The following set of arterial blood gases were obtained from a patient admitted to the ICU after a suicide attempt.
|
Parameter |
Patient Value |
Normal Adult Range |
|
|
pH |
6.84* |
7.36 – 7.44 |
|
|
PCO2 |
94 mmHg* (12.3 kPa) |
36 – 44 (4.6 – 5.9) |
|
|
PO2 |
140 mm Hg (18.4 kPa) |
||
|
P50 |
24 mm Hg |
||
|
Standard base excess |
-16.0 mmol/L* |
-2.0 – +2.0 |
|
|
FiO2 |
0.4 |
||
a) In addition to the hypercapnia and the acidosis, what anomaly do you notice in the blood gas report?
b) List two other investigations you would perform to elucidate the cause of the anomaly.
College Answer
a)
A left shifted curve despite a high PCO2 and a low pH.
b)
CoHb
Measure temperature
Measure 2,3 DPG
Discussion
This question can be rephrased as "what are the causes of a shifted oxygen-haemoglobin dissociation curve"? The college called attention to the obvious abnormality by putting a p50 value into the question, whereas all the other ABGs in the exam never report this variable.
Anyway: In the adult, the normal p50 should be 24-28mmHg.
- Causes of a right shift in the oxygen-hemoglobin dissociation curve
- Acidosis
- Increased PaCO2 (the Bohr Effect)
- Increased temperature
- Increased 2,3-DPG (eg. in pregnancy)
- Sulfhaemoglobin
- Causes of a left shift in the oxygen-hemoglobin dissociation curve
- Alkalosis
- decreased PaCO2
- Decreased temperature
- Decreased 2,3-DPG (eg. in stored blood)
- Carboxyhaemoglobin
- Methaemoglobin