Pulse oximetry is based on optical physics principles developed in the 18th and 19th centuries. Many high school students may recognize these principles as the Beer-Lambert Law. In 1860 German scientists used these ideas to develop analytical spectroscopy, the use of light to measure the chemical composition of solids, liquids, and gases. In the 1930s George Stokes applied analytical spectroscopy to the different wavelengths of light (color) reflected by hemoglobin in its oxygenated and deoxygenated states. For the first time in history the oxygen saturation of hemoglobin could be quantitatively measured, but the device had several practical limitations. Over the next 40 years those practical limitations were solved by the US military and physiologists. Then in 1972 a young Japanese bioengineer named Takuo Aoyagi figured out how to use arterial pulsations to better calibrate the device.
Modern pulse oximetry was born- although Aoyagi’s employer, Nihon Kohden Corporation, did not see the potential in his innovation other people did. In 1979 Biox Technology was founded in Denver, CO, USA and in 1981 Nellcor Company in Hayward, CA, USA. Nellcor had an anesthesiologist Dr. Mark Yelderman, who introduced the pulse oximetry device at the 1983 ASA annual conference. By 1986 the American Society of Anesthesiologists endorsed pulse oximetry as a standard monitor. Interestingly there are no clinical trials showing pulse oximetry decreases mortality during surgery. Keep in mind there are also no clinical trials showing parachutes decrease death during skydiving!
The first time I understood the important of a pulse oximeter was during My 4th year of medical school during my pediatric anesthesiology rotation. I was with a PGY-3 resident for the day. He extubated an adorable 3 year old girl after a hernia operation. No one was in the operating room besides him, the patient, and myself. Suddenly her oxygen saturation dropped to the 60s, dangerous low. The resident immediately instructed me to hold an ambu-bag to her face with both hands. He quickly moved to the anesthesia machine turned a few dials then squeezed a green bag. I remember thinking: “I really hope this guy knows what he is doing, because I have no idea what is happening!!” After about 15 seconds she turned blue, then suddenly her color returned and she was breathing normally. She woke up 10 minutes later without further complications. I was speechless.
That resident expertly managed a laryngospasm, a complication most commonly seen in pediatric patients when the vocal cords become irritated then spontaneously close without warning. Without the pulse oximeter we might have intervened too late. Our first sign of a problem would have been the child turning blue. No one even knew this resident saved this girl’s life; not her, not her parents, not the surgeon, and not the nurse. Anesthesiology is a solitary act of defiance against Nature. You must perform at the highest level to even have the chance of success.
End-tidal carbon dioxide (EtCO2) is the other monitor that changed the landscape of anesthesiology. The measurement of CO2 from human breath was first reported by John Tyndall in 1865. Multiple practical advances led to the first volumetric capnogram in 1928 by American physicist John Aitken and English physician Archibald Clark-Kennedy. After wartime applications the technology entered clinical application in 1955 when the first capnographic profiles of human respiration appeared in anesthesiology literature followed by the 1970s when EtCO2 changes were linked to clinical changes in patients. In 1978 clinical capnography was introduced in the United States at the World Congress of Critical Care medicine. Initially it was thought to be “of little value”. Then a Canadian malpractice insurer granted massive discounts to anesthesiologists who used capnography during their cases because it decreased malpractice premiums paid out for esophageal intubation claims. The technology became widespread in the 1990s. It eliminated mortality associated with esophageal intubation and greatly improved the safety of sedation especially in endoscopy procedures.
I have accidentally intubated the esophagus twice in my residency. Both times the mistake was immediately recognized due to capnography. I re-intubated both patients safety and both cases proceeded without further complications or patient harm. If I practiced in the 1970s those two patients might have suffered brain damage or death. Today, I confirm my endotracheal tube placement with direct or indirect visualization, bilateral breath sounds, pulse oximetry, and capnography.
Our five senses are also important for detecting complications early. As a resident I had the privilege of training with Dr. Anahat Dhillon. Her kind demeanor betrays her depth of expertise and high expectations for herself and her residents. One day I was working with her during a robotic pancreatic resection. She asked me close my eyes then guess the heart rate and oxygen saturation. Because the tone of the heart rate changes based on the oxygen saturation, an anesthesiologist can know both values just be listening. After about 30 minutes of trying I could be within 5 beats per minute of the true value for the heart rate and within 2% of the oxygen saturation.
Since that day with Dr. Dhillon I learned my ears can give me information about the entire operating room. I learned to listen to the surgeon’s voice, the electrocautery, the suction, the circulating nurse, and my own monitors in order to grasp all activity around me. This is like sitting in silence in the middle of the forest. At first you will hear nothing. Eventually you will become aware of a symphony of activity around you: the sounds of the leaves rustling, animals walking, water running, and birds calling. I learned to isolate and interpret the individual sounds in real time. Now looking over the curtain seemed like cheating.
A year after my case with Dr. Dhillon I did a liver transplant with Dr. Avner Gereboff. Initially the case went well: the failing liver was removed, new liver inserted in its place then the supra-hepatic vena cava (1), infra-hepatic inferior vena cava (2), portal vein (3), and hepatic artery (3) were anastomosed to the patient’s native vasculature. Before working on the common bile duct (5) they repaired the femoral vein after decannulation from veno-veno bypass.
As the surgeons were suturing the femoral vein I noticed their yankauer suction catheters near the liver were making a characteristic sound. I asked them to explore the liver bed for an arterial bleed. At first they resisted then when they looked to humor me, immediately as they lifted the liver, multiple bright red jets of arterial blood appeared. My junior resident demanded I tell her how I knew about the arterial bleed! I smiled then said: “close your eyes and tell me what the heart rate is”.