Internal jugular veins are the major cerebral venous outflow pathway in supine humans. In upright humans the positioning of these veins above heart level causes them to collapse. An alternative cerebral outflow pathway is the vertebral venous plexus. We set out to determine the effect of posture and central venous pressure (

The xBRS method should be considered for experimental and clinical use, because it yielded values that correlated strongly with and were close to the EUROBAVAR averages, yielded more values per minute, had lower within-patient variance and measured baroreflex delay.

Type 2 diabetes is associated with an increased risk of endothelial dysfunction and microvascular complications with impaired autoregulation of tissue perfusion. Both microvascular disease and cardiovascular autonomic neuropathy may affect cerebral autoregulation. In the present study, we tested the hypothesis that, in the absence of cardiovascular autonomic neuropathy, cerebral autoregulation is impaired in subjects with DM+ (Type 2 diabetes with microvascular complications) but intact in subjects with DM- (Type 2 diabetes without microvascular complications). Dynamic cerebral autoregulation and the steady-state cerebrovascular response to postural change were studied in subjects with DM+ and DM-, in the absence of cardiovascular autonomic neuropathy, and in CTRL (healthy control) subjects. The relationship between spontaneous changes in MCA V(mean) (middle cerebral artery mean blood velocity) and MAP (mean arterial pressure) was evaluated using frequency domain analysis. In the low-frequency region (0.07-0.15 Hz), the phase lead of the MAP-to-MCA V(mean) transfer function was 52+/-10 degrees in CTRL subjects, reduced in subjects with DM- (40+/-6 degrees ; P<0.01 compared with CTRL subjects) and impaired in subjects with DM+ (30+/-5 degrees ; P<0.01 compared with subjects with DM-), indicating less dampening of blood pressure oscillations by affected dynamic cerebral autoregulation. The steady-state response of MCA V(mean) to postural change was comparable for all groups (-12+/-6% in CTRL subjects, -15+/-6% in subjects with DM- and -15+/-7% in subjects with DM+). HbA(1c) (glycated haemoglobin) and the duration of diabetes, but not blood pressure, were determinants of transfer function phase. In conclusion, dysfunction of dynamic cerebral autoregulation in subjects with Type 2 diabetes appears to be an early manifestation of microvascular disease prior to the clinical expression of diabetic nephropathy, retinopathy or cardiovascular autonomic neuropathy.

Noninvasive cardiac output (CO) measured by arterial pulse analysis was compared with that measured by inert gas rebreathing in six healthy male volunteers. Pulse contour analysis was applied to the pressure wave output of a Finapres, which noninvasively measures continuous arterial pressure in a finger. Data were collected before, during, and after a 10-day 6 degrees head-down tilt experiment. Intravenous saline loading and lower body negative pressure stimuli varied CO over 2.8-9.6 l/min, as measured by the rebreathing technique. Because pulse contour provides only relative changes in CO, to obtain absolute values it must be calibrated against another measurement. Pulse contour data were calibrated every measurement day against the mean of two to four control rebreathing CO measurements before the lower body negative pressure or intravenous saline loading stimuli. Using one averaged calibration factor per subject for a total of 27 days, we compared the results of both methods. The linear regression between pulse contour (Pc CO) and rebreathing CO (Rebr CO) was Pc CO = 0.15 + 0.98(Rebr CO) (r = 0.96). The standard deviation of the difference of the two methods was 0.5 l/min (n = 205), excluding data used for calibration. By monitoring pulse contour CO before and during rebreathing, the rebreathing maneuver itself was shown to produce a substantial increase in CO that was mainly related to an increase in heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Standing up shifts blood to dependent parts of the body, and blood vessels in the leg become filled. The orthostatic blood volume accumulation in the small vessels is relatively unknown, although these may contribute significantly. We hypothesized that in healthy humans exposed to the upright posture, volume accumulation in small blood vessels contributes significantly to the total fluid volume accumulated in the legs. Considering that near-infrared spectroscopy (

A dip in blood pressure (BP) in response to head-up tilt (HUT) or active standing might be due to rapid pooling in the veins below the heart (preload) or muscle activation-induced drop in systemic vascular resistance (afterload). We hypothesized that, in the cardiovascular response to passive HUT, where, in contrast to active standing, little BP dip is observed, features affecting the preload play a key role. We developed a baroreflex model combined with a lumped-parameter model of the circulation, including viscoelastic stress-relaxation of the systemic veins. Cardiac contraction is modeled using the varying-elastance concept. Gravity affects not only the systemic, but also the pulmonary, circulation. In accordance with the experimental results, model simulations do not show a BP dip on HUT; the tilt-back response is also realistic. If it is assumed that venous capacities are steady-state values, the introduction of stress-relaxation initially reduces venous pooling. The resulting time course of venous pooling is comparable to measured impedance changes. When venous pressure-volume dynamics are neglected, rapid (completed within 30 s) venous pooling leads to a drop in BP. The direct effect of gravity on the pulmonary circulation influences the BP response in the first ϳ5 s after HUT and tilt back. In conclusion, the initial BP response to HUT is mainly determined by the response of the venous system. The time course of lower body pooling is essential in understanding the response to passive HUT. baroreflex; cardiovascular system; modeling; tilt table A MODELING APPROACH to the study of the blood circulation and its response to postural changes can provide insight into the underlying physiology. Existing models approximate certain aspects of the circulation and its response to orthostatic stress, but the transients seen on passive head-up tilt (HUT) have proven difficult to capture.Transient response of the cardiovascular system to active standing and passive HUT has been the focus of various studies (3,34,45,47,51,59). The steady-state response to active standing and HUT is usually comparable, but there is a difference in the blood pressure (BP) and heart rate (HR) responses in the first 30 s (Fig. 1). On passive HUT, Rossberg and Martinez (34) found initial increases in HR comparable to the response to active standing. In later studies, however, a BP dip on active standing was reduced (3, 47) or absent (45,51,59) in passive HUT responses. The initial BP dip on standing is thought to be due to a decrease in peripheral resistance resulting from the active part of the maneuver (45). Sprangers et al. (45) point out two mechanisms: 1) the central (autonomic) command that accompanies active muscle contraction and 2) the displacement of large amounts of venous blood to the right atrium by the massive muscle action induced by active standing, eliciting a cardiopulmonary (CP) reflex effect on the systemic circulation. In passive tilt, if abrupt muscle contraction in response to sudden tilt maneuvers is avoided, perip...

