What is the interpretation of perfusion index (PI) on a pulse oximeter (pulse oxygen saturation monitor)?

From the Guidelines

The perfusion index (PI) on a pulse oximeter should be interpreted with caution, considering its limitations in patients with poor peripheral perfusion, as highlighted in the 2010 American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care 1. When interpreting PI, it's essential to establish a baseline for each patient and monitor trends rather than focusing solely on absolute values. A higher PI value suggests better peripheral perfusion, with values above 1.0-1.4% generally considered normal in adults. However, it's crucial to consider the clinical context, as low PI values (below 0.5%) may indicate poor peripheral perfusion due to conditions such as hypovolemia, vasoconstriction, hypothermia, or peripheral vascular disease. Some key points to consider when interpreting PI include:

• The measurement works by calculating the ratio between pulsatile and non-pulsatile blood flow, with the pulse oximeter's infrared light signal detecting these differences.
• PI can be particularly useful in critical care settings to assess the effectiveness of interventions aimed at improving tissue perfusion.
• Clinicians should be aware of the limitations of pulse oximetry, including its unreliability in patients with severe hypoxemia (SpO2 below 85%) or those with conditions affecting pulsatile flow, as noted in the 2017 BTS guideline for oxygen use in adults in healthcare and emergency settings 1.
• It's essential to interpret PI alongside other clinical parameters, such as heart rate, blood pressure, capillary refill time, and the patient's overall clinical condition, for a comprehensive assessment.

From the Research

Perfusion Index Interpretation

The perfusion index (PI) is a measure derived from pulse oximetry, defined as the ratio of the pulse wave of the pulsatile portion (arteries) to the non-pulsatile portion (venous and other tissues) 2. It is used as an indicator of peripheral microcirculatory perfusion and has been supported by clinical studies for guiding hemodynamic management and serving as an indicator of outcome and organ function.

Clinical Applications

• The PI can detect fluid responsiveness in patients with septic shock, with a moderate ability to predict fluid responders 3.
• The changes in PI can be used as a non-invasive surrogate of cardiac output to detect fluid responsiveness 3.
• The PI measured on different fingers may show variation, with the highest reading found on the middle finger 4.
• The PI value can affect the agreement between noninvasive arterial blood pressure monitors and arterial catheter-based blood pressures 5.

Limitations and Considerations

• Further clinical trials are required to clarify the normal and critical values of PI for different monitoring devices in various clinical conditions 2.
• The PI may not be reliable in all clinical scenarios, and its use should be considered with caution 4.
• The state of perfusion to the fingers, as reflected by the PI, can affect the accuracy of noninvasive arterial blood pressure monitors 5.
• Fluid responsiveness should be assessed in conjunction with other clinical indicators, as the PI alone may not be sufficient to predict fluid responsiveness 6.