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 |
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.
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.
- The A-a gradient is normal.
PAO2 = (0.4 × 713) - (28 × 1.25) = 250.2
Thus, A-a = ( 250.2 - 189) = 61.2mmHg. - There is alkalaemia
- The PaCO2 is contributing to the alkalaemia
- The SBE is not supplied, but the bicarbonate is 29, suggesting that there is a metabolic alkalosis.
- 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.