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Biomedical subjects

R M Heethaar

Publications and source records attributed to R M Heethaar.

90 records · Page 5Linked to original sources
PubMed1982

Cardiac function, fiber shortening, and dynamic geometry.

Many models for the study of the pump function of the heart emphasize the importance of cardiac geometry and detailed dimensional data. Because of the lack of accurate measuring techniques, approximate geometries such as shells of revolution have been applied. In this study, methods are presented that measure the dynamic geometry of the working, isolated canine heart by means of ultrasound-velocity tomography techniques. In addition, cardiac dimensions, intramural deformations, and fiber shortening have been measured dynamically in the in situ canine heart throughout the cardiac cycle with implanted radiopaque markers and biplane roentgen techniques. Results of regional contraction and relaxation patterns are presented. Epicardial fiber shortening between apex and base were computed and found to be dependent on the duration of the preceding RR interval.

Animals↗
PubMed1982

Clinical relevance of postextrasystolic potentiation.

The significance and clinical relevance of postextrasystolic potentiation resides in the fact that any change in duration of the cardiac cycle results in a change in the contractile behavior of the heart. Since neither a normal nor a diseased heart ever beats strictly regularly, postextrasystolic potentiation is continuously operative, and all hemodynamic or circulatory data obtained in the intact organism are affected by it. This means that postextrasystolic potentiation should be taken into account when cardiovascular clinical data are analyzed and subsequently used for diagnosis or treatment. Postextrasystolic potentiation used as an intervention for the evaluation of residual viable myocardium in scar tissue areas in patients with coronary heart disease may contribute to out insights in the prognosis of and therapeutic judgments in those patients. Postextrasystolic potentiation is a fundamental physiologic property of all myocardium under all circumstances and as such does not allow for integrating or averaging data obtained during more than one cardiac cycle, if cardiac rhythm has not been strictly regular. This fact is insufficiently appreciated in nuclear cardiology and two-dimensional echocardiography.

Animals↗
PubMed1981

Ultrasound velocity tomography, an imaging method.

Analysis of the geometry of the beating heart may yield important information about its condition and function, and may reveal physiologically and clinically relevant information about the influence of regional malfunctioning on the integral heart performance. In this study, a method has been developed which will ultimately allow the determination of the three-dimensional geometry of an isolated working dog-heart throughout the cardiac cycle. The technique will be called 'ultrasound velocity tomography'. Ultrasound velocity tomograms are reconstructions of the spatial distribution of the ultrasound velocity in cross-sections of the object under study. This velocity is to a certain extent characteristic for a tissue, so that various tissues and structures can be identified in a tomogram. The velocities are mathematically reconstructed from thousands of measured times-of-flight of ultrasound pulses which have travelled along different path-ways through the object. By detecting the blood-heart muscle transitions in a tomogram and by combining tomograms from different cross-sections of the heart, a three-dimensional heart-geometry may be obtained. Preliminary results on a formalin-fixed heart are shown and discussed.

Animals↗
PubMed1981

Method for measuring cardiac dimensions and intramural deformations.

Information about the geometry and intramural deformations of the left ventricle of the heart throughout the cardiac cycle is necessary for the analysis of contractile function of the intact heart. In this study a method is developed to measure dimensions and intramural deformations of the left ventricle of a dog heart during the cardiac cycle. Radiopaque markers (platinum spheres of 1mm diameter) were implanted in the wall of the left ventricle. The spatial positions of these markers were determined from pairs of perpendicular X-ray projections, obtained with the aid of rotational X-ray equipment. Preliminary results of ventricular deformations are in agreement with measurements by other techniques.

Animals↗
PubMed1981

Determination of coronary artery diameter and contrast medium concentration from angiograms - possibilities and limitations.

The quantification of coronary arterial stenosis is a great clinical importance. The coronarograms obtained with the usual X-ray systems are severely distorted. The principal effects, underlying this distortion are the focal spot dimensions, blurring due to object movements and noise due to individual X-ray photons. Some general assessment, as well as a simplified procedure are given. Under the assumptions of a cylindrical artery and a Gaussian curve for its density graph, a relation can be derived to calculate the artery diameter. Application to test objects show that this is a practicable method.

