Biomedical subjects

P Steendijk

Publications and source records attributed to P Steendijk.

At least 55 records · Page 3Linked to original sources

PubMed1993

Cardiac sympathetic denervation does not change the load dependence of the left ventricular end-systolic pressure/volume relationship in dogs.

It has been shown that in the intact canine heart the left-ventricular end-systolic pressure/volume relation (ESPVR) depends on loading conditions: an increase in arterial vascular resistance causes a leftwards shift and a steeper slope of the ESPVR, suggesting an increased inotropic state. Our purpose was to investigate the possible contribution of the sympathetic nervous system to this load sensitivity of the ESPVR, using intact, but denervated, hearts with normal coronary perfusion and afterload. We used two types of loading intervention: venous volume infusion and gradual occlusion of the descending aorta. ESPVRs were obtained in six anaesthetized open-chest dogs, both before and after bilateral ablation of the stellate ganglia. To exclude the influence of heart rate changes, bilateral vagotomy was performed and the heart was paced. The absence of (unpaced) heart rate changes in response to pressure alterations was used to confirm total denervation. Left ventricular pressure was measured with a micromanometer and volume with a conductance catheter. ESPVRs were essentially linear and characterized by their slope (Ees) and volume intercept at 12 kPa (V12). We found that Ees (P < 0.0001) and V12 (P < 0.05) were both significantly different during pressure and volume interventions (0.67 +/- 0.29 and 0.41 +/- 0.18 kPa/ml for Ees and 16.2 +/- 8.2 and 18.2 +/- 8.4 ml for V12 respectively). Denervation did not significantly affect the parameters of the ESPVR obtained by either volume infusion or aortic occlusion. Two-way analysis of variance revealed no significant interactive effect between denervation and intervention, indicating that the sympathetic nervous system does not influence the load dependency of the ESPVR.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗

PubMed1993

The four-electrode resistivity technique in anisotropic media: theoretical analysis and application on myocardial tissue in vivo.

Several aspects of the four-electrode resistivity technique were studied with special emphasis on the theoretical determination of penetration depth and sample volume in anisotropic media such as (cardiac) muscle. Moreover, the presence of a thin disturbing layer on top of the medium under study was analyzed. A four-electrode sensor was developed for the measurement of local myocardial resistivity in two orthogonal directions. The sensor was applied to the epicardium of anesthetized open chest dogs and, as an example, results are given on the frequency dependence (5-60 kHz) and the changes during the cardiac cycle of longitudinal and transverse myocardial resistivity.

Animals↗

PubMed1993

Left ventricular stroke volume by single and dual excitation of conductance catheter in dogs.

The conductance method employs a multielectrode catheter to measure intracavitary electric conductance from which left ventricular volume is estimated. A dual-excitation method introduced by us uses a more homogeneous electric field and thereby should enable more accurate volume estimation. In six anesthetized open-chest dogs we compared stroke volume obtained from electromagnetic flow probes with the conventional single-excitation method and with the new dual-excitation conductance method. Caval occlusion and left atrial hemorrhage were used to obtain a wide range of stroke volumes. The slope of the relation between stroke volume calculated from the flow probes and from the conductance catheter increased significantly (P < 0.001) from 0.635 with single excitation to 0.835 with dual excitation, but the interanimal variability was not reduced. The linearity of the relation was substantially improved.

Animals↗

PubMed1993

Cerebral hemodynamics and oxygenation in preterm infants after low-vs. high-dose surfactant replacement therapy.

In thirteen preterm infants receiving surfactant (Curosurf) replacement therapy, changes in cerebral hemodynamics and oxygenation were investigated by near infrared spectroscopy. Surfactant instillation led to an instantaneous increase in cerebral blood volume (CBV) in all infants, which was primarily due to an increase in deoxygenated hemoglobin. Five infants received a low dose (100 mg/kg = 1.25 ml/kg) of surfactant and 8 a high dose (200 mg/kg = 2.50 ml/kg). A significantly larger increase in CBV was observed in the infants receiving a high dose compared to those receiving a low dose of surfactant. We conclude that cerebral perfusion is affected more after the instillation of a high dose compared to a low dose of surfactant.

