Effect of Increased Blood Flow on Pulmonary Circulation Before and During High Altitude Acclimatization

The physiological basis for individualized oxygenation targets in critically ill patients with circulatory shock

BACKGROUND

Circulatory shock, defined as decreased tissue perfusion, leading to inadequate oxygen delivery to meet cellular metabolic demands, remains a common condition with high morbidity and mortality. Rapid restitution and restoration of adequate tissue perfusion are the main treatment goals. To achieve this, current hemodynamic strategies focus on adjusting global physiological variables such as cardiac output (CO), hemoglobin (Hb) concentration, and arterial hemoglobin oxygen saturation (SaO2). However, it remains a challenge to identify optimal targets for these global variables that best support microcirculatory function. Weighting up the risks and benefits is especially difficult for choosing the amount of oxygen supplementation in critically ill patients. This review assesses the physiological basis for oxygen delivery to the tissue and provides an overview of the relevant literature to emphasize the importance of considering risks and benefits and support decision making at the bedside.

PHYSIOLOGICAL PREMISES

Oxygen must reach the tissue to enable oxidative phosphorylation. The human body timely detects hypoxia via different mechanisms aiming to maintain adequate tissue oxygenation. In contrast to the pulmonary circulation, where the main response to hypoxia is arteriolar vasoconstriction, the regulatory mechanisms of the systemic circulation aim to optimize oxygen availability in the tissues. This is achieved by increasing the capillary density in the microcirculation and the capillary hematocrit thereby increasing the capacity of oxygen diffusion from the red blood cells to the tissue. Hyperoxia, on the other hand, is associated with oxygen radical production, promoting cell death.

CURRENT STATE OF RESEARCH

Clinical trials in critically ill patients have primarily focused on comparing macrocirculatory endpoints and outcomes based on stroke volume and oxygenation targets. Some earlier studies have indicated potential benefits of conservative oxygenation. Recent trials show contradictory results regarding mortality, organ dysfunction, and ventilatory-free days. Empirical studies comparing various targets for SaO2, or partial pressure of oxygen indicate a U-shaped curve balancing positive and negative effects of oxygen supplementation.

CONCLUSION AND FUTURE DIRECTIONS

To optimize risk-benefit ratio of resuscitation measures in critically ill patients with circulatory shock in addition to individual targets for CO and Hb concentration, a primary aim should be to restore tissue perfusion and avoid hyperoxia. In the future, an individualized approach with microcirculatory targets will become increasingly relevant. Further studies are needed to define optimal targets.

Beta-adrenergic blockade increases pulmonary vascular resistance and causes exaggerated hypoxic pulmonary vasoconstriction at high altitude: a physiological study

BACKGROUND

An increasing number of hypertensive persons travel to high altitude (HA) while using antihypertensive medications such as beta-blockers. Nevertheless, while hypoxic exposure initiates an increase in pulmonary artery pressure (Ppa) and pulmonary vascular resistance (PVR), the contribution of the autonomic nervous system is unclear. In animals, beta-adrenergic blockade has induced pulmonary vasoconstriction in normoxia and exaggerated hypoxic pulmonary vasoconstriction (HPV) and both effects were abolished by muscarinic blockade. We thus hypothesized that in humans, propranolol (PROP) increases Ppa and PVR in normoxia and exaggerates HPV, and that these effects of PROP are abolished by glycopyrrolate (GLYC).

METHODS

In seven healthy male lowlanders, Ppa was invasively measured without medication, with PROP and PROP + GLYC, both at sea level (SL, 488 m) and after a 3-week sojourn at 3454 m altitude (HA). Bilateral thigh-cuff release manoeuvres were performed to derive pulmonary pressure-flow relationships and pulmonary vessel distensibility.

RESULTS

At SL, PROP increased Ppa and PVR from (mean ± SEM) 14 ± 1 to 17 ± 1 mmHg and from 69 ± 8 to 108 ± 11 dyn s cm-5 (21% and 57% increase, P = 0.01 and P < 0.0001). The PVR response to PROP was amplified at HA to 76% (P < 0.0001, P[interaction] = 0.05). At both altitudes, PROP + GLYC abolished the effect of PROP on Ppa and PVR. Pulmonary vessel distensibility decreased from 2.9 ± 0.5 to 1.7 ± 0.2 at HA (P < 0.0001) and to 1.2 ± 0.2 with PROP, and further decreased to 0.9 ± 0.2% mmHg-1 with PROP + GLYC (P = 0.01).

CONCLUSIONS

Our data show that beta-adrenergic blockade increases, and muscarinic blockade decreases PVR, whereas both increase pulmonary artery elastance. Future studies may confirm potential implications from the finding that beta-adrenergic blockade exaggerates HPV for the management of mountaineers using beta-blockers for prevention or treatment of cardiovascular conditions.

