Acute respiratory distress syndrome (ARDS) is an acute respiratory failure due to inflammation triggered by pulmonary and non-pulmonary injuries that lead to altered permeability of the alveolar-capillary barrier and profound hypoxemia. ARDS was redefined by a panel of experts in Berlin in 2012.1 Although the Berlin criteria improved the validity and reliability of the definition, they did not make it possible to diagnose ARDS in specific settings.2 The Kigali modification proposed a mechanism to identify ARDS in resource-limited settings because the Berlin definition requires positive pressure ventilation, arterial blood gas measurements, and chest X-rays. These requirements mean that many ARDS patients cannot be diagnosed in many parts of the world, which is important because a lack of recognition leads to treatment failure.3

Since all estimates of the incidence of ARDS come from developed countries, which have significant differences vs developing countries on the availability of intensive care unit (ICU) beds, human and material resources (mechanical ventilation, gas determination, etc.), and decision-making criteria such as end-of-life decisions, Kigali proved important. The Kigali modification proposed replacing the ratio of partial pressure of oxygen (PaO2) to the fraction of inspired oxygen (FiO2) with the oxygen saturation (SpO2) to FiO2 ratio when blood gas determination is not possible, forgoing the requirement of mechanical ventilation with PEEP 5cmH2O in places where ventilation capacity is low, allowing the use of lung ultrasound in addition to chest X-ray to define bilateral opacities, validating alternative diagnostic modalities in resource-rich settings, and preventing ARDS by screening patients in all hospital areas, not just ICUs.2,3

Another specific setting is altitude. Critical care medicine at altitude (CCA) is the medicine that manages critically ill patients residing at an altitude of 1500m above sea level (masl) or higher; this affects 2% of the world's population. The consideration of 1500 masl is because adaptations derived from the progressive decrease in barometric pressure, following the ascent to altitude, and changes to arterial blood gas tests are observed from that altitude upwards. The highest altitude ICUs in the world are located in Cerro de Pasco, Peru at 4380masl and El Alto, Bolivia at 4150masl.4 From a practical standpoint, it has been proposed to categorize altitude related to CCA into 3 levels: medium altitude (1500masl up to 2500masl), high altitude (2500masl up to 3500masl), and very high altitude (3500masl up to 5800masl).4

The CCA framework is very similar to that outlined in the Kigali modification: low economic resources, lack of ICU beds, specialized personnel, limited material resources, etc., with the addition of altitude.5 At altitudes above 1500masl, variables such as barometric pressure, inspired O2 pressure (PIO2), and PaO2 are decreased, and the higher the altitude, the greater the decrease (Table 1).4,6 Another important factor differentiating it from sea level is the mean pulmonary arterial pressure, which at altitudes above 1600masl is ≥ 18mmHg, and in the presence of high-altitude ARDS, can far exceed 20mmHg, variables that must be considered in treatment and use of PEEP.7

If ARDS were diagnosed at high altitude (HA-ARDS) using the Berlin criteria, the clinical application of strategies such as high-flow nasal cannula oxygenation would delay its diagnosis. The Berlin Consensus stratified the severity levels of ARDS but was aware of the difficulty in applying it at altitude and proposed a correction formula to diagnose in populations located above 1000 masl ((PaO2/FiO2×barometric pressure)/760).1 However, the PaO2/FiO2 ratio measured in various studies and calculated using the Berlin Consensus formula does not seem to be consistent with reality,4 and in acclimatized patients on invasive mechanical ventilation, this adjustment equation appears to be inaccurate.8

Therefore, the diagnostic criteria for HA-ARDS remain controversial: severity stratification must be discussed and reconsidered, especially in poorly adapted patients, as well as the replacement of the SpO2/FiO2 ratio with the PaO2/FiO2 ratio, as has been proposed in low-income countries.2,3 In a study conducted at 2600 masl, the SpO2/FiO2 and PaO2/FiO2 ratios were correlated, and it was concluded that both have similar diagnostic performance to treat severely hypoxemic ventilated patients.9

In a consensus conference on HA-ARDS diagnostic criteria in Western China, Zhang et al. noted that, although it has the same etiology, disease, and physiology as low-altitude ARDS, HA-ARDS is influenced, from >1500 masl, by the progressive increase in altitude, the drop in PIO2, and the environmental factors of high altitudes, which result in significant pathophysiological changes, clinical signs and symptoms, and arterial blood gas parameters.10

In conclusion, people living at high altitudes exhibit physiological and anatomical differences derived from physiological adaptation mechanisms. Therefore, HA-ARDS also has its own pathophysiological features. The diagnostic and management strategies for HA-ARDS are different from those for ARDS in lower altitude areas and remain unclear. The clinical definition of ARDS complicates the management of this syndrome regarding quantification in resource-limited settings or at altitude. It is necessary to develop methods to identify and treat ARDS in the overall context, even if it may require adapting the current Berlin definition and validate potential changes to avoid missing out on the opportunity to effectively diagnose and treat these patients and improve their outcomes in the mentioned settings.

Table 2 shows the bases for managing HA-ARDS by the High-Altitude Critical Care Medicine Expert Committee of the Pan American and Iberian Federation of Critical Care and Intensive Therapy.