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Biomedical subjects

A Noordergraaf

Publications and source records attributed to A Noordergraaf.

At least 19 recordsLinked to original sources
PubMed1995

Valvular and systemic arterial hemodynamics in aortic valve stenosis. A model-based approach.

BACKGROUND: Assessment of the severity of a stenotic aortic valve has been confounded by reports of flow dependence of stenosis severity. We hypothesized that the pressure gradient at the stenotic aortic valve would be dependent on the characteristics of the arterial circulation. Therefore, clinically useful measures of the severity of aortic valve stenosis may have to take this dependence into account. METHODS AND RESULTS: We developed an analog model of the systemic arterial circulation in the presence of a stenotic aortic valve. The model clearly describes the dependence of stenosis severity (described by coefficients A and B) on the resistive and capacitive properties of the arterial system. We used high-fidelity pressure recordings obtained at the time of diagnostic cardiac catheterizations and found that a highly significant relation between the measured mean transvalvular gradient and that predicted by the model was demonstrated both at rest (r2 = .90) and with exercise (r2 = .80). Furthermore, the relative constancy of stenosis coefficients A and B was validated. CONCLUSIONS: Transvalvular hemodynamics in patients with aortic valve stenosis are dependent on the properties of the arterial system. The current model describes such behavior, correctly predicts the transvalvular gradient from model parameters, and may be useful in the assessment of stenosis severity under various clinical and physiological conditions.

Aged↗
PubMed1995

Effect of non-steady-state perfusion on xenon-133 cerebral blood flow measurements: an analytical study.

Activation studies employing the noninvasive xenon-133 technique are widely used to investigate the cerebral circulation. Typical examples are the investigation of hemispheral specialization of higher cortical function with cognitive activation or the assessment of the hemodynamic reserve in occlusive cerebrovascular disease by CO2 inhalation. Traditionally, in studies using this technique, there is the requirement of a circulatory steady state during the measurement. Due to limitations in the duration of the stimulus or habituation to the stimulus, the basic assumption is often violated. In this study we investigated with the aid of a computer model to what extent blood flow measurement results are affected by non-steady-state blood flow. The findings indicate that cortical activation need not extend throughout the whole measurement to be detectable. Maintenance of activation for at least 5 min is sufficient for a successful measurement. In addition, the results show that the activation should be fully established when the measurement starts to achieve maximal sensitivity. Delay in activating the circulation will result in attenuated responses, especially if the stimulus is delayed beyond 2 min.

Cerebrovascular Circulation↗
PubMed1994

Pressure generation in a contracting myocyte.

The central hypothesis of this investigation is that a shortening myocyte generates a time-varying transmural pressure, or intracellular pressure. A mathematical model was formulated for a single myocyte, consisting of a fluid-filled cylindrical shell with axially arranged contractile filaments, to quantitate the fiber-fluid interaction. In this model, the intracellular pressure mediates the interaction between myofilament force, cell shortening, and the mechanical properties of the sarcolemma. Shortening of myofibrils, which are embedded in the fluid-filled myocytes, deforms the myocyte, thereby altering its transmural fluid pressure. This increase in transmural pressure counteracts fiber shortening, hence constituting an internal load to shortening. The shortening of the myocyte is accompanied by thickening, due to the incompressible nature of its contents. Consequently, the overall contractile performance of the cell is integrally linked to the generation of intracellular pressure. The model manifests a positive transmural pressure during shortening, but not without shortening. The pressure in the myocyte, therefore, is not a direct function of the force generated, but rather of shortening. Intracellular pressure was measured through a fluid-filled glass micropipette (5 mu ID) employing a servo-nulling pressure transducer in a standard micropuncture technique. Measured intracellular pressure in a contracting isolated skeletal myocyte of the giant barnacle is observed to be dynamically related to shortening, but not to tension without shortening. The relation between the force of contraction, cell shortening, and intracellular pressure was assessed during both isotonic and isometric contractions. The results support the prediction that isometric, or nondeforming, contractions will not develop intracellular pressure and identify a reason for relengthening of the myocytes during relaxation.

