Investigation of a continuous heating/cooling technique for cardiac output measurement

Cardiac output is frequently measured to assess patient hemodynamic status in the operating room and intensive care unit. Current research for measuring cardiac output includes continuous sinusoidal heating and synchronous detection of thermal signals. This technique is limited by maximum heating element temperatures and background thermal noise. A continuous heating and cooling technique was investigated in vitro to determine if greater thermal signal magnitudes could be obtained. A fast responding thermistor was employed to measure consecutive ejected temperature plateaus in the thermal signal. A flow bath and mechanical ventricle were used to simulate the cardiovascular system. A thermoelectric module was used to apply heating and cooling energy to the flow stream. Trials encompassing a range of input power, input frequency, and flow rate were conducted. By alternating heating and cooling, thermal signal magnitude can be increased when compared to continuous heating alone. However, the increase was not sufficient to allow for recording in all patients over the expected normal range of cardiac output. Consecutive ejected temperature plateaus were also measured on the thermal signal and ejection fraction calculations were made.     

Extraction of the Two-Dimensional Cardiothoracic Ratio from Digital PA Chest Radiographs: Correlation with Cardiac Function and the Traditional Cardiothoracic Ratio

The purpose of our study was to extract the two-dimensional (2D) cardiothoracic ratio from digital chest radiographs using image analysis software on a Magic View 300 system. We also wanted to investigate its correlation with cardiac function, as defined by left ventricular ejection fraction from MUGA scanning, and with the traditional one-dimensional cardiothoracic ratio. One hundred patients undergoing radionuclide ventriculography and concurrent digital PA chest radiography using a commercial selenium detector system were studied. The 2D cardiothoracic ratio was extracted by defining a region of interest around the cardiac and thoracic areas and calculating a ratio of the two pixel counts obtained. The one-dimensional cardiothoracic ratio was calculated in the traditional manner. Regression analysis was used to study the association between these ratios and the ejection fraction. The 2D ratio could be achieved on all radiographs after image manipulation. The traditional linear cardiothoracic ratio showed an inverse correlation with left ventricular ejection fraction (r = –0.45). The 2D cardiothoracic ratio showed an improved correlation (r = –0.52). Assuming that a left ventricular ejection fraction of 55% or more indicates normal cardiac function, the normal 2D cardiothoracic ratio is 0.23 or less and a ratio of 1:4 is suggested. The ratio of cardiac and thoracic area correlates better with cardiac function as assessed by left ventricular ejection fraction than the traditional linear cardiothoracic ratio. Two-dimensional cardiothoracic ratio is easily extracted and improves the data yield from digital PA chest radiographs on PACS systems.     

An analogue circuit for the computation of stroke volume by the electrical impedance method

An analogue circuit is described for use in conjunction with thedZ/dt, e.c.g. and phonocardiogram signals from an IFM Impedance Cardiograph to measure stroke volume. This is displayed on a digital meter. The circuit can be extended to measure cardiac output.     

Implanted strain gauge and EMG amplifier to record motor behavior in unrestrained rats

We report a technique for recording EMG from a specific axial muscle with concomitant recording of an independent measure of axial movement during behavioral activity elicited by stimulation applied to awake unrestrained rats. EMG signals are led from bipolar teflon-coated tungsten wires (50 μ diameter) to a well-insulated FET input operational amplifier implanted subcutaneously. At the same time, changes are recorded in the resistance of a small mercury-in-silastic strain gauge stretched between appropriate vertebral dorsal spines. Differential amplification of the EMG in situ has been found to provide a high gain output signal which is relatively free from movement-related artifact. Satisfactory recordings have been obtained for up to 10 days from individual animals and both devices can be re-used.     

Enhanced cardiac thermal dilution measurement of cardiac output, volumes, regurgitation, valve effective diameters, ventricular power and efficiency -- Feasibility analysis using digital simulation.

