The purpose of this study was to determine whether the biphasic arterial blood pressure responses... more The purpose of this study was to determine whether the biphasic arterial blood pressure responses elicited by static muscle contraction of decerebrate rabbits are mediated, at least in part, by an initial decrease and a subsequent increase in sympathetic outflow. Renal sympathetic nerve activity (RSNA) was used as an index of sympathetic outflow. Static contraction of the triceps surae muscle (n = 14) initially decreased mean arterial blood pressure (MAP) -20 +/- 3 mmHg and heart rate (HR) -15 +/- 5 beats/min (nadir values). After this initial decrease, MAP increased 12 +/- 2 mmHg (peak increase) above baseline and there was a tendency for HR to be elevated (6 +/- 3 beats/min). The changes in RSNA during muscle contraction (n = 6) mirrored the nadir and peak responses of MAP (-50 +/- 9 and 32 +/- 11%). Muscle stretch (n = 11) also evoked similar nadir and peak responses of MAP (-20 +/- 5 and 9 +/- 1 mmHg), HR (-17 +/- 7 and 3 +/- 3 beats/min), and RSNA (-43 +/- 9 and 46 +/- 15%). Th...
The purpose of this study was to determine if the initial ventilatory and phrenic nerve responses... more The purpose of this study was to determine if the initial ventilatory and phrenic nerve responses to isometric contraction of the triceps surae muscle of anesthetized cats are influenced by the pattern of the contraction. To address this, three different types of muscle contraction were evoked: (1) a high tension, continuous tetanic (HT-CT) contraction; (2) a moderate tension, continuous tetanic (MT-CT) contraction; and (3) high tension, intermittent tetanic (HT-IT) contractions. The duration of each contraction period was 60 sec. The MT-CT and HT-IT contractions increased minute volume (VE; 19-4-4% and 15-/-5%, respectively) within the first 15 sec. These increases were the result of rises in breathing frequency and tidal volume. However, only the MT-CT contraction increased phrenic activity (p'f'E) in the first 15 sec. By contrast, ventilation and phrenic nerve activity failed to increase within the first 15 sec of the HT-CT contraction. If fact, 'tidal' phrenic activity (pVT;-14 4-5%) decreased during the first 5 sec, and there was a tendency for tidal volume ('~T;-8-4-5%), 'V'E (-8 4-6%), and p'QE (-16 4-8%) to fall. These data suggest that stimulation of muscle afferent fibers by static contraction can initially inhibit phrenic nerve activity, provided the activation is sustained and of sufficient intensity.
During the acute transition from lying supine to standing upright, a large volume of blood sudden... more During the acute transition from lying supine to standing upright, a large volume of blood suddenly moves from the chest into the legs. To prevent fainting, the blood pressure control system senses this change immediately, and rapidly adjusts flow (by increasing heart rate) and resistance to flow (by constricting the blood vessels) to restore blood pressure and maintain brain blood flow. If this system is inadequate, the brain has a backup plan. Blood vessels in the brain can adjust their diameter to keep blood flow constant. If blood pressure drops, the brain blood vessels dilate; if blood pressure increases, the brain blood vessels constrict. This process, which is called autoregulation, allows the brain to maintain a steady stream of oxygen, even when blood pressure changes. We examined what changes in the blood pressure control system or cerebral autoregulation contribute to the blood pressure control problems seen after spaceflight. We asked: (1) does the adaptation to spacefli...
Exposure to microgravity alters the distribution of body fluids and the degree of distension of c... more Exposure to microgravity alters the distribution of body fluids and the degree of distension of cranial blood vessels, and these changes in turn may provoke structural remodelling and altered cerebral autoregulation. Impaired cerebral autoregulation has been documented following weightlessness simulated by head-down bed rest in humans, and is proposed as a mechanism responsible for postspaceflight orthostatic intolerance. In this study, we tested the hypothesis that spaceflight impairs cerebral autoregulation. We studied six astronauts approximately 72 and 23 days before, after 1 and 2 weeks in space (n = 4), on landing day, and 1 day after the 16 day Neurolab space shuttle mission. Beat-by-beat changes of photoplethysmographic mean arterial pressure and transcranial Doppler middle cerebral artery blood flow velocity were measured during 5 min of spontaneous breathing, 30 mmHg lower body suction to simulate standing in space, and 10 min of 60 deg passive upright tilt on Earth. Dynamic cerebral autoregulation was quantified by analysis of the transfer function between spontaneous changes of mean arterial pressure and cerebral artery blood flow velocity, in the very low- (0.02-0.07 Hz), low- (0.07-0.20 Hz) and high-frequency (0.20-0.35 Hz) ranges. Resting middle cerebral artery blood flow velocity did not change significantly from preflight values during or after spaceflight. Reductions of cerebral blood flow velocity during lower body suction were significant before spaceflight (P < 0.05, repeated measures ANOVA), but not during or after spaceflight. Absolute and percentage reductions of mean (+/- s.e.m.) cerebral blood flow velocity after 10 min upright tilt were smaller after than before spaceflight (absolute, -4 +/- 3 cm s(-1) after versus -14 +/- 3 cm s(-1) before, P = 0.001; and percentage, -8.0 +/- 4.8% after versus -24.8 +/- 4.4% before, P…
... aerobic power or other aspects of human physiology may be less likely to limit demanding acti... more ... aerobic power or other aspects of human physiology may be less likely to limit demanding activities such as extra-vehicular activity (EVA) than the physical capabilities (eg heat rejection, CO 2 scrubbing, mass) of the EVA suit itself. ... Iellamo F, Di Rienzo M, Lucini D, Legramante ...
