A 43 year old man, with no history of previous illnesses is admitted with septic shock requiring administration of high dose vasopressor. His blood results on 40% oxygen, pressure support ventilation are as follows:

Parameter

Value

Normal range

pH

7.64

7.35-7.45

PaCO2

28 mmHg (3.7 kPa)

35-45 mmHg (4.7-6.0 kPa)

PaO2

189 mmHg (25.2 kPa)

75-98 mmHg (10.0-13.0 kPa)

Actual
bicarbonate

29 mmol/l

22-26 mmol/l

Sodium

147 mmol/l

134-145 mmol/l

Potassium

3.5 mmol/l

3.5-5.1 mmol/l

a. Describe the acid-base  abnormality
b. List 3 likely causes of each acid-base  abnormality in this patient.

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College Answer

a. Describe the acid-base abnormality

Respiratory and metabolic alkalosis

b. List 3 likely causes of each acid-base abnormality in this patient. Respiratory alkalosis: Hyperventilation – spontaneous or IPPV induced, septic
encephalopathy, pneumonia
Metabolic alkalosis: Diuretics, volume contraction, upper GI losses, steroids.

Discussion

Let us dissect these results systematically.

  1. The A-a gradient is normal.
    PAO2 = (0.4 × 713) - (28 × 1.25) = 250.2
    Thus, A-a = ( 250.2 - 189) = 61.2mmHg.
  2. There is alkalaemia
  3. The PaCO2 is contributing to the alkalaemia
  4. The SBE is not supplied, but the bicarbonate is 29, suggesting that there is a metabolic alkalosis.
  5. If the metabolic alkalosis is the primary disorder, then there is no respiratory compensation- the expected PaCO2(29 × 0.7) + 20 = 40.3mmHg. Alternatively, if we assume that acute respiratory alkalosis is the primary disorder, then the expected bicarbonate is 26.4mmol/L: 
    (measured bicarbonate + 2mmol/L per every 10mmHg decrease in PaCO2). Thus, there are two coexisting disorders - a metabolic alkalosis and a respiratory alkalosis - irrespective of how you look at it.

What the hell is driving this derangement in this septic patient?

The disturbances are not connected. Each likely has a separate cause.

The respiratory alkalosis can be driven by spontaneous hyperventilation, or by excessive mechanical ventilation (i.e. somebody has increased the pressure support level to a point where the patient is generating unnecessarily massive tidal volumes). Alternative explanations include pain, encephalopathy (of whatever cause) and pneumonia.

The  causes of metabolic alkalosis are numerous; those relevant to this question can include the following:

  • Gastric losses by vomiting or drainage
  • Diuretics: loop diuretics or thiazides
  • Diarrhoea 
  • Posthypercapneic state (hypercapnea recently reversed by mechanical ventilation)
  • Recent use of corticosteroids
  • Hypoalbuminaemia

References

Sankaran, Ramkumar T., et al. "Laboratory abnormalities in patients with bacterial pneumonia."CHEST Journal 111.3 (1997): 595-600.