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 72 Bic 10 BE -26.0 pO2 24 Sat 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 55 Bic 19 BE -10.2 pO2 62 Sat 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!