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
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:
 The Aa gradient cannot be calculated; all that can be said is that this gentleman is not hypoxic
 There is acidaemia
 The PaCO_{2} is compensatory (34mmHg)
 The SBE is 16.1, suggesting a severe metabolic acidosis
 The respiratory compensation is inadequate  the expected PaCO_{2}( 8 × 1.5) + 8 = 20mmHg, and thus there is also a respiratory acidosis
 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 acidbase physiology.
 Strong ion difference
 Partial CO2 tension
 Total concentration of weak acid (ATOT)
The Stewart approach can be found heavily marketed at acidbase.org. The original paper, describing the dependent and independent variables, can be found here but is available as an abstract only. Fortunately, the acidbase.org people have published Stewart's original book online, for free.
References
Figge, James, et al. "Anion gap and hypoalbuminemia." Critical care medicine26.11 (1998): 18071810.
Stewart, Peter A. "Independent and dependent variables of acidbase control." Respiration physiology 33.1 (1978): 926.