Simultaneous determination of wave speed and arrival time of reflected waves using the pressure-velocity loop

This is the post print version of the article. The official published version can be found at the link below.In a previous paper we demonstrated that the linear portion of the pressure–velocity loop (PU-loop) corresponding to early systole could be used to calculate the local wave speed. In this paper we extend this work to show that determination of the time at which the PU-loop first deviates from linearity provides a convenient way to determine the arrival time of reflected waves (Tr). We also present a new technique using the PU-loop that allows for the determination of wave speed and Tr simultaneously. We measured pressure and flow in elastic tubes of different diameters, where a strong reflection site existed at known distances away form the measurement site. We also measured pressure and flow in the ascending aorta of 11 anaesthetised dogs where a strong reflection site was produced through total arterial occlusion at four different sites. Wave speed was determined from the initial slope of the PU-loop and Tr was determined using a new algorithm that detects the sampling point at which the initial linear part of the PU-loop deviates from linearity. The results of the new technique for detecting Tr were comparable to those determined using the foot-to-foot and wave intensity analysis methods. In elastic tubes Tr detected using the new algorithm was almost identical to that detected using wave intensity analysis and foot-to-foot methods with a maximum difference of 2%. Tr detected using the PU-loop in vivo highly correlated with that detected using wave intensity analysis (r 2 = 0.83, P < 0.001). We conclude that the new technique described in this paper offers a convenient and objective method for detecting Tr, and allows for the dynamic determination of wave speed and Tr, simultaneously

Sequentially based analysis versus image based analysis of Intima Media Thickness in common carotid arteries studies - Do major IMT studies underestimate the true relations for cardio- and cerebrovascular risk?

<p>Abstract</p> <p>Background</p> <p>Image-based B-mode ultrasound has gained popularity in major studies as a non-invasive method of measuring cardio- and cerebrovascular risk factors. However, none of the major studies appears to have paid sufficient attention to the variation in end diastolic wall process. By using sequentially based analyses (SBA) of Intima-Media Thickness (IMT), the general purpose of this study was to show that the current image based (ECG tracked) analysis (IBA) has some major variations and might underestimate the true relations for cardiovascular events and stroke for IMT measurement.</p> <p>Method</p> <p>The study group consisted of 2500 healthy male subjects aged between 35 to 55 years. 4 sequences (300 images) were analyzed per subject. 750,000 images were analysed throughout the course of this study.</p> <p>Results</p> <p>IBA showed significantly lower mean, maximal, and minimal values for IMT in CCA than for SBA. The correlation analysis between IBA and SBA with the cardio- and cerebrovascular risk factors showed a higher correlation of SBA for all risk factors. The Pearson coefficient was 0.81, p < 0.01, for SBA versus Framingham CHD risk level (FCRL) and 0.49, p = 0.01, for IBA versus FCRL.</p> <p>Conclusion</p> <p>IBA did not measure the true maximal values of the IMT in this study. Together with the correlation analysis, this indicates that IBA might underestimate the true relations for IMT and risk factors.</p

Intradialytic versus home based exercise training in hemodialysis patients: a randomised controlled trial

Background: Exercise training in hemodialysis patients improves fitness, physical function, quality of life and markers of cardiovascular disease such as arterial stiffness. The majority of trials investigating this area have used supervised exercise training during dialysis (intradialytic), which may not be feasible for some renal units. The aim of this trial is to compare the effects of supervised intradialytic with unsupervised home-based exercise training on physical function and arterial stiffness

Using a human cardiovascular-respiratory model to characterize cardiac tamponade and pulsus paradoxus

<p>Abstract</p> <p>Background</p> <p>Cardiac tamponade is a condition whereby fluid accumulation in the pericardial sac surrounding the heart causes elevation and equilibration of pericardial and cardiac chamber pressures, reduced cardiac output, changes in hemodynamics, partial chamber collapse, pulsus paradoxus, and arterio-venous acid-base disparity. Our large-scale model of the human cardiovascular-respiratory system (H-CRS) is employed to study mechanisms underlying cardiac tamponade and pulsus paradoxus. The model integrates hemodynamics, whole-body gas exchange, and autonomic nervous system control to simulate pressure, volume, and blood flow.</p> <p>Methods</p> <p>We integrate a new pericardial model into our previously developed H-CRS model based on a fit to patient pressure data. Virtual experiments are designed to simulate pericardial effusion and study mechanisms of pulsus paradoxus, focusing particularly on the role of the interventricular septum. Model differential equations programmed in C are solved using a 5^th-order Runge-Kutta numerical integration scheme. MATLAB is employed for waveform analysis.</p> <p>Results</p> <p>The H-CRS model simulates hemodynamic and respiratory changes associated with tamponade clinically. Our model predicts effects of effusion-generated pericardial constraint on chamber and septal mechanics, such as altered right atrial filling, delayed leftward septal motion, and prolonged left ventricular pre-ejection period, causing atrioventricular interaction and ventricular desynchronization. We demonstrate pericardial constraint to markedly accentuate normal ventricular interactions associated with respiratory effort, which we show to be the distinct mechanisms of pulsus paradoxus, namely, series and parallel ventricular interaction. Series ventricular interaction represents respiratory variation in right ventricular stroke volume carried over to the left ventricle via the pulmonary vasculature, whereas parallel interaction (via the septum and pericardium) is a result of competition for fixed filling space. We find that simulating active septal contraction is important in modeling ventricular interaction. The model predicts increased arterio-venous CO₂due to hypoperfusion, and we explore implications of respiratory pattern in tamponade.</p> <p>Conclusion</p> <p>Our modeling study of cardiac tamponade dissects the roles played by septal motion, atrioventricular and right-left ventricular interactions, pulmonary blood pooling, and the depth of respiration. The study fully describes the physiological basis of pulsus paradoxus. Our detailed analysis provides biophysically-based insights helpful for future experimental and clinical study of cardiac tamponade and related pericardial diseases.</p

Influence of the central-to-peripheral arterial stiffness gradient on the timing and amplitude of wave reflections

In individuals with compliant aortas, peripheral muscular artery stiffness exceeds central elastic artery stiffness. With ageing, central stiffness increases, with little change in peripheral stiffness, resulting in a reversal of the normal stiffness gradient. This reversal may reduce wave reflection amplitude, due to movement of the major “effective” reflection site further from the heart. To test this, we investigated the relationship among arterial stiffness gradients (normal and reversed), wave reflection amplitude and reflection site distance. Subjects aged ≥50years were recruited from the Anglo-Cardiff Collaborative Trial. Central stiffness was assessed by carotid-femoral pulse wave velocity (cfPWV). In study 1, peripheral PWV was also measured in the arm (carotid-radial, crPWV), and in study 2 in the leg (femoral- dorsalis pedis, fpPWV). Reflection site distance was calculated from cfPWV and reflected wave travel time. Subjects were dichotomized into those with a normal stiffness gradient (peripheral>central PWV), or a reversed gradient (peripheral<central PWV). In study 1, reflection site distance was greater in subjects with a reversed gradient (P<0.01), whereas time to reflection was lower (P<0.001). Both augmentation pressure (P<0.001) and augmentation index (P<0.05) were greater in subjects with a reversed gradient. In study 2, augmentation pressure, augmentation index and reflection site distance were greater in subjects with a reversed stiffness gradient (P<0.01, P<0.05 and P<0.01, respectively), and time to reflection was not different between groups. A reversed arterial stiffness gradient is associated with increased reflection site distance and a paradoxical increase in reflected wave amplitude, and augmentation index