Introduction

Pulse oximetry is a non-invasive monitor that measures the oxygen saturation in the blood by shining light at specific wavelengths through tissue (most commonly the fingernail bed). Deoxygenated and oxygenated hemoglobin absorb light at different wavelengths (660 nm and 940 nm respectively), and the absorbed light is processed by a proprietary algorithm in the pulse oximeter to display a saturation value. It is a standard monitor for all anesthesia cases in most developed countries [1][2] and also used in emergency departments, hospital wards, and ambulances to assess blood oxygenation in patients with respiratory difficulties or monitor for respiratory depressant effects of pain medications. Since its widespread use in hospitals, the incidence of unrecognized desaturations has decreased significantly [3]. In addition to hospital-grade pulse oximeters, newer and much smaller consumer-grade models are rapidly gaining popularity in the sports, private aviation, mountain climbing, and other recreational activity communities. Because of the size and low cost of these consumer grade models, many patients with chronic respiratory illnesses are purchasing these to titrate either their medications or oxygen flow at home. However, most of those consumer-grade devices have not been evaluated by the Food and Drug Administration to diagnose or treat diseases.

Specimen Collection

Pulse oximeters are non-invasive and well tolerated. Tissue beds most commonly used are finger or toenail beds. Since clinicians are most interested in arterial saturation, the algorithm of the machine is looking for arterial pulsations, which are very small in the arteriolar/capillary tissue beds. Subsequently, in patients with low perfusion to or excessive movement of the extremities, a reliable signal may not be easily obtained. Other application sites such as earlobes and forehead [4], nasal alar or lip have been used with success in those cases. Site-specific sensors have been developed and calibrated to those specific sites and should be used for those applications wherever available. In addition to collecting light absorption spectra, most pulse oximeters also capture the pulse wave activity and display the plethysmographic waves from which the clinician can gain additional valuable information.  

Procedures

Pulse oximeter probes have a light emitter and a sensor that should be aligned to capture the light on the other side of the tissue bed or the reflection of light from a site such as the forehead. They either come in single-use adhesive probes or as multi-use clips [5]. However, due to infection control concerns, recently single patient use clips have been introduced to the market for certain sites. Different application sites have different probes that are optimized for that site and should be used whenever possible.     

Indications

Pulse oximetry is one of the standard American Society of Anesthesiologists monitor parameters for all anesthesia cases. In addition, it should be used for hospitalized patients that receive medications that may impair their respiration (mostly opioids). All patients with acute respiratory problems should be monitored with pulse oximetry whether in an emergency room, intensive care unit, general hospital or pre-hospital ambulance setting. Pulse oximetry is not only used to rapidly diagnose hypoxia but also to titrate the treatment for hypoxia such as ventilator support parameters and supplemental oxygen in order to avoid hyperoxia, which can be detrimental in neonates but as recent publications also suggest in adults suffering from myocardial infarction and possibly other diseases. 

Potential Diagnosis

Conventional pulse oximeters diagnose hypoxia. However, recent developments in certain pulse oximeters now can also assess other parameters such methemoglobin and carboxyhemoglobin levels, total hemoglobin, and even oxygen levels above 100% saturation. With the display of the pulse wave activity parameters, volume status in intubated patients, as well as respiratory rate, can be displayed in certain pulse oximeters by looking at the pulse pressure variation with the respiratory cycles [6]. 

The use of an index of perfusion has been used to diagnose the success of increased blood flow after sympathectomy. [7]

Normal and Critical Findings

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. 

Interfering Factors

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 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.

Patients with dark skin may have their oxygen saturation overestimated by ~2%. This varies by which device is used. Pulse oximeters can be less accurate at cold temperatures. Warm site temperature (about 33°C) should be maintained for reliable readings.

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. It 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 accounts 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. 

Complications

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.

Patient Safety and Education

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. 

Clinical Significance

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 monitor the monitor continuously. 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.

The normal oxygen levels in a pulse oximeter usually range from 95% to 100%. Hypoxemia is oxygen saturation less than 90%