Acid base teaching
Assessment of Acid-Base Disturbance
This module is based on six clinical scenarios. It was written by Dr John Neary. In each case describe the acid-base disturbance, assess whether any compensation has occurred, and discuss the causes, in terms of the brief history of each patient.
The cases will open in a separate window/tab so that you can keep the hints and guidance on this page open. Click on the acid-base diagram to show a larger figure.
- Case 1 – An oliguric patient after a road accident
- Case 2 – A tired 18 year-old
- Case 3 – A semi-conscious hyperventilating young woman
- Case 4 – A confused patient with respiratory disease
- Case 5 – An elderly man with collapse and hypotension
- Case 6 – A vomiting baby
Open the cases in a separate browser window so you can keep flipping back to this.
- First look at the hydrogen-ion concentration (pH): is there acidaemia or alkalaemia? [Strictly the terms “acidaemia” and “alkalaemia” should be used rather than acidosis/alkalosis, as acidosis/alkalosis can occur without a change in [H+], if complete compensation (which would be unusual) has also occured.][H+] and pH are two methods for measuring hydrogen ion concentration. pH = The negative logarithm of [H+]
On your phone/calculator, enter [H+] (32 nmol = 0.000 000 032 molar), then Log, then change the sign to get pH (and vice versa). A kind chemical engineer at Rensselaer Polytechnic Institute posted this calculator.
- Next look at pCO2 changes which indicate a respiratory effect.
If the altered pCO2 would change [H+] in the direction of the observed abnormality. It is likely to be the primary disturbance; if it would change [H+] in the direction opposite to that observed, it is likely to be compensatory.
- Now look at the bicarbonate [HCO3] concentration.
Changes in this indicate metabolic effect, which again may be primary or compensatory. Reference to whether there is acidaemia or alkalaemia should allow you to decide which.
- In general, if only one of these parameters is changed, with a corresponding change in [H+], a primary disturbance is indicated, eg respiratory alkalaemia or metabolic acidaemia.
But if both parameters are changed, compensation has occurred, eg respiratory acidaemia with metabolic compensation.
- Finally, calculate the anion gap, if the data are available. If it is raised there is an unmeasured anion present.
Anion Gap = ([Na+] + [K+]) – ([Cl–] + [HCO3–]). Quoted normal values for AG vary; some say <16 meq/l (<11 or 12 if calculated without [K]), but in the table below we’ve allowed up to 20. Anion gap can be useful, but it isn’t very reliable for spotting unmeasured anions when it is only moderately elevated, and is sometimes not as high as you’d expect, for example in some patients with lactic acidosis. Renal failure increases it.
*** Open the cases in a separate tab or browser window so that you can keep flipping back to the these hints ***