Question 7.2

Highest mark

9.3

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

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)

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.

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