Mechanisms underpinning sympathoexcitation in hypoxia

Sympathoexcitation is a hallmark of hypoxic exposure, occurring acutely, as well as persisting in acclimatised lowland populations and with generational exposure in highland native populations of the Andean and Tibetan plateaus. The mechanisms mediating altitude sympathoexcitation are multifactorial, involving alterations in both peripheral autonomic reflexes and central neural pathways, and are dependent on the duration of exposure. Initially, hypoxia-induced sympathoexcitation appears to be an adaptive response, primarily mediated by regulatory reflex mechanisms concerned with preserving systemic and cerebral tissue O2 delivery and maintaining arterial blood pressure. However, as exposure continues, sympathoexcitation is further augmented above that observed with acute exposure, despite acclimatisation processes that restore arterial oxygen content (C a O 2 ${C_{{\mathrm{a}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ). Under these conditions, sympathoexcitation may become maladaptive, giving rise to reduced vascular reactivity and mildly elevated blood pressure. Importantly, current evidence indicates the peripheral chemoreflex does not play a significant role in the augmentation of sympathoexcitation during altitude acclimatisation, although methodological limitations may underestimate its true contribution. Instead, processes that provide no obvious survival benefit in hypoxia appear to contribute, including elevated pulmonary arterial pressure. Nocturnal periodic breathing is also a potential mechanism contributing to altitude sympathoexcitation, although experimental studies are required. Despite recent advancements within the field, several areas remain unexplored, including the mechanisms responsible for the apparent normalisation of muscle sympathetic nerve activity during intermediate hypoxic exposures, the mechanisms accounting for persistent sympathoexcitation following descent from altitude and consideration of whether there are sex-based differences in sympathetic regulation at altitude.

Echocardiography and extravascular lung water during 3 weeks of exposure to high altitude in otherwise healthy asthmatics

Background: Asthma rehabilitation at high altitude is common. Little is known about the acute and subacute cardiopulmonary acclimatization to high altitude in middle-aged asthmatics without other comorbidities. Methods: In this prospective study in lowlander subjects with mostly mild asthma who revealed an asthma control questionnaire score >0.75 and participated in a three-week rehabilitation program, we assessed systolic pulmonary artery pressure (sPAP), cardiac function, and extravascular lung water (EVLW) at 760 m (baseline) by Doppler-echocardiography and on the second (acute) and last day (subacute) at a high altitude clinic in Kyrgyzstan (3100 m). Results: The study included 22 patients (eight male) with a mean age of 44.3 ± 12.4 years, body mass index of 25.8 ± 4.7 kg/m2, a forced expiratory volume in 1 s of 92% ± 19% predicted (post-bronchodilator), and partially uncontrolled asthma. sPAP increased from 21.8 mmHg by mean difference by 7.5 [95% confidence interval 3.9 to 10.5] mmHg (p < 0.001) during acute exposure and by 4.8 [1.0 to 8.6] mmHg (p = 0.014) during subacute exposure. The right-ventricular-to-pulmonary-artery coupling expressed by TAPSE/sPAP decreased from 1.1 by -0.2 [-0.3 to -0.1] mm/mmHg (p < 0.001) during acute exposure and by -0.2 [-0.3 to -0.1] mm/mmHg (p = 0.002) during subacute exposure, accordingly. EVLW significantly increased from baseline (1.3 ± 1.8) to acute hypoxia (5.5 ± 3.5, p < 0.001) but showed no difference after 3 weeks (2.0 ± 1.8). Conclusion: In otherwise healthy asthmatics, acute exposure to hypoxia at high altitude increases pulmonary artery pressure (PAP) and EVLW. During subacute exposure, PAP remains increased, but EVLW returns to baseline values, suggesting compensatory mechanisms that contribute to EVLW homeostasis during acclimatization.

Pulmonary vascular reactivity to supplemental oxygen in Sherpa and lowlanders during gradual ascent to high altitude

NEW FINDINGS

What is the central question of this study? How does hypoxic pulmonary vasoconstriction and the response to supplemental oxygen change over time at high altitude? What is the main finding and its importance? Lowlanders and partially de-acclimatized Sherpa both demonstrated pulmonary vascular responsiveness to supplemental oxygen that was maintained for 12 days' exposure to progressively increasing altitude. An additional 2 weeks' acclimatization at 5050 m altitude rendered the pulmonary vasculature minimally responsive to oxygen similar to the fully acclimatized non-ascent Sherpa. Additional hypoxic exposure at that time point did not augment hypoxic pulmonary vasoconstriction.

