This prospective, single-arm, observational study is reported according to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) consensus guidelines. This study was approved by the Institutional Review Board (2019-0532: Ube Industries Central Hospital, Japan) and registered in the University Hospital Medical Information Network Clinical Trials Registry (UMIN 000037866). Written informed consent was obtained from all patients.
We selected patients from 31 August 2019 to 26 November 2019, who were scheduled for hip surgery and had chosen ultrasound-guided SFIB followed by general anesthesia. Exclusion criteria were age <20 years, peripheral neurological or vascular diseases, general contraindications to local anesthesia (e.g., increased risk of bleeding, infection at the injection site, allergy to local anesthetics), inability to communicate (e.g., due to dementia or other neuropsychiatric diseases), and refusal to participate.
Ultrasound-guided supra-inguinal fascia iliaca block
Ultrasound-guided SFIB was performed as described by Hebbard et al. [10] A liner ultrasound probe (HFL38 6–13 MHz, S-nerve ultrasound System, Sonosite Inc., Bothell, WA, USA) was placed in the sagittal plane to obtain an image of the anterior superior iliac spine. The fascia iliaca and sartorius, iliopsoas, and oblique internal muscles were identified by sliding the probe medially. After identifying the bow-tie sign formed by the muscle fasciae, a 100-mm 20G nerve block needle (UNIEVER DISPOSABLE NERVE BLOCKADE NEEDLE WITH HUBER POINT, UNISIS Corp., Saitama Japan) was introduced 1-cm cephalad to the inguinal ligament (IL). Using an in-plane approach, the fascia iliaca was penetrated and separated from the iliac muscle by hydrodissection. Within the space created by hydrodissection, the needle was advanced further in a cranial and slightly dorsal direction. Upward movement of the overlying deep circumflex artery upon injection was used as a sign of successful fascia iliaca penetration. A total volume of 30 ml levobupivacaine 0.25% was injected. Injection was considered successful if spread of local anesthesia was observed cranial to the point where the iliac muscle passes under the abdominal muscles. If spread was insufficient, the injection was stopped and the needle repositioned until adequate spread was obtained. All SFIB procedures were performed by the same experienced anesthesiologist (M.Y).
General anesthesia was administered after assessing the temperature. A supraglottic airway was inserted after the induction of anesthesia with sevoflurane, remifentanil, and rocuronium. Anesthesia was maintained with sevoflurane and remifentanil. Fentanyl was used in the range of 0–2 μg/kg.
Block assessment
To avoid interference with skin temperature measurements, sensory block was assessed immediately following the final infrared thermographic image acquisition using the cold test. Tests were conducted in succession at the femoral, obturator, and LFCN sites.
Infrared thermographic imaging
Infrared thermographic images were acquired using a FLIR ONE infrared camera (FLIR Systems, Wilsonville, US) with 0.1°C resolution. During measurements, the camera was set according to the actual distance from the patient (1.0 m), room temperature (20.0°C), air humidity, and the emission coefficient for the human body (0.9). Three pictures were taken from the lateral thigh, thigh, and medial bottom side of the hip perpendicular to the surface at distance of 0.8 m. No interventions were performed during infrared imaging, and the patients were instructed not to move their legs during the measurement period.
Temperature assessment
Primary outcome was skin temperature after block compared before block. Infrared thermographic images of the hip and thigh were acquired before and at 5-min intervals after ultrasound-guided SFIB for up to 15-min post-injection. A preliminary assessment of skin temperature distribution was conducted in the operating room, and then, absolute values were derived after surgery at femoral, obturator, and LFCN sites (measurement area of 150 × 100 pixels) using a computer software (FLIR Systems, Wilsonville, OR, USA). Heat maps were created for each site with a temperature increase of >0°C defined as the success between baseline and 5, 10, or 15 min.
Statistical analysis
We used data from a past study by Park et al. [11] to estimate the sample size needed to detect a significant difference in skin temperature. Assuming a mean change in skin temperature of 2.0°C and standard deviation of 0.8, 20 patients would be required for 80% statistical power (beta) at an alpha level (two-sided) of 5%. All statistical analyses were conducted using SPSS version 24 for Windows (SPSS, Chicago, IL, USA).
We analyzed differences among measurement times and sites using linear mixed-effects models accounting for repeated measures. Least square means and 95% confidence intervals (CI) are reported for these models. Bonferroni contrasts were calculated for comparison among measurement times. Data are expressed as mean ± standard deviation. A p <0.05 was considered statistically significant for all tests.
Results
Twenty-eight consecutive patients were deemed eligible during the study period from August to November 2019, of which four patients were excluded due to dementia and three refused to participate (Fig. 1), and the remaining 20 patients (2 males and 18 females, aged 82 ± 4 years) were included. The number of patients with ASA physical status 1, 2, and 3 was 1, 4, and 15, respectively. Thermographic images are shown in Fig. 2. The temperature mean increased from 35.5 to 36.7 °C after 5 min, to 36.7°C after 10 min, and to 36.4°C after 15 min. The mean temperature including the skin area of the three nerves 5, 10, and 15 min after nerve block was significantly increased compared with the baseline (P < 0.01 for all, Fig. 3). The cold test response was reduced in all cases at the femoral and LFCN sites and in 13 cases at the obturator nerve site. Figure 4 displays heat maps for each site with a temperature >0°C defined as the success between baseline and 5, 10, or 15 min. The sensitivity and specificity of the temperature increase to the cold loss were 96% and 63%, respectively when we defined >0°C as the clinical threshold.