PROCEDURE FOR VENOUS BLOOD GAS SAMPLING


  1. Refer to Procedure for Blood
    Sampling from an Indwelling Line
  2. Site Selection and Considerations for Venous Blood Gas Samples
  3. Document


Procedures for drawing blood from an indwelling line or measurement of blood gases can be found in the Procedures menu.

This procedure reviews the types of venous blood gas samples, indications and specific considerations.

PROCEDURE

RATIONALE FOR PROCEDURE

 1.

Refer to Blood Sampling and Blood Gas Procedures

Follow the procedures for Blood Gas Measurement and Blood Sampling from an Indwelling Line.

Follow LHSC policies for hand hygiene and infection control before, during and after procedure. Perform hand hygiene and don a mask with eye shield and non-sterile gloves prior to blood sampling.

LHSC Hand Hygiene Policy

LHSC Routine Practices

LHSC Donning and Doffing Policy

Confirm 2 patient identifiers as per LHSC Policy for Patient Identification



 1.

The flush solution within the catheter dead space must be removed to prevent dilution of the sample.

 2.

VENOUS BLOOD GASES: SITE SELECTION AND CONSIDERATIONS

Venous blood gases can be drawn via several different methods. The location and method of sampling should always be considered when interpreting the results.

Assessment of Carbon Dioxide, Bicarbonate and pH

In the absence of an arterial line, a venous blood gas sample can be used to evaluate carbon dioxide, pH and bicarbonate. Venous blood gases do not evaluate arterial oxygenation, therefore, they must be combined with pulse oximetry values to fully assess ventilation. The role of venous blood gas samples in the evaluation of oxygen delivery is discussed below.

The preferred method for assessment for assessment of acid-base balance is to draw the sample from a central venous catheter. A central venous catheter provides a broader reflection of systemic pH than a peripheral stab. The correlation of central venous blood gases to arterial blood gases (Gold Standard) is also supported by the most research evidence and clinical experience.

A central venous sample is still limited to reflect only the venous return from organs captured by the catheter location (e.g., central venous catheter reflects upper extremity and brain).

In the absence of a central venous catheter, a peripheral venipuncture stab can be used for blood gas measurements for the purpose of screening for systemic acidosis. Attempt to draw sample with < 1 minute of tournequet time**.

Laboratory orders and clinical documentation should clearly identify the origin of a venous sample for blood gas analysis. For example, there should be clear documentation to differentiate whether a venous sample was drawn from a peripheral stab or an indwelling central venous catheter. Central line samples should also identify the site of measurement to differentiate central venous (IJ, PICC, SC), mixed venous (pulmonary artery lumen of PA catheter) or femoral venous sites to aid in the interpretation of results. Gases may also be impacted if a patient has a regional blood flow problem if that area is captured in the blood gas sample (e.g., a blood gas from a right subclavian line in a patient with an ischemic right arm).

 

Correlation between central venous and arterial blood gases:

  Central Venous Peripheral Venous
pH 0.03-0.05 below arterial
0.02-0.04 below arterial
PCO2 4-5 mmHg above arterial 3-8 mmHg above arterial
HCO3 almost same as arterial 1-2 mmol/L above arterial
PO2/SO2 No correlation to arterial No correlation to arterial

 

**Correlation between venous and arterial gases may deteriorate in shock, therefore, arterial confirmation is recommended in hypotensive or critically ill patients. Intermittent correlation between arterial and venous gases is recommended when venous gases are used for serial trending**

 

Confirmation of Venous Access Placement:

Mixed venous (SvO2), central venous (ScvO2) and femoral venous gases may be used to confirm venous placement of a central venous catheter (rule out inadvertent arterial placement).  When using the venous oxygen saturation to rule out arterial placement, be cautious to compare the results to a known arterial sample. A low oxygen level during severe shock could lead to incorrect interpretation.

 

Assessment of Extraction:

Mixed Venous Gases (SvO2 ):

  • Abbreviated SvO2
  • Drawn from the pulmonary artery port of the pulmonary artery catheter
  • Captures blood from the superior and inferior vena cavae and the coronary sinus to reflect a true mixture of all of the venous blood coming back to the right side of the heart
  • Venous blood entering the pulmonary artery is already "mixed" or "averaged", but has not yet been reoxygenated at the pulmonary capillary
  • Reflects the amount of oxygen "leftover" after all of the tissues of the body have extracted oxygen but before the blood is reoxygenated at the lung
  • Is the "Gold Standard" for assessment of oxygen extraction
  • Normal value is 60-80%

Low ScvO Readings:

  • A low SvO2 suggests that tissue oxygen extraction is increased (there is less oxygen leftover)
  • A low SvO2 is most suggestive of increased extraction if it occurs in the setting of a relatively normal arterial oxygen saturation (extraction is truly the difference between arterial and venous oxygen content)
  • Extraction increases when cardiac output alone is insufficient to meet tissue oxygen demand. It is therefore our "second compensatory response".
  • A low venous oxygen saturation (suggesting increased oxygen extraction) is an indication to increase the patient's cardiac output (and oxygen delivery)

Oxygen Delivery = Cardiac Output X Oxygen Content (Hb X SaO2)

