The effect of hyperoxia on muscle sympathetic nerve activity: a systematic review and meta-analysis.

PURPOSE: We conducted a meta-analysis to determine the effect of hyperoxia on muscle sympathetic nerve activity in healthy individuals and those with cardio-metabolic diseases. METHODS: A comprehensive search of electronic databases was performed until August 2022. All study designs (except reviews) were included: population (humans; apparently healthy or with at least one chronic disease); exposures (muscle sympathetic nerve activity during hyperoxia or hyperbaria); comparators (hyperoxia or hyperbaria vs. normoxia); and outcomes (muscle sympathetic nerve activity, heart rate, blood pressure, minute ventilation). Forty-nine studies were ultimately included in the meta-analysis. RESULTS: In healthy individuals, hyperoxia had no effect on sympathetic burst frequency (mean difference [MD] - 1.07 bursts/min; 95% confidence interval [CI] - 2.17, 0.04bursts/min; P = 0.06), burst incidence (MD 0.27 bursts/100 heartbeats [hb]; 95% CI - 2.10, 2.64 bursts/100 hb; P = 0.82), burst amplitude (P = 0.85), or total activity (P = 0.31). In those with chronic diseases, hyperoxia decreased burst frequency (MD - 5.57 bursts/min; 95% CI - 7.48, - 3.67 bursts/min; P < 0.001) and burst incidence (MD - 4.44 bursts/100 hb; 95% CI - 7.94, - 0.94 bursts/100 hb; P = 0.01), but had no effect on burst amplitude (P = 0.36) or total activity (P = 0.90). Our meta-regression analyses identified an inverse relationship between normoxic burst frequency and change in burst frequency with hyperoxia. In both groups, hyperoxia decreased heart rate but had no effect on any measure of blood pressure. CONCLUSION: Hyperoxia does not change sympathetic activity in healthy humans. Conversely, in those with chronic diseases, hyperoxia decreases sympathetic activity. Regardless of disease status, resting sympathetic burst frequency predicts the degree of change in burst frequency, with larger decreases for those with higher resting activity.

The effect of hypoxemia on muscle sympathetic nerve activity and cardiovascular function: a systematic review and meta-analysis.

We conducted a systematic review and meta-analysis to determine the effect of acute poikilocapnic, high-altitude, and acute isocapnia hypoxemia on muscle sympathetic nerve activity (MSNA) and cardiovascular function. A comprehensive search across electronic databases was performed until June 2021. All observational designs were included: population (healthy individuals); exposures (MSNA during hypoxemia); comparators (hypoxemia severity and duration); outcomes (MSNA; heart rate, HR; and mean arterial pressure, MAP). Sixty-one studies were included in the meta-analysis. MSNA burst frequency increased by a greater extent during high-altitude hypoxemia [P < 0.001; mean difference (MD), +22.5 bursts/min; confidence interval (CI) = -19.20 to 25.84] compared with acute poikilocapnic hypoxemia (P < 0.001; MD, +5.63 bursts/min; CI = -4.09 to 7.17) and isocapnic hypoxemia (P < 0.001; MD, +4.72 bursts/min; CI = -3.37 to 6.07). MSNA burst amplitude was only elevated during acute isocapnic hypoxemia (P = 0.03; standard MD, +0.46 au; CI = -0.03 to 0.90), and MSNA burst incidence was only elevated during high-altitude hypoxemia [P < 0.001; MD, 33.05 bursts/100 heartbeats; CI = -28.59 to 37.51]. Meta-regression analysis indicated a strong relationship between MSNA burst frequency and hypoxemia severity for acute isocapnic studies (P < 0.001) but not acute poikilocapnia (P = 0.098). HR increased by the same extent across each type of hypoxemia [P < 0.001; MD +13.81 heartbeats/min; 95% CI = 12.59-15.03]. MAP increased during high-altitude hypoxemia (P < 0.001; MD, +5.06 mmHg; CI = 3.14-6.99), and acute isocapnic hypoxemia (P < 0.001; MD, +1.91 mmHg; CI = 0.84-2.97), but not during acute poikilocapnic hypoxemia (P = 0.95). Both hypoxemia type and severity influenced sympathetic nerve and cardiovascular function. These data are important for the better understanding of healthy human adaptation to hypoxemia.

The sympathetic nervous system in healthy and hypertensive pregnancies: physiology or pathology?

NEW FINDINGS: What is the topic of this review? Sympathoexcitation in both healthy and hypertensive pregnancies, and concurrent adaptations along the neurovascular cascade. What advances does it highlight? Known and plausible adaptations along the neurovascular cascade which may offset elevated MSNA in normotensive pregnancy while also highlighting knowledge gaps regarding understudied pathways. ABSTRACT: The progression from conception through to the postpartum period represents an extraordinary period of physiological adaptation in the mother to support the growth and development of the fetus. Healthy, normotensive human pregnancies are associated with striking increases in both plasma volume and sympathetic nerve activity, yet normal or reduced blood pressure; it represents a unique period of apparent healthy sympathetic hyperactivity. However, how this normal blood pressure is achieved in the face of sympathoexcitation, and the mechanisms responsible for this increased activity are unclear. Importantly, sympathetic activation has been implicated in hypertensive pregnancy disorders - the leading causes of maternal-fetal morbidity and mortality in the developed world. An understudied link between pregnancy and the development of maternal hypertension may lie in the sympathetic nervous system regulation of blood pressure. This brief review presents the latest data on sympathoexcitation in both healthy and hypertensive pregnancies, and concurrent adaptations along the neurovascular cascade.

