A 23-year-old male of Southeast Asian descent is admitted to the Emergency Department following an episode of syncope. He is currently resident in a medical research facility, taking part in a phase 1 trial of a novel anti-inflammatory agent. An extensive pre-trial health questionnaire did not reveal any concerns.
In the Emergency Department he is alert and comfortable with no respiratory distress, and noted to have an SpO2 of 83% on 15L O2 via a non-rebreathe mask.
His initial arterial blood gas results are shown below:
Parameter | Patient Value | Adult Normal Range | |||||||||||||
FiO2 | 0.9 | ||||||||||||||
pH | 7.52* | 7.35 – 7.45 | |||||||||||||
pO2 | 200 mmHg (26.3 kPa) | ||||||||||||||
pCO2 | 31.0 mmHg (4.1 kPa)* | 35.0 – 45.0 (4.6 – 6.0) | |||||||||||||
SaO2 | 100% | ||||||||||||||
Hb | 118 g/L* | 135 – 170 | |||||||||||||
FO2Hb | 90.6%* | 94.0 – 97.0 | |||||||||||||
FMetHb | 7.8%* | 0.0 – 1.5 | |||||||||||||
FCOHb | 2.1%* | 0.0 – 1.5 |
He has a baseline and repeat full blood count (at 24 hours), shown below:
Parameter | Patient Value | Adult Normal Range | |||||||||||||
On admission | At 24 hours | ||||||||||||||
Hb | 122 g/L | 77 g/L* | 120 – 160 | ||||||||||||
MCV | 91 | 92 | 80 – 100 | ||||||||||||
Platelets | 262 | 214 | 150 – 350 | ||||||||||||
WCC | 15.5 x 109/L* | 18.9 x 109/L* | 4.0 – 11.0 | ||||||||||||
Reticulocytes | - | 192 x 109/L* | 20 – 100 | ||||||||||||
Mega/myelocytes | 0.19 x 109/L* | 0.7 x 109/L* | 0.00 – 0.06 | ||||||||||||
Nucleated RBC | 1.1 | 6.4 | |||||||||||||
Film | Blister (helmet) cells ++, bite cells ++, polychromasia Occasional Howell-Jolly bodies |
a) List three potential causes of the discrepancy between SpO2 and SaO2 in this patient.
(15% marks)
b) List the likely diagnosis and underlying aetiology suggested by his investigations.
(30% marks)
c) List the additional tests you would order. List the results you would expect from these tests.
(25% marks)
d) List three potential causes for the elevated MetHb. (15% marks)
e) List three potential causes for the elevated COHb. (15% marks)
Not available.
a)
b)
Southeast Asian? Haemolysis? Surely this must G6PD deficiency, the most common hereditary enzymopathy among people from that part of the world. How did this happen?
c) Additional tests you would order:
d) Three potential causes for the elevated MetHb are asked for, but it is not clear whether they asked for causes in this patient or in general. Noting that the first part of this question specifies "in this patient", one can only conclude that generic causes of methaemoglobinaemia are asked for. But then, it asks for three potential causes for the elevated MetHb, not just an elevated MetHb. What are we to make of this?.. To be safe, the following differentials were tailored to the case scenario:
e) Three potential causes for the elevated COHb:
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Luzzatto, Lucio, and Elisa Seneca. "G6 PD deficiency: a classic example of pharmacogenetics with on‐going clinical implications." British journal of haematology 164.4 (2014): 469-480.
Clark, Byron B., Robert W. Morrissey, and Dorothy Blair. "Relation of methemoglobin to hemolysis." Blood 6.6 (1951): 532-543.
Ata, Fateen, et al. "Favism Induced Methemoglobinemia in G6DP Deficient Patients: Case Series and Review of Literature." Blood 136.1 (2020): 11-12.
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