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Friday, January 13, 2012

History of the fifth vital sign

Back in 1991 I visited the emergency room because my asthma was acting up.  It was my first visit to the ER since I was discharged from my six months stay at the asthma hospital (NJH/NAC) in 1985.  An RT came into my room to give me a breathing treatment and he slipped this thing over my finger.  That was my first exposure to pulse oximetry -- the fifth vital sign.

I had been to emergency rooms many, many times for asthma in my life, and never had I seen such an object.  A temperature, blood pressure, and heart rate check were common, but to slip something that glows red, and causes a red number to appear on this little box the RT held, was something new.

"What is that?" I asked back then.  

The RT attempted an explanation that zoomed over my head.  "It's called a pulse oximeter.  It tells me how well you're oxygenating and gives me your heart rate."  

I wasn't satisfied with that answer.  I wanted to know more.  What was it?  How did it work?  What significance was it?  What did the number 98 mean anyway?  I asked these questions and, once again, the answer wafted over me like a cool breeze.

In the summer of 1993 I was a journalist for a small weekly newspaper.  The local fire department received funding to get a new piece of equipment that was supposed to help them help sick people.  The objects were quite bulky, pretty heavy, and covered by a large, black case.  

A cord loomed from both, and the EMT I was interviewing slipped the probe over my finger.  "This is what we call a pulse oximeter," he said.  He attempted an explanation, yet again the answer flew over my head.  I ended up taking a picture of the EMT holding the new pulse oximeters.  On the caption under I wrote:  "EMT Josh Paramedic is holding two new pulse oximeters purchased with money from a Grant by..."

My next exposure to the 5th vital sign came as an RT student.

What is pulse oximetry

Simply put, pulse oximetry is basically sends two lights through your finger (or ear lobe or toe), through your artery, and then the light is collected by a sensor on the other side of your finger.  The red light you can see, and the infared light you cannot see.

By calculating the amount of infrared light returning to the sensor, the pulse oximeter will tell you what percentage of hemoglobin is carrying oxygen.  Since it's measurement is made by the pulse of arterial blood, it can also give you a heart rate.

A normal pulse oximeter reading is 98%, however, anything greater than 90% is deemed acceptable.  In this way, supplemental oxygen can be administered and adjusted accordingly to maintain an adequate hemoglobin saturation.  The reading is measured as SpO2 (S = saturation, p = pulse oximetry, O2 = oxygen).

How did the pulse oximeter come to be?  

When I was a kid and was having an asthma attack, the respiratory therapist would place a nasal cannula over my face, and I'd promptly proceed to peel it off.  He'd get mad at me saying, "You really need to keep this on.  We need you to get oxygen."

Yet was my oxygen level really low?  The only way to tell would be to either do an invasive arterial blood gas, something that was rarely done on a kid.  I don't recall an ABG being drawn on me until I was at least in my upper teens.

Other than an ABG, the only way to know if a patient was oxygenating well was to use objective measurements such as skin color.  Usually if a person isn't oxygenating well blood will be shunted from fingertips and lips, and these areas will appear grayish or blue.  This is referred to as acrocyanosis, and is not life threatening.  Usually a low dose of oxygen -- say 2lpm -- will be all that's needed to resolve the problem.

If the core of the the body is blue or gray in color, this is referred to as central cyanosis.  Central cyanosis is critical, meaning oxygen isn't getting into that person's body at all, and oxygenated blood -- what little there is -- is shunted directly to vital organs such as the heart, kidneys and lungs.  In these situations, usually large amounts of oxygen are needed to remedy the problem, and perhaps even some form of positive pressure breathing such as from an Ambu-Bag, BiPAP or ventilator.

Obviously if you see cyanosis you know oxygen is needed.  Still, how much oxygen do you give?  When do you taper it off?  And what if the patient isn't oxygenating well yet doesn't show any cyanosis?  What do you do then?  Chances are, in the days prior to pulse oximetry, you winged it.  Some patients got too much oxygen, and some didn't get enough.  Outside of doing an invasive ABG, you really didn't have any means of monitoring and adjusting oxygen.

This was pretty much how it was even up to the early 2000s at some institutions.  Yet then came along the 5th vital sign.  This vital sign -- pulse oximetry -- made the job of respiratory therapists "much easier," notes Gennie Ridlen in her article Pulse Oximetry:  A Historical Perspective.

She notes the history of the pulse oximetry can be traced back to 1862:
  • In 1862 Hoppe-Seyler disovered that oxygen was transported by hemoglobin, and he and he referred to the oxygen-hemoglobin compound as oxyhemoglobin
  • In 1864 Stokes proved that oxygen was transported in the blood by hemoglobin
  • In 1862 von Vierodt invented the first pulse oximeter.  He measured oxygen consumption using transmitted light by wrapping a rubber band around his wrist to cut off circulation and shining a light on his hand, he saw the two bands of oxyhemoglobin disappeared and a band of deoxyhemoglobin appeared.  Using reflected light from a spectrometer, he measured the oxygen consumption of the living tissues by noting the time that elapsed as oxyhemoblogin changed into deoxyhemoglobin."
Between 1862 and 1977 the pulse oximeter was improved upon to the point that the first pulse oximeter was marketed in the U.S. in 1977.  The oximeter used a finger probe with fibre optic cables that were very sensitive to motion.

By the late 1970s new probes were invented to solve the problem with motion, and a heart rate tracker was added and the device was precalibrated to make it more accurate.

As with any new discovery in medicine, it took another 10 years for the pulse oximeter to make any real impact in the medical industry.  Interest finally started growing in the early 1980s as the device was determined useful when a patient was sedated to monitor oxygen saturation, and by 1987 it became standard practice on sedated patient, operating rooms, critical care units and emergency rooms. 

The devices continued to improve, and by 1995 you had a choice between bulky bedside monitors and finger probes that could fit into your pocket.

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