A 39-year-old man comes into the A&E complaining of vomiting for the past five hours. His respiratory rate is 7 breaths per minute. Blood results from ABG show: 


||||
|--------------|:-------:|------------------|
|pH|8.25|7.35 - 7.45|
|PaO₂|11 kPa|11 - 15|
|PaCO₂|8.1 kPa|4.6 - 6.4|
|Bicarbonate|35 mmol/L|22 - 30|


What is the acid-base disturbance seen?

Options:
explanation: This is not the correct answer. In metabolic acidosis, the pH would be low and the bicarbonate level would be low. The PaCO2 may be low if compensation due to hyperventilation is present. In this case, the clinical picture is that of metabolic alkalosis as the pH is high and the bicarbonate levels are high. There is also partial compensation due to hypoventilation present as the pH is still not in the normal range but the PaCO2 level is elevated. Some examples of causes of metabolic alkalosis with raised anion gap include lactic acidosis, diabetic ketoacidosis and starvation. Metabolic acidosis with a normal anion gap is usually secondary to excessive loss of HCO3-, causes of which include diarrhoea, adrenal insufficiency, and use of spironolactone.
name: Metabolic acidosis
label: b
answer: no
explanation: This is not the correct answer. There is alkalosis present as the pH is high, however, in a respiratory alkalosis the PaCO2 will be low while in a metabolic alkalosis, as seen in the scenario, the bicarbonate level is high. There is also partial compensation due to hypoventilation present as the pH is still not in the normal range but the PaCO2 level is elevated. Some examples of causes of respiratory alkalosis include respiratory causes (pulmonary oedema, pneumonia, pneumothorax, chronic obstructive pulmonary disease etc.), hyperventilation, salicylate poisoning and compensation for metabolic acidosis.
name: Respiratory alkalosis
label: c
answer: no
explanation: This is not the correct answer. In respiratory acidosis, the pH would be low and the PaCo2 would be high. The bicarbonate level would be normal, or high if compensation is present. In this case, the pH is high, implying an alkalosis, and the bicarbonate level is high which implies a metabolic cause. There is also partial compensation due to hypoventilation present as the pH is still not in the normal range but the PaCO2 level is elevated.
name: Respiratory acidosis
label: d
answer: no
explanation: This is not the correct answer. The clinical picture above is indeed that of a patient with metabolic alkalosis. However, since the pH is still not within the normal range, there is insufficient compensation. Hypoventilation increases the arterial partial pressure of carbon dioxide and compensates for metabolic alkalosis, which is seen in this case with the elevated PaCO2. As such, there is partial compensation.
name: Metabolic alkalosis with compensation
label: e
answer: no
explanation: This is the correct answer. The pH is high, implying an alkalosis, and the bicarbonate level is high which implies a metabolic cause. Metabolic alkalosis can be compensated for by hypoventilation as carbon dioxide is a limiting factor for compensation, which is seen in this case with the reduced respiratory rate and elevated PaCO2 as hypoventilation increases the arterial partial pressure of carbon dioxide. Since the pH is still abnormal, there is insufficient compensation, however, since there is an increase in PaCO2, there is partial compensation. Some examples of causes of metabolic alkalosis include vomiting, diuretic use, Cushing syndrome and Conn's syndrome.
name: Metabolic alkalosis with partial compensation
label: a
answer: yes

Correct answer: a