Electromagnetic Flow Measurements Research Articles

New signal processing techniques for improved precision of noninvasive impedance cardiography.

Impedance cardiographic determination of clinically important cardiac parameters such as systolic time intervals, stroke volume, and related cardiovascular parameters has not yet found adequate application in clinical practice, since its theoretical basis remains controversial, and the precision of beat-to-beat parameter estimation has until recently suffered under severe shortcomings of available signal processing techniques. High levels of noise and motion artifacts deteriorate signal quality and result in poor event detection. To improve the precision of impedance cardiography, new techniques for event detection and parameter estimation have been developed. Specifically, matched filtering and various signal segmentation and decomposition techniques have been tested on impedance signals with various levels of artificially superimposed noise and on actual recordings from subjects in a laboratory study of cardiovascular response to a cognitive challenge. Substantial improvement in the precision of impedance cardiography was obtained using the newly developed signal processing techniques. In addition, some preliminary evidence from comparisons of the impedance cardiogram with invasive aortic electromagnetic flow measurement in anesthetized rabbits is presented to address questions relating to the origin of the impedance signal.

The effect of velocity profile on electromagnetic flow measurement

Electromagnetic flowmeters are commonly known to be quite insensitive to velocity profile effects. However, the test results show that even the best meters are sensitive to disturbances in the flow. In practice it is very difficult to test meters in all possible pipe configurations. Numerical methods are therefore suggested as a solution to this problem. In any case, accurate mathematical modelling of the flow and the meters is needed. The magnetic field of an electromagnetic flowmeter has been measured and the model for the meter has been computed. Measured and computed values of the velocity profile of water flow have been used to estimate the error.

Minimising stimulator artefacts in electromagnetic flow signals.

Simultaneous electrical stimulation of tissue with the measurement of blood flow using an electromagnetic flowmeter system almost invariably results in large flow measurement inaccuracies. These inaccuracies are because the electrical energy from stimulating artefacts is amplified along with the flow signals. The paper describes the building and use of an inexpensive circuit to remove stimulation artefacts from electromagnetic flow measurements.

Splenic artery and superior mesenteric artery blood flow: nonsurgical Doppler US measurement in healthy subjects and patients with chronic liver disease.

The accuracy of the duplex Doppler ultrasound system in the measurement of blood flow in the splenic artery and the superior mesenteric artery was evaluated in seven anesthetized dogs by comparing blood flow recordings obtained simultaneously with the electromagnetic flowmeter, with those obtained with the combination of B-mode and M-mode scanning. Various flow rates were produced by the infusion of dopamine. Splenic artery blood flow and superior mesenteric artery blood flow measured with the duplex system exhibited significant correlations with corresponding values obtained with electromagnetic flow measurements (r = .93 and r = .93, respectively). Changes in splanchnic circulation with the progression of chronic liver disease in humans were then investigated with the use of the duplex system. Splenic artery blood flow and superior mesenteric artery blood flow were significantly increased in patients with cirrhosis compared with patients with chronic hepatitis and healthy subjects. These results indicate that a hyperdynamic circulatory state may develop in the splanchnic circulation of the intestine and spleen in cirrhotic patients.

Separate lung blood flow in anesthetized dogs: a comparative study between electromagnetometry and SF6 and CO2 elimination.

Anesthetized, prone dogs were intubated with a double-lumen endobronchial tube, and the lungs were ventilated independently. Three methods of recording differential blood flow were compared during unilateral lung hypoxia: electromagnetic flow measurement, flow probes being fitted onto each main pulmonary artery after thoracotomy (QPr); SF6 elimination from each lung, the inert gas being continuously infused into a central vein (QSF6); and CO2 elimination (QCO2). During control conditions (100% O2 to both lungs), the test lung QPr was 54% of cardiac output, and corresponding QSF6 and QCO2 were 56% and 52%, respectively. Hypoxic challenge with 8% O2 to the test lung reduced QPR, QSF6, and QCO2 by 25%, 27%, and 7%, respectively. Ventilation of the test lung with pure nitrogen reduced its blood flow further, QPr, QSF6, and QCO2 being reduced by 39%, 42%, and 23%, respectively, from initial control. A strong correlation between test lung QPr and QSF6 was seen with a slope of 0.90 (r:0.89, P less than 0.001). Only 60% of the reduction in test lung blood flow was detected by CO2 elimination, as compared to electromagnetic flow measurement or SF6 elimination. The poor results obtained with CO2 elimination can be explained by its dependence on the ventilation-perfusion ratio and the effect of oxygen tension on the CO2 binding capacity of blood (Haldane effect). The findings emphasize the necessity of using an inert, poorly soluble gas for the measurement of separate lung blood flow.

