Comparison of cardiovascular responses between lower body negative pressure and head-up tilt.

To investigate local blood-flow regulation during orthostatic maneuvers, 10 healthy subjects were exposed to -20 and -40 mmHg lower body negative pressure (LBNP; each for 3 min) and to 60 degrees head-up tilt (HUT; for 5 min). Measurements were made of blood flow in the brachial (BF(brachial)) and femoral arteries (BF(femoral)) (both by the ultrasound Doppler method), heart rate (HR), mean arterial pressure (MAP), cardiac stroke volume (SV; by echocardiography), and left ventricular end-diastolic volume (LVEDV; by echocardiography). Comparable central cardiovascular responses (changes in LVEDV, SV, and MAP) were seen during LBNP and HUT. During -20 mmHg LBNP, -40 mmHg LBNP, and HUT, the following results were observed: 1) BF(brachial) decreased by 51, 57, and 41%, and BF(femoral) decreased by 40, 53, and 62%, respectively, 2) vascular resistance increased in the upper limb by 110, 147, and 85%, and in the lower limb by 76, 153, and 250%, respectively. The increases in vascular resistance were not different between the upper and lower limbs during LBNP. However, during HUT, the increase in the lower limb was much greater than that in the upper limb. These results suggest that, during orthostatic stimulation, the vascular responses in the limbs due to the cardiopulmonary and arterial baroreflexes can be strongly modulated by local mechanisms (presumably induced by gravitational effects).     

Causal relationships between heart period and systolic arterial pressure during graded head-up tilt

In physiological conditions, heart period (HP) affects systolic arterial pressure (SAP) through diastolic runoff and Starling's law, but, the reverse relation also holds as a result of the continuous action of baroreflex control. The prevailing mechanism sets the dominant temporal direction in the HP-SAP interactions (i.e., causality). We exploited cross-conditional entropy to assess HP-SAP causality. A traditional approach based on phases was applied for comparison. The ability of the approach to detect the lack of causal link from SAP to HP was assessed on 8 short-term (STHT) and 11 long-term heart transplant (LTHT) recipients (i.e., less than and more than 2 yr after transplantation, respectively). In addition, spontaneous HP and SAP variabilities were extracted from 17 healthy humans (ages 21-36 yr, median age 29 yr; 9 females) at rest and during graded head-up tilt. The tilt table inclinations ranged from 15 to 75° and were changed in steps of 15°. All subjects underwent recordings at every step in random order. The approach detected the lack of causal relation from SAP to HP in STHT recipients and the gradual restoration of the causal link from SAP to HP with time after transplantation in the LTHT recipients. The head-up tilt protocol induced the progressive shift from the prevalent causal direction from HP to SAP to the reverse causality (i.e., from SAP to HP) with tilt table inclination in healthy subjects. Transformation of phases into time shifts and comparison with baroreflex latency supported this conclusion. The proposed approach is highly efficient because it does not require the knowledge of baroreflex latency. The dependence of causality on tilt table inclination suggests that "spontaneous" baroreflex sensitivity estimated using noncausal methods (e.g., spectral and cross-spectral approaches) is more reliable at the highest tilt table inclinations.     

Dynamic time course of hemodynamic responses after passive head-up tilt and tilt back to supine position.

