Is there any link between RQ and V/Q ratio?

[E.S.Prakash]

Khoo Piggy (Year 2 Student) asks: Is respiratory quotient the same as ventilation-perfusion ratio?

Reply by Prakash: 

No, Respiratory Quotient is the ratio of the volume of CO2 produced by the oxidation of an energy substrate to the volume of oxygen consumed for oxidation. In the case of carbohydrates, this ratio is 1; whereas fatty acids require a greater amount of oxygen for complete oxidation (RQ for fatty acids is around 0.7) and it is around 0.85 for protein. On a mixed diet, average RQ is around 0.8. 

Ventilation-Perfusion Ratio is the ratio of alveolar ventilation to pulmonary blood flow. In the resting state, if alveolar ventilation were about 4 liters/min and pulmonary blood flow about 5 L/min, then the V/Q ratio would be around 0.8. 

I wish to take this opportunity to note that 'Respiratory Exchange Ratio (RER)' is related to RQ; it is the ratio of the volume of CO2 excreted across the lungs to the volume of oxygen taken up across the lungs per unit time. In the resting state, for a healthy adult weighing 70 kg, this is about 200 ml/250 ml = 0.8. Thus, RER reflects the "average in vivo RQ" when the gas exchange function of lungs is normal. There are at least 2 exceptions to this simplification: during intense exercise, RQ exceeds 1 and may hit 1.5. The high RQ additionally reflects the CO2 that is formed when hydrogen ions accumulating during exercise are disposed by hyperventilation along with HCO3 in plasma as "CO2". Similarly when an oxygen debt is repaid following intense exercise, RER might be as low as 0.5. That is the oxygen taken up replenishes oxygen lost from intracellular stores in combination with myoglobin. 

Best

Prakash