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Blood gases provide information concerning the
oxygenation, ventilatory, and acid-base status of the patient. Blood
gas results are usually given as pH, Po2,
Pco2, [HCO3–],
base excess or deficit (base difference), and O2 saturation.
This test gives information on acid–base homeostasis (pH,
Pco2, [HCO3–],
and base difference) and on blood oxygenation (Po2,
O2 saturation). Arterial blood gases (ABG) are most commonly
measured; venous, mixed venous, and capillary blood gases are measured
less frequently. Indications for blood gas determinations are as
follows (Respir Care 2001;46:498–505):
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- To determine a patient’s ventilatory (Paco2), acid–base (pH
and Paco2), and oxygenation
and O2-carrying capacity (Pao2 and o2Hb)
- To quantitate the response to therapeutic intervention (eg,
supplemental O2 administration, mechanical ventilation)
or diagnostic evaluation (eg, exercise desaturation)
- Monitoring the severity and progression of documented disease
processes (eg, COPD)
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Normal values for blood gas analysis are given in Table
8–1, and capillary blood gases are discussed in a
following section. Mixed venous blood gases are reviewed in Chapter 20. The bicarbonate
concentration ([HCO3–]) from
the blood gas is a calculated value and should not be used in interpretation
of blood gases; the [HCO3–] from
a concurrent chemistry panel should be used.
Note: The HCO3– values
on the chemistry panel and
those calculated from the blood gases should be about the same.
A major discrepancy (> 10% difference) means one or more
of the three values is in error (pH, Pco2,
or [HCO3–]). The most
common cause of discrepancies is drawing the blood gas and chemistry
panel samples at different times. ABGs and chemistry panels [HCO3–] should
be obtained at the same time for the most accurate interpretation.
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There is little difference between arterial and venous pH and [HCO3–] (except
in severe CHF and shock). Venous blood gas levels may occasionally
be used to assess acid–base status, but venous O2 levels
are significantly less than arterial values (see Table
8–1).
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A CBG is obtained from a highly vascularized capillary bed. CBG
is often ...