A 60 year old woman was admitted with lobar pneumonia. She was on a thiazide diuretic for 9 months following a previous admission with congestive cardiac failure. The admission arterial blood results were:
Arterial Blood Gases
pCO2 32 mmHg
pO2 75 mmHg
HCO3 33 mmol/l
K+ 2.1 mmol/l
The severe hypokalaemia requires urgent K+ replacement therapy.
The clinical history suggests the following as possibilities:
A mixed alkalosis: A metabolic alkalosis due to to the thiazide diuretic therapy and a respiratory alkalosis
The metabolic alkalosis is probably chronic as the patient has been on these drugs for some time. The hypokalaemia is assumed to be related to this.
Correction of the hypokalaemia should commence early with IV replacement therapy, but should not be aggressive because the hypokalaemia has probably been present for some time (& thus is better tolerated) and because of the risk of hyperkalaemia because of the small ECF K+ content.
A respiratory alkalosis is present. This is probably secondary to the dyspnoea from decreased pulmonary compliance due to the pneumonia. If the plasma [K+] were to drop further, there is a risk of generalised muscle weakness. This can result in respiratory muscle failure and development of a respiratory acidosis.
Overall: The situation here is consistent with a lady with a pre-existing chronic metabolic alkalosis (related to thiazide therapy) who develops pneumonia which results in hyperventilation (acute respiratory alkalosis) is response to the decreased pulmonary compliance.
The combination of hyperventilation and thiazide diuretics is a common cause of a mixed alkalosis with hypokalaemia.
Most such patients would not have arterial blood gases collected but clues to the presence of a metabolic alkalosis are an electrolyte profile showing hypokalaemia and an elevated bicarbonate level.