The effect of supine rotation on left ventricular dimensions in man: A study using radio-opaque epicardial markers

It has been reported that patient rotation into the left lateral decubitus position (30° rao) produces significant changes in the regression equations used for left ventricular volume determination and that normal values for echocardiographic left ventricular dimensions obtained from supine patients differ from those obtained after rotation. The purpose of this study was to establish whether patient rotation is associated with changes in left ventricular size and systolic function. The distances between left ventricular epicardial markers attached at the time of cardiac surgery were measured using biplane cineradiography in 14 patients in order to determine left ventricular dimensions before and after rotation. Supine epicardial left ventricular dimensions were not significantly different from those obtained with the patient rotated 30° rao. These results suggest that reported changes in invasive and noninvasive measurements of left ventricular function associated with patient rotation are not primarily due to changes in left ventricular size.     

Potential Improvement in Shelf Life Using the Prodrug Approach. II. A Systematic Examination of Kinetic Requirements

The utilization time (UT) for a solution of a prodrug that is rapidly and completely converted to drug in the blood may be longer than the time for 10% loss of the initial concentration. The UT for an intravenous prodrug solution is the period during which the total prodrug and drug concentration exceeds 90% of the initial concentration. The influence of the rate of prodrug degradation (k _nc), its conversion (k _c) to drug, and the subsequent drug degradation (k _h) on the UT of a stored solution was examined by simulating the prodrug and drug concentration–time courses. The ratio of the shelf life of a prodrug solution to that of the parent drug (UT_ratio) was calculated using a wide range of values for the three rate constants. Three-dimensional plots relating the UT_ratio to the k _c, k _nc, and k _h values provide a basis for making a priori assessments of kinetic requirements for designing a prodrug to increase storage time. A parenteral prodrug intended to increase storage time may have a larger overall rate of loss than the parent drug, but it must have a smaller degradation rate (k _nc < k _h) to be successful. The UT for an oral prodrug solution depends upon the bioavailability of the prodrug relative to the drug in addition to the values for k_nc, k _c, and k _h. Two ampicillin prodrugs were used as models to calculate actual UT_ratio versus pH profiles. Intravenous solutions showed modest gains in the UT_ratio in the acid region, whereas oral solutions reached a UT_ratio as high as 22 by combining favorable rate constants with increased bioavailability. These actual UT_ratio versus pH profiles were interpreted in terms of the theory established using the simulations.     

A computer model for simulating ultrasonic scattering in biological tissues with high scatterer concentration

Scattering of ultrasonic waves by biological tissues at different scatterer concentrations is investigated using one- and two-dimensional computer simulation models. The backscattered power as a function of scatterer concentrations is calculated using two types of incident waves, a Gaussian shaped pulse and a continuous wave (CW). The simulation results are in good agreement with the Percus-Yevick packing theory within the scatterer concentrations, from 0% to 100% in one-dimensional (1D) space, and 0% to 46% in two-dimensional (2D) space. In all cases, the simulation results from a pulsed incident wave show a much smaller standard deviation (SD) than those from an incident CW. The simulation can serve as a useful tool to verify scattering theories, simulate different experimental conditions, and to investigate the interaction between the scatterer properties and the scattering of ultrasonic waves. More importantly, the 2D) simulation procedure serves as an initial step toward the final realization of a true three-dimensional (3D) simulation of ultrasonic scattering in biological tissues.     

Magnetization-prepared MR angiography with fat suppression and venous saturation.

Magnetization-prepared magnetic resonance (MR) angiography (MPMRA) is an inflow-based two-dimensional (2D) imaging sequence in which a preparation phase precedes rapid image acquisition. For maximal blood/tissue contrast, an inversion-recovery preparation nulls signal from static tissue. If needed, a second inversion suppresses signal from fat. Fully magnetized blood flows in after the inversion pulse(s), providing high signal intensity. The centric phase-encoding order, which ensures that the initial contrast is reflected in the image set, requires the use of a modified venous saturation technique. The sequence is described and its performance assessed with regard to (a) depiction of in-plane flow, (b) fat suppression, and (c) venous saturation. Phantom and volunteer studies showed good performance in all three areas. MPMRA images, acquired in just 2-4 seconds per image, had a blood/tissue contrast-to-noise ratio nearly twice that of standard 2D time-of-flight MR angiograms, acquired in 5-7 seconds. The technique is promising for restless patients and in anatomic areas plagued by motion degradation.     

