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The Autors' Department was
created in 1993 by the former Cardiovascular Engineering Group
existing since early 80s. Research activity is presently devoted to
the study of cardiac output regulation and the interaction between
the ventricle and circulatory network. Details on research activity
are reported at www.cardio.itbm.rm.cnr.it. |
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Cardiovascular engineering includes several research topics
characterized by the use of engineering techniques. These topics
include hemodynamics, signal analysis, active and passive
cardiovascular prosthetics components, automated diagnostic
procedures, and others. The research activities of Cardiovascular
Engineering Department of our Institute are, in general, oriented
to the study of interaction between the ventricle and circulatory
network. To this aim, researchers devote special attention to
hemodynamics evaluation under different physiological and
pathological conditions. Initially, studies are usually done with
hydraulic and biological (sheep) models. These methodologies are
then transferred into clinical environments. In these environments,
there is a large demand for real-time monitoring systems. Such
systems must be flexible, easily reconfigurable to accommodate new
experiments, and must consider the possibility of interfacing to
different medical devices, such as patient monitors, biological
amplifiers, and so on. Furthermore in order to introduce these new
methodologies in daily clinical activities, data presentation must
be designed considering standards used by traditional medical
devices.
Materials
The proposed system resides on a Windows-based PC implemented
with a ADC/DAC conversion board from National Instrument (6040E and
6041E families). The PC uses a Pentium II 366 MHz processor with
132 MB RAM and 5 GB HD to allow the system to use computational
capability; however, less powerful machines can also be used.
The software is designed using National Instrument's LabVIEW, an
environment operating under Windows that creates programs in
block-diagram forms. LabVIEW offers a built-in library for data
acquisition, processing, analysis, and display. The library is
implemented in the graphical language G. LabVIEW programs are
called virtual instruments (VIs), because their appearence and
operation imitate actual instruments. However functions from the G
library are analogous to functions from conventional language
programs.
Generally speaking, VIs comprise a block diagram (Figure
1) and user interface or front panel (Figures 2 and 3).
The block diagram gives a pictorial solution to a programming
problem and contains the source code for VIs shown as icons and
connectors. The front panel simulates a panel of physical
instruments and contains both controls and indicators in such a way
that the operator can input data using keyboard and mouse and then
then view results on a computer screen.
Figure 1: This block diagram of the proposed VI
system includes a data acquisition system with an ADC system.
The core of the software driving the system is the acquisition
section. The G code for this part of the software is represented in
Figure 1. Standard LabVIEW functions set up the system's
hardware capabilities before running ADC conversions. The ADC board
is designed to acquire data in DMA using a 20,000-sample circular
buffer. Data retrieval occurs in a while loop structure (the thick
line in Figure 1). During each iteration, ten samples are
transferred in a LabVIEW array for computation.
Using this approach objects performing different functions can
be added using visual wiring capabilities. Non-expert users can
create simple objects based on system library VIs. More complicated
analysis can be implemented by an expert programmer and simply
wired into the main program. Furthermore, since LabVIEW supports
custom codes written in other languages, you can integrate writen
graphical language software with source code written in C++,
interfaced to the main program using some special components of
these tools (for example, the Code Interface Node or CIN).
A further capability of our hardware/software set up is to
communicate, or network, with other processes. This allows our
system to be applied in a LAN or to be used as an Internet server
to share real-time data on the Web.
Results
Innovative Patient Monitoring
Patient monitoring and consequent data interpretation offers the
possibility to have both information on patient's status and on the
changes produced by therapy. By this point of view, the combination
of different signals, even if by simple computations, is useful in
clinical practice. The implementation of these methodologies
supports the extraction of further information on hemodynamics and
energetic parameters from existing signals. This capability can
reduce the global invasiveness of the measurement system and
permits an easier uptake of research results from bioengineering to
the clinical routine
. Starting from these considerations, we have
developed an application able to implement an innovative
signal-processing application in intensive care and during
surgery
. In this occurrence the system is based on a
battery powered notebook PC to avoid problems regarding safety
according to IEC 601 safety requirements. Visualization and simple
manipulation of data coming from different devices can be easily
performed. In this way it is possible to test new and simple
multimodal analysis. The on-line study of the interaction of
respiratory and cardiovascular systems has been implemented to
reduce invasiveness in left-ventricular preload assessment
.
Figure 2: Our instrument's
output for in vivo studies. The figure shows, from top to bottom,
ECG, arterial pressure, venous pressure, and expired
CO2. The right side of the figure shows required on-line
computations.
Benchmark for Mechanical Heart Assist Devices
In the study and in the evaluation of mechanical heart assist
devices (such as IABP and VAD) special attention is given to both
the device mechanical properties and to their effect on the
circulatory system under different hemodynamic conditions.
Modeling these phenomena can be helpful in a wide range of
clinical and research applications and frequency domain analysis
has been widely applied
. In particular, frequency domain representation
of arterial impedance as well as pressure transfer function of
different vascular districts are used to evaluate mechanical
properties in the presence of IABP and VAD. In this context,
hydraulic circuits to simulate different conditions of the heart
and different arterial loads are used.
This section of the article summarizes how we have developed a
set of virtual instruments to perform real-time frequency-domain
data analysis to characterize cardiovascular devices and their
loads
.
Figure 3: Output of our system for in vitro
benchmarks. On left side acquired signals (two pressures and flow);
on right side computed input impedance and pressure transfer
function (modulus and phase waveforms).
Figure 3 is the front panel of the
system computing harmonic response (in terms of input impedance and
pressure transfer function) of an hydraulic network simulating
systemic circulation. The acquisition of pressure and flow signals
is triggered to cover a whole cardiac cycle.
In benchmarking it is moreover often necessary to integrate
numerical and hydraulic models and allow the exchange of data
between them. The problem of real time data acquisition and
analysis is clearly central also in this context. It must be
considered that often numerical models are described using
different languages like for example C. We found a solution to this
problem using the real time capabilities of LabVIEW and integrating
it with source codes written in C++.
Conclusions
The proposed system is useful in cardiovascular engineering for
innovative prototyping and/or limited production of new
devices.
