ANALOG STUDIES OF HUMAN SYSTEMIC ARTERIAL HEMODYNAMICS

Abstract not availabl

ANALOG STUDIES OF HUMAN SYSTEMIC ARTERIAL HEMODYNAMICS

Abstract not availabl

Waves and Windkessels reviewed

Pressure and flow are travelling waves and are reflected at many locations. The forward and reflected waves, obtained by wave separation, are compound waves. This compounded character of the reflected wave explains why its magnitude decreases with increased peripheral resistance, why it appears to run forward rather than backward, and why its return time relates poorly with aortic wave speed. A single tube (aorta) with distal reflection is therefore an incorrect arterial model. Wave Intensity Analysis (WIA) uses time derivatives of pressure and flow, augmenting rapid changes and incorrectly suggesting a ‘wave free period’ in diastole. Assuming a ‘wave free period’, the Reservoir-Wave Approach (RWA) separates pressure into a ‘waveless’ reservoir pressure, predicted by Frank's Windkessel, and excess pressure, accounting for wave phenomena. However, the reservoir pressure, being twice the backward pressure, and location dependent, is a wave. The Instantaneous wave Free pressure Ratio distal and proximal of a stenosis, iFR, also assumes a ‘wave free period’, and is based on an instantaneous pressure-flow ratio, an incorrect resistance measure since Ohm's law pertains to averaged pressure and flow only. Moreover, this ratio, while assumed minimal, was shown to decrease with vasodilation. Windkessel models are descriptions of an arterial system at a single location using a limited number of parameters. Windkessels can be used as model but the actual arterial system is not a Windkessel. Total Peripheral Resistance and Total Arterial Compliance, (the 2-element, Frank Windkessel), supplemented with aortic characteristic impedance (3-element Windkessel) mimics the arterial system well

Snapshots of hemodynamics: An aid for clinical research and graduate education

This new edition is written in the same quick reference style as its predecessor to help clinical and basic researchers, as well as graduate students, understand hemodynamics. Hemodynamics makes it possible to characterize, in a quantitative way and often with noninvasive techniques, the function of the heart and the arterial system, individually and in combination. Snapshots of Hemodynamics provides a thorough grounding in the discipline that will help any medical professional and researcher in the field. The authors have designed each chapter such that it gives a succinct overview of individual topics in a concise and understandable format. Each chapter of this new edition has been extensively updated while new chapters have been included on pulmonary hemodynamics and wave intensity analysis. The new edition presents the newest current information on hemodynamics in this ever-changing field

Location of a reflection site is elusive: consequences for the calculation of aortic pulse wave velocity

Aortic pulse wave velocity (PWV), a measure of aortic stiffness, is an important indicator of cardiovascular risk. Derivation of PWV from uncalibrated proximal aortic or carotid pressure alone has practical advantages. However, when the time of return of the reflected wave, (Delta)t, is used to calculate PWV, inaccurate data are obtained. With aging PWV increases but (Delta)t hardly decreases, suggesting that the reflection site moves toward the periphery. We hypothesized that the forward and reflected waves in the distal aorta are not in phase, leading to an undefined reflection site. We derived forward and backward waves, at the entrance and distal end of a uniform tube, with length "L." With the tube closed at the end, forward and reflected waves are there in phase, and PWV=2L/(Delta)t. When the tube is ended with the input impedance of the lower body, forward and backward waves at its end are not in phase, and (Delta)t is increased, suggesting that the reflection site is further away (tube seems longer), and PWV calculated from 2L/(Delta)t is underestimated. Using an anatomically accurate model of the human arterial system, we show that the forward and backward waves in the distal aorta are not in phase. When aortic PWV increases, (Delta)t changes only little, and the reflection site appears to move to the periphery, similar to what is observed in humans. We conclude that to define the location of a reflection site is elusive and that PWV cannot be calculated from time of return of the reflected wav

Modeling Arterial Pulse Pressure From Heart Rate During Sympathetic Activation by Progressive Central Hypovolemia

