Haemodynamic changes during neck pressure and suction in seated and supine positions

Heat stress enhances arterial baroreflex control of muscle sympathetic nerve activity via increased sensitivity of burst gating, not burst area, in humans

The relationship between muscle sympathetic nerve activity (MSNA) and diastolic blood pressure has been used to describe two sites for arterial baroreflex control of MSNA. By determining both the likelihood of occurrence for sympathetic bursts and the area of each burst for a given diastolic blood pressure, both a 'gating' and an 'area' control site has been described in normothermic humans. Assessing the effect of heat stress on these mechanisms will improve the understanding of baroreflex control of arterial blood pressure under this thermal condition. Therefore, the purpose of this study was to test the hypothesis that heat stress enhances arterial baroreflex control of burst gating and area. In 10 normotensive subjects (age, 32+/-2 years; mean+/-s.e.m.), MSNA (peroneal) was assessed using standard microneurographic techniques. Five minute periods of data were examined during normothermic and whole-body heating conditions. The burst incidence (i.e. number of sympathetic bursts per 100 cardiac cycles) and the area of each burst were determined for each cardiac cycle and were placed into 3 mmHg intervals of diastolic blood pressure. During normotheric conditions, there was a moderate, negative relationship between burst incidence and diastolic blood pressure (slope=-2.49+/-0.38; r(2)=0.73+/-0.06; mean+/-s.e.m.), while area per burst relative to diastolic blood pressure exhibited a less strong relationship (slope=-1.13+/-0.46; r(2)=0.45+/-0.09). During whole-body heating there was an increase in the slope of the relationship between burst incidence and diastolic blood pressure (slope=-4.69+/-0.44; r(2)=0.84+/-0.03) compared to normothermia (P<0.05), while the relationship between area per burst and diastolic blood pressure was unchanged (slope=-0.92+/-0.29; r(2)=0.41+/-0.08) (P=0.50). The primary finding of this investigation is that, at rest, whole-body heating enhanced arterial baroreflex control of MSNA through increased sensitivity of a 'gating' mechanism, as indicated by an increase in the slope of the relationship between burst incidence and diastolic blood pressure. This occurrence is likely to afford protection against potential decreases in arterial blood pressure in an effort to preserve orthostatic tolerance during heat stress.

Effects of Sevoflurane Anesthesia on Carotid-Cardiac Baroreflex Responses in Humans

Sevoflurane depresses cardio-vagal baroreflex gain (ability of vagally mediated R-R interval response to arterial blood pressure change). We examined the effects of sevoflurane anesthesia on maximum buffering capacity of vagally mediated hemodynamic control (baroreflex range) by examining the entire stimulus-response baroreflex relation. Electrocardiogram and invasive arterial blood pressure were monitored in 11 healthy volunteers. Carotid-cardiac baroreflex responses were elicited by increasing neck chamber pressure (external pressure applied over the bilateral carotid sinuses) to 40 mm Hg for 5 heartbeats followed by decreasing chamber pressure by successive 15-mm Hg R-wave triggered decrements to -65 mm Hg during held expiration. R-R intervals were plotted as functions of preceding carotid distending pressure. Range, maximum gain, and operational point (relative position of the resting set point within the entire baroreflex response curve) were determined at conscious baseline, during 2% (end-tidal) sevoflurane anesthesia, without and with phenylephrine infusion to maintain conscious arterial blood pressure, and at 30, 60, 120, and 180 min after emergence from anesthesia. Sevoflurane anesthesia significantly depressed maximum gain (from 3.84 +/- 0.99 to 1.04 +/- 0.40 ms/mm Hg [mean +/- sd]; P < 0.001) and range (from 207 +/- 43 to 52 +/- 19 ms; P < 0.001) of the reflex relation, both of which recovered at 120 and 180 min after emergence. Phenylephrine infusion only partially restored these variables. The operational point was unchanged throughout the study. Our results indicate that maximum cardio-vagal compensatory response to buffer hemodynamic perturbation is depressed during sevoflurane anesthesia. Sevoflurane-induced hypotension, which produced vagal withdrawal, did not play an important role in depressing cardio-vagal reflex function.