Cerebrovascular and blood pressure responses during voluntary apneas are larger than rebreathing.

Both voluntary rebreathing (RB) of expired air and voluntary apneas (VA) elicit changes in arterial carbon dioxide and oxygen (CO2 and O2) chemostimuli. These chemostimuli elicit synergistic increases in cerebral blood flow (CBF) and sympathetic nervous system activation, with the latter increasing systemic blood pressure. The extent that simultaneous and inverse changes in arterial CO2 and O2 and associated increases in blood pressure affect the CBF responses during RB versus VAs are unclear. We instrumented 21 healthy participants with a finometer (beat-by-beat mean arterial blood pressure; MAP), transcranial Doppler ultrasound (middle and posterior cerebral artery velocity; MCAv, PCAv) and a mouthpiece with sample line attached to a dual gas analyzer to assess pressure of end-tidal (PET)CO2 and PETO2. Participants performed two protocols: RB and a maximal end-inspiratory VA. A second-by-second stimulus index (SI) was calculated as PETCO2/PETO2 during RB. For VA, where PETCO2 and PETO2 could not be measured throughout, SI values were calculated using interpolated end-tidal gas values before and at the end of the apneas. MAP reactivity (MAPR) was calculated as the slope of the MAP/SI, and cerebrovascular reactivity (CVR) was calculated as the slope of MCAv or PCAv/SI. We found that compared to RB, VA elicited ~ fourfold increases in MAPR slope (P < 0.001), translating to larger anterior and posterior CVR (P ≤ 0.01). However, cerebrovascular conductance (MCAv or PCAv/MAP) was unchanged between interventions (P ≥ 0.2). MAP responses during VAs are larger than those during RB across similar chemostimuli, and differential CVR may be driven by increases in perfusion pressure.

Duration at high altitude influences the onset of arrhythmogenesis during apnea.

PURPOSE: Autonomic control of the heart is balanced by sympathetic and parasympathetic inputs. Excitation of both sympathetic and parasympathetic systems occurs concurrently during certain perturbations such as hypoxia, which stimulate carotid chemoreflex to drive ventilation. It is well established that the chemoreflex becomes sensitized throughout hypoxic exposure; however, whether progressive sensitization alters cardiac autonomic activity remains unknown. We sought to determine the duration of hypoxic exposure at high altitude necessary to unmask cardiac arrhythmias during instances of voluntary apnea. METHODS: Measurements of steady-state chemoreflex drive (SS-CD), continuous electrocardiogram (ECG) and SpO2 (pulse oximetry) were collected in 22 participants on 1 day at low altitude (1045 m) and over eight consecutive days at high-altitude (3800 m). SS-CD was quantified as ventilation (L/min) over stimulus index (PETCO2/SpO2). RESULTS: Bradycardia during apnea was greater at high altitude compared to low altitude for all days (p < 0.001). Cardiac arrhythmias occurred during apnea each day but became most prevalent (> 50%) following Day 5 at high altitude. Changes in saturation during apnea and apnea duration did not affect the magnitude of bradycardia during apnea (ANCOVA; saturation, p = 0.15 and apnea duration, p = 0.988). Interestingly, the magnitude of bradycardia was correlated with the incidence of arrhythmia per day (r = 0.8; p = 0.004). CONCLUSION: Our findings suggest that persistent hypoxia gradually increases vagal tone with time, indicated by augmented bradycardia during apnea and progressively increased the incidence of arrhythmia at high altitude.

Global REACH 2018: volume regulation in high-altitude Andeans with and without chronic mountain sickness.

