Question 30.3

Created on Tue, 05/12/2015 - 02:35
Last updated on Fri, 04/28/2017 - 15:55

Pass rate: 80%
Highest mark: 9.0

Topic

Respiratory Medicine and Mechanical Ventilation

Other SAQs in this paper

Question 30.2

Other SAQs on this topic

Question 28 Question 21.1

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.

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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

Deranged Physiology