Carotid-Cardiac Baroreflex Function Does Not Influence Blood Pressure Regulation during Head-Up Tilt in Humans

The influence of the carotid-cardiac baroreflex on blood pressure regulation was evaluated during supine rest and 40 degrees head-up tilt (HUT) in 9 healthy young subjects with and without full cardiac vagal blockade. The carotid baroreflex responsiveness, or maximal gain (G(MAX)), was assessed from the beat-to-beat changes in heart rate (HR) and mean arterial pressure (MAP) by the variable neck pressure and suction technique ranging in pressure from +40 to -80 Torr, with and without glycopyrrolate (12.0 +/- 1.0 microg/kg body weight; mean +/- SE). In the supine position, glycopyrrolate increased the HR to 91 +/- 3 bpm, from 54 +/- 3; MAP to 89 +/- 2 mmHg, from 76 +/- 2; and cardiac output to 6.8 +/- 0.3 l.min(-1), from 4.9 +/- 0.3 (P < 0.05). The G(MAX) of the carotid baroreflex control of HR was reduced to -0.06 +/- 0.01 bpm.mmHg(-1), from -0.30 +/- 0.02 (P < 0.05) with no significant effect on the G(MAX) of the carotid baroreflex control of MAP. During HUT the carotid baroreflex control of MAP was unchanged, though the G(MAX) of the carotid baroreflex control of HR was increased (P < 0.05). During HUT, central blood volume, assessed by electrical thoracic admittance, and total vascular conductance were decreased with and without glycopyrrolate. Furthermore, glycopyrrolate reduced G(MAX) of the carotid baroreflex control of HR during HUT (P < 0.05) with no significant effect on G(MAX) of the carotid baroreflex control of MAP. These data suggest that during supine rest and HUT-induced decreases in central blood volume, the carotid baroreflex control of HR is mediated primarily via parasympathetic activity. Furthermore, the maintenance of arterial blood pressure during postural stress is primarily mediated by arterial and cardiopulmonary reflex regulation of sympathetic activity and its effects on the systemic vasculature.

Arterial baroreflex resetting during exercise: a current perspective

Within the past 20 years numerous animal and human experiments have provided supportive evidence of arterial baroreflex resetting during exercise. In addition, it has been demonstrated that both the feedforward mechanism of central command and the feedback mechanism associated with skeletal muscle afferents (the exercise pressor reflex) play both independent and interactive roles in the resetting of the arterial baroreflex with exercise. A fundamental alteration associated with baroreflex resetting during exercise is the movement of the operating point of the reflex away from the centring point and closer to the threshold, thereby increasing the ability of the reflex to buffer hypertensive stimuli. Recent studies suggest that central command and the cardiopulmonary baroreceptors may play a role in this movement of the operating point on the baroreflex-heart rate and baroreflex-blood pressure curve, respectively. Current research is focusing on the investigation of central neural mechanisms involved in cardiovascular control, including use of electrophysiological and molecular biological techniques in rat and mouse models to investigate baroreflex resetting as well as use of state of the art brain imaging techniques in humans. However, the purpose of this review is to describe the role of the arterial baroreflex in the regulation of arterial blood pressure during physical activity from a historical perspective with a particular emphasis on human investigations.

The effect of changes in cardiac output on middle cerebral artery mean blood velocity at rest and during exercise

We examined the relationship between changes in cardiac output and middle cerebral artery mean blood velocity (MCA V(mean)) in seven healthy volunteer men at rest and during 50% maximal oxygen uptake steady-state submaximal cycling exercise. Reductions in were accomplished using lower body negative pressure (LBNP), while increases in were accomplished using infusions of 25% human serum albumin. Heart rate (HR), arterial blood pressure and MCA V(mean) were continuously recorded. At each stage of LBNP and albumin infusion was measured using an acetylene rebreathing technique. Arterial blood samples were analysed for partial pressure of carbon dioxide tension (P(a,CO2). During exercise HR and were increased above rest (P < 0.001), while neither MCA V(mean) nor P(a,CO2) was altered (P > 0.05). The MCA V(mean) and were linearly related at rest (P < 0.001) and during exercise (P = 0.035). The slope of the regression relationship between MCA V(mean) and at rest was greater (P = 0.035) than during exercise. In addition, the phase and gain between MCA V(mean) and mean arterial pressure in the low frequency range were not altered from rest to exercise indicating that the cerebral autoregulation was maintained. These data suggest that the associated with the changes in central blood volume influence the MCA V(mean) at rest and during exercise and its regulation is independent of cerebral autoregulation. It appears that the exercise induced sympathoexcitation and the change in the distribution of between the cerebral and the systemic circulation modifies the relationship between MCA V(mean) and .