Mechanisms involved in the control of arterial pressure during postural changes were studied by analysis of the dynamic time course of cardiovascular changes during head-up tilt (HUT) and tilt back to supine position (TB). Beat-to-beat values of cardiovascular variables were recorded continuously before, during, and after passive HUT to 30 degrees in seven healthy humans. Left cardiac stroke volume (SV, Doppler ultrasound), mean arterial blood pressure (MAP), heart rate (HR), cardiac output (CO), and total peripheral conductance (TPC) were recorded. During HUT, MAP at the level of the carotid baroreceptors decreased by approximately 5 mmHg. There was a striking asymmetry between the time courses of cardiovascular changes on HUT and on TB. Adjustments generally took up to 30 s after HUT, whereas most changes were completed during the first 10 s after TB. Cardiovascular reflex adjustments of HR and TPC were more symmetrical. After HUT, SV was maintained during the first 4-6 s and then decreased steadily during the next 30 s to a stable level approximately 25% below its pretilt value. However, after TB, SV increased rapidly to its pretilt value in <10 s. This asymmetry in SV dynamics may be explained in part by a more rapid change in left cardiac filling after TB than after HUT. On TB, there must be a rapid inflow of stagnant blood from the legs, whereas venous valves will impede backward filling of veins in the lower body on HUT. In conclusion, we have revealed a characteristic asymmetry in cardiovascular responses to inverse variations in gravity forces in humans. This asymmetry can be explained in part by nonlinear, hydrodynamic factors, such as the one-way effect of venous valves in the lower part of the body.     

Age dependency of cardiovascular autonomic responses to head-up tilt in healthy subjects.

In elderly subjects, heart rate responses to postural change are attenuated, whereas their vascular responses are augmented. Altered strategy in maintaining blood pressure homeostasis during upright position may result from various cardiovascular changes, including age-related cardiovascular autonomic dysfunction. This exploratory study was conducted to evaluate impact of age on cardiovascular autonomic responses to head-up tilt (HUT) in healthy subjects covering a wide age range. The study population consisted of 63 healthy, normal-weight, nonsmoking subjects aged 23-77 yr. Five-minute electrocardiogram and finger blood pressure recordings were performed in the supine position and in the upright position 5 min after 70 degrees HUT. Stroke volume was assessed from noninvasive blood pressure signals by the arterial pulse contour method. Heart rate variability (HRV) and systolic blood pressure variability (SBPV) were analyzed by using spectral analysis, and baroreflex sensitivity (BRS) was assessed by using sequence and cross-spectral methods. Cardiovascular autonomic activation during HUT consisted of decreases in HRV and BRS and an increase in SBPV. These changes became attenuated with aging. Age correlated significantly with amplitude of HUT-stimulated response of the high-frequency component (r = -0.61, P < 0.001) and the ratio of low-frequency to high-frequency power of HRV (r = -0.31, P < 0.05) and indexes of BRS (local BRS: r = -0.62, P < 0.001; cross-spectral baroreflex sensitivity in the low-frequency range: r = -0.38, P < 0.01). Blood pressure in the upright position was maintained well irrespective of age. However, the HUT-induced increase in heart rate was more pronounced in the younger subjects, whereas the increase in peripheral resistance was predominantly observed in the older subjects. Thus it is likely that whereas the dynamic capacity of cardiac autonomic regulation decreases, vascular responses related to vasoactive mechanisms and vascular sympathetic regulation become augmented with increasing age.     

Differential blood pressure and heart rate responses to supramedullary brain stimulation in cats

The purpose of this study was to compare the cardiovascular responses to electrical stimulation of different supramedullary brain regions. Arterial blood pressure (BP) and heart rate (HR) effects were elicited by electrical stimulation of the lateral hypothalamus (LH), mamillary bodies (Mm), substantia nigra (SN), globus pallidus (GP), and the subthalamic nucleus (Sub) in conscious, freely moving cats. Pressor responses were obtained from all of these regions. The higher intensity of stimulation the higher increase in BP and HR was obtained. However, clear-cut differences occurred in the effects both during and after the termination of stimulations. Namely, a continuous increase in BP and HR was obtained from the LH and SN. In contrast, the initial increase in BP and HR was followed by a reduction compared to the peak value of the effects of stimulation in the GP and the Sub. However, the BP and HR never reduced to the pre-stimulaion level during the stimulation. Also the changes following the cessation of stimulation at the different brain loci were dissimilar. The BP and HR either returned gradually to the pre-stimulation level, or long-lasting oscillation occurred. The electrical activity of the nucleus of the solitary tract (NTS) and the vagus nerve co-varied with the changes in BP and HR. It is concluded that the supramedullary stimulations produce differential cardiovascular effects, and these effects are modified by the baroreflexes that are activated by the electrically elicited rise in blood pressure.     

