Pulse Oximeters

Pulse oximeters are an interesting device. They measure the percentage of blood or hemoglobin that is saturated with oxygen in a patient’s body. An LED light source gets shined through a thin part of the body such as a fingertip or infants foot and picked up by a phototransistor on the other side. Because hemoglobin has a different color depending on the amount of oxygen it’s saturated with the pulse oximeter computer chip analyzes the difference in the wavelength of the light and computes the percentage of oxygen in the blood.

The pulse oximeter also detects heart rate or pulse and produces an audible beep with every pulse. Because the pulse oximeter is very dependent on pulsatile flow there are situations where it may not work well. Most of the time the problems we get in our shop involve the sensor attached to it. There have been rare cases when the sensor was fine and the complaint was that it would give erratic readings or suspiciously low saturation levels of a patient. With a little investigation it turned out that the patient had been brought in for hypothermia where they had been shoveling snow for an extended period of time.

Because the probe is attached to a finger most of the time, the patient had very little blood flow into their digits. Remember the pulse oximeter works on the basis of pulsatile flow. No flow, no SAT reading. After the patient warmed up a little while the pulse oximeter was tried again and worked fine. Certain heart conditions and medications that restrict vascular flow will also cause erroneous readings with the machine. Because the pulse oximeter shines a light through the patient I have seen instances where bright fluorescent lights in an emergency room cause false readings or in a few cases no readings. Simply covering the probe with a blanket or other object to restrict ambient light cured the problem.

Anesthesiologists depend heavily on pulse oximeters during surgical procedures. Because a patient has a mixture of several different gases being pumped into their lungs through the ventilator the oxygen amount in the blood becomes critical. An insufficient amount of oxygen for a prolonged surgical procedure will cause brain damage or paralysis to a patient. The anesthesiologist is constantly monitoring the patient’s oxygen saturation levels and adjusting the air, oxygen, nitrous and anesthetic gas accordingly to maintain a specific percentage of oxygen in the patient’s blood. There is no calibration for the machine but a PM generally using a simulator to simulate the pulse and oxygen percentages and compare them against the readout on the pulse oximeter screen. Standard deviation for most machines is 1% +/- of the displayed readings between the simulator and the pulse oximeter.

If a simulator is not available then it becomes necessary to test 3 or 4 pulse oximeters as a group with your own finger in a controlled situation and monitor the variance in machines. To do this correctly use the same probe for all 4 machines, and do them all in the same location by the same person with only one specific chosen finger, like the index finger of either hand. I like to use the index finger of my left hand because it’s closest to the heart and has the strongest pulse flow. If all machines measure within 1% of each other then it’s a suitable indication that the saturation measurements are accurate. The pulse rate will be more difficult to measure accurately with this method because subtle changes in temperature, body movement, external stimuli, (phone ringing, people talking) all will change the heart rate a little. I generally go with a 5% deviation in pulse rate when I use this method. For example a resting pulse rate of 70 beats per minute shouldn’t change more than 3.5 beats per minute +/- for the short duration of testing for these machines. If they fall within this range they pass.