Biological Products↗

PubMed1993

Myocardial perfusion and performance after indomethacin administration in newborn lambs.

Indomethacin is a drug widely used to achieve pharmacologic closure of a patent ductus arteriosus in the premature infant. In several vascular beds (brain, kidney, intestine), indomethacin has been shown to cause vasoconstriction. Possible negative effects on myocardial blood flow and performance could be deleterious in premature infants with limited cardiac reserve. Before, during, and 30 and 60 min after administration of 1 mg.kg-1 of indomethacin in nine newborn lambs, we measured coronary blood flow velocity (Doppler flow probe around the left circumflex coronary artery), left ventricular (LV) pressure (by tip manometer) and volume (by conductance catheter technique), cardiac output, arterial pressure, arterial and venous saturations and calculated systemic and coronary vascular resistance, LV systolic function by the end-systolic pressure-volume relationship, and myocardial oxygen extraction. To investigate the effect of indomethacin on the flow regulation of the coronary vascular bed, we measured coronary flow and LV function under different levels of myocardial demand, achieved by stepwise occluding of the descending aorta. During indomethacin infusion, coronary and systemic vascular resistance increased significantly (by 43 and 76%, respectively), resulting in an increase in arterial pressure from 10.2 to 16.9 kPa, whereas neither coronary flow nor LV systolic function changed despite the increase in afterload. Thirty and 60 min after indomethacin, coronary and systemic vascular resistance had returned to baseline levels and LV systolic function remained unchanged. The relationship between coronary flow and cardiac demand was not different before or after indomethacin.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

The influence of indomethacin on the autoregulatory ability of the cerebral vascular bed in the newborn lamb.

Prevention of hyperperfusion of the brain in the perinatal period has been thought to be an important mechanism by which indomethacin reduces the risk for severe periventricular-intraventricular hemorrhage. The present study investigated whether an indomethacin-induced enhancement of the upper limit of cerebral vascular autoregulatory ability in the neonate contributed to this reduction in cerebral blood flow. In seven anesthetized newborn lambs, we measured temporal blood flow velocity (TMFV) in the carotid artery over a wide range of mean aortic blood pressures (MABP) before and 30 min after an i.v. dose of 1 mg/kg indomethacin. TMFV in the carotid artery was used as an estimate for changes in cerebral blood flow. Stepwise changes in MABP of approximately 10 mm Hg were achieved by progressive balloon occlusion of the thoracic aorta or by progressive bleeding. Multiple linear regression analysis of TMFV versus MABP, indomethacin, and the possible interactive effects confirmed that, at MABP values up to 86 mm Hg, indomethacin lowered TMFV of the carotid artery. Above 86 mm Hg, indomethacin reduced the slope of the TMFV-MABP relationship, indicating an improvement of the autoregulatory ability of the cerebral vascular bed. There was a significant interanimal variability. Thus, indomethacin may reduce the risk for PIVH by limiting cerebral blood flow, especially during increased cerebral perfusion pressures, which often occur after birth asphyxia.

Animals↗

PubMed1992

Cerebral blood flow velocity: the influence of myocardial contractility on the velocity waveform of brain supplying arteries.

Indices of Doppler-derived velocity waveforms of arteries perfusing the brain are used as relative measures of neonatal brain blood flow. Using a dog model, we investigated the influence of changes in myocardial contractility, induced by dobutamine, on the blood flow velocity waveform of the vertebral artery. The following indices of the velocity waveform were investigated during control states and during 5 or 10 micrograms/kg/min dobutamine infusion: peak systolic flow velocity (PSFV), temporal mean flow velocity (TMFV), end-diastolic flow velocity (EDFV) and acceleration time (ACC-time). PSFV and ACC-time of the vertebral artery showed a strong relationship with myocardial contractile state. These results indicate that PSFV of an artery supplying the brain or indices which combine PSFV with MFV or EDFV should not be used for noninvasive assessment of brain blood flow or cerebral vascular resistance. ACC-time may prove to be very useful in assessing changes in myocardial contractile state.

Animals↗

PubMed1992

Left ventricular segmental volume by conductance catheter and Cine-CT.