The volume indifferent point (VIP) is the point within the circulation where blood volume does no... more The volume indifferent point (VIP) is the point within the circulation where blood volume does not change with changes in posture. Because both volume and pressure are unaffected by posture at this point, its location should dictate the filling gradient to the heart. Previously we identified a contribution of the splanchnic circulation to its location. We experimentally manipulated blood volume in the splanchnic region to quantify changes in the VIP. Furthermore, we determined the relationship between the VIP and an individual's tolerance to an orthostatic stress. In Protocol 1, we found that administration of the somatostatin analog octreotide acetate, which elicits relatively selective splanchnic vasoconstriction, induced a superior shift in the VIP (+1.9 +/- 3.3 cm, P = 0.03). This finding corroborates previous reports of improvements in tilt tolerance after octreotide and suggests it might be related to relocation of the VIP. In Protocol 2, application of -20 mmHg lower body negative pressure (LBNP) induced splanchnic pooling and moved the VIP inferiorly (-6.0 +/- 7.2 cm, P < 0.01). LBNP combined with head-up tilt significantly decreased tilt tolerance (median tilt time: 28.0 vs. 4.2 min; Chi (2) = 14.29, P < 0.01); the change in the VIP predicted the reduction in tilt time (Deltatilt time = 3.05 + 0.12 DeltaVIP, P = 0.03). Thus, individuals with the largest inferior shift in the VIP also demonstrated the largest decrease in tilt table tolerance. We conclude that the splanchnic circulation plays an important role in determining the location of the VIP and the location of the VIP is a determinant of tolerance to orthostatic stress.
Ten male firefighters were tested on a treadmill to determine their heart rate (HR) x oxygen cons... more Ten male firefighters were tested on a treadmill to determine their heart rate (HR) x oxygen consumption (VO2) relationship. These men then performed a simulated fire suppression protocol during which HR and VO2 were measured simultaneously by a portable physiological monitoring system. Average VO2 in the simulated setting was 31.0 +/- 7.0 ml.kg-1.min-1 at a HR of 176 +/- 9 bpm. This VO2 was significantly (p less than or equal to 0.05) less than the VO2 that would have been predicted by treadmill testing (38.9 +/- 5.0 ml.kg-1.min-1) at a corresponding HR. Fifty-nine per cent of this variability could be accounted for by regression analysis. Firefighters worked on average at 73 +/- 10% VO2 max with a range of 54% to 88%. There was a significant (-0.82; p less than or equal to 0.05) inverse relationship between performance time of the fire suppression protocol and the relative intensity of VO2 max at which the firefighters worked. These findings indicate that the prediction of energy expenditure from HR is not straightforward in fire suppression settings. Furthermore, the relative intensity of work firefighters self-select is variable and should be considered as an additional physiological determinant of work behaviour.
Background The cardiovascular adaptation to bed rest leads to orthostatic intolerance, characteri... more Background The cardiovascular adaptation to bed rest leads to orthostatic intolerance, characterized by an excessive fall in stroke volume (SV) in the upright position. We hypothesized that this large fall in SV is due to a change in cardiac mechanics. Methods and Results We measured pulmonary capillary wedge pressure (PCWP), SV, left ventricular end-diastolic volume (LVEDV), and left ventricular mass (by echocardiography) at rest, during lower-body negative pressure, and after saline infusion before and after 2 weeks of bed rest with −6° head-down tilt (n=12 subjects aged 24±5 years). Pressure (P)-volume (V) curves were modeled exponentially by P= ae k V + b and logarithmically by P=− S ln[(V m −V)/(V m −V 0 ], where V 0 indicates volume at P=0, and the constants k and S were used as indices of normalized chamber stiffness. Dynamic stiffness (dP/dV) was calculated at baseline LVEDV. The slope of the line relating SV to PCWP during lower-body negative pressure characterized the stee...
