D. Limitations

1. Decreased accuracy when arterial saturation is <65%

 Pulse oximetry will overestimate SpO2 at this level;

therefore, blood gas confirmation is imperative (9–11).

2. Not a sensitive indicator for hyperoxemia (10)

 Pulse oximeter accuracy does not allow for precise

estimation of PO2 at saturations >90%. Small changes

in O2 saturation (1% to 2%) may be associated with

large changes in PO2 (6 to 12 mm Hg) (10).

3. Because pulse oximeters rely on pulsatile fluctuations

in transmitted light intensity to estimate SpO2, they are

all adversely affected by movement (9–11)

 In some cases, the pulse oximeter may calculate an

SpO2 value for signals caused by movement, or it may

reject the signal and not update the display. Usually,

the heart rate output from the oximeter will reflect the

detection of nonarterial pulsations, indicating either

“0” saturation or “low-quality signal” (3). Advances in

microprocessor technology have led to improved signal

processing, which makes it possible to minimize motion

artifact and monitor saturation more accurately during

motion or low-perfusion states. (10)

4. Significant levels of carboxyhemoglobin or methemoglobin can yield erroneous readings (carboxyhemoglobin absorbs light at the 660-nm wavelength) (10).

However, carboxyhemoglobin levels of <3% will not

affect the accuracy of the instrument.

5. SpO2 may be overestimated in darkly pigmented

infants.

 Some oximeters will give a message such as “insufficient signal detected” if a valid signal is not obtained

(10–13)

6. Erroneous readings can occur in the presence of high

fetal hemoglobin (14)

 A smaller effect on accuracy is noted when fetal

hemoglobin levels are <50% (14). With a predominance of fetal hemoglobin, an SpO2 of >92% may be

associated with hyperoxemia (14). However, whereas

saturations may appear adequate, PO2 may be low

enough to produce increased pulmonary vascular resistance (SpO2/PO2 curve shift to the left).

 Infants with chronic lung disease and prolonged oxygen dependence are older and have less fetal hemoglobin; therefore, SpO2 readings obtained from these

patients may be more accurate than those obtained

from neonates with acute respiratory disorders at an

earlier age (14). The same situation exists in infants

who have undergone exchange transfusion because of

decreased levels of fetal hemoglobin.

7. Light sources that can affect performance include surgical lights, xenon lights, bilirubin lamps, fluorescent

lights, infrared heating lamps, and direct sunlight.

 Although jaundice does not account for variability in

pulse oximeter accuracy (15), phototherapy can interfere with accurate monitoring. Therefore, appropriate

precautions should be taken, such as covering the

probe with a relatively opaque material (1).

8. Do not correlate SpO2 values with laboratory hemoximeters (15).

 Most laboratory oximeters measure fractional oxygen

saturation (all hemoglobin including dysfunctional

hemoglobin) as opposed to functional oxygen saturation (oxyhemoglobin and deoxyhemoglobin excluding

all dysfunctional hemoglobin).

 Use of normal adult values for hemoglobin,

2,3-diphosphoglycerate, and, in some cases, PCO2 can

lead to errors in the algorithm to calculate SpO2 with

some blood gas analysis instruments (15).

9. Although pulse oximeters can detect hyperoxemia, it is