Answering your questions

QUESTION

As part of our ICU’s sepsis protocol, we need to obtain a ScvO₂ measurement. We have been asked to build a ScvO₂ test in our LIS so it can be ordered, but we have not been provided with much information on it from a lab perspective, such as: what measurement are we doing? I have done some research, and, according to what I’ve read, it seems to be the O₂ saturation measured on central venous blood, obtained using a distal lumen port off of a triple lumen catheter, which is blood returning to the right atrium via the super vena cava.  The ICU Director wants us to report the entire blood gas analysis (pH, pCO₂, etc.) in addition to the O₂ sat.
Can you please tell me, first, am I correct in my conclusion about the laboratory measurement of ScvO₂; and, second, what reference ranges should we use for this type of specimen?

We have been instructed not to use our existing venous blood gas test order, but to build a specific test for this particular specimen.

ANSWER

Each year, approximately 750,000 patients develop severe sepsis. At least a third die, and sepsis is the leading cause of death in non-cardiac ICUs.^1,2 Concerns about tissue hypoxia play a key role in patient management, requiring constant monitoring. To reduce patient mortality, a campaign was initiated through the Institute for Healthcare Improvement (IHI), creating a “Sepsis Resuscitation Bundle.” The components of this protocol have been shown to play a significant role in dealing with patients with signs of septic shock/sepsis and high risk surgical patients.^1-3,5 There are two parts to this procedure, as follows.³

Severe sepsis three-hour resuscitation bundle:

Lactate level measurement

Blood cultures prior to antibiotics

Administer antibiotics

Administer 30 mL/kg crystalloid or lactate ≥ 4 mmol/L

Six-hour septic shock bundle:

Vasopressor is appropriate

Maintain arterial pressure ≥ 65 mm Hg

If appropriate, measure central
venous pressure (CVP)

If appropriate, measure central
venous oxygen saturation (ScvO₂)

Re-measure lactate level

Not all bundle conditions may be needed if patient responds to initial efforts.

Venous oxygen saturation requires a balance between oxygen delivery (DO₂) and oxygen consumption (VO₂) expressed as a normal oxygen extraction ratio of 25% to 35%.⁴ Central venous oxygen saturation (ScvO₂) refers to the venous oxygen saturation of the blood from the superior vena cava prior to entering the right ventricle of the heart. Appropriate saturation levels have been reported to be 70% to 75%.^1,3,5 Mixed venous oxygen saturation (SvO₂) occurs when the blood enters the pulmonary artery and mixes with venous blood from the inferior vena cava (normal is 65%).¹ 

SvO₂ is generally measured by acquiring a specimen from a PAC (pulmonary
artery catheter), while the ScvO₂ is collected from a central venous catheter placed in the internal jugular or the subclavian veins. Measurement of the ScvO₂ is performed on a venous blood gas from the distal port of the central venous catheter.⁴ Central venous catheters also allow for continuous monitoring through the use of fiber optic technology. Fiber optic probes are used to determine oxygen saturation by measuring the amount of transmitted light reflected off of hemoglobin and reported as a percentage. 

Venous oxygen saturation levels differ depending on the organ and its relative oxygen needs. ScvO₂ primarily reflects oxygen supply and demand for the brain and the upper body and thus is a useful tool in monitoring septic patients. In cases of severe sepsis or septic shock, early-on measurement of ScvO₂ has been strongly advised. Patients who present with low saturation values have been associated with increased mortality rates.^2,5 Based on various studies, the IHI recommends that ScvO₂ be performed within six hours of sepsis presentation.³ 

When oxygen delivery (DO₂) is compromised, cell metabolism undergoes an
anaerobic phase and produces the by-product lactate. Lactate is a marker for non-localized tissue hypoxia.⁴ Lactate levels
> 4 mmol/L (normal < 2 mmol/L) have been associated with poor patient outcome.⁶ Lactate clearance levels have been shown to reflect a poor outcome if greater than 48 hours (less than 24 hours is the target).⁷ Another laboratory test useful in monitoring for cardiovascular collapse is B-type natriuretic peptide (BNP). It is elevated in severe sepsis due to ventricular dilatation.¹ 

These procedures are all part of the process known as Early Goal-Directed Therapy (EGDT), whereby the chances of survival of patients with symptoms of sepsis/septic shock or high-risk surgical patients may improve if specific protocols are implemented in a timely fashion. A dialogue between the laboratory and the ICU seems the most appropriate action to identify the most appropriate procedures to implement. Measuring ScvO_w in certain patients appears to be a good clinical marker in addressing tissue hypoperfusion so that fluid resuscitation can be started within a three-hour period, thus possibly reducing patient mortality.⁸

References

1. McNeill B. ScvO₂ and early goal-directed therapy: The 6-hour window of opportunity. Hospira; ProCE.  www.icumed.com/media/130877/McNeill%20-%20The%20Scv02%20and%20Early%20Goal-Directed%20Therapy.pdf.  Accessed July 17, 2013.
2. O’Neill R, Morales J, Jule M. Early goal-directed therapy (EGDT) for severe sepsis/septic shock. J Emerg Med.  2012;42(5):503-510.
3. Institute for Healthcare Improvement. Severe sepsis bundles. http://www.ihi.org/knowledge/Pages/SevereSepsisBundles.aspx. Accessed July 17, 2013.
4. Nguyen HB, Walters EL. Hemodynamic and perfusion monitoring. In: Farcy DA, Chiu WC, Flaxman A, Marshall JP, eds. Critical Care Emergency Medicine. New York, NY: McGraw-Hill; 2012:Chapter 12.
5. Collaborative Study Group on Perioperative ScvO₂ Monitoring. Multicentre study on peri- and postoperative central venous oxygen saturation in high-risk surgical patients. Medscape. 2007;10(6). 
6. Perez DI, Scott HM, Duff J, et al. The significance of lacticacidemia in the shock syndrome. Ann N Y Acad Sci. 1965;119:1133-1141.
7. McNelis J, Marini CP, Jurkiewicz A, et al. Prolonged lactate clearance is associated with increased mortality in the surgical intensive care unit. Am J Surg. 2001;182:481-485.
8. Melville NA. Early fluid resuscitation reduces sepsis mortality. Medscape. January 21, 2013.
   

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