Effect of hydration on some orthostatic and haematological responses to head-up tilt

Experiments were undertaken to determine the effects of hydration status on a) orthostatic responses, and on b), relative changes in intravascular volume and protein content, during 70 degrees head-up tilt (HUT). Six men underwent 45 min of HUT, preceded by 45 min supine, first dehydrated, and again 105 min later after rehydration with water. Heart rate was consistently lower following rehydration (p less than 0.01), while supine diastolic pressure was higher (p less than 0.02). Systolic pressure fell during dehydrated HUT (p less than 0.01), but not during rehydrated HUT. Postural haemoconcentration, which was reduced after rehydration (p less than 0.001), was accompanied by a decrease in intravascular albumin content (p less than 0.05). Two subjects experienced severe presyncopal symptoms during dehydrated HUT, but not during rehydrated HUT. Thus, it appears that rehydration after fluid restriction improves orthostatic tolerance. Furthermore, extravascular hydration status may be more important than intravascular hydration status in determining orthostatic tolerance.     

Effects of exercise and passive head-up tilt on fractal and complexity properties of heart rate dynamics

tk;1Passive head-up tilt and exercise result in specific changes in the spectral characteristics of heart rate (HR) variability as a result of reduced vagal and enhanced sympathetic outflow. Recently analytic methods based on nonlinear system theory have been developed to characterize the nonlinear features in HR dynamics. This study was designed to assess the changes in the fractal and complexity measures of HR behavior during the passive head-up tilt and during exercise. Fractal exponent (alpha(1)) and approximate entropy (ApEn), measures of short-term correlation properties and overall complexity of HR, respectively, along with spectral components of HR variability were analyzed during a passive head-up tilt test (n = 10) and a low-intensity steady-state exercise (n = 20) in healthy subjects. We observed that alpha(1) increased during the tilt test (from 0.85 +/- 0.22 to 1.48 +/- 0.20; P < 0.001) and during the exercise (from 1.00 +/- 0.22 to 1.37 +/- 0. 14; P < 0.001). ApEn also increased during the exercise (from 1.04 +/- 0.11 to 1. 11 +/- 0.08; P < 0.05), but it did not change during the tilt test. The normalized high-frequency spectral component decreased and the low-frequency component increased similarly during both the exercise and the tilt test (P < 0.001 for all). Exercise and passive tilt result in an increase of short-term fractal correlation properties of HR dynamics, which is related to changes in the balance between the low- and high-frequency oscillations in controlled situations. Overall complexity of HR dynamics increases during exercise but not during passive tilt.     

The blood pressure and heart rate chronome of centenarians.

Rhythm characteristics of blood pressure (BP) and heart rate (Hr) of 11 healthy centenarians and 66 medical students are described. Each subject ambulatorily monitored measured BP and HR around the clock at 15-min intervals for 48 hours. Least-squares spectra were obtained by the fit of cosine curves (cosinor) and compared between the two populations. Confounding by geographic differences seems to be ruled out by comparisons with results from international data bases. A shift in prominence from the circadian domain to higher frequency harmonics was found for the BP but not for the HR of centenarians. In clinically mostly healthy centenarians, markers of primary aging may consist of a relatively low circadian BP and HR amplitude and a tendency toward internal and external desynchronization. Whether these chronobiologic changes with age are desirable, indifferent or undesirable can now be elucidated by outcome studies, in the light of the reference standards provided herein.     

