Question: How is InSpectra™ StO₂ different from SvO₂ and ScvO₂ ?

Background

InSpectra™ StO₂ is a measure of the percent hemoglobin oxygen saturation in the capillaries, venules, and arterioles of the microcirculation of peripheral tissue, usually in the muscle of the thenar eminence. The microcirculation is where oxygen leaves hemoglobin bound for the tissue cells. InSpectra StO₂ therefore changes dynamically with changes in oxygen delivery to the tissue and oxygen consumption by the tissue. Changes in tissue perfusion status as a result of reduced blood flow to peripheral muscle, such as occurs in hemorrhagic shock, are reflected by direct, real-time changes in InSpectra StO₂.

What is SvO₂? SvO₂, mixed venous oxygen saturation, is the percent oxygen saturation of hemoglobin in venous blood that has returned and been pooled, or “mixed,” from the entire body. It is therefore a global indicator of oxygen utilization from the body as a whole, including the heart. It cannot, however, indicate where in the body inadequate delivery or consumption may exist.

SvO₂ is measured from a pulmonary artery (PA) catheter, also called a Swan Ganz catheter. The PA catheter is introduced through a large vein, often the subclavian, femoral or internal jugular vein. From this entry site, it is threaded, often with the aid of fluoroscopy, through the right atrium of the heart, into the right ventricle, and subsequently into the pulmonary artery. The pulmonary artery contains all venous blood that has completed a circuit through the body and is now being delivered to the lungs to become re-oxygenated. Usually the primary purpose of a PA catheter is to measure pressures in the pulmonary artery, from which a number of cardiac function parameters are calculated. In addition, a measurement of SvO₂ may be obtained either by drawing a sample of blood from the catheter and analyzing it in the clinical laboratory, or by using a specialized catheter with NIRS capability to measure the oxygen saturation continuously in situ.

What is ScvO₂? ScvO₂, central venous oxygen saturation, requires placement of a catheter in the superior or inferior vena cava just outside the right atrium of the heart. It is also sometimes measured in the right atrium. When placed in the superior vena cava, it represents systemic circulation above the heart. When placed in the inferior vena cava, it represents systemic circulation below the heart. When placed in the right atrium it represents upper and lower body systemic circulation. None of these measurement locations include the coronary circulation. ScvO₂ is measured more often than SvO₂ because this technique is less challenging to perform.

Comparison: PA catheters measuring SvO₂ or central line catheters measuring ScvO₂ are used in critical care medicine to obtain direct measurements of oxygen levels in venous blood, and are used far less commonly in the ED/trauma bay than in the ICU. The normal range for SvO₂ is 60%-80%, and ScvO₂ is often slightly higher because the higher oxygen extraction of cardiac muscle is not included in ScvO₂.

InSpectra StO₂, SvO₂ and ScvO₂ are all sensitive to hypoperfusion. Hence in some clinical situations they will track with hypoperfusion and resuscitation similarly. The primary difference between these measures is that since InSpectra StO₂ monitors a peripheral local site, one may expect InSpectra StO₂ to signal the onset of hypoperfusion earlier than either SvO₂ or ScvO₂ since blood flowing to peripheral muscle is the first segment of the circulation to be shut down during hypovolemia. A clinical study published by McKinley et al1 shows an example in which InSpectra StO₂ starts low during hypovolemic shock and increases to normal levels as resuscitation proceeds. SvO₂, measured at the same time, neither showed an initial deficit nor changes during resuscitation.

The use of these highly invasive systemic oxygen consumption indicators is unusual in early stages of resuscitation. Insertion of these catheters is resource intensive, technically challenging, requires a sterile field, critical care monitoring, and comes with an added risk of complications. InSpectra StO₂ is noninvasive, and it is quick, and easy to place the sensor. Peripheral changes in InSpectra StO₂ are an early indicator of systemic changes as a result of hemorrhage or hypovolemia.

References

1. McKinley BA, et al. Tissue hemoglobin O₂ saturation during resuscitation of traumatic shock monitored using near infrared spectroscopy. J Trauma. 2000; 48:647-642.
2. Podbregar M, et al. Skeletal muscle oxygen saturation does not estimate mixed venous oxygen saturation in patients with severe left heart failure and additional severe sepsis or septic shock. Crit Care Med. 2007;11(1):116.

3. Crawford J, Otero R, Goyal N, Sankey S, Rivers E. Near infrared spectroscopy to assess systemic perfusion in the critically ill. Crit Care Med. 2007; 35(12)Suppl:A255.