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Biomedical subjects

S Sideman

Publications and source records attributed to S Sideman.

At least 19 recordsLinked to original sources
PubMed1999

Regulation of energy consumption in cardiac muscle: analysis of isometric contractions.

The well-known linear relationship between oxygen consumption and force-length area or the force-time integral is analyzed here for isometric contractions. The analysis, which is based on a biochemical model that couples calcium kinetics with cross-bridge cycling, indicates that the change in the number of force-generating cross bridges with the change in the sarcomere length depends on the force generated by the cross bridges. This positive-feedback phenomenon is consistent with our reported cooperativity mechanism, whereby the affinity of the troponin for calcium and, hence, cross-bridge recruitment depends on the number of force-generating cross bridges. Moreover, it is demonstrated that a model that does not include a feedback mechanism cannot describe the dependence of energy consumption on the loading conditions. The cooperativity mechanism, which has been shown to determine the force-length relationship and the related Frank-Starling law, is shown here to provide the basis for the regulation of energy consumption in the cardiac muscle.

Animals↗
PubMed1998

Thermographic imaging in the beating heart: a method for coronary flow estimation based on a heat transfer model.

Intraoperative thermographic imaging in open-chest conditions can provide the surgeon with important qualitative information regarding coronary flow by utilizing heat transfer analysis following injection of cold saline into the aortic root. The heat transfer model is based on the assumption that the epicardial temperature changes are mainly due to convection of heat by the blood flow, which may, therefore, be estimated by measuring the temperature variations. Hearts of eight dogs were exposed and imaged by a thermographic camera. Flow in the left arterial descending (LAD) coronary branch was measured by a transit-time flowmeter. 20 ml of cold saline were injected into the aortic root (just after the aortic valve) and the epicardial temperature images were recorded at end-diastole, for 20-30 s. Different flow rates were achieved by 1 min occlusion of the LAD, which affected a reactive hyperemic response. The dynamics of the temperature in the arterial coronary tree was obtained by averaging the temperature over an edge-detected arterial segment for each frame. The heat transfer equation was curve-fitted, and the flow-dependent heat transfer index was correlated with the experimentally determined coronary flow (r = 0.69, p < 0.001). In summary: a method for quantitative estimation of coronary blood flow by thermography and heat transfer analysis was developed and tested in animal experiments. This method can provide important information regarding coronary blood flow during open-chest surgical procedures.

Animals↗
PubMed1998

Estimation of coronary blood flow by ECG gated cardiac thermography in open-chest conditions.

Thermography is suggested as a tool to estimate myocardial and coronary epicardial flow in open-chest heart surgery. To test the feasibility and compare various methods for coronary flow estimation in open-chest surgery, thermographic imaging was applied to eight open-chest dogs which were injected with cold saline into the aortic root. Blood flow in the left arterial descending (LAD) coronary vessel was measured by a transit-time flowmeter. ECG gated images were acquired for 20-30 s, while the cold saline (20 ml) was injected into the aortic root. Several flow levels were achieved during repeated hyperaemic response to transient occlusions of the LAD. A temperature response curve for each flow level was obtained by averaging over an edge-detected arterial segment for each image frame. Several indices were calculated from the temperature curve and correlated with the measured coronary flow. These include: an index based on a corrective heat transfer model (r = 0.69, p < 0.001), the slope of the descending part of the response curve (r = 0.76, p < 0.001), the peak temperature difference (r = 0.66, p < 0.001), and the area above the temperature response curve (r = 0.61, p < 0.01). As shown, coronary flow can be estimated quantitatively by intraoperative epicardial thermography, and may therefore provide important on-line information regarding blood flow during open-chest surgical procedures. Further studies are required for optimal application of this technique so as to increase its potential as a valid clinical tool.

Animals↗
PubMed1997

Dynamic interaction between myocardial contraction and coronary flow.

Phasic coronary flow is determined by the dynamic interaction between central hemodynamics and myocardial and ventricular mechanics. Various models, including the waterfall, intramyocardial pump and myocardial structural models, have been proposed for the coronary circulation. Concepts such as intramyocardial pressure, local elastance and others have been proposed to help explain the coronary compression by the myocardium. Yet some questions remain unresolved, and a new model has recently been proposed, linking a muscle collagen fibrous model to a physiologically based coronary model, and accounting for transport of fluids across the capillaries and lymphatic flow between the interstitial space and the venous system. One of the unique features of this model is that the intramyocardial pressure (IMP) in the interstitial space is calculated from the balance of forces and fluid transport in the system, and is therefore dependent on the coronary pressure conditions, the myocardial function and the transport properties of the system. The model predicts a wide range of experimentally observed phenomena associated with coronary compression.

Animals↗
PubMed1997

Integrative and interactive studies of the cardiac system: deciphering the cardionome.

