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Acid base teaching

Educational resources for renal medicine

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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.  

HintsAn approach to interpretation of acid-base data | Normal values

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. 

An approach to interpretation of acid-base data

Open the cases in a separate browser window so you can keep flipping back to this. 

  1. 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

  2. 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.
     
  3. 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.
     
  4. 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.
     
  5. 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.


Normal values

Normal values

Arterial [H+] 36 - 44 nmol/L (40±4)
pH  7.36 - 7.44 (7.4±.04)
Arterial PCO2 4.8 - 6.0 kPa
Arterial [HCO3-] 21.0 - 27.5 mmol/L
Total CO2 24 - 30 mmol/L
[Na+] 132 - 144 mmol/L
[K+]  3.6 - 5.1 mmol/L
[CI-] 95 - 107 mmol/L
Anion gap 10 - 20 mmol/

Acid-base diagram


(Click for larger image)

***  Open the cases in a separate tab or browser window so that you can keep flipping back to the these hints ***

 
Page last modified 17.06.2012, 12:35 by Administrator. edrep and edren are produced by the Renal Unit at the Royal Infirmary of Edinburgh and Univ. Edinburgh. CAUTIONS and Contact us. Note that the information published here is primarily intended for education, not for clinical care.