The concentration-dependent effects of general anesthesia on spontaneous baroreflex indices and their correlations with pharmacological gains

Beat-to-beat assessment of spontaneously occurring fluctuations in heart rate and arterial blood pressure allows noninvasive determination of cardiovagal function, but little is known regarding the effects of general anesthesia on spontaneous baroreflex (SBR) indices. We examined (a) concentration-dependent effects of sevoflurane on SBR indices, heart rate variability (HRV), and blood pressure variability and (b) correlation and agreement between pharmacological baroreflex gains and SBR indices during sevoflurane anesthesia. Continuous electrocardiogram and invasive arterial blood pressure were monitored in nine healthy volunteers before, during, and for 3 h after sevoflurane anesthesia, during which end-tidal sevoflurane was maintained at 0.7%, 1.4%, and 2.0% in random sequences. We derived three SBR indices (sequence method, alpha-index, and low-frequency transfer function) and compared them with pressor and depressor test gains by the pharmacological method. HRV and blood pressure variability were analyzed at a fixed respiratory rate (12 breaths/min) in awake and anesthetized conditions. Except for low-frequency transfer function, SBR indices were depressed by sevoflurane and remained depressed for 30 min after emergence from anesthesia, compared with the conscious baseline value. Spontaneous sequence indices and high- and low-frequency powers of HRV demonstrated concentration-dependent depression. Pharmacological gains and SBR indices during anesthesia generally correlated well, but Bland-Altman analysis revealed that SBR indices had limits of agreement as large as the baroreflex gain itself. These data suggest that spontaneous indices are inadequate estimates of, and are inconsistent with, the pharmacological baroreflex gain during sevoflurane anesthesia.

Whole-body heating slows carotid baroreflex response in human subjects

Heat stress increases sympathetic activity and decreases parasympathetic activity to the heart. To test the hypothesis that carotid baroreflex responses of heart rate (HR) and systemic blood pressure become slowed with altered autonomic nerve activities during whole-body heat stress, we determined changes in HR and mean arterial pressure (MAP) in response to approximately 5 s of 40 mmHg neck pressure (NP) and of -65 mmHg neck suction (NS) in normothermia and during whole-body heating produced by a hot water-perfused suit. The NP and NS stimuli were triggered by R waves of an ECG during held expiration in the supine position. Whole-body heating did not alter the onset time of the HR and MAP responses during NP and NS. Whole-body heating significantly increased the time from onset of the HR response until peak of the response during NP (2.53 +/- 0.33 s in normothermia and 3.46 +/- 0.28 s during heating, P<0.05) and NS (1.20 +/- 0.23 s and 2.24 +/- 0.29 s, P<0.05). Whole-body heating significantly increased the time from onset of the MAP response until peak of the response during NP (4.31+/-0.46 s in normothermia, 6.67 +/- 0.56 s during heating, P<0.05) but not during NS (5.06 +/- 0.47 s and 4.50 +/- 0.60 s). These findings suggest that heat stress prolongs the response time of carotid-cardiac and carotid-vasomotor baroreflexes.

