Autonomic regulation in different hypertensive phenotypes - the HELIUS study.

Background: Hypertension can be classified into different phenotypes based on systolic and diastolic blood pressure (BP) that carry a different prognosis and may therefore be differently associated with sympathetic activity. We assessed the association between cardiac autonomic function determined from continuous finger BP recordings and hypertensive phenotypes. Methods: We included 10,221 individuals aged between 18-70 years from the multi-ethnic HELIUS study. Finger BP was recorded continuously for 3-5 minutes from which cross-correlation baroreflex sensitivity (xBRS) and heart rate variability (HRV) were determined. Hypertension was classified into isolated systolic (ISH; ≥140/<90), diastolic (IDH; <140/≥90) and combined systolic and diastolic hypertension (SDH; ≥140/≥90). Differences were assessed after stratification by age (younger: ≤40, older: >40 years) and sex, using regression with correction for relevant covariates. For xBRS, values were log-transformed. Results: In younger adults with ISH, xBRS was comparable to normotensive individuals in men (ratio 0.92; 95%CI 0.84-1.01) and women (1.00; 95%CI 0.84-1.20), while xBRS was significantly lower in IDH and SDH (ratios between 0.67 and 0.80). In older adults, all hypertensive phenotypes had significantly lower xBRS compared to normotensives. We found a similar pattern for HRV in men, while in women HRV did not differ between phenotypes. Conclusions: In younger men and women ISH is not associated with a shift towards increased sympathetic control, while IDH and SDH in younger and all hypertensive phenotypes in older participants were associated with increased sympathetic control. This suggests that alterations in autonomic regulation could be a contributing factor to known prognostic disparities between hypertensive phenotypes.

Hypertension can be classified into different phenotypes based on systolic and diastolic blood pressure (BP) that carry a different prognosis. Impaired autonomic regulation is important in the pathogenesis of hypertension and independently associated with adverse cardiovascular outcomes.We analyzed 3-5 minutes continuous non-invasive finger blood pressure recordings performed in over 10.000 individuals participating in the HELIUS cohort study. From these measurements, short term heart rate variability (HRV) and cross correlation baroreflex sensitivity (xBRS) were determined using an automatic algorithm.In our analysis we observed pronounced differences in the relation between autonomic regulation and hypertensive phenotypes that depend on age and sex.Younger men and women (age 18-40 years) with isolated systolic hypertension had similar values for xBRS and HRV compared to normotensives, while isolated diastolic hypertension was associated with a shift towards increased sympathetic control. In contrast to our findings in younger individuals, all hypertensive phenotypes were associated with increased sympathetic control in older participants (age 40-70 years).This supports earlier studies showing prognostic differences and suggests that alterations in sympathovagal balance could be a contributing factor to the disparities between phenotypes.

Automated analysis of finger blood pressure recordings provides insight in determinants of baroreflex sensitivity and heart rate variability-the HELIUS study.

Sympathovagal balance is important in the pathogenesis of hypertension and independently associated with mortality. We evaluated the value of automated analysis of cross-correlation baroreflex sensitivity (xBRS) and heart rate variability (HRV) and its relationship with clinical covariates in 13,326 participants from the multi-ethnic HELIUS study. Finger blood pressure (BP) was continuously recorded, from which xBRS, standard deviation of normal-to-normal intervals (SDNN), and squared root of mean squared successive difference between normal-to-normal intervals (RMSDD) were determined. A subset of 3356 recordings > 300 s was used to derive the minimally required duration by comparing shortened to complete recordings, defined as intraclass correlation (ICC) > 0.90. For xBRS and SDNN, 120 s and 180 s were required (ICC 0.93); for RMSDD, 60 s (ICC 0.94) was sufficient. We included 10,252 participants (median age 46 years, 54% women) with a recording > 180 s for the regression. xBRS, SDNN, and RMSDD decreased linearly up to 50 years of age. For xBRS, there was a signification interaction with sex, with for every 10 years a decrease of 4.3 ms/mmHg (95%CI 4.0-4.6) for men and 5.9 ms/mmHg (95%CI 5.6-6.1) for women. Using splines, we observed sex-dependent nonlinearities in the relation with BP, waist-to-hip-ratio, and body mass index. Future studies can help unravel the dynamics of these relations and assess their predictive value. Panel 1 depicts automatic analysis and filtering of finger BP recordings, panel 2 depicts computation of xBRS from interpolated beat to beat data of systolic BP and interbeat interval, and (IBI) SDNN and RMSDD are computed directly from the filtered IBI dataset. Panel 3 depicts the results of large-scale analysis and relation of xBRS with age, sex, blood pressure and body mass index.

