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Scvo2 in Sepsis: A Measurement Provided by Respiratory Care Practitioners

Herbert Patrick, MD, MSEE, FCCM
Dennis Disney, RRT, ACCS


When treating patients with sepsis, intensive care unit (ICU) and emergency department clinicians have learned that improved outcomes occur when staff members work together as a team. This article outlines the value of central venous oxygen saturation (Scvo2) measurement in sepsis patients, emphasizing the role of the respiratory care practitioner in measuring Scvo2 using blood gas oximetry.

The measurement of Scvo2 has a controversial role in patients with sepsis. To review its value, remember that this measure is a surrogate for tissue oxygen consumption in the upper body. Decreased Scvo2 reflects an increased difference in oxygen leaving the left ventricle minus oxygen returning to the right ventricle. The normal value for Scvo2 is 65% to 70%. The lower the Scvo2, the more inadequate the
blood supply to tissues.

To further understand Scvo2, we’ll first need to review the equation for oxygen delivery (Do2), where arterial oxygen saturation (Sao2) from the arterial blood gas measurement or arterial pulse oxygen saturation (Spo2) from arterial pulse oximetry is a factor.

Do2 = CO x Cao2 = CO x ((1.34 x Hgb x Sao2) +( 0.0031 x Pao2))

Where CO is cardiac output (L/min); Cao2 is content of arterial oxygen (g/dL); the constant 1.34 is the amount of oxygen (mL) at one atmosphere bound to 1 gram of hemoglobin (Hgb); the constant 0.0031 is the amount of oxygen dissolved in 1 milliliter of plasma; and Pao2 is the partial pressure of oxygen (mm Hg). As the two constants indicate, the majority of oxygen content is bound versus dissolved. Recall that this remains true even though Sao2 is inserted in the equation as Sao2/100.

As oxygen delivery accounts for the oxygen provided to the tissues, the Scvo2 reflects the consumption of oxygen by the tissues. Although tissue oxygen consumption cannot be easily measured in an ICU
patient, the difference between oxygen delivery and consumption is reflected by the Scvo2. Oxygen leaves the left ventricle, travelling throughout the body where the organs take oxygen as needed. As the blood returns to the right atrium, the reduction in oxygen saturation (from Sao2 to Scvo2) reflects the oxygen consumed by the organs. Therefore, the difference between Sao2 and Scvo2 is a measure of oxygen consumption, specifically in the internal jugular (IJ) or the subclavian (SC) vein. Of course, the blood gas sample for Svo2 cannot be drawn from either a femoral or a peripheral vein if the goal is to measure total tissue oxygen consumption.

In patients with sepsis, oxygen delivery must meet tissue consumption to avoid hypoxic injury to tissues. The patient with sepsis can be assessed for adequacy of oxygen delivery versus consumption using the Scvo2 value. For example, if Scvo2 is 65% to 70%, delivery versus consumption is normal; if Scvo2 is below 65%, the consumption exceeds delivery; if Scvo2 is below 40%, consumption exceeds delivery by such a large amount that hypoxic damage to the tissues increases and the risk of mortality increases.

Technique for drawing a blood gas sample for Scvo2 measurement 

For those interested in accurate Scvo2 measurements, attention must be given to drawing the blood gas sample, avoiding the common error of introducing an air bubble into the specimen. Any blood gas sample mixed with an air bubble will have an altered oxygen saturation value, because that bubble of room air contains a partial pressure of oxygen of 149 mm Hg, per the alveolar gas equation. This is higher than the normal venous oxygen pressure of 35 to 40 mm Hg in a sample drawn from a central venous catheter (CVC). Depending on the volume of the air bubble and the volume of the blood sample, a falsely elevated Scvo2 value can result. To minimize air bubbles from contact with the central venous sample, the ICU respiratory care practitioners have standardized this technique for drawing each Scvo2 sample.

