Cardiac assist by intrathoracic and abdominal pressure variations: a mathematical study.
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Abdomen↗
Biomedical subjects
R Beyar
Publications and source records attributed to R Beyar.
Model for left ventricular contraction combining the force length velocity relationship with the time varying elastance theory.
A model for the contraction of the left ventricle (LV) is developed for a spheroidal geometry. The classical force-length-velocity relationship for a single muscle fiber is assumed. The linear maximum pressure volume relationship (maximum elastance), a measure of muscle contractility, is further extended into a time-varying function. This is achieved by utilizing a mechanical activation function, assumed as half a sinusoidal wave, to describe the time-dependent isometric stress for the activated cardiac muscle. This, in turn, results in the time-varying elastance function and represents the instantaneous activity of the muscle contractile proteins. The model is tested for a set of boundary conditions that determine preload, afterload, and the inherent properties of the muscle, i.e., the contractility. The computed results of the isovolumic contraction, auxotonic contraction, and isovolumic relaxation are in agreement with the expected behavior of the LV. The relations between the simulated variations on preload, afterload, and contractility, and the set of performance indexes of the LV, are presented and discussed.
Animals↗
Heart-sound processing by average and variance calculation--physiologic basic and clinical implications.
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Analysis of Variance↗
Jaundice, the circulation and the kidney.
(Fig. 1) Cholemia per se (i.e. independent of parenchymal liver damage) causes a profound disturbance of systemic hemodynamics. This includes decrease in total peripheral vascular resistance, and possible impairment of left ventricular performance. These, in turn, lead to a decrease in effective blood volume, a tendency to hemorrhagic shock and prerenal failure. Early in the course of cholemia , the natriuretic effects of bile salts in the circulation may aggravate the hypovolemia. In marked contrast to the decrease in total peripheral vascular resistance, the regional vascular beds of the kidney and the brain constrict during cholemia . The combined effect of cholemia may thus lead to redistribution of cardiac output away from the kidney and the brain. When parenchymal liver damage complicates obstructive jaundice, the tendency to arterial hypotension is aggravated. The overall interrelationship between jaundice and circulatory homeostasis is depicted in figure 1.
Animals↗
A computer study of the left ventricular performance based on fiber structure, sarcomere dynamics, and transmural electrical propagation velocity.
A model of the left ventricle which combines a spheroidal geometry with a spatial fiber angle distribution is presented. The mechanics of each muscle fiber is described by its passive stress-strain relationship, active stress-strain relationship, and an activation function (half a sinusoid) which represents the time-dependent degree of activation of the fiber. A stress-strain rate relationship which characterizes the muscle fibers is used to calculate the mechanics of left ventricular contraction during ejection. Furthermore, a radial electrical signal propagation from the endocardium to the epicardium is used here as a first approximation to the actual depolarization sequence. The model is used to describe the process of contraction throughout the systole. The different calculated parameters and indices of left ventricular function are presented and discussed for different preloading, afterloading and contractility conditions. The maximum elastance is found to be an optimal macroscale parameter of contractility, as it is completely preload and afterload independent, and is a good reflection of the active microscale sarcomere stress-strain relationship.
Computers↗
Cardiopulmonary resuscitation by intrathoracic pressure variations--in vivo studies and computer simulation.
The effect of intrathoracic pressure variations on the hemodynamics of dogs with cardiac arrest were studied experimentally and simulated on a computer. High intrathoracic pressure (up to 90 mm Hg) was generated by lung inflation with passive and active modes of external fixation. Abdominal binding was found to be essential for the generation of high intrathoracic pressure. Remarkable Doppler flow signals were detected over the femoral artery with each lung inflation. Blood gases measured after 30 minutes of cardiac fibrillation in dogs together with intrathoracic pressure variations showed well oxygenated arterial blood with metabolic acidosis. A computer model was used to explore the effects of intrathoracic pressure variations over a large range of parameters. For intrathoracic pressure of 50/0 mm Hg. the mathematical model predicted maximal flow of 663 ml/min, occurring at a rate of 115 cpm, with a duty cycle of 58%. The heart showed only minor volume changes during the cycle, indicating its main function as a passive conduit during cardiopulmonary resuscitation. The data show that intrathoracic pressure variation with no direct heart compression can cause systemic blood flow of the magnitude occurring in most cardiopulmonary resuscitation techniques.
Animals↗
Noninvasive cardiac assist by extension of phase D compression cardiopulmonary resuscitation (PCCPR).
