Low ETCO2 (<35 mmHg) most commonly indicates which physiological change?

• A. Decreasing respiratory rate
• B. Hypothermia
• C. Increasing respiratory rate / hyperventilation
• D. Increased cardiac output

Answer: (C)

End-tidal CO2 represents the CO2 level in alveolar gas at the end of a breath, which drops when ventilation increases faster than CO2 is produced. Breathing rapidly or hyperventilating blows off CO2 more quickly, so the ETCO2 falls below the normal range (about 35–45 mmHg). That makes increased respiratory rate / hyperventilation the most common reason for a low ETCO2 value. Conversely, slowing breathing would retain more CO2 and raise ETCO2; hypothermia lowers CO2 production but isn’t the typical acute cause; and higher cardiac output usually brings more CO2 to the lungs, increasing ETCO2 rather than decreasing it.