Tissue thickness can make the transmission of light waves more difficult in the extremely obese and thus make pulse oximetry readings unreliable. However, pulse oximetry in the moderately obese is generally accurate. In morbidly obese patients, the earlobe should be used instead of the finger for probe placement, as the earlobe is usually thinner than the digit, with higher perfusion, and the probe stays in place more easily. If use of an earlobe probe is not possible, use of the fifth digit of the hand or foot can be tried. Other areas of placement include the nose, lip, or temporal artery. 17
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