Supplies:

1.    Three-way stopcock (1)
2.    Luer-Lok syringes, 10-12 mL (2)
3.    Blood gas sample syringe, no needle, with air purge cap (1)
4.    Saline flush, 10 mL (1)

Technique:

5.    Select the port of the CVC for the blood gas sample; stop any medication infusion.
6.    Close the slide valve of the CVC port.
7.    Remove the intravenous medication tubing.
8.    Attach a three-way stopcock to the CVC port, with the stopcock to the port off.
9.    Open the slide valve
10.    Connect one Luer-Lok syringe (waste) to the stopcock.
11.    Turn the stopcock off to the 90-degree open port.
12.    Remove waste blood, 5 mL.
13.    Remove the waste syringe and discard.
14.    Connect the second Luer-Lok syringe (sample) to the stopcock.
15.    Remove sample blood, 5 mL.
16.    Turn the stopcock off to the CVC port.
17.    Press the sample syringe to push blood into the open stopcock port until the air in that port is expelled.
18.    Press all air out of the blood gas syringe by depressing the plunger to empty the air.
19.    Place the blood gas syringe securely onto the stopcock
20.    Depressing the sample syringe, but NOT pulling on the plunger of the blood gas syringe, transfer 1-2 mL of blood through the stopcock into the blood gas syringe.
21.    Remove the blood gas syringe from the stopcock and apply the cap.
22.    Express air from the cap.
23.    Place the syringe in a biohazard bag.
24.    Close the slide valve of the CVC port.
25.    Remove the stopcock.
26.    Flush the CVC port with saline.
27.    Reconnect the intravenous medication tubing

Reluctance to Use Scvo2 Values
Why aren’t a greater number of Scvo2 samples drawn from patients with sepsis? In the 2001 trial of early goal-directed therapy by Rivers et al, the Scvo2 was measured in each patient.1 Many discussions and debates followed over the inclusion and value of Scvo2 in the sepsis resuscitation bundle. Between 2014 and 2015, three studies—ProCESS,2 ARISE,3 and ProMISe,4 all published in the New England Journal of Medicine—concluded that Scvo2 does not improve outcomes in sepsis patients. These three articles dampened enthusiasm and subsequent use of Scvo2, but many advocates have remained vocal in their support. Vincent and De Backer published an editorial5 accompanying a 2018 article,6 noting that the ProCESS,2 ARISE,3 and ProMISe4 studies did not include patients as sick as those in the study by Rivers et al. Vincent and De Backer pointed out that the average Scvo2 for the Rivers patients was in the 40% range, while the average Scvo2 for the patients in the subsequent three studies was approximately 70%. They proposed that Scvo2 should have greater use as a marker for the severity of sepsis (ie, the greater the oxygen consumption, the lower the Scvo2 and the higher the mortality). Methods to raise the Scvo2 value and lower the mortality rate include transfusion with packed red blood cells and/or inotropes.

Conclusion
The respiratory care practitioner who measures and reports Scvo2 shows an understanding of the physiology of sepsis, oxygen delivery, and oxygen consumption. Future use of the Scvo2 value is expected to increase as more clinicians initiate controlled studies and present results in patients with sepsis.

References: 

  1. Rivers E, Nguyen B, Havstad S, et al. for the Early Goal-Directed Therapy Collaborative Group. Early Goal-Directed Therapy In The Treatment Of Severe Sepsis And Septic Shock. N Engl J Med 2001; 345(19):1368-1377. 
  2. Yearly DM, Kellum JA, Huang DT, et al. for the ProCESS Investigators. A Randomized Trial of Protocol-Based Care for Early Septic Shock. N Engl J Med 2014; 370(18):1683-1693. 
  3. Bailey M, Rinaldo B, Cameron P, et al. for the ARISE Investigators and the ANZICS Clinical Trials Group. Goal-Directed Resuscitation For Patients With Early Septic Shock. N Engl J Med 2014; 371(16):1496-1506. 
  4. Mouncey PR, Osborn TM, Power GS, et al. for the ProMISe Investigators. Trial of Early, Goal-Directed Resuscitation For Septic Shock. N Engl J Med 2015; 372(14):1301-1311. 
  5. Vincent J-L, Daniel De Backer D. From Early Goal-Directed Therapy To Late(r) SCVO2 Checks. Chest 2018; 154(6):1267-1269. 
  6. Protti A, Masson S, Latini R, et al. Persistence of Central Venous Oxygen Desaturation During Early Sepsis is Associated With Higher Mortality: A Retrospective Analysis Of The ALBIOS Trial. Chest 2018; 154(6):1291-1300.