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