Distribution of facial and nasal blood flow in the cold loaded dog

1. 1 Electromagnetic flow measurements and the tracer microspheres technique were combined to measure total blood flow of the internal maxillary artery (IMA-FLOW) and its flow distribution to the arteriovenous anastomoses (AVA-FLOW) and the capillaries (CAP-FLOW) of the left and right side of the nose and forehead regions together with perfusion pressure (PP) in 9 anaesthetized dogs at blood temperatures ( blood ) of 38, 36 and 34°C. 2. 2 Decrease of T blood to 34°C by internal and external body cooling induced shivering and a significant decrease of IMA-FLOW on either side without any significant effect on PP. The reduction of IMA-FLOW resulted from decreases of CAP-FLOW and of AVA-FLOW. 3. 3 There was an extreme differentiation in CAP-FLOW of the different vascular compartments under normothermic and hypothermic conditions. Lowering T blood produced reductions of CAP-FLOW mainly in those tissues of the face (skin, lips, nasal plane) and nose (alar folds, nasal cartilages) which are normally affected directly by a cold environment. 4. 4 IMA-FLOW as well as AVA-FLOW were linearly correlated with each other and both were related to PR-IMA ina hyperbolic fashion, while CAP-FLOW did not so clearly indicate such a relationship at normal and at reduced T blood . 5. 5 It is concluded that the adjustment of PR-IMA and thus of IMA-FLOW and AVA-FLOW respectively, is achieved for various requirements solely by small neurogenically induced changes in diameter of the resistance vessels. 6. 6 The fact that CAP-FLOW does not exhibit this clear relationship to PR-IMA suggests additional independent influences, either neurogenic or autoregulatory.

A simplified mitral valve method for two-dimensional echo Doppler blood flow calculation: Validation in an open-chest canine model and initial clinical studies

Quantitative two-dimensional Doppler echocardiography is increasingly used for the calculation of cardiac blood volumes. This study evaluates a simplified method for measuring volume flow at the mitral valve orifice. In four open-chest dogs the simplified mitral valve method was compared to a previously described mitral orifice method for Doppler volume flow calculation and to data obtained by electromagnetic flow measurement. Cardiac output varied from 1.4 to 7.7 L/min. Correlation for both the Doppler method and the electromagnetic meter volume flows was r = 0.98 to 0.99, and there was no significant difference between the two methods. In a clinical pilot study, 10 children were studied in the catheterization laboratory, and the simplified Doppler flow method was compared to thermodilution cardiac output measurements with good results. The study indicates that the simplified mitral valve method provides acceptable accuracy for Doppler cardiac output measurement.

Correlation between fluorescein flowmetry, 133Xenon clearance and electromagnetic flow measurement: a study in the intestine of the pig

Correlation between fluorescein flowmetry, ¹³³Xenon clearance and electromagnetic flow measurement: a study in the intestine of the pig

Geotraverse across the Sikhote Alin ? The Sea of Japan ? The Honshu Island ? The Pacific

The paper presents the results of geological-geophysical research carried out during the Soviet-Japanese cooperative study of the structure and dynamics of the Earth's crust and upper mantle in the transition zone from the Pacific Ocean to the Asian continent. The 300 km deep geological-geophysical section of the tectonosphere (geotraverse) has been compiled on the basis of combined interpretations of seismic, geological, petrographic, gravimetric, magnetometric, electromagnetic and heat flow measurements. Estimates were made of deep temperatures along the geotraverse and of the depths of the partial melting level that can be identified with the upper boundary of the asthenosphere.