The high-altitude maladaptation syndrome known as chronic mountain sickness (CMS) is characterized by polycythemia and is associated with proteinuria despite unaltered glomerular filtration rate. However, it remains unclear if indigenous highlanders with CMS have altered volume regulatory hormones. We assessed NH2-terminal pro-B-type natriuretic peptide (NT pro-BNP), plasma aldosterone concentration, plasma renin activity, kidney function (urinary microalbumin, glomerular filtration rate), blood volume, and estimated pulmonary artery systolic pressure (ePASP) in Andean males without (n = 14; age = 39 ± 11 yr) and with (n = 10; age = 40 ± 12 yr) CMS at 4,330 m (Cerro de Pasco, Peru). Plasma renin activity (non-CMS: 15.8 ± 7.9 ng/mL vs. CMS: 8.7 ± 5.4 ng/mL; P = 0.025) and plasma aldosterone concentration (non-CMS: 77.5 ± 35.5 pg/mL vs. CMS: 54.2 ± 28.9 pg/mL; P = 0.018) were lower in highlanders with CMS compared with non-CMS, whereas NT pro-BNP was not different between groups (non-CMS: 1394.9 ± 214.3 pg/mL vs. CMS: 1451.1 ± 327.8 pg/mL; P = 0.15). Highlanders had similar total blood volume (non-CMS: 90 ± 15 mL·kg-1 vs. CMS: 103 ± 18 mL·kg-1; P = 0.071), but Andeans with CMS had greater total red blood cell volume (non-CMS: 46 ± 10 mL·kg-1 vs. CMS: 66 ± 14 mL·kg-1; P < 0.01) and smaller plasma volume (non-CMS: 43 ± 7 mL·kg-1 vs. CMS: 35 ± 5 mL·kg-1; P = 0.03) compared with non-CMS. There were no differences in ePASP between groups (non-CMS: 32 ± 9 mmHg vs. CMS: 31 ± 8 mmHg; P = 0.6). A negative correlation was found between plasma renin activity and glomerular filtration rate in both groups (group: r = -0.66; P < 0.01; non-CMS: r = -0.60; P = 0.022; CMS: r = -0.63; P = 0.049). A smaller plasma volume in Andeans with CMS may indicate an additional CMS maladaptation to high altitude, causing potentially greater polycythemia and clinical symptoms.

A sympathetic view of blood pressure control at high altitude: new insights from microneurographic studies.

NEW FINDINGS: What is the topic of the review? Sympathoexcitation and sympathetic control of blood pressure at high altitude. What advances does it highlight? Sustained sympathoexcitation is fundamental to integrative control of blood pressure in humans exposed to chronic hypoxia. The largest gaps in current knowledge are in understanding the complex mechanisms by which central sympathetic outflow is regulated at high altitude. ABSTRACT: High altitude (HA) hypoxia is a potent activator of the sympathetic nervous system, eliciting increases in sympathetic vasomotor activity. Microneurographic evidence of HA sympathoexcitation dates back to the late 20th century, yet only recently have the characteristics and underpinning mechanisms been explored in detail. This review summarises recent findings and highlights the importance of HA sympathoexcitation for the regulation of blood pressure in lowlanders and indigenous highlanders. In addition, this review identifies gaps in our knowledge and corresponding avenues for future study.

The 2018 Global Research Expedition on Altitude Related Chronic Health (Global REACH) to Cerro de Pasco, Peru: an Experimental Overview.

NEW FINDINGS: What is the central question of this study? Herein, a methodological overview of our research team's (Global REACH) latest high altitude research expedition to Peru is provided. What is the main finding and its importance? The experimental objectives, expedition organization, measurements and key cohort data are discussed. The select data presented in this manuscript demonstrate the haematological differences between lowlanders and Andeans with and without excessive erythrocytosis. The data also demonstrate that exercise capacity was similar between study groups at high altitude. The forthcoming findings from our research expedition will contribute to our understanding of lowlander and indigenous highlander high altitude adaptation. ABSTRACT: In 2016, the international research team Global Research Expedition on Altitude Related Chronic Health (Global REACH) was established and executed a high altitude research expedition to Nepal. The team consists of ∼45 students, principal investigators and physicians with the common objective of conducting experiments focused on high altitude adaptation in lowlanders and in highlanders with lifelong exposure to high altitude. In 2018, Global REACH travelled to Peru, where we performed a series of experiments in the Andean highlanders. The experimental objectives, organization and characteristics, and key cohort data from Global REACH's latest research expedition are outlined herein. Fifteen major studies are described that aimed to elucidate the physiological differences in high altitude acclimatization between lowlanders (n = 30) and Andean-born highlanders with (n = 22) and without (n = 45) excessive erythrocytosis. After baseline testing in Kelowna, BC, Canada (344 m), Global REACH travelled to Lima, Peru (∼80 m) and then ascended by automobile to Cerro de Pasco, Peru (∼4300 m), where experiments were conducted over 25 days. The core studies focused on elucidating the mechanism(s) governing cerebral and peripheral vascular function, cardiopulmonary regulation, exercise performance and autonomic control. Despite encountering serious logistical challenges, each of the proposed studies was completed at both sea level and high altitude, amounting to ∼780 study sessions and >3000 h of experimental testing. Participant demographics and data relating to acid-base balance and exercise capacity are presented. The collective findings will contribute to our understanding of how lowlanders and Andean highlanders have adapted under high altitude stress.

Global REACH 2018: Andean highlanders, chronic mountain sickness and the integrative regulation of resting blood pressure.

