Venous Blood Gas Versus

Arterial Blood GasAnalysis

Ping-Wei ChenPGY-2

Emergency Medicine

It’s Go Time…

• 25 yo female • Single vehicle rollover near Sundre• Intubated for deteriorating GCS• In the ED:– BP 70 palp, HR 122– Not responding to painful stimuli

• 01:23 - VBG ordered as part of workup• 01:41 – ABG ordered as part of workup

Objectives

• Controversy• Can VBGs replace ABGs?• When are VBGs and ABGs different?• When might I want an ABG?• NOT covered– Electrolytes– Lactate

What’s all the fuss about?

Arterial Blood Gas• PAINFUL• Arterial injury• Thrombosis with distal

ischemia• Hemorrhage/hematoma• Aneurysm formation• Median nerve damage• Infection• Needlestick injury• Reflex sympathetic dystrophy

Venous Blood Gas• Samples can be drawn

simultaneously at time of venipuncture

• Should be done without tourniquette

• More difficult to obtain in pulseless patients

• Controversy regarding level of agreement with arterial values

• Prospective, observational study• 218 subjects, ED population

dyspnea, DKA, renal failure, seizures, ↓LOC, ingestions, ischemic colitis

• A priori definition of clinically important difference

• Pearson correlation coefficient• Bland-Altman plots

Rang et al. 2002. Can J Emerg Med 4(1):7

Results

• Excellent correlation pH (r= 0.913) pCO2 (r=0.921) calculated HCO3 (r=0.953)

Results• Clinically Important Differences

26/45 physicians responded

Result

• Mean Differences pH 0.036 (0.030-0.042) pCO2 6.0 mm Hg (5.0-7.0)

HCO3 1.5 mEq/L (1.3-1.7)

• Prospective, observational study• 246 subjects, ED population

acute respiratory disease, suspected metabolic disorder

• pH only• Results:

Excellent correlation r=0.92 Mean difference: 0.04 pH units (-0.11 to +0.04)

Kelly et al. 2001. Emerg Med J. 18:340

• Prospective, observational study• 95 patients, ED population

AECOPD, pneumonia, sepsis, ARF/CRF, DKA, ACS, acute gastroenteritis, SLE, toxic ingestion

Bland-Altman Analysis• Results: ABG compared to VBG

Mean Difference 95% Limits of Agreement

pH 0.015 -0.1 to 0.13

PCO2 -3 -7.6 to 6.8

HCO3 -0.74 -5.8 to 4.3

PO2 65 -32.9 to 145.3

• Review article: 6 studies• pH and HCO3

• Results:– Mean difference • pH: 0.02 (-0.009 to 0.021), n=258, DKA patients only• pH: 0.037 (-0.11 to 0.04), n =763, respiratory/metabolic

illness• HCO3: -1.88 mEq/L (N/A), n =21, DKA patients only• HCO3: -0.99 mEq/L (-2.73 to 5.13), n=763,

respiratory/metabolic illness

When are they different?

• Weil et al. 1986. Difference in acid-base state between venous and arterial blood during cardiopulmonary resuscitation. NJEM. 315:153-6.– Prospective, observational study (n=16) – ICU/CCU patients – Arteriovenous gradient

Pre-Arrest Arrest

pH 0.06±0.02 0.30±0.05

pCO2 11±2 mmHg 36±6mmHg

So when might I want an ABG?

• Unable to establish IV access• Inability to obtain sample• Inability to obtain O2 saturation by pulse

oximeter– Peripheral vasoconstriction– Abnormal hemoglobins • Carboxyhemoglobin• Methemoglobin• Sickle hemoglobin

N = 1

• VBG at 01:32– pH 7.11/pCO2 41/HCO3 14/lactate 6.6

• ABG at 01:41– pH 7.12/pCO2 34/HCO3 11/lactate 6.1

Conclusions

• VBGs not interchangeable with ABGs BUT– Excellent correlation with ABG values– Reasonable agreement on VBG for clinical

decision making in ED• pH 0.02-0.04 lower• PCO2 3-6 mmHg higher

• HCO3 essentially the same

• Consider ABG in: – Inability to obtain sample– Inability to utilize pulse oximeter

Questions?