ABSTRACT

Prolonged alveolar hypoxia leads to pulmonary vascular remodelling. We examined the time course at altitude, over which hypoxic pulmonary vasoconstriction goes from being acutely reversible to potentially irreversible. Study subjects were lowlanders (n = 20) and two Sherpa groups. All Sherpa were born and raised at altitude. One group (ascent Sherpa, n = 11) left altitude and after de-acclimatization in Kathmandu for ∼7 days re-ascended with the lowlanders over 8-10 days to 5050 m. The second Sherpa group (non-ascent Sherpa, n = 12) remained continuously at altitude. Pulmonary artery systolic pressure (PASP) and pulmonary vascular resistance (PVR) were measured while breathing ambient air and following supplemental oxygen. During ascent PASP and PVR increased in lowlanders and ascent Sherpa; however, with supplemental oxygen, lowlanders had significantly greater decrease in PASP (P = 0.02) and PVR (P = 0.02). After ∼14 days at 5050 m, PASP decreased with supplemental oxygen (mean decrease: 3.9 mmHg, 95% CI 2.1-5.7 mmHg, P < 0.001); however, PVR was unchanged (P = 0.49). In conclusion, PASP and PVR increased with gradual ascent to altitude and decreased via oxygen supplementation in both lowlanders and ascent Sherpa. Following ∼14 days at 5050 m altitude, there was no change in PVR to hypoxia or O2  supplementation in lowlanders or either Sherpa group. These data show that both duration of exposure and residential altitude influence the pulmonary vascular responses to hypoxia.

The Impact of Breathing Hypoxic Gas and Oxygen on Pulmonary Hemodynamics in Patients With Pulmonary Hypertension

Background

Pure oxygen breathing (hyperoxia) may improve hemodynamics in patients with pulmonary hypertension (PH) and allows to calculate right-to-left shunt fraction (Qs/Qt), whereas breathing normobaric hypoxia may accelerate hypoxic pulmonary vasoconstriction (HPV). This study investigates how hyperoxia and hypoxia affect mean pulmonary artery pressure (mPAP) and pulmonary vascular resistance (PVR) in patients with PH and whether Qs/Qt influences the changes of mPAP and PVR.

Study Design and Methods

Adults with pulmonary arterial or chronic thromboembolic PH (PAH/CTEPH) underwent repetitive hemodynamic and blood gas measurements during right heart catheterization (RHC) under normoxia [fractions of inspiratory oxygen (FiO2) 0.21], hypoxia (FiO2 0.15), and hyperoxia (FiO2 1.0) for at least 10 min.

Results

We included 149 patients (79/70 PAH/CTEPH, 59% women, mean ± SD 60 ± 17 years). Multivariable regressions (mean change, CI) showed that hypoxia did not affect mPAP and cardiac index, but increased PVR [0.4 (0.1–0.7) WU, p = 0.021] due to decreased pulmonary artery wedge pressure [−0.54 (−0.92 to −0.162), p = 0.005]. Hyperoxia significantly decreased mPAP [−4.4 (−5.5 to −3.3) mmHg, p &lt; 0.001] and PVR [−0.4 (−0.7 to −0.1) WU, p = 0.006] compared with normoxia. The Qs/Qt (14 ± 6%) was &gt;10 in 75% of subjects but changes of mPAP and PVR under hyperoxia and hypoxia were independent of Qs/Qt.

Conclusion

Acute exposure to hypoxia did not relevantly alter pulmonary hemodynamics indicating a blunted HPV-response in PH. In contrast, hyperoxia remarkably reduced mPAP and PVR, indicating a preserved vasodilator response to oxygen and possibly supporting the oxygen therapy in patients with PH. A high proportion of patients with PH showed increased Qs/Qt, which, however, was not associated with changes in pulmonary hemodynamics in response to changes in FiO2.

Right Ventricular Response to Acute Hypoxia Exposure: A Systematic Review

Background: Acute hypoxia exposure is associated with an elevation of pulmonary artery pressure (PAP), resulting in an increased hemodynamic load on the right ventricle (RV). In addition, hypoxia may exert direct effects on the RV. However, the RV responses to such challenges are not fully characterized. The aim of this systematic review was to describe the effects of acute hypoxia on the RV in healthy lowland adults.

Methods: We systematically reviewed PubMed and Web of Science and article references from 2005 until May 2021 for prospective studies evaluating echocardiographic RV function and morphology in healthy lowland adults at sea level and upon exposure to simulated altitude or high-altitude.

Results: We included 37 studies in this systematic review, 12 of which used simulated altitude and 25 were conducted in high-altitude field conditions. Eligible studies reported at least one of the RV variables, which were all based on transthoracic echocardiography assessing RV systolic and diastolic function and RV morphology. The design of these studies significantly differed in terms of mode of ascent to high-altitude, altitude level, duration of high-altitude stay, and timing of measurements. In the majority of the studies, echocardiographic examinations were performed within the first 10 days of high-altitude induction. Studies also differed widely by selectively reporting only a part of multiple RV parameters. Despite consistent increase in PAP documented in all studies, reports on the changes of RV function and morphology greatly differed between studies.