NEW FINDINGS: What is the central question of this study? Does chronic mountain sickness (CMS) alter sympathetic neural control and arterial baroreflex regulation of blood pressure in Andean (Quechua) highlanders? What is the main finding and its importance? Compared to healthy Andean highlanders, basal sympathetic vasomotor outflow is lower, baroreflex control of muscle sympathetic nerve activity is similar, supine heart rate is lower and cardiovagal baroreflex gain is greater in mild CMS. Taken together, these findings reflect flexibility in integrative regulation of blood pressure that may be important when blood viscosity and blood volume are elevated in CMS. ABSTRACT: The high-altitude maladaptation syndrome chronic mountain sickness (CMS) is characterized by excessive erythrocytosis and frequently accompanied by accentuated arterial hypoxaemia. Whether altered autonomic cardiovascular regulation is apparent in CMS is unclear. Therefore, during the 2018 Global REACH expedition to Cerro de Pasco, Peru (4383 m), we assessed integrative control of blood pressure (BP) and determined basal sympathetic vasomotor outflow and arterial baroreflex function in eight Andean natives with CMS ([Hb] 22.6 ± 0.9 g·dL-1 ) and seven healthy highlanders ([Hb] 19.3 ± 0.8 g·dL-1 ). R-R interval (RRI, electrocardiogram), beat-by-beat BP (photoplethysmography) and muscle sympathetic nerve activity (MSNA; microneurography) were recorded at rest and during pharmacologically induced changes in BP (modified Oxford test). Although [Hb] and blood viscosity (7.8 ± 0.7 vs. 6.6 ± 0.7 cP; d = 1.7, P = 0.01) were elevated in CMS compared to healthy highlanders, cardiac output, total peripheral resistance and mean BP were similar between groups. The vascular sympathetic baroreflex MSNA set-point (i.e. MSNA burst incidence) and reflex gain (i.e. responsiveness) were also similar between groups (MSNA set-point, d = 0.75, P = 0.16; gain, d = 0.2, P = 0.69). In contrast, in CMS the cardiovagal baroreflex operated around a longer RRI (960 ± 159 vs. 817 ± 50 ms; d = 1.4, P = 0.04) with a greater reflex gain (17.2 ± 6.8 vs. 8.8 ± 2.6 ms·mmHg-1 ; d = 1.8, P = 0.01) versus healthy highlanders. Basal sympathetic vasomotor activity was also lower compared to healthy highlanders (33 ± 11 vs. 45 ± 13 bursts·min-1 ; d = 1.0, P = 0.08). In conclusion, our findings indicate adaptive differences in basal sympathetic vasomotor activity and heart rate compensate for the haemodynamic consequences of excessive erythrocyte volume and contribute to integrative blood pressure regulation in Andean highlanders with mild CMS.

Sympathetic neurovascular transduction following acute hypoxia.

PURPOSE: Following an acute exposure to hypoxia, sympathetic nerve activity remains elevated. However, this elevated sympathetic nerve activity does not elicit a parallel increase in vascular resistance suggesting a blunted sympathetic signaling [i.e. blunted sympathetic neurovascular transduction (sNVT)]. Therefore, we sought to quantify spontaneous sympathetic bursts and related changes in total peripheral resistance following hypoxic exposure. We hypothesized that following hypoxia sNVT would be blunted. METHODS: Nine healthy participants (n = 6 men; mean age 25 ± 2 years) were recruited. We collected data on muscle sympathetic nerve activity (MSNA) using microneurography and beat-by-beat total peripheral resistance (TPR) via finger photoplethysmography at baseline, during acute hypoxia and during two periods of recovery (recovery period 1, 0-10 min post hypoxia; recovery period 2, 10-20 min post hypoxia). MSNA burst sequences (i.e. singlets, doublets, triplets and quads+) were identified and coupled to changes in TPR over 15 cardiac cycles as an index of sNVT for burst sequences. A sNVT slope for each participant was calculated from the slope of the relationship between TPR plotted against normalized burst amplitude. RESULTS: The sNVT slope was blunted during hypoxia [Δ 0.0044 ± 0.0014 (mmHg/L/min)/(a.u.)], but unchanged following termination of hypoxia [recovery 1, Δ 0.031 ± 0.0019 (mmHg/L/min)/(a.u.); recovery 2, Δ 0.0038 ± 0.0014 (mmHg/L/min)/(a.u.) compared to baseline (Δ 0.038 ± 0.0015 (L/min/mmHg)/(a.u.)] (main effect of group p = 0.012). CONCLUSIONS: Contrary to our hypothesis, we have demonstrated that systemic sNVT is unchanged following hypoxia in young healthy adults.

Blunted sympathetic neurovascular transduction is associated to the severity of obstructive sleep apnea.

