The autonomic nervous system at high altitude

The effect of altitude-induced hypoxia on heart disease: do acute, intermittent, and chronic exposures provide cardioprotection?

With the global prevalence of heart disease continuing to increase and large populations living at altitude around the world, we review the concept of altitude and cardioprotection. Current epidemiologic data, as well as the basic science and molecular mechanisms involved in acute, intermittent, and chronic exposure to altitude, are discussed. Intermittent and chronic exposures have been demonstrated to increase coronary vasculature, decrease infarction size, and provide more efficient metabolism and better cardiac functional recovery postischemia. Mechanisms demonstrated in these situations include those mediated by the hypoxia inducible factor, as well as reactive oxygen species, certain ion channels, and protein kinases. Although current epidemiologic studies are difficult to interpret owing to many confounders, many studies point to the possibility that living at altitude provides cardiovascular protection. Further research is needed to determine if the bench studies showing mechanisms consistent with cardioprotection translate to the population living at altitude.

Cross Tolerance to Environmental Stressors: Effects of Hypoxic Acclimation on Cardiovascular Responses of Channel Catfish (Ictalurus punctatus) to a Thermal Challenge

Hypoxia and temperature are two major, interactive environmental variables that affect cardiovascular function in fishes. The purpose of this study was to determine if acclimation to hypoxia increases thermal tolerance by measuring cardiovascular responses to increasing temperature in two groups of channel catfish. The hypoxic group was acclimatized to moderate hypoxia (50% air saturation, a P(O2) of approximately 75 torr) at a temperature of 22° C for seven days. The normoxic (i.e. control) group was maintained the same, but under normoxic conditions (a P(O2) of approximately 150 torr). After acclimation, fish were decerebrated, fitted with dorsal aorta cannulae, and then exposed to increasing temperature while cardiovascular variables were recorded. The end point (critical thermal maximum, CTMax) was defined as a temperature at which heart rate and blood pressure sharply decreased indicating cardiovascular collapse. Fish acclimatized to moderate hypoxia had higher resting heart rate than controls. Hypoxic acclimatized fish had a significantly higher CTMax. Acclimation to hypoxia increases the cardiovascular ability of channel catfish to withstand an acute temperature increase.

Perioperative opioid requirements are decreased in hypoxic children living at altitude

OBJECTIVES

To investigate the effect of altitude on perioperative opioid requirements in otherwise healthy children.

AIM

To investigate whether children living and having surgery at high altitude received different doses of fentanyl than those living and having surgery at sea level.

BACKGROUND

Recent studies in animals (Anesthesiology, 105, 2006 and 715) and children with obstructive sleep apnea (Anesthesiology, 105, 2006 and 665; Anesthesiology 100, 2004 and 806) suggest that analgesic effects of exogenous opioids are enhanced by hypoxia. However, the effects of hypoxia on perioperative narcotic requirements in otherwise healthy children have not been previously reported.

METHODS/MATERIALS

We reviewed retrospectively the opioid requirements of pediatric patients who underwent cleft lip or palate surgery during Smile Network International mission trips to Cusco and Lima, Peru between 2007 and 2009. Patients who had surgery at high altitude were compared to those who had surgery at sea level. All patients received a standardized anesthetic with intravenous fentanyl as the only perioperative opioid.

RESULTS

Hundred and two patients had surgery at 3399 m above sea level (masl) (Cusco) and 169 patients had surgery at 150 masl (Lima). Patients at high altitude had significantly lower baseline oxygen saturations (92 ± 4% vs 98 ± 3%; P < 0.001) and received 40% less opioid (1.2 ± 0.8 vs 2.0 ± 1.4 μg·kg(-1) per h; P < 0.001) compared to patients at sea level.

CONCLUSIONS

Opioid administration was reduced in otherwise healthy children with altitude-induced chronic hypoxia when compared to non-hypoxic children undergoing similar operations under similar anesthetic regimens. Whether this difference is due to altitude or altitude-induced hypoxia, requires further study.

A catecholamine crisis on Mount Kilimanjaro: a hypoxia effect?

