Lab case 319 interpretation

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Question 1:

PH = 7.25, That is moderate acidaemia

PCO2 = 84, tat is high. So we have respiratory acidosis.

From the history, this patient has COPD. \however, it is not clear if she is a CO2 retainer or not. Accordingly, the CO2 elevation can be acute, chronic or acute on chronic.

For acute respiratory acidosis, we expect HCO3 to increase by 1 for every 10 PCO2 above 40. For a case of acute respiratory acidosis with PCO2 of 84, the expected HCO3 should be 24 + 0.1 x (84 – 40) = 28.4 (way less than 40) So, we most have chronic respiratory process going on.

For chronic respiratory acidosis, HCO3 should increase by 4 for every 10 PCO2 above 40. Accordingly, the expected HCO3 for chronic respiratory acidosis should be like the following: 24 + 0.4 x (84 – 40) = 41.6, that is close to 40. However, for for pure chronic respiratory acidosis (with full compensation), we expect PH to be close to 7.35.

Do we have additional metabolic process???

We can check the anion gap = 134 -( 87+40) = 7. That is normal anion gap.

The other two possibilities are acute on chronic respiratory acidosis or chronic respiratory acidosis combined with NAGMA. From the history, most probably it is acute on chronic respiratory acidosis.

Other findings:

PaO2 = 92 mmHg. This patient is on 6L O2, that is FiO2 of 44%. Expected PaO2 usually equals 4-5 x FiO2. Accordingly, it should be 176 to 220 mmHg. Accordingly, this patient is hypoxia (relative hypoxia).

We can calculate the A-a gradient.

Expected A-a gradient =  [age in years/4] + 4 = 26.

To calculate A-a gradient for this patient. We need to calculate the PAO2 first.

PAO2 = (760 – pH2O) x FiO2 – PCO2/0.8 = 208.

Then A-a Gradient for this patient will be 208 – 92 = 116, way higher than the expected for this patient that is 26.

Other abnormal findings:

  •  Cl = 87, that is a usual finding with alkalosis to maintain the electric charge
  • Lactate = 2.8, moderate lactosis, this can be caused by  poor tissue perfusion, however in this case it is most probably related to salbutamol
  • Cr = 150, in a 88 year old lady, this give her a GFR of 28. That is stage 4 Chronic Kidney disease/dysfunction.
  • Glucose = 9.6, mildly elevated.. Possibly per-diabetes, As part of stress reaction or due to steroid administration in this patient

 

 Question 2:

Management of this patient,

” Because of COVID these days, this patient needs to be managed in an isolation room”.

First step is to maintain oxygenation and tissue perfusion. (Maintain blood pressure/ MAP around 65-70).

Specific management of the airways.

  • improve ventilation, we need to consider BiPAP for this patient.
  • Continuous bronchodilators and a dose of steroids’ to improve the air flow and reduce work of breathing.
  • Treat the cause, consider infection, if any suspension. Start the patient on  broad spectrum antibiotics and cover for the atypical. (Consider pseudomonas colonisation in patients with severe COPD).
  • This patient has high HCO3, most probably due to deteriorating lungs function over longer period. this patient might not benefit from BiPAP in that case we might need to consider discussion of the End of Life options with this patient and her family.