The cardiac system, denoted as the Cardionome, represents one of the most exciting challenges to human ingenuity. Critical to our survival, it consists of a tantalizing array of interacting phenomena, from ionic transport, membrane channels and receptors through cellular metabolism, energy production, fiber mechanics, microcirculation, and electrical activation to the clinically observed global functions. These are measured by pressure, volume, shape, coronary flow, heart rate, and other changes. It is a complex interactive system requiring the intense efforts of capable scientists in the life sciences, including medicine, exact sciences, engineering and biomedical technology devoted to address these multivariable, multidisciplinary challenges, so as understand and control the pathologies involved. Here we present some of our past interactive studies and highlight two new models, one demonstrating micro to macro integration, and one involving tissue-organ interaction of various parameters. These models yield new insights into cardiac performance.

Animals↗
PubMed1997

Assessing medical technology in less-developed countries.

Less developed countries (LDCs) are limited in medical resources. Medical technology and the management talent required to handle it play a particularly major role in their national health care and has significant economic, political, and ethical ramifications. This study of the assessment process of medical technology in the LDCs proposes a limited framework for the analysis of the major parameters involved, i.e., stakeholders, boundaries and constraints, goals and objectives, criteria to be met, performance measures, and measurement of performance. The importance of the intangible factors is elucidated.

Decision Making, Organizational↗
PubMed1997

Regional right ventricular endocardial motion in normal hearts and in hearts with left ventricular aneurysm: a three dimensional study.

We mapped the three dimensional (3D) regional right ventricular (RV) motion using Cine-CT in 9 normal subjects and compared it to data from 10 patients with left ventricular (LV) aneurysm. The endocardial borders were traced and the RV's reconstructed in 3D. Regional perpendicular RV systolic motion was evaluated by our 3D stroke-volume-element approach, and the circumferential and longitudinal variations determined. The normal RV is characterized by higher endocardial motion in the posterior relative to the anterior regions (p < 0.0001), and no longitudinal (apex-to-base) gradient. In hearts with LV aneurysms, similar circumferential variations in wall motion are accompanied with a longitudinal increase in systolic motion, from apex to the base (p < 0.0001). Therefore, a 3D method for measurement of RV regional motion was developed and applied to normal and LV aneurysm patients, showing that LV aneurysm causes RV motion abnormality at the apex, compensated by an increased basal motion.

Adult↗
PubMed1997

A targeting-and-extracting technique to enhance hearing in the presence of competing speech.

A targeting-and-extracting procedure of speech enhancement for hearing aids in the presence of background noise, especially competing speech, is proposed. The procedure is composed of two steps: targeting by a fixed (or deterministic) beamforming array, followed by a post-targeting extracting step. Emphasis is placed on the extracting step, which performs noise cancellation based on the acoustic difference between the desired speech and interfering speech. Either comb filtering or attenuation is applied to the signal in accordance with the current voiced/unvoiced/silence state of the desired signal. The comb filter design is based on the fundamental pitch frequency of the desired speech. Algorithms for deciding the voiced/unvoiced/silence state and determining the fundamental frequency are developed. The performance of the system is evaluated through computer simulation. The simulation results indicate significant noise cancellation and intelligibility improvement.

Amplifiers, Electronic↗
PubMed1996

Simulation of fixed microphone arrays for directional hearing aids.

Microphone arrays with fixed optimum weights are known to suppress the background noise and reverberation that severely reduce the effectiveness of conventional hearing aids. By means of a general technique for digital frequency-domain implementation of optimum broadband arrays that was developed recently [C. Liu and S. Sideman, J. Acoust. Soc. Am. 98, 241-247 (1995)], a practically promising system is proposed to realize the arrays with the well-known sensitivity-constrained superdirective beamforming weights, and with five identical omnidirectional, cardioid, hypercardioid, or dipole microphones, respectively, in the endfire or broadside configurations, which were theoretically proposed by Stadler and Rabinowitz [J. Acoust. Soc. Am. 94, 1332-1342 (1993)]. The digital broadband frequency-domain beamforming system allows the broadband superdirective beamforming weights to be faithfully and independently applied to each frequency component of the signal. The practical application of this technique is demonstrated through computer simulation of the system in anechoic situations. Furthermore, its performance in simulated reverberant environments is evaluated.

Computer Simulation↗
PubMed1996

Effect of cellular inhomogeneity on cardiac tissue mechanics based on intracellular control mechanisms.

