You are called to the Emergency Department to see a 56-year-old female with a diagnosis of acute severe asthma .

She has been given 4 sequential salbutamol nebulisers (5 mg dose), 200 mg IV hydrocortisone, and 500 mcg of subcutaneous adrenaline with no improvement.

When you arrive in the Emergency  Department,  she has  some  inspiratory  stridor,  and is  only  able to  talk in single words. She is flushed, in sinus rhythm at 125 beats/minute, and has a blood pressure  of 160/90 mmHg (no paradox). She is breathing room air with an O2 saturation of 100%.

Auscultation reveals symmetrical breath sounds. There are no signs of heart failure. 

An arterial blood gas analysis shows the following results:

Parameter

Patient Value

Adult Normal Range

F i O2

0.21

pH

7.56*

7.35 - 7.45

pO2

117 mmHg

pCO2

16.0 mmHg

35.0 - 45.0 (4.6 - 6.0)

SpO2

100%

Bicarbonate

14.0 mmol/L*

22.0 - 26.0

Base Excess

-8.7 mmol/L*

-2.0- +2.0

Lactate

5.2 mmol/L*

0.5 - 1.6

Sodium

141 mmol/L

135 - 145

Potassium

3.5 mmol/L

3.5 - 5.0

Chloride

112 mmol/L*

95 - 105

Glucose

11.0 mmol/L*

3.5 - 6.0

Ionised calcium

1.21 mmol/L

1.10-1.35

a)    Interpret the arterial blood gas provided above. (20% marks)

b)    What are the disturbances in physiology contributing to her breathlessness? (30% marks)
 

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College answer

  1. Interpret the arterial blood gas provided above
  • normal A-a gradient of 13
  • severe respiratory alkalosis
  • Bicarbonate is lower than would be expected for an acute process, suggesting either a co-existing acidosis, or a chronic compensation.
  • Elevated lactate, slightly elevated anion gap (15)
  1. What are the disturbances in physiology contributing to her breathlessness?
  • hyperventilation
  • probable vocal cord dysfunction syndrome
  • beta-agonist toxicity (lactaemia, increased Vco2)

Discussion

Let's do this ABG first:

  • The A-a gradient is (713 × 0.21) - (16 / 0.8)  - 117 = 12.7, i.e. completely normal.
  • There is alkalaemia
  • There is a metabolic acidosis, albeit mild: the SBE is -8.7 
  • The CO2 is unhelpful: the expected CO2 for this level of acidosis would be around 31 by the SBE method, or (1.5 × 14) + 8 = 29 if using the Boston rules. So, there is quite a severe respiratory alkalosis
  • The anion gap is trivially elevated: (141-112-14) = 15
  • The delta ratio is therefore (15-12)/(24-14) = 0.3, i.e. this mild acidosis is almost completely a non-anion gap affair
  • The lactate is elevated (one could blame the adrenaline and salbutamol)
  • There is trivial hyperglycaemia

So, this "asthmatic". The asthma history does not sound plausible, as asthma usually is not associated with stridor. Also, one might expect some wheeze on auscultation. Could this be something else?

Possible causes of respiratory alkalosis include:

  • Respiratory control centre
    • Head injury, stroke
    • Anxiety, fear, stress, pain
    • Salicylates
    • Pregnancy
    • Chronic liver disease
    • Hypoxia
  • Pulmonary receptors
    • Pulmonary embolism
    • Pneumonia
    • Asthma
    • Pulmonary oedema

Possible differentials include:

  • Psychogenic - which would explain the failure to improve with treatment - however the college specifically asks for disturbances in physiology, and strictly speaking that would be a disturbance in psychology.
  • Anaphylaxis
  • Carcinoid syndrome crisis: they often hyperventilate, look flushed and present with wheeze
  • Thyrotoxicosis (the stridor is the goitre!)
  • Septic shock, because sepsis could look like anything, right?...
  • A PE would probably be expected to give rise to some hypoxia, but one cannot rule this out as a differential

References

Grahame-Smith, D. G. "Progress report: the carcinoid syndrome." Gut 11.2 (1970): 189.