Losartan Has No Effect on High Altitude Diuresis or Acute Mountain Sickness in Well-Acclimatizing Individuals

Wilderness Medical Society Clinical Practice Guidelines for the Prevention, Diagnosis, and Treatment of Acute Altitude Illness: 2024 Update

To provide guidance to clinicians about best practices, the Wilderness Medical Society (WMS) convened an expert panel to develop evidence-based guidelines for prevention, diagnosis, and treatment of acute mountain sickness, high altitude cerebral edema, and high altitude pulmonary edema. Recommendations are graded based on the quality of supporting evidence and the balance between the benefits and risks/burdens according to criteria put forth by the American College of Chest Physicians. The guidelines also provide suggested approaches for managing each form of acute altitude illness that incorporate these recommendations as well as recommendations on how to approach high altitude travel following COVID-19 infection. This is an updated version of the original WMS Consensus Guidelines for the Prevention and Treatment of Acute Altitude Illness published in Wilderness & Environmental Medicine in 2010 and the subsequently updated WMS Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness published in 2014 and 2019.

Regulation of haemoglobin concentration at high altitude

Lowlanders sojourning for more than 1 day at high altitude (HA) experience a reduction in plasma volume (PV) that increases haemoglobin concentration and thus restores arterial oxygen content. If the sojourn extends over weeks, an expansion of total red cell volume (RCV) occurs and contributes to the haemoconcentration. While the reduction in PV was classically attributed to an increased diuretic fluid loss, recent studies support fluid redistribution, rather than loss, as the underlying mechanism. The fluid redistribution is presumably driven by a disappearance of proteins from the circulation and the resulting reduction in oncotic pressure exerted by the plasma, although the fate of the disappearing proteins remains unclear. The RCV expansion is the result of an accelerated erythropoietic activity secondary to enhanced renal erythropoietin release, but a contribution of other mechanisms cannot be excluded. After return from HA, intravascular volumes return to normal values and the normalisation of RCV might involve selective destruction of newly formed erythrocytes, although this explanation has been strongly challenged by recent studies. In contrast to acclimatised lowlanders, native highlanders originating from the Tibetan and the Ethiopian plateaus present with a normal or only mildly elevated haemoglobin concentration. Genetic adaptations blunting the erythropoietic response to HA exposure have been proposed as an explanation for the absence of more pronounced haemoconcentration in these populations, but new evidence also supports a contribution of a larger than expected PV. The functional significance of the relatively low haemoglobin concentration in Tibetan and Ethiopian highlanders is incompletely understood and warrants further investigation.