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!