Lab case 236 Interpretation

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Question 1: Answer

PH = 7.31 (less than 7.35) that is mild acidemia.

PO2 = 31, that is low for the patient age. (Estimated normal PaO2 = 100 mmHg – (0.3) age in years), Accordingly, expected PO2 = 75 mmHg. So we have respiratory failure

PCO2 = 76 so we have respiratory acidosis. Also because PCO2 is elevated, this patient has type 2 respiratory failure.

Next step we calculate compensation.

For acute respiratory acidosis HCO3 should increase by 1 mmol/L for every 10 mmHg of CO2 above 40. Accordingly, expected HCO3 should be (24 + 3.6) = 27.7.

For chronic respiratory acidosis HCO3 should increase by 4 mmol/L for every 10 mmHg of CO2 above 40. Accordingly, expected HCO3 should be (24 + 14.4) = 38.4

HCO3 is 34 (between the 2 calculated values)so we have acute on chronic respiratory acidosis.

However, chronic respiratory acidosis with acute metabolic acidosis is still a possibility. To exclude that we calculate the anion gap. ( 135 – (95 + 34) = 6 ) that is normal anion gap. Accordingly we can confirm acute on chronic respiratory acidosis.

Blood glucose is slightly elevated, 11.2 mmol/L that is mild hyperglycemia.

Lactate is slightly elevated 2.4, mild hyperlactatemia

Cr = 163, consistent with chronic kidney disease.

The final conclusion is: Type 2 respiratory failure, acute on chronic respiratory acidosis on the background of chronic renal failure with mild hyperglycemia and mild hyperlactatemia.

 

Question 2: Answer

when ever we get arterial blood gas values, it is a good practice to calculate the  (A – a gradient). (A-a gradient) is a measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen.  ( PAO2 – PaO2).

To Calculate PAO2 we use the alveolar gas equation: PAO2 = (Barometric pressure – water vapor pressure) x FiO2− (PaCO2/R), for people breathing air O2 at sea level, the equation can be simplified to (150 – PaCO2/ 0.8). So, for this patient, PAO2 = 150 – 76 x 1.25 = 55 mmHg.

So A-a Gradient for this patient is 55 – 31 = 24.

expected A-a Gradient for age is calculated as [age in years/4] + 4 so, for this patient the expected gradient is 83/4 + 4 = 25. So we have normal A-a gradient.

A-a gradient help us to narrow the differential diagnosis of hypoxia.

  • If A-a gradient is within expected range for age then the cause of hypoxemia is either alveolar hypoventilation (increase PCO2) or low PiO2 (FiO2 < 0.21 or barometric pressure < 760 mmHg).

  • if A-a gradient is higher than expected for the age then the cause cane be either Diffusion defect, V/Q mismatch or Right-to-Left shunt.

 

Question 3: Answer

The treatment for acute respiratory acidosis is treating the cause and consider ventilation (BiPAP) to reverse hypercapnia. In severe cases we can consider intubation and mechanical ventilation.

Patients with chronic respiratory acidosis rarely experiences symptoms, as their systems usually compensate for the changes in acid/base balance.

In Patients with chronic respiratory acidosis, PCO2 is high due to decrease responsiveness to hypoxia and hypercapnia.

Patients with chronic COPD have increase dead space volume which leads to increase ventilation perfusion mismatch. Usually they have reduced diaphragm function due to fatigue and hyperinflation.

Accordingly, in patients with chronic respiratory acidosis with very high HCO3 we should consider palliative measures.