A practical guide to active stand testing and analysis using continuous beat-to-beat non-invasive blood pressure monitoring.

PURPOSE: The average adult stands approximately 50-60 times per day. Cardiovascular responses evoked during the first 3 min of active standing provide a simple means to clinically assess short-term neural and cardiovascular function across the lifespan. Clinically, this response is used to identify the haemodynamic correlates of patient symptoms and attributable causes of (pre-)syncope, and to detect autonomic dysfunction, variants of orthostatic hypotension, postural orthostatic tachycardia syndrome and orthostatic hypertension. METHODS: This paper provides a set of experience/expertise-based recommendations detailing current state-of-the-art measurement and analysis approaches for the active stand test, focusing on beat-to-beat BP technologies. This information is targeted at those interested in performing and interpreting the active stand test to current international standards. RESULTS: This paper presents a practical step-by-step guide on (1) how to perform active stand measurements using beat-to-beat continuous blood pressure measurement technologies, (2) how to conduct an analysis of the active stand response and (3) how to identify the spectrum of abnormal blood pressure and heart rate responses which are of clinical interest. CONCLUSION: Impairments in neurocardiovascular control are an attributable cause of falls and syncope across the lifespan. The simple active stand test provides the clinician with a powerful tool for assessing individuals at risk of such common disorders. However, its simplicity belies the complexity of its interpretation. Care must therefore be taken in administering and interpreting the test in order to maximise its clinical benefit and minimise its misinterpretation.

The effect of haemodynamic and peripheral vascular variability on cardiac output monitoring: thermodilution and non-invasive pulse contour cardiac output during cardiothoracic surgery.

While haemodynamic variability interferes with the assumption of constant flow underlying thermodilution cardiac output calculation, variability in (peripheral) arterial vascular physiology may affect pulse contour cardiac output methods. We compared non-invasive finger arterial pressure-derived continuous cardiac output measurements (Nexfin® ) with cardiac output measured using thermodilution during cardiothoracic surgery and determined the impact of cardiovascular variability on either method. We compared cardiac output derived from non-invasive finger arterial pressure with cardiac output measured by thermodilution at four grades (A-D) of cardiovascular variability. We defined Grade A data as heart rate and mean arterial pressure variability < 5% and the absence of arrhythmias (implying stable flow) and Physiocal® interval (as measure of variability in finger arterial physiology) > 30 beats. Grade B included all levels of heart rate/mean arterial pressure variability and arrhythmias (Physiocal < 30 excluded). Grade C included all Physiocal intervals (heart rate/mean arterial pressure variability > 5% and arrhythmias excluded). Grade D included all data. Comparison results were quantified as percentage errors. We analysed measurements in 27 patients undergoing coronary artery bypass surgery. Before extracorporeal circulation, the percentage error was 23% (n = 14 patients) in grade A, 28% (n = 20) in grade B, 32% (n = 22) in grade C and 37% (n = 26) in grade D, with a significant increase in variance (p = 0.035). Bias did not differ between grades. After extracorporeal circulation (n = 27), percentage errors became larger, but were not different between grades. Variability during cardiothoracic surgery affected the comparison between thermodilution and non-invasive finger arterial pressure-derived cardiac output. When the main sources of variability were included, percentage errors were large. Future cardiac output methodology comparison studies should report haemodynamic variability.