Continuous measurement of left ventricular volume in animals and humans by conductance catheter.

An eight-electrode conductance catheter previously developed by us and used to determine stroke volume in dogs was applied in human beings and dogs to measure absolute left ventricular volume quantitatively. For calibration we developed the formula V(t) = (1/alpha)(L2/sigma b)G(t) - Vc, where V(t) is time-varying left ventricular volume, alpha is a dimensionless constant, L is the electrode separation, sigma b is the conductivity of blood obtained by a sampling cuvette, and G(t) is the measured conductance within the left ventricular cavity. Vc is a correction term caused by the parallel conductance of structures surrounding the cavity and is measured in two ways. The first method, applicable in the anesthetized animal, consists of temporary reduction of volume to zero by suction. The second method uses a transient change in sigma b by injection of a small bolus of hypertonic saline (dogs) or 10 ml of cold glucose (humans) into the pulmonary artery. The validity of the formula was previously established for the isolated postmortem canine heart. The predicted linearity, slope constant alpha, and accuracy of Vc for the left ventricle in vivo were investigated by comparing the conductance volume data with results from independent methods: electromagnetic blood flow measurement for stroke volume and indicator dilution technique for ejection fraction (dogs), thermal dilution for cardiac output (12 patients), and single-plane cineventriculography for V(t) (five patients). In all comparisons, linear regression showed high correlation (from r = .82 [n = 46] to r = .988 [n = 20]) while alpha, with one exception, ranged from 0.75 to 1.07 and the error in Vc ranged from 0.5% to 16.5% (mean 7%). After positioning of the catheter, no arrhythmias were observed. It is concluded that the conductance catheter provides a reliable and simple method to measure left ventricular volume, giving an on-line, time-varying signal that is easily calibrated. Together with left ventricular pressure obtained through the catheter lumen, the instrument may be used for instantaneous display of pressure-volume loops to facilitate assessment of left ventricular pump performance.

Methodology and basic problems related to blood flow studies in the human fetus

A method was developed for non-invasive measurement of human fetal blood flow. The method combines real-time ultrasonography with 2 MHz pulsed Doppler technique. The blood flow is calculated from the blood velocity, estimated from the Doppler spectrum, and the vessel diameter, measured in the real-time image. Time-distance recording was applied for measurements of the pulsatile diameter changes in the fetal aorta. The method proved to possess a good accuracy and reproducibility when tested in vitro experiments and in a comparison with electromagnetic flow measurements in animals. Possible sources of error were analysed and recommendations for minimizing the risk of errors are presented.

Prostaglandin I2 supports blood flow to hypoxic alveoli in anesthetized dogs.

In an animal model of unilateral alveolar hypoxia, inhibition of cyclooxygenase activity, estimates of immunoreactive 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha), and administration of prostaglandin I2 (PGI2) were used to evaluate the hypothesis that endogenous PGI2 opposes hypoxic pulmonary vasoconstriction, thereby producing redistribution of blood flow to hypoxic alveoli and reductions in systemic PO2. In anesthetized dogs, one lung was ventilated with 100% N2 and the other with 100% O2. Thermal dilution coupled with electromagnetic flow measurements permitted estimates of blood flow to each lung. Indomethacin or meclofenamate reduced flow to the N2-ventilated lungs (P less than 0.05) and increased systemic PO2 (P less than 0.05). Simultaneously, aortic concentrations of immunoreactive 6-keto-PGF1 alpha decreased 63 +/- 8% (P less than 0.001). Following cyclooxygenase inhibition, incremental doses of PGI2 (0.01, 0.025, and 0.10 micrograms X kg-1 X min-1) increased flow to the N2-ventilated lungs and reduced systemic PO2 (P less than 0.001) without affecting mixed venous PO2. These results suggest that systemic PO2 was reduced because of increased venous admixture. We conclude that PGI2 attenuates hypoxic vasoconstriction which allows flow to be maintained to hypoxic alveoli, resulting in reduced systemic PO2.