Cardiac output is measured by the thermal dilution method which uses a quadruple lumen catheter, with a thermistor on the tip, through the right atrium, right ventricle and into the pulmonary artery. Cold saline is injected into the right atrium and the resulting pulmonary artery temperature profile is integrated. The same procedure performed with three thermistors and three pressure sensors located on the catheter to measure temperature and pressure in the atrium, ventricle and artery respectively will produce a set of temperature and pressure curves with shapes determined by injectate temperature, injectate volume, heart rate, systolic time interval, body temperature, cardiac output, volumes, flow rates and valve openings. A digital computer program has been developed to optimize the fit of a lumped parameter model to the thermodilution curves in order to determine heart rate, systolic time as a fraction of cardiac cycle, right atrial systolic and diastolic volumes, ventricular systolic and diastolic volumes, cardiac output, inflow valve forward and reverse flow rates and effective diameters, outflow valve forward and reverse flow rates and effective diameters, ventricular power and efficiency. The program has been tested over a range of operating conditions including noise in the temperature and pressure signals, randomly varying heart rate and cardiac cycle. All of the data for the tests were produced by a digital computer simulation of a pulsatile artificial heart. The results of these tests indicate that the enhanced thermal dilution analysis method is feasible.     

Scatter-glare corrections in quantitative dual-energy fluoroscopy

Previous attempts to use time subtraction intravenous digital subtraction angiography for ventricular imaging have been hampered by artifacts due to cardiac and respiratory motion. We have previously reported a motion-immune dual-energy technique in which kVp is switched between 60 and 120, at 300-500 mA, 30 times/s. In order to quantitate parameters such as ejection fraction and left ventricular volume, it is necessary to correct for scatter and veiling glare (SVG), which are the major sources of nonlinearities in videodensitometric digital subtraction angiography (DSA). In this report, a convolution filtering method has been investigated to estimate SVG in DSA images. In the first step, a grey level transformation of the detected image is utilized to get an estimated SVG image. In the second step this image is convolved to produce an image with appropriate spatial frequency content. Estimates of SVG in several Humanoid chest phantom images were obtained using Gaussian convolution kernels with a full width at half-maximum (FWHM) of 51-125 pixels. The root-mean-square (rms) percentage error of these estimates was obtained by comparison with direct SVG measurement. A convolution kernel with a FWHM of 75 pixels in each dimension applied to 16 Humanoid phantom images with various projections, thicknesses, and beam energies resulted in an average rms percentage error of 9.7% in the SVG estimate, for the 16 cases studied. The SVG estimation consisting of grey scale-to-SVG fraction lookup table (LUT) is made based on previous measurements. The x-ray settings required for each patient are utilized to alter the LUT in order to account for patient thickness variations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Impulse response of a segment of artery derived from transcutaneous blood-velocity measurements

A new method is described for analysing the Doppler-shift signals obtained from transcutaneous ultrasonic flow-velocity meters, in particular from the diseased superficial femoral artery. Using the input and output maximum-blood-velocity/time information from the arterial segment, the impulse response of the segment is obtained by harmonic-analysis methods. This impulse response has clinical significance and can be used in the derivation of the transfer function of the diseased segment. It is also shown that, in certain cases, the impulse response may be derived by correlation techniques, and for these cases the transfer function may be obtained directly from the input-output crosspower density spectrum. A discussion is presented on the errors involved in the computation of the impulse response and how these were minimised. A flow chart for the computation and accuracy-check procedure is included.     

Assessment of a procedure for left ventricle volumetry

A three-step radiographic procedure is described for the determination of left ventricle volumes, based on one lateral and two angled single plane images. The first angled image is taken to yield a minimally foreshortened LV image and the second image, at the same angle, is taken of a calibration sphere. Five sources of operator-related, or 'input', error are analysed, two of which are normally removable. The remaining three input errors are assessed using a commercial digital subtraction angiography system and comparisons with the analysis are given. A summary result is that relative errors in end-systolic and end-diastolic LV volumes, as well as cardiac output, should normally be less than 13% under reasonable operator care. The input errors should not contribute to errors in calculated ejection fractions.     

