Biochemistry at The Biology Project

Clinical Correlates of pH Levels
Problem Set

Problem 9: Hyperventilation

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

Patient's
Value
Normal
Value
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 occurence 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 decreased decreased decreased decreased
HCO3-:CO2 ratio decreased decreased decreased decreased
[HCO3-] increased decreased decreased decreased decreased decreased
pCO2 decreased decreased decreased decreased decreased increased
total CO2 decreased decreased decreased decreased decreased decreased
increased = increased
decreased = decreased
= no major change
U = uncompensated
C = compensated
increasedincreased = Red arrows indicate the primary defect.
increasedincreased = 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 bicarb 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...

Why does this work??

[Problem 9] [Answer]

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The Biology Project
The University of Arizona
January 19, 1999
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