Acid Base Learner Series: Respiratory Acidosis in detail

A respiratory acidosis is a primary acid-base disorder in which arterial pCO₂ rises to a level higher than expected.

At onset, the acidosis is designated as an 'acute respiratory acidosis'. The body's initial compensatory response is limited during this phase.
As the body's renal compensatory response increases over the next few days, the pH returns towards the normal value and the condition is now a 'chronic respiratory acidosis'.
The differentiation between acute and chronic
is determined by time but occurs because of the renal compensatory response (which is slow).

 Causes of Respiratory Acidosis:
The arterial pCO₂ is normally maintained at a level of about 40 mmHg by a balance between production of CO₂ by the body and its removal by alveolar ventilation. If the inspired gas contains no CO₂ then this relationship can be expressed by:

paCO₂ is proportional to V_CO2 / V_A

where:
V_CO2 is CO₂ production by the body
V_A is Alveolar ventilation

An increase in arterial pCO₂ can occur by one of three possible mechanisms:

• Presence of excess CO₂ in the inspired gas
• Decreased alveolar ventilation
• Increased production of CO₂ by the body

CO₂ gas can be added to the inspired gas or it may be present because of rebreathing : Anaesthetists are familiar with both these mechanisms. In these situations, hypercapnia can be induced even in the presence of normal alveolar ventilation and normal carbon dioxide production by the body.
An adult at rest produces about 200mls of CO₂ per minute