QuestionSBA
QuestionChoice
This is highly unlikely given nothing in the history is mentioned about possible overdose or suicidal ideation. Furthermore, ethylene glycol poisoning would result in a high anion gap metabolic acidosis.
32802
e
Ethylene glycol poisoning
QuestionChoice
Uraemia could occur as a result of an acute kidney injury secondary to the vomiting and diarrhoea. However, this would cause an increased anion gap metabolic acidosis and the anion gap here is normal.
32801
d
Uraemia
QuestionChoice
Lactic acidosis would cause a metabolic acidosis with a low pH and patients with renal impairment can get a metformin induced lactic acidosis. However, this would cause a raised anion gap metabolic acidosis, and the anion gap here is normal.
32800
c
Lactic acidosis
QuestionChoice
Diabetic ketoacidosis would cause a metabolic acidosis with a low pH, but the anion gap would be high as there is excess acid in the blood. The anion gap in this case is normal.
32799
b
Diabetic ketoacidosis
QuestionChoice
The pH here is below the normal range, implying an acidosis. The carbon dioxide also also below the normal range, implying that this must be a metabolic acidosis with partial respiratory compensation.

The anion gap is calculated as follow: [Na+] - ( [Cl-] + [HCO3-] ). A normal anion gap is 8-12 mmol/L. The anion gap in this case is 137-(117+10)=10. This represents a normal anion gap acidosis. One of the most common causes of a normal anion gap metabolic acidosis is diarrhoea, due to the significant bicarbonate loss from the gut.
32798
a
Gastroenteritis
Concept
Chapter
# Summary

Metabolic acidosis is a type of acid-base imbalance where blood pH is low due to an increase in hydrogen ions (H+), a decrease in bicarbonate (HCO3-) or both. Causes can be split into those that cause a raised anion gap (calculated as sodium plus potassium minus both chloride and bicarbonate) and those where the anion gap is normal. Causes of a raised anion gap metabolic acidosis include diabetic ketoacidosis (DKA), uraemia, lactic acidosis and medications such as salicylates or isoniazid. Causes of a normal anion gap metabolic acidosis include diarrhoea, renal tubular acidosis and medications such as acetazolamide and spironolactone. The key diagnostic investigation is a blood gas, with further investigations and management tailored based on the clinical presentation and suspected cause.

# Definition

A metabolic acidosis refers to a state where the blood pH is less than 7.35 (i.e. an acidosis) with a low bicarbonate (less than 22 mmol/L). It occurs due to a variety of causes that derange the normal acid-base balance.

# Classification

Metabolic acidoses are classified depending on the anion gap. This is calculated using the following formula:

Anion Gap = (Sodium + Potassium) - (Bicarbonate + Chloride)

A normal anion gap is between 10-20 mmol/L (or 8-16 mmol/L if potassium is excluded from the equation)

The anion gap is a way of quantifying anions that are not measured by a blood gas machine, for example ketones in diabetic ketoacidosis. 

Normal anion gap metabolic acidoses are caused by bicarbonate loss, and high anion gap metabolic acidoses are caused by accumulation of organic acids or reduced excretion of hydrogen ions.

# Signs and Symptoms

Clinical presentation depends on the underlying cause, for example fruity smelling breath in diabetic ketoacidosis. 

Dyspnoea and tachypnoea are common features due to respiratory compensation (hyperventilation causes ""blowing off"" of carbon dioxide as a way to reduce blood pH).

Nausea, vomiting and anorexia are also common symptoms. 

In severe metabolic acidosis, patients may be lethargic or comatose, and hypotension may occur secondary to myocardial suppression.