Animals↗
PubMed1994

Differential effects of wave reflections and peripheral resistance on aortic blood pressure: a model-based study.

It has been generally accepted that arterial system wave reflections act to increase aortic blood pressure and the load placed on the left ventricle. Using a mathematical model of the coupled left ventricle-arterial system, we predict that this is not the case. With the model, two aspects of wave reflection, the global reflection coefficient [TG(omega)] and the pulse wave velocity (cph), were adjusted independently. In addition, TG(omega) and cph could be altered independently of the direct-current properties of the arterial system model. Reduction of TG(omega) yielded increases in stroke volume (SV) as well as in peak systolic (Ps), diastolic (Pd), and mean aortic (Pao) pressures and, hence, increased the load on the left ventricle. SV and Pao increased only in the range where strong reflection occurs. Reduced cph also yielded higher pressures, whereas increased cph resulted in reduced Pao and Pd but increased Ps. The changes in pressures and SV in response to altered TG(omega) and cph were relatively small compared with absolute levels. Simulated vasoconstriction and vasodilation further demonstrated the much greater importance of peripheral resistance on pressure and SV levels and lead to the prediction that pressure reduction in vasodilation occurs not because of, but in spite of, reduced wave reflections. We conclude that these results have not yet been observed experimentally, because reflection cannot yet be separated from the direct-current properties of the arterial system; therefore wave reflections themselves have not yet been adequately studied in the intact animal.

Animals↗
PubMed1994

Heart rate variability as a prognostic tool in cardiology. A contribution to the problem from a theoretical point of view.

BACKGROUND: Recent clinical studies have proposed standard deviation of heart rate as a diagnostic tool for the outcome of cardiac infarction. Mathematical analysis of heart rate variability shows that heart rate is influenced by different frequency components derived from different parts of the autonomous nervous system. In the experimental part of this study, we investigated the possibility of calculating a variable describing the parasympathetic branch of the autonomous nervous system exclusively. METHODS AND RESULTS: In 60 healthy volunteers, heart rate was measured to 1 millisecond during two different conditions: 5 minutes of rest, and 5 minutes of intermittent handgrip dynamometry; the latter is known to increase sympathetic arousal selectively. Heart rate was found to be lower at rest (65.9 +/- 9.7 beats per minute) than during dynamometry (72.8 +/- 10.4 beats per minute, P < .001). Respiratory sinus arrhythmia (RSA) calculated from the mean absolute differences between successive heart beats showed no significant change (3.01 +/- 1.62 beats per minute at rest versus 2.97 +/- 1.30 beats per minute during dynamometry). In contrast, standard deviation increased from 5.19 +/- 1.98 to 9.22 +/- 3.56 beats per minute (P < .001). CONCLUSIONS: It can be concluded from these data as well as from other plots presented in this article that RSA is a measure of the parasympathetic vagal tone, whereas standard deviation is increased by both sympathetic and parasympathetic arousal. Clinical evidence and data from physiological experiments are presented to show that a selective measure of vagal tone like RSA may offer advantages over standard deviation as a prognostic tool in cardiology.

Autonomic Nervous System↗
PubMed1993

Quantitative analysis of the Landis method.

Landis developed the single capillary micro-occlusion method for the determination of the capillary filtration coefficient, Lp. More recently, several modified versions of Landis's original procedure have been introduced in an attempt to measure Lp with greater ease and accuracy. Each of the techniques assumes that the capillary is a uniformly cylindrical indistensible tube. Since recently obtained experimental data have demonstrated that the capillary is compliant, a distributed mathematical model of the capillary was developed to reveal the extent to which capillary distensibility influences the measurement of Lp at different sites along the capillary. After assigning physiologic values to this modeled capillary, including Lp and the capillary wall's modulus of elasticity, Landis's method and two of its modified versions were performed on the modeled capillary, with each yielding values for Lp at 10 different locations along the capillary. Comparison of these 10 computed Lp values with the corresponding Lp values defined in the modeled capillary demonstrates that capillary distensibility can introduce significant error into the measurement of Lp.

