1. Pulse Ox Awesomeness
Pulse oximetry is one of our favorite vital signs, in addition to measuring the percentage of oxygen bound to hemoglobin it:
- Verifies the pulse with a mechanical heart rate.
- Can verify capture when using transcutaneous pacing.
- Measures peripheral perfusion.
Principles of Pulse Oximetry:
- Emits a near-infrared and a red electromagnetic light source that is detected by a photodiode.
- The application of the Beer-Lambert law in pulse oximetry explains the absorption of the two light frequencies (infrared and red) by hemoglobin.
- The Beer portion describes how the amount of light absorbed is proportional to the concentration of the light absorbing substance (i.e., hemoglobin concentration).
- The Lambert portion describes how the amount of light absorbed is proportional to the distance the light travels within the absorbing substance (i.e., vasodilation causing more hemoglobin molecules to be present and absorb light).
- Oxyhemoglobin absorbs infrared light better at 940 nm, while deoxyhemoglobin absorbs red light better at 660 nm.
- An adequate pulse is necessary for pulse oximeters to work because the amount of red and infrared light fluctuates with the cardiac cycle within arterial blood.
Advantages of pulse oximetry:
- Saturation number – provides a value for oxygenation that is easy to comprehend (1-100%)
- Auditory tone – lets you know (without looking at the number) when the oxygen is falling.
- Provides a photoplethysmograph (Pleth or PPG) waveform that indicates the “mechanical” heart rate.
- Presence of a pleth waveform during transvenous or transcutaneous pacing indicated effective pacing.
- What can you do if you do not have a reliable tracing?
- Assure correct probe placement, reposition the probe and eliminate ambient light.
- If there is still a poor tracing consider poor peripheral perfusion (hypovolemia, vasoconstriction, PVD) or even hypoxemia!
- 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.
- The perfusion index is a numerical value that measures the strength of the pulsations read by the pulse ox (indicating the strength of peripheral perfusion). It’s more useful as a trend instead of a single number.
- 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.
- Optimal pulse oximetry monitoring requires a site with dense capillaries like the fingertips, toes, forehead, nose, or earlobe.
- There is work being done on newer generations of pulse oximetry. Future uses of this technology have the potential to evaluate for respiratory rates, detection of carboxyhemoglobin and methemoglobin values, pulsus paradoxus and fluid responsiveness.
Disadvantages of pulse oximetry:
- Reusable pulse ox probes are disgusting and we should use single use probes.
- Wilkins MC. Residual bacterial contamination on reusable pulse oximetry
sensors. Respir Care. 1993 Nov;38(11):1155-60.
- There is a several second lag-time between the real oxygen saturation and pulse oximeter reading.
- Poor peripheral perfusion (such as vasoconstriction, hypothermia, and hypotension) cause difficulty with pulse oximetry readings.
- Carboxyhemoglobin is read as if it were oxyhemoglobin, which gives a falsely elevated pulse ox reading for a victim suffering from carbon monoxide poisoning and hypoxemia.
What do you do with a patient that has nail polish? Turn the pulse ox sideways 90o so that the emitter and the detector on the side of the fingertip, find another site, or clean off the nail polish.
- 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.
Causes and mechanisms of unreliable SpO2 readings
Intermittent drop-outs or inability to read SpO2
Poor perfusion (hypovolemia, vasoconstriction, hypothermia, pump failure or ysrhythmias)
Falsely normal or falsely elevated SpO2
Carbon monoxide poisoning, Sickle cell vasoocclusive crisis
Falsely low SpO2
Venous pulsations, excessive movement, intravenous pigmented dyes, inherited forms of abnormal hemoglobin, fingernail polish (?), severe anemia
Falsely low or high SpO2
Methemoglobinemia, sulfhemoglobinemia, poor probe positioning, sepsis or septic shock
CASE: 54 year old female scheduled for a laparoscopic assisted vaginal hysterectomy for uterine cancer. Toward the end of the procedure the surgeons request a dose of methylene blue during cystoscopy to assess for ureter compliance. Patient has a history of hypothyroidism, sickle cell disease, CAD and PVD.
- Five minutes after the injection of the methylene blue the patient’s oxygen saturation decreases from 99% to 91%.
- Utilizing pulse oximetry data only here are some potential differential diagnoses:
- False reduction in saturation caused by methylene blue
- Pulse oximeter is not secured to finger well (check finger)
- Accidental extubation of patient (check ETT placement and capnography)
- Mucus plug or obstruction of ETT (suction ETT)
- Heart failure or dysrhythmia resulting in hypotension (check BP, ECG, capnography)
- Sickle cell crisis (check ABG, BP, ECG)
- Poor peripheral perfusion (change probe from finger to forehead)
- Cannesson M, et al. Relation between respiratory variations in pulse oximetry plethysmographic waveform amplitude and arterial pulse pressure in ventilated patients. Crit Care. 2005 Jul; 9:R562-R568
- Chan ED, et al. Pulse oximetry: understanding its basic principles
facilitates appreciation of its limitations. Respir Med. 2013 Jun;107(6):789-99.
- DeMeulenaere, Susan. "Pulse oximetry: uses and limitations." The Journal for Nurse Practitioners 3.5 (2007): 312-317.
- 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):1210-3Mannheimer PD. The light-tissue interaction of pulse oximetry. Anesth Analg. 2007 Dec;105(6 Suppl):S10-7. Review.
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|Jul 01, 2019