Conclusion: This systematic review revealed that the study reports on the effects of acute hypoxia on the RV are controversial and inconclusive. This may be the result of significantly different study designs, non-compliance with international guidelines on RV function assessment and limited statistical power due to small sample sizes. Moreover, the potential impact of other factors such as gender, age, ethnicity, physical activity, mode of ascent and environmental factors such as temperature and humidity on RV responses to hypoxia remained unexplored. Thus, this comprehensive overview will promote reproducible research with improved study designs and methods for the future large-scale prospective studies, which eventually may provide important insights into the RV response to acute hypoxia exposure.

Influence of Exposure at Different Altitudes on the Executive Function of Plateau Soldiers-Evidence From ERPs and Neural Oscillations

This study investigates the changes in soldiers' brain executive function at different altitude environments and their relationship with blood oxygen saturation. Stratified sampling was conducted in different altitude 133 active-duty soldiers who were stationed in Weinan (347 m, n = 34), Nyingchi (2,950 m, n = 32), Lhasa (3,860 m, n = 33), and Nagqu (4,890 m, n = 34) for 2 years. The Go/NoGo paradigm with event-related potentials (ERPs) and event-related oscillations (EROs) was used to explore the time and neural oscillation courses of response inhibition. Behavioral results revealed that at the 4,890-m altitude area, the soldiers had the highest false alarm rate, the longest reaction time, and the slowest information transmission rate. The electrophysiological results revealed that NoGo-N2 and N2d decreased with increasing altitude, with significant changes at 3,860 m; the amplitudes of NoGo-P3 and P3d in plateau groups were significantly more negative than the plain and changed significantly at 2,950 m. The results of correlation analysis showed that NoGo-P3 was negatively correlated with altitude (r = −0.358, p = 0.000), positively correlated with SpO₂ (r = 0.197, p = 0.041) and information translation rate (ITR) (r = 0.202, p = 0.036). P3d was negatively correlated with altitude (r = −0.276, p = 0.004) and positively correlated with ITR (r = 0.228, p = 0.018). N2d was negatively correlated with ITR (r = 0.204, p = 0.034). The power spectrum analysis of NoGo-N2 and NoGo-P3 showed that the power of δ and θ bands at the plateau area was significantly lower than the plain area and showed a significant step-by-step decrease; the α-band power increases significantly only in the area of 4,890 m. The effect of chronic hypoxia exposure at different altitudes of the plateau on the response inhibition of soldiers was manifested: 3,860 m was the altitude at which the brain response inhibition function decreased during the conflict monitoring stage, and 2,950 m was the altitude at which it dropped during the response inhibition stage. In addition, the soldier's brain's executive function was closely related to SpO₂, and a reduction in SpO₂ may lead to a decline in response inhibition.

Plasma volume contraction reduces atrial natriuretic peptide after four days of hypobaric hypoxia exposure

We investigated whether low arterial oxygen tension ([Formula: see text]) or hypoxia-induced plasma volume (PV) contraction, which reduces central blood volume (BV) and atrial distension, explain reduction in circulating atrial natriuretic peptide (ANP) after prolonged hypoxic exposure. Ten healthy males were exposed for 4 days to hypobaric hypoxia corresponding to an altitude of 3,500 m. PV changes were determined by carbon monoxide rebreathing. Venous plasma concentrations of midregional proANP (MR-proANP) were measured before and at the end of the exposure. At the latter time point, the measurement was repeated after 1) restoration of [Formula: see text] by breathing a hyperoxic gas mixture for 30 min and 2) restoration of BV by fluid infusion. Correspondingly, left ventricular end-diastolic volume (LVEDV), left atrial area (LAA), and right atrial area (RAA) were determined by ultrasound before exposure and both before and after fluid infusion at the end of the exposure. Hypoxic exposure reduced MR-proANP from 37.9 ± 18.5 to 24.5 ± 10.3 pmol/L (P = 0.034), LVEDV from 107.4 ± 33.5 to 91.6 ± 26.3 mL (P = 0.005), LAA from 15.8 ± 4.9 to 13.3 ± 4.2 cm² (P = 0.007), and RAA from 16.2 ± 3.1 to 14.3 ± 3.5 cm² (P = 0.001). Hyperoxic breathing did not affect MR-proANP (24.8 ± 12.3 pmol/L, P = 0.890). Conversely, fluid infusion restored LVEDV, LAA, and RAA to near-baseline values (108.0 ± 29.3 mL, 17.2 ± 5.7 cm², and 17.2 ± 3.1 cm², respectively, P > 0.05 vs. baseline) and increased MR-proANP to 29.5 ± 13.3 pmol/L (P = 0.010 vs. preinfusion and P = 0.182 vs. baseline). These findings support that ANP reduction in hypoxia is at least partially attributed to plasma volume contraction, whereas reduced [Formula: see text] does not seem to contribute.

