Pulse oximetry: basic principles and applications in aerospace medicine.

INTRODUCTION: Pulse oximeters are reliable, objective, and noninvasive monitors that have broad application in aerospace medicine. New technology enables pulse oximeters to perform well in adverse environments and measure additional parameters. Small, battery-powered devices can be used to monitor oxyhemoglobin saturation while in flight. THEORY OF OPERATION: Pulse oximeters use spectrophotometry to measure the ratio of oxyhemoglobin (Hbo2) to reduced hemoglobin (Hb) in arterial blood. This value is displayed as oxyhemoglobin saturation (Spo2). A plethysmographic waveform that resembles arterial waveform is also frequently displayed and may indicate relative changes in perfusion and blood volume. Loss and subsequent reappearance of this waveform during occlusion with a cuff has been used to measure systolic blood pressure during helicopter flight. APPLICATIONS: Accurate determination of oxygen saturation requires a high quality arterial signal and is limited by errors resulting from calibration, motion and vibration, and dyshemoglobinemias. Vasoconstriction may result in decreased pulse amplitude and also impair accurate measurement. Conventional fingertip probes may interfere with the performance of required duties, while helmets and other restrictive clothing can impede the use of sensors on the forehead or ear. Recently introduced devices answer some of these limitations and enable measurement of additional parameters. For example, new probe designs permit more freedom of movement and include a contactless camera and a sensor that fits around a finger like an ordinary ring. This article explains the theory of operation and limitations of pulse oximetry, offers an update on new technology, and discusses applications of this technology in aerospace medicine.