Carotid baroreflex function during and following voluntary apnea in humans

Sympathetically mediated increases in cardiac output, not restraint of peripheral vasodilation, contribute to blood pressure maintenance during hyperinsulinemia

Vasodilatory effects of insulin support the delivery of insulin and glucose to skeletal muscle. Concurrently, insulin exerts central effects that increase sympathetic nervous system activity (SNA), which is required for the acute maintenance of blood pressure (BP). Indeed, in a cohort of young healthy adults, herein we show that intravenous infusion of insulin increases muscle SNA while BP is maintained. We next tested the hypothesis that sympathoexcitation evoked by hyperinsulinemia restrains insulin-stimulated peripheral vasodilation and contributes to sustaining BP. To address this, a separate cohort of participants were subjected to 5-s pulses of neck suction (NS) to simulate carotid hypertension and elicit a reflex-mediated reduction in SNA. NS was conducted before and 60 min following intravenous infusion of insulin. Insulin infusion caused an increase in leg vascular conductance and cardiac output (CO; P < 0.050), with maintenance of BP (P = 0.540). As expected, following NS, decreases in BP were greater in the presence of hyperinsulinemia compared with control (P = 0.045). However, the effect of NS on leg vascular conductance did not differ between insulin and control conditions (P = 0.898). Instead, the greater decreases in BP following NS in the setting of insulin infusion paralleled with greater decreases in CO (P = 0.009). These findings support the idea that during hyperinsulinemia, SNA-mediated increase in CO, rather than restraint of leg vascular conductance, is the principal contributor to the maintenance of BP. Demonstration in isolated arteries that insulin suppresses α-adrenergic vasoconstriction suggests that the observed lack of restraint of leg vascular conductance may be attributed to sympatholytic actions of insulin.NEW & NOTEWORTHY We examined the role of sympathetic activation in restraining vasodilatory responses to hyperinsulinemia and sustaining blood pressure in healthy adults. Data are reported from two separate experimental protocols in humans and one experimental protocol in isolated arteries from mice. Contrary to our hypothesis, the present findings support the idea that during hyperinsulinemia, a sympathetically mediated increase in cardiac output, rather than restraint of peripheral vasodilation, is the principal contributor to the maintenance of systemic blood pressure.

Physiology of static breath holding in elite apneists

NEW FINDINGS

What is the topic of this review? This review provides an up-to-date assessment of the physiology involved with extreme static dry-land breath holding in trained apneists. What advances does it highlight? We specifically highlight the recent findings involved with the cardiovascular, cerebrovascular and metabolic function during a maximal breath hold in elite apneists.

ABSTRACT

Breath-hold-related activities have been performed for centuries, but only recently, within the last ∼30 years, has it emerged as an increasingly popular competitive sport. In apnoea sport, competition relates to underwater distances or simply maximal breath-hold duration, with the current (oxygen-unsupplemented) static breath-hold record at 11 min 35 s. Remarkably, many ultra-elite apneists are able to suppress respiratory urges to the point where consciousness fundamentally limits a breath-hold duration. Here, arterial oxygen saturations as low as ∼50% have been reported. In such cases, oxygen conservation to maintain cerebral functioning is critical, where responses ascribed to the mammalian dive reflex, e.g. sympathetically mediated peripheral vasoconstriction and vagally mediated bradycardia, are central. In defence of maintaining global cerebral oxygen delivery during prolonged breath holds, the cerebral blood flow may increase by ∼100% from resting values. Interestingly, near the termination of prolonged dry static breath holds, recent studies also indicate that reductions in the cerebral oxidative metabolism can occur, probably attributable to the extreme hypercapnia and irrespective of the hypoxaemia. In this review, we highlight and discuss the recent data on the cardiovascular, metabolic and, particularly, cerebrovascular function in competitive apneists performing maximal static breath holds. The physiological adaptation and maladaptation with regular breath-hold training are also summarized, and future research areas in this unique physiological field are highlighted; particularly, the need to determine the potential long-term health impacts of extreme breath holding.

Ventilation inhibits sympathetic action potential recruitment even during severe chemoreflex stress

