Clinical Correlates of pH Levels Problem Set

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 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...

Why does this work??

The Biology Project
Department of Biochemistry and Molecular Biophysics
The University of Arizona
January 1999
Revised: October 2004
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