Effect of motion artefact on pulse oximeters: evaluation of four instruments and finger probes. The ability of the Ohmeda 3700, Nellcor N200, Datex Satlite Plus and Simed S100 pulse oximeters to detect induced hypoxaemia in the presence of motion artefact was assessed, under conditions of controlled vibration using an industrial vibration facility. Vibration at 4 Hz and 8 Hz induced increases in detection time for hypoxaemia and spurious decreases in the displayed SaO2 in some of the oximeters tested. Finger-dependent differences in oximeter performance and pulse rate registration were noted especially in those oximeters without ECG linkage (Ohmeda 3700 and Simed S100). Subsequently, eight different pulse oximeter finger probes were assessed for those characteristics that may predispose to motion artefact. There were marked differences in the mass of the probes, the forces exerted on the test finger and in the force required to displace the probes from the subject's finger. Differences in both the microprocessor programmes and the physical characteristics of the finger probes may explain the observed differences in function. Similar studies should form part of the standard evaluation of new pulse oximeters. 