I would be cautious in assuming that all low pulse oximetry values in
infants are false negatives. Whether an oxygen saturation of 95% in a sleeping
child is "abnormal" or clinically relevant is unknown. Also young infants with
gastroenteritis and dehydration are prone to methemoglobinemia. Small
percentages (less than approximately 15%) of MHB will lower the pulse ox reading
but will not make the infant appear cyanotic. In addition, an arterial blood gas
will give a normal oxygen saturation in MHB since most blood gas machines use
potentiometry to measure PO2 and calculate the oxygen saturation from the pH and
PO2. Pulse oximeters which display the plethysmographic waveform allow
clinicians to determine if the machine is reading the changes in pulse volume
associated with arteriolar bloodflow and hence whether the reading is reliable.
Instructing nurses to check the waveform before recording the pulse ox reading
would likely cut down on falsely abnormal or even falsely normal readings.
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