People travelling to high altitude who have preexisting systemic hypertension may find their blood pressure to rise during the first week to 10 days at altitude. Wu et al. measured this in the workers on the Qinghai-Tibet railroad (Wu et al. 2007) Since systemic hypertension is most of the time asymptomatic, symptoms of headache are most likely being caused by acute mountain sickness rather than to an increase in systemic blood pressure. Wu et al. did not find an increase in the incidence of AMS on arrival at altitude in the systemic hypertension patients compared to the normotensive subjects.

This figure shows that the systolic blood pressure rises during the first week and thereafter gradually declines to baseline values except for a group who develops hypertension at altitude. These people probably have borderline hypertension at sea level and detoriate at altitude.