Barbara J. Morgan

Fatiguing inspiratory muscle work causes reflex reduction in resting leg blood flow in humans

1 We recently showed that fatigue of the inspiratory muscles via voluntary efforts caused a time‐dependent increase in limb muscle sympathetic nerve activity (MSNA) ( St Croix et al. 2000 ). We now asked whether limb muscle vasoconstriction and reduction in limb blood flow also accompany inspiratory muscle fatigue. 2 In six healthy human subjects at rest, we measured leg blood flow (Q̇L) in the femoral artery with Doppler ultrasound techniques and calculated limb vascular resistance (LVR) while subjects performed two types of fatiguing inspiratory work to the point of task failure (3‐10 min). Subjects inspired primarily with their diaphragm through a resistor, generating (i) 60 % maximal inspiratory mouth pressure (PM) and a prolonged duty cycle (TI/TTOT= 0.7); and (ii) 60 % maximal PM and a TI/TTOT of 0.4. The first type of exercise caused prolonged ischaemia of the diaphragm during each inspiration. The second type fatigued the diaphragm with briefer periods of ischaemia using a shorter duty cycle and a higher frequency of contraction. End‐tidal PCO2 was maintained by increasing the inspired CO2 fraction (FI,CO2) as needed. Both trials caused a 25–40 % reduction in diaphragm force production in response to bilateral phrenic nerve stimulation. 3 Q̇ L and LVR were unchanged during the first minute of the fatigue trials in most subjects; however, Q̇L subsequently decreased (‐30 %) and LVR increased (50‐60 %) relative to control in a time‐dependent manner. This effect was present by 2 min in all subjects. During recovery, the observed changes dissipated quickly (< 30 s). Mean arterial pressure (MAP; +4‐13 mmHg) and heart rate (+16‐20 beats min−1) increased during fatiguing diaphragm contractions. 4 When central inspiratory motor output was increased for 2 min without diaphragm fatigue by increasing either inspiratory force output (95 % of maximal inspiratory pressure (MIP)) or inspiratory flow rate (5 × eupnoea), Q̇L, MAP and LVR were unchanged; although continuing the high force output trials for 3 min did cause a relatively small but significant increase in LVR and a reduction in nQ̇L. 5 When the breathing pattern of the fatiguing trials was mimicked with no added resistance, LVR was reduced and Q̇L increased significantly; these changes were attributed to the negative feedback effects on MSNA from augmented tidal volume. 6 Voluntary increases in inspiratory effort, in the absence of diaphragm fatigue, had no effect on Q̇L and LVR, whereas the two types of diaphragm‐fatiguing trials elicited decreases in Q̇L and increases in LVR. We attribute these changes to a metaboreflex originating in the diaphragm. Diaphragm and forearm muscle fatigue showed very similar time‐dependent effects on LVR and Q̇L.

Fatiguing inspiratory muscle work causes reflex sympathetic activation in humans

1 We tested the hypothesis that reflexes arising from working respiratory muscle can elicit increases in sympathetic vasoconstrictor outflow to limb skeletal muscle, in seven healthy human subjects at rest. 2 We measured muscle sympathetic nerve activity (MSNA) with intraneural electrodes in the peroneal nerve while the subject inspired (primarily with the diaphragm) against resistance, with mouth pressure (PM) equal to 60 % of maximal, a prolonged duty cycle (TI/TTot) of 0.70, breathing frequency (fb) of 15 breaths min−1 and tidal volume (VT) equivalent to twice eupnoea. This protocol was known to reduce diaphragm blood flow and cause fatigue. 3 MSNA was unchanged during the first 1–2 min but then increased over time, to 77 ± 51 % (s.d.) greater than control at exhaustion (mean time, 7 ± 3 min). Mean arterial blood pressure (+12 mmHg) and heart rate (+27 beats min−1) also increased. 4 When the VT, fb and TI/TTot of these trials were mimicked with no added resistance, neither MSNA nor arterial blood pressure increased. 5 MSNA and arterial blood pressure also did not change in response to two types of increased central respiratory motor output that did not produce fatigue: (a) high inspiratory flow rate and fb without added resistance; or (b) high inspiratory effort against resistance with PM of 95 % maximal, TI/TTot of 0.35 and fb of 12 breaths min−1. The heart rate increased by 5–16 beats min−1 during these trials. 6 Thus, in the absence of any effect of increased central respiratory motor output per se on limb MSNA, we attributed the time‐dependent increase in MSNA during high resistance, prolonged duty cycle breathing to a reflex arising from a diaphragm that was accumulating metabolic end products in the face of high force output plus compromised blood flow.

