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ETCO2-PaCO2 Gradient Dead Space — FRCA Final MCQ

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ModerateIntensive Care MedicineETCO2-PaCO2 Gradient Dead SpaceFRCA Final

A 60-year-old man on ICU has a PaCO2 of 9.5 kPa on a ventilator set to deliver VT 500 mL at RR 14 (MV 7 L/min). His weight is 80 kg (IBW 75 kg). The ETCO2-PaCO2 gradient is 2.5 kPa (elevated). What does an increased ETCO2-PaCO2 gradient indicate?

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Correct answer: D — Increased alveolar dead space ventilation

An increased ETCO2-PaCO2 gradient (normal <0.5-1.0 kPa) indicates increased alveolar dead space ventilation: ventilated but unperfused alveoli dilute the CO2 in exhaled gas, reducing ETCO2 relative to PaCO2. Causes include: pulmonary embolism (acute increase in dead space), low cardiac output states (reduced pulmonary perfusion), high PEEP (overdistension compressing capillaries), COPD/emphysema (V/Q mismatch), and hypovolaemia. This finding should prompt investigation for the underlying cause, particularly PE and low CO states.

Reference: RCOA – 2023 – Respiratory physiology: dead space; ICS – 2023 – Ventilation monitoring