Search PubMed⌕ Search

Biomedical subjects

N Westerhof

Publications and source records attributed to N Westerhof.

At least 145 records · Page 8Linked to original sources
PubMed1984

Heat transport in the canine left ventricular wall.

The rate of rise of local myocardial temperature (dT/dt) evoked by left coronary artery main stem occlusion has been proposed in the literature as a measure of local metabolic heat production, assuming heat loss due to diffusion to be negligible. In a previous study (ten Velden, G. H. M., G. Elzinga, and N. Westerhof. Circ. Res. 50: 63-73, 1982), we showed that this assumption was not valid. With this information in mind, in an attempt to study local metabolism, we compared, in anesthetized dogs, the dT/dt with the temperature distribution over the left ventricular wall. We found the value of dT/dt to be reproducible in time and to reproducibly depend on location. Negative values as well as positive values were measured; values even higher than the maximal possible temperature slope, calculated from the energy equivalent of left ventricular oxygen consumption and the specific heat of cardiac tissue, were found. Transmural distribution of the dT/dt showed positive values epicardially and negative values endocardially, while, as previously shown, a parabola-like shape of the transmyocardial temperature distribution existed. Our findings demonstrate that dT/dt by left coronary main stem occlusion cannot be used as a measure of local myocardial heat production.

Animals↗
PubMed1984

Input impedance of the pulmonary arterial system in normal man. Effects of respiration and comparison to systemic impedance.

Input impedance of the pulmonary arterial system was determined in 10 subjects undergoing elective cardiac catheterization. No cardiovascular or pulmonary disease was found in these patients. In five of the subjects, systemic arterial impedance was also obtained, so that both systems could be compared. Pulmonary and systemic peripheral resistances were 79 +/- 9 dynes sec/cm5 (mean +/- SEM) and 1016 +/- 50 dynes sec/cm5, respectively. Characteristic impedance of the pulmonary circulation was lower than the characteristic impedance of the systemic circulation: 20 +/- 1 dynes sec/cm5 vs. 47 +/- 9 dynes sec/cm5, respectively. Pulmonary pressure and flow spectra for both systems are also presented. The amplitudes of the harmonics of pressure and flow are smaller for the pulmonary circulation, which is consistent with the lower pressures and more rounded waveforms of the normal pulmonary circulation. In all 10 subjects, input impedance of the pulmonary system was examined during both the inspiratory and expiratory phases of respiration. There was no difference between inspiration and expiration in either pulmonary vascular resistance (77 +/- 10 dynes sec/cm5 vs. 80 +/- 9 dynes sec/cm5, respectively), characteristic impedance (20 +/- 1 dynes sec/cm5 vs. 20 +/- 1 dynes sec/cm5) or in the overall impedance spectrum. Quiet respiration, thus, has no effect on the pulmonary arterial load, and changes in pressure and flow must result from alterations in right ventricular performance.

Adult↗
PubMed1984

Does the history of contraction affect the pressure-volume relationship?

A comparison is made between two descriptions of the hemodynamic properties of the heart: the ventricular pressure-volume (P-V) relationship, modeled by a time-varying elastance, and the ventricular pump function graph, i.e., the relationship between mean ventricular pressure and output. It appears that, although both descriptions seem to reflect the same myocardial properties, an unexplained discrepancy exists between the assumed linear nature of the time-varying elastance and the curvature of the pump function graph. It is argued that this discrepancy may reflect deviations from the ideal time-varying elastance behavior, such as those observed in isolated cardiac muscle, where the history of contraction affected the P-V relationship.

Animals↗
PubMed1983

Steady state and instantaneous pressure-flow relationships: characterisation of the canine abdominal periphery.

