Respiratory quotient (RQ) measures the ratio of CO2 produced to O2 consumed. In a patient receiving intravenous lipid emulsion as the sole caloric source, the expected RQ and its implication for ventilatory requirement is:

• A RQ = 0.7, indicating fat oxidation with lowest CO2 production per unit oxygen consumed ✓
• B RQ = 1.0, indicating carbohydrate oxidation predominates regardless of substrate provided
• C RQ >1.0, indicating de novo lipogenesis from glucose administered alongside the lipid
• D RQ = 0.85, the mixed substrate value applicable to all IV nutrition formulations

Explanation

The RQ for pure fat oxidation is approximately 0.7 (CO2 produced/O2 consumed = 0.7), reflecting the high hydrogen-to-oxygen ratio of fatty acids requiring more oxygen per carbon but producing less CO2. For carbohydrates RQ = 1.0, for protein ~0.82. An RQ > 1.0 occurs during de novo lipogenesis (excess carbohydrate feeding) because NADPH-dependent fatty acid synthesis produces CO2 in excess of O2 consumed. In critically ill patients on ventilators, a high-carbohydrate diet (RQ close to or above 1.0) increases CO2 burden, making weaning more difficult; fat-predominant nutrition reduces this burden.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.