The PCCPR technique was experimentally and theoretically extended to serve as a cardiac assist procedure, and the potential of this method as a cardiac assist device was studied. Preliminary studies in dogs verify the applicability of this technique.
Animals↗
Characterization of regional left ventricular contraction by curvature difference analysis.
A method which characterizes the contraction of the left ventricle (LV) by changes in the LV endocardial contour curvatures is presented. A normalized curvature difference function (NCDF) is defined by the difference between the (normalized) curvature functions of end diastolic (ED) and end systolic (ES) contours. Unlike wall-motion based procedures, NCDF is independent of any reference system and of the method used for ED-ES shape alignment. Normal and pathological diagnosis criteria were first established based on right anterior oblique (RAO) projection ventriculograms of a study group which included 58 normal and 28 abnormal patients. Patients with an infarcted myocardial region differed from the characteristic NCDF pattern of normals and exhibited regionally defined irregularities. The diagnosis criteria were then applied to a total of 159 patients in two groups in two independent laboratories. One group (in Israel) included 49 cases (20 normals, 29 abnormals); the second (in France) included 108 cases (48 normals, 60 abnormals). These two groups yielded similar sensitivity (97% and 97%) and specificity (90% and 100%) in detection of abnormality of the ventricle. When tested against other quantitative wall motion techniques, the NCDF shwos a regional sensitivity of 95%, indicating that curvature difference analysis is a potential tool for the automatic and objective diagnosis of regional LV function abnormalities.
Heart↗
Blood gas and acid-base balance during cardiopulmonary resuscitation by intrathoracic and abdominal pressure variations.
The blood gases and acid-base balance in a modified cardiopulmonary resuscitation (CPR) technique, based on intrathoracic and abdominal pressure variations by means of circumferential chest and abdominal balloon inflation, were examined in seven mongrel dogs. CPR proceeded for periods lasting 30 min or more and was monitored by measurements of aortic and right ventricular pressures and carotid blood flow during the compression (artificial systole) and the relaxation phase (artificial diastole). The carotid blood flow was 21.7 +/- 7.8 (mean +/- SD) ml/min, which was 0.18 +/- 0.6 (mean +/- SD) of the baseline mean carotid flow. Arterial blood was well oxygenated throughout the experiments, and low PCO2 levels (5-9 mm Hg) caused an initial severe alkalosis (pH = 7.94). However, a gradual decline in the pH was observed, reaching a value of 7.34 +/- 0.11 in the arterial blood after 30 min of CPR. The venous blood had a very low oxygen content (less than 25.5%) with a low PO2 and a normal PCO2 (43.7 +/- 7.3 mm Hg) throughout the experiment. A gradually developing metabolic acidosis was reflected in the pH values, and an increase in base deficit from 2.25 +/- 5.6 meq/1 prior to CPR to 16.7 +/- 3.2 meq/1 after 30 min of CPR was observed. High arteriovenous differences in oxygen content (greater than 66.4%) and CO2 tension (30.1-41.5 mm Hg) with a slowly developing metabolic acidosis were noted. Thus, CPR by thoracic and abdominal pressure variations is associated with a slowly developing metabolic acidosis which is the result of the combination of hyperventilation and a low perfusion state.
Abdomen↗
Dynamic analysis of left-ventricular shape based on curvature function.
The local curvature function is defined as the change in curvature around the circumference of the LV silhouette. The local instantaneous curvature function is used here to quantify regional left-ventricular (LV) performance throughout the cardiac cycle. Left ventriculography images, taken in the right anterior oblique (RAO) view from nine patients with normal ventricular contraction, and eight patients with anterior hypokinesis (AHK) are used. The local curvature around the circumference of the LV is calculated for each heart throughout the ejection period. The dynamic increase in the curvature of the apex, defined as apical sharpening, is a typical feature of LV contraction. Apical sharpening from end-diastole to end-systole is closely related to the degree of hypokinesis. Normal hearts show larger apical sharpening (128 +/- 57%, SD) than do AHK hearts (46 +/- 13%, p = 0.002). The ratio between apical and anterior curvatures at ES has been found to be 7 +/- 3.5 for normal hearts and 2.3 +/- 0.6 for AHK hearts (p = 0.003). Linear regression between the ventricular volume and apical curvature yields a significant relationship for the normal hearts (r = 0.82 +/- 0.06, average p = 0.07), but not for the AHK hearts (r = 0.72 +/- 0.2, average p = 0.24). Thus, the information inherent in the local curvature of the LV and its dynamic change throughout the cardiac cycle may be used to distinguish between normal and anterior hypokinetic hearts.
