Local Warming to Improve Pulse Oximetry Accuracy in Critically Ill Patients

Pulse oximetry is a fundamental component of monitoring and oxygen titration in critically ill patients. Its accuracy may, however, be impaired by reduced peripheral perfusion. Low peripheral perfusion is common in mechanically ventilated intensive care patients and may contribute to clinically relevant differences between peripheral oxygen saturation measured by pulse oximetry (SpO₂) and arterial oxygen saturation measured by blood gas analysis (SaO₂).

Local warming increases cutaneous blood flow and is routinely used to improve peripheral perfusion during procedures such as capillary blood sampling. Preliminary single-center data suggest that local warming may also improve pulse oximetry accuracy in critically ill patients with low peripheral perfusion. Whether this effect is present when evaluated in a randomized controlled trial is unknown.

This is a prospective, multicenter, parallel-group randomized controlled strategy trial conducted in four intensive care units. Mechanically ventilated adult ICU patients with an arterial catheter and a finger perfusion index (PFI) <1.0 will be randomized in a 1:1 ratio to a protocolized local warming strategy or usual care without protocolized warming. Randomization will be stratified by study site and Fitzpatrick skin phototype.

In the intervention group, local warming will be applied to the dorsum of the hand and distal forearm for 15 minutes, with a target interface temperature of 41-42°C. Repeat warming is permitted during follow-up if finger PFI decreases below 0.5 according to the prespecified study strategy. The control group will receive usual care without protocolized local warming.

SpO₂, finger PFI, earlobe PFI, and arterial SaO₂ will be assessed at baseline, 15 minutes, and 60 minutes. Additional PFI measurements will be obtained at 120 and 240 minutes. Oxygen therapy will otherwise be managed according to standard clinical care.

The primary objective is to compare pulse oximetry bias between the randomized groups at 15 minutes. Bias is defined as SpO₂ minus SaO₂, expressed in percentage points. The primary analysis will compare the groups using an analysis of covariance adjusted for baseline SpO₂-SaO₂ bias and study site.

Secondary objectives include assessment of pulse oximetry accuracy at 60 minutes, the proportion of measurements within ±2 percentage points of SaO₂, changes in peripheral perfusion during 4 hours of follow-up, the local versus systemic perfusion response assessed using finger and earlobe PFI, the response to repeat warming, safety outcomes, and possible treatment-effect modification according to Fitzpatrick skin phototype.

The planned sample size is 240 participants, with 120 participants allocated to each study group.

Inclusion criteria

• Age 18 years or older.
• Admitted to an intensive care unit.
• Receiving invasive mechanical ventilation via endotracheal tube or tracheostomy.
• Existing arterial catheter placed for a clinical indication.
• Objectively verified low peripheral perfusion, defined as finger perfusion index (PFI) <1.0 at screening.
• Stable baseline pulse oximetry signal permitting calculation of the SpO2-SaO2 difference.
• Clinical condition considered sufficiently stable to allow study measurements without interfering with ongoing treatment.

Exclusion criteria

• Absence of a stable baseline pulse oximetry signal on all available sensors, preventing calculation of the SpO2-SaO2 difference.
• Skin injury, burn, infection, or other local skin condition of the hand or forearm that prevents safe application of the warming pad or pulse oximetry sensor.
• Clinical instability for which the treating physician considers study participation likely to interfere with acute treatment or patient safety.
• Known hypersensitivity to materials used in the warming pad or sensors.
• Participation in another ongoing interventional study that could affect the primary outcome.