Pulse oximetry

The human eye is poor at recognizing hypoxemia. Even under idealconditions, skilled observers cannot consistently detect hypoxemia until theoxygen (O2) saturation is below 80% [1]. Thedifficulty that physicians have in detecting hypoxemia was recently exemplifiedin a study of over 14000 patients being evaluated at the UCLA EmergencyDepartment [2]. Patients were monitored by oximetry butrecordings were given to physicians only after they completed their initialassessment. Changes in diagnostic testing and treatment were most likely at anO2 saturation of 89%, and changes were actually less common at lowersaturations, probably because the physicians were able to detect evidence ofhypoxemia without requiring a pulse oximeter.With the proliferation of pulse oximeters in different locations ofthe hospital throughout the 1980s, several investigators demonstrated thatepisodic hypoxemia is much more common than previously suspected with anincidence ranging from 20-82% [3,4,5] (Fig. 1). Thesignificance of episodic desaturation on patient outcome is largely unknown[6]. In patients admitted to a general medical service,Bowton et al. [7] found that O2saturation < 90% of at least 5 min duration occurred in 26% of the patients.On follow-up over the next 4-7 months, those patients experiencing hypoxemiaduring the first 24 h of hospitalization had more than a threefold highermortality than patients who did not desaturate. Although episodic desaturationmay simply be a marker of increased risk rather than the direct cause ofdecreased survival, an increased mortality rate was still observed in patientswith episodic hypoxemia when the investigators corrected for severity ofillness. Whether or not the early detection and treatment of episodic hypoxemiacan affect patient outcome remains unknown.Pulse oximetry is based on two physical principles: (a) the presenceof a pulsatile signal generated by arterial blood, which is relativelyindependent of non-pulsatile arterial blood, venous and cap