Comparing volume-clamp method and intra-arterial blood pressure measurements in patients with atrial fibrillation admitted to the intensive or medium care unit.

International guidelines highlight the importance of blood pressure (BP) in patients with atrial fibrillation (AF). However, BP measurement in AF is complicated by beat-to-beat fluctuation. Automated BP measurement devices are not validated for patients with AF and no consensus exists on how to measure BP in AF manually. Beat-to-beat BP measurement using the volume-clamp method (VCM) could represent a non-invasive method to accurately assess BP, but has not been validated in AF. 31 admitted patients with sustained AF and 10 control patients with sinus rhythm underwent simultaneous intra-arterial and non-invasive BP measurement using a VCM monitor (Nexfin®, BMEYE, Amsterdam, The Netherlands). Patients with compromised peripheral perfusion, high doses of vasopressor drugs or peripheral edema were excluded. Differences in systolic, diastolic and mean BP of 5 (standard deviation; SD 8) mmHg (accuracy and precision) between both methods were considered acceptable. Additionally, the magnitude of beat-to-beat fluctuations in systolic BP of both methods was compared. In AF, the differences between noninvasive and invasive BP were -4 (SD 12), +1 (SD 7) and 0 (SD 8) mmHg for systolic, diastolic and mean BP respectively. Absolute differences in beat-to-beat BP fluctuations were 1.5 (IQR 0.8-3.8) mmHg. Accuracy of VCM in AF was similar to sinus rhythm. In conclusion, in patients with AF, accurate and precise measurement of non-invasive beat-to-beat BP measurement using the VCM is possible, the one exception being the precision of systolic BP. Beat-to-beat variability can be accurately reproduced.

Noninvasive beat-to-beat finger arterial pressure monitoring during orthostasis: a comprehensive review of normal and abnormal responses at different ages.

Over the past 30 years, noninvasive beat-to-beat blood pressure (BP) monitoring has provided great insight into cardiovascular autonomic regulation during standing. Although traditional sphygmomanometric measurement of BP may be sufficient for detection of sustained orthostatic hypotension, it fails to capture the complexity of the underlying dynamic BP and heart rate responses. With the emerging use of noninvasive beat-to-beat BP monitoring for the assessment of orthostatic BP control in clinical and population studies, various definitions for abnormal orthostatic BP patterns have been used. Here, age-related changes in cardiovascular control in healthy subjects will be reviewed to define the spectrum of the most important abnormal orthostatic BP patterns within the first 180 s of standing. Abnormal orthostatic BP responses can be defined as initial orthostatic hypotension (a transient systolic BP fall of >40 mmHg within 15 s of standing), delayed BP recovery (an inability of systolic BP to recover to a value of >20 mmHg below baseline at 30 s after standing) and sustained orthostatic hypotension (a sustained decline in systolic BP of ≥20 mmHg occurring 60-180 s after standing). In the evaluation of patients with light-headedness, pre(syncope), (unexplained) falls or suspected autonomic dysfunction, it is essential to distinguish between normal cardiovascular autonomic regulation and these abnormal orthostatic BP responses. The prevalence, clinical relevance and underlying pathophysiological mechanisms of these patterns differ significantly across the lifespan. Initial orthostatic hypotension is important for identifying causes of syncope in younger adults, whereas delayed BP recovery and sustained orthostatic hypotension are essential for evaluating the risk of falls in older adults.

Initial orthostatic hypotension in teenagers and young adults.

