Pulse ox in the clinical setting

As patient status can change rapidly during anesthesia, a qualified anesthesia provider should be present continuously to monitor the patient and provide anesthetic care. The American Society of Anesthesiologists has determined standards for basic anesthetic monitoring, which state that “during all anesthetics, the patient’s oxygenation, ventilation, circulation and temperature shall be continually evaluated.” 

Regarding blood oxygenation and SpO2 measurement during anesthesia, ASA standards state that “a quantitative method of assessing oxygenation such as pulse oximetry” should be used at all times. It is important that the volume, pitch, and low threshold alarm noises be audible to the anesthesia care team personnel throughout the duration of anesthesia.

References: ASA Monitoring Requirements

Keywords: anesthesia, monitoring, frequency

Monitoring SpO2 is a critical part of managing patients with respiratory failure. The frequency of monitoring should take into consideration the severity of the patient’s illness and be tailored to the individual patient at the provider’s discretion. Here are some considerations:

• For patients with mild disease, SpO2 should be checked on initial assessment.
• For patients with moderate disease, SpO2 should be monitored intermittently (about every four hours).
• For patients with severe or critical disease, SpO2 should be monitored continuously or as frequently as possible.

For more information on routine monitoring and patient care, use this Charting Tool from the OpenCriticalCare.org project. 

References: Charting Tool Templates for COVID19 Care

Keywords: monitoring SpO2, respiratory failure, COVID

While SpO2 can be useful in many cases, there are certain situations where an arterial blood gas (ABG) should be drawn and analyzed. If the pulse oximeter shows a tracing that is dampened or erratic, or low PI or signal quality indicator, this may indicate that the SpO2 readings are unreliable and an ABG is warranted. Also, if there are any other factors present that might reduce the pulse oximeter’s accuracy (such as poor perfusion, low body temperature, etc.), an ABG should be obtained. 

Other reasons to get an arterial blood gas include if there is a clinical suspicion of Met-Hb, CO-Hb, S-Hb, or other hemoglobin types. Additionally, pulse oximetry does not provide information about ventilation or acid-base status, so an ABG is needed in situations where this information is also needed.

Keywords: ABG, arterial, blood gas

Pulse oximeters can be used to measure many different clinically important values. Some (but not all) pulse oximeters can measure the following:

• Respiratory rate
• Perfusion
• Carboxyhemoglobin
• Methemoglobin
• Hemoglobin concentration
• Pulsatility variation

Keywords: measurement, respiratory rate, pulsatility variation

‘Differential bias’ (sometimes referred to as disparate bias) attempts to describe how much a pulse oximeter performs differently in people with light skin pigment as compared to dark skin pigment. To determine this, we compare the device’s bias (how much it over or under-reads SpO2) for people with very light skin and people with very dark skin*. 

Differential bias is calculated independently for two different SpO2 ranges (70-85% and 85-100%). Thresholds for allowable differential bias are not yet established though proposals have targeted 3.5-4% for the SpO2 70-85% range, and 1.5-2% for the SpO2 85-100% range. 

For example:

• Scenario 1: If a pulse oximeter on a person with light skin reads 1% higher than their true blood oxygen saturation (e.g., the oximeter shows 91% when the actual blood oxygen saturation is 90%), and on a person with dark skin it reads 2% higher (e.g., it shows 92% when the actual blood oxygen saturation is also 90%), the differential bias between the two is 1%.
• Scenario 2: If the pulse oximeter on a person with light skin reads 2% lower than their true blood oxygen saturation (e.g., the device shows 88% when the actual blood oxygen saturation is 90%), and on a person with dark skin it reads 1% lower (e.g., it shows 89% when the true value is 90%), the differential bias here is also 1%, but in the opposite direction.
• Scenario 3: If the pulse oximeter on a person with light skin shows no bias (e.g., reads exactly 90% when the actual blood oxygen saturation is 90%), but on a person with dark skin it reads 5% higher (e.g., the device shows 95% when the actual value is 90%), the differential bias is 5%.

These examples demonstrate how differential bias can manifest as either higher or lower readings depending on skin pigment.  

*Using modeling of skin pigment and SpO2 data, very light skin and very dark skin are defined as having an difference in ITA of 100. Read more about ITA here.

The optimal SpO2 for patients with respiratory failure has not been well established and is still being evaluated. The World Health Organization (WHO) guidance for patients with hypoxemic respiratory failure due to COVID-19 recommends the following targets: Initial SpO2 of >94% for stabilization, then >90% for stable patients who are not pregnant or 92-95% for stable patients who are pregnant. It is important not to make SpO2 goals too high, as this can cause oxygen toxicity and will deplete the oxygen supply more quickly. For more discussion on optimal SpO2 goals in patients with respiratory failure, please read more here.

References: WHO SARI Toolkit 

Keywords: target SpO2, goal, respiratory failure, COVID-19