Top EMS Game Changers – #4: Pulse oximetry
SpO2, formerly a nice-to-have metric, has become the sixth vital sign
Pulse oximeters and me, we got started in EMS around the same time. I was a new EMT in 1994 when I attended an in-service for an early prehospital version of the device – a battery-powered, plastic unit the size of a Cracker Jack box.
Those gadgets had cables with spring-loaded plastic probes for attachment to peripheral body parts. You’d clamp the sensor onto a finger, toe or earlobe, push a button and wait for the machine to show the heart rate and hemoglobin saturation, usually an indication of oxygenation.
Patients whose readings were in the 95-98 percent range often seemed encouraged by our assurances that they were "doing fine." Those with persistent chest pain were more skeptical.
The science of BLS
Other than AEDs, pulse oximeters were the only high-tech tools we BLS providers had. They became a source of self-respect, not to mention multitasking; we figured we’d know how effectively patients were breathing while we saw their heart rates digitally displayed. Automating those two vital signs meant we’d have more time for SAMPLE, OPQRST and maybe a little TLC.
It wasn’t long before I noticed colleagues hooking up the pulse ox before even speaking with their patients. Not knowing how the darn things worked merely added to their appeal.
It turned out the science behind oximetry wasn’t complicated: Tiny beams of red and infrared light are transmitted by the probe through skin. The amount of infrared light collected by a sensor determines the peripheral saturation, or SpO2 – a close approximation and painless alternative to the arterial value. The procedure is non-invasive and makes even a novice caregiver look as cool as Mr. Spock scanning a sub-species of Denebian slime devil.
The limitations of technology
The disadvantages of relying on pulse oximetry for assessment soon became clear, though. While the state-of-the-art hardware did measure hemoglobin saturation, it didn’t tell us what our patients’ hemoglobin was saturated with. We started to hear about SpO2 percentages falsely inflated by carboxyhemoglobin, a byproduct of carbon monoxide. Readings could also be distorted by methemoglobin, poor perfusion, movement, nail polish or even dirt under a fingernail.
Digital heart rates were problematic, too. However precise at the instant recorded, those numbers failed to convey the strength or regularity of pulses – often more important indicators of health than beats per minute. I knew I was progressing prehospitally to a more knowledgeable level of ignorance when I started insisting my partners palpate patient pulses the old-fashioned way, and not make oximetry priority number one.
Portability versus integration
Boxy pulse oximeters weren’t mainstream for very long. My agency switched to units not much bigger than the fingertip probes we’d been using. The heart rate and SpO2 would flash on a tiny integrated screen after the usual brief pause to sample the signal.
Those key-fob-size devices were less than spectacular: their plastic clamps weren’t as strong and batteries were harder to replace. The miniature units also got lost and stepped-on a lot.
There was even a rumor at my garage that someone had left one of the new oximeters in a pants pocket on laundry day. Without confirming or denying such a story, let me just suggest that the ability of high-tech circuitry to withstand water, detergent and spin cycles is novel research.
By the early 2000s, we started to see oximetry integrated with EKGs. That’s become standard. The finger clamp looks the same as it used to, but now it plugs into your cardiac monitor instead of a separate device.
The sixth vital sign?
More important than deciding how to package pulse oximetry was the growing recognition that hyperoxia secondary to unrestrained O2 administration could adversely affect MI and stroke outcomes. The theory is that free radicals associated with excess circulating oxygen further damages ischemic coronary or brain tissue. Consequently, the American Heart Association added SpO2 guidelines to ACLS algorithms. Now we’re supposed to withhold oxygen unless the sat is less than 90 percent in the setting of acute coronary syndromes and below 94 percent for everything else.
So what’s the verdict on pulse oximetry? SpO2, formerly a nice-to-have metric, has become the sixth vital sign. Clinically, though, the signs and symptoms of patients who need oxygen haven’t changed. Perhaps the main purpose of pulse oximetry is to discourage those of us who should know better from giving O2 for no particular reason.
- Frea R. History of Pulse Oximetry and the 5th vital sign. Available at: www.hardluckasthma.blogspot.com.
- Barker S, Hay B, Poets C, et al. History of Pulse Oximetry. Available atL www.oximetry.org
- Cydulka RK, Shade B, Emerman CL, et al. Prehospital pulse oximetry: useful or misused? Annals of Emergency Medicine. 1992 Jun;21(6):675-9.
- Witt C. Vital signs are vital: The history of pulse oximetry. ACP Hospitalist. May 2014.
- American Heart Association. Advanced Cardiac Life Support Provider Manual. 2016.