Lab Case 166 Interpretation

40 year old female presents to ED complaining of anxiety and feeling panicked. She has recently been diagnosed with anxiety by her GP and started on prn lorazepam.

Alkalaemia

pCO2 17 – Primary respiratory alkalosis

Expected HCO3= 24- (2x{40-Measured pCO2}/10) =19.4. Actual pCO2 is 17. Therefore there is a co –existing metabolic acidosis.

AG = 19. Therefore coexisting HAGMA

Expected PAO2 = 150 –(pCO2x1.25) = 128.75

A-a gradient = 128.75 – 126=2.75 (Normal Age/4+4 ). Therefore no significant A-a gradient

Lactate is raised.

Causes of Respiratory Alkalosis:

Central Causes – head injury, Stroke, Anxiety-hyperventilation syndrome (psychogenic), Other ‘supra-tentorial’ causes (pain, fear, stress, voluntary), Various endogenous compounds (eg progesterone during pregnancy, cytokines during sepsis, toxins in patients with chronic liver disease)

Hypermetabolic Pregnancy (Progesterone)(Secondary to reduced FRC), Sepsis (fever) (often before metabolic acidosis), Thyrotoxicosis

Environmental- HYPERthermia

Drugs salicylate OD, Progesterone, Stimulants

Liver failure (encephalopathy) with hyperammonaemia (ammonia)

Iatrogenic mechanical ventilation

Hypoxaemia – Respiratory stimulation via peripheral chemoreceptors

Pulmonary Causes – (via intrapulmonary receptors) Pneumonia, Pulmonary oedema, Asthma ,Pulmonary Embolism

In the context of the above case one would have to consider the cause of a mixed acid base disorder ie respiratory alkalosis and metabolic acidosis. The likely differentials in a patient with weight loss and recent diagnosis of anxiety, would be thyrotoxicosis or stimulant drug use. Sepsis should be considered, but a pulmonary cause is unlikely as the A-a gradient is not raised. Other toxicological causes would be salicylate OD.

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