Pulse oximeter index offers non-invasive guides for fluid therapy

A prospective study conducted at Avicenna University Hospital (Cadi Ayyad University) suggests that the plethysmographic perfusion index (PPI), a non-invasive parameter derived from pulse oximetry, can help identify fluid responsiveness in critically ill patients with acute circulatory failure. The research is published in the Journal of Intensive Medicine.

Why it matters?
When patients develop shock or acute circulatory failure, deciding whether to administer intravenous fluids is a crucial step in their care. If too little fluid is given, organs may remain underperfused and risk injury. But if too much fluid is administered, it can accumulate in the lungs and tissues, worsening oxygenation and leading to complications such as pulmonary edema. Traditionally, advanced hemodynamic monitors or echocardiography are used to guide these decisions—but such tools may not always be available, especially in the hectic early hours of emergency care or in resource-limited environments.

What is PPI?
The PPI is a simple index automatically generated by most modern pulse oximeters. It reflects the ratio of pulsatile to non-pulsatile blood flow detected in the fingertip or earlobe. Because it is influenced by changes in stroke volume and peripheral circulation, clinicians have proposed that changes in PPI after a fluid bolus might mirror changes in cardiac output. This would make it a low-cost, non-invasive way to help predict whether patients are “fluid responsive.”

What the study did?
The Moroccan research team conducted a prospective observational study in a 10-bed intensive care unit. Fifty adult patients with acute circulatory failure were enrolled between February and September 2024. Each patient received a standardized 500-mL intravenous fluid bolus.

Fluid responsiveness was determined using transthoracic echocardiography, the current reference method, by measuring changes in the velocity–time integral (VTI) across the left ventricular outflow tract. A patient was classified as fluid responsive if VTI increased by 15% or more after the fluid challenge. PPI was recorded from the bedside monitor before and after the bolus, and the researchers calculated the relative change (ΔPPI).

What the study found?
Two-thirds of the patients (66%) were fluid responders by echocardiography. A 33% increase in PPI correctly identified responders with 70% sensitivity and 82% specificity. The overall diagnostic accuracy was moderate, with an area under the ROC curve of 0.78. Importantly, a “gray zone” of inconclusive values between 0% and 88% encompassed about 30% of patients, highlighting that ΔPPI should be interpreted alongside other clinical signs.

Although ΔPPI did not correlate perfectly with echocardiographic changes, a directional analysis showed 70% agreement between the two measures—suggesting that PPI does capture real trends in stroke volume changes.

Implications for practice
Because PPI is automatically calculated by standard pulse oximeters, it requires no additional equipment, cost, or technical training. This makes it especially attractive for resource-limited hospitals and emergency situations, where more sophisticated cardiac output monitoring is unavailable.

“Because PPI is derived from the standard pulse oximeter, it is widely accessible, simple, and non-invasive,” explains corresponding author Dr. Younes Aissaoui, intensivist-anesthesiologist at Cadi Ayyad University. “Our findings support the use of ΔPPI as a pragmatic adjunct for fluid management, especially in resource-limited and emergency settings where advanced monitoring is often unavailable.”

Global relevance
The study adds to a growing body of evidence that PPI can help guide resuscitation decisions in intensive care. It is also one of the few prospective studies from North Africa exploring simple, non-invasive monitoring strategies in critically ill patients. This underlines the importance of pragmatic tools that can be used worldwide—not just in high-income settings.

Looking ahead
The authors caution that larger multicenter trials are needed to confirm these results and refine thresholds for clinical use. They also emphasize that ΔPPI should not replace—but rather complement—clinical judgment and, where available, echocardiography.

Future technological refinements and integration with other hemodynamic indicators may further enhance its diagnostic performance and clinical utility.

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Reference
DOI: 10.1016/j.jointm.2025.07.004

About the Author
Dr. Younes Aissaoui is an intensivist-anesthesiologist and faculty member at the Faculty of Medicine and Pharmacy of Marrakech, Cadi Ayyad University, Morocco. His research focuses on sepsis, hemodynamic monitoring, and pragmatic diagnostic tools for resource-limited settings. He has led multiple prospective ICU studies on fluid responsiveness, echocardiography, and innovative monitoring approaches, and is actively involved in teaching and international critical care initiatives to advance both science and ethics in intensive care medicine.