The purpose of this study was to determine whether the biphasic arterial blood pressure responses... more The purpose of this study was to determine whether the biphasic arterial blood pressure responses elicited by static muscle contraction of decerebrate rabbits are mediated, at least in part, by an initial decrease and a subsequent increase in sympathetic outflow. Renal sympathetic nerve activity (RSNA) was used as an index of sympathetic outflow. Static contraction of the triceps surae muscle (n = 14) initially decreased mean arterial blood pressure (MAP) -20 +/- 3 mmHg and heart rate (HR) -15 +/- 5 beats/min (nadir values). After this initial decrease, MAP increased 12 +/- 2 mmHg (peak increase) above baseline and there was a tendency for HR to be elevated (6 +/- 3 beats/min). The changes in RSNA during muscle contraction (n = 6) mirrored the nadir and peak responses of MAP (-50 +/- 9 and 32 +/- 11%). Muscle stretch (n = 11) also evoked similar nadir and peak responses of MAP (-20 +/- 5 and 9 +/- 1 mmHg), HR (-17 +/- 7 and 3 +/- 3 beats/min), and RSNA (-43 +/- 9 and 46 +/- 15%). Th...
The purpose of this study was to determine if the initial ventilatory and phrenic nerve responses... more The purpose of this study was to determine if the initial ventilatory and phrenic nerve responses to isometric contraction of the triceps surae muscle of anesthetized cats are influenced by the pattern of the contraction. To address this, three different types of muscle contraction were evoked: (1) a high tension, continuous tetanic (HT-CT) contraction; (2) a moderate tension, continuous tetanic (MT-CT) contraction; and (3) high tension, intermittent tetanic (HT-IT) contractions. The duration of each contraction period was 60 sec. The MT-CT and HT-IT contractions increased minute volume (VE; 19-4-4% and 15-/-5%, respectively) within the first 15 sec. These increases were the result of rises in breathing frequency and tidal volume. However, only the MT-CT contraction increased phrenic activity (p'f'E) in the first 15 sec. By contrast, ventilation and phrenic nerve activity failed to increase within the first 15 sec of the HT-CT contraction. If fact, 'tidal' phrenic activity (pVT;-14 4-5%) decreased during the first 5 sec, and there was a tendency for tidal volume ('~T;-8-4-5%), 'V'E (-8 4-6%), and p'QE (-16 4-8%) to fall. These data suggest that stimulation of muscle afferent fibers by static contraction can initially inhibit phrenic nerve activity, provided the activation is sustained and of sufficient intensity.
During the acute transition from lying supine to standing upright, a large volume of blood sudden... more During the acute transition from lying supine to standing upright, a large volume of blood suddenly moves from the chest into the legs. To prevent fainting, the blood pressure control system senses this change immediately, and rapidly adjusts flow (by increasing heart rate) and resistance to flow (by constricting the blood vessels) to restore blood pressure and maintain brain blood flow. If this system is inadequate, the brain has a backup plan. Blood vessels in the brain can adjust their diameter to keep blood flow constant. If blood pressure drops, the brain blood vessels dilate; if blood pressure increases, the brain blood vessels constrict. This process, which is called autoregulation, allows the brain to maintain a steady stream of oxygen, even when blood pressure changes. We examined what changes in the blood pressure control system or cerebral autoregulation contribute to the blood pressure control problems seen after spaceflight. We asked: (1) does the adaptation to spacefli...
Exposure to microgravity alters the distribution of body fluids and the degree of distension of c... more Exposure to microgravity alters the distribution of body fluids and the degree of distension of cranial blood vessels, and these changes in turn may provoke structural remodelling and altered cerebral autoregulation. Impaired cerebral autoregulation has been documented following weightlessness simulated by head-down bed rest in humans, and is proposed as a mechanism responsible for postspaceflight orthostatic intolerance. In this study, we tested the hypothesis that spaceflight impairs cerebral autoregulation. We studied six astronauts approximately 72 and 23 days before, after 1 and 2 weeks in space (n = 4), on landing day, and 1 day after the 16 day Neurolab space shuttle mission. Beat-by-beat changes of photoplethysmographic mean arterial pressure and transcranial Doppler middle cerebral artery blood flow velocity were measured during 5 min of spontaneous breathing, 30 mmHg lower body suction to simulate standing in space, and 10 min of 60 deg passive upright tilt on Earth. Dynamic cerebral autoregulation was quantified by analysis of the transfer function between spontaneous changes of mean arterial pressure and cerebral artery blood flow velocity, in the very low- (0.02-0.07 Hz), low- (0.07-0.20 Hz) and high-frequency (0.20-0.35 Hz) ranges. Resting middle cerebral artery blood flow velocity did not change significantly from preflight values during or after spaceflight. Reductions of cerebral blood flow velocity during lower body suction were significant before spaceflight (P < 0.05, repeated measures ANOVA), but not during or after spaceflight. Absolute and percentage reductions of mean (+/- s.e.m.) cerebral blood flow velocity after 10 min upright tilt were smaller after than before spaceflight (absolute, -4 +/- 3 cm s(-1) after versus -14 +/- 3 cm s(-1) before, P = 0.001; and percentage, -8.0 +/- 4.8% after versus -24.8 +/- 4.4% before, P…
... aerobic power or other aspects of human physiology may be less likely to limit demanding acti... more ... aerobic power or other aspects of human physiology may be less likely to limit demanding activities such as extra-vehicular activity (EVA) than the physical capabilities (eg heat rejection, CO 2 scrubbing, mass) of the EVA suit itself. ... Iellamo F, Di Rienzo M, Lucini D, Legramante ...