Autonomic nervous system influence on arterial baroreflex control of heart rate during exercise in humans

A combination of sympathoexcitation and vagal withdrawal increases heart rate (HR) during exercise, however, their specific contribution to arterial baroreflex sensitivity remains unclear. Eight subjects performed 25 min bouts of exercise at a HR of 90, 120, and 150 beats min-1, respectively, with and without metoprolol (0.16 +/- 0.01 mg kg(-1); mean +/- S.E.M.) or glycopyrrolate (12.6 +/- 1.6 microg kg-1). Carotid baroreflex (CBR) function was determined using 5 s pulses of neck pressure (NP) and neck suction (NS) from +40 to -80 Torr, while transfer function gain (GTF) was calculated to assess the linear dynamic relationship between mean arterial pressure and HR. Spontaneous baroreflex sensitivity (SBR) was evaluated as the slope of sequences of three consecutive beats in which systolic blood pressure and the R-R interval of the ECG either increased or decreased, in a linear fashion. The beta-1 adrenergic blockade decreased and vagal cardiac blockade increased HR both at rest and during exercise (P < 0.05). The gain at the operating point of the modelled reflex function curve (GOP) obtained using NP and NS decreased with workload independent of beta-1 adrenergic blockade. In contrast, vagal blockade decreased GOP from -0.40 +/- 0.04 to -0.06 +/- 0.01 beats min-1 mmHg-1 at rest (P < 0.05). Furthermore, as workload increased both GOP and SBR, and GOP and GTF were correlated (P < 0.001), suggesting that the two dynamic methods applied to evaluate arterial baroreflex (ABR) function provide the same information as the modelled GOP. These findings suggest that during exercise the reduction of arterial baroreceptor reflex sensitivity at the operating point was a result of vagal withdrawal rather than an increase in sympathetic activity.

Muscle metaboreflex modulates the arterial baroreflex dynamic effects on peripheral vascular conductance in humans

We aimed to investigate the interaction between the arterial baroreflex and muscle metaboreflex [as reflected by alterations in the dynamic responses shown by leg blood flow (LBF: by the ultrasound Doppler method), leg vascular conductance (LVC), mean arterial blood pressure (MAP), and heart rate (HR)] in humans. In 12 healthy subjects (10 men and 2 women), who performed sustained 1-min handgrip exercise at 50% maximal voluntary contraction followed immediately by an imposed postexercise muscle ischemia (PEMI), 5-s periods of neck pressure (NP; 50 mmHg) or neck suction (NS; -60 mmHg) were used to evaluate carotid baroreflex function both at rest (Con) and during PEMI. First, the decreases in LVC and LBF and the augmentation of MAP elicited by NP were all greater during PEMI than in Con (DeltaLVC, -1.2 +/- 0.2 vs. -1.9 +/- 0.2 ml.min(-1).mmHg(-1); DeltaLBF, -97.3 +/- 11.2 vs. -177.0 +/- 21.8 ml/min; DeltaMAP, 6.7 +/- 1.2 vs. 11.5 +/- 1.4 mmHg, Con vs. PEMI; each P < 0.05). Second, in Con, NS significantly increased both LVC and LBF (DeltaLVC, 0.9 +/- 0.2 ml.min(-1).mmHg(-1); DeltaLBF, 46.6 +/- 9.8 ml/min; significant change from baseline: each P < 0.05), and, whereas during PEMI no significant increases in LVC and LBF occurred during NS itself (DeltaLVC, 0.2 +/- 0.1 ml.min(-1).mmHg(-1); DeltaLBF, 10.8 +/- 9.6 ml/min; each P > 0.05), a decrease was evident in each parameters at 5 s after the cessation of NS. Third, during PEMI, the decrease in MAP elicited by NS was smaller (DeltaMAP, -8.4 +/- 1.0 vs. -5.8 +/- 0.4 mmHg, Con vs. PEMI; P < 0.05), and it recovered to its initial level more quickly after NS (vs. Con). Finally, however, the HR responses to NS and NP were not different between PEMI and Con. These results suggest that during muscle metaboreflex activation in humans, the arterial baroreflex dynamic effect on peripheral vascular conductance is modulated, as exemplified by 1) an augmentation of the NP-induced LVC decrease, and 2) a loss of the NS-induced LVC increase.

