Altitude Cardiac Stress Index (ACSI) Calculator – High-Altitude Athletes
Altitude Cardiac Stress Index (ACSI): Explanation and Clinical Context
The ACSI is a pragmatic, heuristic tool intended to summarize cardiac stress loads for athletes training or competing at altitude. It integrates key determinants of hypoxic cardiac strain: absolute altitude and ascent rate (as proxies for ambient hypoxia burden), acclimatization time, resting oxygen saturation, heart rate, blood pressure, and hematologic concentration; it also flags prior altitude illness, known cardiac disease, age, sex, and planned training intensity. Higher scores reflect cumulatively higher physiological stress and potential hazard for ischemia, arrhythmia, right ventricular afterload, and maladaptive hemodynamics during exertion. This calculator is not a validated risk score and should supplement—not replace—clinical judgment, shared decision-making, and sport cardiology guidance.
Why these variables?
Acute hypoxia increases cardiac output and heart rate at rest and during exertion; with acclimatization, these responses partially normalize, but higher altitudes sustain greater stress. Lower resting SpO2 at altitude reflects more severe hypoxemia and correlates with physiologic strain. Rapid ascent and inadequate acclimatization increase cardiopulmonary load and altitude illness risk. Elevated systolic pressure and very high hemoglobin (polycythemia) can augment afterload and viscosity, respectively, while pre-existing cardiac disease substantially elevates risk. Masters male athletes have a slightly higher background risk of exercise-related cardiac events. Planned vigorous intensity further amplifies demand. These concepts are consistent with contemporary statements and reviews on exercise at altitude and sport cardiology practice.
Risk categories and use
The ACSI provides qualitative categories (Low, Moderate, High, Very High) to guide training decisions: proceed and monitor (Low), consider moderating intensity and extending acclimatization (Moderate), reduce intensity and consider specialist evaluation (High), and avoid vigorous training pending optimization and clearance (Very High). For competitive athletes or those with cardiac diagnoses, pre-participation evaluation (including ECG and, when indicated, exercise testing or echocardiography) is advisable.
Reference:
American Heart Association Scientific Statement on exercise at altitude summarizes physiologic responses to hypoxia and practical recommendations for athletes. Cornwell WK III, et al. J Am Heart Assoc. 2021;10:e023225. doi:10.1161/JAHA.121.023225.
European Society of Cardiology 2020 guidance outlines risk stratification principles for athletes, applicable when considering training at altitude. Sharma S, et al. 2020 ESC Guidelines on sports cardiology and exercise in patients with cardiovascular disease. Eur Heart J. 2020;41:4101–4110.
Reviews on high-altitude cardiovascular physiology describe increased heart rate/cardiac output, pulmonary pressures, and the importance of acclimatization. Khodaee M, et al. Sports Health. 2016;8:124–130. Santiváñez JCA, et al. 2024 literature review and update.
Emerging research proposes prediction models for high-altitude myocardial ischemia and explores stress echocardiography utility in athletes at altitude, supporting individualized risk appraisal (e.g., Chen Y, et al. 2024; Wang Y, et al. 2024).
Important note: No peer-reviewed publication currently defines an original, validated “Altitude Cardiac Stress Index” with fixed coefficients. The above scoring weights are deliberately conservative, derived from established pathophysiology and consensus guidance, and should be refined or replaced if/when validated models become available for athletes training at altitude.