A Prospective Observational Study of Perfusion Index in Predicting Hypotension Following Sub-Arachnoid Block in Patients Undergoing Lower Limb Orthopedic Surgeries. (Preprint)

Abstract

BACKGROUND Sub-arachnoid block (SAB) is often associated with hypotension due to changes in intravascular volume, tone of vessels and cardiac output. Sympatholytic effect of SAB causes hypotension which is exaggerated in elderly, hypovolemic and cardiac patients, thus increasing the incidence of myocardial ischemia. Perfusion index (PI), derived from pulse oximeter, is a relatively newer and non-invasive parameter to measure the vascular tone (1,2). Changes in baseline peripheral vascular tone may affect the degree of hypotension, and patients with low baseline vascular tone may be at an increased risk of hypotension (3-6). This study was conducted to investigate correlation between baseline PI and incidence of hypotension following SAB in patients undergoing lower-limb orthopedic surgeries and to determine a new baseline cut-off to assess the chances of developing hypotension. OBJECTIVE To investigate the correlation between baseline PI and incidence of hypotension following SAB, in patients undergoing lower-limb orthopedic surgeries. METHODS This single center, prospective, observational study was conducted between June 2018 and February 2020, at a tertiary care center, after obtaining Institutional Ethics Committee clearance. Adult patients (18-60 years) undergoing lower-limb orthopedic surgery under SAB and belonging to American Society of Anesthesiologists (ASA) grade I and II were included. Patient not willing for surgery under regional anesthesia (RA) or having contraindications to RA due to allergy to local anesthetics (LA), procedure site infection, any coagulation disorder, ASA grade III-IV, mental health issues were excluded. After detailed pre-anesthetic check-up, a written informed consent was taken. Standard fasting guidelines were followed. All the patients were given tablet lorazepam 2 mg in the night and morning before surgery. After wheeling the patient into the theatre, intravenous access was taken, standard monitors were attached. Each patient was pre-hydrated with 500 mL of Ringer Lactate before SAB followed by maintenance fluid. A total of 75 patients were enrolled in the study initially and 15 were excluded because of exclusion criteria. Remaining 60 patients were assessed for baseline PI and were divided into two groups. The PI was measured in the supine position using specific pulse oximeter probe attached to the left index finger of all patients to ensure uniformity in measured PI values. Patients with baseline PI ≤ 3.5 fell into group A and those with PI > 3.5 fell into group B. SAB was performed by an anesthesiologist blinded to the baseline PI values, using 25-gauge Quincke's spinal needle in sitting position with 15 mg of injection 0.5% bupivacaine (hyperbaric) at L3–L4 interspace. The level of sensory blockage was checked using a cold swab. Maximum cephalad spread was checked 20min after SAB. Non-invasive blood pressure (NIBP), heart rate (HR), Saturation (SpO2) and PI were recorded at 2 min intervals up to 20 minutes after SAB, followed by every 5 minutes interval for rest 40 minutes. Hypotension was defined as a decrease in Mean Arterial Pressure (MAP) of 20% from baseline and was treated with intra-venous bolus of injection Mephentermine 6 mg and 100 ml of Ringer lactate. Hypotension within first 60 min following SAB was considered for anesthesia-induced hypotension. Bradycardia (HR <55 beats/minutes), if any, was treated with injection atropine 0.6 mg. Statistical analysis was done by using Statistical Package for Social Sciences version 21.0 (IBM Corporation, Armonk, New York, USA). Data was entered in Microsoft excel 2016 for Windows. Statistical analysis was performed using unpaired t-test for comparison between the groups and Pearson’s chi-square test was applied to compare categorical variables (frequency and percentages). Receiver operating characteristic (ROC) curve was plotted for baseline PI and occurrence of hypotension. A P-value <0.05 was considered statistically significant. RESULTS A total of 75 patients were enrolled in the study initially. 15 patients were excluded as they were not following inclusion criteria, 3 patients were excluded due to inadequate level of spinal blockade. A total of 28 patients felt in Group A and 29 patients in Group B, for final analysis (figure 1). Systolic blood pressure (SBP), diastolic blood pressure (DBP) and MAP were significantly higher in group A as compared to group B for the first 25 minutes after SAB (figure 2, 3). Overall, incidence of post-SAB hypotension was 68.97% in group B and 10.71 % in group A (figure 4). Intra-operatively, heart rate was comparable between the two groups. The ROC analysis revealed that baseline PI was suitable for detecting risk for hypotension (AUC = 0.854, P < 0.001) (figure 5) (Table 2). The baseline PI cut-off point that predicted hypotension as determined by the ROC analysis was 3.1 with a sensitivity of 100% (95% CI= 85.20% - 100.00%), a specificity of 73.53% (95% CI = 55.60–87.10%), a positive predictive value of 71.88% (95% CI= 59.34% - 81.74%), and a negative predictive value of 100.00% (95% CI = NA) (figure 5). CONCLUSIONS In patients undergoing lower limb orthopedic surgeries, Perfusion index (PI) can be used as a tool for predicting hypotension following SAB. Baseline PI >3.5 is associated with higher incidence of developing hypotension than PI <3.5.