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Received Jan 5, 2018; Accepted Feb 25, 2018
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1. Introduction
Acute and chronic physiologic reactions to altitude are observed starting at altitudes of 1400 m and have been studied in detail in the last decades [1]. Maladaptive diseases such as acute and chronic mountain sickness or high-altitude pulmonary edema and hypertension are observed at altitudes above 2500 m (high altitude) [2, 3]. The main causative factor for adaptive and maladaptive reactions is hypoxia. However, atmospheric pressure is also relevant, as differences between reactions to normobaric and hypobaric hypoxia have been demonstrated [4–7].
Acute hypoxia causes hypoxic pulmonary vasoconstriction (HPV) leading to an increase in pulmonary arterial pressure (PAP) with the level of altitude being inversely related to arterial oxygen saturation (SaO2) and directly related to PAP [8]. Mean (m)PAP increases from 10 to 15 mmHg at lowland to values of 20–25 mmHg at altitudes of 2600–3600 m [9, 10]. Under chronic hypoxia, vascular remodeling occurs in the pulmonary arteries with thickening of the adventitia and media and muscularization of formerly nonmuscularized precapillary vessels, causing a sustained increase in PAP even weeks after return to normoxia [11].
PAP increases upon exertion because of rising cardiac output (CO). Yet, this increase is moderate in healthy subjects even under maximal exercise, because vasodilation and vascular recruitment are decreasing pulmonary vascular resistance (PVR) at the same time. A disproportionately high rise in PAP under exertion is considered a sign of early stages of pulmonary vascular remodeling and disease [12]. At altitude, PAP rises more sharply with the increase in CO upon exercise compared to sea level [13]. Thus, exercise at high altitude may unmask pulmonary vascular alterations that might not be noted at rest or at lowland.
A further important physiologic response to acute hypoxia is a rise in breathing rate and tidal volume, termed hypoxic ventilatory response (HVR). This hyperventilation increases alveolar and arterial oxygen pressure and decreases carbon dioxide pressure and is sustained during chronic exposure in healthy high-altitude dwellers, while a decrease in hyperventilation during chronic hypoxia is seen as an initial mechanism of maladaptation and development of chronic...