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Biomedical subjects

R M Heethaar

Publications and source records attributed to R M Heethaar.

At least 73 records · Page 4Linked to original sources
PubMed1989

Low Reynolds number steady state flow through a branching network of rigid vessels: I. A mixture theory.

A mixture theory has been used to formulate a theory of blood perfusion. By means of a formal averaging procedure the discrete network of microvessels is transformed into a continuum. During this procedure, the distinction between arterioles, capillaries and venules is preserved by means of an arteriovenous parameter. In this paper, two equations are derived for the case of low Reynolds number steady-state flow through a rigid vessel network: the extended Darcy equation and the continuity equation. A verification of the theory is presented, on the basis of a network analysis.

Mathematics↗
PubMed1987

Geometry of the aortic bifurcation measured by computed tomography.

Apart from metabolic risk factors haemodynamic disturbances play a role in atherogenesis. The haemodynamic factors depend among other things on the geometry of vessels including the aortic bifurcation. The geometry of the aortic bifurcation can be expressed by the diameter of the distal abdominal aorta, the area ratio of the bifurcation and the convergence of the aorta. Data on the geometry have been reported of both post-mortem and in-vivo investigations by angiography and echography. Because of the disadvantages of angiographic and echographic measurements, the aortic bifurcation has been assessed by CT-scanning in a series of 50 patients. The mean diameter of the distal aorta measured 15.9 mm for men and 13.0 mm for women. The calculated area ratio was 0.74 for men and 0.81 for women. These data are in agreement with the angiographic measurements. A convergence was found of 30.6 percent for men and 45.4 percent for women. There is a discrepancy between these values and those reported in the literature.

Aorta, Abdominal↗
PubMed1986

Fluid shear as a possible mechanism for platelet diffusivity in flowing blood.

Platelet transport theory is based on convection diffusion and describes adequately the influence of wall shear rate, platelet concentration and axial (down stream) position. Until now, the influence of the predominant factors affecting platelet adherence, the hematocrit and the red cell size, was not included in this theory. Their role remained hidden in the platelet diffusivity (Dw), which was assumed to be related to the shear rate (gamma) expressed in s-1 by a power law function Dw = m gamma n, in which m and n were thought to be constants. We have determined platelet diffusivity directly from platelet adherence to subendothelium as a function of axial distance in an in vitro perfusion system. Our results indicate that m is a constant with a value of (1.05 +/- 0.05) 10(-9) cm2 s-1 and that n is a function of the hematocrit (h) which is best approximated by a quadratic equation n = 0.297 + 1.29 h - 0.90 h2. The effect of red cell size was introduced by correcting the hematocrit containing factors in this quadratic equation for the square of the red cell diameter. This correction was made on the basis of theoretical considerations. The theoretically derived platelet adherence correlated closely with the previous experimental data regarding the effect of red cell size in which we found that the hemodynamic effect of red cells on platelet adherence decreases with decreasing red cell diameter.

Biomechanical Phenomena↗
PubMed1986

Feto-placental circulatory competence.

Doppler assessment of the pulsatility index (PI) of umbilical artery blood velocities offers promise as a simple, non-invasive method for early diagnosis of placental failure. The present report identifies the hemodynamic factors determining this PI, based on current knowledge of feto-placental physiology. It is postulated that the umbilical artery PI is mainly determined by the ratio of capillary to arterial resistances in the fetal placental circulation. However, the biological PI variations on a minute by minute basis are caused by short-term variations in blood pressure pulsatility. The ratio of resistances declines markedly with advancing pregnancy, indicating that the site of predominant resistance determining the volume of placental blood flow gradually migrates from the peripheral capillary to the arterial compartment. After 28 wk of gestation, umbilical artery PI values decrease below 1, reflecting the normal reserve capacity of the functional capillary bed of the placenta. PI values above 1 in the last trimester reflect a critical enhancement of placental capillary resistance, which must impede blood flow to functional chorionic villi. It is concluded that this Doppler technique may provide a simple method for clinical assessment of placental circulatory competence.