Test of dynamic closed-loop baroreflex and autoregulatory control of total peripheral resistance in intact and conscious sheep

This is the first study able to examine and delineate the actual actions of the physiological mechanisms responsible for the dynamic couplings between cardiac output (CO), arterial pressure (Pa), right atrial pressure (PRA), and total peripheral resistance (TPR) in an individual subject without altering the underlying regulatory mechanisms. Eight conscious male sheep were used, where both types of baroreceptors were independently exposed to simultaneous beat-to-beat pressure perturbations under intact closed-loop conditions while CO, Pa, PRA, and TPR were measured. We applied the cardiovascular system identification method proposed in a companion paper ( 4 ) to quantitatively characterize the dynamic closed-loop transfer relations CO→Pa, PRA→Pa, Pa→TPR, and PRA→TPR from the measured signals. To validate the dynamic properties of the estimated transfer relations, the essential parts of the linear dynamics of the model were independently and comprehensively evaluated via error model cross-validation, and the overall model's steady-state behavior was compared with a separate random effects regression approach. In addition to numerous physiological findings, we found that the cardiovascular system identification results were exceptionally consistent with the analytically derived solutions previously discussed in Ref. 4 . In conclusion, this study presents the first time validation of a cardiovascular system identification method by means of experimentally acquired animal data in the intact and conscious animal and offers a set of powerful quantitative tools essential to advancing our knowledge of cardiovascular regulatory physiology.

Carotid baroreflex control of leg vasculature in exercising and non-exercising skeletal muscle in humans

Carotid baroreflex (CBR) function was examined in five men and three women (25 +/- 1 years) using the variable-pressure neck collar technique at rest and during dynamic, one-legged knee extension exercise at 7 W and 25 W. The CBR exhibited control of leg vascular conductance (LVC) at rest and during exercise in both an exercising leg (EL) and a non-exercising leg (NEL) across a wide range of pressures from +40 Torr neck pressure (NP) to -80 Torr neck suction (NS). Specifically, increases in LVC (% change) in response to NS were no different across -20 to -80 Torr in either EL or NEL compared to rest, P > 0.05. However, CBR-mediated decreases in percentage LVC in response to NP were attenuated in EL at both 7 W (16 +/- 1%) and 25 W (12 +/- 1%) compared to rest (40 +/- 3%; P < 0.05) as well as compared to responses in the NEL (36 +/- 6% at 7 W and 36 +/- 7% at 25 W; P < 0.05). This decrease in vascular responsiveness in EL was associated with a reduction in the gain of the percentage muscle sympathetic nerve activity (%MSNA)-%LVC relationship compared to rest (P < 0.05). Collectively, these data indicate that, despite a clear attenuation of the vascular response to MSNA in the exercising leg, CBR-mediated changes in mean arterial pressure were no different between rest and exercise.

Arterial blood pressure and carotid baroreflex function during arm and combined arm and leg exercise in humans

AIM

During arm cranking (A) blood pressure is higher than during combined arm and leg exercise (A + L), while the carotid baroreflex (CBR) is suggested to reset to control a higher blood pressure in direct relation to work intensity and the engaged muscle mass.

METHOD

This study evaluated the function of the CBR by using neck pressure and neck suction during upright A, L and A + L in 12 subjects and, in order to evaluate a potential influence of the central blood volume on the CBR, also during supine A in five subjects. Exercise intensities for A and L were planned to elicit a heart rate response of c. 100 and 120 beats min(-1), respectively, in the upright position and both workloads were maintained during A + L and supine A.

RESULTS

The CBR operating point, corresponding to the pre-stimulus blood pressure, was 88 +/- 6 mmHg (mean +/- SE) at rest. During upright A, L and A + L and supine A it increased to 109 +/- 9, 95 +/- 7, 103 +/- 7 and 104 +/- 4 mmHg, respectively, and it was thus higher during upright A than during A + L and supine A (P < 0.05). In addition, the CBR threshold and saturation pressures, corresponding to the minimum and maximum carotid sinus pressure, respectively, were higher during upright A than during supine A, A + L, L and at rest (P < 0.05) with no significant change in the maximal reflex gain.

CONCLUSION

These findings demonstrate that during combined arm and leg and exercise in the upright position the CBR resets to a lower blood pressure than during arm cranking likely because the central blood volume is enhanced by the muscle pump of the legs.

