NREMT distinguishes respiratory distress from respiratory failure. Respiratory distress includes compensatory signs such as tachypnea, accessory muscle use, and diaphoresis. Respiratory failure occurs when those compensatory mechanisms fail and the body can no longer maintain adequate oxygenation or ventilation.

Option D (Altered mental status) is the most reliable indicator of respiratory failure. According to NREMT, hypoxia and hypercapnia directly affect brain function, leading to confusion, agitation, lethargy, or unresponsiveness. This indicates that oxygen delivery to the brain is no longer adequate.

Option A is common in early respiratory distress.

Option B indicates increased work of breathing but not failure.

Option C reflects sympathetic activation, not failure.

NREMT teaches that mental status changes are late and ominous signs, requiring immediate airway and ventilatory support.