Aorta↗
PubMed1986

Endocardial catheter mapping: validation of a cineradiographic method for accurate localization of left ventricular sites.

To guide surgical therapy for ventricular tachycardia by preoperative endocardial catheter mapping, accurate anatomic localization of arrhythmogenic sites is mandatory. For this reason we developed a mathematical cineradiographic method to compute left ventricular sites relative to three anatomic reference points: the centers of aortic and mitral valve ostia and the left ventricular apex. To validate the method 14 epicardial left ventricular markers were implanted in four dogs to simulate arrhythmogenic sites. Distances between markers and the anatomic references were calculated and the results were compared with postmortem measurements. The difference between calculated and measured distances was 0.5 +/- 3.1 mm (mean +/- SD), confirming accurate localization of anatomic marker sites. However, in surgery the results have to be displayed in a practically applicable, unambiguous way. Therefore, wire skeletons were constructed to represent calculated endocardial marker sites relative to the anatomic reference points. To validate this approach, 14 markers were implanted in the left ventricular subendocardium in four dogs. Wire skeletons were constructed, one for each marker site, and inserted postmortem into the left ventricular cavity via a 2 cm incision. In all cases the correct indication of a marker site by the corresponding wire skeleton was confirmed by fluoroscopic inspection in multiple projections. This wire skeleton technique may enhance the practical usefulness of preoperative endocardial catheter mapping.

Animals↗
PubMed1986

Increased red blood cell deformability due to isoxsuprine administration decreases platelet adherence in a perfusion chamber: a double-blind cross-over study in patients with intermittent claudication.

Platelet transport towards the vessel wall is influenced by the hematocrit, red blood cell (RBC) size, and shape. Recent in vitro studies have indicated that RBC deformability may also influence platelet transport. The observation that isoxsuprine, a known vasodilating drug, caused increased RBC deformability in vitro and decreased platelet transport in vitro prompted us to study the effects of this drug in vivo. The study was performed in a double-blind cross-over study of isoxsuprine v placebo in ten patients with peripheral arterial insufficiency. RBC deformability was estimated from viscosity measurements using the blood viscosity equation of Dintenfass and expressed as T value. Platelet transport was studied in an annular perfusion chamber according to Baumgartner. Human umbilical arteries were used as blood vessels. Perfusion studies were performed with whole blood or with RBCs of the patients mixed with normal platelets and plasma at a standardized hematocrit and platelet count. An increase in RBC deformability concomitant with a decrease in platelet adherence was observed in patients on isoxsuprine with a drop in T value of approximately 0.06 (from 0.91 toward 0.86), and a concomitant decrease in platelet adherence of 20% to 40%. These observations differed significantly from the results in the placebo group and showed a significant group-period interaction at the cross-over of medication (analysis of variance). The effects on platelet adherence were observed at high vessel wall shear rate (1,800 s-1) with perfusates consisting of patients' RBCs and donor plasma and platelets at standardized hematocrit and platelet count. No differences were observed under these conditions at a shear rate of 300 s-1. When whole blood of patients was used, nonsignificant effect was observed at shear rates of 300 s-1 and 1,800 s-1. This was probably caused by the added noise due to variations in hematocrit and platelet number. These data demonstrate that isoxsuprine increases RBC deformability, and they suggest the possibility of decreasing platelet-vessel wall interaction in vivo by manipulation of RBC deformability.

Adult↗
PubMed1984

A computer-aided method to evaluate the function of implanted Björk-Shiley prosthetic heart valves.

Evaluation of the function of implanted prosthetic valves is important in early diagnosis of valve dysfunction. Björk-Shiley valves contain two radiopaque rings, which are projected as ellipses in cineradiography. From these ellipses the actual valve opening angle can be calculated. A computer-aided method was developed that enables measurement of ellipse characteristics reliably, independent of projection angle and valve opening. It is demonstrated that calculated and real opening angles differ less than 2 degrees with this method. Using the same technique the tilting angle of the valve ring during the cardiac cycle is computed for evaluation of progression of valve dehiscense in case of a paravalvular leak. Application of the method to patient data is illustrated by three cases. The method is suitable for use by technicians. It can be implemented on a small microprocessor system. The method proved to be a powerful tool in the evaluation of patients with implanted Björk-Shiley valves.

