The following arterial blood gas was obtained from a 19-year-old chemistry student who presents with dyspnoea, cyanosis and feeling unwell. He has been handling chemicals today and admits to recreational drug use but takes no prescribed medicines. His observations on 8 L/min oxygen via a Hudson mask are as follows:
Heart rate: 130 beats/min sinus
Blood Pressure: 140/73 mmHg
Oxygen Saturation: 82%
His initial arterial blood gas analysis is given below:
|Parameter||Patient Value||Adult Normal Range|
|pH||7.45||7.35 – 7.45|
|pO2||219 mmHg (29.2 kPa)|
|pCO2||37.8 mmHg (5.04 kPa)||35.0 – 45.0 (4.60 – 6.00)|
|Bicarbonate||26.6 mmol/L*||22.0 – 26.0|
|Base Excess||2.6 mmol/L*||-2.0 – +2.0|
|Lactate||0.9 mmol/L||0.5 – 1.6|
|Sodium||139 mmol/L||135 – 145|
|Potassium||3.9 mmol/L||3.5 – 5.0|
|Chloride||107 mmol/L*||95 – 105|
|Ionised Calcium||1.18 mmol/L||1.15 – 1.29|
|Glucose||7.1 mmol/L*||3.5 – 6.0|
a) What is the diagnosis? (10% marks)
b) Name two recreational drugs and two chemical compounds that can cause this condition other than prescribed medicines. (20% marks)
c) What emergency treatment would you give? (20% marks)
d) In what circumstance would your treatment of choice be contraindicated? (10% marks)
a) This is methaemoglobinaemia. The chemistry student is hopped up on nitrites. The specific clues to this diagnosis are
Thus, this is methaemoglobinaemia.
Now, one should point out that when you write "SpO2", the little "p" usually means "pulse", and so it is confusing and weird to see it in the ABG results. Usually the blood gas analyser will report an "SaO2", where the "a" stands for "arterial". It may seem pedantic to pick on these minor elements, but then some might say that ICU is all about being thorough and detailed. It is especially confusing to find this mistake in a question where specifically the difference between the pulse oximeter and the arterial oximeter is a critically important part of the diagnosis.
b) Recreational drugs which cause methaemoglobinaemia are:
Non-recreational chemical compounds are numbered in the countless thousands; though the author confesses he has not personally audited this list to make sure none of these substances has any enjoyable effects. Briefly:
You could also have used use nitric oxide, so long as you mention that it is a part of internal combustion engine exhaust (even though it could theoretically also be used medically to treat pulmonary hypertension). In case it is necessary in some future version of this question, prescribed medicines which cause methaemoglobinaemia include:
c) Emergency treatment of methaemoglobinaemia:
This is discussed in much more detail in Question 1 from the second paper of 2004, where it was a full 10-mark question. Here, for 20% of the mark, you would only have time to write this much:
d) "your treatment of choice be contraindicated" if the patient has G6PD deficiency. If your treatment of choice was ascobic acid, you would be just fine.
Wright, Robert O., William J. Lewander, and Alan D. Woolf. "Methemoglobinemia: etiology, pharmacology, and clinical management."Annals of emergency medicine 34.5 (1999): 646-656.
ROSEN, PETER J., et al. "Failure of methylene blue treatment in toxic methemoglobinemiaAssociation with glucose-6-phosphate dehydrogenase deficiency." Annals of internal medicine 75.1 (1971): 83-86.