Issac,
Pulse oximetry is a good way of monitoring the patients oxygen
saturation.However, peripheral circulation and several other factors
influence pulse oximetry. Blood gases are more reliable. However in a clinic
situation,
pulse oximetry readings are more convenient and hence commonly
used.Sometimes I find it better to use an ear probe as opposed to a finger
probe.I cannot recall the reference but apparently finger probes are more
reliable
than the ear probe.
Now coming to your question, we usually have a cut off as 92% at rest and
90% with activity. You may want to remember that a lot of COPD
patientsdepend on their hypoxic drive to regulate respiration as opposed to
hypercapnic drive since they retain CO2 and this drive becomes blunted.If
you use supplemental oxygen with exercise,it may be a good idea to determine
how much oxygen the patient needs at rest and with activity and advice the
patient very clearly about this. In the U.S. oxygen has to be prescribed bya
physician. Apparently Medicare ( USA) guidelines do not approve home oxygen
unless the patient drops to 87% with activity.
It seems like the vasoconstriction you are talking about is hypoxic
pulmonary vasoconstriction( HPV )This is in contrast to systemic vascular
beds where a decrease in oxygen availability causes vasodilatation. I have
not seen any references which state that SPO2 below 92% leads to HPV. (
Reference Grover RF: Chronic hypoxic pulmonary hypertension. In: Fishman
AP,ed.' The Pulmonary Circulation: Normal and Abnormal. Philadelphia: Univ.
Pennsylvania Press; 1990:83-299 )
Hope this helps.
Nazir Ahmed P.T.
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