Monitoring of the central blood pressure waveform via a conformal ultrasonic device.

Continuous monitoring of the central-blood-pressure waveform from deeply embedded vessels, such as the carotid artery and jugular vein, has clinical value for the prediction of all-cause cardiovascular mortality. However, existing non-invasive approaches, including photoplethysmography and tonometry, only enable access to the superficial peripheral vasculature. Although current ultrasonic technologies allow non-invasive deep-tissue observation, unstable coupling with the tissue surface resulting from the bulkiness and rigidity of conventional ultrasound probes introduces usability constraints. Here, we describe the design and operation of an ultrasonic device that is conformal to the skin and capable of capturing blood-pressure waveforms at deeply embedded arterial and venous sites. The wearable device is ultrathin (240 μm) and stretchable (with strains up to 60%), and enables the non-invasive, continuous and accurate monitoring of cardiovascular events from multiple body locations, which should facilitate its use in a variety of clinical environments

Contemporary Review of Hemodynamic Monitoring in the Critical Care Setting

Hemodynamic assessment remains the most valuable adjunct to physical examination and laboratory assessment in the diagnosis and management of shock. Through the years, multiple modalities to measure and trend hemodynamic indices have evolved with varying degrees of invasiveness. Pulmonary artery catheter (PAC) has long been considered the gold standard of hemodynamic assessment in critically ill patients and in recent years has been shown to improve clinical outcomes among patients in cardiogenic shock. The invasive nature of PAC is often cited as its major limitation and has encouraged development of less invasive technologies. In this review, the authors summarize the literature on the mechanism and validation of several minimally invasive and noninvasive modalities available in the contemporary intensive care unit. They also provide an update on the use of focused bedside echocardiography

Noninvasive Assessment of Preclinical Atherosclerosis

Initially considered as a semipermeable barrier separating lumen from vessel wall, the endothelium is now recognised as a complex endocrine organ responsible for a variety of physiological processes vital for vascular homeostasis. These include the regulation of vascular tone, luminal diameter, and blood flow; hemostasis and thrombolysis; platelet and leucocyte vessel-wall interactions; the regulation of vascular permeability; and tissue growth and remodelling. The endothelium modulates arterial stiffness, which precedes overt atherosclerosis and is an independent predictor of cardiovascular events. Unsurprisingly, dysfunction of the endothelium may be considered as an early and potentially reversible step in the process of atherogenesis and numerous methods have been developed to assess endothelial status and large artery stiffness. Methodology includes flow-mediated dilatation of the brachial artery, assessment of coronary flow reserve, carotid intimamedia thickness, pulse wave analysis, pulse wave velocity, and plethysmography. This review outlines the various modalities, indications, and limitations of available methods to assess arterial dysfunction and vascular risk

Pressure and Flow Wave Propagation in Patient-Specific Models of the Arterial Tree

