A method to estimate the depth of the sciatic nerve during subgluteal block by using thigh diameter as a guide.

BACKGROUND AND OBJECTIVE: The subgluteal approach is common for sciatic nerve block. Although the surface landmarks are clear, the depth of this nerve at this level is difficult to judge. The purpose of this study is to establish a method of estimating the sciatic nerve depth using the anteroposterior (AP) diameter of the thigh as a marker. METHODS: The study was undertaken in 2 phases. Phase 1 entailed review of 100 magnetic resonance images (MRIs) of the pelvis and proximal lower extremity of patients. Measurements were taken of the AP diameter of the thigh at the midpoint of the lesser trochanter and then compared with distances of the sciatic nerves from the skin of the posterior aspect of the thigh at the same level. Phase 2 involved enrolling 40 patients undergoing lower-extremity surgery for whom subgluteal sciatic nerve blocks were indicated. The AP diameters of the thighs were measured from the subgluteal groove to the inguinal groove with the patient in the supine position. Placing the patient in the lateral position, the subgluteal sciatic block was then performed by using a stimulating needle. The distances from the skin at which the sciatic nerves were actually found, as estimated by maximum motor response to stimulus, were noted. RESULTS: Phase 1 showed a mean AP diameter of 18.94 cm +/- 2.61 cm (mean +/- standard deviation [SD]), mean nerve depth of 6.51 cm +/- 1.46 cm (mean +/- SD), and a linear regression slope of 0.48. Phase 2 showed a mean AP diameter of 16.28 cm +/- 2.73 cm (mean +/- SD), a mean nerve depth of 6.99 cm +/- 1.39 cm (mean +/- SD), and a linear regression slope of 0.43. The thigh diameters differed (P < .001) between the groups, but there was no difference in the depth to the sciatic nerve between the 2 groups (P = .07). CONCLUSIONS: Comparing phase 1 and phase 2 datasets shows the slopes of linear regression lines are nearly parallel. The clinical data from phase 2 verify the anatomical data collected in phase 1 and show that the sciatic nerve depth to AP diameter ratio is 0.43 or the depth of the sciatic nerve is approximately 43% of thigh diameter if the patient is positioned in the lateral decubitus position.

Noninvasive intraoperative monitoring of carbon dioxide in children: endtidal versus transcutaneous techniques.

BACKGROUND: The current study prospectively compares the accuracy of the intraoperative use of transcutaneous (Tc) and endtidal (PE) CO2 monitoring during surgical procedures in 30 paediatric patients, ranging in age from 6 months to 15 years (6.15 +/- 4.35 years) and in weight from 4.7 to 73 kg (24.9 +/- 18.2 kg). METHODS: Following calibration and an equilibration time for the TcCO2 monitor, arterial blood gas samples were obtained as clinically indicated. A total of 64 sample sets (PaCO2, PECO2 and TcCO2) were obtained from the 30 patients. RESULTS: The PECO2 to PaCO2 difference was 0.6-0.9 kPa (4.4 +/- 7.1 mmHg) while the TcCO2 to PaCO2 difference was 0.36-0.38 kPa (2.8 +/- 2.9 mmHg) (P=NS). The difference between the PaCO2 and PECO2 was 0.4 kPa (3 mmHg) or less in 37 of 64 sample sets while the difference between the PaCO2 and TcCO2 was 0.4 kPa (3 mmHg) or less in 49 of 64 sample sets (P=0.038). Linear regression analysis of PECO2 vs. PaCO2 revealed a slope of 0.434, r=0.8761, r2=0.7676. Linear regression analysis of TcCO2 vs. PaCO2 revealed a slope of 0.914, r=0.9472, r2=0.8972. CONCLUSIONS: Although in most circumstances, both noninvasive monitors of PCO2 provided a clinically acceptable estimate of PaCO2, TCCO2 provided a slightly more accurate estimate of PaCO2 during intraoperative anaesthetic care in children.