Automated measurement of cardiac output in patients by use of a digital computer

A digital computer system has been developed to repetitively measure cardiac output on-line. This system, which also monitors arterial pressure, heart rate, and other physiological variables, derives the values for stroke volume, vascular resistance, appearance and mean circulation time of dye, central blood volume, and cardiac work. Measurements, computation and print-out are accomplished within a 5-min period. Summary reports of these variables are generated by a typewriter. A complete set of data is recorded on punched cards for detailed analysis.     

Online measurement of cardiac indices from frequency transformed TAV Doppler ultrasound signals

A system is described for the analysis of blood flow signals in the aortic artery which enables indices of stroke volume and cardiac output to be derived. A commercial Doppler ultrasound monitor is used, the demodulated return signals are digitised, and frequency analysis is performed in real time using an FFT signal processing circuit. A Z-80 microprocessor controls the synchronisation of the data acquisition, transformation and display of the signals. Algorithms have been developed to identify the maximum velocity profile of each heartbeat and perform the necessary calculations to produce indices of stroke volume and cardiac output. The system has been evaluated against an existing offline method for a series of recordings on normal subjects and has demonstrated good repeatability. It has also been used in a clinical study on the effects of anaesthesia on chronic spinal injury patients. The results have shown that the system can be used to follow serial changes in cardiac performance within individual subjects.     

脑电信号与经穴输入信号相关分析的实验研究

利用不同频率电脉冲信号作用于人体不同经穴上，通过对经穴输入信号与脑电波输出信号之间的相关性研究，探索一种经穴优化输入的新方法，确立经络与大脑之间的动态关联性。实验研究表明30Hz的断续波电脉冲刺激信号与其诱发的脑电α2波相关性最强，使用30Hz的断续波电脉冲刺激天枢、大肠俞这组穴位有利于诱发大脑的有益波态α波。     

Determination of the impedance of the guinea pig skin

The impedance of guinea-pig skin has been studied using anin vitro method which consists of placing the recording chamber containing a strip of guinea-pig skin in the feedback branch of a closed-loop amplifier having an adjustable resistor in its input circuit. The chamber consists of two cylinders filled with a circulating Krebs solution at 37°C and two platinum electrodes facing one another inside the cylinders. At a given frequency, the series resistor is adjusted to obtain a gain of unity between input and output sinewave voltages (gain=10). The value of the series resistor is then equal to the modulus of the impedance in the feedback loop and its argument is calculated by a phase/amplitude convertor on the basis of the input and output phase shift. The impedance measurement is made separately for the recording chamber and the whole system in which the tissue is inserted. Substracting vectorially both values obtained enables the impedance of the tissue itself to be deduced. This procedure is repeated over a wide frequency band and the resulting values are represented on a Bode diagram of phase and amplitude. The Bode diagram shows the variations of the electrical characteristics of the tissues as the frequency varies, and indicates nonlinear filtering especially for low-frequency values (10^?2 Hz).     

Data acquisition and analysis of pulsatile signals using a personal computer: an application in cardiovascular physiology

Software is described which collects and analyzes pulsatile analog signals using an APPLE PC and a modestly priced analog-digital interface. The program will input and output data, store and retrieve data to disk and display data on the monitor. Four types of data analysis determine mean, maximum and minimum values and frequency in various combinations. For example, the present program analyzes cardiovascular signals for heart rate, stroke volume, cardiac output, systolic, diastolic and mean pressures over a representative 10 s period. Simple modifications to the program are described which extend its range of application.     

Predictors of low cardiac output in decompensated severe heart failure

OBJECTIVE:

To identify predictors of low cardiac output and mortality in decompensated heart failure.

INTRODUCTION:

Introduction: Patients with decompensated heart failure have a high mortality rate, especially those patients with low cardiac output. However, this clinical presentation is uncommon, and its management is controversial.

METHODS:

We studied a cohort of 452 patients hospitalized with decompensated heart failure with an ejection fraction of <0.45. Patients underwent clinical‐hemodynamic assessment and Chagas disease immunoenzymatic assay. Low cardiac output was defined according to L and C clinical‐hemodynamic profiles. Multivariate analyses assessed clinical outcomes. P<0.05 was considered significant.