A reduced xylem pressure altered the electric and growth responses in cucumber hypocotyls

This study measured the electric and growth responses in excised cucumber hypocotyls and compared them with those in intact seedlings. Root excision (first severing cut) eliminated most of the positive xylem pressure (P_x) in the hypocotyl, caused a rapid, transient drop in the hypocotyl growth rate (GR) and some small, local depolarization near the cut site. Although accompanied by a smaller decrease in P_x, a second, severing cut in the basal hypocotyl caused a decrease in GR which was no longer transient and a depolarization which was increased in both size and extent. These changes were not wound effects because they could be simulated by root incubation in mannitol. The reduced GR recovery occurred also in the absence of electric changes after a second increase in the mannitol concentration incubating the root of intact seedlings. Increased electric sensitivity and altered growth response therefore appear to be two independent examples of physiological changes resulting from a lowered P_x.     

Differences between heart rate and blood pressure variability in schizophrenia.

Heart rate and blood pressure variability parameters were assessed to determine the risk of cardiac mortality in schizophrenia. We investigated 21 acute, unmedicated patients with paranoid schizophrenia and 21 matched controls. Cardiovascular parameters obtained included heart rate variability, blood pressure variability, cardiac output and left ventricular work index. All parameters investigated were analyzed using linear and non-linear techniques. These investigations revealed increased left ventricular work index and reduced heart rate variability. Furthermore, blood pressure was significantly higher compared to controls, whereas its variability was unchanged. We conclude that our results reflect autonomic cardiovascular dysregulation in acute schizophrenia.     

Blood pressure and heart rate responses to sudden change of posture during 20 days of simulated microgravity (-6 degrees head-down tilt)

The cardiovascular function buffering the disturbance of blood pressure caused by postural changes may be deconditioned after exposure to microgravity (microG). However, total picture of the deconditioning including its longitudinal process is still unknown. The aim of this study was to determine time-dependent changes in the feedback regulation system of blood pressure as exposed to simulated microG (-6 degrees head-down tilt (HDT)) for 20 days.     

Changes in blood pressure, heart rate and blood constituents during heat exposure in men with elevated blood pressure

Although the vascular volume response of hypertensive men during exercise has been rather well characterized, the effect of resting heat exposure in this patient population has not been examined. This was done in the present report in seven men with high blood pressure (BP) (i.e., diastolic pressure greater than 12 kPa (90 mmHg) upon initial interview) and 5 normotensive control subjects. 50 min after each subject had consumed an amount of water equal to 1% of his body weight, he reclined on a cot. 10 min later the subject was carried into an environmental chamber equilibrated at Tdb = 45 degrees C, Twb = 28 degrees C. Free-flowing venous blood samples were obtained from a cubital vein, and BP and heart rate were measured, before the heat exposure and at 15 min intervals during the experiment. Within 30 min systolic, diastolic and mean BP of the high BP subjects had decreased to normal levels; no BP changes were detected in normotensive subjects. Accompanying this depressor response was an exaggerated elevation in plasma glucose concentration. No alterations were found with haematocrit, plasma osmolality or electrolytes, or total protein and albumin. The data suggest that heat exposure may have been more stressful for the subjects with high BP than for their controls. This finding implies that phasic depressor responses may be as important as phasic pressor episodes in the aetiology of established essential hypertension.     

Intrapericardial denervation: radial artery blood flow and heart rate responses to LBNP

