A 45 year old man was admitted to the intensive care unit after sustaining 40% BSA burns  in a house  fire.  He was  transported  initially  to a local  hospital where initial resuscitation  was commenced  including  mechanical  ventilation for suspected inhalational injury.  On arrival in your ICU an arterial blood gas was taken which is shown below:

Patient value

Normal range

pH

7.14

pCO2

34 mmHg         (4.5 kPa)

pO2

195 mmHg

Bicarbonate

8 mmol/L

24 – 32

Standard Base Excess

-16.1 mmol/L

-2.0 – +2.0

Chloride

120 mmol/L

98 – 108

Sodium

145 mmol/L

133 – 145

Potassium

4.8 mmol/L

3.2 – 4.5

Haemoglobin

180 g/L

115 – 160

Arterial Lactate

3.8 mmol/L

< 1.5

a)  List four potential contributing causes of the metabolic derangement

b)  How would you classify the acid base derangement and explain your reasoning?

c) The serum albumin is 18g/L. Outline how would this affect the anion gap.

d)  Whilst on your ward round the RMO asks your opinion on the Stewart approach to acid base physiology. List the 3 independent variables that comprise this approach

[Click here to toggle visibility of the answers]

College Answer

a)  List four potential contributing causes of the metabolic derangement

•    Shock/Underesuscitation/hypovolaemia (elevated Hb and Lactate)
•    Normal (0.9%) Saline fluid resuscitation
•    Carbon monoxide poisoning
•    Cyanide toxicity from smoke inhalation (elevated anion gap acidosis)
•    Other   missed   injuries   e.g.  abdominal   trauma,   bleeding   etc  leading   to  hypo perfusion/shock
•    Potential concurrent ingestions e.g. methanol, ethylene glycol

b)  How would you classify the acid base derangement and explain your reasoning?

•    Mixed metabolic acidosis

(Note: CO2 is also high for pH but less relevant because patient on IPPV)
Delta ratio indicates a greater fall in [HCO3-] than expected given increase in AG. This can be explained  by a mixed metabolic  acidosis,  i.e. a combined high anion
gap and normal anion gap acidosis.

c) The serum albumin is 18g/L. Outline how would this affect the anion gap.

•    The   plasma   proteins   are   the   major   source   of   unmeasured   anions.   Hypo albuminemia may mask an increased concentration  of gap anions by lowering the value of the anion gap. Adjustment of the anion gap can be made by the application of correction factors (see Figge et al, CCM 1998).

d)  Whilst on your ward round the RMO asks your opinion on the Stewart approach to acid base physiology. List the 3 independent variables that comprise this approach

Strong ion difference
Partial CO2 tension
Total concentration of weak acid (ATOT)

Discussion

Logically, (b) is the question one ought to answer first, in order to answer (a).

Thus: a systematic approach:

  1. The A-a gradient cannot be calculated; all that can be said is that this gentleman is not hypoxic
  2. There is acidaemia
  3. The PaCO2 is compensatory (34mmHg)
  4. The SBE is -16.1, suggesting a severe metabolic acidosis
  5. The respiratory compensation is inadequate - the expected PaCO2( 8 × 1.5) + 8 = 20mmHg, and thus there is also a respiratory acidosis
  6. The anion gap is raised:
    (145) - (120 + 8) = 17, or 21.8 when calculated with potassium
    The delta ratio, assuming a normal anion gap is 12 and a normal bicarbonate is 24, would therefore be (17 - 12) / (24 - 8) = 0.31
    This delta ratio suggests that there is a mixed high anion gap and normal anion gap metabolic acidosis here.

 If we adjust the anion gap to compensate for the low albumin (which is discussed later), the "normal" anion gap value ends up being about 6.5, which means the delta ratio is in fact 0.65. 

Now, for question (a): the following are potential causes of a mixed metabolic acidosis in a severe burns patient:

  • Shock due to hypovolemia (hemoconcentration is apparent from the high Hb level)
  • Shock due to sepsis
  • High lactate due to carbon monoxide poisoning
  • High lactate due to cyanide poisoning

c) Albumin is a negatively charged protein, and thus it contributes to the anion gap in solution. Indeed for every 4 grams per litre of albumin, the "normal" anion gap value changes by 1. Thus, if we assume that the "normal" albumin value is about 40, this patient has an "albumin deficit" of 22g/L, and the normal value for anion gap should actually be 6.5 (12 - 22/4).

d) The college asks the candidate to list the three independent variables of Stewart's interpretation of acid-base physiology.

  • Strong ion difference
  • Partial CO2 tension
  • Total concentration of weak acid (ATOT)

The Stewart approach can be found heavily marketed at acid-base.org. The original paper, describing the dependent and independent variables, can be found here but is available as an abstract only. Fortunately, the acid-base.org people have published Stewart's original book online, for free.

References

References

Figge, James, et al. "Anion gap and hypoalbuminemia." Critical care medicine26.11 (1998): 1807-1810.

Stewart, Peter A. "Independent and dependent variables of acid-base control." Respiration physiology 33.1 (1978): 9-26.