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Three Part Question

In [patients with an Acute Exacerbation of COPD] can a [normal venous blood gas CO2] [rule out arterial hypercarbia]?

Clinical Scenario

A 74 year old male patient with known COPD presents acutely breathless with widespread wheeze. He refuses an arterial blood gas (ABG) and complains that last time he was here it took a long time to get the sample and it was very painful. You have already obtained a venous blood gas which has a PaCO2 of 5.5kPa. You wonder if this is sufficient to rule out arterial hypercarbia, and therefore, is an ABG in this patient an unnecessary test?

Search Strategy

Ovid MEDLINE(R) 1948 to week 4 November 2014. Embase and CINHAL databases via the Athens gateway. Cochrane database of systematic reviews. Date of search is 31/07/2013.
Medline:[chronic obstructive pulmonary disease.ti,ab OR chronic obstructive airway* disease.ti,ab OR COPD.ti OR COAD.ti OR BRONCHITIS OR BRONCHITIS, CHRONIC OR EMPHYSEMA OR PULMONARY EMPHYSEMA] AND [venous blood gas*.ti,ab OR vbg*.ti,ab OR venous co2.ti,ab OR [exp BLOOD GAS ANALYSIS AND VENOUS] OR [hypercarbia.ti,ab AND VEOUS] OR [exp HYPERCAPNIA AND VENOUS] OR [exp CARBON DIOXIDE/bl [bl=Blood] AND VENOUS] LIMIT to English Language

EMBASE: [chronic obstructive pulmonary disease.ti,ab OR chronic obstructive airway* disease.ti,ab OR COPD.ti OR COAD.ti OR BRONCHITIS OR BRONCHITIS, CHRONIC OR EMPHYSEMA OR PULMONARY EMPHYSEMA] AND [venous blood gas*.ti,ab OR vbg*.ti,ab OR venous co2.ti,ab OR [exp BLOOD GAS ANALYSIS AND VENOUS] OR [hypercarbia.ti,ab AND VEOUS] OR [exp HYPERCAPNIA AND VENOUS] OR [exp CARBON DIOXIDE/bl [bl=Blood] AND VENOUS] LIMIT to English Language

Search Outcome

17 papers where identified in total. 11 were of sufficient quality and relevance for inclusion.

Relevant Paper(s)

Author, date and country	Patient group	Study type (level of evidence)	Outcomes	Key results	Study Weaknesses
Elborn et al. 1991 Ireland	48 Inpatients with COPD	Single centre prospective study	Difference between arterial and venous CO2 values	No significant difference between the arterial and venous CO2 tensions (PaCO2 41+/- 9.5mmHg, PvCO2 42+/-10.6mmHg), with the two being closely related (r=0.84, p<0.001)	Small sample. No power calculation. Single centre. No inclusion or exclusion criteria stated.
Rang et al. 2002 Canada	218 patients presenting to a single centre requiring blood gas analysis for any reason.	Single centre prospective convenience study	Correlation between arterial and venous CO2	r=0.921	Small cohort. Single centre. Cohort not limited to COPD.
Kelly et al. 2002 Australia	201 patients presenting with 'acute respiratory illness or potential ventilatory compromise'	Single centre prospective convenience study	Sensitivity and specificity of venous pCO2 to detect arterial normocarbia with a cut off value of 6kPa	Sensitivity 100%. Specificity 57.1%	Single centre. No power calculation. Population not limited to patients with COPD.
Kelly et al. 2005 Australia	107 patients presenting to a single centre with COPD	Single centre prospective validation study	Sensitivity of venous pCO2 to detect arterial normocarbia with a cut off value of 6kPa	Sensitivity 100%. 95% CI; 91-100%	Single centre. Small sample size. No power calculation. Limited demographics available for interrogation.
Ak et al. 2006 Turkey	132 patients presenting with AECOPD	Single centre prospective observational study using convenience sampling.	Correlation between arterial and venous CO2	r=0.908	Single centre. No power calculation. No CI provided for statistics. Cohort limited to COPD but not excluding other metabolic disorders. Possible confounders: high altitude and skewed populations towards men.
			Equation to estimate arterial pCO2 from venous pCO2 using linear regression	Arterial pCO2 = 0.873 x venous pCO2
			utility of venous pCO2 to detect arterial hypercarbia (>46mmHg)	100% sensitivity and NPV. 47% specifity
Razi and Moosavi. 2007 Iran	107 patients presenting with COPD and type 2 respiratory failure (pCO2 > 45mmHg). Convenience sampling used.	Single centre prospective observational study.	Correlation between arterial and venous CO2	r=0.761	Single centre. No power calculation. Cohort limited to hypercarbic COPD patients. Excessive exclusion criteria. Skewed population towards men.
Lim and Kelly 2010 Australia	Meta-analysis of the literature on the use of peripheral VBGs in ED patients with COPD.	Meta-analysis of prospective observational studies	The weighted average difference for pCO2	5.92mmHg	Limits of heterogenous individual studies included in analysis. Only 6 studies. 3 of which not limited to COPD population. 1 author of meta-analysis is also author of 2 of included studies.
Ibrahim et al. 2011 Singapore	122 patients requiring ABG analysis as decided by treating physician	Single centre cross-sectional study	Sensitivity and negative predictive value of excluding arterial hypercarbia with a venous PCO2 of below 30mmHg	100% sensitivity and 100% NPV	Single centre. No power calculation. Cohort not limited to COPD. Possible interpreter bias – pCO2 threshold calculated retrospectively.
McCanny et al. 2012 Australia	89 patients presenting with COPD. Paired arterial and venous blood gas analyses.	Single centre prospective observational study	Sensitivity of venous pCO2 to detect arterial normocarbia with a cut off value of 6kPa	100% sensitivity	Single centre. Underpowered sample size. Small cohort. Convenience sampling used.
Kelly 2013 Australia	529 Patients presenting with COPD across 4 studies	Systematic review	Sensitivity of venous pCO2 to detect arterial normocarbia with a cut off value of 45mmHg	100% sensitivity	Reliance on validity of included studies.
Sur 2013 Scotland	Patients presenting with an acute exacerbation of COPD to a Scottish urban ED had arterial and venous blood gas analyses. 68 paired samples were compared over a 2 month period.	Prospective observational study	Correlation between arterial and venouspCO2	Pearson's r=0.973, but 95% LOA -4.94 to 14.26 mmHg	Results presented as a poster. Blood gas analysis performed at physician's discretion rather that according to preset criteria.
			Detection of arterial hypercarbia, pCO2>45 mmHg	31 patients (46%), all cases detected by venous pCO2>45 mmHg. Sensitivity 100% (95% CI 89-100%), specificity 86% (95% CI 71-95%).