Aortic Valve↗
PubMed1984

Velocity profiles in annular perfusion chamber measured by laser-Doppler velocimetry.

p6e fluid flow in the annular perfusion chamber of Baumgartner developed to study platelet vessel wall interaction was examined with laser-Doppler velocimetry. A laminar and stable flow with a Reynolds number of less than or equal to 50 was measured at flow rates up to 3 ml s-1. No turbulence was found. The wall shear rate directly determined from measured velocity profiles agreed well with theory. The experiments underlined the necessity to work with vessels of uniform thickness and a smooth surface.

Blood Flow Velocity↗
PubMed1984

Characteristics of flow velocities in the umbilical arteries, assessed by Doppler ultrasound.

Measuring blood velocities by Doppler ultrasound has introduced a new noninvasive technique into obstetrical diagnostics. This study evaluates the information about fetoplacental hemodynamics that can be extracted from Doppler signals obtained from umbilical arteries. The most significant blood flow characteristic is blood flow pulsatility expressed as pulsatility index (PI). The PI is the difference between systolic and diastolic velocity, divided by the mean velocity. The PI strongly correlates with downstream impedance to flow. It is demonstrated that sophisticated spectral analysis of the Doppler signals has no practical advantages over the simple zero-crossing technique for establishing PI values, since blood flow in umbilical arteries has a blunt-to-flat profile. This study shows that the PI is a gestational-age-related, intra- and inter-observer-reproducible variable. The significant decrease of PI values with advancing gestational age reflects a progressive reduction of placental vascular resistance to umbilical blood flow. The PI holds promise as a simple method for early diagnosis of impaired umbilical circulation.

Blood Flow Velocity↗
PubMed1984

The concept of apparent cardiac arrest as a prerequisite for coronary digital subtraction angiography.

This study was undertaken to evaluate the possible use of digital subtraction applied after selective coronary arteriography. An identical position of the objects with and without contrast medium is an absolute requirement for the application of subtraction techniques. Because coronary arteries are in continuous motion, the subtraction technique cannot be applied without certain precautions. In our study, only images from corresponding moments in the cardiac cycle before and after contrast injection were matched for subtraction, that is, the concept of apparent cardiac arrest. To prevent variations in cardiac contractions due to varying RR intervals, heart rate was controlled by regular right atrial stimulation. Moreover, the stimulation rate and cine frequency were in synchrony, which was effected by triggering both on the frequency of the electric main alternating current (50 cycles/s). In this way, each cardiac cycle contains exactly the same number of frames at corresponding moments. A combination of the application of the concept of apparent cardiac arrest with the subtraction technique in 12 patients resulted in good quality images. Furthermore, better visualization of capillary filling with contrast material was obtained than with conventional coronary arteriography.

Capillaries↗
PubMed1984

Nonrandom ventricular rhythm in horses with atrial fibrillation and its significance for patients.

RR interval sequences during spontaneous atrial fibrillation in eight horses were analyzed as in previous studies in patients and dogs using histograms and serial auto-correlograms. In patients and dogs with spontaneous atrial fibrillation, ventricular rhythms were always random. In the horses, the histograms were skewed with median RR intervals of approximately 1,000 ms. A striking finding in these animals was the presence of long RR intervals up to 5,000 ms in duration. The shortest RR intervals lasted 400 to 600 ms. In contrast to findings in dogs and patients, the serial autocorrelograms showed periodicity that was reenforced by digitalis (n = 3), but eliminated by quinidine (n = 2) and atropine (n = 2). Quinidine and atropine eliminated the longer RR intervals, whereas digitalis increased the number of long RR intervals. In one horse, it was possible to measure intraarterial pressure, and large fluctuations in pressure were observed as the RR intervals varied from over 3,000 to less than 500 ms. It is postulated that these changes in blood pressure are associated with baroreceptor responses that may alter the electrophysiologic behavior of the atria and atrioventricular node. These changes cause the nonrandom patterns of ventricular rhythm in the horse. Because such very long RR intervals do not occur in human beings or dogs during atrial fibrillation, the random ventricular rhythm in these groups is maintained even during digitalis treatment.