Eight rhesus monkeys were used to study responses of radial artery blood flow velocity (RABFV) and heart rate (HR) to low (0 to -20 mmHg) and high (0 to -60 mmHg) ramp exposures during supine lower body negative pressure (LBNP). These levels were chosen to separate peripheral vascular responses associated with stimulation of low- and high-pressure baroreceptors. Four monkeys had efferent and afferent cardiac denervation by use of the Randall procedure with pharmacological (phenylephrine and atropine) verification. Animals were studied 3 wk after surgery to avoid reinnervation. Findings were compared with those of four identically treated intact animals. Denervated animals showed no change in RABFV or HR during low-level LBNP; however, HR increased significantly (P less than 0.05) when LBNP reached -50 mmHg and blood flow velocity also fell (P less than 0.05) starting at -30 mmHg pressure. In contrast, intact animals showed steady decreases in RABFV during both high- and low-pressure protocols, with HR showing a 6-beat/min increase (P less than 0.05) starting at -20 mmHg pressure. As with denervated animals, intact animals showed a more pronounced increase in HR after reaching a level of -60 mmHg suction. Cardiac output (electromagnetic flowmeter, ascending aorta) fell significantly in both groups starting at -30 mmHg pressure. Left ventricular pressure (Konigsberg pressure cell) in three intact animals showed a progressive fall in systolic pressure starting at -10 mmHg suction, which became significant at -55 mmHg pressure. These results demonstrate that cardiac denervation by use of the Randall technique significantly affects RABFV and HR responses to LBNP in rhesus monkeys. The lack of RABFV change during LBNP in denervated animals suggests that these changes coupled with HR response can be used as an effective method to verify the completeness of denervation of low-pressure baroreceptors in animals that have undergone intrapericardial denervation.     

Heart rate and blood pressure responses to isometric exercise in young and older men

The aim of this study was to examine the isometric endurance response and the heart rate and blood pressure responses to isometric exercise in two muscle groups in ten young (age 23–29 years) and seven older (age 54–59 years) physically active men with similar estimated forearm and thigh muscle masses. Isometric contractions were held until fatigue using the finger flexor muscles (handgrip) and with the quadriceps muscle (one-legged knee extension) at 20%, 40%, and 60% of the maximal voluntary contraction (MVC). Heart rate and arterial pressure were related to the the individual's contraction times. The isometric endurance response was longer with handgrip than with one-legged knee extension, but no significant difference was observed between the age groups. The isometric endurance response averaged 542 (SEM 57), 153 (SEM 14), and 59 (SEM 5) s for the handgrip, and 276 (SEM 35), 94 (SEM 10) and 48 (SEM 5) s for the knee extension at the three MVC levels, respectively. Heart rate and blood pressure became higher during one-legged knee extension than during handgrip, and with increasing level of contraction. The older subjects had a lower heart rate and a higher blood pressure response than their younger counterparts, and the differences were more apparent at a higher force level. The results would indicate that increasing age is associated with an altered heart rate and blood pressure response to isometric exercise although it does not affect isometric endurance. Accepted: 23 October 1997     

Central muscarinic modulation of fetal blood pressure and heart rate

It has previously been demonstrated that the fetal lamb cardiovascular system can respond to peripheral muscarinic stimulation. However the role of central muscarinic mechanisms in modulating fetal cardiovascular function has not been described. Pilocarpine is a muscarinic agonist that readily crosses the blood-brain barrier and was therefore employed to examine both central and peripheral muscarinic mechanisms in modulating fetal cardiovascular function. Fetal lambs were prepared for chronic intrauterine recording of fetal blood pressure (FBP) and heart rate (FHR). Direct administration of pilocarpine to the fetus resulted in an immediate dose-dependent decrease in both systolic and diastolic blood pressure and a rapid fall in FHR. The initial phase of hypotension was very short-lived (1-2 min) and was subsequently followed by a significant increase in systolic, diastolic and pulse pressures (30-60 min). Fetal heart rate gradually returned to control levels by 30 min after pilocarpine administration. Atropine pretreatment was effective in completely blocking the cardiovascular actions of pilocarpine, while methylatropine was only able to block the initial hypotensive and bradycardiac response. A prolonged tachycardia was also unmasked by methylatropine pretreatment. These data suggest that the initial hypotension and bradycardia in response to pilocarpine administration are mediated via peripheral muscarinic receptors, while stimulation of central muscarinic receptors result in hypertension and tachycardia. These data confirm that, as in the adult, central cholinergic mechanisms are involved in the modulation of cardiovascular function in the developing fetus.