Pulse oximetry enables continuous monitoring of functional oxyhemoglobin saturation using an accurate, noninvasive, and real-time probe. A continuous pulse oximeter reading allows the early warning sign of hypoxia. As a result, the loss of airway patency, potential loss of oxygen supply from the anesthetic machine, or intrinsic shunting can be clinically detected early to prevent any disastrous outcome.
The basic principle of pulse oximetry depends on two components: the generation of an arterial pulsatile waveform and the ability to differentiate two different wavelengths. The pulse oximeter emits two light-measuring wavelengths, 660 nm (red) and 940 nm (near infrared [IR]), for the calculation of fraction of oxygenated blood (FHbO2) and ultimately oxygen saturation. Oxygenated blood (O2Hb) absorbed more IR light whereas deoxygenated blood (deoxyHb) absorbs more red light. This phenomenon is generally observed with the naked eye: O2Hb is seen as red because it scatters the red light more than deoxyHb does. As a result, deoxyHb appears less red because these molecules actually absorb more of the red waveform. Most pulse oximeters also provide plethysmographic waveforms to help distinguish between a true or artificial signal.
Conventional pulse oximetry uses two waveforms to measure the hemoglobin saturation through tissue bed. Pulse oximetry can be used on the finger, ears, or other skin tissue. The tissue bed is composed of bone, soft tissue, capillary blood that can affect the accurate absorbance reading. To distinguish arterial blood from tissue, most pulse oximetry will distinguish between a nonpulsatile or direct current (DC) component and a pulsatile or alternating current (AC) component. The fixed DC absorbance results from solid tissues, venous and capillary blood, and nonpulsatile arterial blood. The AC component is caused by pulsations in the arterial blood volume. During the AC component, systolic volume expands the arteriolar bed, thus producing an increase in optical path for an increased light absorbance. Most pulse oximeters assume that arterial blood is the only pulsatile absorber. Each wavelength (660 nm and 940 nm) measures its corresponding AC and DC components. AC component of the wavelength is divided by the corresponding DC component to calculate the absorbance (S): S660 = AC660/DC660 and S940 = AC940/DC940. As a result, the pulse oximeter divides the absorbance ratio between the two wavelengths to establish a Red:IR modulation ratio (R):
“R-value” is plotted on a calibration curve created by directly measuring arterial blood oxygen saturation (SaO2) in healthy volunteers. The result is stored in a digital microprocessor. Increased red light absorbance (increased R) is associated with increased deoxyHb, that is, lower SpO2. Therefore, a normal SpO2 calls for a low “R-value” ratio. The value of R varies from roughly ...