Influence of cerebrovascular function on the hypercapnic ventilatory response in healthy humans

An important determinant of [H+] in the environment of the central chemoreceptors is cerebral blood flow. Accordingly we hypothesized that a reduction of brain perfusion or a reduced cerebrovascular reactivity to CO2 would lead to hyperventilation and an increased ventilatory responsiveness to CO2. We used oral indomethacin to reduce the cerebrovascular reactivity to CO2 and tested the steady‐state hypercapnic ventilatory response to CO2 in nine normal awake human subjects under normoxia and hyperoxia (50% O2). Ninety minutes after indomethacin ingestion, cerebral blood flow velocity (CBFV) in the middle cerebral artery decreased to 77 ± 5% of the initial value and the average slope of CBFV response to hypercapnia was reduced to 31% of control in normoxia (1.92 versus 0.59 cm−1 s−1 mmHg−1, P < 0.05) and 37% of control in hyperoxia (1.58 versus 0.59 cm−1 s−1 mmHg−1, P < 0.05). Concomitantly, indomethacin administration also caused 40–60% increases in the slope of the mean ventilatory response to CO2 in both normoxia (1.27 ± 0.31 versus 1.76 ± 0.37 l min−1 mmHg−1, P < 0.05) and hyperoxia (1.08 ± 0.22 versus 1.79 ± 0.37 l min−1 mmHg−1, P < 0.05). These correlative findings are consistent with the conclusion that cerebrovascular responsiveness to CO2 is an important determinant of eupnoeic ventilation and of hypercapnic ventilatory responsiveness in humans, primarily via its effects at the level of the central chemoreceptors.

Respiratory influences on sympathetic vasomotor outflow in humans

We have attempted to synthesize findings dealing with four types of respiratory system influences on sympathetic outflow in the human. First, a powerful lung volume-dependent modulation of muscle sympathetic nerve activity (MSNA) occurs within each respiratory cycle showing late-inspiratory inhibition and late-expiratory excitation. Secondly, in the intact human, neither reductions in spontaneous respiratory motor output nor voluntary near-maximum increases in central respiratory motor output and inspiratory effort, per sec, influence MSNA modulation within a breath, MSNA total activity or limb vascular conductance. Thirdly, carotid chemoreceptor stimuli markedly increase total MSNA; but most of the MSNA response to chemoreceptor activation appears to be mediated independently of increased central respiratory motor output. Fourthly, repeated fatiguing contractions of the diaphragm or expiratory muscles in the human show a metaboreflex mediated time-dependent increase in MSNA and reduced vascular conductance and blood flow in the resting limb. Recent evidence suggests that these respiratory influences contribute significantly to sympathetic vasomotor outflow and to the distribution of systemic vascular conductances and blood flow in the exercising human.

Role of Respiratory Motor Output in Within-Breath Modulation of Muscle Sympathetic Nerve Activity in Humans

We measured muscle sympathetic nerve activity (MSNA, peroneal microneurography) in 5 healthy humans under conditions of matched tidal volume, breathing frequency, and end-tidal CO(2), but varying respiratory motor output as follows: (1) passive positive pressure mechanical ventilation, (2) voluntary hyperventilation, (3) assisted mechanical ventilation that required the subject to generate -2.5 cm H(2)O to trigger each positive pressure breath, and (4) added inspiratory resistance. Spectral analyses showed marked respiratory periodicities in MSNA; however, the amplitude of the peak power was not changed with changing inspiratory effort. Time domain analyses showed that maximum MSNA always occurred at end expiration (25% to 30% of total activity) and minimum activity at end inspiration (2% to 3% of total activity), and the amplitude of the variation was not different among conditions despite marked changes in respiratory motor output. Furthermore, qualitative changes in intrathoracic pressure were without influence on the respiratory modulation of MSNA. In all conditions, within-breath changes in MSNA were inversely related to small changes in diastolic pressure (1 to 3 mm Hg), suggesting that respiratory rhythmicity in MSNA was secondary to loading/unloading of carotid sinus baroreceptors. Furthermore, at any given diastolic pressure, within-breath MSNA varied inversely with lung volume, demonstrating an additional influence of lung inflation feedback on sympathetic discharge. Our data provide evidence against a significant effect of respiratory motor output on the within-breath modulation of MSNA and suggest that feedback from baroreceptors and pulmonary stretch receptors are the dominant determinants of the respiratory modulation of MSNA in the intact human.