RESULTS:

The mean age was 60.1 years; 245 (54.2%) patients were >60 years, and 64.6% were men. Low cardiac output was present in 281 (63%) patients on admission. Chagas disease was the cause of heart failure in 92 (20.4%) patients who had higher B type natriuretic peptide levels (1,978.38 vs. 1,697.64 pg/mL; P = 0.015). Predictors of low cardiac output were Chagas disease (RR: 3.655, P<0.001), lower ejection fraction (RR: 2.414, P<0.001), hyponatremia (RR: 1.618, P = 0.036), and renal dysfunction (RR: 1.916, P = 0.007). Elderly patients were inversely associated with low cardiac output (RR: 0.436, P = 0.001). Predictors of mortality were Chagas disease (RR: 2.286, P<0.001), ischemic etiology (RR: 1.449, P = 0.035), and low cardiac output (RR: 1.419, P = 0.047).

CONCLUSIONS:

In severe decompensated heart failure, predictors of low cardiac output are Chagas disease, lower ejection fraction, hyponatremia, and renal dysfunction. Additionally, Chagas disease patients have higher B type natriuretic peptide levels and a worse prognosis independent of lower ejection fraction.     

Automatic correction for drift in an integrator for phasic signals

An electronic integrator for phasic signals has been designed that does not undergo slow changes in output voltage (drift). Output is stabilised by means of negative feedback using an active 5-pole lowpass filter. Accurate integration of phasic signals with frequencies above 0·1 Hz is obtained. The integrator has been used for continuous determination of cyclincal volume changes, with a pneumotachograph as an input source, in the plotting of respiratory pressure-volume loops.     

Correlation Between Negative Near-Wall Shear Stress in Human Aorta and Various Stages of Congestive Heart Failure

The critical effect of advanced congestive heart failure is reduced blood flow in descending aorta resulting from mild to severe reduction in cardiac output, usually accompanying low ejection fraction. In these patients the heart tries to compensate by beating faster, but reduced blood flow combined with increased heart rate can lead to retrograde flow and negative shear stress along the vessel walls during each cardiac cycle. Our studies show that near-wall negative shear stress can result from an entire-retrograde flow at normal heart rates or a Womersley-type phase delayed near-wall retrograde flow at high heart rate and low ejection fraction conditions. In our experiments, a compliant aortic loop with appropriate pressure and flow instrumentation was used, running on either various aqueous glycerin solutions or property filtered, anticoagulated diluted bovine blood. The flow field was mapped using a General Electric Vingmed System 5 platform. The resulting images were analyzed with Caltech's digital ultrasound speckle image velocimetry technique. We showed the occurrence of near-wall retrograde flow under certain aortic flow rates and frequencies, charted via an empirical relationship between Reynolds and Womersley numbers. Also, we demonstrated a strong correlation between retrograde flow level and transition from preliminary to advanced congestive heart failure patients. © 2003 Biomedical Engineering Society. PAC2003: 8719Hh, 8719Rr, 8719Uv, 4380Qf, 8763Df     

Cardiac signal extraction in patients with Implantable Cardioverter Defibrillators

According to the guidelines the indication for Implantable Cardioverter Defibrillator (ICD) implantation is based on the ejection fraction. However, only a fraction of patients with implanted ICD shows live threatening arrhythmic events followed by adequate shocks. For this reason, further research is needed to find a more sensitive risk stratificator for patients prone to ventricular tachycardia or fibrillation. Unfortunately, standard prospective studies are time consuming.An alternative approach is to perform retrospective studies on patients with already implanted ICDs. So far, an implanted ICD is an exclusion criterion for Magnetic Field Imaging (MFI) studies. To overcome this problem several Blind Source Separation (BSS) algorithms have been tested to find out whether it is possible to separate the disturbances from the cardiac signals, in spite of the extreme difference in amplitude. Not all the methods are able to separate cardiac signal and disturbances. Temporal Decorrelation source Separation (TDSEP) is found to be superior both from a separation and performing point of view.For the first time it is possible to extract cardiac signals from measurements disturbed by an ICD, offering the possibility for a QRS-fragmentation analysis in patients with already implanted ICDs.     

