Bloedgas
Workshop
Laura Kater
Emergency Physician
Rode Kruis Ziekenhuis, Beverwijk
ROD
17.01.2013
What will we discuss
Warming up
Aa gradient
Arterial vs venous blood gas
1kPa = 7.5 mmHg
Warming up
35 yo female
Dyspnea
FiO2 100%
ABG:
pH 6.76
pCO2
Bic
BE
pO2
Sat
72
10
-26.0
24
94
31 yo male
Dyspnea
FiO2 100%
ABG:
pH
6.72
pCO2 87
Bic 11
BE
-28.7
pO2 301
Sat 97
94 yo female
Dyspnea
FiO2 4 ltr O2 = ongeveer 30%?
ABG:
pH 7.15
pCO2
Bic
BE
pO2
Sat
55
19
-10.2
62
83
Aa
Gradient
Aa gradient =
alveolar minus arterial oxygen pressure
pAO2 – paO2
pAO2: calculation
paO2: measurement in arterial blood gas
Why is this important?
pAO2
pAO2 (mmHg) = 7x %O2 – paCO2 – 10
So at room air: 7x21 – 40 – 10 = 97 mmHg
Example
65 yo, room air (FiO2 21%)
ABG 7.44 / 29 / 88 / 19 / -3 / 95%
pAO2 = 7x21 – 29 – 10 = 108 mmHg
paO2 in ABG = 88 mmHg
Difference = Aa gradient = 20 mmHg
Is that normal???
Aa gradient
Aa max = age / 3 + pAO2 / 5 – 23
Our pt:
65 / 3 + 108 / 5 – 23 =
21.67 + 21.6 – 23
= 20.27
Calculated gradient was 20.
Another one
61 yo, FiO2 30%
ABG 7.02 / 22 / 146 / 6 / -24 / 98%
pAO2 = 7x30 – 22 – 10 = 178 mmHg
paO2 = 146
Aa gradient = 32
Aa max for this age:
61/3 + 178/5 – 23
=
20.3 + 35.6 – 23
=
32.9
Conclusion: normal Aa gradient
En nu?
30 yo, non rebreather (FiO2 +/-80%)
ABG: 7.40 / 40 / … / 25 / 0 / 100%
What pO2 do you expect?
pAO2 = 7x80 – 40 – 10 = 510 mmHg
Max Aa gradient= 30/3 + 510 / 5 – 23
= 10 + 102 – 23 = 89
Expected paO2 in ABG is about 421 mmHg
Last…
74 yo, room air
ABG: 7.42 / 39 / 62 / 25 / 1 / 90%
pAO2 = 7x21 – 39 – 10 = 98 mmHg
paO2 = 62
Aa gradient = 36
Aa max = 74/3 + 98/5 – 23
=
24.7 + 19.6 – 23
= 21
Aa gradient 15 mmHg to high = low paO2
Pulm.problem?, shunt / VQ mismatch?
Lifeinthefastlane.com
Arterial
or
Venous
Use bloodgas in ED
acid-base status
pH, bicarbonaat
respiratory function
pCO2, sometimes pO2
Why venous?
Painful arterial punction
Hematoma after art.punction
Easy to sample a venous one
when you’re already drawing
blood for standard labs
BUT
Is a venous bloodgas clinical
equivalent to arterial?????
EMRAP june 2008 summarizes a few
publications:
(www.emrap.org)
A few publications in:
Annals of EM april 1998
In diabetic ketoacidosis in adults the venous
blood gas measurements accurately
demonstrate the degree of acidosis. Mean
difference between arterial and venous pH was
0.03 (range 0.0-0.11)
EM Journal sept 2001
strong correlation between arterial and venous
pH, difference 0.4.
Journal EM jan 2002
Very good agreement in pH with venous
samples being -/-34 units lower than arterial
samples.
pCO2 on average 5.8 mmHg higher in venous
samples
Canadian Journal EM 2002
pH art an venous difference 0.36
pCO2 6 mmHg
HCO3- 1.5
Annals of EM 2005
very good correlation between arterial and
venous pH and HCO3-
EM Australasia feb 2006
in pts with DKA the weighted average
differences between arterial and venous
pH was 0.02
bic -1.88
For details look at the studies
Resus.me
Cliff Reid:
Professor Anne-Maree Kelly, June 2009
pH - Close enough agreement for clinical
purposes in DKA, isolated metabolic disease; more
work needed in shock, mixed disease
Bicarbonate - Close enough agreement for clinical
purposes in most cases; more work needed in shock, mixed
disease, calculated vs measured gap
pCO2 – NOT enough agreement for clinical purposes;
potential as a screening test
Base excess – Insufficient data
Lim and Kelly
Eur J of EM 2010
Available evidence suggests that
there is good agreement for pH and HCO3
values between arterial and pVBG results in
patients with COPD, but not for pO2 or pCO2.
Widespread clinical use is limited because of the
lack of validation studies on clinical outcomes
Questions???
Take home message
When in doubt of a pulmonary problem
causing hypoxia, use the Aa gradient to
calculate if the oxygen you give your patiënt
correlates with the paO2 in the ABG
Think before you ask for an ABG. What do you
need to know, will a venous blood gas do?
When in need of an ABG: local anesthesia.
Thank you!