Spontaneous beat-by-beat fluctuations of total peripheral and cerebrovascular resistance in response to tilt

Beat-by-beat estimates of total peripheral resistance (TPR) can be obtained from continuous measurements of cardiac output by using Doppler ultrasound and noninvasive mean arterial blood pressure (MAP). We employed transfer function analysis to study the heart rate (HR) and vascular response to spontaneous changes in blood pressure from the relationships of systolic blood pressure (SBP) to HR (SBP→HR), MAP to total peripheral resistance (TPR) and cerebrovascular resistance index (CVRi) (MAP→TPR and MAP→CVRi), as well as stroke volume (SV) to TPR in nine healthy subjects in supine and 45° head-up tilt positions. The gain of the SBP→HR transfer function was reduced with tilt in both the low- (0.03–0.15 Hz) and high-frequency (0.15–0.35 Hz) regions. In contrast, MAP→TPR transfer function gain was not affected by head-up tilt, but it did increase from low- to high-frequency regions. The phase relationships between MAP→TPR were unaffected by head-up tilt, but, consistent with an autoregulatory system, changes in MAP were followed by directionally similar changes in TPR, just as observed for the MAP→CVRi. The SV→TPR had high coherence with a constant phase of 150–160°. Together, these data that showed changes in MAP preceded changes in TPR, as well as a possible link between SV and TPR, are consistent with complex interactions between the vascular component of the arterial and cardiopulmonary baroreflexes and intrinsic properties such as the myogenic response of the resistance arteries.

Dynamic carotid baroreflex control of the peripheral circulation during exercise in humans

We sought to determine the dynamic relationship between carotid baroreflex (CBR)-mediated control and local control of the skeletal muscle vasculature during dynamic exercise. In 12 subjects (18-35 years old), oscillatory neck pressure (NP, +40 mmHg) was applied at 0.1 Hz (i.e. 5 s on, 5 s off) for 5 min to determine the degree of CBR control over heart rate (HR), arterial blood pressure (ABP), muscle sympathetic nerve activity (MSNA), femoral blood velocity and skeletal muscle tissue oxygenation at rest and during 7 W dynamic knee-extension exercise. Skeletal muscle tissue oxygenation measurements of both the exercising and nonexercising leg were evaluated. Fast Fourier transformation was performed on 5 min segments to calculate spectral power of the R-R interval (RRI), ABP, MSNA, femoral blood velocity and tissue oxygenation time series, and the low-frequency (LF, 0.085-0.115 Hz) power spectra were compared to evaluate the degree of CBR-mediated entrainment for each variable. At rest, sinusoidal NP significantly increased LF spectral power of RRI, ABP, MSNA and femoral blood velocity. During exercise, sinusoidal NP provoked a similar increase in spectral power for RRI and MSNA, while CBR-mediated changes in ABP and femoral blood velocity were attenuated compared to rest. Changes in spectral power of skeletal muscle tissue oxygenation during sinusoidal NP were similar between the exercising and nonexercising leg at rest. However, during exercise the changes in skeletal muscle tissue oxygenation power were significantly less in the exercising leg, while changes in the nonexercising leg were similar to rest. We have demonstrated simultaneous entrainment of all CBR end-organ measurements, ranging from cardiac chronotropic effects to alterations at the level of the skeletal muscle microcirculation. Moreover, we have identified a significant and specific attenuation of end-organ responsiveness to CBR-mediated sympathoexcitation in the vasculature of the exercising muscle. However, despite a shift towards more predominant local control over the exercising muscle vasculature, systemic arterial blood pressure was well preserved.

Modulation of arterial baroreflex dynamic response during mild orthostatic stress in humans

We tested the hypothesis that in humans, carotid-baroreflex dynamic responses (evaluated by examining the time course of the carotid-baroreflex-induced alterations in muscle sympathetic nerve activity (MSNA), mean arterial blood pressure (MAP) and heart rate (HR)) would be altered during mild orthostatic stress in ways that serve to limit orthostatic hypotension. In 12 healthy subjects (10 male, 2 female), 5-s periods of neck pressure (NP) (50 mmHg) and neck suction (NS) (-60 mmHg) were used to evaluate carotid baroreflex function at rest (CON) and during lower body negative pressure (LBNP) (-15 mmHg). During LBNP (as compared with CON) (a) the augmentations in MSNA and MAP elicited by NP were greater, (b) the NS-induced period of MSNA suppression was, if anything, shorter, (c) the peak decrement in MAP elicited by NS, although not different in amplitude, occurred earlier and recovered to its initial level more quickly after NS, and (d) the HR responses to NP and NS were greater. These results suggest that during mild orthostatic stress, carotid-baroreflex dynamic responses are modulated in ways that should help maintain blood pressure and limit orthostatic hypotension.

