Venous Blood Gas Versus Arterial Blood Gas Analysis

1
Venous Blood Gas VersusArterial Blood
GasAnalysis

• Ping-Wei Chen
• PGY-2
• Emergency Medicine

2
Its 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
• 0123 - VBG ordered as part of workup
• 0141 ABG ordered as part of workup

3
Objectives

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

4
Whats all the fuss about?

• Arterial Blood Gas

• Venous Blood Gas

• PAINFUL
• Arterial injury
• Thrombosis with distal ischemia
• Hemorrhage/hematoma
• Aneurysm formation
• Median nerve damage
• Infection
• Needlestick injury
• Reflex sympathetic dystrophy

• 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

5

• 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
6
Results

• Excellent correlation
• pH (r 0.913)
• pCO2 (r0.921)
• calculated HCO3 (r0.953)

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Results

• Clinically Important Differences
• 26/45 physicians responded

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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)

9

• Prospective, observational study
• 246 subjects, ED population
• acute respiratory disease, suspected metabolic
  disorder
• pH only
• Results
• Excellent correlation r0.92
• Mean difference 0.04 pH units (-0.11 to 0.04)

Kelly et al. 2001. Emerg Med J. 18340
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• 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
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• Review article 6 studies
• pH and HCO3
• Results
• Mean difference
• pH 0.02 (-0.009 to 0.021), n258, 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), n763,
  respiratory/metabolic illness

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When are they different?

• Weil et al. 1986. Difference in acid-base state
  between venous and arterial blood during
  cardiopulmonary resuscitation. NJEM. 315153-6.
• Prospective, observational study (n16)
• ICU/CCU patients
• Arteriovenous gradient

Pre-Arrest Arrest
pH 0.060.02 0.300.05
pCO2 112 mmHg 366mmHg
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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

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N 1

• VBG at 0132
• pH 7.11/pCO2 41/HCO3 14/lactate 6.6
• ABG at 0141
• pH 7.12/pCO2 34/HCO3 11/lactate 6.1

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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

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Questions?