Angiography↗
PubMed1978

Postextrasystolic relaxation in the dog heart.

Left ventricular relaxation was studied in 8 dogs using parameters derived from the left ventricular pressure: the fastest pressure fall and the time constant of pressure decline. Effects of extrasystolic rhythm interventions were examined on the relaxation parameters of the post-relative to the preextrasystolic beat. Postextrasystolic potentiation of these parameters could not be demonstrated. Possible influences of physiologic variables as peak left ventricular pressure, endiastolic aortic and enddiastolic left ventricular pressure on relaxation mechanism were evaluated. The nonselective description of myocardial relaxation by pressure derived parameters is discussed.

Animals↗
PubMed1976

Atrial rhythm during ventricular fibrillation in the dog.

Depolarization of the atrioventricular junctional tissues and of the atrial septum was examined in the perfused dog heart before and during ventricular fibrillation by (1) recording the potentials from the junctional tissues in the regions of the interatrial and interventricular septum and examining the relationship of activity at these sites to atrial depolarization, (2) computing histograms and autocorrelograms of atrial firing intervals to study atrial rhythmicity, and (3) plotting the sequence of atrial septal depolarization. The junctional tissue was generally randomly depolarized by the fibrillating ventricles. The histograms and autocorrelograms indicate that during ventricular fibrillation the atrial depolarization intervals do not remain constant, but vary widely. This seems to be due to the retrograde excitation from the junctional tissues. Plots of the depolarization sequences of the interatrial septum also indicate that retrograde depolarization takes place. The junctional tissues decrease the number of impulses that can pass from ventricle to atrium, and they similarly decrease the number of impulses that pass in an antegrade direction during atrial fibrillation.

Animals↗
PubMed1975

Site of initial excitation and current threshold as a function of electrode radius in heart muscle.

End-diastolic current thresholds have been measured in 13 open chested dogs as a function of electrode radius by stimulating the left ventricle with epicardial disc electrodes ranging in a radius from 0-3 mm to 9 mm. Thresholds for cathodal rectangular short stimuli as well as specifically for cathodal make stimulation, proved to be proportional to the electrode radius to the power 1-5. This relationship between radius and threshold can be explained theoretically, assuming that electrical stimulation results in a propagated depolarization front if a critical current density is reached somewhere in the myocardium. The current distribution measured over the electrode and the site of initial depolarization in the tissue are in accordance with this theoretical explanation.

Animals↗
PubMedDate not supplied

MRI evaluation of right ventricular pressure overload in chronic obstructive pulmonary disease.

In chronic obstructive pulmonary disease (COPD), the development of pulmonary hypertension is common. This study was performed to assess the signs of right ventricular (RV) pressure overload and RV failure in COPD. In 8 COPD patients without primary cardiac disease, RV wall thickness, mass, and end-diastolic volume were measured by cardiac-triggered cine MRI. MR phase-contrast velocity quantification was used to measure stroke volume and the patterns of flow into and out of the RV. Data of patients were tested versus those of a control group matched for age (n = 8). Results showed that the RV wall thickness was increased (.6 +/- 0.1 vs 0.4 +/- 0.1 cm, P < .001). RV mass was increased (67 +/- 11 vs 57 +/- 5 g, P < .005). RV stroke volume was decreased (57 +/- 13 vs 71 +/- 13 ml, P < .01), but RV ejection fraction was not different. In the main pulmonary artery flow, the quotient of acceleration time divided by ejection time was decreased (33 +/- 5% vs 38 +/- 4%, P < .05), which is indicative of pulmonary hypertension. In conclusion, this MRI protocol provides a tool to assess the effects of RV pressure overload in COPD before heart failure has become manifest.

Aged↗
PubMedDate not supplied

Flow profiles in the left anterior descending and the right coronary artery assessed by MR velocity quantification: effects of through-plane and in-plane motion of the heart.

