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