Our earlier description of the intracellular control (IC) of contraction of a single cell, based on coupling calcium kinetics with cross-bridge cycling, is extended here to study the performance of a multicellular inhomogeneous tissue common in pathophysiological situations. Inhomogeneity in calcium affinity or in cross-bridge kinetics is first simulated by analyzing two fiber segments connected as parallel or serial duplexes. The calculated characteristics of the parallel duplex are tested against our experimental data with two parallel nonuniform rat papillary fibers. The predicted serial duplex behavior is compared with reported experimental data of the effects of segmental hypoxia along a papillary fiber. Fiber inhomogeneity leads to polyphasic contraction of the fiber segments, reduces muscle length shortening, and affects the control of relaxation. We next investigated the force generated by a nonuniform tissue containing small areas of necrosis, evident in subendocardial infarction. Theoretical analysis suggests that the IC mechanism decreases the extension of cell necrosis by lowering the energy consumption of the viable cells in the ischemic zone. The study emphasizes the importance of IC in determining the global and local function of the inhomogeneous myocardium.

Animals↗
PubMed1995

A model of Ca2+ release from the sarcoplasmic reticulum.

Various functions in the myocyte depend on Ca2+ transport, yet the control of these processes is still obscure. In order to better understand the intracellular Ca2+ processes, a model of Ca2+ release from the cardiac sarcoplasmic reticulum (SR) is suggested, in which the release of Ca2+ from the SR is mainly regulated by the kinetics of Ca2+ channels within the SR membrane. These kinetics are controlled by changes in the concentration of free Ca2+ near the openings of Ca2+ channels, and are affected by Ca2+ competitors, e.g., ryanodine. The control mechanism is based on a combination of positive and negative control loops, associated with two respective types of Ca2+ binding sites located on the SR membrane: 1) activating sites with low affinity to Ca2+ and high binding rate, and 2) inactivating sites with high affinity but low binding rate. The model also assumes that the activation of the Ca2+ channels depends on the preceding stimulation pattern (short term memory), an additional activation mechanism which is Ca2+ independent. This report describes the cytoplasmatic Ca2+ concentration in response to Ca2+ release from the SR, including the dependence on the beat intervals, either in the steady state or during response to premature and delayed beats. The analysis of ryanodine intervention supports a control mechanism based on two feedback loops, and available interval-dependent data favors inclusion of the short-term memory mechanism in the proposed model.

Animals↗
PubMed1995

Crossbridge dynamics in muscle contraction.

The study deals with the description of muscle contraction based on biochemical studies and describes four major approaches for coupling calcium kinetics with crossbridge (Xb) cycling. The analysis illuminates two controversial points: 1) the relationship between Xb attachment/detachment and Xb cycling, i.e., the transition between weak to strong conformations, and 2) the effect of calcium on Xb function: does it regulate Xb kinetics or Xb recruitment.

Animals↗
PubMed1995

Integration of structure, function and mass transport in the myocardium.

A left ventricular (LV) model that integrates muscle mechanics, coronary flow, and fluid transport, and accounts for the three-phase (fiber-blood-interstitium) myocardial structure and composition, is used to study the interactions between the mechanics, coronary flow and fluid and mass transport in the myocardium. Theoretical simulations elucidate the effects of ventricular load, coronary perfusion pressure, and fluid and mass transport on ventricular performance and coronary dynamics. The analysis yields a direct relation between cardiac function and structure to cardiac mechanics, coronary flow, and intramyocardial fluid (and mass) transport, and allows to study the interactions between coronary flow, ventricular and myocardial mechanics and intramyocardial fluid shifts.

Animals↗
PubMed1995

Modulation of the arterial coronary blood flow by asynchronous activation with ventricular pacing.

UNLABELLED: This study aims to test the assumptions that: (1) coronary arterial flow is attenuated in an early activated region by ventricular pacing; (2) asynchronous mechanical activation caused by ventricular pacing under controlled perfusion pressure and intact coronary tone is associated with reduced coronary flow compared to atrial pacing; and (3) abolishment of vascular tone under controlled perfusion pressure diminishes the expected difference in blood flow between atrial and ventricular pacing. Blood flow velocity (BFV) in the left anterior descending (LAD) and the left circumflex arteries (CFX) and a wall thickening index were measured in 14 open-chest dogs under normal conditions, and constant perfusion pressure. Four pacing sites were used: right atrium (RAp), mid-right ventricle (RVp), mid-left ventricle (LVp), and left ventricular apex (Apexp). Pacing modes were either sequential ventriculoatrial (VA) (protocol A, n = 7), or sequential atrioventricular (AV) (protocol B, n = 7), with a shorter AV difference (30 msec) than normal. RESULTS: BFV was decreased in the LAD during RVp and Apexp pacing by 9.7%-12.9% versus RAp and by 11.6%-14.6% versus LVp (P < 0.05). No BFV variations were observed in the CFX. Flow velocity conductance (FVC = mean blood flow velocity divided by the mean aortic pressure) was higher by 16%-28% in the CFX for the three ventricular pacing sites versus the atrial pacing, and higher by 14.1% +/- 6.1% only in LVp versus RAp pacing in the LAD (P < 0.05). Wall thickening index reduced during ventricular pacing in all three ventricular sites by 50%-64% (P < 0.05) compared to atrial pacing. Under constant perfusion pressure, LAD blood flow decreased with ventricular pacing as compared to right atrial pacing; this was particularly pronounced during the diastolic phase (16.6%-45.5%, P < 0.02). Normalized oscillatory flow amplitude (OFAn) was reduced in RVp pacing compared to RAp and LVp pacing (16.2 +/- 3.5 and 21.7% +/- 4.1%, respectively, P < 0.03). The variations in blood flow and OFAn disappeared with adenosine-mediated maximum vasodilatation. SUMMARY: (1) Mean and phasic flows are reduced in the early activated LAD region by ventricular pacing (RVp, Apexp). (2) Under controlled perfusion pressure and intact vascular tone, ventricular pacing compromises blood flow compared with atrial pacing. (3) This effect disappears when vascular tone is eliminated by intracoronary injection of adenosine, suggesting that the coronary autoregulation is responsible for some of the effects.

