What does this research mean for the field?

A reduction in cardiac parasympathetic activity, rather than increased sympathetic activity, is the primary mechanism underlying the elevated resting heart rate during chronic exposure to hypoxia. Novelty: ClaimNovelty.NOVEL_FINDING. Consensus alignment: ConsensusAlignment.NEUTRAL.

What question did this study set out to answer?

To determine whether the elevation in resting heart rate during chronic hypoxia is driven by increased sympathetic nervous activity, reduced parasympathetic nervous activity, or non-autonomic mechanisms.

September 10, 2026

Parasympathetic withdrawal increases heart rate after 2 weeks at 3454 m altitude

Q: What are the key findings of this study?

Parasympathetic inhibition with glycopyrrolate abolished the high altitude-induced increase in resting heart rate (110 at sea level vs 112 beats/min at altitude, P=0.28), unlike sympathetic inhibition.

Q: What is the clinical evidence from this study?

Study design: Other. Population: Healthy (n=7). Intervention: Pharmacological autonomic inhibition (propranolol and/or glycopyrrolate) vs. No drug intervention (control). Primary outcome: Resting heart rate (p=0.006).

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Key result

Parasympathetic inhibition with glycopyrrolate abolishes high altitude-induced resting heart rate increases, unlike sympathetic inhibition.

P=0.006
n=7

Why the study?

Chronic hypoxia increases resting heart rate, but the underlying mechanism remains incompletely understood.

Does parasympathetic withdrawal mediate the increase in resting heart rate during chronic hypoxia in healthy lowlanders?

Population

Seven healthy lowlanders

Comparison

Sea level vs high altitude with propranolol, glycopyrrolate, both, or no drug

Follow-up

15-18 days

Authors

CSChristoph SiebenmannInstitute of Mountain Emergency Medicine PRPeter Rasmussen MHMike HugUniversity of Copenhagen

Discussion

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Implication

Parasympathetic withdrawal drives altitude tachycardia in healthy lowlanders; extends autonomic mechanisms but leaves open relevance to patients or interventions.

Key Points

To determine whether the elevation in resting heart rate during chronic hypoxia is driven by increased sympathetic nervous activity, reduced parasympathetic nervous activity, or non-autonomic mechanisms.
Assessed resting heart rate and circulating noradrenaline in 7 healthy lowlanders at sea level and after 15–18 days at high altitude (3454 m).
Administered pharmacological receptor blockades: control, propranolol (beta-adrenergic blockade), glycopyrrolate (muscarinic blockade), and combined propranolol plus glycopyrrolate.
Circulating noradrenaline concentration increased from 0.9 ± 0.4 nmol l⁻¹ at sea level to 2.7 ± 1.5 nmol l⁻¹ at high altitude (P = 0.03).
Resting heart rate increased at high altitude under control conditions (64 ± 10 to 74 ± 12 beats min⁻¹, P = 0.007) and during propranolol treatment (52 ± 4 to 59 ± 5 beats min⁻¹, P < 0.001).
Parasympathetic blockade prevented the altitude-induced heart rate increase, showing no significant difference between sea level and high altitude with glycopyrrolate (110 ± 7 vs. 112 ± 5 beats min⁻¹, P = 0.28) or combined blockade (83 ± 5 vs. 85 ± 5 beats min⁻¹, P = 0.25).

Structured PICO

Does parasympathetic withdrawal mediate the increase in resting heart rate during chronic hypoxia in healthy lowlanders?

Population

7 healthy lowlanders evaluated at sea level and after 15-18 days of exposure to 3454 m high altitude.

Intervention

15-18 days of exposure to 3454 m high altitude with intravenous administration of propranolol, glycopyrrolate, or both

Comparator

Measurements at sea level under the same conditions (control and pharmacological inhibition)

Outcome

Resting heart ratesurrogate

Main Result

p-value: p=0.006

Reduced cardiac parasympathetic activity is the primary mechanism underlying elevated resting heart rate after 2 weeks of exposure to chronic hypoxia.

Cite This Study

Siebenmann et al. (2017) studied Healthy (n=7). Pharmacological autonomic inhibition (propranolol and/or glycopyrrolate) vs. No drug intervention (control) was evaluated on Resting heart rate (p=0.006). Parasympathetic inhibition with glycopyrrolate abolished the high altitude-induced increase in resting heart rate (110 at sea level vs 112 beats/min at altitude, P=0.28), unlike sympathetic inhibition.

synapsesocial.com/papers/6aa269eaa1e750260650a017 https://doi.org/10.1113/jp273726

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Implication

Parasympathetic withdrawal drives altitude tachycardia in healthy lowlanders; extends autonomic mechanisms but leaves open relevance to patients or interventions.

Parasympathetic withdrawal increases heart rate after 2 weeks at 3454 m altitude

Discussion

Member takes

Implication

Key Points

Structured PICO

Main Result

Cite This Study

Parasympathetic withdrawal increases heart rate after 2 weeks at 3454 m altitude

Discussion

Member takes

Implication

Key Points

Structured PICO

Main Result

Cite This Study

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