Venous blood gas
Background
The venous blood gas (VBG) is a multi-component serum assessment of pH, blood gas tensions (PvO2 and PvCO2), bicarbonate (HCO3), and the base excess.
- can be drawn from an IV catheter along with other bloodwork, unlike an arterial blood gas (ABG),
- does not accurately reflect (PaO2)
- arterial values of pH, CO2, and HCO3 can be extrapolated with varying levels of accuracy [1] [2][3]
Components
pH
- measurement of acidity/alkalinity: normal range 7.37-7.45
- pH > 7.45 = alkalosis
- pH< 7.35 = acidosis
- correlates well with arterial pH [4]
PvO2
- measures the partial pressure of oxygen dissolved in the serum; that is, the dissolved oxygen not carried by hemoglobin
- significantly lower than and nonindicative of arterial oxygen pressure (PaO2)
PvCO2
- The amount of carbon dioxide dissolved in arterial blood. The normal range is 35 to 45 mm Hg.
- PvCO2 >45 = primary respiratory acidosis
- PvCO2 <35 = primary respiratory alkalosis
- correlates with controversial accuracy with PaCO2: see discussion below
HCO3
- calculated from PvCO2 and pH with Henderson-Hasselbalch equation
- correlates well with serum HCO3 [5]
Base Excess
- an estimation of the metabolic component of acid/base status: how much "extra base" there is in the system
- the theoretical number of H+ needed to bring pH to 7.40 at pCO2=40
- reference range -2 to +2
- positive in metabolic alkalosis, negative in metabolic acidosis
- can be expressed as "base deficit", with the same but opposite values
- e.g. base excess=-8 in a case of lactic acidosis; base deficit=+8
Comparison with ABG
Disadvantages
- no information about PaO2
- poor correlation with PaCO2
- one study in COPDers shows PvCO2 exceeds PaCO2 by 5.4, but only unreliably so with 95%i nterval -8.8 to +20.5 mmHg[6]
- sensitivity for elevated arterial hypercapnea was 100% in this study with PvCO2>45; authors suggest that PvCO2 can be used to screen for hypercapnea
- metaanalysis shows 95% prediction interval PvCO2 of -10.7 mm Hg to +2.4 mm Hg [7]
- one study in COPDers shows PvCO2 exceeds PaCO2 by 5.4, but only unreliably so with 95%i nterval -8.8 to +20.5 mmHg[6]
Advantages
- convenience
- no extra, painful radial artery access to draw ABG
- can be easily obtained with IV access
- pH very reliable
- some studies do show
- in the era of continuous pulse ox, meaningful information about oxygenation is always available
- the PvCO2 is internally consistent
- can trend CO2 response to treatment; just don't know exact starting point
External Links
http://www.clinicalcorrelations.org/?p=5608 https://lifeinthefastlane.com/ccc/vbg-versus-abg/
References
- ↑ Byrne AL, Bennett M, Chatterji R, Symons R, Pace NL, Thomas PS. Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology. 2014 Feb;19(2):168-75. doi: 10.1111/resp.12225. Epub 2014 Jan 3. Review. PubMed PMID: 24383789.
- ↑ Malatesha G, Singh NK, Bharija A, Rehani B, Goel A. Comparison of arterial and venous pH, bicarbonate, Pco2 and Po2 in initial emergency department assessment. Emergency Medicine Journal : EMJ. 2007;24(8):569-571. doi:10.1136/emj.2007.046979.
- ↑ 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(6):896-900.
- ↑ 1: Brandenburg MA, Dire DJ. Comparison of arterial and venous blood gas values in the initial emergency department evaluation of patients with diabetic ketoacidosis. Ann Emerg Med. 1998 Apr;31(4):459-65. PubMed PMID: 9546014.
- ↑ Nargis W, Rahman AS, Ahamed BU, Hossain MZ. Comparison of values of traditionally measured venous bicarbonate with calculated arterial bicarbonate in intensive care unit patients of a hospital in a third-world country. Nigerian Medical Journal : Journal of the Nigeria Medical Association. 2014;55(4):285-288. doi:10.4103/0300-1652.137186.
- ↑ Kelly AM, Kyle E, McAlpine R. Venous pCO(2) 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. PubMed PMID: 11809551.
- ↑ Byrne AL, Bennett M, Chatterji R, Symons R, Pace NL, Thomas PS. Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology. 2014 Feb;19(2):168-75. doi: 10.1111/resp.12225. Epub 2014 Jan 3. Review. PubMed PMID: 24383789.