PURPOSE: Obstructive sleep apnea (OSA) is a common disorder (~ 4%) that augments sympathetic nerve activity (SNA) and elevates blood pressure. The relationship between sympathetic vasomotor outflow and vascular responsiveness, termed sympathetic neurovascular transduction (sNVT), has been sparsely characterized in patients with OSA. Therefore, we sought to quantify spontaneous sympathetic bursts and related changes in diastolic pressure. METHODS: Twelve participants with variable severities of OSA were recruited. We collected muscle sympathetic nerve activity (MSNA) (microneurography) and beat-by-beat diastolic pressure (finger photoplethysmography) during normoxia (FiO2 = 0.21) and hyperoxia (FiO2 = 1.0) to decrease MSNA burst frequency. MSNA burst sequences (i.e. singlets, doublets, triplets and quadruplets) were identified and coupled to changes in diastolic pressure over 15 cardiac cycles as an index of sNVT. sNVT slope for each individual was calculated from the slope of the relationship between peak responses in outcome plotted against normalized burst amplitude. RESULTS: sNVT slope was unchanged during hyperoxia compared to normoxia (normoxia 0.0024 ± 0.0011 Δ mmHg total activity [a.u.]-1 vs. hyperoxia 0.0029 ± 0.00098 Δ mmHg total activity [a.u.]-1; p = 0.14). sNVT slope was inversely associated with burst frequency during hyperoxia (r = -0.58; p = 0.04), but not normoxia (r = -0.11; p = 0.71). sNVT slope was inversely associated with the apnea-hypopnea index (AHI) (r = -0.62; p = 0.030), but not after age was considered. CONCLUSIONS: We have demonstrated that the prevailing MSNA frequency is unmatched to the level of sNVT, and this can be altered by acute hyperoxia.

The Impact of Hospitalization on Physical Activity During Pregnancy.

The concept of prenatal physical activity has transitioned from a potentially risky behaviour to a prescription to improve maternal-fetal health. Pregnant women who are hospitalized because of a complication should continue to engage in light activities of daily living given the known adverse effects of maternal bedrest. Because the amount physical activity performed by pregnant women while in hospital not been documented; we compared levels of physical activity between women who were and were not hospitalized during pregnancy. We found that hospitalized pregnant women self-impose bedrest even in the absence of a medical recommendation.

Acute intermittent hypercapnic hypoxia and sympathetic neurovascular transduction in men.

KEY POINTS: Intermittent hypoxia leads to long-lasting increases in muscle sympathetic nerve activity and blood pressure, contributing to increased risk for hypertension in obstructive sleep apnoea patients. We determined whether augmented vascular responses to increasing sympathetic vasomotor outflow, termed sympathetic neurovascular transduction (sNVT), accompanied changes in blood pressure following acute intermittent hypercapnic hypoxia in men. Lower body negative pressure was utilized to induce a range of sympathetic vasoconstrictor firing while measuring beat-by-beat blood pressure and forearm vascular conductance. IH reduced vascular shear stress and steepened the relationship between diastolic blood pressure and sympathetic discharge frequency, suggesting greater systemic sNVT. Our results indicate that recurring cycles of acute intermittent hypercapnic hypoxia characteristic of obstructive sleep apnoea could promote hypertension by increasing sNVT. ABSTRACT: Acute intermittent hypercapnic hypoxia (IH) induces long-lasting elevations in sympathetic vasomotor outflow and blood pressure in healthy humans. It is unknown whether IH alters sympathetic neurovascular transduction (sNVT), measured as the relationship between sympathetic vasomotor outflow and either forearm vascular conductance (FVC; regional sNVT) or diastolic blood pressure (systemic sNVT). We tested the hypothesis that IH augments sNVT by exposing healthy males to 40 consecutive 1 min breathing cycles, each comprising 40 s of hypercapnic hypoxia ( PETCO2 : +4 ± 3 mmHg above baseline; PETO2 : 48 ± 3 mmHg) and 20 s of normoxia (n = 9), or a 40 min air-breathing control (n = 7). Before and after the intervention, lower body negative pressure (LBNP; 3 min at -15, -30 and -45 mmHg) was applied to elicit reflex increases in muscle sympathetic nerve activity (MSNA, fibular microneurography) when clamping end-tidal gases at baseline levels. Ventilation, arterial pressure [systolic blood pressure, diastolic blood pressure, mean arterial pressure (MAP)], brachial artery blood flow ( Q̇BA ), FVC ( Q̇BA /MAP) and MSNA burst frequency were measured continuously. Following IH, but not control, ventilation [5 L min-1 ; 95% confidence interval (CI) = 1-9] and MAP (5 mmHg; 95% CI = 1-9) were increased, whereas FVC (-0.2 mL min-1  mmHg-1 ; 95% CI = -0.0 to -0.4) and mean shear rate (-21.9 s-1 ; 95% CI = -5.8 to -38.0; all P < 0.05) were reduced. Systemic sNVT was increased following IH (0.25 mmHg burst-1  min-1 ; 95% CI = 0.01-0.49; P < 0.05), whereas changes in regional forearm sNVT were similar between IH and sham. Reductions in vessel wall shear stress and, consequently, nitric oxide production may contribute to heightened systemic sNVT and provide a potential neurovascular mechanism for elevated blood pressure in obstructive sleep apnoea.

Evidence for a physiological role of pulmonary arterial baroreceptors in sympathetic neural activation in healthy humans.

