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

Other SAQs in this paper

Other SAQs on this topic

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
  • 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