This study was performed to characterise a vascular bed in terms of pressure-flow relationships. Steady state and instantaneous relationships were obtained in the flow perfused isolated femoral beds of six mongrel dogs. The steady state pressure-flow relations were obtained by applying a series of stepwise changes of flow in random order. The relations were found to be straight and to have a zero-flow pressure intercept (P0). The slope of this relation is the differential resistance (Rd). On each steady state flow level a ramp-flow was superimposed. The pressure response was measured between 1.5 and 5 s after the start of the ramp-flow, to exclude compliance effects and (auto) regulatory effects, respectively. In this way instantaneous pressure-flow relations were obtained, the slope of this relation is the instantaneous resistance (Ri). The instantaneous resistance expresses the true physical resistance value at a working point of the steady state pressure-flow relation before the bed has performed its (auto)regulatory adaptation after a change in flow. Instantaneous resistance therefore characterises the vascular state that exists at that particular working point. After this particular vascular state has been modified by (auto)-regulation the steady state pressure-flow relation is reached again. Instantaneous resistance increases with increasing flow thereby approximating the value of the differential resistance. At the same flow a vasodilator decreases and a vasoconstrictor increases instantaneous resistance. The gain (G) of the system, that characterises the (auto)regulatory capability, was calculated as G = 1--Ri/Rd and was found to decrease with increasing flow. The (partial) reflection of travelling waves depends on both the characteristic impedance (Zc) and the instantaneous resistance rather than on differential or peripheral resistance. Furthermore it is the product of the instantaneous resistance (Ri) and vascular compliance (C) that determines the time constant of a vascular bed.

Adenosine↗
PubMed1982

Left ventricular energetics. Heat loss and temperature distribution of canine myocardium.

The sum of total left ventricular heat loss and left ventricular mean total external power was compared with the product of oxygen consumption and its energy equivalent. Myocardial blood flow, measured with 15 +/- 3 micrometers radioactive microspheres, was multiplied by the transcoronary arteriovenous temperature difference and by oxygen content difference to obtain coronary heat loss and oxygen consumption, respectively. Since only part of the heat is carried away by the coronary system a thermodilution technique was used to obtain the ratio between the heat removed by the coronary system and the external heat loss. A correction was made for the endothermic reactions of hemoglobin deoxygenation and carbon dioxide reactions with blood. Left ventricular oxygen consumption corresponded to 2.26 +/- 0.66 W/100 g, and for the sum of total left, ventricular heat loss and external power, 2.09 +/- 0.51 W/100 g was found (n = 14). In a second series, the measured transmyocardial temperature distribution was compared with the calculated temperature distribution, assuming that heat production in the myocardium is uniform and that heat is lost by coronary flow and diffusion. When thoracic and luminal myocardial surface temperatures were about equal, blood flow was found to be about the same in the various layers of the heart, whereas myocardial temperature was found to be highest near the middle of the wall (0.36 +/ 0.07 degrees C warmer than luminal temperature (n = 6). When thoracic surface temperature was increased or decreased (by + 1.56 +/- 0.99 degrees and -1.10 +/- 0.59 degrees C, respectively), consistent changes were seen for the temperature distribution in the myocardium, but not for the local flow (endo/epi ratio: 1.06 +/- 0.29 and 0.96 +/- 0.21, respectively). These data suggest that myocardial blood flow is independent of tissue temperature.

Animals↗
PubMed1982

Isolated cat trabeculae in a simulated feline heart and arterial system. Contractile basis of cardiac pump function.

Isolated cat trabeculae were studied under conditions resembling those present for the muscle fibers in the wall of the left ventricle. To obtain such a situation experimental animals, perfusion fluid, temperature, stimulation frequency, peak stress values, contraction sequence, length, and force control were chosen with respect to that criterion. Results were compared with those described for the intact feline heart in previous studies. Special emphasis was placed on determinants of the pump function graph, i.e., the relationship between mean ventricular pressure and output. It was found that peak isometric stress values measured in the trabeculae were about twice as high as those existing on average at the base of the intact left ventricle in the circumferential direction. However, the duration of the mechanical activity, as measured in iso(volu)metric contractions, was in the isolated trabeculae (206 msec) significantly less (P less than 0.01) than found in intact right (292 msec) or intact left ventricle (344 msec). Furthermore the (maximum) output of the intact left ventricle at end-diastolic pressure could not be accounted for in a simple manner by the maximum amount of shortening found in isolated trabeculae. The points of the pump function graph obtained by varying the input impedance of the loading arterial system over a wide range of compliance and resistance values in the steady state deviated only little from the graph obtained from a series of constant pressure levels applied in a beat-to-beat fashion. Therefore, the insensitivity of the pump function graph to the nature of the arterial load is found in the intact heart as well as in isolated cardiac muscle.

Animals↗
PubMed1982

Pulse wave reflection: can it explain the differences between systemic and pulmonary pressure and flow waves? A study in dogs.