Differential responses to CO2 and sympathetic stimulation in the cerebral and femoral circulations in humans

The relative importance of CO2 and sympathetic stimulation in the regulation of cerebral and peripheral vasculatures has not been previously studied in humans. We investigated the effect of sympathetic activation, produced by isometric handgrip (HG) exercise, on cerebral and femoral vasculatures during periods of isocapnia and hypercapnia. In 14 healthy males (28.1 ± 3.7 (mean ±s.d.) years), we measured flow velocity (; transcranial Doppler ultrasound) in the middle cerebral artery during euoxic isocapnia (ISO, +1 mmHg above rest) and two levels of euoxic hypercapnia (HC5, end‐tidal PCO2, P  ET,CO 2 , =+5 mmHg above ISO; HC10, P  ET,CO 2 =+10 above ISO). Each PET,CO2 level was maintained for 10 min using the dynamic end‐tidal forcing technique, during which increases in sympathetic activity were elicited by a 2‐min HG at 30% of maximal voluntary contraction. Femoral blood flow (FBF; Doppler ultrasound), muscle sympathetic nerve activity (MSNA; microneurography) and mean arterial pressure (MAP; Portapres) were also measured. Hypercapnia increased and FBF by 5.0 and 0.6% mmHg−1, respectively, and MSNA by 20–220%. Isometric HG increased MSNA by 50% and MAP by 20%, with no differences between ISO, HC5 and HC10. During the ISO HG there was an increase in cerebral vascular resistance (CVR; 20 ± 11%), while remained unchanged. During HC5 and HC10 HG, increased (13% and 14%, respectively), but CVR was unchanged. In contrast, HG‐induced sympathetic stimulation increased femoral vascular resistance (FVR) during ISO, HC5 and HC10 (17–41%), while there was a general decrease in FBF below ISO. The HG‐induced increases in MSNA were associated with increases in FVR in all conditions (r= 0.76–0.87), whereas increases in MSNA were associated with increases in CVR only during ISO (r= 0.91). In summary, in the absence of hypercapnia, HG exercise caused cerebral vasoconstriction, myogenically and/or neurally, which was reflected by increases in CVR and a maintained . In contrast, HG increased FVR during conditions of ISO, HC5 and HC10. Therefore, the cerebral circulation is more responsive to alterations in PCO2, and less responsive to sympathetic stimulation than the femoral circulation.

Chronic intermittent hypoxia augments chemoreflex control of sympathetic activity: role of the angiotensin II type 1 receptor.

Chronic exposure to intermittent hypoxia (CIH) increases carotid sinus nerve activity in normoxia and in response to acute hypoxia. We hypothesized that CIH augments basal and chemoreflex-stimulated sympathetic outflow through an angiotensin receptor-dependent mechanism. Rats were exposed to CIH for 28 days: a subset was treated with losartan. Then, lumbar sympathetic activity was recorded under anesthesia during 20-s apneas, isocapnic hypoxia, and potassium cyanide. We measured carotid body superoxide production and expression of angiotensin II type-1 receptor, neuronal nitric oxide synthase, and NADPH oxidase. Sympathetic activity was higher in CIH vs. control rats at baseline, during apneas and isocapnic hypoxia, but not cyanide. Carotid body superoxide production and expression of angiotensin II type 1 receptor and gp91(phox) subunit of NADPH oxidase were elevated in CIH rats, whereas expression of neuronal nitric oxide synthase was reduced. None of these differences were evident in animals treated with losartan. CIH-induced augmentation of chemoreflex sensitivity occurs, at least in part, via the renin-angiotensin system.

Peripheral Chemoreflex and Baroreflex Interactions in Cardiovascular Regulation in Humans

We tested the hypothesis that activation of peripheral chemoreceptors with acute isocapnic hypoxia resets arterial baroreflex control of both heart rate and sympathetic vasoconstrictor outflow to higher pressures, resulting in increased heart rate and muscle sympathetic nerve activity without changes in baroreflex sensitivity. We further hypothesized that this resetting would not occur during isocapnic hyperpnoea at the same breathing rate and depth as during isocapnic hypoxia. In 12 healthy, non‐smoking, normotensive subjects (6 women, 6 men, 19‐36 years), we assessed baroreflex control of heart rate and muscle sympathetic nerve activity using the modified Oxford technique during normoxia, isocapnic hyperpnoea, and isocapnic hypoxia (85 % arterial O2 saturation). While isocapnic hyperpnoea did not alter heart rate, arterial pressure, or sympathetic outflow, hypoxia increased heart rate from 61.9 ± 1.8 to 74.7 ± 2.7 beats min−1 (P < 0.05), increased mean arterial pressure from 97.4 ± 2.0 to 103.9 ± 3.3 mmHg (P < 0.05), and increased sympathetic activity 22 ± 13 % relative to normoxia and 72 ± 21 % (P < 0.05) relative to hyperpnoea alone. The sensitivity for baroreflex control of both heart rate and sympathetic activity was not altered by either hypoxia or hyperpnoea. Thus, it appears that acute activation of peripheral chemoreceptors with isocapnic hypoxia resets baroreflex control of both heart rate and sympathetic activity to higher pressures without changes in baroreflex sensitivity. Furthermore, these effects appear largely independent of breathing rate and tidal volume.

