From Essentials of Emergency Medicine NYC 2017, Reuben Strayer explains how the pulse ox might be the most useful bit of tech in the ED. Pearls: The pulse ox waveform is an excellent indicator of mechanical heart rate and peripheral perfusion. For patients breathing room air, pulse oximetry can be used to monitor for hypoventilation. Nail polish has minimal impact on the accuracy of pulse oximetry. If you are unable to get a good pulse ox waveform by adjusting or repositioning the probe, be concerned that the patient is poorly perfused. “The respiratory rate is the most vital of the vital signs.” Experienced doctors look at a patient who seems well, but understands that they’re not truly well, because they subconsciously notice tachypnea. Subconsciously is the only way to notice tachypnea, because respiratory rate is often not measured accurately. Since we don’t always have access to reliable respiratory rate, Strayer’s go-to vital sign is the oxygen saturation. “Reusable pulse oximeter probes are gross.” One study found that even when these probes are cleaned by standard procedure, ⅔ had bacteria cultured from them. Strayer recommends using single use probes in your department. Wilkins MC. Residual bacterial contamination on reusable pulse oximetry
sensors. Respir Care. 1993 Nov;38(11):1155-60. PubMed PMID: 10145923. Data is conflicting about the effect of nail polish on pulse oximetry readings, but overall it is felt that the impact is minimal. Earlier data suggested that nail polish decreased sat readings by 2-10%, but more recent studies found minimal effect. If it seems that the waveform is affected by nail polish, you can remedy the situation by turning the probe 90 degrees, so it goes sideways through the finger. Yamamoto LG, et al. Nail polish does not significantly affect pulse oximetry measurements in mildly hypoxic subjects. Respir Care. 2008 Nov;53(11):1470-4. PubMed PMID: 18957149. As long as a patient is breathing room air, pulse ox can monitor ventilation and function as a hypoventilation alarm. Significantly hypercapnic patients saturate less than 95% when they’re breathing room air. So if you need to monitor a patient for hypoventilation, such as due to intoxication or procedural sedation, the pulse ox will do a great job of telling you if the patient is still breathing. If you need to give supplemental oxygen, then use capnography to monitor respirations. The pulse oximeter does so much more than provide oxygen saturation. It provides the photoplethysmogram (PPG) which is a waveform that tells you the “mechanical” heart rate. While telemetry gives the electrical heart rate, what really matters to your organs is the mechanical rate. This can be especially helpful during transvenous or transcutaneous pacing. When you have reliable tracing, the pulse ox heart rate is more reliable than the telemetry heart rate. The pulse ox can measure the peripheral perfusion index which is a more sensitive and earlier indicator of hypoperfusion than blood pressure. This is a numerical value which indicates the strength of the pulsations read by the pulse oximeter. It is based on the amplitude of the pulse ox waveform and expressed as a number between 1 (low) and 10 (high). The perfusion index dips before the stroke volume drops and long before the heart rate rises. Many monitors will report the perfusion index in tiny print after the word PERF. Lima AP, Beelen P, Bakker J. Use of a peripheral perfusion index derived from
the pulse oximetry signal as a noninvasive indicator of perfusion. Crit Care Med.
2002 Jun;30(6):1210-3. PubMed PMID: 12072670. van Genderen ME, et al. Peripheral perfusion index as an early predictor for central hypovolemia in awake healthy volunteers. Anesth Analg. 2013 Feb;116(2):351-6. PubMed PMID: 23302972. What if you don’t have a reliable pulse ox tracing? Most of the time this is because the probe is poorly positioned, the patient is moving too much, or there’s a lot of ambient light. If you’ve corrected for these problems and you still don’t have a good tracing, you should be concerned that the patient is poorly perfused. One study of 20,000 anesthesia cases showed that pulse ox failure was directly related to worsening physical status. Moller JT, et al. Randomized evaluation of pulse oximetry in 20,802 patients: I. Design, demography, pulse oximetry failure rate, and overall complication rate. Anesthesiology. 1993 Mar;78(3):436-44. PubMed PMID: 8457044. How does the pulse ox measure oxygen saturation and what is the best way to position the oximeter probe on the finger? One side of the pulse ox puts emits visible (red) light and infrared light. On the other side is the detector. The percent oxygen saturation is calculated based on the different way in which oxyhemoglobin absorbs visible and infrared light compared with deoxyhemoglobin. The pulse ox measures carboxyhemoglobin as if it were oxyhemoglobin, giving a falsely elevated pulse ox reading for a victim of carbon monoxide poisoning. The best spot for a peripheral pulse ox is a place with a lot of capillaries and arterioles, like the fingertips, earlobes, nose, or forehead. Functionally, it doesn’t seem to matter whether the emitter is on the dorsum, volar aspect, or even side of the finger. For convenience sake, most find it ergonomically superior to have the cord and emitter on the dorsum of the finger. Mannheimer PD. The light-tissue interaction of pulse oximetry. Anesth Analg.
2007 Dec;105(6 Suppl):S10-7. Review. PubMed PMID: 18048891 Vegfors M, Lennmarken C. Carboxyhaemoglobinaemia and pulse oximetry. Br J
Anaesth. 1991 May;66(5):625-6. PubMed PMID: 2031826 DeMeulenaere, Susan. "Pulse oximetry: uses and limitations." The Journal for Nurse Practitioners 3.5 (2007): 312-317. Link. Chan ED, et al. Pulse oximetry: understanding its basic principles
facilitates appreciation of its limitations. Respir Med. 2013 Jun;107(6):789-99.