Question: How is InSpectra™ StO₂ different from SpO₂ and SaO₂?

(Also see FAQ: How is InSpectra StO2 different from SvO2 and ScvO2?)

Background

InSpectra StO₂ is primarily a measure of the percent hemoglobin oxygen saturation in the capillaries, venules, and arterioles of the microcirculation of peripheral tissue, usually the muscle of the thenar eminence. The microcirculation is where oxygen leaves hemoglobin bound for the tissue cells. 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, realtime changes in StO₂.

SpO₂, measured with a pulse oximeter, is also a measure of hemoglobin oxygen saturation. The primary difference is that SpO₂ is a measure of arterial hemoglobin oxygen saturation. SpO₂ measured on the fingertip or the big toe will have similar readings because there is no oxygen exchange between the thick-walled, pulsing arteries and tissue cells. SpO₂ primarily reflects whether blood is being adequately oxygenated by the lungs. SpO₂ is not a measure of the adequacy of oxygen supply to the tissue, only the adequacy of oxygen loading in the lungs. Significant blood loss may occur before there is a change in SpO₂.  

In addition, because a pulse oximeter is dependent on detecting the pulse, or surge, of hemoglobin in the artery as the heart pumps blood, there may not be sufficient pulse for an SpO₂ reading in hypothermic patients. The InSpectraTM StO₂ Monitor, which measures oxygen saturation in the tiny, non-pulsing vessels, is unaffected by hypothermia in its ability to make accurate measurements. Note: InSpectra StO₂ may be low or normal in a hypothermic patient, accurately reflecting inadequate perfusion. (see FAQ: How is StO₂ affected by hypothermia?)

SaO₂ is also a measure of percent arterial hemoglobin oxygen saturation. SaO₂ is measured intermittently via an arterial blood sample that is analyzed in a blood gas analyzer or co-oximeter.

Both InSpectra StO₂ and SpO₂ utilize optical technology (near infrared spectroscopy) and report a percent of hemoglobin oxygen saturation. However, the difference in where the measurement is made changes the insight into a patient’s physiology. Under normal circumstances, SpO₂ or SaO₂ are close to 100%, reflecting that 100% of hemoglobin molecules are fully ‘saturated’ with oxygen. Because StO₂ is measuring where oxygen is being exchanged with tissue, StO₂ is a lower value. StO₂’s normal threshold is 75% and reflects the oxygen currently available to the tissue cells i.e. about 25% of the oxygen has left hemoglobin to be consumed by the cells.

InSpectra StO₂ changes dynamically with changes in a patient’s perfusion status. This is clearly demonstrated in patients with hypovolemic shock; while InSpectra StO₂ declines, SpO₂ levels remain normal until lung function is compromised. InSpectra StO₂ also responds to the interventions which improve perfusion status during resuscitation, while SpO₂ may not change during fluid and blood product administration.

References

1. Cohn SM, et al. Tissue Oxygen Saturation Predicts the Development of Organ
   Dysfunction During Traumatic Shock Resuscitation. J Trauma. 2007; 62:44-55.
2. Moyle JTB, et al. Pulse Oximetry 1994. Chapter 3: How Pulse Oximetry Works
3. Guyton AC, et al. Textbook of Medical Physiology 2000. Chapter 24: Circulatory
   Shock and the Physiology of Its Treatment.
4. Five case studies from the StO₂ Trauma Study, 2008 revisions.