Pulse Oximetry is Not a Monitor of Ventilation

Pulse oximetry provides an analysis of adequate oxygenation, but no direct information about ventilation. Critically ill patients pose a challenge for pulse oximetry with reduced tissue perfusion combined with motion.  Low perfusion is the product of reduced peripheral blood flow and subsequent reduction in the detectable signal at the sensor site.Feb 7, 2007 PULSE OXIMETRY AND LOW PERFUSION https://www.rtmagazine.com/public-health/healthcare-policy/home-care/pulse-oximetry-and-low-perfusion/

Paula, when ill, and Kraepelin’s patients showed signs of vasoconstriction with cold, pale feet, hands and lips [periphery]. Does vasoconstriction decrease blood flow, meaning reduced tissue perfusion? When blood vessels constrict, the flow of blood is restricted or decreased, thus retaining body heat or increasing vascular resistance. … Generalized vasoconstriction usually results in an increase in systemic blood pressure, but it may also occur in specific tissues, causing a localized reduction in blood flow. from Vasoconstriction – Wikipedia en.wikipedia.org › wiki › Vasoconstriction

Pulse oximeters will not be accurate under conditions of low tissue perfusion. In addition pulse oximeters will not pick up abnormal measures of PCO2. Only an arterial blood gas test can give this measurement. And only PCO2 measurement can diagnose respiratory failure. [although an abnormal respiratory rate at rest will give you a good idea of when an arterial blood gas test is necessary.

Evaluation of Tissue Perfusion can be tricky;

Tissue perfusion

Tissue perfusion is a critical parameter for tissue survival and function, [ including Brain Tissue] and both relative and absolute perfusion assessments are highly relevant for both diagnosis and evaluation of the therapy response. From: Methods in Enzymology, 2004 https://www.sciencedirect.com/topics/immunology-and-microbiology/tissue-perfusion

Evaluation of tissue perfusion can be done by considering gum or lip mucous membrane colour, the capillary refill time, and the blood pressure. High mean arterial pressure does not guarantee adequate tissue perfusion. For example, when blood pressure increases during anaesthesia in response to a surgical stimulus, cardiac output may be decreased due to increased afterload from peripheral vasoconstriction.

Tissue perfusion is usually decreased when the gums are pale, rather than pink, sometimes when very pink, and the capillary refill time (CRT) exceeds 1.5 seconds, or the mean arterial pressure (MAP) is less than 60 mmHg. When MAP [mean arterial pressure] is above 60 mmHg, palpation of the strength of the peripheral pulse and observation of oral membrane colour and CRT [capillary refill time] should be used to assess adequacy of peripheral perfusion and cardiac output. Patient monitoring and clinical measurement In Veterinary Anaesthesia (Eleventh Edition), 2014 https://www.sciencedirect.com/topics/immunology-and-microbiology/tissue-perfusion

Different conditions will impact cardiac output and tissue perfusion differently. Paula and Kraepelin’s patients, in addition to their slow breathing had lowered body temperature [perhaps because of their slow breathing rate and their lethargy]….

Mild hypothermia is associated with increased cardiac output, peripheral vasoconstriction and tachycardia due to sympathetic stimulation. Also produces grey colour due to combination of pallor and cyanosis, with skin cold to touch even in areas not usually exposed to cold and with possible puffy face.HYPOTHERMIA https://www.aic.cuhk.edu.hk/web8/hypothermia.htm

Paula and Kraepelin’s patients similar to Paula had all these signs…ALL OF THEM!

These are terrible conditions under which to use a pulse oximeter because it will be inaccurate and misleading, and besides, you still need to do an ABG [arterial blood gas test] because of the toxic appearance of the patient, the altered mood and mental status [ask them to remember 7 digits forwards and backward-Digit Span Test], and because of their too slow breathing.

This is a little complicated but is essentially BASIC FIRST AID.

Pulse oximeters will fail.


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