Question 15.2

Highest mark

A 41 year old man is admitted to your Emergency Department, unconscious, with the first set of blood results. The second set of blood gases are taken 1 hour later.

Parameter	Initial values	1 hour later	Normal range
pH	7.05*	7.35	7.35 – 7.45
PaCO2	34 mmHg (4.6 kPa)	39 mmHg (5.2 kPa)	35 – 45 (4.7-6.0 kPa)
PaO2	203 mmHg (33.6 kPa)	94 mmHg (12.5 kPa)	75 – 98 (10.0-13.0 kPa)
Actual bicarbonate	9 mmol/l*	21 mmol/l	22 – 26
Sodium	137 mmol/l		134 –145
Potassium	4.2 mmol/l		3.5 – 5.1
Glucose*	11.2 mmol/l		4 – 6
Ionised Calcium	1.21 mmol/l		1.15 – 1.35
Chloride	105 mmol/L		95 – 105

a) Describe the initial acid-base disturbance.

b) List 3 clinical scenarios which may produce such a pattern of arterial blood gas derangement?

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College Answer

a) Describe the initial acid-base disturbance.

The initial acid-base disturbance is a mixed metabolic and respiratory acidosis with a raised anion gap.

b) List 3 clinical scenarios which may produce such a pattern of arterial blood gas derangement?

Seizures
Resuscitated cardiac arrest
Near drowning
Near hanging

Discussion

Let us dissect these initial results systematically.

The A-a gradient cannot be calculated - the FiO₂ is not supplied
There is acidaemia
The PaCO2 is vaguely compensatory
The SBE is not supplied, but the bicarbonate is 9 , suggesting a severe metabolic acidosis
The respiratory compensation is inadequate - the expected PaCO2(9 × 1.5) + 8 = 21.5mmHg, and thus there is also a respiratory acidosis
The anion gap is raised: (137) - (105 + 9) = 23, or 27.2 when calculated with potassium. The delta ratio, assuming a normal anion gap is 12 and a normal bicarbonate is 24, would therefore be (23 - 12) / (24 - 9) = 0.73, suggesting that there is a mixed high and normal anion gap metabolic acidosis.

In the subsequent gas, the acidosis is almost completely corrected- and it only took one hour.

What could cause this sort of picture?

Well.

Whatever the extra anion was, it has either been buffered well, or it has been eliminated in some way, or it has been metabolised.

Thus, the following things may have happened:

The patient received some IV bicarbonate
The anion was lactate and virtually all of it has been metabolised; that means the cause was reversible, and has been reversed.
- Circulatory failure (eg. cardiac arrest, tension pneumothoroax, cardiac tamponade)
- Hypoxia (asphyxiation of some sort, be it drowning, hanging, etc)
- Seizures

References

Phypers, Barrie, and JM Tom Pierce. "Lactate physiology in health and disease." Continuing Education in Anaesthesia, Critical Care & Pain 6.3 (2006): 128-132.

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