The ability of the conductance catheter method to measure left ventricular segmental and total volume was evaluated by comparison with the Cine-CT technique. In the seven dogs studied, 19 conductance catheter and simultaneous Cine-CT runs were obtained. High correlation coefficients were found for total volume and segmental volumes, except in the basal segment. However, in most cases there was a significant variability in slope and intercept between animals. Both methods are promising tools for estimating dynamic segmental left ventricular volume, each having specific advantages such as a continuous signal (conductance catheter) or anatomic detail (Cine-CT). However, the results also show the need for further improvement of both methods.

Animals↗

PubMed1992

Ventricular pressure-volume relations in vivo.

A number of fundamental mechanical properties of cardiac muscle as well as of the total ventricle are discussed. These include: Starling's law, shortening deactivation, homeometric autoregulation, and so-called hyperactivation associated with a small amount of ejection. Whenever appropriate, muscle properties are related to those of the intact ventricle. The phenomenon of load-dependence of the end-systolic pressure-volume relationship (ESPVR) receives particular attention and is placed within the framework of shortening deactivation and homeometric autoregulation. However, this effect also leads to a more basic definition of myocardial contractility, based on the observed, rather parallel shift of the ESPVR with different afterload conditions. Using this definition, the conclusion is drawn that an increase in afterload is, in fact, met by the left ventricle increasing its inotropic state. Finally, within this approach, it is proposed that the value of end-systolic volume at a chosen constant end-systolic pressure (e.g. 13 kPa or 100 mmHg) be used to characterize changes in myocardial contractility in patients after an intervention. The way of obtaining this parameter is explained.

Animals↗

PubMed1992

Single and dual excitation of the conductance-volume catheter analysed in a spheroidal mathematical model of the canine left ventricle.

The conductance method employs a multi-electrode catheter to generate an electrical field and measure intracavitary segmental conductances. Left ventricular (LV) volumes are calculated using an algorithm which assumes the electrical field to be homogeneous. This assumption may be violated leading to a non-linear relation between conductance-derived and true volumes. In addition, this relation may vary between segments. A new method is introduced which uses a more homogeneous field. Volume estimates using the conventional single excitation and the new dual excitation method were compared in a mathematical model of a canine LV, which was varied over a large volume range. With single excitation the slope factors, relating conductance-derived and true volumes, varied from 0.50 to 0.76 between segments and was 0.65 for total LV volume. Using dual excitation the segmental slope variability was reduced (range: 0.74-0.77) and the slope factor for total volume increased to 0.76. The linearity of the relation between conductance-derived and true volume was improved with dual excitation and extended over a larger range.

Animals↗

PubMed1992

The end-systolic pressure-volume relationship in young animals using the conductance technique.

Evaluation of ventricular performance by the end-systolic pressure-volume relationship (ESPVR) has been extensively performed in the adult heart using the conductance technique. We undertook this study to validate the conductance technique and to generate ESPVRs in the small heart. To validate the technique, we simultaneously measured left ventricular volume by the conductance catheter and biplane cineangiography in nine piglets during changes in volume and contractility. Raw conductance volumes correlated highly with cineangiographic volumes (R = 0.97), and the slope was near identity (1.11 +/- 0.04). However, 'alpha Vc-corrected' volumes correlated less well (R = 0.85), probably because of errors induced by the saline technique for alpha Vc. We evaluated the ESPVR in nine lambs by inferior vena cava (IVC) occlusion, aortic occlusion, and volume infusion at rest and during changes in contractility. Reliable and linear ESPVRs were obtained in almost all IVC and aortic occlusions but not in volume infusions. Neither slope (Ees) nor position (V14) significantly changed over time or with dobutamine, but both changed after propranolol, supporting studies showing a limited contractile reserve in the newborn. However, Ees was 25% less steep when generated by IVC occlusion as compared to aortic occlusion. We conclude that the ESPVR can be reliably generated in the small heart using the conductance technique, but that it is sensitive to the loading technique.

Age Factors↗

PubMed1991

Nonlinearity and load sensitivity of end-systolic pressure-volume relation of canine left ventricle in vivo.

