aka Metabolic Muddle 008
You are asked to review a pregnant patient (30 weeks gestation) who has been diagnosed with pneumonia. The referring doctor is concerned that she has a metabolic acidosis. She is currently receiving high flow oxygen with an FiO2 of 0.5.
This is her arterial blood gas:
Q1. Describe and interpret the A-a gradient.
Answer and interpretation
- A-a gradient (mmHg) = PAO2 – PaO2
- Using the alveolar gas equation, PAO2 = PiO2 – PaCO2/R; where are is the respiratory quotient, typically with a value of 0.8
- PiO2 = FiO2 x (PB – PH20) = 0.5 x (760 -47) = 357 mmHg; where FiO2 is the fraction of inspirated oxygen, PB is barometric pressure and PH20 is the stand vapour pressure of water. In other words, how much oxygen is in the lungs depends on the percentage of oxygen being breathed in, the atmospheric pressure and how much gas is displaced by the inspired gas being saturated with water.
- PaCO2/R = 29/0.8 = 36 mmHg
- So, PAO2 = PiO2 – PaCO2/R = 357 mmHg – 36 mmHg = 321 mmHg
- And finally the A-gradient is PAO2 – PaO2 = 321 – 123 = 198 mmHg
- The A-a gradient is high. The normal value varies with age and FiO2 as follows:
— At FiO2 0.21: 7 mmHg in young, 14 mmHg in elderly
— At FiO2 1.0: 31 mmHg in young, 56 mmHg in elderly
- The high A-gradient is due to her pneumonia, with the underlying pathophysiological mechanisms being V/Q mismatch and/ or shunt.
In a written exam it is important to perform the full calculation when describing the A-a gradient. On the fly a useful rule of thumb is this:
- The expected PaO2 = FiO2 x 500
In this case the expected PaO2 = 250 mmHg, which is substantially higher than the measured PaO2 of 123 mmHg, also indicating a high A-a gradient.
Finally, another rule of thumb: to estimate the normal A-a gradient you can use the same equation we use for calculating ETT size in children:
- Approximate expected A-a gradient = age/4 + 4
Q2. Describe and interpret the metabolic parameters on the ABG.
Answer and interpretation
- pH 7.42 is normal, but leans towards an alkalemia for the purpose of further acid base interpretation.
- PCO2 29 mmHg indicates a respiratory alkalosis
- HCO3 20 mmol/L indicates appropriate metabolic compensation for the respiratory alkalosis, not primary metabolic acidosis. A drop of ~10 mmHg in PaCO2 from normal (40 mmHg), if there is appropriate compensation, will lead to a fall in HCO3 of 2 mmol/L acutely, or 5 mmol/L chronically from the normal concentration (25 mmol/L)
- Mild hyponatremia, hypokalemia, hypergylcemia
- Anaemia (Hb 84 g/L)
- The apparent metabolic acidosis is actually a compensated respiratory alkalosis, which is normal in the third trimester of pregnancy. Progesterone induces a respiratory alkalosis and the kidney compensates by excreting HCO3.
- The electrolyte abnormalities may have various causes including SIADH due to pneumonia, dehydration, fluid administration, etc, etc.
- Mild hyperglycemia is not unexpected given the relatively insulin resistant state of the third trimester of pregnancy and the stress of an inter-current illness.
- Relative anaemia is expected in pregnancy as the 30-40% expansion in plasma volume exceeds the 20–25 % increase in red cell mass that occurs in pregnancy. However, an Hb <105 g/L in the third trimester suggests other causes may be at play, usually iron deficiency.
- Acid-Base Physiology — Acid base aspects of Pregnancy