Animals↗
PubMed1993

Peripheral vascular effects on auscultatory blood pressure measurement.

Experiments were conducted to examine the accuracy of the conventional auscultatory method of blood pressure measurement. The influence of the physiologic state of the vascular system in the forearm distal to the site of Korotkoff sound recording and its impact on the precision of the measured blood pressure is discussed. The peripheral resistance in the arm distal to the cuff was changed noninvasively by heating and cooling effects and by induction of reactive hyperemia. All interventions were preceded by an investigation of their effect on central blood pressure to distinguish local effects from changes in central blood pressure. These interventions were sufficiently moderate to make their effect on central blood pressure, recorded in the other arm, statistically insignificant (i.e., changes in systolic [p < 0.3] and diastolic [p < 0.02]). Nevertheless, such alterations were found to modify the amplitude of the Korotkoff sound, which can manifest itself as an apparent change in arterial blood pressure that is readily discerned by the human ear. The increase in diastolic pressure for the cooling experiments was statistically significant (p < 0.001). Moreover, both measured systolic (p < 0.004) and diastolic (p < 0.001) pressure decreases during the reactive hyperemia experiments were statistically significant. The findings demonstrate that alteration in vascular state generates perplexing changes in blood pressure, hence confirming experimental observations by earlier investigators as well as predictions by our model studies.

Adolescent↗
PubMed1993

Repeated reflection of waves in the systemic arterial system.

Traditional analysis of pulse-wave propagation and reflection in the arterial system treats measured pressure and flow waves as the sum of a single forward wave (traveling away from the heart) and a single backward wave (traveling toward the heart). The purpose of this study was to develop a more general wave reflection theory that allows repeated reflection of these waves. The arterial system was modeled as a uniform viscoelastic tube terminating in a complex load with reflections occurring at the tube load interface and the heart tube interface. The resulting framework considers the forward wave to be the sum of an initial wave plus a series of antegrade waves. Similarly, the backward wave is the sum of a series of retrograde waves. This repeated reflection theory contains within it the traditional forward/backward wave reflection analysis as a special case. In addition, the individual antegrade and retrograde waves, at the tube entrance, are shown to be independent of the tube length. Aortic pressure and flow data, from dog experiments, were used to illustrate the phenomenon of repeated reflections. Alteration of the arterial system loading conditions, brought about through pharmacological intervention, affected the number and morphology of repeated waves. These results are compared with those found in traditional forward/backward reflection analysis.

Animals↗
PubMed1992

Right ventricular-pulmonary arterial interactions.

The application of pulsatile models to hemodynamic data has made possible a more complete understanding of the relationship of pulmonary pressure and flow. To review the genesis of these concepts, the unique characteristics of the pulmonary artery and right ventricle are outlined as a basis for understanding why differences in their pulsatile properties from the systemic circuit must exist. The pulmonary impedance spectrum is introduced and the concept of optimal right ventricular-pulmonary artery coupling is explored based on a review of extensive experimental data. Finally, available studies of normal pulmonary impedance in man and abnormal impedance in human disease states are reviewed, with emphasis on disturbances in optimal ventricular-vascular coupling. The important implications of these concepts for understanding and treatment of cardiovascular disease are developed.

Coronary Disease↗
PubMed1992

Cardiac adaptation of sarcomere dynamics to arterial load: a model of hypertrophy.

