What is the respiratory quotient (RQ)?

From the Guidelines

The respiratory quotient (RQ) is a critical parameter in assessing metabolic function, calculated as the ratio of carbon dioxide production to oxygen consumption, with normal values ranging from 0.7 to 1.0, depending on the primary energy source being metabolized. The RQ provides valuable information about which macronutrients are being used for energy at a given time, with carbohydrates resulting in an RQ of 1.0, proteins around 0.8, and fats approximately 0.7 1. This parameter is essential in clinical settings to assess nutritional status, metabolic disorders, and respiratory function.

Key Points to Consider

• The RQ is calculated by dividing carbon dioxide production by oxygen consumption (VCO2/VO2) 1
• Indirect calorimetry is the preferred method for measuring RQ, as it provides a more accurate assessment of energy expenditure compared to predictive equations 1
• Understanding a patient's RQ can guide nutritional interventions and monitor metabolic responses to treatment, making it a useful parameter in both research and clinical settings
• The type and rate of substrate utilization are reflected by the RQ, with different values for carbohydrates, proteins, and fats 1

Clinical Applications

• Assessing nutritional status and metabolic disorders
• Monitoring metabolic responses to treatment
• Guiding nutritional interventions
• Evaluating respiratory function The most recent and highest quality study on this topic is from 2021, which emphasizes the importance of indirect calorimetry in measuring RQ and assessing energy expenditure 1. Therefore, it is recommended to use indirect calorimetry to measure RQ whenever possible, as it provides a more accurate assessment of metabolic function and energy expenditure.

From the Research

Definition and Measurement of Respiratory Quotient

• The respiratory quotient (RQ) is defined as the ratio of carbon dioxide production (VCO2) to oxygen consumption (VO2) 2.
• RQ is affected by extremes of substrate use by the body, with underfeeding promoting the use of endogenous fat stores and decreasing RQ, while overfeeding results in lipogenesis and increases RQ 2.
• RQ can be measured using indirect calorimetry, which provides an estimate of energy expenditure and substrate utilization 3, 4, 5.

Clinical Use of Respiratory Quotient

• The clinical use of RQ is limited to a marker of test validity and a marker for respiratory tolerance of the nutrition support regimen 2.
• RQ should not be used to finely adjust the nutrition support regimen due to its low sensitivity and specificity in indicating over- or underfeeding 2.
• Elevation of overall measured RQ > or = 1.0 may be associated with reduced tolerance and mild respiratory compromise 2.

Factors Affecting Respiratory Quotient

• The route of nutrient intake does not influence energy nutrient utilization, as determined by RQ 3.
• Carbohydrate caloric intake is correlated with CO2 production, with a stronger relationship in patients with RQ > 1 5.
• High-fat diets result in a higher RQ, indicating increased fat oxidation, while high-carbohydrate diets result in a lower RQ, indicating increased carbohydrate oxidation 6.

Respiratory Quotient in Different Patient Populations

• In head-injured critically ill patients, RQ and resting energy expenditure (REE) remain comparable with continuous vs. intermittent enteral feeding regimens 4.
• In critically ill patients, high caloric loads result in increased CO2 production, exhaled minute ventilation, and deadspace ventilation, with a correlation between CO2 production and alveolar ventilation 5.