Influence of menstrual cycle on baroreflex control of heart rate: comparison with male volunteers

This study was designed to determine baroreflex control of heart rate (HR) to hypotensive and hypertensive stimuli during the early follicular (EF), preovulation (PreOV), and midluteal (ML) phases of the menstrual cycle and to test the hypothesis that cardiovagal reflex responses to hypertensive stimuli would be altered depending on the plasma estradiol levels in healthy women. In addition, these results were compared with those of male volunteers. Fifteen healthy women with regular menstrual cycles and thirteen male volunteers were recruited. Cardiovagal baroreflex sensitivity was defined as the slope of the linear portion relating R-R interval and systolic blood pressure triggered by bolus injections of nitroprusside and phenylephrine, from the overshoot phase of the Valsalva maneuver, and during spontaneous fluctuations. Three measurements were averaged in each test as a representative at each phase, and the order of phases was counterbalanced. Baroreflex sensitivities by the phenylephrine pressor test and Valsalva maneuver during the PreOV phase were significantly greater than those during the EF and ML phases but were similar to those of men. Depressor test sensitivities by nitroprusside and down-sequence spontaneous cardiac baroreflex sensitivity during the EF phase were significantly greater than those of the ML phase and of men. Significant correlations were observed between plasma estradiol concentrations and baroreflex sensitivities assessed by phenylephrine and the Valsalva maneuver. Our results indicate that baroreflex control of HR is altered during the regular menstrual cycle, and estradiol appears to exert cardiovagal modulation in healthy women.

Carotid baroreflex responsiveness to head-up tilt-induced central hypovolaemia: effect of aerobic fitness

This investigation examined the interaction between carotid baroreflex (CBR) responsiveness during head-up tilt (HUT)-induced central hypovolaemia and aerobic fitness. Seven average fit (AF) individuals, with a mean maximal oxygen uptake (VO2max) of 49 +/- 1 (ml O2) kg-1 min-1, and seven high fit (HF) individuals, with a VO2max of 61 +/- 1 (ml O2) kg-1 min-1, voluntarily participated in the investigation. After 10-15 min supine, each subject was exposed to nine levels of progressively increasing HUT by 10 deg increments from -20 deg to +60 deg. During the final 3 min of each stage of HUT, the CBR responsiveness was measured using a rapid pulse (500 ms) train of neck pressure (NP) and neck suction (NS) ranging from +40 to -80 Torr. The maximal gain of the carotid-HR (Gmax-HR) and carotid-MAP (Gmax-MAP) baroreflex function curves was identified as measures of CBR responsiveness. During HUT-induced decreases in thoracic admittance, an index of central blood volume (CBV), the Gmax-HR and Gmax-MAP of the AF subjects increased more than the Gmax-HR and Gmax-MAP of the HF subjects (P < 0.05). The data demonstrate that the increase in the CBR responsiveness during a tilt-induced progressive unloading of the cardiopulmonary baroreceptors was attenuated in endurance-trained subjects. These findings provide an explanation for the predisposition to orthostatic hypotension and intolerance in well-trained athletes.

Backrest angle and cardiac output measurement in critically ill patients

BACKGROUND

Cardiac output is an extremely important measurement in the care of critically ill patients, but the accuracy of measurement is unknown when patients are in positions other than flat and supine.

OBJECTIVE

The purpose of this study was to compare the effects of varying degrees of backrest elevation on continuous cardiac output measurements in critically ill patients at head-of-bed angle of 0 degrees, 30 degrees, and 45 degrees, and at time points of 0 minutes, 5 minutes, and 10 minutes after each position change.