The effects of mechanical changes in loading conditions on the left ventricular end-systolic pressure-volume relation (ESPVR) were studied in nine open-chest dogs, including three dogs studied before and after beta-adrenergic blockade. Left ventricular pressure was measured with a micromanometer, and left ventricular volume was measured with a conductance catheter. ESPVRs were obtained by increasing left atrial inflow over wide volume ranges (as much as threefold) under three different conditions: control or high or low aortic impedance. High impedance was obtained by occlusion of the descending aorta, and low impedance was obtained by a shunt between the subclavian artery and the left atrium. In the unblocked animals in 21 of 28 runs, a second-order polynomial equation gave a better fit for the ESPVR than a linear relation. To quantify the effects of the changes in aortic impedance on the ESPVR, we calculated from the quadratic equation its volume intercept (V18) and its local slope (E18) at an end-systolic pressure (Pes) of 18 kPa. In the unblocked animals, a statistically significant difference was found in V18 between low impedance (21.50 +/- 6.27 ml) and high impedance (14.10 +/- 8.98 ml; p less than 0.005) and between control (19.14 +/- 9.58 ml) and high impedance (p less than 0.05). In most dogs, E18 was increased at high and decreased at low impedance, but not significantly. In the additional experiments with beta-blockade, the nonlinearity diminished somewhat, but the load dependency of the ESPVR remained present after beta-blockade because the same leftward shift of the ESPVR with high aortic impedance was found. Two other relations, namely, of dP/dtmax and of stroke work versus end-diastolic volume, were also investigated, which on the whole showed the same behavior as the ESPVR. These results indicate that the ESPVR and dP/dtmax-Ved and stroke work-end-diastolic volume relations, when studied over a wide volume range, are nonlinear and that changes in loading conditions influence indexes of contractility derived from these relations, especially the volume intercepts, in such a way that an increase in aortic impedance may be interpreted as an increase in contractility. Blocking the beta-adrenergic receptors did not influence the load dependency of the ESPVR but, in some cases, tended to decrease the nonlinearity in concordance with the relation between contractility and nonlinearity in isolated hearts.

Adrenergic beta-Antagonists↗

PubMed1991

The end-systolic pressure-volume relationship in the newborn lamb: effects of loading and inotropic interventions.

Indices of global systolic performance of the newborn left ventricle exceed those of the adult, despite isolated tissue studies showing immature contractile mechanisms. To evaluate contractility in situ, we investigated the end-systolic pressure-volume relationship (ESPVR) by the conductance technique in nine newborn lambs. After percutaneous placement of catheters, we generated ESPVR by inferior vena cava occlusion, aortic occlusion, and volume infusion in two control states, during three levels of dobutamine infusion, and after propranolol. We performed linear and nonlinear regression analyses of the end-systolic points and derived the slope (Ees) and volume at 14 kPa pressure. We found that reliable ESPVR could be obtained in almost all inferior vena cava and aortic occlusions (50 of 51 in each), but in only 18 of 27 volume infusions. Overall, linear regressions adequately defined the ESPVR (75 of 102 were not statistically different than nonlinear regressions; of those different, the mean linear R2 was 0.934 +/- 0.048). By multiple regression analysis, neither Ees nor volume at 14 kPa significantly changed with dobutamine, but both changed after propranolol (23% less than control and 54% greater, respectively), supporting previous studies showing a limited contractile reserve in the newborn secondary to high resting beta-adrenergic tone. Neither Ees nor volume at 14 kPa was different between control states. However, Ees was 25% less steep when generated by inferior vena cava than by aortic occlusion. We conclude that the ESPVR can be generated reliably and reproducibly in the newborn lamb and is relatively linear and sensitive to changes in contractility, but that it is also sensitive to the technique of load intervention.

Animals↗

PubMed1991

Acceleration of blood flow velocity in the carotid artery and myocardial contractility in the newborn lamb.