In the past, the dynamics of the left ventricle were studied by its response to altered venous and arterial load for a given heart. This led researchers to propose the concept of an arterioventricular match or optimal point of function. The model of this paper reverses that idea by fixing preload and afterload while computing cardiac function due to altered left ventricular size or shape, resulting from modification of the number of parallel and series sarcounits. A mathematical model of physiological hypertrophy is introduced. Series and parallel arrangements of sarcounits constitute a cylindrical model of the left ventricle. Filling occurs from a venous reservoir with constant pressure through a valve, while ejection takes place into a three-element model of the systemic arterial system through another valve. It is found that the dynamics of the myofibrils can be matched to those of the left ventricle by choosing a ventricular shape that results in a minimum in myocardial O2 consumption (MVO2) for any constant ventricular load. A unique solution for the size of the ventricle results if the rate of MVO2 is specified. The model is able to predict correctly hypertrophy due to hypoxia and due to pressure (concentric) and volume (eccentric) overloads.

Adaptation, Physiological↗
PubMed1991

Thresholds for premature ventricular contractions in frog hearts exposed to lithotripter fields.

Piezoelectrically generated lithotripter shocks were shown to produce premature ventricular contractions of the frog heart. Anesthetized grass frogs, Rana pipiens, were studied following implantation of an aortic catheter and EKG leads. The most sensitive phase of the heart cycle for the generation of premature ventricular contractions with lithotripter shocks at 30 MPa peak pressure was found to be the T-P segment. During this phase of the heart cycle, the minimum peak-positive pressure shock wave necessary to produce a premature ventricular contraction in a frog heart was between 5 MPa and 10 MPa.

Animals↗
PubMed1991

Similar pressure pulse propagation and reflection characteristics in aortas of mammals.

Similar pressure and flow waveforms recorded in mammalian aortas suggest that pulse transmission characteristics may also be similar. We examined the validity of this hypothesis, utilizing allometric equations of pertinent hemodynamic parameters and a model of the arterial system. Results show that both the reflection coefficient and the propagation constant times the aortic length are essentially invariant across the mammalian species investigated. Resolved forward and reflected propagating waves are also similar. These findings suggest that the arterial system in these mammals indeed functions in a similar manner.

Animals↗
PubMed1990

Effect of interfacial tension on flow of fluorochemicals in the vasculature of the lung: a theoretical and experimental study.

Perfluorocarbons can be selectively imaged using magnetic resonance. When introduced in the vasculature they do not flow beyond a certain level. This level depends on the driving pressure. We consider here such flow stoppage in the case of the vascular bed of rat lung. A theoretical analysis based on the assumption that interfacial tension is primarily responsible for this phenomenon leads to a formula that predicts a "critical" radius of the vessels at where the flow stops. This radius depends on the driving pressure. The predicted result was verified experimentally using direct measurements on histological sections and was found to confirm the hypothesis.

Animals↗
PubMed1990

Theoretical and experimental analysis of right ventricular bypass and univentricular circulatory support.

In this paper we examine the dynamic coupling between cardiac pump events and vascular arterial-venous factors that regulate the rate of blood flow around the circulation. A series of experiments were designed to test the feasibility of maintaining vascular and pulmonary function in the absence of the right heart and to characterize the physiologic and hemodynamic consequence of such an exclusion. Theoretical analysis of the cardiovascular system (excluding neuro-humoral factors) using both lumped time invariant and distributed compartmental mathematical equivalent representations, demonstrated that a change in cardiac output (Q) has an inverse-linear effect on venous and direct-linear effect on arterial pressure. A single blood-pump, in a form of a mechanical substitute or the biologic left-heart, alone can support the circulation. Cardiac output reserve is limited (50 percent of normal) because of the rapidly diminishing pulmonary venous-pressure as outflow is increased, irrespective of the pump's specific characteristics. Experiments in animals combined with mock-circulatory studies and computer modeling confirm that near normal flow can be sustained by increasing the stressed blood volume or reducing selectively the systemic venous compliance (i.e., inflatable pressure suit, venous constriction, intra-abdominal compression maneuvers, etc.). The right heart is not essential for normal pulmonary circulation but serves to maintain low systemic venous pressure and relatively high left-heart flow reserve. Purely mechanical properties of the vascular system determine the control and stability of the circulation.