  • SvO2 can be used to titrate therapies aimed at raising the cardiac output (e.g., HR, preload, contractility, afterload, SaO2 or Hb manipulation)

High SvO2 Readings:

    • SvO2 may be falsely elevated if the tip of the pulmonary artery catheter is wedged, distally placed or if excessive vacuum has been applied to the sampling syringe. Any of these technical problems can cause oxygenated blood to be pulled from the pulmonary capillary into the syringe,falsely elevating the SvO2 result.
    • Aspiration of air into the blood gas syringe during sampling, or the presence of an air bubble are potential causes for false elevation of SvO2
    • The normal mixed venous PO2 (PvO2) is 40 mmHg. This generally produces an SvO2 of ~70%.  If the PvO2 rises above 60 mmHg, the SvO2  may rise to arterial saturation levels. A high arterial PaO2 during administration of 100% oxygen can produce abnormally high PvO2 and SvO2  values. 
    • Rarely, a high SvO2  reading may indicate failure of the cells to extract. This could occur in end stage multi organ failure or with cell toxins such as cyanide (e.g., in house fires or nitroprusside toxicity). These scenarios would be accompanied by lactic acidosis (final compensation when extraction is also inadequate).
    • High readings are most commonly due to sampling issues or high arterial oxygen concentrations

Central Venous Gases (ScvO2 ):

  • Abbreviated ScvO2
  • Drawn from an internal jugular or subclavian or PICC line
  • Reflects the amount of oxygen "leftover" that is coming from just the head and upper extremities
  • Is a surrogate for SvO2  but it misses the inferior vena cava blood (gut, kidney and low extremities) and coronary sinus, therefore may not correlate with SvO2 during shock
  • Interpret results and utilize to evaluate therapies aimed at improving oxygen delivery as per SvO2 , trends in ScvO2 are more valuable than the absolute value
  • If a patient has a pulmonary artery catheter in place, it is useful to measure SvO2  and ScvO2 just prior to pulmonary artery catheter removal to identify the correlation between these two values prior to switching to ScvO2  monitoring alone

    Low ScvO2 Readings:

    • A low ScvO2 suggests that tissue oxygen extraction is increased (there is less oxygen leftover)
    • A low ScvO2 is most suggestive of increased extraction if it occurs in the setting of a relatively normal arterial oxygen saturation (extraction is truly the difference between arterial and venous oxygen content)
    • Extraction increases when cardiac output alone is insufficient to meet tissue oxygen demand. It is therefore our "second compensatory response".
    • A low venous oxygen saturation (suggesting increased oxygen extraction) is an indication to increase the patient's cardiac output (and oxygen delivery)

    Oxygen Delivery = Cardiac Output X Oxygen Content (Hb X SaO2)

    • SvO2 can be used to titrate therapies aimed at raising the cardiac output (e.g., HR, preload, contractility, afterload, SaO2 or Hb manipulation)

    High ScvO2 Readings:

      • Is not subject to the catheter placement or aspiration technique challenges of the pulmonary artery catheter
      • Aspiration of air into the blood gas syringe during sampling, or the presence of an air bubble are potential causes for false elevation of ScvO2
      • The central venous PO2 (PcvO2) is 40 mmHg. This generally produces an ScvO2 of >70%.  If the PcvO2 rises above 60 mmHg, the ScvO2  may rise to arterial saturation levels. A high arterial PaO2  during administration of 100% oxygen can produce abnormally high PcvO2 and ScvO2  values. 
      • Rarely, a high ScvO2  reading may indicate failure of the cells to extract. This could occur in end stage multiorgan failure or with cell toxins such as cyanide (e.g., in house fires or nitrorusside toxicity).These scenarios would be accompanied by lactic acidosis (final compensation when extraction is also inadequate)
      • An increase in ScvO2   could represent regional extraction failure such as in neurological death or near neurological death.

Femoral Venous Blood Gases:

Femoral venous gases represent the "leftover" oxygen from the lower extremities and sometimes the gut. These values are usually much lower than ScvO2  or SvO2 values and it is unclear how these values should be interpreted.

Femoral venous gases do not correlate to ScvO2 , and in shock when there is gut ischemia, may demonstrate very low oxygen levels. The absolute value is rarely helpful, but the trend in venous gas measurement can be used as a marker of cardiac output adequacy and/or treatment response.

 

Peripheral Venous Gases :

  • Peripheral venous gases are not used to evaluate extraction or oxygenation.
  • They can be used to determine acid-base balance or follow acid-base balance trends.

9.

Document

Document results in the clinical record and communicate any significant findings to the physician and respiratory therapist.

Ensure that documentation in the graphic record and core lab or GEM orders correctly identifies the type of sample (e.g., peripheral venous, central venous or mixed venous)

 9.

To ensure appropriative interpretation and interventions are made.

 

References:

Theordore, A., Manaker, S., and Finlay, G. (March 20, 2013). Venous blood gases and other alternatives to arterial blood gases. www.uptodate.com

Developed: November 28, 1988 (Morgan, B)

Updated: June 30, 2016

Brenda Morgan RN BScN MSc CNCC, CCTC

 

LHSCHealth Professionals

Last Updated June 30, 2016 | © 2007, LHSC, London Ontario Canada