Pulse Oximetry

Oxygen saturation levels around 96% to 100% are considered normal at sea level. Normal individuals living at higher elevations may have lower oxygen saturation levels. Pulse oximeters are usually calibrated to a range of saturation from 70% to 100% with an accuracy of 2% to 4%, which means that pulse oximeter is reading lower than 70%, may not be accurate compared to the gold standard invasive blood gas measurements. While they may not demonstrate absolute accuracy, they usually still track the oxygen saturation and display lower numbers. There are some technical reasons for the 70% calibration; however, by that time, the patients usually have clinical signs of hypoxemia that would not need invasive clinical confirmation and treatment to reverse that level of hypoxia would not differ from the treatment for a patient with a saturation of 70%. Critical findings that would prompt intervention for most patients would likely be in the mid to high 80% range at sea level as the partial pressure of oxygen would be in the 60 mmHg range. However, the critical numbers that require treatment may be lower at high altitude or in patients with hypoxic heart defects where venous blood get mixed with arterial blood going into the systemic circulation. 

Pulse oximetry relies on light absorption through a tissue bed with pulsating blood. Therefore factors that interfere with those parameters can interfere with the reading of pulse oximeters. One of the common examples of interfering factors is nail polish and artificial fingernails [8][9]. Numerous publications have covered this topic, but with varying conclusions implicating mostly blue or black colored nails. Artificial fingernails have also been reported as either detrimental or having no effect. Because of the ever-changing fashion trends, it will be difficult to make a general statement which nail treatments will be safe and which should ideally be avoided. Placing the sensor sideways on a finger that does not give a reading has sometimes been used with success, but it should be remembered that this will be outside the calibration of that sensor.

Intravenous dyes such as methylene blue or indocyanine green that are sometimes used for surgical or diagnostic procedures will color the serum in the blood and may interfere with the light absorption spectrum and give low readings [10][11].

Dyshemoglobinemias, such as carboxyhemoglobin, methemoglobin, and others, will change the color and absorption spectrum of blood and give false readings. In that case, a confirmation with a Co-oximeter should be obtained. Some newer pulse oximeters using multiple wavelengths can display some methemoglobinemias.

Light pollution into the sensor part of the probe can be another interfering factor to an accurate reading, such as certain ambient light or other probes emitting light in a similar spectrum in the vicinity of the pulse oximeter probe, and should be avoided by covering the site or the probe.

Pulsating blood is another prerequisite for an accurate reading. The pulse amplitude in tissue beds is very small and accounting for about only 5% of the pulse oximeter signals to be available for analysis. Any further decrement in pulse wave amplitude such as severe hypotension, cold extremities, Raynaud disease or other factors such as excessive motion may interfere with an accurate reading. Hospital grade pulse oximeters usually can read through perfusing cardiac arrhythmias such as atrial fibrillation and premature atrial or ventricular contractions. Most pulse oximeters display the plethysmographic waveform in addition to the saturation number to give the clinician another parameter to interpret the saturation number.

Pulse oximeter manufacturers are trying to mitigate these factors by different strategies with hardware sensor and software algorithm improvements. Therefore, publications reporting limitations of certain pulse oximeters may be specific to that manufacturer or make and model. 

Pulse oximeters are very well tolerated and are used in all age groups ranging from neonates to geriatric populations. Energy transfer to patients is low and, while heat blisters with prolonged use of certain sensors are possible, most of the complaints come from an allergic reaction to the adhesives or the mechanical adhesive properties of the single-use sensors. Pressure points from clip sensors are possible, but also rare and can be minimized by using adhesive probes.

No special education is usually needed beyond explaining that the sensor measures the oxygen level in the patient’s blood, that the risk of these monitors is very low, and to report any discomfort to the patient care provider. 

Pulse oximetry is the minimum standard intra-operative monitoring by the ASA, the World Federation of Societies of Anesthesiologists, and the World Health Organization [12] . With the use of pulse oximeters as a standard of care for all anesthetics and for most patients in emergency [13] and urgent care settings as well as monitoring a significant percentage of hospitalized patients, the rate of undiscovered hypoxic events leading to bad outcomes has decreased significantly as we can monitor oxygen saturation continuously. Spot checks in hospitalized patients are less optimal, as breathing and oxygenation is a dynamic process and can change rapidly. The challenge with continuously monitored parameters is that someone has to continuously monitor the monitor. However, there are now sophisticated monitoring stations available from multiple vendors that will not only alarm when a critical value is reached, but also notify the patient care provider by phone or pager. In order to minimize false alarms and associated alarm fatigue, individualized alarm settings for each patient should be used for the pulse oximeter alarm parameters.