Animals↗
PubMed1990

Model-based analysis of transmural vessel impedance and myocardial circulation dynamics.

The basic structure of a model of the coronary circulation has been developed to explain the relationship between transmural perfusion dynamics and intramyocardial mechanics. The model is in the form of a topologically isomorphic network representation and incorporates experimentally measured time-varying perfusion and intramyocardial pressure sources as driving inputs to the model. The intramyocardial vessels are treated as nonlinear impedance elements possessing regional external pressure-dependent resistance and capacitance. Three circuit branches, perfusing the epicardial, subepicardial, and subendocardial muscle layers, are mathematically modeled and are used to predict time-dependent flow within the left ventricular myocardium. The phasic coronary blood flow characteristics predicted by the model exhibit waveform patterns that correlate qualitatively with those patterns measured experimentally. In addition, the pressure-dependent vascular capacitance induces a sustained (out of phase with arterial inflow) venous systolic flow. The model also exhibits retrograde systolic subendocardial flow and stop-flow pressure, which are dependent on coronary resistive and capacitive properties and on the perfusion pressure decay time constant. Furthermore, the results predict an abrupt decrease in subendocardial flow with perturbation of either arteriolar or capillary bed compliance. The model describes time-dependent intramyocardial properties that have been confusing and controversial in the understanding of coronary circulation dynamics. Several steps are identified that are expected to improve and refine the model significantly.

Animals↗
PubMed1990

Estimation of total systemic arterial compliance in humans.

Systemic arterial compliance, a major component of aortic input impedance, was determined in 10 patients with congestive heart failure secondary to idiopathic dilated cardiomyopathy and 11 age-matched control subjects found free of detectable cardiovascular disease. Total arterial compliance was determined from high-fidelity ascending aortic pressure and velocity recordings using 1) the traditional monoexponential aortic diastolic pressure decay and 2) the direct solution of the equation, which describes the three-element windkessel model of the arterial system. Resting values for total arterial compliance (x10(-3) cm5/dyn) derived from method 1 were significantly correlated with compliance derived from method 2 (r = 0.89, P less than 0.01). However, method 1 values (control mean 1.15 +/- 0.27, heart failure mean 1.18 +/- 0.54) were consistently and significantly lower (P less than 0.001) than method 2 values (control mean 1.59 +/- 0.50, heart failure mean 1.38 +/- 0.60). Resting total arterial compliance in heart-failure patients was not significantly different from control subjects. Total arterial compliance did not significantly change with exercise in either group despite increases in arterial pressure. However, nitroprusside administration in the heart-failure group increased total arterial compliance both at rest and on exercise compared with the unmedicated state. These different methodological approaches to the estimation of total arterial compliance in humans resulted in significantly different absolute values for compliance, although both methods provided concordant results with respect to the response of arterial compliance to physiological and pharmacological interventions.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1989

The Korotkoff sound.

As the auscultatory method of blood pressure measurement relies fundamentally on the generation of the Korotkoff sound, identification of the responsible mechanisms has been of interest ever since the introduction of the method, around the turn of the century. In this article, a theory is proposed that identifies the cause of sound generation with the nonlinear properties of the pressure-flow relationship in, and of the volume compliance of the collapsible segment of brachial artery under the cuff. The rising portion of a normal incoming brachial pressure pulse is distorted due to these characteristics, and energy contained in the normal pulse is shifted to the audible range. The pressure transient produced is transmitted to the skin surface and stethoscope through deflection of the arterial wall. A mathematical model is formulated to represent the structures involved and to compute the Korotkoff sound. The model is able to predict quantitatively a range of features of the Korotkoff sound reported in the literature. Several earlier theories are summarized and evaluated.

Auscultation↗