Cardiomyopathies↗
Evaluation of regional load in acute ischemia by three-dimensional curvatures analysis of the left ventricle.
Geometric remodeling of the left ventricule (LV) following myocardial infarction and ischemic insult is associated with myocardial load redistribution. Regional curvatures based on 3-D reconstructions of the LV are used to calculate the regional loads. The technique uses surface normals to derive local circumferential and meridional curvatures. Following the validation of the procedure on simple geometric shapes, the effect of acute ischemia on the regional load redistribution was studied in six open chest dogs. Short axis magnetic resonance imaging (MRI) scans were used to reconstruct end-diastolic (ED) and end-systolic (ES) LV images by applying our helical shape descriptor, before and after acute coronary occlusion. Regional curvatures as well as local wall thickness by the volume element method were calculated before and after acute ischemia, and were used to approximate regional loads, by a regional stress index (sigma/P). Postmortem evaluation using monastral blue staining was used to divide each LV to normal (NZ), ischemic (IZ), and border (BZ) zones in the ischemic case, and to the anatomically matched regions in the preischemic LVs. Ischemia affects the local curvatures and loads both at ED and ES. At ED, sigma/P rose significantly only in the IZ. Similarly, at ES, the highest increase in load was detected in the IZ, but increases in circumferential and meridional load were seen in all regions. Identifying the load redistribution following acute ischemia helps delineate the mechanisms affecting geometric LV remodeling following myocardial infarction.
Animals↗
Effects of myocardial contraction on coronary blood flow: an integrated model.
The effects of myocardial contraction on the coronary flow are studied by means of an integrated structural model of left ventricular (LV) mechanics, coronary flow, and fluid and mass transport. This model relates global LV performance, and in particular coronary flow dynamics, to myocardial composition and structure and contractile sarcomere activity. Extravascular pressure is identified with hydrostatic tissue pressure, i.e., intramyocardial pressure (IMP), and is determined by the dynamics of myocardial contraction and fluid transport. Consistent with available experimental data, changes in myocardial function and contractile state are simulated by changing the sarcomere contractile properties or changing the LV loading conditions. The model's predictions are successfully compared with a wide range of experimental studies; all but one were performed at a constant coronary perfusion pressure and maximal vasodilation. The results indicate a dominant effect of the myocardial contractile state on coronary flow and a dissociation between coronary compression and LV cavity pressure (LVP) when the pressure is controlled by load changes. However, when active sarcomere contraction is regionally impaired by lidocaine, LVP plays an important role in the coronary flow characteristics. The model adequately predicts observations on the effect of cardiac contraction on systolic and diastolic coronary flows, as well as the role of LVP at different loading and contractile conditions. The analysis supports the hypothesis that coronary compression, as mediated through IMP, is independent of LV loading conditions and depends on myocardial contractility and coronary perfusion pressure.
Biomechanical Phenomena↗
Coronary flow patterns in normal and ischemic hearts: transmyocardial and artery to vein distribution.
The dynamics of the transmyocardial coronary flow patterns during normal and ischemic conditions are complex and relatively inaccessible to measurements. Therefore, theoretical analyses are needed to help in understanding these phenomena. The proposed model employs compartmental division to three layers, each with four vessel-size compartments which are characterized by resistance and compliance. These compartments are subjected to the extravascular compressive pressure (ECP) generated by cardiac contraction, which by modifying the transmural pressure causes changes in cross-sectional area of the vessels in each compartment continuously determining the resistance and capacitance values. Autoregulation and collaterals are also included in order to simulate the flow patterns during regional ischemia. Using these features, the model predicts the typical out of phase arterial and venous flow patterns. Systolic collapse of the large intramyocardial veins during the normal cycle, as well as systolic arteriolar collapse during ischemia are predicted. The transmural flow during ischemia is characterized by alternating flows between the layers. The ECP is considered here is two ways: (a) as a function of left ventricle (LV) pressure, decreasing linearly from endocardium to epicardium and (b) as the interstitial fluid pressure, employing a multilayer muscle-collagen model of the LV. While both of these approaches can describe the dynamics of coronary flow under normal conditions, only the second approach predicts the large compressive effects due to high ECP obtained at very low cavity pressure, resulting from significant muscle shortening and radial collagen stretch. This approach, combining a detailed description of transmural coronary circulation interacting with the contracting myocardium agrees with many observations on the dynamics of coronary flow and suggests that the type of LV mechanical model is important for that interaction.