OBJECTIVE: To assess: (1) the frequency of an abnormally large fall in blood pressure (BP) upon standing from supine in patients with initial orthostatic hypotension (IOH); (2) the underlying hemodynamic mechanisms of this fall in BP upon standing from supine and from squatting. METHODS: In a retrospective study of 371 patients (≤30 years) visiting the syncope unit, the hemodynamic response to standing and squatting were studied in 26 patients who were diagnosed clinically with IOH, based on history taking only. In six patients changes in cardiac output (CO) and systemic vascular resistance (SVR) were determined, and the underlying hemodynamics were analyzed. RESULTS: 15/26 (58 %) patients with IOH had an abnormally large initial fall in systolic BP (≥40 mmHg). There was a large scatter in CO and SVR response after arising from supine [ΔCO at BP nadir median -8 % (range -37, +27 %); ΔSVR at BP nadir median -31 % (range -46, +10 %)]. The hemodynamic response after squatting showed a more consistent pattern, with a fall in SVR in all six patients [ΔCO at BP nadir median +23 % (range -12, +31 %); ΔSVR at BP nadir median -42 %, (range -52, -35 %)]. INTERPRETATION: The clinical diagnosis of IOH is based on history taking, as an abnormally large fall in systolic BP can only be documented in 58 %. For IOH upon standing after supine rest, the hemodynamic mechanism can be either a large fall in CO or in SVR. For IOH upon arising from squatting a large fall in SVR is a consistent finding.

Minimally invasive intraoperative estimation of left-ventricular end-systolic elastance with phenylephrine as loading intervention.

BACKGROUND: Left-ventricular end-systolic elastance (Ees) is an index of cardiac contractility, but the invasive nature of its assessment has limited perioperative application. We explored the feasibility of a minimally invasive method of Ees estimation for perioperative assessment of cardiac function and evaluated the suitability of phenylephrine as a loading intervention. METHODS: In 17 surgical patients, Ees was determined as the slope of the end-systolic pressure-volume relation, which was obtained from non-invasive or invasive continuous arterial pressure measurements and left-ventricular volume determinations using transoesophageal echocardiography (TOE). Ees was determined using as loading interventions preload reduction by inferior vena cava compression (IVCC) and afterload increase by phenylephrine administration. RESULTS: Median invasive Ees determined with phenylephrine estimated 1.05 (0.59-1.21) mm Hg ml(-1) and with IVCC 0.58 (0.31-1.13) mm Hg ml(-1). Bland-Altman analysis to evaluate the level of agreement between minimally invasive and invasive Ees estimation revealed a bias of -0.03 (0.12) mm Hg ml(-1) with limits of agreement from -0.27 to 0.21 mm Hg ml(-1) and the percentage error was 33%. Agreement between Ees obtained with phenylephrine and IVCC revealed a bias of 0.15 (0.69) mm Hg ml(-1) with limits of agreement from -1.21 to 1.51 mm Hg ml(-1) and a percentage error of 149%. CONCLUSIONS: It is feasible to determine Ees combining continuous non-invasive arterial pressure measurements and left-ventricular volume determinations with TOE. However, administration of phenylephrine cannot substitute IVCC as a loading intervention, indicating that estimation of Ees in the intraoperative setting remains a challenge.

Prolonged post-faint hypotension can be reversed by dynamic tension.

A severe variant of vasovagal syncope, observed during tilt tests and blood donation has recently been termed "prolonged post-faint hypotension" (PPFH). A 49-year-old male with a life-long history of severe fainting attacks underwent head-up tilt for 20 min, and developed syncope 2 min after nitroglycerine spray. He was unconscious for 40 s and asystolic for 22 s. For the first 2 min of recovery, BP and HR remained low (65/45 mmHg and 40 beats/min) despite passive leg-raising. Blood pressure (and symptoms) only improved following active bilateral leg flexion and extension ("dynamic tension"). During PPFH, when vagal activity is extreme, patients may require central stimulation as well as correction of venous return.

Pulse contour cardiac output derived from non-invasive arterial pressure in cardiovascular disease.

