VBG's are commonly used as a guide for therapy for patients presenting with SOB, but there has not been much evidence looking at the correlation of pH and PCO2 between arterial and venous samples.
  • Kelly and Klim have looked at the correlation of pH and PCO2 between arterial and venous samples in a group of 53 patients on NIV with 89 sample pairs being evaluated.
  • These were predominantly in patients with COPD (43%) and APO (40%)
  • The mean difference in pH was 0.04 (95% CI -0.02 - 0.11)
  • The mean difference in PCO2 was -8.02 mmHg (95% CI -22.63 - 6.58)
  • They concluded that there was good correlation with the pH, but poor correlation with the PCO2.

Take Home Message - in patients with respiratory distress from COPD and APO on NIV, use an arterial sample to accurately ascertain respiratory status (PCO2).

Reference
Kelly AM, Klim S. Agreement between arterial and venous pH and pCO2 in patients undergoing non-invasive ventilation in the emergency department. EMA 2013 (Early Online - DOI: 10.1111/1742-6723.12066)
 


Comments

04/14/2013 8:23am

Hi,

Thanks for summary on this recent article.

Dr. Kelly actually did a nice review on this in 2010 - http://www.ncbi.nlm.nih.gov/pubmed/21143397

A similar conclusion was reached in that arterial pCO2 and venous pCO2 don't correlate very well (the patient populations was mixed, not just APO/COPD). However venous gases are easy to obtain and usually obtained on first cannulation/blood draw and can be used as a screener (if it's normal on venous, the patient is very unlikely to be hypercarbic - as reflected by the confidence interval in the paper you have reviewed).

If however CO2 is raised on the venous draw, it can certainly assist us in either starting NIV or prompting an arterial sample for proper measurement, but in my experience, getting an arterial sample is of less import in these situations than just getting some NIV on, oxygenating and calming the patient.

I don't think the arterial sample is unnecessary, but I find it less useful in an emergent setting

05/22/2013 7:13pm

The study says they don't correlate doesn't say which one is right, it assumes Arterial is "correct". In an ED setting close clinical observation is better than either and supplemented with the gas and for most of what we do venous seems to work just fine.


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