Measurement of systolic and diastolic flow rates in the coronary artery system by x-ray densitometry.

The reliability of a modified videodensitometric and photodensitometric sampling technique for measuring phasic flow rates in the coronary artery system was examined. Electromagnetic flow measurements were performed in a circulatory model with continuous and pulsatile flow and intraoperatively in aortocoronary bypass grafts; cineangiograms were made simultaneously. Based on the front velocities of injected boluses of contrast medium, the densitometric measurement overestimated the electromagnetically measured flow systematically by about 20%. Systolic and diastolic flow rates in aortocoronary bypass grafts and coronary arteries determined from biplane cineangiograms in 34 patients generally revealed the typical pulsatile flow pattern familiar from electromagnetic and ultrasonic flow measurements. Flow velocities in unstenosed coronary arteries were nearly identical before and after branchings of the vessels, whereas the corresponding flow rates were higher in proximal than in distal segments. The identical flow velocities in different branches of the same vessel and the low variability of this parameter in different patients may be a suitable index of the effect of stenoses on coronary arterial blood flow.

Coronary artery spasm in intact dogs induced by potassium and serotonin.

Although coronary artery spasm has been implicated as an important cause of myocardial ischemia in humans, an animal model of reversible segmental coronary constriction has not been described. To provoke coronary spasm in open-chest dogs, selected vasoconstricting agents adsorbed to viscous ion exchange gels were applied topically to the surface of epicardial coronary arteries. The procedure provided a sustained localized release of drug, and minimized effects on contiguous myocardium or on the systemic circulation. Segmental arterial constrictor responses were evaluated by sonomicrometry, arteriography, and electromagnetic flow measurements. Potassium evoked sustained constrictions or spasms, and concomitantly reduced flow by -42 +/- 4% (SE; n = 34). Serotonin likewise produced sustained decreases in flow of -22 +/- 6% (SE; n = 5). Other constrictors, including norepinephrine and angiotensin, failed to evoke sustained constrictions. Spasms nearly abolished reactive hyperemic responses elicited by temporary complete occlusion of the artery. Intravenous nitroglycerin and dihydropyridine calcium antagonists promptly relieved the spasms. Scanning electronmicroscopic examination of the intimal surface of arteries undergoing sustained spasm revealed no platelet thrombi. Thus, nonthrombotic, vasodilator-sensitive segmental coronary spasms were elicited by endogenous constrictors which may play a role in regulating flow to ischemic myocardium.

Nuclear magnetic resonance and transcutaneous electromagnetic blood flow measurement.

Static and alternating magnetic fields are employed in blood flowmeters using nuclear magnetic resonance (NMR) principles and electromagnetic induction by a moving conductor (TEM). Both techniques require high steady magnetic fields, obtained either from permanent magnets or from electromagnets. A relatively homogeneous magnetic field is needed for NMR, but, though important for calibration, homogeneity is not critical for TEM. NMR is more complex than TEM since it requires radio-frequency and audio-frequency magnetic fields. However, the TEM method requires surface electrodes in contact with the skin, or needle electrodes placed subcutaneously, whereas NMR is contactless. The NMR flowmeter can be calibrated directly, but appropriate and approximate models must be assumed and then solved by computer to quantify blood flow by the TEM flowmeter. Flow in individual vessels is measured a priori in the TEM flowmeter by virtue of the assumed models. To measure flow in individual vessels by NMR, a scanning or ranging method is required, which logically leads to blood flow imaging. The levels of steady, radio-frequency, and audio-frequency magnetic fields used in the two types of flowmeters are low enough so as not to cause any apparent stimulus to human volunteers and patients tested.

The role of adrenergic mechanisms in thermoregulatory control of blood flow through capillaries and arteriovenous anastomoses in the sheep hind limb.