Pulse contour methods determine cardiac output semi-invasively using standard arterial access. This study assessed whether cardiac output can be determined non-invasively by replacing the intra-arterial pressure input with a non-invasive finger arterial pressure input in two methods, Nexfin CO-trek and Modelflow , in 25 awake patients after coronary artery bypass surgery. Pulmonary artery thermodilution cardiac output served as a reference. In the supine position, the mean (SD) differences between thermodilution cardiac output and Nexfin CO-trek were 0.22 (0.77) and 0.44 (0.81) l.min(-1) , for intra-arterial and non-invasive pressures, respectively. For Modelflow, these differences were 0.70 (1.08) and 1.80 (1.59) l.min(-1) , respectively. Similarly, in the sitting position, differences between thermodilution cardiac output and Nexfin CO-trek were 0.16 (0.78) and 0.34 (0.83), for intra-arterial and non-invasive arterial pressure, respectively. For Modelflow, these differences were 0.58 (1.11) and 1.52 (1.54) l.min(-1) , respectively. Thus, Nexfin CO-trek readings were not different from thermodilution cardiac output, for both invasive and non-invasive inputs. However, Modelflow readings differed greatly from thermodilution when using non-invasive arterial pressure input.

Noninvasive haemodynamic monitoring using finger arterial pressure waveforms.

Haemodynamic monitoring may potentially lead to improved quality of care in haemodynamic compromised patients. However, the usefulness of invasive techniques using the pulmonary artery catheter is questioned. Noninvasive techniques which provide data on haemodynamics might provide a good alternative. New techniques have been developed in recent years to monitor cardiac output and other parameters of cardiac performance continuously and noninvasively. Recently, a new technique has become available that assesses these haemodynamic data from finger arterial pressure waveforms obtained noninvasively. Although an invasively derived calibration is still needed to obtain absolute data on cardiac output, relative changes in cardiac output can be accurately monitored using this method. Currently, the device can be used in patients to continuously monitor haemodynamic data and guide therapy. Furthermore, it might have a role in clinical research to noninvasively assess cardiac output, as a surrogate endpoint, before and after interventions. Although this new method seems promising, the clinical value has to be proven.

RC time constant of single lung equals that of both lungs together: a study in chronic thromboembolic pulmonary hypertension.

The product of resistance, R, and compliance, C (RC time), of the entire pulmonary circulation is constant. It is unknown if this constancy holds for individual lungs. We determined R and C in individual lungs in chronic thromboembolic pulmonary hypertension (CTEPH) patients where resistances differ between both lungs. Also, the contribution of the proximal pulmonary arteries (PA) to total lung compliance was assessed. Patients (n=23) were referred for the evaluation of CTEPH. Pressure was measured by right heart catheterization and flows in the main, left, and right PA by magnetic resonance imaging. Total, left, and right lung resistances were calculated as mean pressure divided by mean flow. Total, left, and right lung compliances were assessed by the pulse pressure method. Proximal compliances were derived from cross-sectional area change DeltaA and systolic-diastolic pressure difference DeltaP (DeltaA/DeltaP) in main, left, and right PA, multiplied by vessel length. The lung with the lowest blood flow was defined "low flow" (LF), the contralateral lung "high flow" (HF). Total resistance was 0.57+/-0.28 mmHg.s(-1).ml(-1), and resistances of LF and HF lungs were 1.57+/-0.2 vs. 1.00+/-0.1 mmHg.s(-1).ml(-1), respectively, P<0.0001. Total compliance was 1.22+/-1.1 ml/mmHg, and compliances of LF and HF lung were 0.47+/-0.11 and 0.62+/-0.12 ml/mmHg, respectively, P=0.01. Total RC time was 0.49+/-0.2 s, and RC times for the LF and HF lung were 0.45+/-0.2 and 0.45+/-0.1 s, respectively, not different. Proximal arterial compliance, given by the sum of main, right, and left PA compliances, was only 19% of total lung compliance. The RC time of a single lung equals that of both lungs together, and pulmonary arterial compliance comes largely from the distal vasculature.