Animals↗
PubMed1995

An integrated model of LV muscle mechanics, coronary flow, and fluid and mass transport.

An integrated left ventricular (LV) model that accounts for the three-phase (fiber-blood-interstitium) myocardial structure and composition is used to study the interactions among myocardial mechanics, coronary flow, and fluid and mass transport. Effects of ventricular load, coronary perfusion pressure, and fluid and mass transport on ventricular performance and coronary dynamics are studied here. In agreement with experimental observations, the analysis shows that 1) coronary flow impediment is not significantly affected by changes in the afterload and preload at constant coronary perfusion pressures, 2) an increase in coronary perfusion pressure increases the intramyocardial pressure (IMP) as well as the mean flow and oscillatory flow amplitude, 3) contractility has a direct effect on IMP and coronary flow impediment, and 4) changes in blood osmolarity and lymphatic outflow, which may cause myocardial edema, affect both ventricular mechanics and coronary flow. Clearly, accounting for fluid and mass transport allows to study the interactions among coronary flow, ventricular and myocardial mechanics, and intramyocardial fluid shifts.

Animals↗
PubMed1994

Analysis of coronary circulation under ischaemic conditions.

Coronary flow patterns and pressure/flow relationships in coronary vessels with arterial stenoses are examined by using a model that combines the flow in the epicardial arterial tree with the intramyocardial perfusion. By using appropriate resistive elements, the model allows for the autoregulation of the vascular bed and for the development of coronary collaterals. Arterial flow predictions are compared to canine data. Coronary stenosis is simulated by a local pressure drop caused by a combination of viscous and inertial forces; stenosis with a constant cross-sectional area is compared to a dynamic stenosis in which the cross-sectional area is a function of the instantaneous transmural pressure. Simulation results predict that the normal phasic flow patterns in the epicardial arteries are unaffected up to 73% reduction in cross-sectional area, while the average flow remains unchanged up to 90% area reduction. At the critical level of 90% rigid stenosis, the autoregulation is saturated and the phasic nature of the arterial flow is severely damped. Dynamic stenoses demonstrate hysteresis loops of the instantaneous pressure/flow relationship. Theoretical predictions of local and global values are in excellent agreement with experimental measurements, indicating that the proposed approach can be used to realistically describe the coronary flow in the ischemic heart.

Animals↗
PubMed1994

Analysis of flow in coronary epicardial arterial tree and intramyocardial circulation.

A mathematical model combining the coronary flow in the epicardial arterial tree and the intramyocardial circulation is presented. The epicardial arterial tree is represented by a resistive capacitive network based on its realistic anatomy. The intramyocardial flow is affected by the pump action of the contracting myocardium through the extravascular compressive pressure (ECP), which, in turn, affects the dynamic resistance and compliance changes based on the relationship between the transmural pressure and the cross-sectional area of a vessel. The model accounts for the autoregulatory mechanism of the intramyocardial compartments (arteriolar, microvascular and venular) and is structured according to the epicardial coronary anatomy. Realistic coronary epicardial arterial flow patterns are obtained, which compare well to experimentally measured data in six dogs under basal conditions and during reactive hyperemic response. Simulations of the average transmural flow in the three intramyocardial vascular compartments show that the flow in the arterial side is predominantly diastolic, with a systolic retrograde component, and is dominantly systolic antegrade flow in the venular side, consistent with experimental data. Interestingly, the transmurally average microcirculatory flow is continuous, with very small change throughout the cardiac cycle, and is practically insensitive to changes in the model parameters. The model presents a quantitative tool that describes the dynamic patterns of coronary flow in relationship to muscular and extravascular parameters.

Animals↗