You are asked to review a drowsy 80-year-old male with chronic obstructive pulmonary disease, 6 hours after internal fixation of a fractured hip. He is normotensive, and rousable with stimulation. The following are data from arterial blood.

Barometric pressure

760mm Hg

FiO2

0.4

pH

7.47

pO2

170mm Hg 22.6 (kPa)

pCO2

65mm Hg 8.6 (kPa)

HCO3-

46.6mmol/L

Standard base excess

20.9mmol/L

a) Describe the acid- base status.
b) List four measures which might improve his acid-base status (apart from mechanical ventilation).

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College Answer

Q7.2a) Metabolic alkalosis and respiratory acidosis.
Q7.2b) Cease narcotics; Naloxone (cautious); Reduce FiO2 and titrate to SpO2 90-95%; Reverse metabolic alkalosis (acetazolamide, KCl if hypokalaemia).

Discussion

Let us dissect these results systematically.
 

  1. The A-a gradient is almost normal.
    PAO2 = (0.4 × 713) - (65 × 1.25) = 203.95
    Thus, A-a = ( 203.95 - 170) = 33.95mmHg.
  2. There is no acidaemia
  3. The PaCO2 is raised, suggesting a respiratory acidosis. The history suggests that this is probably the primary disorder.
  4. The SBE is 20.9, suggesting a metabolic alkalosis
  5. The metabolic compensation is excessive. The PaCO2 increase is (65 - 40)/10 = 2.5; if this were an acute respiratory acidosis we would expect a HCO3around 26.5 mmol/L (bicarbonate + 2.5 × 1), and if it were chronic we would expect 34 mmol/L (bicarbonate + 2.5 × 4). There must also be a metabolic alkalosis. 
    If the metabolic alkalosis were thought to be the primary disorder, the respiratory compensation would appear to be excessive: the expected PaCO(0.7 × 46.6) + 20 = 52.6mmHg. Thus, any way you look at it, there is both a metabolic alkalosis and a respiratory acidosis.

So. This 80-year-old has enjoyed a nice dose of perioperative opiates, and is now drowsy. The metabolic alkalosis is preventing acidaemia from developing, and the CO2 climbs ever higher, fogging up the level of consciousness. This is not helped by the generous oxygenation, which has abolished the normal contribution of hypoxic respiratory drive. Imagine: if this patient was on room air, the alveolar gas equation yields a PAOof only 68mmHg; and with his elderly lungs in charge of gas exchange the PaO2 would be in the 50s, driving the respiratory effort.

Thus, there are various ways of dealing with this acid-base disturbance.

  • Stop the opiates.
  • Stop the oxygen. Go with room air, or whatever is required to maintain an SpO2 of 88-90% (the college generously recommends 90-95%)
  • Reverse the opiates with naloxone
  • Stop the diuretics
  • Administer sodium chloride, potassium chloride or ammonium chloride.
  • Administer acetazolamide