Animals↗
PubMed1984

Red blood cell deformability influences platelets--vessel wall interaction in flowing blood.

Hematocrit and red cell size are important factors for the transport of blood platelets toward subendothelium in flowing blood. We report that red cell deformability also influences platelet transport. Red cell deformability was estimated with Couette-flow viscosimetry at a shear rate of 130 s-1 and expressed as a 'T' factor--a dimensionless parameter relating the relative viscosity and the hematocrit derived from the relationship: T = (1 - mu -0.4 rel)/H, where mu rel is the relative viscosity and H is the hematocrit. The normal value of T was within a narrow range (0.91 +/- 0.02). Treatment of normal red cells with isoxsuprine and chlorpromazine caused decreased rigidity and decreased T. Cholesterol loading and treatment with diamide increased rigidity and increased T. In vitro perfusion experiments in an annular perfusion system with everted human umbilical arteries were performed with perfusates to which such treated red blood cells were added to investigate their influence on platelet adherence to artery subendothelium. Platelet adherence was well correlated with red cell rigidity, with increased adherence at increased rigidity and vice versa. A change in T of 0.10 corresponded to a change in platelet adherence of approximately 50%. These effects were more pronounced at a wall shear rate of 1,800 s-1 than at 300 s-1.

Blood Platelets↗
PubMed1983

Assessment of the opening angle of implanted Björk-Shiley prosthetic valves.

A method has been developed in which cineradiography is used for the assessment of the opening angle of implanted Bjork-Shiley prosthetic valves. The method is based on the fact that the ring and the disc, which are known to be circular, appear to be elliptical on x-ray films. The spatial position of the valve can be retrieved from the characteristics of these ellipses when vectoranalysis is applied. The method's accuracy does not depend on the position of the patient with respect to the direction of the x-ray beam. The accuracy of the method was demonstrated with the use of a phantom valve. The difference between the measured and the real opening angle was -0.7 +/- 1.8 degrees (mean +/- SD). Results were reproducible in patients to within -0.1 +/- 1.8 degrees. In 18 patients with normally functioning valves it could be demonstrated with frame-by-frame analysis (interval between frames 20 msec) that the valves opened very rapidly up to about 60 degrees. Closing patterns varied. In one of our patients with valvular thrombosis insufficient valvular opening could be demonstrated by our method before the patient's complaints drew attention to the valvular dysfunction.

Cineradiography↗
PubMed1983

Red blood cell size is important for adherence of blood platelets to artery subendothelium.

The hematocrit is one of the main factors influencing platelet adherence to the vessel wall. Raising the hematocrit causes an increase of platelet accumulation of about an order of magnitude. Our studies concern the role of red cell size. We have studied this effect using an annular perfusion chamber, according to Baumgartner, with human umbilical arteries and a steady-flow system. Normal human red blood cells (MCV 95 cu mu) increased platelet adherence sevenfold, as the hematocrit increases from 0 to 0.6. Small erythrocytes from goats (MCV 25 cu mu) caused no increment in adherence in the same hematocrit range. Rabbit erythrocytes (MCV 70 cu mu) caused an intermediate increase in adherence. Red blood cells from newborns (MCV 110-130 cu mu) caused a larger increase in platelet adherence than normal red cells at hematocrit 0.4. These results were further confirmed with large red blood cells from two patients. Experiments with small red cells (MCV 70 cu mu) of patients with iron deficiency showed that platelet adherence was similar to normal red cells, provided the red cell diameter was normal. Small red blood cells of a patient with sideroblastic anemia caused decreased adherence. These data indicate that red cell size is of major importance for platelet adherence. Red cell diameter is more important than average volume. However, for size differences in the human range, the hematocrit remains the dominant parameter.

Animals↗