ABG's can tell us some
important things about the body's oxygenation. Every living cell in the body
needs oxygen. Hemoglobin binds and then releases O2 in response to physiologic
- At sea level, pH of 7.4,
37 degrees C, and room air, only 3% of the body's oxygen is dissolved in the
- PaO2 is a
measurement of the partial pressure of oxygen dissolved in the plasma
only. It is measured in mm Hg (millimeters of mercury).
- The PaO2 does
not tell us about the body's total oxygen content, but it does
indicate how much oxygen was available in the alveoli to dissolve in the
measures the total oxygen in the blood
- The remainder of the
body's oxygen is carried attached to hemoglobin molecules.
- SaO2, or
oxygen saturation, measures the degree to which oxygen is bound
- Sa02 is expressed
as a percentage, which is calculated by dividing the maximum oxygen capacity
into the actual oxygen content and multiplying by 100.
- Oxygen's affinity
for hemoglobin changes depending on pH and temperature (see Oxyhemoglobin Dissociation
- Each hemoglobin molecule
has four oxygen binding sites. When those sites are occupied, the hemoglobin
molecule cannot hold any more. This is the oxyhemoglobin molecule.
- Hemoglobin binding
sites can hold molecules other than oxygen.
- For example,
smokers and people exposed to smoke, automobile exhaust, or other
chemicals can have hemoglobin saturated with carbon monoxide (CO).
(HbCO) is a hemoglobin molecule that has carbon monoxide where
the oxygen should attach. The blood will have a cherry red color.
(MetHb) is produced when certain poisons or a genetic condition affect
the iron portion of the hemoglobin subunit. It changes blood to a
- The presence of either
carbon monoxide or methemoglobin changes the affinity of oxygen for hemoglobin.
Oxygen is much less likely to be carried on the hemoglobin molecule.
- The hemoglobin
molecule in these conditions is unusable. If enough hemoglobin is
inactivated like this, it can cause tissue hypoxia.
- Hypoxia, or reduced
oxygen supply to the tissues, can occur even in the presence of 100%
oxygen. This can be a life-threatening condition.
carboxyhemoglobin molecule carries the same amount of oxygen as oxyhemoglobin.
- Hypoxia can result from ventilation or
How and when do body tissues become
|| Too little oxygen
reaches the alveoli
|| Too little blood
reaches the alveoli
term shunt is used to describe a situation where there is ventilation
without oxygenation. Some textbooks call a mild shunt a "mismatch."
can be done to determine the extent of the problem.
- It is normal for
a small percentage of air in the lungs not to reach blood. This is "dead
space". It's an anatomical necessity. Air in the nasopharynx, trachea
and bronchi does not reach the alveoli before exhalation.
- More than this
amount of "dead space" can lead to hypoxia.
- Circulatory problems
that result in a shunt are called "V-Q shunt "
or "V-Q mismatch", where:
- V represents
- Q represents
usually stands for "pulmonary", so apparently the next letter
in the alphabet was used for "perfusion.")
- V and
Q can be compared directly using a nuclear medicine procedure
called a "V-Q scan" or a "lung scan."
- When dealing with
shunts and mismatches, you may also come upon a value known as "A-a gradient."
- This value has
traditionally been used to compare oxygenation of the alveoli and
that of the arteries.
- Recent studies
suggest that using "A/a ratio" can give a more accurate prediction of V-Q
- Newer studies
have shown a much less complex formula, FiO2-PaO2, may be just as accurate.
Points to remember:
- PaO2 is oxygen
dissolved in plasma, not total O2.
- SaO2 is saturated
hemoglobin minus HbCO and MetHb.
- Ventilation does not
- A shunt is normal alveolar
O2, but low blood O2.
hypoxia can result from either a ventilation or a circulatory problem.