## Clinical Correlates of pH LevelsProblem Set

### Problem 9: Hyperventilation

A 19-year-old male presents to the emergency room with the following blood gases:

 Patient'sValue NormalValue pH 7.55 7.35-7.45 pCO2 25 mm Hg 35-45 mm Hg [HCO3-] 22 mmol/liter 24-28 mmol/liter

His companions report that he was "trying to set the world record for holding his breath underwater", but had fainted after hyperventilating for 15 minutes.

What condition does this patient appear to have?

### Tutorial

Respiratory alkalosis

Respiratory alkalosis is not a common occurrence and, in this case, was due to a deliberate (if misguided!) act of prolonged hyperventilation. This is actually rather dangerous since the brain is sensitive to elevated pH and also because the bicarbonate buffering system is less help at pH >7.4.

 respiratory metabolic acidosis alkalosis acidosis alkalosis U C U C U C U C pH HCO3-:CO2 ratio [HCO3-] pCO2 total CO2
 = increased = decreased = no major change
 U = uncompensated C = compensated = Red arrows indicate the primary defect. = Green arrows indicate compensation mechanisms.

Being a respiratory problem, the primary defect shows up in the pCO2 levels (see table above). The low pCO2 pulls the bicarbonate equilibrium to the left, lowering [H+] and thus raising pH:

 CO2 + H2O [H2CO3] H+ + HCO3-

Compensation & treatment
 The body's way of compensating for a respiratory problem is by kidney action (in this case, expelling HCO3-), as indicated by the green arrow above. This pulls the equilibrium back to the right, moving the [H+] back closer to the normal range. The kidney also has a limited ability to retain H+ also, but this all takes time. As a short-term (and very low tech) aid to your patient, you can simply... answer Put a paper bag over his head! or, a more dignified option, give him an oxygen mask not hooked up to anything Why does this work?? answer Expelled CO2 is retained and reinhaled by the patient, restoring pCO2

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