Extravascular lung water and cardiac function assessed by echocardiography in healthy lowlanders during repeated very high-altitude exposure

BACKGROUND

High-altitude pulmonary edema is associated with elevated systolic pulmonary artery pressure (sPAP) and increased extravascular lung water (EVLW). We investigated sPAP and EVLW during repeated exposures to high altitude (HA).

METHODS

Healthy lowlanders underwent two identical 7-day HA-cycles, where subjects slept at 2900 m and spent 4-8 h daily at 5050 m, separated by a weeklong break at low altitude (LA). Echocardiography and EVLW by B-lines were measured at 520 m (baseline, LA₁), on day one, two and six at 5050 m (HA_1-3) and after descent (LA₂).

RESULTS

We included 21 subjects (median 25 years, body mass index 22 kg/m², SpO₂ 98%). SPAP rose from 21 mmHg at LA₁ to 38 mmHg at HA₁, decreased to 30 mmHg at HA₃ (both p < 0.05 vs LA₁) and normalized at 20 mmHg at LA₂ (p = ns vs LA₁). B-lines increased from 0 at LA₁ to 6 at HA₂ and 7 at HA₃ (both p < 0.05 vs LA₁) and receded to 1 at LA₂ (p = ns vs LA₁). Overall, in cycle two, sPAP did not differ (mean difference (95% confidence interval) -0.2(-2.3 to 1.9) mmHg, p = 0.864) but B-lines were more prevalent (+2.3 (1.4-3.1), p < 0.001) compared to cycle 1. Right ventricular systolic function decreased significantly but minimally at 5050 m.

CONCLUSIONS

Exposure to 5050 m induced a rapid increase in sPAP. B-lines rose during prolonged exposures to 5050 m, despite gradual decrease in sPAP, indicating excessive hydrostatic pressure might not be solely responsible for EVLW-development. Repeated HA-exposure had no acclimatization effect on EVLW. This may affect workers needing repetitive ascents to altitude and could indicate greater B-line development upon repeated exposure.

Pulmonary Hypertension in Acute and Chronic High Altitude Maladaptation Disorders

Alveolar hypoxia is the most prominent feature of high altitude environment with well-known consequences for the cardio-pulmonary system, including development of pulmonary hypertension. Pulmonary hypertension due to an exaggerated hypoxic pulmonary vasoconstriction contributes to high altitude pulmonary edema (HAPE), a life-threatening disorder, occurring at high altitudes in non-acclimatized healthy individuals. Despite a strong physiologic rationale for using vasodilators for prevention and treatment of HAPE, no systematic studies of their efficacy have been conducted to date. Calcium-channel blockers are currently recommended for drug prophylaxis in high-risk individuals with a clear history of recurrent HAPE based on the extensive clinical experience with nifedipine in HAPE prevention in susceptible individuals. Chronic exposure to hypoxia induces pulmonary vascular remodeling and development of pulmonary hypertension, which places an increased pressure load on the right ventricle leading to right heart failure. Further, pulmonary hypertension along with excessive erythrocytosis may complicate chronic mountain sickness, another high altitude maladaptation disorder. Importantly, other causes than hypoxia may potentially underlie and/or contribute to pulmonary hypertension at high altitude, such as chronic heart and lung diseases, thrombotic or embolic diseases. Extensive clinical experience with drugs in patients with pulmonary arterial hypertension suggests their potential for treatment of high altitude pulmonary hypertension. Small studies have demonstrated their efficacy in reducing pulmonary artery pressure in high altitude residents. However, no drugs have been approved to date for the therapy of chronic high altitude pulmonary hypertension. This work provides a literature review on the role of pulmonary hypertension in the pathogenesis of acute and chronic high altitude maladaptation disorders and summarizes current knowledge regarding potential treatment options.