KEY POINTS: In an anaesthetised animal model, independent stimulation of baroreceptors in the pulmonary artery elicits reflex sympathoexcitation. In humans, pulmonary arterial pressure is positively related to basal muscle sympathetic nerve activity (MSNA) under conditions where elevated pulmonary pressure is evident (e.g. high altitude); however, a causal link is not established. Using a novel experimental approach, we demonstrate that reducing pulmonary arterial pressure lowers basal MSNA in healthy humans. This response is distinct from the negative feedback reflex mediated by aortic and carotid sinus baroreceptors when systemic arterial pressure is lowered. Afferent input from pulmonary arterial baroreceptors may contribute to sympathetic neural activation in healthy lowland natives exposed to high altitude. ABSTRACT: In animal models, distension of baroreceptors located in the pulmonary artery induces a reflex increase in sympathetic outflow; however, this has not been examined in humans. Therefore, we investigated whether reductions in pulmonary arterial pressure influenced sympathetic outflow and baroreflex control of muscle sympathetic nerve activity (MSNA). Healthy lowlanders (n = 13; 5 females) were studied 4-8 days following arrival at high altitude (4383 m; Cerro de Pasco, Peru), a setting that increases both pulmonary arterial pressure and sympathetic outflow. MSNA (microneurography) and blood pressure (BP; photoplethysmography) were measured continuously during ambient air breathing (Amb) and a 6 min inhalation of the vasodilator nitric oxide (iNO; 40 ppm in 21% O2 ), to selectively lower pulmonary arterial pressure. A modified Oxford test was performed under both conditions. Pulmonary artery systolic pressure (PASP) was determined using Doppler echocardiography. iNO reduced PASP (24 ± 3 vs. 32 ± 5 mmHg; P < 0.001) compared to Amb, with a similar reduction in MSNA total activity (1369 ± 576 to 994 ± 474 a.u min-1 ; P = 0.01). iNO also reduced the MSNA operating point (burst incidence; 39 ± 16 to 33 ± 17 bursts·100 Hb-1 ; P = 0.01) and diastolic operating pressure (82 ± 8 to 80 ± 8 mmHg; P < 0.001) compared to Amb, without changing heart rate (P = 0.6) or vascular-sympathetic baroreflex gain (P = 0.85). In conclusion, unloading of pulmonary arterial baroreceptors reduced basal sympathetic outflow to the skeletal muscle vasculature and reset vascular-sympathetic baroreflex control of MSNA downward and leftward in healthy humans at high altitude. These data suggest the existence of a lesser-known reflex input involved in sympathetic activation in humans.

Global REACH 2018: renal oxygen delivery is maintained during early acclimatization to 4,330 m.

Early acclimatization to high altitude is characterized by various respiratory, hematological, and cardiovascular adaptations that serve to restore oxygen delivery to tissue. However, less is understood about renal function and the role of renal oxygen delivery (RDO2) during high altitude acclimatization. We hypothesized that 1) RDO2 would be reduced after 12 h of high altitude exposure (high altitude day 1) but restored to sea level values after 1 wk (high altitude day 7) and 2) RDO2 would be associated with renal reactivity, an index of acid-base compensation at high altitude. Twenty-four healthy lowlander participants were tested at sea level (344 m, Kelowna, BC, Canada) and on day 1 and day 7 at high altitude (4,330 m, Cerro de Pasco, Peru). Cardiac output, renal blood flow, and arterial and venous blood sampling for renin-angiotensin-aldosterone system hormones and NH2-terminal pro-B-type natriuretic peptides were collected at each time point. Renal reactivity was calculated as follows: (Δarterial bicarbonate)/(Δarterial Pco2) between sea level and high altitude day 1 and sea level and high altitude day 7. The main findings were that 1) RDO2 was initially decreased at high altitude compared with sea level (ΔRDO2: -22 ± 17%, P < 0.001) but was restored to sea level values on high altitude day 7 (ΔRDO2: -6 ± 14%, P = 0.36). The observed improvements in RDO2 resulted from both changes in renal blood flow (Δ from high altitude day 1: +12 ± 11%, P = 0.008) and arterial oxygen content (Δ from high altitude day 1: +44.8 ± 17.7%, P = 0.006) and 2) renal reactivity was positively correlated with RDO2 on high altitude day 7 (r = 0.70, P < 0.001) but not high altitude day 1 (r = 0.26, P = 0.29). These findings characterize the temporal responses of renal function during early high altitude acclimatization and the influence of RDO2 in the regulation of acid-base balance.

Sex differences in dynamic blood pressure regulation: beat-by-beat responses to muscle sympathetic nerve activity.

