Bloedgas Workshop
Laura Kater Emergency Physician Rode Kruis Ziekenhuis,
BeverwijkROD 17.01.2013
What will we discuss
Warming up
Aa gradient
Arterial vs venous blood gas
1kPa = 7.5 mmHg
Warming up
35 yo femaleDyspneaFiO2 100%ABG: pH 6.76
pCO2 72Bic 10 BE -26.0 pO2 24 Sat 94
31 yo maleDyspneaFiO2 100%ABG: pH 6.72
pCO2 87Bic 11BE -28.7pO2 301Sat 97
94 yo femaleDyspneaFiO2 4 ltr O2 = ongeveer 30%?ABG: pH 7.15
pCO2 55Bic 19BE -10.2pO2 62Sat 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 mmHgpaO2 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 mmHgpaO2 = 146
Aa gradient = 32Aa 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 mmHgMax Aa gradient= 30/3 + 510 / 5 – 23
= 10 + 102 – 23 = 89
Expected paO2 in ABG is about 421 mmHg
Last…
74 yo, room airABG: 7.42 / 39 / 62 / 25 / 1 / 90%pAO2 = 7x21 – 39 – 10 = 98 mmHgpaO2 = 62Aa gradient = 36Aa max = 74/3 + 98/5 – 23 =
24.7 + 19.6 – 23 = 21Aa 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 0.36pCO2 6 mmHgHCO3- 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 0.02bic -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!