Question 24.1

A 38-year-old female presents to the Emergency Department with complaints of lower abdominal pain and vaginal bleeding. On examination, she is confused and, with cool peripheral perfusion and patchy ecchymoses over her extremities. Vaginal examination reveals clots, with tissue resembling products of conception. She is tachypneic, tachycardic with a non-invasive blood pressure of 88/42 mmHg.

(Parts 24.1 and 24.2 of the question are related to the initial blood results obtained from this patient.)

A peripheral venous blood sample including a venous blood gas analysis shows the following results:

Parameter Patient Value Adult Normal Range
FiO2 0.21  
pH 7.36 7.35 – 7.45
pO2 46.0 mmHg (6.0 kPa)  
pCO2 20.0 mmHg (2.6 kPa)*     45.0 – 51.0 (5.5 – 6.8 KPa)
SpO2 82%  
Bicarbonate      11.0 mmol/L* 23.0 – 29.0
Base Excess -12.0 mmol/L* -2.0 – +2.0
Sodium 134 mmol/L* 135 – 145
Potassium 2.9 mmol/L* 3.5 – 5.0
Chloride 100 mmol/L 95 – 105
Glucose 5.4 mmol/L 3.5 – 6.0

a)    Explain the acid base status, including your mathematical calculations where appropriate.
(20% marks)

b)    List the most likely source of the metabolic acidosis in this patient.    (10% marks)

c)    List the most likely clinical diagnosis and underlying pathophysiology in this patient.
(10% marks)

d)    Outline the advantages and disadvantages of a peripheral venous blood gas in critically ill patients.
(20% marks)

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College answer

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"Including your mathematical calculations where appropriate"? I thought you'd never ask!

  1. The A-a gradient should not be calculated, as this is a venous gas.
  2. There is no acidaemia.
  3. The PaCO2 is appropriately low, at 20. 
  4. The SBE is -12.0, suggesting a moderate metabolic acidosis. The bicarbonate is 11.
  5. If we assume the primary disorder is metabolic acidosis, then there is also a mild respiratory alkalosis: the expected CO2 is 40+(-12.0) = 28.0, or (1.5 × 11) + 8 = 24.5 mmHg. 
  6. The anion gap is raised:
    134-(100+11) = 23
  7. The delta ratio is
    (23-12)/(24-11) = 0.84
    This suggests a mixed high and normal anion gap metabolic acidosis

Now, this could well be a triple disorder (mixed high and normal anion gap metabolic acidosis as well as a respiratory alkalosis), but this is a venous gas, which really should not be used to assess compensation in a patient with such a terrible circulatory failure. The venous CO2 could be raised because of the extremely poor cardiac output, with an increased arteriovenous CO2 gradient, i.e. the respiratory alkalosis might be even more severe if you had an arterial sample.


b)    List the most likely source of the metabolic acidosis in this patient. 

They did not give us a lactate, but it's probably raised, and - not to cheat or anything - but on the same page, the next question about the same stem gives us a set of haematology values which is clearly demonstrating DIC. Thus, this looks like septic shock, and lactate is the most likely reason for the acidosis. 

c)   List the most likely clinical diagnosis and underlying pathophysiology in this patient.

The patient is falling apart from septic shock, in DIC, and there's "tissue resembling products of conception" in her vagina? What else could this be, if not septic abortion. There's barely anything to list. If it were not for those products, one could also consider toxic shock syndrome, eg. from a retained tampon. The pathophysiology is usually a polymicrobial infection by the normal flora of the vagina and endocervix.

d)   The advantages and disadvantages of a peripheral venous blood gas in critically ill patients, in two minutes (because that's how long you have for this little 20% fragment), would have to be pretty brief:

  • Advantages: 
    • Requires minimal skill to collect
    • Can be acquired in the process of routine cannulation to gain access
    • Gives almost all the necessary information regarding blood biochemistry
    • Venous blood gas values and arterial values are usually closely correlated
  • Disadvantages:
    • "Requires minimal skill to collect" is probably incorrect, as gaining venous access in this half-dead septic patient is not without its challenges. It may actually be easier to feel the arterial pulse.
    • Prognostic and diagnostic value of lactate, SBE, acidosis, etc, are all validated in arterial gases, which makes it difficult to interpret
    • Arterial blood is essentially the same no matter where you sample it, whereas venous blood will vary regionally in its CO2 and lactate content


Finkielman, Javier Daniel, et al. "The clinical course of patients with septic abortion admitted to an intensive care unit." Intensive care medicine 30.6 (2004): 1097-1102.

Stubblefield, Phillip G., and David A. Grimes. "Septic abortion." New England Journal of Medicine 331.5 (1994): 310-314.

Awasthi, Shilpi, Raka Rani, and Deepak Malviya. "Peripheral venous blood gas analysis: An alternative to arterial blood gas analysis for initial assessment and resuscitation in emergency and intensive care unit patients." Anesthesia, essays and researches 7.3 (2013): 355.