The volume indifferent point (VIP) is the point within the circulation where blood volume does no... more The volume indifferent point (VIP) is the point within the circulation where blood volume does not change with changes in posture. Because both volume and pressure are unaffected by posture at this point, its location should dictate the filling gradient to the heart. Previously we identified a contribution of the splanchnic circulation to its location. We experimentally manipulated blood volume in the splanchnic region to quantify changes in the VIP. Furthermore, we determined the relationship between the VIP and an individual's tolerance to an orthostatic stress. In Protocol 1, we found that administration of the somatostatin analog octreotide acetate, which elicits relatively selective splanchnic vasoconstriction, induced a superior shift in the VIP (+1.9 +/- 3.3 cm, P = 0.03). This finding corroborates previous reports of improvements in tilt tolerance after octreotide and suggests it might be related to relocation of the VIP. In Protocol 2, application of -20 mmHg lower body negative pressure (LBNP) induced splanchnic pooling and moved the VIP inferiorly (-6.0 +/- 7.2 cm, P < 0.01). LBNP combined with head-up tilt significantly decreased tilt tolerance (median tilt time: 28.0 vs. 4.2 min; Chi (2) = 14.29, P < 0.01); the change in the VIP predicted the reduction in tilt time (Deltatilt time = 3.05 + 0.12 DeltaVIP, P = 0.03). Thus, individuals with the largest inferior shift in the VIP also demonstrated the largest decrease in tilt table tolerance. We conclude that the splanchnic circulation plays an important role in determining the location of the VIP and the location of the VIP is a determinant of tolerance to orthostatic stress.
Ten male firefighters were tested on a treadmill to determine their heart rate (HR) x oxygen cons... more Ten male firefighters were tested on a treadmill to determine their heart rate (HR) x oxygen consumption (VO2) relationship. These men then performed a simulated fire suppression protocol during which HR and VO2 were measured simultaneously by a portable physiological monitoring system. Average VO2 in the simulated setting was 31.0 +/- 7.0 ml.kg-1.min-1 at a HR of 176 +/- 9 bpm. This VO2 was significantly (p less than or equal to 0.05) less than the VO2 that would have been predicted by treadmill testing (38.9 +/- 5.0 ml.kg-1.min-1) at a corresponding HR. Fifty-nine per cent of this variability could be accounted for by regression analysis. Firefighters worked on average at 73 +/- 10% VO2 max with a range of 54% to 88%. There was a significant (-0.82; p less than or equal to 0.05) inverse relationship between performance time of the fire suppression protocol and the relative intensity of VO2 max at which the firefighters worked. These findings indicate that the prediction of energy expenditure from HR is not straightforward in fire suppression settings. Furthermore, the relative intensity of work firefighters self-select is variable and should be considered as an additional physiological determinant of work behaviour.
Background The cardiovascular adaptation to bed rest leads to orthostatic intolerance, characteri... more Background The cardiovascular adaptation to bed rest leads to orthostatic intolerance, characterized by an excessive fall in stroke volume (SV) in the upright position. We hypothesized that this large fall in SV is due to a change in cardiac mechanics. Methods and Results We measured pulmonary capillary wedge pressure (PCWP), SV, left ventricular end-diastolic volume (LVEDV), and left ventricular mass (by echocardiography) at rest, during lower-body negative pressure, and after saline infusion before and after 2 weeks of bed rest with −6° head-down tilt (n=12 subjects aged 24±5 years). Pressure (P)-volume (V) curves were modeled exponentially by P= ae k V + b and logarithmically by P=− S ln[(V m −V)/(V m −V 0 ], where V 0 indicates volume at P=0, and the constants k and S were used as indices of normalized chamber stiffness. Dynamic stiffness (dP/dV) was calculated at baseline LVEDV. The slope of the line relating SV to PCWP during lower-body negative pressure characterized the stee...
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