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).