Introduction
Blood gas analysis is probably one of the most used tests for diagnosis and therapeutic guidance in the emergency departments (EDs) and intensive care units (ICUs).
The evaluation of arterial blood gas (ABG) analysis is commonly used to estimate acid-base status, oxygenation and concentration of carbon dioxide (CO2) in critically ill patients. However, arterial blood (AB) may be difficult to obtain due to weak pulses or movement of the patient. Furthermore, because the thick walls and their innervation, it is more painful for the patient.
Therefore, venous blood gas (VBG) analysis is an alternative to estimate pH and other values in a quicker and easier way.
Venous blood gas analysis
Venous blood (VB) can be obtained from different places. You should always consider the location and the sampling method to interpret the results.
VBG analysis is an alternative for ABG in situations of low peripheral perfusion such as shock states of any etiology.
VBG has been studied in critically ill patients as an alternative in patients who do not have a central venous catheter (CVC) (Tavakol, 2013; Byrne, 2014). If a tourniquet is used to facilitate venous puncture, it should be released approximately a minute before the extraction in order to avoid changes induced by ischemia. (Cengiz, 2009). However, VB is preferred from a CVC given its higher correlation with AB. The values obtained from a VBG and an ABG are interchangeable in clinical practice, in both central VB (Malinoski, 2005; Walkey, 2010; Mallat, 2015) and peripheral VB (Malatesha, 2007; Chu, 2003; Kelly, 2001), except for the values of oxygen saturation (SaO2) and partial pressure of oxygen (PaO2).
| VB Central | VB Peripheral | |
|---|---|---|
|
pH |
0.03 – 0.05 below arterial values |
0.02 – 0.04 below arterial values |
|
PCO2 |
4 – 5 mmHg above arterial values |
3 – 8 mmHg above arterial values |
|
HCO3 |
Minimal variation |
1 – 2 mEq/L above arterial values |
|
PaO2 / SaO2 |
No correlation |
No correlation |
Table 1 – Correlation between venous blood gases and arterial blood gases
Mixed VB (obtained from a pulmonary artery catheter) gives similar results to the values obtained from a CVC. (Ladakis, 2001; Tsaousi, 2010). One should be cautious when interpreting VBG, it has to be always correlated to the clinical state of the patient and if it is necessary, it should be confirmed with an ABG.
Central venous gas analysis
Central VBG analysis allows us to assess the metabolic state of a patient with a good correlation with ABG. Even though central VB is not adequate to assess oxygenation efficacy, this can be estimated by pulse oximetry. Likewise, central VBG analysis gives us central venous oxygen saturation (SatvO2), which is a very sensitive marker of the respiratory, hemodynamic and metabolic homeostatic variations. (Gattinoni, 2017).
Any change in the pulmonary, hemodynamic, metabolic or oxygen transport functions will affect SatvO2. In other words, when we assess SatvO2 value, we are analyzing the result of the interaction between all its determinants:
1) Oxygen input (respiratory system)
2) Oxygen transport (hemoglobin)
3) Oxygen availability DO2 (cardiac output)
4) Oxygen consumption VO2 (tissues).
Gasometric assessment of a central VB sample and its relation with the pulse oximetry will provide us with more information than an ABG analysis.
Global tissue perfusion
In recent year it has been shown that the difference between the value of CO2 obtained from mixed venous blood or central venous blood sample and the value of CO2 obtained from an arterial blood sample is correlated with an increased anaerobic cellular metabolism when the result shows values above 6mmHg. This increase in the veno-arterial CO2 difference is given by an increase of hydrogen in plasma coming from the intracellular environment because of anaerobic metabolism; these hydrogen molecules are buffered in plasma and metabolized to CO2. The causes of the increase in the veno-arterial CO2 difference are mainly due to hypoperfusion secondary to the inadequate cardiac output of mitochondrial dysfunction. (Ospina-Tascón, 2016). Likewise, the quotient of the veno-arterial CO2 difference and the arterio-venous O2 difference has been related with higher accuracy of the tissue perfusion status.
Conclusion
During the assessment of critically ill patients, the analysis of blood gases stands up as a fundamental step in the process of attention. A VBG analysis and SpO2 can give us enough information to make decisions even if there is no ABG analysis available, besides being easy to obtain a sample, implies less pain and less punctures in general. An indication of taking an AB sample is to assess tissue perfusion in severely ill patients.
References
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