Abstract-Type 2 diabetes mellitus is associated with microvascular complications, hypertension, and impaired dynamic cerebral autoregulation. Intensive blood pressure (BP) control in hypertensive type 2 diabetic patients reduces their risk of stroke but may affect cerebral perfusion. Systemic hemodynamic variables and transcranial Doppler-determined cerebral blood flow velocity (CBFV), cerebral CO 2 responsiveness, and cognitive function were determined after 3 and 6 months of intensive BP control in 17 type 2 diabetic patients with microvascular complications (T2DMϩ), in 18 diabetic patients without (T2DMϪ) microvascular complications, and in 16 nondiabetic hypertensive patients. Cerebrovascular reserve capacity was lower in T2DMϩ versus T2DMϪ and nondiabetic hypertensive patients (4.6Ϯ1.1 versus 6.0Ϯ1.6 [PϽ0.05] and 6.6Ϯ1.7 [PϽ0.01], ⌬% mean CBFV/mm Hg). After 6 months, the attained BP was comparable among the 3 groups. However, in contrast to nondiabetic hypertensive patients, intensive BP control reduced CBFV in T2DMϪ (58Ϯ9 to 54Ϯ12 cm ⅐ s Ϫ1 ) and T2DMϩ (57Ϯ13 to 52Ϯ11 cm ⅐ s Ϫ1 ) at 3 months, but CBFV returned to baseline at 6 months only in T2DMϪ, whereas the reduction in CBFV progressed in T2DMϩ (to 48Ϯ8 cm ⅐ s Ϫ1 ). Cognitive function did not change during the 6 months. Static cerebrovascular autoregulation appears to be impaired in type 2 diabetes mellitus, with a transient reduction in CBFV in uncomplicated diabetic patients on tight BP control, but with a progressive reduction in CBFV in diabetic patients with microvascular complications, indicating that maintenance of cerebral perfusion during BP treatment depends on the progression of microvascular disease. We suggest that BP treatment should be individualized, aiming at a balance between BP reduction and maintenance of CBFV. (Hypertension. 2011;57:738-745.) • Online Data Supplement

We investigated the quantitative contribution of all local conduit arterial, blood, and distal load properties to the pressure transfer function from brachial artery to aorta. The model was based on anatomical data, Young's modulus, wall viscosity, blood viscosity, and blood density. A three-element windkessel represented the distal arterial tree. Sensitivity analysis was performed in terms of frequency and magnitude of the peak of the transfer function and in terms of systolic, diastolic, and pulse pressure in the aorta. The root mean square error (RMSE) described the accuracy in wave-shape prediction. The percent change of these variables for a 25% alteration of each of the model parameters was calculated. Vessel length and diameter are found to be the most important parameters determining pressure transfer. Systolic and diastolic pressure changed Ͻ3% and RMSE Ͻ1.8 mmHg for a 25% change in vessel length and diameter. To investigate how arterial tapering influences the pressure transfer, a single uniform lossless tube was modeled. This simplification introduced only small errors in systolic and diastolic pressures (1% and 0%, respectively), and wave shape was less well described (RMSE, ϳ2.1 mmHg). Local (arm) vasodilation affects the transfer function little, because it has limited effect on the reflection coefficient. Since vessel length and diameter translate into travel time, this parameter can describe the transfer accurately. We suggest that with a, preferably, noninvasively measured travel time, an accurate individualized description of pressure transfer can be obtained. blood pressure; transfer function; personalization; brachial artery; aorta AORTIC PRESSURE appears a better indicator of cardiovascular morbidity and mortality than peripheral, e.g., brachial or radial, pressure (5,15,16,37,38). Also, ascending aortic pressure defines the systolic load on the heart through ventricular wall stress. Effects of therapy are preferably studied using aortic pressure (19). However, whereas peripheral pressures can be obtained noninvasively, measurement of aortic pressure requires invasive techniques. Transfer functions that calculate aortic pressure from peripheral pressure circumvent this invasive measurement and provide an opportunity to noninvasively obtain this cardiovascular information.Several approaches have been taken to arrive at transfer functions. Chen et al. (4) and Fetics et al. (7) used a special mathematical transformation to obtain a transfer function from human data, and the averaged transfer function of a group of patients was used as "standard" transfer. Karamanoglu et al.(18) used a segmented model of the arterial tree to derive the transfer function. Gizdulich et al. (9, 10) proposed a method to obtain brachial pressure from finger pressure by fitting a second order filter to averaged data. Stergiopulos et al. (27) showed that splitting the brachial pressure in its backward and forward waves and by shifting these waves with respect to each other over the travel time between aorta and brach...