Heart rate (HR) has an impact on the central blood pressure (BP) wave shape and is related to pulse wave velocity and therefore to timing and duration of systole and diastole. This study tested the hypothesis that in healthy subjects both in rest and during sympathetic stimulation the relation between HR and pulse pressure (PP) is described by a linear effect model. Forty-four healthy volunteers were subjected to sympathetic stimulation by continuous lower body negative pressure (LBNP) until the onset of pre-syncopal symptoms. Changes in PP and HR were tracked non-invasively and modeled by linear mixed effect (LME) models. The dataset was split into two groups: the first was used for creating a model and the second for its evaluation. Models were created on the data obtained during LBNP. Model performance was expressed as absolute median error (1st; 3rd quantiles) and bias with limits of agreement (LOA) between modeled and measured PP. From rest to sympathetic stimulation, mean BP was maintained while HR increased (~30%) and PP decreased gradually (~20%). During baseline, PP could be modeled with an absolute error of 6 (4; 10) mm Hg and geometric mean ratio of the bias was 0.97 (LOA: 0.8-1.1). During LBNP, absolute median model error was 5 (4; 8) mmHg with geometric mean ratio 1.02 (LOA: 0.8-1.3). In conclusion, both during rest and during sustained sympathetic outflow induced by progressive central hypovolemia, a LME model of HR provides for an estimate of PP in healthy young adults

Modeling Arterial Pulse Pressure From Heart Rate During Sympathetic Activation by Progressive Central Hypovolemia

Heart rate (HR) has an impact on the central blood pressure (BP) wave shape and is related to pulse wave velocity and therefore to timing and duration of systole and diastole. This study tested the hypothesis that in healthy subjects both in rest and during sympathetic stimulation the relation between HR and pulse pressure (PP) is described by a linear effect model. Forty-four healthy volunteers were subjected to sympathetic stimulation by continuous lower body negative pressure (LBNP) until the onset of pre-syncopal symptoms. Changes in PP and HR were tracked non-invasively and modeled by linear mixed effect (LME) models. The dataset was split into two groups: the first was used for creating a model and the second for its evaluation. Models were created on the data obtained during LBNP. Model performance was expressed as absolute median error (1st; 3rd quantiles) and bias with limits of agreement (LOA) between modeled and measured PP. From rest to sympathetic stimulation, mean BP was maintained while HR increased (~30%) and PP decreased gradually (~20%). During baseline, PP could be modeled with an absolute error of 6 (4; 10) mm Hg and geometric mean ratio of the bias was 0.97 (LOA: 0.8–1.1). During LBNP, absolute median model error was 5 (4; 8) mmHg with geometric mean ratio 1.02 (LOA: 0.8–1.3). In conclusion, both during rest and during sustained sympathetic outflow induced by progressive central hypovolemia, a LME model of HR provides for an estimate of PP in healthy young adults

Noninvasive Blood Pressure Measurement by the Nexfin Monitor During Reduced Arterial Pulsatility: A Feasibility Study

Noninvasive blood pressure measurements are difficult when arterial pulsations are reduced, as in patients supported by continuous flow left ventricular assist devices (cf-LVAD). We evaluated the feasibility of measuring noninvasive arterial blood pressure with the Nexfin monitor during conditions of reduced arterial pulsatility. During cardiopulmonary bypass (CPB) in which a roller pump based or a centrifugal pump based heart-lung machine generated arterial blood pressure with low pulsatility, noninvasive arterial pressures (NAP) measured by the Nexfin Monitor were recorded and compared with invasively measured radial artery pressures (IAP). We also evaluated NAP in 10 patients with a cf-LVAD during a pump speed change procedure (PSCP). During CPB in 18 patients, the NAP-IAP average difference was -1.3 +/- 6.5 mm Hg. The amplitude of pressure oscillations were 4.3 +/- 3.8 mm Hg measured by IAP. Furthermore, in the cf-LVAD patients, increase in pump speed settings led to an increase in diastolic and mean arterial pressures (MAP) while the NAP acquired a sinusoidal shape as the aortic valve become permanently closed. In conclusion, NAP was similar to IAP under conditions of reduced arterial pulsatility. The device also measured the blood pressure waveform noninvasively in patients supported by a cf-LVAD. ASAIO Journal 2010; 56:221-22