Cardiac Acclimatization at High Altitude in Two Different Ethnicity Groups

Gaur, Priya, Meerim Sartmyrzaeva, Abdirashit Maripov, Kubatbek Muratali Uulu, Supriya Saini, Koushik Ray, Krishna Kishore, Almaz Akunov, Akpay Sarybaev, Bhuvnesh Kumar, Shashi Bala Singh, and Praveen Vats. Cardiac acclimatization at high altitude in two different ethnicity groups. High Alt Med Biol. 22:58-69, 2021. Introduction: High altitude (HA) exposure causes substantial increase in pulmonary artery pressure (PAP) and resistance. However, the effects of HA hypoxia exposure on cardiac function remain incompletely understood. Studies evaluating interethnic differences in cardiac functions in response to HA exposure are lacking. We aimed to compare the cardiac performance in Indian versus Kyrgyz healthy lowland subjects over the course of a 3-week HA exposure at 4,111 m. Methodology: Ten Indians and 20 Kyrgyz subjects were studied to assess cardiac acclimatization noninvasively by echocardiography in two different ethnic groups for 3 weeks of stay at HA. Pulmonary hemodynamics, right and left ventricular functions were evaluated at basal and on days 3, 7, 14, and 21 of HA exposure and on day 3 of deinduction. Results: HA exposure significantly increased PAP, pulmonary vascular resistance, cardiac output (CO), and heart rates (HRs) in both groups. Tricuspid regurgitant gradient increased significantly in both the group at day 3 versus basal; 38.9 mmHg (31.8, 42.9) versus 21.9 mmHg (19.5, 22.6) in Kyrgyz; and 34.1 mmHg (30.2, 38.5) versus 20.4 mmHg (19.7, 21.3) in Indians. HR increased significantly in Indians at day 3 and 7, whereas in Kyrgyz throughout exposure. CO increased significantly in both groups at day 3 versus basal with 5.9 L/min (5.5, 6.4) versus 5.1 L/min (4.4, 5.9) in Kyrgyz, and 5.7 L/min (5.56, 5.98) versus 4.9 L/min (4.1, 5.3) in Indians. Both groups exhibited preserved right ventricular diastolic and systolic functions at HAs. HA exposure changed the left ventricular diastolic parameters only in Kyrgyz subjects with impaired mitral inflow E/A, but not in Indian subjects. All cardiac changes induced at HAs have been recovered fully upon deinduction in both, except lateral-septal A', which remained low in Indians. Conclusion: Although pulmonary hemodynamics responses were similar in both groups, there were differences in cardiac functional parameters between the two in response to HA exposure that may be accounted to ethnic variation.

Residence at moderately high altitude and its relationship with WHO Group 1 pulmonary arterial hypertension symptom severity and clinical characteristics: the Pulmonary Hypertension Association Registry

BACKGROUND

WHO Group 1 pulmonary arterial hypertension is a progressive and potentially fatal disease. Individuals living at higher altitude are exposed to lower barometric pressure and hypobaric hypoxemia. This may result in pulmonary vasoconstriction and contribute to disease progression. We sought to examine the relationship between living at moderately high altitude and pulmonary arterial hypertension characteristics.

METHODS

Forty-two US centers participating in the Pulmonary Hypertension Association Registry enrolled patients who met the definition of WHO Group 1 pulmonary arterial hypertension. We utilized baseline data and patient questionnaire responses. Patients were divided into two groups: moderately high altitude residence (home ≥4000 ft) and low altitude residence (home <4000 ft) based on zip-code. Clinical characteristics, hemodynamic data, patient demographics, and patient reported quality of life metrics were compared.

RESULTS

Controlling for potential confounders (age, sex at birth, body mass index, supplemental oxygen use, race, 100-day cigarette use, alcohol use, and pulmonary arterial hypertension medication use), subjects residing at moderately high altitude had a 6-min walk distance 32 m greater than those at low altitude, despite having a pulmonary vascular resistance that was 2.2 Wood units higher. Additionally, those residing at moderately high altitude had 3.7 times greater odds of using supplemental oxygen.

CONCLUSION

Patients with pulmonary arterial hypertension who live at moderately high altitude have a higher pulmonary vascular resistance and are more likely to need supplemental oxygen. Despite these findings, moderately high altitude Pulmonary Hypertension Association Registry patients have better functional tolerance as measured by 6-min walk distance. It is possible that a "high-altitude phenotype" of pulmonary arterial hypertension may exist. These findings warrant further study.

Outcomes of the Non-fenestrated Fontan Procedure at High Altitude

Background:

Although high altitude has been considered a risk factor for the Fontan operation, and an indication for fenestration, there is a paucity of data to support its routine use. Fenestration, with its necessary right to left induced shunt, together with the lower partial pressure of oxygen found with progressive altitude, can significantly decrease hemoglobin oxygen saturation, and therefore, it would be desirable to avoid it.

Objective:

To analyze immediate and medium-term results of the non-fenestrated, extracardiac, Fontan procedure at high altitude.

Methods:

Retrospective analysis of data from consecutive patients who underwent non-fenestrated, extracardiac, Fontan procedure at two institutions located in Mexico City at 2,312 m (7,585 ft) and 2,691 m (8,828 ft) above sea level. High altitude was not considered a risk factor.

