RQ (Respiratory Quotient) Calculators

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The respiratory quotient (RQ) is the ratio of CO₂ produced to O₂ consumed: RQ = VCO₂/VO₂. It indicates which metabolic substrate is being oxidized. Carbohydrate oxidation gives RQ = 1.0; fat oxidation gives RQ = 0.71; protein gives RQ ≈ 0.82. RQ > 1.0 indicates net lipogenesis (excess carbohydrate converting to fat). Measured by indirect calorimetry, RQ is used in ICU nutritional support to avoid overfeeding and in exercise physiology to assess substrate utilization at different intensities.

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RQ Values by Substrate

Glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O; RQ = 6/6 = 1.00. Palmitic acid: C₁₆H₃₂O₂ + 23O₂ → 16CO₂ + 16H₂O; RQ = 16/23 = 0.70. Protein: RQ ≈ 0.82. Mixed diet at rest: RQ ≈ 0.82–0.87.

Clinical Significance (ICU)

  • RQ 0.85–1.0: target range for nutritional support
  • RQ > 1.0: overfeeding → net lipogenesis → excess CO₂ production → increased ventilatory demand → may prevent ventilator weaning
  • RQ < 0.7: starvation or measurement error

Weir Equation

REE (kcal/day) = (3.941 × VO₂ + 1.106 × VCO₂) × 1440; VO₂ and VCO₂ in L/min.

Exercise

At rest: RQ ≈ 0.75–0.82 (fat dominant). With increasing intensity: RQ rises toward 1.0 (CHO dominant). Above lactate threshold: RER > 1.0 due to bicarbonate buffering of lactate (not true lipogenesis).

Glossary

Respiratory Quotient (RQ)
VCO₂/VO₂; carbohydrate = 1.0; fat = 0.71; protein = 0.82; RQ > 1.0 = lipogenesis or overfeeding; target 0.85–1.0 for ICU nutrition; measured by indirect calorimetry.
Weir Equation
REE (kcal/day) = (3.941×VO₂ + 1.106×VCO₂)×1440; calculates resting energy expenditure from indirect calorimetry; VO₂ and VCO₂ in L/min.
RER (Respiratory Exchange Ratio)
RQ measured at the mouth; equals RQ at rest; exceeds RQ during intense exercise due to bicarbonate buffering of lactate (not true lipogenesis); used to identify ventilatory threshold.

Frequently Asked Questions

RQ = VCO₂/VO₂. Each substrate has a characteristic RQ: Carbohydrate = 1.00 (equal moles CO₂ produced and O₂ consumed). Fat = 0.70–0.71 (fat is more reduced; needs more O₂ per CO₂). Protein ≈ 0.82. Mixed diet at rest ≈ 0.82–0.87. RQ > 1.0 = lipogenesis — excess carbohydrate is being converted to fat, producing more CO₂ than is consumed in O₂. Measured by indirect calorimetry (metabolic cart measuring expired gas concentrations).

When ICU patients are overfed with carbohydrates, RQ rises above 1.0 because excess glucose is converted to fat (lipogenesis), producing CO₂ faster than O₂ is consumed. This elevates VCO₂ → increases the respiratory drive and required minute ventilation. In patients with marginal respiratory reserve, this extra CO₂ load can prevent successful weaning from mechanical ventilation. Intervention: reduce total calories or shift from carbohydrate to fat calories to bring RQ into the target range (0.85–1.0).

The Weir equation: REE (kcal/day) = (3.941 × VO₂ + 1.106 × VCO₂) × 1440. VO₂ = O₂ consumed (L/min); VCO₂ = CO₂ produced (L/min); 1440 = minutes per day. Measure VO₂ and VCO₂ for 20–30 min at steady state with a metabolic cart. REE sets daily caloric targets — typically 25–30 kcal/kg/day for most ICU patients, but indirect calorimetry provides individualized values.

At rest: RQ ≈ 0.75–0.82 (fat is the dominant fuel). As exercise intensity increases: more carbohydrate is oxidized → RQ rises toward 1.0. Above the lactate threshold: RER (measured at the mouth) exceeds 1.0 due to bicarbonate buffering of lactate (HCO₃⁻ + H⁺ → H₂O + CO₂), releasing extra CO₂ — not true lipogenesis. This non-metabolic CO₂ is used to identify the ventilatory threshold (≈ lactate threshold) in exercise testing.