PURPOSE: The purpose of this work was to compare the temporal profiles of volume flow in the left anterior descending artery (LAD) and the right coronary artery (RCA) and to assess the effect of through-plane and in-plane myocardial motion. METHOD: In eight healthy volunteers, MR phase-difference velocity quantification was applied with prospective ECG triggering, pixel size of 1.16 x 0.98 mm2 (LAD) or 1.25 x 0.98 mm2 (RCA), velocity sensitivity of 40 cm/s, and data acquisition time window of 64 ms for LAD (3 ky lines per heartbeat) and 24 ms for RCA. In-plane motion was measured from the magnitude images. RESULTS: In the LAD, systolic peak and mean flow values were 0.94+/-0.28 and 0.30 +/-0.22 ml/s, respectively. Diastolic peak and mean flows were 2.42+/-0.56 and 1.38+/-0.43 ml/s. The systolic to diastolic ratio was 0.37+/-0.12 for peak flow and 0.22+/-0.15 for mean flow. Mean flow through the cardiac cycle was 59.1+/-15.0 ml/min. In the RCA, systolic peak and mean flow values were 1.96+/-0.69 and 0.74+/-0.31 ml/s, respectively. Diastolic peak and mean flows were 1.80+/-0.53 and 0.83+/-0.20 ml/s. The systolic to diastolic ratio was 0.97+/-0.58 for peak flow and 0.85+/-0.39 for mean flow. Mean flow through the cardiac cycle was 38.4+/-10.8 ml/min. The in-plane velocity of the coronary artery cross-section was 6.4+/-1.8 cm/s for the LAD and 14.9 +/-4.0 cm/s for the RCA (given by peak values in diastole). CONCLUSION: It is confirmed noninvasively with MR that the LAD shows a predominantly diastolic flow, whereas the RCA shows about equal flow values in systole and diastole. Through-plane motion correction is required for assessing the true flow patterns. The in-plane velocities of the coronary artery cross-sections imply a maximum data acquisition time window, estimated at 58 ms for the LAD and at 23 ms for the RCA.

Adult↗
PubMedDate not supplied

Effects of flow pulsatility on platelet adhesion to subendothelium.

Platelet adhesion in the annular perfusion system developed by Baumgartner was studied under pulsatile, oscillatory, or steady flow conditions. To investigate in what way pulsatile flow affects platelet adhesion, we developed a flow system that produces a sinusoidal laminar flow superimposed on a constant component in the annular perfusion chamber. Frequencies and amplitudes of this sinusoidal flow were in the physiological range. Pulse frequencies varied between 30 and 120 beats/minute, and different amplitudes of the wall shear rate in the range 75 to 1000 s-1 were studied. Shear rates resulting from the constant flow component were between 500 s-1 and 1800 s-1. Under these conditions, no significant differences in platelet adhesion were observed between steady flow and pulsatile flow. In the case of an oscillatory flow (absence of constant component), a clear dependence of platelet adhesion on the amplitude of the pulse was seen. These data indicate that platelet adhesion in larger blood vessels, such as the aorta and larger arteries where backflow is limited, is not essentially influenced by the pulsatility in these vessels.

Biophysical Phenomena↗
PubMedDate not supplied

Blood platelets are concentrated near the wall and red blood cells, in the center in flowing blood.

Hematocrit and vessel wall shear rate are important factors in the transport and subsequent adherence of platelets to vessel wall subendothelium. When mass transport theory is applied to platelets in flowing blood, the blood is usually considered to be a fluid with platelet and red cell wall concentrations similar to the average tube concentration. With the laser-Doppler technique, we found how red blood cell ghosts and platelets were distributed radially for various hematocrits and wall shear rates. Red cell ghosts are crowded near the axis of the tube, with a local hematocrit higher than the average tube hematocrit, and they decrease steadily toward the wall. In the absence of ghosts, platelets exhibit the 'tubular pinch' effect (rigid particles crowding at 0.6 x tube radius). In the presence of ghosts, the platelets are expelled toward the wall region. This high concentration at the wall increases with higher average tube hematocrit and wall shear rates. Increasing the average tube platelet concentration 10 times causes the wall concentration to increase only three times. The increase in platelet adherence observed with increasing hematocrit and increasing wall shear rate can be partially ascribed to increased platelet concentration near the wall. The observation that the increased platelet concentration does not fully explain the platelet adherence data suggests that platelet transport may also be enhanced by a shear rate-dependent rotary motion.

Blood Flow Velocity↗