This study investigated the influence of ventilation on sympathetic action potential (AP) discharge patterns during varying levels of high chemoreflex stress. In seven trained breath-hold divers (age 33 ± 12 yr), we measured muscle sympathetic nerve activity (MSNA) at baseline, during preparatory rebreathing (RBR), and during 1) functional residual capacity apnea (FRC_Apnea) and 2) continued RBR. Data from RBR were analyzed at matched (i.e., to FRC_Apnea) hemoglobin saturation (HbSat) levels (RBR_Matched) or more severe levels (RBR_End). A third protocol compared alternating periods (30 s) of FRC and RBR (FRC-RBR_ALT). Subjects continued each protocol until 85% volitional tolerance. AP patterns in MSNA (i.e., providing the true neural content of each sympathetic burst) were studied using wavelet-based methodology. First, for similar levels of chemoreflex stress (both HbSat: 71 ± 6%; P = NS), RBR_Matched was associated with reduced AP frequency and APs per burst compared with FRC_Apnea (both P < 0.001). When APs were binned according to peak-to-peak amplitude (i.e., into clusters), total AP clusters increased during FRC_Apnea (+10 ± 2; P < 0.001) but not during RBR_Matched (+1 ± 2; P = NS). Second, despite more severe chemoreflex stress during RBR_End (HbSat: 56 ± 13 vs. 71 ± 6%; P < 0.001), RBR_End was associated with a restrained increase in the APs per burst (FRC_Apnea: +18 ± 7; RBR_End: +11 ± 5) and total AP clusters (FRC_Apnea: +10 ± 2; RBR_End: +6 ± 4) (both P < 0.01). During FRC-RBR_ALT, all periods of FRC elicited sympathetic AP recruitment (all P < 0.001), whereas all periods of RBR were associated with complete withdrawal of AP recruitment (all P = NS). Presently, we demonstrate that ventilation per se restrains and/or inhibits sympathetic axonal recruitment during high, and even extreme, chemoreflex stress.NEW & NOTEWORTHY The current study demonstrates that the sympathetic neural recruitment patterns observed during chemoreflex activation induced by rebreathing or apnea are restrained and/or inhibited by the act of ventilation per se, despite similar, or even greater, levels of severe chemoreflex stress. Therefore, ventilation modulates not only the timing of sympathetic bursts but also the within-burst axonal recruitment normally observed during progressive chemoreflex stress.

Carotid baroreflex function at the onset of cycling in men

Arterial baroreflex function is important for blood pressure control during exercise, but its contribution to cardiovascular adjustments at the onset of cycling exercise remains unclear. Fifteen healthy male subjects (24 ± 1 yr) performed 45-s trials of low- and moderate-intensity cycling, with carotid baroreceptor stimulation by neck suction at −60 Torr applied 0–5, 10–15, and 30–35 s after the onset of exercise. Cardiovascular responses to neck suction during cycling were compared with those obtained at rest. An attenuated reflex decrease in heart rate following neck suction was detected during moderate-intensity exercise, compared with the response at rest ( P < 0.05). Furthermore, compared with the reflex decrease in blood pressure elicited at rest, neck suction elicited an augmented decrease in blood pressure at 0–5 and 10–15 s during low-intensity exercise and in all periods during moderate-intensity exercise ( P < 0.05). The reflex depressor response at the onset of cycling was primarily mediated by an increase in the total vascular conductance. These findings evidence altered carotid baroreflex function during the first 35 s of cycling compared with rest, with attenuated bradycardic response, and augmented depressor response to carotid baroreceptor stimulation.

Respiratory modulation of human autonomic function on Earth

KEY POINTS

We studied healthy supine astronauts on Earth with electrocardiogram, non-invasive arterial pressure, respiratory carbon dioxide concentrations, breathing depth and sympathetic nerve recordings. The null hypotheses were that heart beat interval fluctuations at usual breathing frequencies are baroreflex mediated, that they persist during apnoea, and that autonomic responses to apnoea result from changes of chemoreceptor, baroreceptor or lung stretch receptor inputs. R-R interval fluctuations at usual breathing frequencies are unlikely to be baroreflex mediated, and disappear during apnoea. The subjects' responses to apnoea could not be attributed to changes of central chemoreceptor activity (hypocapnia prevailed); altered arterial baroreceptor input (vagal baroreflex gain declined and muscle sympathetic nerve burst areas, frequencies and probabilities increased, even as arterial pressure climbed to new levels); or altered pulmonary stretch receptor activity (major breathing frequency and tidal volume changes did not alter vagal tone or sympathetic activity). Apnoea responses of healthy subjects may result from changes of central respiratory motoneurone activity.

ABSTRACT

We studied eight healthy, supine astronauts on Earth, who followed a simple protocol: they breathed at fixed or random frequencies, hyperventilated and then stopped breathing, as a means to modulate and expose to view important, but obscure central neurophysiological mechanisms. Our recordings included the electrocardiogram, finger photoplethysmographic arterial pressure, tidal volume, respiratory carbon dioxide concentrations and peroneal nerve muscle sympathetic activity. Arterial pressure, vagal tone and muscle sympathetic outflow were comparable during spontaneous and controlled-frequency breathing. Compared with spontaneous, 0.1 and 0.05 Hz breathing, however, breathing at usual frequencies (∼0.25 Hz) lowered arterial baroreflex gain, and provoked smaller arterial pressure and R-R interval fluctuations, which were separated by intervals that were likely to be too short and variable to be attributed to baroreflex physiology. R-R interval fluctuations at usual breathing frequencies disappear during apnoea, and thus cannot provide evidence for the existence of a central respiratory oscillation. Apnoea sets in motion a continuous and ever changing reorganization of the relations among stimulatory and inhibitory inputs and autonomic outputs, which, in our study, could not be attributed to altered chemoreceptor, baroreceptor, or pulmonary stretch receptor activity. We suggest that responses of healthy subjects to apnoea are driven importantly, and possibly prepotently, by changes of central respiratory motoneurone activity. The companion article extends these observations and asks the question, Might terrestrial responses to our 20 min breathing protocol find expression as long-term neuroplasticity in serial measurements made over 20 days during and following space travel?