Comment(s)

Arterial blood gas analysis is conventionally a routine test in the assessment of patients with AECOPD. One has to question this practice. Arterial blood gas analysis has many complications including severe pain, failure of procedure, haematoma formation, aneurysm formation, arterial laceration, sepsis and rarely loss of limb. This must be weighed up with the benefit of the procedure, and the principle of 'first do no harm' born into mind. These studies demonstrate that if the venous pCO2 is within range then this excludes arterial hypercarbia. One may argue an arterial sample is still required to assess the pO2, however, the BTS guidelines support using transcutanous oxygen saturations to titrate O2 therapy.

Editor Comment

AECOPD, acute exacerbation of chronic obstructive pulmonary disease; COPD, chronic obstructive pulmonary disease; ED, emergency department; LOA, limits of agreement; NPV, negative predictive value; pCO2, partial pressure of carbon dioxide; VBG, venous blood gas.

Clinical Bottom Line

In patients presenting with AECOPD, if they have a normal pCO2 on a VBG they do not need an ABG to exclude hypercarbia.

References

1. Elborn JS, Finch MB, Stanford CF. Non-arterial assessment of blood gas status in patients with chronic pulmonary disease. The Ulster Med J 1991; 60(2): 164-167.
2. Rang LCF, Murray HE, Wells GA, MacGougan CK. Can peripheral venous blood gases replace arterial blood gases in emergency department patients? CJEM 2002;4(1): 7-15.
3. Kelly AM, Kyle E, McAlpine R. Venous pCO2 and pH can be used to screen for significant hypercarbia in emergency patients with acute respiratory disease J Emerg Med 2002 Jan; 22(1): 15-9.
4. Kelly AM, Kerr D, Middleton P. Validation of venous pCO2 to screen for arterial hypercarbia in patients with chronic obstructive airways disease J Emerg Med 2005; 28(4): 377-379.
5. Ak A, Ogun CO, Bayor A et al. Prediction of arterial blood gas values from venous blood gas values in patients with acute exacerbation of chronic obstructive pulmonary disease Tohoku J Exp Med 2006; 210: 285-290.
6. Razi E, Moosavi GA. Comparison of arterial and venous blood gases analysis in patients with exacerbation of chronic obstructive pulmonary disease Saudi Med J 2007; 28(6): 862-865.
7. Lim BL, Kelly AM. A meta-analysis on the utility of peripheral venous blood gas analysis in exacerbations of chronic obstructive pulmonary disease in the emergency department Eur J Emerg Med 2010; 17: 246-248.
8. Ibrahim I, Ooi SBS, Huak CY, Sethi S. Point-of-care bedside gas analyzer: limited use of venous pCO2 in emergency patients J Emerg Med 2011 41(2): 117-123.
9. McCanny P, Bennett K, Staunton P, McMahon G. Venous vs. arterial blood gases in the assessment of patients presenting with an exacerbation of chronic obstructive pulmonary disease Am J Emerg Med 2012; 30: 896-900.
10. Kelly AM. Agreement between arterial and venous blood gases in emergency medical care: a systematic review Hong Kong J Emerg Med 2013 20(3): 166-171.
11. Sur, E COPD: is it all in the vein? Thorax 2013: 68 (Suppl 3): P182