NICU: Respiratory: Blood Gas Interpretation
Blood gases are obtained from arteries, veins and capillary beds to assess
the acid-base status of the blood, and the amount of carbon dioxide and
oxygen in the blood. In addition to these direct measurements, bicarbonate
ion may be either measured or calculated. In addition, the base excess
is calculated based on these values. Certain blood gas machines also
provide information such as hematocrit, electrolytes, ionized calcium and
glucose concentrations. An arterial blood gas measurement is the
most useful for measuring oxygenation. A venous blood gas is useful
for examining tissue extraction of oxygen, and capillary blood gases are
a convenient way of monitoring the patient's response to ventilator weaning
when arterial access is not available.
A blood gas is traditionally written as:
| pH |
pCO2 |
pO2 |
HCO3- |
B.E. |
FiO2 |
The normal values are:
| Type of Blood Gas |
pH |
pCO2 |
pO2 |
HCO3- |
BE |
| ABG |
7.35-7.45 |
35-45 |
depends |
depends |
-2 to 2 |
| CBG |
|
|
|
|
|
| VBG |
|
|
|
|
|
Samples ABGs
Example 1:
History: DOL 7 in 26 weeker.
Interpretation: The pH indicates a severe acidosis, and a pCO2
of 72 is definite hypercarbia. Recall that each change in pCO2 of
10 mm Hg results in a pH change of 0.08 in the opposite direction.
So, a rise of 30 mmHg in pCO2 should lower the pH by 0.24 (=3 * 0.08) and
7.16 is close to 7.17. A quick look at the small BE of -1.5 (or base
deficit of 1.5) reveals little metabolic contribution to this respiratory
acidosis. Note also the hypoxia. Better check the ET tube.
Example 2:
History: 12 hour old, septic 28 weeker.
Interpretation: This patient is acidotic too, but the pCO2 is
not high enough to account for this. Note the base excess of -10.6
and the very low bicarbonate. This is a metabolic acidosis.
The pO2 is acceptable.
Example 3:
History: DOL 104 in a 24 weeker.
Interpretation: The pH is mildly low, but certainly not as low
as you would expect with a pCO2 of 68. Putting the picture together,
you note the high HCO3 and BE. This is a chronic respiratory acidosis
with metabolic compensation. You will see ABGs like this in older
preemies with severe BPD. Again, the
pO2 is acceptable.
See also: Adjusting
vent settings to optimize ABG values
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