Background <p>Hypotension is the most common complication of spinal anesthesia, particularly in older patients, where the incidence and potential adverse effects are increasing. This study aims to investigate the role of the perfusion index (PI) in predicting spinal anesthesia-induced hypotension (SAIH) during orthopedic lower extremity surgery and its relationship with age.</p> Methods <p>We conducted a single-center, prospective, observational study of 120 elective patients over 18 years of age, classified as ASA I-II-III risk groups, undergoing lower extremity surgery in the supine position under spinal anesthesia. Demographic characteristics, baseline perfusion index (PI) values, and hemodynamic parameters of all patients were assessed and recorded. Patients were divided into elderly (group E, over 65 years) and non-elderly (group NE, under 65 years) groups. A saline infusion, limited to a maximum of 400 mL, was initiated prior to spinal anesthesia. All patients received 10-12.5&#xa0;mg 0.5% hyperbaric bupivacaine for spinal anesthesia and the dermatomal distribution of the block was documented. Intraoperatively, hemodynamic variables and PI values were monitored, along with any potential complications.</p> Results <p>The final analysis comprised 101 patients. Except for age, ASA, and operation type, demographic characteristics were similar in both groups. To further evaluate the independent predictive value of the PI for SAIH, a multivariable logistic regression analysis was conducted, adjusting for age, BMI, comorbidities, ASA score, and surgical factors. The analysis identified ASA score as a significant predictor of hypotension, with higher ASA scores associated with increased risk (<i>p</i> = 0.024). Notably, the preoperative PI was found to be a significant independent predictor of hypotension (<i>p</i> = 0.020). A high baseline PI was directly related to SAIH in all patients and especially in group NE (AUROC = 0.675 (0.568–0.781), <i>p</i> = 0.002; AUROC = 0.727 (0.579–0.875), <i>p</i> = 0.006). In group E, baseline PI did not predict SAIH (AUROC = 0.579 (0.417–0.740), <i>p</i> = 0.336). In all patients, a PI cut-off of 2.25 predicted SAIH with 65% sensitivity and specificity, while in group NE the cut-off was 1.75 (68% sensitivity, 64% specificity). There were no significant differences between groups in PI and hemodynamic data at different time intervals (<i>p</i> &gt; 0.05). Group NE used more bupivacaine and fluids than group E (<i>p</i> = 0.013, <i>p</i> = 0.014 respectively). Both groups showed increased rates of hypotension and ephedrine use above the cut-off values (<i>p</i> &gt; 0.05).</p> Conclusions <p>PI is considered a non-invasive method that can be used to predict SAIH in patients undergoing orthopedic lower extremity surgery (cut-off value = 2.25). However, the same effectiveness was not observed in the elderly patient population.</p> Trial registration <p>NCT06427382|https//www.clinicaltrials.gov/(19/05/2024).</p>