When inspired oxygen is collected in the alveolar sac, perfusion occurs through the capillary network surrounding the alveoli. This perfused oxygen is transported by the red blood cell to the surrounding tissues. As blood is traveling through the capillary bed, oxygen is released from the RBC to the respective tissue site. Concurrently, the tissue releases CO2 through metabolic processes into the red blood cell, delivering it to the lungs.

Normal Respiration

• Vol CO2 = 200 mL/min and Vol O2 = 250 mL/min, resulting in 0.8 respiratory ratio

In the presence of macronutrients, oxygen is required for the breakdown of carbohydrates, fat, and protein. Carbohydrates (C6H12O6 + 6O2) have a 6 carbon chain and metabolize via glycolysis to form 2 pyruvate substrates, releasing CO2 as a by-product when converting to acetyl CoA. CO2 is also a by-product in the Krebs cycle when 2 carbon acetyl CoA reacts with a 4 carbon citrate, making a total of 3 CO2 in each metabolizing cascade. If the starting molecule is a fatty acid, which contains 12, 18, 20, or 22 carbon molecules, it goes through the process of B - oxidation to form acetyl Co-A which does not generate carbon dioxide. Therefore, when using fat over carbohydrate as a form of fuel, less CO2 is generated for every oxygen consumed.

Indirect Calorimetry

To validate RQ, it is important to calculate heat production by measuring pulmonary gas exchange first; this is known as indirect calorimetry. The method for measuring inspired oxygen and expired carbon dioxide is to calculate the resting energy expenditure (REE) and respiratory quotient (RQ). REE allows the nutritionist to understand how much they need to feed their patients.

Respiratory Quotient and Respiratory Exchange Ratio[3]

Respiratory Exchange Ratio (RER) directly measures Vol CO2 released/Vol O2 absorbed at the mouth and does not require invasive procedures. Respiratory quotient, on the other hand, measures directly at the tissue, requiring an arterial and venous catheter while monitoring blood pressure for optimal results.

Carbohydrates are oxidized through aerobic respiration using RER, resulting in an equal ratio of CO2 release and oxygen consumption; this implies that 100% of carbohydrates are consumed to produce ATP.

• C6H12O2 + 6 O2 ? 6O2 + 6 H2O + Energy
•  RQ= 6 CO2 /6 O2 = 1.0

When fat is oxidized and measured using RER, the outcome is reduced CO2 production for every oxygen molecule consumed.

• C16H32O2 + 23 O2 ? 16O2 + 16 H2O + Energy
• RQ= 16 CO2 / 23 O2 = 0.7

When protein is the respiratory substrates, it results in reduced CO2 production for every oxygen molecule consumed.[4]

• C72H112N18O22 + 77 O2 ? 63O2 + 38 H2O + Energy
• RQ= 63 CO2 / 77 O2= 0.9

Chronic Obstructive Pulmonary Disease

Chronic Obstructive Pulmonary Disease (COPD) is a pulmonary disease that causes chronic obstruction of airflow. Chronic inflammation of the bronchioles along with mucous production causes them to become deformed and narrow along with mucous production, limiting the air flow as a person exhales. Because the patient is not able to fully exhale, carbon dioxide remains in the alveoli due to loss of elasticity of the sac. The patients suffer from shortness of breath, productive cough, respiratory acidosis, and complicated pneumonia. 

In 1992, a study showed that carbohydrate-rich food increases production of CO2, leading to an increase in respiratory rate and eventually respiratory failure. On the contrary, fat-rich meals decrease the production of CO2, leading to reduce alveolar ventilation and minor improvement in respiration in patients who initially have an RQ ratio greater than 0.75.[5] 

Non-Insulin Dependent Weight Gain

The respiratory quotient can be used to predict weight gain in non-insulin-dependent diabetic patients. Normally, a diabetic patient has insulin-resistant receptors, which results in hyperglycemia. This prevents metabolization from occurring via glycolysis, therefore, increasing lipolysis. As mentioned, an increase in lipolysis will reduce the production of CO2. A study done in 1998, demonstrated an inverse correlation between RQ ratio and serum glucose levels in non-insulin-dependent diabetes mellitus treated with oral hypoglycemic agents or insulin. Patients who were on the oral hypoglycemic agents and had a higher BMI verified that their RQ ratio was much higher than those of average BMI. After following them for a year, a weight gain of 3 kg was noticed in 50% of the patients along with an increase of RQ ratio. This study concluded that RQ ratio is a valid predictor for weight gain in diabetics treated with oral agents.[6]

Nutrition Guide for Sick Patients

Utilizing the indirect calorimetry and respiratory energy expenditure aids in calculating the ideal kcal patients need to consume per day, especially when unable to estimate caloric requirements, inadequate clinical response to a predicted equation, or clinical sign of over and under-feeding. Patients who are morbidly obese or suffering from sepsis have an alteration in their VCO2 release and VO2 consumption. For this reason, a daily RQ and indirect calorimetry are essential to optimize their diet and reduce hospital stay.[7][8]

• REE = (kcal/d)= (VO2 x 3.94) + (VCO2 x 1.11) x 1440