Blood Gas Analysis Flashcards
what value is considered hypoxemic
PaO2 < 80 mmHg
what value is considered severely hypoxemic
PaO2 < 60 mmHg
what are the causes of hypoxemia
- low PaO2 (low PIO2 or hypoventilation)
- decreased gas exchange (increased venous admixture from V/Q mismatch, R to L shunt, diffusion defects)
are mucus membranes a good indicator of oxygenation
NO - does not turn cyanotic until PaO2 = 40 mmHg or less (severe hypoxemia)
can have PaO2 = 40-80 and still have pink mucus membranes
how does PCV affect cyanosis
anemia: decreases the PaO2 at which the animal becomes cyanotic (PaO2 < 40 mmHg)
polycythemia: increases the PaO2 at which the animal becomes cyanotic (PaO2 > 40 mmHg)
what is the most accurate measure of oxygenation
PaO2
what are two methods you can use to determine the cause of hypoxemia on a patient breathing room air
A-a gradient
120 rule
what are two methods you can use to determine the cause of hypoxemia on a patient breathing supplemental O2
5x rule
P/F ratio
can venous samples be used to assess oxygenation
NO
A-a gradient
PAO2 - PaO2
PAO2: calculate from alveolar gas equation
PaO2: measured on arterial blood gas
normal A-a gradient
< 15 mmHg
cause of hypoxemia with a normal A-a gradient
low PaO2:
1. hypoventilation - assess PaCO2 on blood gas
2. low PIO2 - unlikely on room air
cause of hypoxemia with an increased A-a gradient
increased venous admixture
1. V/Q mismatch
2. R to L shunt
3. Diffusion defect
120 rule
PaO2 + PaCO2
if > 120 mmHg: normal A-a gradient
if < 120 mmHg: increased venous admixture
5x rule
expected PaO2 = 5 x FIO2% of supplemental O2
If on 100% O2 –> expect PaO2 = 500 mmHg
If PaO2 < 500 mmHg –> indicates decreased lung function
PaO2/FIO2 ratio
PaO2 / FIO2 (decimal) = 500 mmHg
normal PaO2/FIO2 ratio
500 mmHg
PaO2/FIO2 ratio for mild lung dysfunction
200-300 mmHg
PaO2/FIO2 ratio for moderate dysfunction
100-200 mmHg
PaO2/FIO2 ratio for severe dysfunction
<100 mmHg
SpO2
pulse oximetry; less accurate but easier to use
SpO2 for severe hypoxemia
< 90%
indicates PaO2 < 60 mmHg
SpO2 for hypoxemia
90-95%
indicates PaO2 = 60-80 mmHg
normal SpO2 for room air
95-98%
indicates PaO2 = 80-120 mmHg
normal SpO2 for supplemental O2
99-100%
indicates PaO2 = 120-500 mmHg
(narrower range of SpO2 due to decreased sensitivity at high PaO2)
what is the most accurate/only measurement of ventilation
PaCO2
hypoventilation
increased PaCO2
hyperventilation
decreased PaCO2
when can you use a venous sample to assess ventilation
only if the patient is hyperventilating
PvCO2 is always higher than PaCO2 - can assume low PvCO2 = low PaCO2 (hyperventilation)
can NOT assume PvCO2 means hypoventilation unless the patient has normal CV function
