Acid base teaching – Case 1

A 28 year old gentleman was  involved in road traffic accident. He had been trapped under a car for several hours prior to being extracted by the fire service.

He sustained bilateral injuries to his legs with femoral and tibial and fibular fractures. Following his injuries he was catheterised and an accurate fluid balance was undertaken.


3 Days following admission following re-introduction of food and fluids the fluid balance above was taken.

Total fluid in in 24 hours 2500ml, output 355ml
Balance = + 2145ml

As above it was noted that his urinary output was low, despite an apparently adequate fluid intake. 

Hb 114               WCC 10.2      Plt 234 

K 5.6                 Na 136           Urea 11.0

Creat  320         eGFR 21

[H+] = 50 nmol/L  pH = 7.3

PaCO2 = 3.2 kPa

[HCO3] = 12 mmol/L

Question 1

The patient has? (choose one of these and then click to see if you are correct)

TRUE – This is metabolic acidaemia with partial respiratory compensation. The metabolic acidaemia is due to renal failure.

FALSE – Typical metabolic alkalaemia presents with increased HC03 – followed by a small compensatory PC02 increase.

FALSE – Typical respiratory acidaemia presents with increased PaC02 and compensatory increased [HC03-].

FALSE – The PaC02 is low, which occurs in respiratory acidaemia, but in this case the low PaC02 is due to compensatory response to metabolic acidaemia.

FALSE – Consider the clinical circumstances. [HC03-] is lowered which indicates acidaemia but the drop in PaC02 is a compensatory effect due to hyperventilation.

Question 2

What is the cause of his acid base disturbance? (choose one of these and then click to see if you are correct)

TRUE – Myoglobin is less soluble in more acidic conditions and precipitates in renal tubules causing renal failure.

TRUE – In rhabdomyolysis muscle cells are damaged with the release of their content.

FALSE – This would cause respiratory acidaemia.

FALSE – Unlikely.


In crush injuries, muscle cells become “leaky” (“rhabdomyolysis”), releasing their contents, including myoglobin and creatine kinase. Myoglobin is relatively soluble at normal physiological [H+], but less so under the more acidic conditions found in the renal tubules. It therefore precipitates, causing renal failure. Rhabdomyolysis can be diagnosed by detecting high plasma creatine kinase levels, or by finding myoglobin in the urine (“myoglobinuria”). Once the condition is established myoglobin will no longer be detectable in the urine since it cannot get through the nephron. Treatment in the early stages is to give large volumes of intravenous fluids to try to flush the myoglobin through the nephron, and sometimes bicarbonate is given to try to alkalinise the urine to prevent precipitation. However, once renal failure has occurred, as here, fluids must be given cautiously. The metabolic acidaemia is caused by a combination of the failure to excrete non-volatile acids, possible increased production of non-volatile acids as the patient enters a catabolic state, and the failure of renal tubular mechanisms for reabsorbing/regenerating bicarbonate

Question 3

What factors do you think about in the management of this individual? (choose one of these and then click to see if you are correct)