We investigated the influence of quantitative changes in myocardial contractile state, reflected by changes in the end-systolic pressure-volume relationship (its slope and volume intercept) and by changes in the slope of the relationship between change in pressure per unit time and end-diastolic volume induced by beta-adrenergic stimulation or inhibition, on the Doppler derived blood flow velocity wave form of the carotid artery, using a newborn lamb model. Acceleration time of the velocity wave form was investigated during control state I, during 4 and 8 micrograms/kg/min dobutamine infusion, during control state II, and during 0.5 mg/kg propranolol infusion, respectively. Using multiple linear regression analysis with dummy variables, confounding effects such as interanimal variability were removed. Acceleration time showed a strong relationship to both the slope and the volume intercept of the end-systolic pressure-volume relationship and to the change in pressure per unit time-end-diastolic volume relationship. The relations appeared to be independent of aortic pressure and relative resistance in the vascular bed of the carotid artery. These results indicate that acceleration of cerebral blood velocity may prove to be useful in assessing changes in myocardial contractile state of the newborn.

Animals↗

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↗

PubMedDate not supplied

Development of an MRI-compatible catheter for pacing the heart: initial in vitro and in vivo results.

An MRI-compatible catheter was developed for pacing the heart during MRI imaging. The device was tested in vitro and in vivo in 10 animal experiments, using spin-echo, gradient-echo, and echo-planar MRI sequences. Images were of good quality in all sequences. Pacing was effective without induced arrhythmias. Therefore, pacing the heart during MRI is feasible and seems to be safe when using dedicated hardware.

Animals↗

PubMedDate not supplied

Dependence of anisotropic myocardial electrical resistivity on cardiac phase and excitation frequency.

Knowledge of myocardial electrical resistivity is of interest because passive electrical properties govern the electrotonic spread of current through the myocardium and influence the shape and velocity of the excitation wave. In addition, measurements of myocardial resistivity may provide information about tissue structure and components. The aim of the present study was to determine the excitation frequency dependence and the changes during the cardiac cycle of anisotropic myocardial electrical resistivity. Longitudinal and transverse myocardial resistivity were measured using an epicardial sensor in four open-chest dogs with excitation frequencies in the range of 5-60 kHz. Mean longitudinal resistivity gradually decreased from 313 +/- 49 omega.cm at 5 kHz to 212 +/- 32 omega.cm at 60 kHz, transverse resistivity decreased from 487 +/- 49 to 378 +/- 53 omega.cm. To analyze the phasic changes, we compared mean resistivity (averaged over the full cardiac cycle) with resistivity during four cardiac phases: pre-ejection, ejection, early diastole and late diastole. Longitudinal resistivity was significantly higher during the ejection phase (+9.6 +/- 4.1 omega.cm) and lower during late diastole (-6.9 +/- 2.9 omega.cm). Transverse resistivity was significantly higher during late diastole (+4.0 +/- 2.3 omega.m). The values during the other cardiac phases were not significantly different from mean resistivity. The phasic changes in longitudinal and transverse resistivity during the cardiac cycle were independent of the excitation frequency. We speculate that these changes are related to geometrical changes, especially to changes in myocardial blood volume.

Animals↗

PubMedDate not supplied

Systolic coronary flow reduction in the canine heart in situ: effects of left ventricular pressure and elastance.

In the externally perfused coronary bed of the isolated heart, LV elastance (Elv) rather than LV pressure (Plv) appears to be the major factor responsible for systolic coronary flow reduction, although effects of both have been demonstrated. However, normal perfusion in the presence of intact autoregulation in the heart in situ may modify these effects. To investigate the systolic coronary flow (Qsyst) responses to changes in end-systolic Elv and in systolic Plv in the intact coronary bed, we studied 7 anesthetized dogs. Dogs were vagotomized, paced and instrumented with Doppler flow probes. Elv, obtained from micromanometer- and conductance-catheters, was changed by dobutamine infusion (10 micrograms/kg/min) and characterized by its slope (Ees) and volume position at 15 kPa (V15). Plv was changed stepwise by volume loading. Correction for metabolism induced changes in Qmean was made. Thus, we found that dobutamine increased Ees by 44% (p < 0.0001), decreased V15 by 43% (p < 0.0001), and reduced corrected Qsyst significantly (p < 0.001) from 22.8 to 13.8 ml/min, while Plv (9.5-19.5 kPa) did not affect corrected Qsyst (p > 0.5). We conclude that systolic coronary flow reduction is related to both elastance parameters, Ees and V15, and not to Plv in the normally perfused canine heart in situ.

Animals↗