Left ventricle load impedance control by apical VAD can help heart recovery and patient perfusion: a numerical study

The aim of this work is to investigate the dependence between left ventricular load impedance control by an apical ventricular assist device (VAD) and the consequent benefits for pathological heart recovery. A pathological left ventricle with 34% contractility has been simulated in the assisted and nonassisted conditions. By means of an extended Kalman filter, left ventricular pressure-volume loops have been partially estimated and ventricular as well as circulatory quantities inferred. The heart operation mode, based on cardiac energetic criteria, is imposed by controlling the VAD filling phase. In the assisted condition, results show that the left ventricle end-diastolic volume, left atrial pressure, and wall stress all decrease; stroke volume, ejection fraction, ventricular efficiency, aortic pressure, and cardiac output all increase. Benefits are also evident for the right ventricle and systemic and pulmonary circulation. The strategy outlined in this work also shows that good results for heart recovery are achievable and a possible way to improve the functional properties of commercial pulsatile VADs.     

Comparison of heart rate variability spectra using generic relationships of their input signals

Spectral analysis of heart rate variability (HRV) is an accepted method for assessment of cardiac autonomic function and its relationship to numerous disorders and diseases. Various non-parametric methods for HRV estimation have been developed. The spectrum of counts, the instantaneous heart rate spectrum and the interval spectrum are mostly practised. Although extensive literature on their respective properties is available, there seems to be a need for a more complete comparison, eventually resulting in recommendations for applicability. The methods for HRV spectral analysis use their specific transforms of the primary R-R interval series into input signals for spectral computation. This is, in fact, the reason for obtaining different spectra. A basis for comparison is established, revealing the generic relationships of these HRV input signals. It allows for a more systematic evaluation and for further development of the considered methods. The results with simulated and real HRV data show better performance by the spectrum of counts and by a proposed instantaneous heart rate spectrum, obtained using a cubic spline interpolated input signal. The modulation depth of the primary signal can influence the accuracy of the HRV analysis methods.     

An impedance cardiography system: A new design

An IBM compatible impedance cardiac output monitoring prototype system has been developed for use at the bedside on patients in the ICU, CCU, ER, Cath. Lab, and OR, etc. This impedance cardiography (ICG) system, whose operation is completely technician-free, provides a continuous display with digital results and four channel color waveforms on an Enhanced Graphics Display screen. The software is written in C language with several special segments in assembly code where speed is essential. In this prototype system, a real-time algorithm was introduced to modify the ensemble averaging technique so that it averages nonperiodic signals such as: ECG, dZ/dT, ΔZ, etc. Also, a real-time algorithm was developed to adaptively detect R spikes from conventional ECG signals. A signal preprocessor was developed to process signals digitally before any further work is done. This procedure reduces muscle noise, 60 Hz interference, and ventilatory movement. A special digital filter was designed to cope with the cases in which pacemakers are used. A special algorithm was also developed to further reduce the ventilation artifacts so that a period of apnea is unnecessary during the performance of the measurements. An anatomically specified electrode configuration has been defined enabling precise and reproducible positioning of the electrodes—hopefully leading to electrode standardization. At the present time, this prototype system has been compared with standard hand calculation and correlated with the clinical “gold standard,” the Swan-Ganz thermodilution cardiac output. Using 144 sets of data from 10 healthy volunteers, 4 critically ill patients, and 8 healthy exercising volunteers, calculations of cardiac output were made using our system and the standard hand calculation of stroke volume, based upon Kubicek's equation; there was a relatively high and stable correlation: r=0.93, p<0.005 (healthy); r=0.94, p<0.002 (ill); r=0.95, p<0.002 (exercise). From 20 patients at two different hospitals all with Swan-Ganz catheters in their hearts 65 correlation studies between our system and the standard thermodilution technique were performed; the results were encouraging in terms of accuracy and consistency (r₁=0.84, p<0.01, n=10 CCU patients), and (r₂=0.93, p<0.01, n=ICU, patients). These results along with a growing body of data from other investigators indicate that this noninvasive and technician-free system for measuring cardiac output could have a significant role in patient care.