An ascent to altitude places considerable demands on the cardiovascular system. Changes in the rate, rhythm, and morphology of the electrocardiogram reflect the fall in the partial pressure of inspired oxygen (PiO2) and the adaptive responses that the human body makes. The effect of hypoxia on the autonomic nervous system results in changes to the heart rate during rest and exercise. Although this is raised during rest and submaximal exercise, maximal heart rate at altitude is reduced as a result of changes in parasympathetic activity. Hypoxic pulmonary vasoconstriction leads to a rise in pulmonary artery pressure and morphological changes on the electrocardiogram. Right axis deviation, right bundle branch block, and changes to P and T wave amplitudes are commonly found on ascent and resolve only after a return to low altitude. Although atrial and ventricular ectopic activity is also common, tachyarrhythmias are rare in healthy individuals. However, in those with significant cardiac disease, the hypoxic environment can be hazardous, exposing individuals to ischemia and the risk of sudden cardiac death.