Mechanisms of the cerebrovascular response to apnoea in humans

We measured ventilation, arterial O2 saturation, end‐tidal CO2 (PET,CO2), blood pressure (intra‐arterial catheter or photoelectric plethysmograph), and flow velocity in the middle cerebral artery (CFV) (pulsed Doppler ultrasound) in 17 healthy awake subjects while they performed 20 s breath holds under control conditions and during ganglionic blockade (intravenous trimethaphan, 4.4 ± 1.1 mg min−1 (mean ±s.d.)). Under control conditions, breath holding caused increases in PET,CO2 (7 ± 1 mmHg) and in mean arterial pressure (MAP) (15 ± 2 mmHg). A transient hyperventilation (PET,CO2−7 ± 1 mmHg vs. baseline) occurred post‐apnoea. CFV increased during apnoeas (by 42 ± 3 %) and decreased below baseline (by 20 ± 2 %) during post‐apnoea hyperventilation. In the post‐apnoea recovery period, CFV returned to baseline in 45 ± 4 s. The post‐apnoea decrease in CFV did not occur when hyperventilation was prevented. During ganglionic blockade, which abolished the increase in MAP, apnoea‐induced increases in CFV were partially attenuated (by 26 ± 2 %). Increases in PET,CO2 and decreases in oxyhaemoglobin saturation (Sa,O2) (by 2 ± 1 %) during breath holds were identical in the intact and blocked conditions. Ganglionic blockade had no effect on the slope of the CFV response to hypocapnia but it reduced the CFV response to hypercapnia (by 17 ± 5 %). We attribute this effect to abolition of the hypercapnia‐induced increase in MAP. Peak increases in CFV during 20 s Mueller manoeuvres (40 ± 3 %) were the same as control breath holds, despite a 15 mmHg initial, transient decrease in MAP. Hyperoxia also had no effect on the apnoea‐induced increase in CFV (40 ± 4 %). We conclude that apnoea‐induced fluctuations in CFV were caused primarily by increases and decreases in arterial partial pressure of CO2 (Pa,CO2) and that sympathetic nervous system activity was not required for either the initiation or the maintenance of the cerebrovascular response to hyper‐ and hypocapnia. Increased MAP or other unknown influences of autonomic activation on the cerebral circulation played a smaller but significant role in the apnoea‐induced increase in CFV; however, negative intrathoracic pressure and the small amount of oxyhaemoglobin desaturation caused by 20 s apnoea did not affect CFV.

Influences of hydration on post‐exercise cardiovascular control in humans

Dehydration is known to decrease orthostatic tolerance and cause tachycardia, but little is known about the cardiovascular control mechanisms involved. To test the hypothesis that arterial baroreflex sensitivity increases during exercise‐induced dehydration, we assessed arterial baroreflex responsiveness in 13 healthy subjects (protocol 1) at baseline (PRE‐EX) and 1 h after (EX‐DEH) 90 min of exercise to cause dehydration, and after subsequent intravenous rehydration with saline (EX‐REH). Six of these subjects were studied a second time (protocol 2) with intravenous saline during exercise to prevent dehydration. We measured heart rate, central venous pressure and arterial pressure during all trials, and muscle sympathetic nerve activity (MSNA) during the post‐exercise trials. Baroreflex responses were assessed using sequential boluses of nitroprusside and phenylephrine (modified Oxford technique). After exercise in protocol 1 (EX‐DEH), resting blood pressure was decreased and resting heart rate was increased. Cardiac baroreflex gain, assessed as the responsiveness of heart rate or R‐R interval to changes in systolic pressure, was diminished in the EX‐DEH condition (9.17 ± 1.06 ms mmHg−1vs. PRE‐EX: 18.68 ± 2.22 ms mmHg−1, P < 0.05). Saline infusion after exercise did not alter the increase in HR post‐exercise or the decrease in baroreflex gain (EX‐REH: 10.20 ± 1.43 ms mmHg−1; P > 0.10 vs. EX‐DEH). Saline infusion during exercise (protocol 2) resulted in less of a post‐exercise decrease in blood pressure and a smaller change in cardiac baroreflex sensitivity. Saline infusion caused a decrease in MSNA in protocol 1. We conclude that exercise‐induced dehydration causes post‐exercise changes in the baroreflex control of blood pressure that may contribute to, rather than offset, orthostatic intolerance.