The possible role of adrenergic mechanisms in thermoregulatory changes in the partition of femoral blood flow between nutrient (capillary) and non-nutrient (arteriovenous anastomoses, AVA) circuits in the hind limb of conscious sheep has been investigated employing radioactive microsphere and electromagnetic blood flow measurement techniques. Constriction of AVAs, normally induced by spinal cooling, could be inhibited by phentolamine, whereas dilatation of AVAs, normally induced by spinal heating, could be inhibited by noradrenaline or methoxamine. AVA constriction could be induced by noradrenaline or methoxamine, or dilatation by phentolamine. Isoprenaline had a small dilator and propranolol a small constrictor effect on AVAs. It is concluded that adrenergic pathways involving predominantly alpha-receptors play a role in thermoregulatory changes in skin blood flow (through AVAs) elicited by manipulation of CNS temperature; under these conditions, beta-receptors do not play any role, although manipulation of their activity will influence AVAs under non-thermoregulatory conditions. Capillary blood flows in skin, bone and fat were sensitive, at different ambient temperatures and to varying degrees, to some alpha- and beta-adrenergic agents.

Theory of electromagnetic flow measurement

A study is made of the influence of an inhoinogeneous profile of the concentration of red blood corpuscles on the sensitivity of an electromagnetic flowmeter under conditions of a pulsating flow. The part played by microscopic correlations and the applicability of the single-velocity approximation in the averaged Ohm's law are examined. On the basis of the solution of an electro-dynamic boundary-value problem, an expression is obtained for the flowmeter sensitivity for arbitrary axisymmetric velocity profile and different conduct'ivities of the core of the flow, the wall layer, and the wall. The solution to the problem of Viscous flow under the influence of an oscillating pressure gradient [8] is generalized to the case of a two-layer distribution of the viscosity. It is shown that the presence of a wall layer of pure plasma leads to a phase shift between the oscillations of the electric intensity and the volume flow rate of the suspension. If the amplitude of the oscillations in the flow rate is comparable with the constant component, the instantaneous sensitivity of the flowmeter changes rapidly with the time and the characteristic of the instrument becomes essentially nonlinear.

Hemodynamic assessment and surgical correction of the kinked internal carotid artery.

A fluid-filled oculoplethysmography (OPG) was used to assess carotid artery flow in 50 patients with symptoms of cerebral ischemia. Based on positioned OPG and angiographic studies, 20 patients had angioplastic procedures. At operation, positional electromagnetic flow measurements were obtained to confirm the preoperative noninvasive findings. Electromagnetic flow reductions were identified in 18 patients with positive preoperative positional OPGs. A high correlation between noninvasive testing data and intraoperative flow measurements suggests that positional OPG testing is helpful in identifying hemodynamically significant kinking of the internal carotid artery.

The Use of Electromagnetic Flow Measurements for Detection of Early Stenosis in Aortocoronary Bypass Grafts

The value of electromagnetic flow measurements for the detection of graft stenosis during aortocoronary bypass and subsequent prevention of early closure by reimplantation of the graft was assessed. Postoperative angiograms were studied in 102 grafts in 55 patients in whom flow was measured during operation. Patients with recurrent angina or insufficient improvement after operation were selected to include many early occluded grafts; 30 grafts became occluded within six months. Applying a criterion of acceptable specificity based on flow pattern analysis, only 6 grafts were found to be abnormal immediately after implantation. It appears that in most instances, early closure is caused by a process of scar tissue formation which takes several months to develop completely. As a rule, the graft is patent after implantation and flow pattern analysis has nothing to offer. The value of flow pattern analysis during operation for the prevention of early closure is limited, and its value for the prediction of long-term prognosis of the grafts remains to be evaluated.

Videodensitometric and electromagnetic flow measurements in aorto coronary bypass grafts (author's transl)

Electromagnetic flow measurements in aorto coronary bypass grafts and cine angiography were performed simultaneously during bypass surgery. Using the front velocities of injected boli of contrast medium the videodensitometric measurement (QVD) overestimates the electromagnetically measured flow (QEM) systematically about 20% (QVD = 1.26 . QEM = 4 ml/min; Syx = 10.8 ml/min; r = 0.97). During the passage of the front of the contrast medium through the videodensitometric measuring windows, the flow is altered by the injection about + 13.6 ml/min on an average.