49
A-2 SpO
2
Measuring principle
Oxygen, bound to hemoglobin, is carried by blood. Hemoglobin that is bound to oxygen is called
oxygenated hemoglobin (HbO
2
), and that is not bound to oxygen is called reduced hemoglobin (Hb).
Arterial oxygen saturation is the percentage of oxygenated hemoglobin in the blood.
Arterial oxygen saturation = HbO
2
/ (Hb+ HbO
2
)
Oxygen-rich arterial blood is red while venous blood
after the oxygen has been removed is dark. The
difference in color comes from the difference in
light-absorbing properties of oxygenated
hemoglobin and reduced hemoglobin. (Refer to
Figure A-1.)
A pulse oximeter uses the difference in
light-absorbing properties of hemoglobin to obtain
arterial oxygen saturation.
When the optical density of the body surface is
obtained optically, it is the characteristic of the
body part that includes tissue, venous blood and
arterial blood. (Refer to Figure A-2.) The area that
the optical density is pulsating due to the heart
pumping function is called the photoelectric pulse
and reflects the arterial blood properties. When
measuring the photoelectric pulse using two
wavelengths, the ratio in amplitudes is obtained
temporarily as the arterial oxygen saturation.
Figure A-1
Figure A-2
