Which of the following is NOT a common site for pulse oximetry sensor placement?

The upper arm is not a common site for pulse oximetry sensor placement primarily due to anatomical and physiological reasons. Pulse oximetry operates on the principle of photoplethysmography, which relies on the detection of changes in light absorption as blood pulses through the microvasculature. Common sites for sensor placement, such as the finger, toe, and earlobe, provide sufficient perfusion and accessibility for accurate readings.

The finger and toe are preferred locations as they have a high concentration of capillaries and are easily accessible, which allows the sensor to function effectively. The earlobe can also provide reliable readings because it is well-vascularized and tends to be less affected by peripheral perfusion issues compared to the more distal extremities.

In contrast, the upper arm is typically not utilized for pulse oximetry because it may have varying levels of perfusion compared to the fingertips and toe, leading to less reliable readings. Additionally, it is often covered obstructively by clothing, making sensor application more cumbersome and potentially impacting the accuracy of the measurement. Hence, while the upper arm can theoretically be used, it does not have the same reliability or commonality as the other sites listed.