Blood flow in the arterial circulation induces hemodynamic forces that play an important role in various forms of vascular diseases. Temporal variation of the wall shear stress seems to play a significant role in atherogenesis and plaque stability. Flow induced wall shear stress has been linked to growth and possibly rupture of the aneurysm wall. Hemodynamic forces are patient-specific and difficult to assess in the clinic. At present, there is no in vivo measurement technique that enables measurement of hemodynamic forces to the degree of precision needed. However, when imaging modalities used frequently in clinical routine re-create high-definition, patient geometric quantification of the blood vessel, they can be employed as a base for creating predictive hemodynamic models. Which in the case of understanding healthy vs. pathologic blood flow within the cerebral or systemic circulation, renders this an interesting approach. First, we developed a "generic 1D" distributed model of the human arterial tree including the primary systemic arteries and coupled this to a heart model. The fluid mechanics equations were solved numerically to obtain pressure and flow throughout the arterial tree. A nonlinear viscoelastic constitutive law for the arterial wall was considered while distal vessels were terminated with a three-element Windkessel model. The coronary arteries were modeled assuming a systolic flow impediment proportional to ventricular varying elastance. The model predictions were validated with noninvasive measurements of pressure and flow performed in young volunteers. Flow in the large arteries was visualized with magnetic resonance imaging, cerebral flow detected with ultrasound Doppler and blood pressure measured with applanation tonometry. Model predictions at different arterial locations compared well to measured flow and pressure waves at the same anatomical points. Thus, the generic 1D model reflected the flow and pressure measurements of the "average subject" of our volunteer population. Following the same approach as the generic 1D model, we built and validated a patient-specific model. In this case, geometric data, flow and pressure measurements were obtained for one person. The model predicted pressure and flow waveforms in good agreement with the in-vivo measurements with regards to wave shape and features. Comparison with a generic 1-D model has shown that the patient-specific model better predicted pressure and flow at specific arterial sites. Overall, the patient-specific 1-D model was able to predict pressure and flow waveforms in the main systemic circulation, whereas this was not always the case for a generic 1-D model. The inherent underestimation of energy losses of the 1-D wave propagation model, due to bifurcations, non-planarity and complex geometry, were examined. The 1-D model was compared to a rigid wall 3-D computational fluid dynamic model. Newtonian and non-Newtonian blood properties were studied and the longitudinal pressure distribution along the arteries was compared with the 1-D patient-specific model mean pressure prediction. The results indicated that pressure drop is significant only in small diameter vessels such as the precerebral and cerebral arteries. In these vessels the 1-D model in comparison to 3-D models consistently underestimated pressure drop. The complex flow patterns resulting from asymmetry and bifurcation yield shear stresses in the 3-D model that were greater than the 1-D model. A 3-D unsteady fluid structure interaction simulation in a patient-specific model was performed to simultaneously capture the flow details, given by the 3-D model, and wave propagation phenomena, provided by the 1-D model. The 3-D unsteady fluid structure interaction approach is the most computationally intense and cumbersome, but it allows physiological simulations with a high level of detail and accuracy. For instance, this approach could be relevant to obtain blood flow details in regions that are prone to atherosclerotic plaques or development of aneurysms. The 3-D fluid structure interaction simulation was performed for a patient-specific aorta. Important clinical parameters such as wall shear stress were quantified and significant differences were found in comparison to the rigid wall 3-D simulation indicating the relevance of a fluid structure interaction approach. A comparison of the fluid structure interaction to an equivalent 1-D model resulted in good reproduction of the pressure and flow waveforms. The effect of a decreased compliance of the arterial tree on hemodynamical parameters has been assessed with the use of a 1-D model. Local, proximal aorta and global stiffening of the arterial tree were modeled and led to two different mechanisms that contribute to the increase in central pulse pressure. They probably both contribute to systolic hypertension and their relative contribution depends on the topology of arterial stiffening and geometrical alterations taking place in aging or in disease. All these patient-specific models are about to being in use in a clinical environment and will be useful for providing better diagnostics and treatment planning in a near future

Thermal-based probe for testing endothelial dysfunction and possible implications for diagnosing atherosclerosis

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (leaves 51-54).Endothelial dysfunction is a precursor to atherosclerosis. Thus, the vascular health of an individual can be assessed if endothelial dysfunction can be readily and unambiguously quantified. A thermal-based approach using temperature and blood perfusion measurements in conjunction with an arterial challenge has the potential to quantitatively assess endothelial dysfunction. This report includes a detailed review of previous attempts to characterize endothelial dysfunction and a preliminary evaluation of a thermal-based approach that relies on temperature and perfusion measurements. Two simple thermal models are used to contextualize results obtained from this technique. Results reveal that this thermal-based method serves as a valid indicator of endothelial assessment while at the same time reducing some of the mitigating factors of existing approaches to identifying endothelial dysfunction. More testing must be performed in order to optimize this thermal-based approach.by Muyinatu A. Lediju.S.B