PROCEDURE FOR VENOUS BLOOD GAS SAMPLING

PROCEDURE FOR VENOUS BLOOD GAS SAMPLING

Ensure that patient and health care provider safety standards are met during this procedure including:

• Risk assessment and appropriate PPE
• 4 Moments of Hand Hygiene
• Procedural Safety Pause is performed
• Two patient identification
• Safe patient handling practices
• Biomedical waste disposal policies

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

PROCEDURE

 1.

Refer to Blood Sampling and Blood Gas Procedures

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

 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 tourniquet 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:
 

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 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.

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