It has been suggested that sex differences in acute blood pressure fluctuations occur during the periods of time between bursts of muscle sympathetic nerve activity. Therefore, we tested the hypothesis that men experience more dynamic changes in mean arterial pressure (Finometer MIDI) than women during acute sympathoinhibition (i.e., slow breathing) in which bursts of sympathetic activity occur more infrequently than at rest. We tested healthy women (n = 9) and men (n = 9) of similar age (22 ± 2 vs. 23 ± 3 yr, P = 0.6). Custom software was used to calculate beat-by-beat changes in blood pressure following sympathetic burst and nonburst sequences (recorded using microneurography) during 10 min of supine rest and a 15-min bout of slow breathing. During slow breathing following nonburst sequences, women demonstrated smaller overall reductions in mean arterial pressure compared with men over the subsequent 15 cardiac cycles (P < 0.01). In addition, following a burst of sympathetic activity, women experienced greater overall increases in mean arterial pressure compared with men over the following 15 cardiac cycles (P < 0.01). Despite these differences, the peak and nadir changes in arterial pressure following burst and nonburst sequences were not different between the sexes (P = 0.45 and P = 0.48, burst and nonburst sequences, respectively). As such, these data suggest that women respond to a burst of sympathetic activity with more sustained increases in blood pressure than men, coupled with improved maintenance of blood pressure during acute periods of sympathetic quiescence. In other words, these findings suggest that men rely more on frequent bursts of sympathetic activity to acutely regulate arterial pressure than women.NEW & NOTEWORTHY We demonstrate that during acute sympathoinhibition, women demonstrate more sustained increases in blood pressure following sympathetic bursts of activity than men. Likewise, during prolonged sympathetic quiescence, blood pressure is less labile in women than men. This suggests that lower overall blood pressure in young women may not be mediated by smaller beat-by-beat changes in blood pressure in response to sympathetic outflow but may instead be mediated by a lower frequency of sympathetic bursts.

Influence of multiparity on sympathetic nerve activity during normal pregnancy.

Recent evidence suggests an elevated risk of cardiovascular disease development in multiparous women. Therefore, we investigated the effects of multiparity on within-pregnancy sympathetic neural regulation in normotensive, pregnant women. We retrospectively analyzed heart rate (HR), blood pressure (BP), and muscle sympathetic nerve activity (MSNA; n = 8) data from 10 women whom participated in microneurographic research studies during two sequential pregnancies (i.e., PREG1 and PREG2). There was no difference in resting BP between pregnancies (P > 0.05), whereas HR trended higher in PREG2 versus PREG1 (P = 0.06). MSNA burst frequency was greater in PREG2 versus PREG1 after adjusting for age (32 ± 12 vs. 22 ± 12 bursts/min; P = 0.049), whereas burst incidence did not differ (40 ± 16 vs. 34 ± 17 bursts/100 heartbeats; P = 0.21). Sympathetic baroreflex sensitivity was not different between PREG1 and PREG2 (P > 0.05). Our results may highlight a possible role of altered within-pregnancy sympathetic neural regulation in the observed relationship in women between parity and future cardiovascular disease risk.NEW & NOTEWORTHY To our knowledge, this is the first study to investigate the effects of multiparity on within-pregnancy sympathetic neural regulation. We observed augmented muscle sympathetic nerve activity in women's second studied pregnancy versus their first. Conversely, blood pressure and sympathetic baroreflex sensitivity did not differ, whereas a trend for increased heart rate was observed. Our results highlight a possible role of altered within-pregnancy sympathetic neural regulation in the relationship between increased parity and cardiovascular disease development.

Highs and lows of sympathetic neurocardiovascular transduction: influence of altitude acclimatization and adaptation.

High-altitude (>2,500 m) exposure results in increased muscle sympathetic nervous activity (MSNA) in acclimatizing lowlanders. However, little is known about how altitude affects MSNA in indigenous high-altitude populations. Additionally, the relationship between MSNA and blood pressure regulation (i.e., neurovascular transduction) at high-altitude is unclear. We sought to determine 1) how high-altitude effects neurocardiovascular transduction and 2) whether differences exist in neurocardiovascular transduction between low- and high-altitude populations. Measurements of MSNA (microneurography), mean arterial blood pressure (MAP; finger photoplethysmography), and heart rate (electrocardiogram) were collected in 1) lowlanders (n = 14) at low (344 m) and high altitude (5,050 m), 2) Sherpa highlanders (n = 8; 5,050 m), and 3) Andean (with and without excessive erythrocytosis) highlanders (n = 15; 4,300 m). Cardiovascular responses to MSNA burst sequences (i.e., singlet, couplet, triplet, and quadruplet) were quantified using custom software (coded in MATLAB, v.2015b). Slopes were generated for each individual based on peak responses and normalized total MSNA. High altitude reduced neurocardiovascular transduction in lowlanders (MAP slope: high altitude, 0.0075 ± 0.0060 vs. low altitude, 0.0134 ± 0.080; P = 0.03). Transduction was elevated in Sherpa (MAP slope, 0.012 ± 0.007) compared with Andeans (0.003 ± 0.002, P = 0.001). MAP transduction was not statistically different between acclimatizing lowlanders and Sherpa (MAP slope, P = 0.08) or Andeans (MAP slope, P = 0.07). When resting MSNA is accounted for (ANCOVA), transduction was inversely related to basal MSNA (bursts/minute) independent of population (RRI, r = 0.578 P < 0.001; MAP, r = -0.627, P < 0.0001). Our results demonstrate that transduction is blunted in individuals with higher basal MSNA, suggesting that blunted neurocardiovascular transduction is a physiological adaptation to elevated MSNA rather than an effect or adaptation specific to chronic hypoxic exposure.NEW & NOTEWORTHY This study has identified that sympathetically mediated blood pressure regulation is reduced following ascent to high-altitude. Additionally, we show that high altitude Andean natives have reduced blood pressure responsiveness to sympathetic nervous activity (SNA) compared with Nepalese Sherpa. However, basal sympathetic activity is inversely related to the magnitude of SNA-mediated fluctuations in blood pressure regardless of population or condition. These data set a foundation to explore more precise mechanisms of blood pressure control under conditions of persistent sympathetic activation and hypoxia.

