Causes of Respiratory Acidosis and Alkalosis
Respiratory acidosis and alkalosis are featured in virtually every paper, and being able to identify a respiratory acid-base disturbance is a vital skill for the CICM fellowship candidate. The SAQs will frequently require the application of the usual rules of compensation to reveal a hidden acid-base disorder, eg. "this patient has a low CO2 but it is not low enough".
Questions which involve respiratory acid-base disturbances are too numerous to list. Some representative examples include the following:
- Question 12.3 from the second paper of 2014
- Question 3.4 from the first paper of 2013
- Question 3.5 from the first paper of 2013
- Question 8.3 from the first paper of 2012
- Question 9.1 from the first paper of 2011
- Question 7.2 from the first paper of 2009
Several CICM fellowship questions revolve around the core question, "what possible causes for this respiratory acid-base disturbance can you think of ?"
The causes can be split into aetiological categories, as below:
Compensation for respiratory acid-base disorders
The general approach to the assessment of metabolic compensation for respiratory acid-base disturbances is discussed in greater detail in the apocryphal chapters devoted to blood gas analysis.
Specifically, the Bedside Rules section discusses a series of empirical formulae which can be used to assess the effectiveness of compensation:
The Boston rules: 1-4-2-5 rule:
The change in HCO3 in response to a 10mHg change in PaCO2
- 1mmol increase with acute respiratory acidosis
- 4mmol increase with chronic respiratory acidosis
- 2mmol decrease with acute respiratory alkalosis
- 5mmol decrease with chronic respiratory alkalosis
The 0.008 rule:
The pH change in response to an acute respiratory acid-base disturbance
pH = 7.40 - ((PaCO2-40) x 0.008))
The 0.003 rule:
The pH change in response to a chronic respiratory acid-base disturbance
pH = 7.40 - ((PaCO2-40) x 0.003))
The Copenhagen Rules: the 0-4-1-6 rules.
0: An acute change in PaCO2 will not change the Standard Base Excess.
4: In chronic disorders, the expected change in SBE will be 0.4 times the change in PaCO2 ... i.e. expected SBE = 0.4 × (40 - PaCO2)
1: In compensation for metabolic acidosis, the compensatory change in PaCO2 will be proportional to the SBE. ..i.e. expected CO2 = 40 + (1.0 × SBE)
6: In compensation for metabolic alkalosis, the compensatory change in PaCO2 will be proportional to 0.6 times the SBE, i.e CO2 = 40 + (0.6 × SBE)