Continuous non-invasive blood pressure monitoring, a validation study of Nexfin in a pregnant population.

OBJECTIVE: To assess the accuracy of a non-invasive beat-to-beat continuous blood pressure monitoring device (Nexfin) in pregnancy according to the International Protocol of the European Society of Hypertension. METHODS: The validation was performed according to the International Protocol of the European Society of Hypertension. The test device (Nexfin, BMEYE, Amsterdam, the Netherlands) calculates beat to beat blood pressure from finger pulse wave analysis. Measurements of systolic and diastolic BP in 33 volunteers were obtained using the mercury sphygmomanometer and the Nexfin alternatingly. RESULTS: The device passed phase 1 as 30 systolic and 32 diastolic readings fell within 5 mmHg (25 required). In addition, the device also passed phase 2.1 as 68 systolic and 67 diastolic readings fell within 5 mmHg (65 required). Finally, it failed to pass phase 2.2 as 24 subjects for systolic and 23 for diastolic had at least 2/3 of their comparisons falling within 5 mmHg (22 required) but 6 subjects for systolic and 8 for diastolic had all three comparisons more than 5 mmHg different from the mercury readings (three allowed). The mean differences were 2.3 mmHg (SD 6.8) for SBP and 0.8 mmHg (SD 6.3) for DBP. CONCLUSION: The Nexfin device passed phase 1 and phase 2.1 but failed to pass phase 2.2. However, adaptation of the data to the more permissive AAMI (mean difference <5 +/- 8 mmHg) and BHS (systolic grade B, diastolic grade A) protocols indicated adequate accuracy for application in research settings or for longitudinal within-patient tracking of blood pressure, given the possibility for continuous monitoring.

Aortic pressure-area relation in Marfan patients with and without beta blocking agents: a new non-invasive approach.

OBJECTIVE: To investigate the heterogeneous response to beta blockade in patients with Marfan syndrome by non-invasive assessment of the aortic pressure-area curve. DESIGN AND PATIENTS: 25 patients with the Marfan syndrome who used beta blocking agents (mean (SD) age, 29 (10) years; 20 men, five women), seven without beta blockade (34 (14) years; five men, two women), and 10 controls (29 (5) years; seven men, three women) underwent magnetic resonance imaging and non-invasive continuous blood pressure measurement. Pressure-area curves were constructed at the level of the descending thoracic aorta. A transition point was defined as the pressure at which the pressure-area relation deviated from its elastic (linear) to the collagen (exponential) course. SETTING: Tertiary referral centre for adult congenital heart disease. RESULTS: In six patients (five with and one without beta blockade), a transition point in the pressure-area curve was observed, indicating that the load bearing component was not only elastin but also collagen. In the remaining 26 Marfan patients and in the control subjects a linear pressure-area relation was observed. CONCLUSIONS: This new non-invasive method to derive aortic pressure-area curves showed that most patients with Marfan syndrome have a similar pressure-area curve to controls with similar blood pressures. Five patients on beta blockade showed a transition point in the pressure-area curve which could play a crucial role in the heterogeneous response to beta blocker treatment in Marfan patients. Patients with a transition at low blood pressures may not benefit from beta blocking agents.

Micromotions of bladder wall in chronic pelvic pain (CPP): a pilot study.

The aim of the study was to investigate the occurrence and duration of micromotions of the bladder wall. Thirty women with CPP and 7 healthy women underwent micromotion detection (MMD). A latex balloon provided with eight electrodes was placed within the bladder through the urethra and filled with saline up to 200 ml. Micromotions (MM), pressure within the balloon, abdominal pressure and respiratory excursions of the abdomen were registered simultaneously. A significant difference in duration as well as frequency of occurrence was found for MM activity between subjects with CPP and controls. For the occurrence of variations in detrusor presure, the difference between groups tended towards significance. We conclude that there are indications that the bladder is involved in CPP.

