Early Detection of Ulnar Sparing After Supraclavicular Block Using Infrared Thermography and Perfusion Index: A Prospective Observational Study

Aim of the Study The aim of this study is to evaluate the effectiveness of early temperature changes measured by infrared thermography and perfusion index monitoring in predicting ulnar nerve sparing following ultrasound-guided supraclavicular brachial plexus block.

Patients and Methods Study Design This study will be a prospective observational diagnostic accuracy study conducted at the Department of Anesthesia, Surgical ICUS &Pain Management in Fayoum University Hospital. The researcher will inform the participants about the objectives of the study, the examination, and the investigation that will be done. Also, the confidentiality of their information and their right not to participate in the study. Written informed consent will be obtained from all patients included in the study.

Study Population Adult patients scheduled for elective upper limb surgery distal to the elbow under ultrasound-guided supraclavicular brachial plexus block.

Inclusion Criteria

1. Adult patients aged 18-65 years.
2. American Society of Anesthesiologists (ASA) physical status I-II.
3. Scheduled for elective unilateral upper limb surgery. Exclusion Criteria

1. Peripheral neuropathy 2. Raynaud's disease or peripheral vascular disease 3. Diabetes mellitus with neuropathy 4. Infection at injection site 5. Allergy to local anesthetics 6. BMI > 35 kg/m² 7. Preexisting temperature asymmetry between limbs Interventions Ninty-two patients will receive ultrasound-guided supraclavicular brachial plexus block using a high-frequency linear probe. A standardized volume (30 mL) of local anesthetic mixture will be injected using in-plane technique. The block will be performed by experienced anesthesiologists.

Measurements Baseline

• Skin temperature at:

• Tip of the little finger (ulnar nerve territory)

• Tip of index finger (median nerve territory)

• Dorsal base of index finger (radial nerve territory)

• Perfusion index at corresponding digits Post-block Assessments Temperature and PI measurements will be recorded at the baseline, every 1 min for the first 10 min after local anesthetic injection, and then every 3 min in the following 21 min.

Temperature will be measured using a calibrated infrared thermographic camera under standardized ambient conditions (room temperature controlled, no external heat sources).

Perfusion index will be recorded using a pulse oximeter probe placed on the finger.

The Perfusion Index Ratio (PIR) is defined as the ratio of the perfusion index measured after an intervention to the baseline perfusion index:

Clinical Block Assessment

Sensory block will be assessed every 5 minutes using cold sensation and pinprick testing in the dermatomal distribution of:

• Ulnar nerve
• Median nerve
• Radial nerve
• Musculocutaneous nerve Block will be considered successful if complete sensory loss occurs within 30 minutes. Ulnar sparing will be defined as preserved sensation in ulnar territory requiring supplemental anesthesia.

Patients with overlapping failure in other nerve territories will be excluded from isolated ulnar sparing analysis when evaluating segment-specific outcomes.

Outcomes Primary Outcome The ability of perfusion index ratio and temperature changes at little fingers for predicting ulnar nerve sparing at 5 mins.

Secondary Outcomes

• The ability of perfusion index ratio and temperature changes at little fingers for predicting ulnar nerve sparing along with other time trends, in the first 30 mins.
• Time needed to reach maximum perfusion index ratio and maximum temperature changes in cases of successful brachial plexus block and in cases of ulnar spared block.
• Sensitivity, specificity, positive predictive value, and negative predictive value.
• Receiver operating characteristic (ROC) curve analysis. Sample size calculation The sample size was determined based on the methodology for comparing the area under the receiver operating characteristic (ROC) curve (AUC), using MedCalc Statistical Software (version 23; MedCalc Software, Belgium). An expected AUC of 0.92 was assumed for infrared thermography, as reported by Gamal et al. (2023). The minimum clinically acceptable AUC was set at 0.79, representing the lower bound of diagnostic performance reported for the perfusion index [4,5].

A two-sided type I error rate (α) of 0.05 and a type II error rate (β) of 0.20 (corresponding to 80% power) were specified. The ratio of positive to negative cases was set at 0.194, assuming an an anticipated ulnar sparing prevalence of 16% reported in the literature (Gamal et al., 2023). Based on these assumptions, the required sample size was calculated to be 77 patients in the successful block group and 15 patients in the ulnar sparing group, yielding a total sample size of 92 patients.