At steady state, there is a dynamic equilibrium between the respiratory exchange ratio in the alveolus, alveolar capillary blood, and tissue. The respiratory exchange ratio at steady state is termed the "respiratory quotient" and is a reflection of cellular metabolism. It is usually designated by the variable "R".
respiratory quotient (R) =
= (carbon dioxide production) / (oxygen consumption)
This can be approximated by the following equation:
respiratory quotient =
= ((partial pressure of oxygen in the inspired gas) - (PAO2)) / (PACO2)
where:
• PACO2 is approximated by PaCO2
• The partial pressure of oxygen in the inspired air is derived in the previous section.
• PAO2 is discussed in the next section.
Interpretation:
• The normal respiratory quotient in resting persons is 0.8, with range 0.7-1.0.
• The respiratory exchange ratio may differ significantly from the respiratory quotient depending on the person's ventilation. If a person hyperventilates then the ratio may be increased above the normal resting level.
• With exercise the ratio tends to increase from resting levels and may exceed 1.00 as carbon dioxide production increases. However, at steady state with exercise for several minutes at a constant workload, the ratio will approximate the respiratory quotient.