Left ventricular wall stress normalization in chronic pressure-overloaded heart: a mathematical model study.

It is generally accepted that the left ventricle (LV) hypertrophies (LVH) to normalize systolic wall stress (sigma(s)) in chronic pressure overload. However, LV filling pressure (P(v)) may be elevated as well, supporting the alternative hypothesis of end-diastolic wall stress (sigma(d)) normalization in LVH. We used an LV time-varying elastance model coupled to an arterial four-element lumped-parameter model to study ventricular-arterial interaction in hypertension-induced LVH. We assessed model parameters for normotensive controls and applied arterial changes as observed in hypertensive patients with LVH (resistance +40%, compliance -25%) and assumed 1) no cardiac adaptation, 2) normalization of sigma(s) by LVH, and 3) normalization of sigma(s) by LVH and increase in P(v), such that sigma(d) is normalized as well. In patients, systolic and diastolic blood pressures increase by approximately 40%, cardiac output (CO) is constant, and wall thickness increases by 30-55%. In scenarios 1 and 2, blood pressure increased by only 10% while CO dropped by 20%. In scenario 2, LV wall thickness increased by only 10%. The predictions of scenario 3 were in qualitative and quantitative agreement with in vivo human data. LVH thus contributes to the elevated blood pressure in hypertension, and cardiac adaptations include an increase in P(v), normalization of sigma(s), and preservation of CO in the presence of an impaired diastolic function.

Total arterial inertance as the fourth element of the windkessel model.

In earlier studies we found that the three-element windkessel, although an almost perfect load for isolated heart studies, does not lead to accurate estimates of total arterial compliance. To overcome this problem, we introduce an inertial term in parallel with the characteristic impedance. In seven dogs we found that ascending aortic pressure could be predicted better from aortic flow by using the four-element windkessel than by using the three-element windkessel: the root-mean-square errors and the Akaike information criterion and Schwarz criterion were smaller for the four-element windkessel. The three-element windkessel overestimated total arterial compliance compared with the values derived from the area and the pulse pressure method (P = 0.0047, paired t-test), whereas the four-element windkessel compliance estimates were not different (P = 0.81). The characteristic impedance was underestimated using the three-element windkessel, whereas the four-element windkessel estimation differed marginally from the averaged impedance modulus at high frequencies (P = 0.0017 and 0.031, respectively). When applied to the human, the four-element windkessel also was more accurate in these same aspects. Using a distributed model of the systemic arterial tree, we found that the inertial term results from the proper summation of all local inertial terms, and we call it total arterial inertance. We conclude that the fourelement windkessel, with all its elements having a hemodynamic meaning, is superior to the three-element windkessel as a lumped-parameter model of the entire systemic tree or as a model for parameter estimation of vascular properties.

Physical basis of pressure transfer from periphery to aorta: a model-based study.

We propose a new method to derive aortic pressure from peripheral pressure and velocity by using a time domain approach. Peripheral pressure is separated into its forward and backward components, and these components are then shifted with a delay time, which is the ratio of wave speed and distance, and added again to reconstruct aortic pressure. We tested the method on a distributed model of the human systemic arterial tree. From carotid and brachial artery pressure and velocity, aortic systolic and diastolic pressure could be predicted within 0.3 and 0.1 mmHg and 0.4 and 1.0 mmHg, respectively. The central aortic pressure wave shape was also predicted accurately from carotid and brachial pressure and velocity (root mean square error: 1.07 and 1.56 mmHg, respectively). The pressure transfer function depends on the reflection coefficient at the site of peripheral measurement and the delay time. A 50% decrease in arterial compliance had a considerable effect on reconstructed pressure when the control transfer function was used. A 70% decrease in arm resistance did not affect the reconstructed pressure. The transfer function thus depends on wave speed but has little dependence on vasoactive state. We conclude that central aortic pressure and the transfer function can be derived from peripheral pressure and velocity.