developed in LabVIEW and consists of modules for Initialization, Data Display, Data Save, Zero pressure, Post processing and Error handling. Each module has a front panel, containing the user interface, and a block diagram which is the actual graphical code that is executed [7]. The coding is done using DAQmx architecture.
Channel
Excitation
Filter
Unit
Range
Arterial Pressure (ABP)
5V
OFF
mmHg
-10 to 300
Left ventricular Pressure (LVP)
5V
OFF
mmHg
-10 to 300
Left Atrial Pressure (LAP)
5V
OFF
mmHg
-10 to 300
Electrocardiogram (ECG)
OFF
10kHz
V
-10 to 10
Heart Valve Sound (HVS)
5V
OFF
V
-1.5 to 1.5
Table 1: Analog input channel configuration details
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Initialization & Configuration: Each physical channel is created logically and configured on a DAQmx task with the settings as given in Table 1. The sampling rate is set to 25kS/s, and the data is read by the software every second from the device allocated buffer in computer memory. The number of samples read can be varied by the user. 25,000 samples per read gives the update time as 1 second. Channels are created with the two DAQmx VIs; ‘Create AI Custom Voltage with Excitation’ or ‘Create AI Custom voltage’ if no excitation is required.
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Display: All the six channels are displayed separately as a strip chart graph. The display can be modified with respect to the time window on x-axis as well as y-axis. Usually autoscale feature is enabled on y-axis. The zero corrected readings are shown and updated every second.
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Save: The data being read can be saved to a file. A time display shows the time period for which the data has been saved. A multichannel graph is also displayed which shows the data being stored in real time.
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Zero: Each of the pressure channels in the system can be zeroed independently as and when required.
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Post processing (convert to text record): All the data stored is in the form of 1second records, each consisting of a waveform array data type. Array size is 5, and the record length is 25,000 which is number of samples per read operation. All these records are converted to a single text record corresponding to the total duration of recording.
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Post processing of saved data Once the data is obtained the following are studied: 1. Arterial pressure gives the systolic and diastolic pressures after correlating with the ECG. 2. ECG gives the heart rate, start of systole and other timing information
3. Transvalvular pressure gradient for explantation study (LV – LA) gives the data regarding forward pressure drop due to blood flow across valve when valve is open, and closing pressures across mitral valve generated during systole. 4. Heart valve sounds of the artificial heart valve are used to determine sound levels and ascertain performance of the valve. Capabilities used 1. DAQmx architecture was used for configuring the Analog to Digital Conversion channels, starting, reading and stopping the ADC process. DAQmx Create Scale VI was used to configure the scaling of input to give output in required units such as mmHg etc. 2. Event handling structure was used for running the VIs based on user input. Property nodes were extensively used to configure display attributes 3. Signal conditioning of SCXI 1520 was used to provide excitation, amplification and low pass filtering capabilities. 4. PXI 6220 DAQ was used to provide ADC of 6 channels at a rate of 25 kilo samples per second.
Fig. 3: Main VI
Fig 5: Display VI
Fig. 4 Initialization & Configuration VI
Fig. 6: Save VI
Fig. 7: Zero Correct VI
Fig. 8: Convert VI
Suitability The system developed was easy to setup and use in the operating theater environment. More than 20 experiments were performed and data captured regarding the performance of the heart valve. Fig. 9 shows the waveforms that were recorded during one of the experiments. Not a single failure was detected in use and the hardware and software components performed satisfactorily. Overall the ease of developing a complicated system using the existing hardware and software components sourced from National Instruments (LabVIEW, PXI) and their rugged execution was highly appreciated.
Figure 9: Waveforms obtained during recording
Conclusion A data acquisition and processing system was developed to capture physiological parameters regarding the functioning of a prosthetic heart valve device during pre-clinical animal trials. The data obtained was of satisfactory quality and was useful in studying the performance of the valve. References 1. International Organization for Standardization [ISO], ISO 5840 : Cardiovascular implants – cardiac valves, 3rd Ed, Ref. No ISO 5840:2005(E), Geneva, 2005 2. Bhuvaneshwar GS, Muraleedharan CV et al., Development of the Chitra tilting disc heart valve prosthesis, J Heart Valve Dis., V5(4):448-58, 1994 3. SCXI – 1520 User Manual, National Instruments Corporation, Part No. 322583B-01, Texas, USA, 2003 4. PXI – 1042 Series User Manual, National Instruments Corporation, Part No. 323351C-01, Texas, USA, 2004 5. Getting Started with SCXI, National Instruments Corporation, Part No. 320515F-01, Texas, USA, 2000 6. Measurements Manual LabVIEW, National Instruments Corporation, Part No. 322661B-01, Texas, USA, 2003 7. LabVIEW User Manual, National Instruments Corporation, Part No. 320999E-01, Texas, USA, Texas, USA, 2004