Results:

Thirty-nine patients were included, with a mean age of 6.7 years. Mean preoperative indexed pulmonary vascular resistance was 1.7 Wood units. Seventy-nine percent of the patients extubated in the operating room. There was one in-hospital death (2.56%) and one at follow-up. Median chest tube drainage time was 6.5 and 6 days for the right and left pleural spaces. Median oxygen saturation at discharge was 90%. At a median follow-up of six months, all survivors, except one, had good tolerance to daily life activities.

Conclusions:

The present study shows good short- and medium-term results for the non-fenestrated, extracardiac, Fontan operation at altitudes between 2,300 and 2,700 m and might favor this strategy over fenestration to improve postoperative oxygen saturation. Further studies to examine the long-term outcomes of this approach need to be considered.

Recruitment of non-perfused sublingual capillaries increases microcirculatory oxygen extraction capacity throughout ascent to 7126 m

KEY POINTS

A physiological response to increase microcirculatory oxygen extraction capacity at high altitude is to recruit capillaries. In the present study, we report that high altitude-induced sublingual capillary recruitment is an intrinsic mechanism of the sublingual microcirculation that is independent of changes in cardiac output, arterial blood pressure or systemic vascular hindrance. Using a topical nitroglycerin challenge to the sublingual microcirculation, we show that high altitude-related capillary recruitment is a functional response of the sublingual microcirculation as opposed to an anatomical response associated with angiogenesis. The concurrent presence of a low capillary density and high microvascular reactivity to topical nitroglycerin at sea level was found to be associated with a failure to reach the summit, whereas the presence of a high baseline capillary density with the ability to further increase maximum recruitable capillary density upon ascent to an extreme altitude was associated with summit success.

ABSTRACT

A high altitude (HA) stay is associated with an increase in sublingual capillary total vessel density (TVD), suggesting microvascular recruitment. We hypothesized that microvascular recruitment occurs independent of cardiac output changes, that it relies on haemodynamic changes within the microcirculation as opposed to structural changes and that microcirculatory function is related to individual performance at HA. In 41 healthy subjects, sublingual handheld vital microscopy and echocardiography were performed at sea level (SL), as well as at 6022 m (C2) and 7042 m (C3), during ascent to 7126 m within 21 days. Sublingual topical nitroglycerin was applied to measure microvascular reactivity and maximum recruitable TVD (TVD_NG ). HA exposure decreased resting cardiac output, whereas TVD (mean ± SD) increased from 18.81 ± 3.92 to 20.92 ± 3.66 and 21.25 ± 2.27 mm mm^-2 (P < 0.01). The difference between TVD and TVD_NG was 2.28 ± 4.59 mm mm^-2 at SL (P < 0.01) but remained undetectable at HA. Maximal TVD_NG was observed at C3. Those who reached the summit (n = 15) demonstrated higher TVD at SL (P < 0.01), comparable to TVD_NG in non-summiters (n = 21) at SL and in both groups at C2. Recruitment of sublingual capillary TVD to increase microcirculatory oxygen extraction capacity at HA was found to be an intrinsic mechanism of the microcirculation independent of cardiac output changes. Microvascular reactivity to topical nitroglycerin demonstrated that HA-related capillary recruitment is a functional response as opposed to a structural change. The performance of the vascular microcirculation needed to reach the summit was found to be associated with a higher TVD at SL and the ability to further increase TVD_NG upon ascent to extreme altitude.

Effects of human relaxin-2 (serelaxin) on hypoxic pulmonary vasoconstriction during acute hypoxia in a sheep model

Purpose

Hypoxia induces pulmonary vasoconstriction with a subsequent increase of pulmonary artery pressure (PAP), which can result in pulmonary hypertension. Serelaxin has shown an increase of pulmonary hemodynamic parameters after serelaxin injection. We therefore investigated the response of pulmonary hemodynamic parameters after serelaxin administration in a clinically relevant model.

Methods

Six controls and six sheep that received 30 μg/kg serelaxin underwent right heart catheterization during a 12-minute hypoxia period (inhalation of 5% oxygen and 95% nitrogen) and subsequent reoxygenation. Systolic, diastolic, and mean values of both PAP (respectively, PAPs, PAPd, and PAPm) and pulmonary capillary wedge pressure (respectively, PCWPs, PCWPd, and PCWPm), blood gases, heart rate (HR), and both peripheral and pulmonary arterial oxygen saturation were obtained. Cardiac output (CO), stroke volume (SV), pulmonary vascular resistance (PVR), pulmonary arterial compliance (PAcompl), and systemic vascular resistance (SVR) were calculated.

