Oximeters have limitations which may result in erroneous readings [15] (Table 1). Because of the sigmoid shape of the oxyhemoglobin dissociation curve, oximetry may not detect hypoxemia in patients with high arterial oxygen tension (PaO 2 ) levels [1,18].

Table 1 Limitations of pulse oximetry Full size table

Conventional pulse oximeters can distinguish only two substances: reduced hemoglobin and oxyhemoglobin; it assumes that dyshemoglobins—such as carboxyhemoglobin (COHb) and methemoglobin (MetHb)—are absent (Fig. 1). Studies showed that the presence of elevated levels of COHb and MetHb could affect the accuracy of SpO 2 readings [1,19]. Accordingly, multiwavelength oximeters that are capable of estimating blood levels of COHb and MetHb have recently been designed [20]. In healthy volunteers, the accuracy of a multiwavelength oximeter (Masimo Rainbow-SET Rad-57 Pulse CO-oximeter; Masimo Corporation, Irvine, CA, USA) in measuring dyshemoglobins was evaluated by inducing carboxyhemoglobinemia (levels range from 0 % to 15 %) and methemoglobinemia (levels range from 0 % to 12 %) [20]. Bias between COHb levels measured with the pulse CO-oximeter and COHb levels measured with the laboratory CO-oximeter (standard method) was −1.22 %; the corresponding precision was 2.19 %. Bias ± precision of MetHB measured with the pulse CO-oximeter and MetHb measured with the laboratory CO-oximeter was 0.0 % ± 0.45 %. The accuracy of pulse CO-oximeters in measuring COHb levels was also assessed during hypoxia [21]. In 12 healthy volunteers, the pulse CO-oximeter was accurate in measuring COHb at an SaO 2 of less than 95 % (bias of −0.7 % and precision of 4.0 %); however, when the SaO 2 dropped below 85%, the pulse CO-oximeter was unable to measure COHb levels. In patients evaluated in the emergency department with suspected carbon monoxide poisoning, the bias between pulse CO-oximetric measurement of COHb and laboratory CO-oximetric measurement of COHb was less than 3 % [22,23]. The limits of agreement between the measurements, however, were large (−11.6 % to 14.14 %) [23], leading some authors to conclude that these new pulse CO-oximeters may not be used interchangeably with standard laboratory measurements of COHb [22–24].

Inaccurate readings with pulse oximetry have been reported with intravenous dyes used for diagnostic purposes, low perfusion states (that is, low cardiac output, vasoconstriction, and hypothermia), pigmented subjects and in patients with sickle cell anemia [1,6,25,26]. Because the two wavelengths (660 and 940 nm) that pulse oximeters use to measure SpO 2 can be produced by various ambient light sources, the presence of such sources could produce false SpO 2 readings. To test the accuracy of pulse oximetry in the presence of ambient light, Fluck and colleagues [27] performed a randomized controlled trial in healthy subjects in which SpO 2 measurements were obtained in a photographic darkroom under five separate light sources: quartz-halogen, infrared, incandescent, fluorescent, and bilirubin light [27]. The largest difference in SpO 2 between the control condition (that is, complete darkness) and any of the five light sources was less than 5%. Nail polish can interfere with pulse oximetry readings [28]. In 50 critically ill patients requiring mechanical ventilation, Hinkelbein and colleagues [29] found that the mean difference between SpO 2 and SaO 2 was greatest for black (+1.6 % ± 3.0 %), purple (+1.2 % ± 2.6 %), and dark blue (+1.1 % ± 3.5 %) nail polish; limits of agreement ranged from 6 % (unpainted fingernail) to 14.4 % (dark blue) (Fig. 5). Rotating the oximeter finger probe by 90 ° did not eliminate the error induced with nail polish.

Fig. 5 Bias of O 2 saturation pulse oximetry (SpO 2 ) and arterial O 2 saturation (SaO 2 ) of various nail polish colors in critically ill patients. Thick horizontal lines represent mean bias, the whiskers represent maximum and minimum bias; the bottom and top of the boxes represent the first and third quartiles. *P < 0.05 ,**P < 0.01 when compared with arterial oxygen saturation. Reprinted with permission from Elsevier Inc. [29] Full size image

Motion artifact is considered an important cause of error and false alarms [30–33]. In the 1990s, several signal processing techniques were incorporated in pulse oximeters in an attempt to reduce motion artifact [34–38]. One such technique is Masimo signal extraction technology (SET™) [39]. During motion and hypoxia, the Masimo SET oximeter performed better than the Agilent Viridia 24C (Agilent Technologies, Santa Clara, CA, USA), the Datex-Ohmeda 3740 (Datex-Ohmeda, Madison, WI, USA), and the Nellcor N-395 (Covidien Corporation, Dublin, Ireland) oximeters [34].