Calculation improves the estimation of needle depth from skin to thoracic epidural space in infants.

Editor,

Epidural anaesthesia in major surgery in infants and children is advantageous due to excellent pain relief and reduction of length of stay on the ICU.[1] The process is challenging and potentially hazardous due to the risk of spinal puncture. To minimise the risk, the use of ultrasound-guided epidural catheter insertion has been propagated by several authors.[2] However, the authors mainly investigated the advantages of identifying anatomical structures and estimating the depth of the epidural space. Here, we investigated the thoracic spine and examined whether a combination of ultrasound-guided measurement of the perpendicular depth of the epidural space corrected by a trigonometric ratio equation is superior in predicting the skin to thoracic epidural space distance compared with estimation by the perpendicular depth. Therefore, 24 children aged 5 to 148 months (median 40 months), body weight 5.7 to 39.5 kg (median 14 kg), undergoing elective surgery with a thoracic epidural catheter (Th 7/8 to Th 11/12) planned for peri-operative pain control were included in a retrospective single group observational study. Ethical approval for this study was provided by the University of Tuebingen Ethical Committee (Chairperson Prof D. Luft) on 15 May 2018 with the project number 302/2018BO2 after written consent of the parents. After induction of general anaesthesia, all patients were placed in a flexed left-sided position. The perpendicular distance to the epidural space was measured by an ultrasound linear probe and a Tuohy needle (BBraun Inc., Melsungen, Hessen, Germany) was inserted. After insertion of the Tuohy needle, the angle (α) between the needle and the skin surface above the spinous processes was defined by use of a sterile protractor and the distance to the epidural space was calculated using Pythagorean triangle geometry [distance to loss of resistance (LOR) = perpendicular distance (ultrasound)/cos (α)]. The epidural needle was then advanced in a midline approach that was verified by ultrasound until loss of resistance to saline was achieved. The distance on the needle from the needle tip to the skin level was read according to the 5 mm graduation on it, compared with the mm graduation of the protractor, and noted in the anaesthesia chart. All ultrasound examinations were performed by the same experienced anaesthesiologist. Correct placement of the epidural catheter was confirmed by either visualisation of the catheter in the epidural space and/or by observation of the local anaesthetic spread. The comparison between the ultrasound-derived distance corrected by a trigonometric ratio equation (calculated epidural depth), the ultrasound-measured perpendicular skin to epidural space distance, and the actual depth of the thoracic epidural space was analysed using Microsoft Excel (version 2013; Microsoft, Redmond, Washington, USA). Subsequently, both groups were compared with linear regression analysis with goodness-of-fit (adjusted R2) by application of SAS JMP (version 13.1.0; SAS Institute Inc., Cary, North Carolina, USA). The distribution of data was tested for normality using the χ2 test. The data were expressed as mean (± SD) with 95% confidence intervals. A value of P less than 0.05 was considered to be statistically significant.

The calculated needle depth from skin to epidural space showed a higher correlation coefficient (R2 = 0.82) compared with the measured distance to LOR than the ultrasound perpendicular distance (R2 = 0.46) (Fig. 1). Deviation from the measured distance to LOR was calculated in percentage. The deviation increased with steepness of the angle of the Tuohy needle. When the needle was inserted at an angle of more than 20° the median deviation can be expected to be approximately 60% of the measured distance. Incidents of dural puncture or postoperative complications were not reported with the epidural cannulation.

Fig. 1

Correlation of the perpendicular measured depth of epidural space (US-distance) and actual distance on the needle (blue) and correlation of depth of the thoracic epidural space corrected by a trigonometric ratio equation and actual distance on the needle (red).

The large R found in this study (R2 = 0.82) implies that this combination of methods is a useful tool to estimate the depth of loss of resistance prior to insertion. The R is in the same range as previously described (R2 = 0.78) for use of a Computed Tomography-derived method for adults.[3] Furthermore, a correction of ultrasound-guided perpendicular measurement is essential as demonstrated by the poor correlation with the actual depth of loss of resistance (R2 = 0.46). The results differ from findings published on epidural cannulation in the lumbar region where the authors described an excellent correlation between the distance measured on ultrasound and the calculated perpendicular epidural depth.[4] Another drawback of not using ultrasound-guided method is that ultrasound can detect anatomical abnormalities such as myelomenigocoeles before puncture.

In conclusion, the correction of the ultrasound-guided perpendicular measurement of the depth of the thoracic epidural space allowed a better estimation of the distance before loss of resistance occurs and is invaluable when the peridural needle is inserted at a steep angle.