What is the primary reason for respiratory distress in a patient being transported in a helicopter at altitude?

The primary reason for respiratory distress in a patient being transported in a helicopter at altitude is low PAO2, which refers to the partial pressure of oxygen in the alveoli. As altitude increases, the atmospheric pressure decreases, leading to a drop in the partial pressure of oxygen available for respiration. This reduction can result in inadequate oxygen delivery to the tissues and can cause symptoms of hypoxia, contributing significantly to the patient's respiratory distress.

While hypercarbia, or elevated carbon dioxide levels, is a concern in certain respiratory conditions, it is often secondary to impaired ventilation rather than the primary cause of distress at altitude. Altitude sickness, although related to changes in altitude, primarily manifests due to other physiological responses and would not be the main reason for respiratory distress in this context. Excessive pulmonary artery pressure can occur under certain conditions but is less likely to be the immediate cause of difficulty in respiration during helicopter transport, particularly in the absence of pre-existing pulmonary issues.

Overall, the decrease in PAO2 is the most direct and impactful factor leading to respiratory distress in this scenario.