# Differential Diagnosis

**Causes of a raised anion gap metabolic acidosis include:**

- Lactic acidosis (e.g. secondary to septic shock, haemorrhage, heart failure, respiratory failure)
- Ketoacidosis (e.g. DKA, alcohol excess, starvation)
- Uraemia (secondary to acute or chronic renal failure)
- Drugs (salicylates, iron, metformin, isoniazid, ciclosporin) 
- Poisoning (methanol, ethylene glycol, sulphur)

**Causes of a normal anion gap metabolic acidosis include:**

- Diarrhoea
- High output ileostomies
- Fistulae (pancreatic, ureteric, biliary, small bowel)
- Renal tubular acidosis
- Adrenal insufficiency
- Chloride excess (e.g. excessive normal saline administration)
- Acetazolamide

# Investigations

**Bedside tests:**

- A **blood gas** is the diagnostic investigation - this may be arterial or venous, with an arterial gas preferred if accurate measurement of PaCO2 and PaO2 is required
- pH is low and bicarbonate is also low
- PaCO2 may also be low due to respiratory compensation
- This also provides the information needed to calculate the anion gap 
- It also measures lactate and glucose (for lactic acidosis and DKA respectively)
- **ECG** looking for arrhythmias which may lead to shock and lactic acidosis
- **Urinalysis and urine MC&S** as part of a septic screen; there may be glycosuria and high urinary ketones in DKA; urine pH is important in differentiating between types of renal tubular acidosis
- **Sputum MC&S** as part of a septic screen if there is a productive cough
- **Stool MC&S, viral PCR and C difficile** if there is diarrhoea

**Blood tests:**

- **FBC** which may show anaemia due to haemorrhage, or raised white cells due to infection
- **U&Es** may be deranged in uraemia
- **LFTs** may be abnormal e.g. in alcohol excess
- **Magnesium** and **bone profile** as electrolytes may be deranged due to diarrhoea/fistulae/high output stomas
- **CRP** looking for evidence of infection
- **Clotting screen** looking for coagulopathy secondary to sepsis or a bleeding diathesis in haemorrhage
- **Group and save** or **cross-match** in cases of haemorrhage in case transfusion is required
- **Blood cultures** if there are signs or symptoms of infection
- **Cortisol** in suspected adrenal insufficiency
- **Salicylate levels** or levels of other medications in suspected toxicity e.g. iron
- **Plasma ketones** in suspected DKA**

**Imaging:**

- **Chest X-ray** looking for evidence of infection or heart failure
- Other imaging may be indicated based on the presentation e.g. **abdominal ultrasound** in suspected biliary sepsis, **CT** or **MRI** for suspected fistulae

# Management

- Management revolves around treatment of the underlying cause and the severity of illness
- Early intensive care involvement should be sought in patients with severe acidosis or reduced levels of consciousness
- Cases of poisoning or medication overdose should be managed with input from toxicology services (e.g. Toxbase, the National Poisons Information Service)
- Additional management options for correcting a severe acidosis include sodium bicarbonate and renal replacement therapy

# Complications

- Hypotension (secondary to both vasodilation and myocardial depression)
- Arrhythmias
- Coma
- Seizures

# References

[Patient UK - Metabolic acidosis](https://patient.info/doctor/metabolic-acidosis)

[Life in the Fast Lane - Metabolic acidosis](https://litfl.com/metabolic-acidosis-evaluation/)

[Life in the Fast Lane - Anion gap](https://litfl.com/anion-gap/)

[BNF - Fluids and electrolytes](https://bnf.nice.org.uk/treatment-summaries/fluids-and-electrolytes/)
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149
Metabolic acidosis
Topic
36
Clinical Chemistry
Condition
1
Acid-base abnormality
UkmlaTopic
1
Acute and emergency
6560
A 63-year-old woman attends A&E. She has a background of type 2 diabetes mellitus and is usually on metformin. She has been experiencing vomiting and diarrhoea since returning from holiday to Sri Lanka. On examination, she is breathing rapidly.


An arterial blood gas (ABG) is performed:


||||
|--------------|:-------:|------------------|
|pH|7.29|7.35 - 7.45|
|PaO₂|11.2 kPa|11 - 15|
|PaCO₂|3.5 kPa|4.6 - 6.4|
|Bicarbonate|10 mmol/L|22 - 30|
|Chloride|117 mmol/L|96-106|
|Sodium|137 mmol/L|134-145|
|Potassium|5.2 mmol/L|3.5 - 5.3|
|Glucose|20 mmol/L|5-8|



What is the most likely cause of these blood gas results?
a