Animals↗
Regional three-dimensional geometry of the normal human left ventricle using cine computed tomography.
The aim of this study is to provide accurate three-dimensional measurements of left ventricular geometrical indices in relation to regional myocardial function. The analysis of the three-dimensional regional geometry and function of left ventricles of ten normal human volunteers is based on three-dimensional reconstructions of the left ventricle from cine computed tomography images, at end diastole and end systole, demonstrating normal left ventricular spatial, geometrical, and functional variability. Regional wall thickness, curvature and surface normals, as well as wall thickening and endocardial wall motion, are calculated and mapped for the entire left ventricle. The circumferential asymmetry of the left ventricle is reflected by the smaller circumferential and meridional curvatures at the septum. Thickening is highest at the anterior and lateral walls. Longitudinally, circumferential curvature increases toward the apex, whereas both wall thickness and wall thickening at end systole are largest at the midventricular level, decreasing toward the apex and base. This study describes the circumferential and apex-to-base variations in regional left ventricular geometric parameters of the normal human left ventricle, using three-dimensional imaging and analysis.
Adult↗
Digital subtraction myocardial contrast echocardiography: design and application of a new analysis program for myocardial perfusion imaging.
Myocardial contrast echocardiography may provide important physiologic information on myocardial perfusion. Most current analysis programs use manual frame grabbing and selecting of the area of interest. This is time-consuming and not highly reproducible. A system for automatic analysis of myocardial contrast echocardiographic studies was developed and evaluated. The program acquires an electrocardiographically gated sequence of end-diastolic images with a frame grabber in a personal computer. The baseline image is subtracted and the videodensity versus time contrast curve parameters are calculated on-line. Fast color-coded analysis is done automatically with a running square window that covers the entire image. A second mode of contrast analysis allows manual selection of multiple regions of interest. The program was evaluated with contrast echo data from open-chest dogs and two demonstrative patients. This myocardial contrast analytic package is an inexpensive, rapid, flexible, convenient, and reproducible on-line method that facilitates myocardial contrast echocardiographic analysis.
Albumins↗
Septal/free wall curvature ratio: a noninvasive index of pulmonary arterial pressure.
The leftward septal shift, a well-recognized feature of pulmonary hypertension, was used to quantify right ventricular pressure in 16 patients with pulmonary hypertension and 11 control patients, all with normal left ventricular function. Pulmonary pressure was calculated from the tricuspid regurgitation jet and left ventricular pressure was taken by arm cuff measurements. Short-axis echocardiographic images were obtained and the midwall curvatures of the septum and the left ventricular free wall were measured for each frame from end diastole to end systole and averaged. The septal/free-wall curvature ratio (CR) was 0.37 +/- 0.19 in the study group compared with 0.79 +/- 0.06 in the control group (p < 0.0001). A tight relationship between the CR and the transseptal/transmural pressure ratio (CR = 0.057 + 0.89 x transseptal/transmural pressure ratio; r = 0.98; p < 0.001) was obtained by linear regression. Given the systolic arterial pressure, the pulmonary systolic pressure is given by: systolic arterial pressure x (1.064-1.12 x CR). Therefore the CR can be used as a noninvasive index that reflects the level of pulmonary pressure in relationship to the systolic arterial pressure.
Blood Pressure↗
Dynamic thermography: analysis of hand temperature during exercise.
Exercise has a noted effect on skin blood flow and temperature. We aimed to characterize the normal skin temperature response to exercise by thermographic imaging. A study was conducted on ten healthy and active subjects (age=25.8+/-0.7 years) who were exposed to graded exercise for determination of maximal oxygen consumption (VO2 max), and subsequently to constant loads corresponding to 50%, 70%, and 90% of VO2 max. The skin temperature response during 20 min of constant load exercise is characterized by an initial descending limb, an ascending limb and a quasi-steady-state period. For 50% VO2 max, the temperature decrease rate was - 0.0075+/-0.001 degrees C/s during a time interval of 390+/-47 s and the temperature increase rate was 0.0055+/-0.0031 degrees C/s during a time interval of 484+/-99 s. The level of load did not influence the temperature decrease and increase rates. In contrast, during graded load exercise, a continuous temperature decrease of -0.0049+/-0.0032 degrees C/s was observed throughout the test. In summary, the thermographic skin response to exercise is characterized by a specific pattern which reflects the dynamic balance between hemodynamic and thermoregulatory processes.
Adult↗