We have studied the effect of changes in pulse wave reflection on the configurations of pressure and flow in systemic and pulmonary circulation. Electromagnetic flow transducers, atrial catheters, and pacing leads were implanted in 10 dogs. In four animals, the flow transducer was placed on the pulmonary artery, in another four on the ascending aorta, and in two additional dogs on both vessels. One week later, ascending aortic and/or pulmonary artery flow and pressure (catheter tip manometer) were measured under general anesthesia (Nembutal, 30 mg/kg, iv). When the pulmonary circulation was studied (six dogs), measurements were made before and during serotonin infusion (0.5-0.75 mg/min). When the systemic circulation was studied (six dogs), measurements were made before and during nitroprusside infusion (50-200 micrograms/min). To quantify the arterial load, we calculated pulmonary and systemic input impedances. To estimate the amount of reflection, we used a reflection index which we defined as the amplitude ratio of reflected and forward wave. Nitroprusside decreased total peripheral resistance, increased total arterial compliance, and decreased the reflection index; similarity between aortic pressure and flow wave shapes increased, and they looked more like their pulmonary counterparts. Serotonin increased pulmonary vascular resistance, decreased pulmonary arterial compliance, and increased the reflection index. Resemblance of pressure and flow waves decreased. The differences in wave shapes can thus be explained by the amount of reflection: the less reflection the more pressure and flow resemble each other.

Animals↗
PubMed1981

Production of chronic heart block in closed-chest dogs: an improved technique.

Chronic complete heart block was produced in 38 closed-chest anesthetized dogs with a special cannula introduced via a jugular vein. A Formalin mixture (0.2-1.0 ml, consisting of equal amounts of 40% Formalin and radiopaque material) was injected through this cannula into the His bundle, which was located with a His-bundle catheter. The entire procedure took approximately 30 min and was successful in 90% of the cases.

Animals↗
PubMed1981

Manipulation of ascending aortic pressure and flow wave reflections with the Valsalva maneuver: relationship to input impedance.

Dramatic changes in the shape of pulsatile ascending aortic pressure and flow wave forms occur during the Valsalva maneuver in man. To study these changes, aortic pressure and flow signals were recorded in eight patients using a multisensor catheter. Aortic input impedance was derived during the control, strain and postrelease phases of the Valsalva maneuver. During control, well-defined minima and maxima occurred in the spectral plots of impedance moduli. This pattern was accentuated during the postrelease phase. In contrast, input impedance during strain was almost equal to the characteristic impedance for all harmonics. These results imply that during the control and postrelease phases, strong reflections return to the ascending aorta, but during the strain phase, reflections are minimal, absent or more diffuse. From wave transmission theory, it also follows that pulsatile pressure and flow wave forms should be similar in shape in the absence of reflections and dissimilar in the presence of reflections. This was observed in all eight patients. By provoking changes in the arterial tree during the Valsalva maneuver, the magnitude and timing of wave reflections were significantly altered, resulting in marked changes in the shape of pulsatile aortic pressure and flow wave forms. This study demonstrates the importance of reflections in determining the shape of the arterial pulse.

Adult↗
PubMed1981

Effects of exercise on aortic input impedance and pressure wave forms in normal humans.

The effects of supine bicycle exercise on the input impedance of the ascending aorta were studied in thirteen male subjects undergoing cardiac catheterization, but in whom no cardiovascular disease was found. Ascending aortic flow velocity and pressure were recorded simultaneously from a multisensor catheter with an electromagnetic flow velocity probe and a pressure sensor mounted at the same location. A second pressure sensor at the catheter tip provided left ventricular pressure. Fick cardiac outputs were used to scale the velocity signal to instantaneous volumetric flow. Input impedance was calculated from 10 harmonics of aortic pressure and flow. For each subject, impedance moduli and phases from a minimum of 15 beats during rest and exercise were averaged. Peripheral resistance decreased from a resting value of 1142 +/- 51 (+/- SE) dynes sec/cm5 to 712 +/- 39 dynes sec/cm5 during exercise. Characteristic impedance remained unchanged with a resting value of 47 +/- 4 dynes sec/cm5 and an exercise value of 45 +/- 4 dynes sec/cm5. These results were associated with an increase in aortic pressure (96 +/- 2 to 111 +/- 2 mm Hg) and pulse wave velocity implying a decrease in aortic compliance. An increase in aortic cross-sectional area apparently offsets the effects of these changes in compliance and pulse wave velocity to result in an unchanged characteristic impedance. The increase in pulse wave velocity caused wave reflections to occur earlier during exercise, but the general characteristics of the pressure wave shapes remained unchanged.

