Venous oxygen saturation monitoring plays a significant role in the management of a wide range of patient populations, helping clinicians proactively manage patients prior to hemodynamic crisis rather than react to late indicators of instability.1 It has been clearly linked with improving outcomes in patients experiencing decreased cardiac output due to myocardial infarction, cardiac surgery, trauma or hemorrhagic shock, high-risk surgeries, and respiratory failure, in addition to its long-time association with early goal-directed therapy (EGDT) for sepsis identification and management (see Table 1).2,3,4,5,6,7
Accurate venous oximetry is key to understanding a patient’s true oxygenation status. Other objective measures used to assess the perfusion of organs and tissues—like mean arterial pressure, heart rate, urine output, and arterial oxygen saturation (SaO2 or SpO2)—can be normal in the presence of global tissue hypoxia and cannot rule out imbalances between oxygen supply and demand.1
TableHemodynamic monitoring helps clinicians ensure adequate tissue perfusion and oxygenation. In the presence of critical illness or injury, the heart’s ability to respond may be limited. As a result, depleted tissue will draw oxygen from the venous oxygen reserve, which can lead to lactic acidosis and global tissue hypoxia.
In recent years, survival rates for acute myocardial infarction, trauma, stroke, and sepsis have improved through early identification of tissue oxygen imbalance using hemodynamic monitoring and tissue perfusion therapy. Early initiation of hemodynamic resuscitation has also been shown to improve mortality rates.8
|Post-Coronary Artery Bypass Graft Surgery (CABG)|
|Myocardial Infarction (MI)|
|Benefit of Monitoring|
|Indication of adequate cardiac output (CO)|
|Decreased length of stay (LOS)|
|Indication of 02 consumption|
|Indication of additional interventional needs|
Venous oxygenation measurements taken at the pulmonary artery are called mixed venous oxygen saturation, or SvO2, while measurements taken at the superior vena cava are called central venous oxygen saturation, or ScvO2.
When a patient is unable to generate cardiac output sufficient to meet the metabolic needs of the tissue, the tissue extracts greater amounts of oxygen, leaving less oxygen present in the venous blood. This is reflected as a decreasing SvO2 or ScvO2 level.
Conversely, when tissue does not extract the oxygen, both SvO2 and ScvO2 levels rise. Various studies have shown that there can be slight differences between ScvO2 and SvO2 values—with either measurement reading 5-6% higher or lower than the other based on patient acuity.9,10,11,12 However, the two have been shown to maintain a high correlation (see Figure 1).9 As a result, ScvO2 is accepted as an appropriate surrogate for SvO2.
Venous oxygen saturation is measured using reflectance spectrophotometry, in which either two or three wavelengths of light are transmitted down a fiber optic filament in a catheter to the blood flowing past the catheter tip.
The current body of literature suggests multiple advantages of three-wavelength technology (TriOx®, ICU Medical, Inc. San Clemente, CA) over two-wavelength systems, including improved accuracy and the elimination of the need for routine calibration (see Figure 2). 14,15
Peripherally inserted central venous catheters (PICCs) have been safely used for many years to access the central venous circulation to administer fluids and facilitate blood draws. Because PICCs access the central venous circulation at the superior vena cava, they provide a less invasive avenue for venous oximetry.
Using a PICC avoids the complications associated with the direct neck puncture of the central venous circulation system with a CVC.16,17,18 and has been shown to have lower catheter-related bloodstream infection (CRBSI)19 and support a longer period of use.20
Because they can be inserted by trained nurses outside the operating room, PICCs reduce cost and decrease treatment delays common in busy operating or intensive care units,21 making them a more convenient, efficient, and cost-effective method to access ScvO2 levels.
The only PICC line with oximetry monitoring is the TriOx-PICC minimally invasive oximetry sensor (ICU Medical Inc., San Clemente CA), which uses three-wavelength oximetry technology to filter noise and artifact caused by cell orientation, vessel wall reflections, and changes in pH.
Venous oximetry has been established as a valuable method to monitor and maintain adequate tissue oxygenation and avoid hemodynamic crisis in critically ill and at-risk patients. ScvO2 has been shown to be a surrogate measurement for SvO2, reducing the reliance on invasive pulmonary artery catheterization. Its broad utility has been demonstrated across a variety of conditions—from cardiac care to trauma, respiratory failure and high-risk surgery—in addition to its origins in the treatment of sepsis.
The technology used to obtain venous oximetry measurements has evolved from pulmonary artery catheterization to less invasive central venous catheterization, with the goal of increasing patient safety, reducing costs, and minimizing procedure time. As the industry continues to migrate toward less invasive and more cost-effective patient monitoring trends, PICC line oximetry will continue to be a viable and efficient method for monitoring ScvO2.