Rapid onset pressor and sympathetic responses to static handgrip in older hypertensive adults

Exaggerated pressor and muscle sympathetic nerve activity (MSNA) responses have been reported during static handgrip in hypertensive (HTN) adults. Recent work suggests that such responses may occur much more rapidly in HTN patients; however, this has not been extensively studied. Thus, we examined the blood pressure (BP) and MSNA responses at the immediate onset of muscle contraction and tested the hypothesis that older HTN adults would exhibit rapid onset pressor and sympathetic responses compared with normotensive (NTN) adults. Heart rate (HR), BP (Finometer) and MSNA (peroneal microneurography) were retrospectively analyzed in 15 HTN (62 ± 1 years; resting BP 153 ± 3/91 ± 5 mm Hg) and 23 age-matched NTN (60 ± 1 years; resting BP 112 ± 1/67 ± 2 mm Hg) subjects during the first 30 s of static handgrip at 30 and 40% of maximal voluntary contraction (MVC). HTN adults demonstrated exaggerated increases in mean BP during the first 10 s of both 30% (NTN: Δ1 ± 1 vs HTN: Δ7 ± 2 mm Hg; P < 0.05) and 40% (NTN: Δ2 ± 1 vs HTN: Δ8 ± 2 mm Hg; P < 0.05) intensity handgrip. Likewise, HTN adults exhibited atypical increases in MSNA within 10 s. Increases in HR were also greater in HTN adults at 10 s of 30% MVC handgrip, although not at 40% MVC. There were no group differences in 10 s pressor or sympathetic responses to a cold pressor test, suggesting no differences in generalized sympathetic responsiveness. Thus, static handgrip evokes rapid onset pressor and sympathetic responses in older HTN adults. These findings suggest that older HTN adults likely have greater cardiovascular risk even during short duration activities of daily living that contain an isometric component.

Effect of repetitive end-inspiration breath holding on very short-term heart rate variability in healthy humans

Very short-term heart rate variability (HRV) is thought to reflect dynamic changes in autonomic nervous activity, which is helpful in understanding the role of autonomic nervous function (ANF) in the mechanisms underlying apnea-induced cardiac arrhythmias. The goal of this study was to investigate the effect of repetitive end-inspiration breath holding on very short-term HRV. A total of 32 young healthy participants took part in the experiments. Three trials were performed, each involving seven repetitive end-inspiration breath holding and a 30 s recovery period between breath holding. Durations of breath holding in the three trials were 1:2:3. The study first evaluated the effect of analyzed data lengths on the stability of HRV indices and determined three HRV indices suitable for very short-term analysis. The results showed that in most cases, during breath holding, the square root of the mean squared differences of successive normal RR intervals (rMSSD) was significantly lower, but normalized units of the power in the low frequency band ranging from 0.04 to 0.15 Hz (nLF) and LF/high frequency (HF) were significantly higher than those during corresponding durations under the normal breathing conditions. On the contrary, during recovery after breath holding, rMSSD was significantly higher but nLF and LF/HF were lower than normal. Moreover, the durations of breath holding had no significant influence on the variations of LF/HF. In addition, as participants repeated the breath holding, HRV indices varied non-linearly. HRV changes may indicate sympathetic activation during breath holding and parasympathetic activation during recovery after breath holding. In conjunction with the existing physiological interpretation based on changes in heart rate, the results may imply that breath holding leads to both cardiac sympathetic and parasympathetic activation simultaneously, which may be a possible pathogenic factor of apnea-induced arrhythmias.