Sympathetic paragangliomas are autonomic nervous system tumors associated with dysregulation of intracellular oxygen metabolism. Exposure to high altitudes is reported to activate the production of catecholamines in the sympathoadrenal system. We describe an individual with a paraganglioma complicated by a catecholamine crisis that occurred on the summit of Mount Kilimanjaro.

Role of mitochondrial-mediated signaling pathways in Alzheimer disease and hypoxia

Development of effective treatments for Alzheimer's disease is complicated by the poor understanding of its pathophysiology. Recent work suggests mitochondria may play a primary role in neurodegeneration, due to alterations in mitochondria turnover and that the brain is specifically susceptible, due to high energy demand. Mitochondria are the major source of cellular energy through oxidative phosphorylation and regulate intracellular calcium levels and survival pathways. Hypoxia has been implicated in several neurodegenerative diseases including Alzheimer's disease. During hypoxic events, mitochondrial complex III produces high levels of reactive oxygen species (ROS). These ROS seem to have a primary role in the regulation of the transcription factor hypoxia inducible factor 1alpha that triggers death effectors. Here we discuss the role of mitochondria in AD putting focus on the activation of hypoxia-mediated mitochondrial pathways, which could eventually lead to cell degeneration and death.

Maturation and long-term hypoxia alters Ca2+-induced Ca2+ release in sheep cerebrovascular sympathetic neurons

The contribution of sympathetic nerves arising from the superior cervical ganglia (SCG) toward the growth and function of cerebral blood vessels is pertinent throughout maturation as well as in response to cardiovascular stress imposed by high-altitude long-term hypoxia (LTH). The function of SCG sympathetic neurons is dependent on intracellular Ca2+ concentration ([Ca2+]i) signaling, which is strongly influenced by a process known as Ca(2+)-induced Ca2+ release (CICR) from the smooth endoplasmic reticulum (SER). In this study, we used the sheep SCG neuronal model to test the hypotheses that maturation decreases CICR and high-altitude LTH depresses CICR in fetal SCG neurons but not in those of the adult. We found that the contribution of CICR to electric field stimulation (EFS)-evoked [Ca2+]i transients was greatest in SCG cells from normoxic fetuses and was abolished by LTH. The decline in CICR was associated with a reduction in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) function in fetal SCG cells during LTH, reducing SER Ca2+ levels below the threshold needed for the coupling of Ca2+ influx and CICR. With respect to the maturation from the fetus to adult, the decrease in CICR may reflect both a reduction in the levels of ryanodine receptor isoforms 2 and 3 and SERCA function. In response to LTH and in contrast to the fetus, CICR function in adult SCG cells is maintained and may reflect alterations in other mechanisms that modulate the CICR process. As CICR is instrumental in the function of sympathetic neurons within the cerebrovasculature, the loss of this signaling mechanism in the fetus may have consequences for the adaptation to LTH in terms of fetal susceptibility to vascular insults.

Systemic hypoxia affects cardiac autonomic activity and vascular hemodynamic control modulated by physical stimulation

This study investigates how various hypoxic interventions affect cardiac autonomic activity and hemodynamic control during posture change and the Valsalva maneuver. Ten healthy sedentary men exposed to 12, 15 and 21% O(2) for 1 h in a normobaric hypoxia chamber in a random order. Before and after various O(2) concentrations were administered, subjects performed the sit-up test and Valsalva maneuver, respectively. An impedance plethysmography was utilized to measure blood pressure (BP) and vascular hemodynamics, whereas spectral analysis of heart rate variability (HRV) was performed to determine cardiac autonomic activity. Analytical results can be summarized as follows: while the patient rests in a supine position, exposure to 12% O(2) reduces the ratio of lower to upper extremity systolic BP, which is accompanied by (1) suppressed arterial reactive hyperemia and increased venous flow resistance, as well as (2) decreased total power and high frequency (HF) and increased low frequency (LF) and the ratio of LF to HF. Moreover, the hypoxia-induced changes of time and frequency domains in HRV at resting supine disappear following the sit-up test, whereas this hypoxic exposure attenuates the BP and heart rate responses to the Valsalva maneuver. Conversely, resting and physical stimuli-mediated HRV and vascular hemodynamic values are unaltered by both 15 and 21% O(2) exposures. We conclude that acute hypoxic exposure affects cardiovascular autonomic functions, with reactions determined by the intervening O(2) concentrations. Moreover, the BP and cardiac autonomic responses to 12% O(2), but not 15% O(2), exposure are depressed while performing posture change and the Valsalva maneuver.