METHOD

A within participants design using a convenience sample (N = 26). Data were collected in a 24-bed adult Medical/Surgical/Trauma Intensive Care Unit. A continuous cardiac output catheter was used for all continuous cardiac measurements and continuous cardiac output values were indexed to continuous cardiac index values.

RESULTS

Four repeated measures analyses of variance (ANOVA) were run, one for each dependent variable (continuous cardiac index, stroke volume, heart rate, and mean arterial pressure). There were two within participant factors with three levels each (time and head-of-bed angle). The results indicated no overall significant differences in continuous cardiac index values at the various head-of-bed angle and time points (p =.715). In addition, no significant differences were found for stroke volume (p =.614), heart rate (p =.289) or mean arterial pressure (p =.246).

CONCLUSION

No differences in the continuous cardiac index values across the nine different measurement conditions were found. An examination of the determinants of cardiac output (stroke volume and heart rate) indicated that the lack of change in continuous cardiac index was not a result of a compensatory change in either stroke volume or heart rate. These data indicate that in daily clinical practice with critical medical surgical patients it may be unnecessary to reposition patients solely for the purpose of obtaining continuous cardiac index measurements. The measurements appear to be reproducible at head-of-bed angle up to 45 degrees.

Baroreflex-mediated changes in cardiac output and vascular conductance in response to alterations in carotid sinus pressure during exercise in humans

We sought to quantify the contribution of cardiac output (Q) and total vascular conductance (TVC) to carotid baroreflex (CBR)-mediated changes in mean arterial pressure (MAP) during mild to heavy exercise. CBR function was determined in eight subjects (25 +/- 1 years) at rest and during three cycle exercise trials at heart rates (HRs) of 90, 120 and 150 beats min-1 performed in random order. Acute changes in carotid sinus transmural pressure were evoked using 5 s pulses of neck pressure (NP) and neck suction (NS) from +40 to -80 Torr (+5.33 to -10.67 kPa). Beat-to-beat changes in HR and MAP were recorded throughout. In addition, stroke volume (SV) was estimated using the Modelflow method, which incorporates a non-linear, three-element model of the aortic input impedance to compute an aortic flow waveform from the arterial pressure wave. The application of NP and NS did not cause any significant changes in SV either at rest or during exercise. Thus, CBR-mediated alterations in Q were solely due to reflex changes in HR. In fact, a decrease in the carotid-HR response range from 26 +/- 7 beats min-1 at rest to 7 +/- 1 beats min-1 during heavy exercise (P = 0.001) reduced the contribution of Q to the CBR-mediated change in MAP. More importantly, at the time of the peak MAP response, the contribution of TVC to the CBR-mediated change in MAP was increased from 74 +/- 14 % at rest to 118 +/- 6 % (P = 0.017) during heavy exercise. Collectively, these findings indicate that alterations in vasomotion are the primary means by which the CBR regulates blood pressure during mild to heavy exercise.

Carotid baroreflex control of leg vascular conductance at rest and during exercise

We sought to test the hypothesis that the carotid baroreflex (CBR) alters mean leg blood flow (LBF) and leg vascular conductance (LVC) at rest and during exercise. In seven men and one woman, 25 +/- 2 (SE) yr of age, CBR control of LBF and LVC was determined at rest and during steady-state one-legged knee extension exercise at approximately 65% peak O(2) uptake. The application of 5-s pulses of +40 Torr neck pressure and -60 Torr neck suction significantly altered mean arterial pressure (MAP) and LVC both at rest and during exercise. CBR-mediated changes in MAP were similar between rest and exercise (P > 0.05). However, CBR-mediated decreases in LVC (%change) to neck pressure were attenuated in the exercising leg (16.4 +/- 1.6%) compared with rest (33 +/- 2.1%) and the nonexercising leg (23.7 +/- 1.9%) (P < 0.01). These data suggest CBR control of blood pressure is partially mediated by changes in leg vascular tone both at rest and during exercise. Furthermore, despite alterations in CBR-induced changes in LVC during exercise, CBR control of blood pressure was well maintained.