Arterial oxygen saturation (SaO2) is a measure of the amount of oxygen bound to hemoglobin. It is measured using non-invasive oximetry techniques and avoids technical and ethical concerns associated with arterial sampling for blood oxygen (O2) level determination.
Oximetry determines SaO2 by analyzing two wavelengths of light (660 nm and 910 nm) that are differentially absorbed by specific hemoglobin structures in the blood (oxygenated hemoglobin [HbO2] and reduced hemoglobin [Hb]). These light waveforms are transmitted by a semiconductor light emitting diode and detected by a photodetector. The relative absorption of these wavelengths reflects the ratio of oxygenated to total hemoglobin and this ratio is expressed as a percentage. As oximetry does not detect the presence of carboxyhemoglobin and methemoglobin, SaO2 must strictly be referred to as SpO2. For a healthy adult, human subject, an SpO2 of 97% to 99% is considered normal.
The oximeter takes approximately 10 seconds to determine an accurate output from the time it is plugged in and the transducer is attached to the subject. It only provides SpO2 output, with no associated pulse rate signal. The oximeter will provide an output signal between 70 to 100% SpO2 (values below 70% will not be registered). It provides one reading per second with a resolution of 1%.
Background
For patients on long-term oxygen therapy, pulse oximetry arterial oxygen saturation (SaO2) measurements are unnecessary except to assess changes in clinical status, or to facilitate changes in the oxygen prescription. Home pulse oximetry is also indicated when there is a need to monitor the adequacy of SaO2 or the need to quantitate the response of SaO2 to a therapeutic intervention.
A National Heart, Lung and Blood Institute/World Health Organization Global Asthma Initiative Report concluded that pulse oximetry was not an appropriate method of monitoring patients with asthma. The report explained that, during asthma exacerbations, the degree of hypoxemia may not accurately reflect the underlying degree of ventilation-perfusion (V-Q) mismatch. Pulse oximetry alone is not an efficient method of screening or diagnosing patients with suspected obstructive sleep apnea. The sensitivity and negative predictive value of pulse oximetry is not adequate to rule out obstructive sleep apnea in patients with mild to moderate symptoms. Therefore, a follow up sleep study would be required to confirm or exclude the diagnosis of obstructive sleep apnea, regardless of the results of pulse oximetry screening.
Home overnight pulse oximetry (OPO) has been used to evaluate nocturnal desaturation in patients with chronic obstructive pulmonary diseases (COPD), However, Lewis et al (2003) found that nocturnal desaturation in patients with COPD exhibited marked night-to-night variability when measured by home OPO. A single home OPO recording may be insufficient for accurate assessment of nocturnal desaturation.