A microcomputer oxygen control system for ventilatory therapy

A computer-based feedback system has been developed to adjust the concentration of oxygen in the inspired gas of a patient under artificial respiration. The system uses a proportional plus integral controller and feedback of arterial oxygen saturation to adjust the inspired oxygen fraction. The effectiveness of the controller has been tested using a dynamic, mathematical model of the human respiratory system. This relatively sophisticated model has been developed and examined in the past, and it has been shown that it can realistically describe the human respiratory system for a wide variety of test conditions. The performance of the oxygen control system has been evaluated using the simulation model. The response of the controller to different disturbances is always stable, with arterial pressure of oxygen returning to normal in less than 12 minutes. Some of the simulation results are presented to illustrate the dynamic behavior and robustness of the controller.     

An integrative approach to intracranial hydraulic physiology. I. Basic concepts, pressure-volume relationships, and infusion studies

A computer simulation of the anatomy of the intradural fluid compartment is used to explore intracranial hydraulic physiology. It examines the mutual interactions of fluid flow rates, volumes, and pressures that control the physiology of the intracranial fluid. The mechanisms of change in intracranial pressure despite the fixed volume constraint are elucidated. The physical origins of the pressure-volume curve and data on the infusion of cerebrospinal fluid are examined. The dynamic behavior of this passive system closely simulates the observed physiologic response to bolus infusion, constant rate infusion, hypercapnia- hypocapnea , and hydrocephalus.     

Twitcher: A Device to Simulate Thumb Twitch Response to Ulnar Nerve Stimulation

Objective. To design and fabricate a device to simulate evoked thumb adduction in response to ulnar nerve stimulation. Methods. We implemented a computer-controlled, motorized thumb (TWITCHER) that responds to ulnar nerve stimulation by an unmodified peripheral nerve stimulator. Clinically realistic response patterns are generated for both depolarizing and non-depolarizing muscle relaxants and three modes of stimulation (single twitch, train-of-four, tetanus). Results. The device has been used in a full-scale patient simulator for the last six years. Discussion. TWITCHER has been well received by participants in simulation exercises including the use of neuromuscular blocking drugs.     

Cardiology patient simulator and computer-assisted instruction technologies in bedside teaching

The design and uses of an innovative technology-based approach which addresses critical problems with bedside teaching during ward rounds in the current health care situation are described. A cardiology patient simulator (HARVEY) and an accompanying computer-based interactive laserdisk system provide medical students, house officers and other health professionals with the opportunity to encounter a wide variety of clinical problems for learning and assessment. A group of cardiologists and medical educators from a consortium of US medical schools has guided the development and formal assessment of the entire system over the past 10 years. The system and simulator can be configured and programmed to provide appropriate heart sounds, laboratory data, and test results upon request; hands-on experience to practise examination skills is also available. The system, whole or in part, can be used in a variety of instructional modes from self-instruction to demonstration teaching, and from self-assessment to the final clinical examination. The system's effectiveness for teaching medical clerks is summarized, as is its use in providing continuing education for primary care doctors in rural practice.     

脉搏波与生理病理变化关系的仿真研究

为探讨脉搏波中所含生理病理信息的机理，采用一个由１２０个数字计算单元组成的人体心血管系统血液动力学分布式仿真模型，对血液粘度和血管弹性的变化所致桡动脉脉搏波的发迹以及心力衰竭、高血压、动脉硬化３种病状所致脉搏波的改变趋势进行了数字仿真研究，取得了初步煌结果。     

流体化学位的MonteCarlo模拟

采用耦合位能函数伞形法解决了流体化学位模拟中Ｗｉｄｏｍ试验粒子法的高密度困难。新方法的实现相当容易，而且同样适用于ＭＤ模拟。通过对Ｌｅｎｎａｒｄ－Ｊｏｎｅｓ流体的模拟，表明这一方法比原始的Ｗｉｎｄｏｗ试验粒子法具有更高的计算效率，其结果更加可靠。