Chronic hypoxia-induced angiogenesis normalizes blood pressure in spontaneously hypertensive rats

We hypothesized that activation of angiogenesis by chronic hypoxia may affect vascular resistance and, subsequently, blood pressure levels in spontaneously hypertensive rats (SHRs). Five-week-old prehypertensive SHRs and age-matched normotensive Wistar-Kyoto (WKY) rats (n=8 per group) were maintained under normobaric normoxic or hypoxic (10% O(2)) conditions for 8 weeks. Three weeks later, the systolic blood pressure was lower by 26% in hypoxic SHRs compared to normoxic SHRs (P<0.05) and remained at the normoxic WKY level. Total peripheral vascular resistance, calculated as the mean arterial pressure/cardiac output (assessed by ultrasound imaging and Doppler), was 30% lower in hypoxic than in normoxic SHRs (P<0.001) and returned to WKY levels. Interestingly, chronic hypoxia also significantly reduced systolic blood pressure in adult 12-week-old SHRs with established hypertension; blood pressure was normalized (versus normoxic WKY rats) after 4 weeks of hypoxia. Changes in hemodynamic parameters were associated with activation of proangiogenic pathways. Protein levels of vascular endothelial growth factor (VEGF)-A in the skeletal muscles were increased by 2.2-fold in hypoxic compared to normoxic SHRs (P<0.001). At the end of the hypoxic period, capillary density in the quadriceps muscle was 1.2-fold higher in hypoxic than in normoxic SHRs (P<0.001). Myocardial capillary density and VEGF-A protein contents were also 1.2- and 2.1-fold higher in hypoxic compared to normoxic SHRs (P<0.001 and P<0.05, respectively). Moreover, treatment with neutralizing VEGF-A antibody abrogated the hypoxia-induced angiogenesis and subsequently worsened arterial hypertension. Therefore, our results suggest that chronic normobaric hypoxia (1) activates VEGF-A-induced angiogenesis and thereafter (2) prevents the occurrence of hypertension in young prehypertensive SHRs and (3) normalizes blood pressure in adult SHRs with established hypertension.

Survey of Acute Mountain Sickness on Mauna Kea

Although thousands of people ascend 4205 m to visit the summit of Mauna Kea each year, there has been no information on the rate of altitude illness triggered by such visits. Two surveys were used: one for tourists driving up to the summit and the other for summit astronomy workers staying at lodging facilities at intermediate altitude. The surveys included the standardized Lake Louise Self-report Acute Mountain Sickness (AMS) Questionnaire that, when scored, gave the Lake Louise Symptoms Score (LLSS). Thirty percent of surveyed day visitors and 69% of surveyed professional astronomy staff had AMS, defined as a LLSS score of 3 or greater, with headache. Nine participants reported "disorientation/confusion" or greater consciousness changes. A majority of astronomy professionals reported fatigue, disturbed sleep, reduced activity, and mental status changes. Few took any AMS medications. The incidence of AMS in visitors to Mauna Kea's summit warrants increased education and increased availability of supplemental oxygen at the summit. The absence of reported serious altitude illness in the community is probably due to the rapid descent available on Mauna Kea, with prompt reversibility of adverse effects.

Treatment of chronic mountain sickness: critical reappraisal of an old problem

A review is made on the different treatment strategies essayed to date in the management of chronic mountain sickness (CMS). After a brief presentation of the epidemiology and of the pathophysiological mechanisms proposed for explaining the disease, the advantages and drawbacks of the different treatment approaches are discussed, along with their pathopysiological rationale. A particular emphasis is dedicated to the scientific foundations underlying the development of acetazolamide and angiotensin-converting enzyme inhibitors as promising therapeutic agents for CMS, as well as the clinical evidence existing so far on their usefulness in the treatment of CMS. Various methodological issues that need to be addressed in future clinical studies on efficacy of therapies for CMS are discussed. There is also a brief discussion on potential treatment options for chronic high altitude pulmonary hypertension. Closing remarks on the need of taking increasingly into account the development and implementation of preventive measures are made.