Muscle sympathetic reactivity to apneic and exercise stress in high-altitude Sherpa.

Lowland-dwelling populations exhibit persistent sympathetic hyperactivity at altitude that alters vascular function. High-altitude populations, such as Sherpa, have previously exhibited greater peripheral blood flow in response to acute stress than Lowlanders, which may be explained through lower sympathetic activity. Our purpose was to determine whether Sherpa exhibit lower sympathetic reactivity to stress than Lowlanders. Muscle sympathetic nerve activity (MSNA; microneurography) was measured at rest in Lowlanders (n = 14; age = 27 ± 6 yr) at 344 m and between 1 and 10 days at 5,050 m. Sherpa (age = 32 ± 11 yr) were tested at 5,050 m (n = 8). Neurovascular reactivity (i.e., change in MSNA patterns) was measured during maximal end-expiratory apnea, isometric hand grip (IHG; 30% maximal voluntary contraction for 2-min), and postexercise circulatory occlusion (PECO; 3 min). Burst frequency (bursts/min) and incidence (bursts/100 heartbeats) and total normalized SNA (arbitrary units/min) were analyzed at rest, immediately before apnea breakpoint, and during the last minute of IHG and PECO. Vascular responses to apnea, IHG, and PECO were also measured. MSNA reactivity to apnea was smaller in Sherpa than Lowlanders at 5,050 m, although blood pressure responses were similar between groups. MSNA increases in Lowlanders during apnea at 5,050 m were significantly lower than at 344 m (P < 0.05), indicating that a possible sympathetic ceiling was reached in Lowlanders at 5,050 m. MSNA increased similarly during IHG and PECO in Lowlanders at both 334 m and 5,050 m and in Sherpa at 5,050 m, while vascular changes (mean brachial arterial pressure, contralateral brachial flow and resistance) were similar between groups. Sherpa demonstrate overall lower sympathetic reactivity that may be a result of heightened vascular responsiveness to potential apneic stress at altitude.

Global REACH 2018: The carotid artery diameter response to the cold pressor test is governed by arterial blood pressure during normoxic but not hypoxic conditions in healthy lowlanders and Andean highlanders.

NEW FINDINGS: What is the central question of this study? What is the impact of oxygen on the circulatory responses to an isocapnic cold pressor test (CPT) in lowlanders and Andean highlanders? What is the main finding and its importance? Overall, the circulatory responses to an isocapnic CPT were largely unaltered with acute normobaric hypoxia and chronic hypobaric hypoxia exposure in lowlanders. However, the relationship between mean arterial pressure and common carotid artery diameter was dampened in hypoxic conditions. Furthermore, there were no differences in the circulatory responses to the CPT between lowlanders and Andean highlanders with lifelong exposure to high altitude. ABSTRACT: The impact of oxygen on the circulatory responses to a cold pressor test (CPT) in lowlanders and Andean highlanders remains unknown. Our hypotheses were as follows: (i) in lowlanders, acute normobaric and hypobaric hypoxia would attenuate the common carotid artery (CCA) diameter response to the CPT compared with normobaric normoxia; (ii) Andean highlanders would exhibit a greater CCA diameter response compared with lowlanders; and (iii) a positive relationship between CCA diameter and blood pressure in response to the CPT would be present in both lowlanders and highlanders. Healthy lowlanders (n = 13) and Andean highlanders (n = 8) were recruited and conducted an isocapnic CPT, which consisted of a 3 min foot immersion into water at 0-1°C. Blood pressure (finger photoplethysmography) and CCA diameter and blood flow (Duplex ultrasound) were recorded continuously. The CPT was conducted in lowlanders at sea level in isocapnic normoxic and hypoxic conditions and after 10 days of acclimatization to 4300 m (Cerro de Pasco, Peru) in hypoxic and hyperoxic conditions. Andean highlanders were tested at rest at high altitude. The main findings were as follows: (i) in lowlanders, normobaric but not hypobaric hypoxia elevated CCA reactivity to the CPT; (ii) no differences in response to the CPT were observed between lowlanders and highlanders; and (iii) although hypobaric hypoxaemia reduced the relationship between CCA diameter and blood pressure compared with normobaric normoxia (P = 0.132), hypobaric hyperoxia improved this relationship (P = 0.012), and no relationship was observed in Andean highlanders (P = 0.261). These data demonstrate that the circulatory responses to a CPT were modified by oxygen in lowlanders, but were unaltered with lifelong hypoxic exposure.