Ventilatory restraint of sympathetic activity during chemoreflex stress

The within-breath modulation of muscle sympathetic nerve activity (MSNA) is well established, with greater activity occurring during expiration and less during inspiration. Whether ventilation per se affects the longer-term (i.e., minute-to-minute) regulation of MSNA has not been determined. We sought to define the specific role of ventilation in regulating sympathetic activation during chemoreflex activation, where both ventilation and MSNA are increased. Ten young healthy subjects performed both asphyxic rebreathing and repeated, rebreathing apneas to cause the same magnitude of chemoreflex stress in the presence or absence of ventilation. Both protocols caused increases in sympathetic burst frequency, burst amplitude, and burst incidence. However, burst frequency was increased more during repeated apneas (12 ± 6 to 25 ± 7 bursts/min) compared with rebreathing (12 ± 5 to 17 ± 7 bursts/min; P < 0.001) due to a greater burst incidence during apneas (36 ± 11 bursts/100 heart beats) vs. rebreathing (26 ± 8 bursts/100 heart beats, P < 0.001). The sympathetic gain to chemoreflex stress was also larger during repeated apneas (2.29 ± 1.29 au/% desaturation) compared with rebreathing (1.44 ± 0.53 au/% desaturation, P < 0.05). The augmented sympathetic response during apneas was associated with a larger pressor response and total peripheral resistance compared with rebreathing. These data demonstrate that ventilation per se restrains sympathetic activation during chemoreflex activation. Further, the augmented sympathetic response during apneas was associated with greater cardiovascular stress and may be relevant to the cardiovascular pathology associated with sleep-disordered breathing.

Effect of repetitive hypoxic apnoeas on baroreflex function in humans

Baroreflex function is impaired in patients with obstructive sleep apnoea. We tested the hypothesis that short-term exposure to repetitive hypoxic apnoeas (RHA) produces prolonged impairment in baroreflex function. Baroreflex function was determined using the modified Oxford technique in 14 subjects (26 +/- 1 years). Baroreflex sensitivity (BRS) was quantified from the R-R interval-systolic blood pressure (BP; cardiovagal BRS), heart rate-systolic BP (HR BRS) and muscle sympathetic nerve activity (MSNA)-diastolic BP (sympathetic BRS) relations. RHA involved subjects performing repetitive end-expiratory apnoeas (20 s) every minute for 30 min during intermittent hypoxia to accentuate oxygen desaturation. After RHA, BP and MSNA at rest were elevated. BRS was measured approximately 7 (Post 1), approximately 30 (Post 2) and approximately 50 min (Post 3) after RHA to provide insight into the temporal pattern of responses. Cardiovagal BRS (16.8 +/- 1.3, 16.5 +/- 1.6, 17.6 +/- 2.0 and 17.4 +/- 1.5 ms mmHg(-1) for Pre, Post 1, Post 2 and Post 3, respectively), HR BRS (-1.1 +/- 0.1, -1.1 +/- 0.1, -1.3 +/- 0.1 and -1.4 +/- 0.1 beats min(-1) mmHg(-1)) and sympathetic BRS (-4.5 +/- 0.6, -4.4 +/- 0.7, -3.7 +/- 0.5 and -4.7 +/- 1.0 arbitrary units (au) beat(-1) mmHg(-1)) were unchanged by RHA. In contrast, the operating points of the baroreflexes were shifted rightward (to higher levels of BP) and upward (to higher levels of heart rate and MSNA) after RHA (P < 0.05). Time control studies performed in five additional subjects showed no change in any of the measured variables over time. Collectively, these data indicate that short-term exposure to RHA shifts ('resets') the baroreflex stimulus-response curve to higher levels of BP without influencing BRS for extended periods of time.

Periods of intermittent hypoxic apnea can alter chemoreflex control of sympathetic nerve activity in humans

Obstructive sleep apnea is associated with sustained elevation of muscle sympathetic nerve activity (MSNA) and altered chemoreflex control of MSNA, both of which likely play an important role in the development of hypertension in these patients. Additionally, short-term exposure to intermittent hypoxic apneas can produce a sustained elevation of MSNA. Therefore, we tested the hypothesis that 20 min of intermittent hypoxic apneas can alter chemoreflex control of MSNA. Twenty-one subjects were randomly assigned to one of three groups (hypoxic apnea, hypercapnic hypoxia, and isocapnic hypoxia). Subjects were exposed to 30 s of the perturbation every minute for 20 min. Chemoreflex control of MSNA was assessed during baseline, 1 min posttreatment, and every 15 min throughout 180 min of recovery by the MSNA response to a single hypoxic apnea. Recovery hypoxic apneas were matched to a baseline hypoxic apnea with a similar nadir oxygen saturation. A significant main effect for chemoreflex control of MSNA was observed after 20 min of intermittent hypoxic apneas (P <0.001). The MSNA response to a single hypoxic apnea was attenuated 1 min postexposure compared with baseline (P <0.001), became augmented within 30 min of recovery, and remained augmented through 165 min of recovery (P <0.05). Comparison of treatment groups revealed no differences in the chemoreflex control of MSNA during recovery (P=0.69). These data support the hypothesis that 20 min of intermittent hypoxic apneas can alter chemoreflex control of MSNA. Furthermore, this response appears to be mediated by hypoxia.