Adult↗
PubMed1981

"Pressure-volume" relations in isolated cat trabecula.

We studied isolated cat trabecula under conditions closely resembling those present for muscle fibers in the left ventricular wall. The purpose of the study was to see if muscle contraction under those circumstances could be described by a time-varying compliance as reported for intact canine left ventricle. We found the time of the end of systole to depend on the history of contraction. This time varied between 100 and 160 msec as measured from the onset of contraction. Similar dependency, although less percentage-wise, was found by reanalysis for intact feline left ventricles. We conclude that the behavior of the canine left ventricle as a time-varying compliance may be related to the complex organization of the cardiac muscle fibers in the wall of the heart rather than to muscle properties.

Animals↗
PubMed1980

Pump function of the feline left heart: changes with heart rate and its bearing on the energy balance.

Pump function of the feline left heart was determined by measuring the relationship between mean left ventricular pressure and mean left ventricular output, obtained by changing the arterial load on a beat-to-beat basis. The effect of a change in heart rate from 120 to 160 beats . min-1 was studied and a parallel shift of the pump function graph was found. Care was taken to keep left ventricular end-diastolic pressure constant with the change in frequency. If the mean pressure and output values obtained at 160 beats . min-1 were multiplied by the ratio between the two frequencies (0.75), almost complete superposition of the two graphs was obtained. Changes in arterial load also caused changes in oxygen consumption, mean external power and external efficiency of the heart. We plotted these variables, altered them as a function of mean left ventricular output for easy comparison with the pump function graph. It was found that oxygen consumption decreases with increasing output. Mean external power and efficiency attain maxima for different values of mean output. If the left heart in the intact animal is controlled to function at its maximum power output, this can therefore not be achieved at the optimum efficiency level. The results of the present study and those obtained earlier were compared with the behaviour of a time varying compliance model.

Animals↗
PubMed1980

Measurement of left ventricular wall stress.

Forces in the myocardial wall can be measured in several ways or calculated using certain simplifying assumptions. In this study we investigated the reliability of two measurement methods, one of which was introduced by Feigl et al (1967), whereas the other method was developed in our laboratory. Both methods were tested in actively contracting skeletal muscle and beating hearts of open-chest dogs by comparing the force transferred from the muscle to the transducer under various circumstances. It appeared that changes in muscle length, be it through initial length changes or through shortening during contractions, had a great influence on the transfer of force to the transducer, for both methods, in both preparations. In the heart a decrease in internal left ventricular diameter of 15% resulted in a 50% reduction of force transferred to the transducer, independent of whether the length took place as a change in filling or as a change in ejection volume. In skeletal muscle the length-dependent effects during shortening were larger and those resulting from initial length changes were more variable than in beating hearts. That the effects of muscle length changes are of such magnitude means that, if no other errors exist, they alone would invalidate that until principally different methods of measuring wall stress in the myocardium are discovered, attempts at accurate calculation of myocardial wall stress are a better approach than wall stress measurements.

Animals↗
PubMed1980

The arterial system characterised in the time domain.

When an impulse of flow is applied to the arterial system then the resulting pressure, the impulse response, is a characterisation of the arterial tree. The impulse is generated by means of an occluder around the ascending aorta. The impulse response shows an initial sharp peak followed by an exponential decay with two peaks superimposed on it. The exponential decay is due to diffuse reflection and is linked to the windkessel properties of the arterial tree. The superimposed peaks arise from two distinct reflection sites in the arterial tree. By means of the pulse wave velocity the location of these reflection sites may be calculated; one is found in the bed distal to the brachiocephalic and subclavian arteries and the other in the bed distal to the descending aorta. The distinct reflections are linked to the reflection sites in the asymmetric T-model of the arterial tree. Vasodilatation (nitroprusside) and vasoconstriction (angiotensin) mainly influence the diffuse reflections, while the locations of the distinct reflection sites appear to be unchanged. Inflation of a balloon in the descending aorta shows up as a sharp peak in the impulse response function. The results obtained are compared with the impulse response function computed from pressure and flow waves in the steady-state.

Animals↗