Results

The key findings of the current study are that serelaxin prevents the rise of PAPs (p≤0.001), PAPm, PCWPm, PCWPs (p≤0.03), and PAPd (p≤0.05) during hypoxia, while it simultaneously increases CO and SV (p≤0.001). Similar courses of decreases of PAPm, PAPd, PAPs, CO, SVR (p≤0.001), and PCWPd (p≤0.03) as compared to hypoxic values were observed during reoxygenation. In direct comparison, the experimental groups differed during hypoxia in regard to HR, PAPm, PVR, and SVR (p≤0.03), and during reoxygenation in regard to HR (p≤0.001), PAPm, PAPs, PAPd, PVR, SVR (p≤0.03), and PCWPd (p≤0.05).

Conclusion

The findings of this study suggest that serelaxin treatment improves pulmonary hemodynamic parameters during acute hypoxia.

Changes in acute pulmonary vascular responsiveness to hypoxia during a progressive ascent to high altitude (5300 m)

NEW FINDINGS

What is the central question of this study? Do the pulmonary vascular responses to hypoxia change during progressive exposure to high altitude and can alterations in these responses be related to changes in concentrations of circulating biomarkers that affect the pulmonary circulation? What is the main finding and its importance? In our field study with healthy volunteers, we demonstrate changes in pulmonary artery pressure suggestive of remodelling in the pulmonary circulation, but find no changes in the acute responsiveness of the pulmonary circulation to changes in oxygenation during 2 weeks of exposure to progressive hypoxia. Pulmonary artery pressure changes were associated with changes in erythropoietin, 8-isoprostane, nitrite and guanosine 3',5'-cyclic monophosphate. We sought to determine whether changes in pulmonary artery pressure responses to hypoxia suggestive of vascular remodelling occur during progressive exposure to high altitude and whether such alterations are related to changes in concentrations of circulating biomarkers with known or suspected actions on the pulmonary vasculature during ascent. We measured tricuspid valve transvalvular pressure gradients (TVPG) in healthy volunteers breathing air at sea level (London, UK) and in hypoxic conditions simulating the inspired O₂ partial pressures at two locations in Nepal, Namche Bazaar (NB, elevation 3500 m) and Everest Base Camp (EBC, elevation 5300 m). During a subsequent 13 day trek, TVPG was measured at NB and EBC while volunteers breathed air and hyperoxic or hypoxic mixtures simulating the inspired O₂ partial pressures at the other locations. For each location, we determined the slope of the relationship between TVPG and arterial oxygen saturation (SaO2) to estimate the pulmonary vascular response to hypoxia. Mean TVPG breathing air was higher at any SaO2 at EBC than at sea level or NB, but there was no change in the slope of the relationship between SaO2 and TVPG between locations. Nitric oxide availability remained unchanged despite increases in oxidative stress (elevated 8-isoprostane). Erythropoietin, pro-atrial natriuretic peptide and interleukin-18 levels progressively increased on ascent. Associations with TVPG were observed only with erythropoietin, 8-isoprostane, nitrite and guanosine 3',5'-cyclic monophosphate. Although the increased TVPG for any given SaO2 at EBC suggests that pulmonary vascular remodelling might occur during 2 weeks of progressive hypoxia, the lack of change in the slope of the relationship between TVPG and SaO2 indicates that the acute pulmonary vascular responsiveness to changes in oxygenation does not vary within this time frame.

Adropin Is a Key Mediator of Hypoxia Induced Anti-Dipsogenic Effects via TRPV4-CamKK-AMPK Signaling in the Circumventricular Organs of Rats

Water intake reduction (anti-dipsogenic effects) under hypoxia has been well established, but the underlying reason remains unknown. Our previous report indicated that activated TRPV4 neurons in SFO are associated with anti-dipsogenic effects under hypoxia. Although low partial pressure of blood oxygen directly activates TRPV4, humoral factors could also be involved. In the present study, we hypothesize that adropin, a new endogenous peptide hormone, was rapidly increased (serum and brain) concomitant with reduced water intake in early hypoxia. Also, the nuclear expression of c-Fos, a marker for neuronal activation, related to water-consumption (SFO and MnPO) was inhibited. These effects were mitigated by a scavenger, rat adropin neutralizing antibody, which effectively neutralized adropin under hypoxia. Interestingly, injection of recombinant adropin in the third ventricle of the rats also triggered anti-dipsogenic effects and reduced c-Fos positive cells in SFO, but these effects were absent when TRPV4 was knocked down by shRNA. Moreover, adropin-activated CamKK-AMPK signaling related to TRPV4 calcium channel in SFO in normoxia. These results revealed that dissociative adropin was elevated in acute hypoxia, which was responsible for anti-dipsogenic effects by altering TRPV4-CamKK-AMPK signaling in SFO.