How is it possible?

That the pulse oximeter always shows normal oxygen levels? Paula has a resting breathing rate of 5 breaths per minute when well and as low as 3 breaths per minute when depressed/confused yet pulse oximeter suggests that her oxygen levels are normal.

It is possible that Paula’s system only tracks hypoxia and corrects her breathing in order to avoid hypoxia, thus giving her normal oxygen levels. The pulse oximeter cannot detect the partial pressure of the carbon dioxide levels in the blood. The brain usually tracks PCO2 levels to control breathing rate, although it also can track oxygen levels secondarily. Perhaps her system no longer is able to respond to carbon dioxide levels in the blood and allows levels to rise or fall as long as acid base levels are only mildly disturbed even though mental confusion results. [and only an invasive test [ABG’s] can give the PCO2 levels of the blood.] The Ins and Outs of Breathing, How We Learnt about the Body’s Most Vital Function by Dr Normal L. Jones 2011

or

Maybe “the patient has a large amount of non- functional hemoglobin,in which case the reading will not accurate and will be misleading. Pulse oximeters usually assume there are no non-functioning hemoglobins in the arterial blood. Non-functional hemo- globin is defined as hemoglobin which is incapable of carrying oxygen, but does include carboxyhemoglobin (HbCO) -ie….. from smoking….and methemoglobin (METHb). Several situations can lead to such large amounts of non-functioning hemoglobin. Carbon monoxide poisoning and even heavy smoking increase the amount of carboxyhemoglobin, a non-functioning hemoglobin.

Functional hemoglobin is defined as hemoglobin capable of carrying oxygen.http://incenter.medical.philips.com/doclib/enc/fetch/586262/586457/Understanding_Pulse_Oximetry.pdf%3Fnodeid%3D586458%26vernum%3D2

or

Anemia An anemic patient may not have enough function- ing hemoglobin in the blood to oxygenate the tissues. The small amount of functioning hemoglobin in the blood may be well saturated with oxygen, so the patient may have a normal SpO2 [pulse oximeter] reading, but the patient may not have enough oxygen going to the tissues and so this will be misleading. . Damage to red blood cells may cause anemia, a lack of red blood cells and thus hemoglobin in the blood. Very low levels of Ferric Iron can also cause mental problems even if anemia cannot be detected yet. http://incenter.medical.philips.com/doclib/enc/fetch/586262/586457/Understanding_Pulse_Oximetry.pdf%3Fnodeid%3D586458%26vernum%3D2

or

The pulse oximeter will not work properly if the patient is vasoconstrictor or if their extremities are cold; http://incenter.medical.philips.com/doclib/enc/fetch/586262/586457/Understanding_Pulse_Oximetry.pdf%3Fnodeid%3D586458%26vernum%3D2 …………………………….both of which occur in depressive insanity……………………

or

Maybe these patients have hemoglobin with a high affinity for oxygen.

C. Hemoglobins with altered oxygen affinity
High/increased oxygen affinity states:
Fetal red cells
Decreased RBC 2,3-BPG
Carboxyhemoglobinemia, HbCO
Structural variants
Low/decreased oxygen affinity states:
Increased RBC 2,3-BPG
Structural variants
D. Methemoglobinemia
Congenital methemoglobinemia:
Structural variants
Cytochrome b5 reductase deficiency

Acquired (toxic) methemoglobinemia Classification of the Disorders of Hemoglobin Bernard G. Forget1 and  H. Franklin Bunn2 Cold Spring Harb Perspect Med. 2013 Feb; 3(2): a011684. doi: 10.1101/cshperspect.a011684

Scientists have been learning a lot this decade.

Now we just have to let the psychiatrists and the neurologists know what they’ve learnt.

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