Original ContributionUltrasound Visibility of Spinal Structures and Local Anesthetic Spread in Children Undergoing Caudal Block
Introduction
Several formulas have been introduced to predict cranial spread of local anesthetics after single-shot caudal block in children (Armitage, 1979, Busoni and Andreuccetti, 1986, Schulte-Steinberg and Rahlfs, 1977, Takasaki et al., 1977). Recently, a low-concentration, high-volume (1.5 mg/kg) local anesthetic regimen for prolonged post-operative analgesia has gained popularity (Lundblad et al., 2011, Lundblad et al., 2012, Silvani et al., 2006). However, the relationship between volume of anesthetic and block level was unreliable, and there was a large discrepancy in predicted levels between formulas, especially at high volumes. A radiographic study that used 1.5 mL/kg 0.15% ropivacaine for caudal blockade reported the median (range) drug spread level to be T6 (T3–T11) in 37 infants and small children (Hong et al. 2009).
As an alternative method to assess local anesthetic spread during caudal block in children, real-time ultrasound has garnered much interest (Brenner et al., 2011, Thomas et al., 2010, Triffterer et al., 2012). In a recent ultrasound study, cranial spread beyond T12 after caudal injection of 1.5 mg/kg local anesthetic was observed in only 25% of 32 infants and toddlers (Lundblad et al. 2011). Another study reported the median drug spread level to be as high as T8 (T4–T11) after caudal injection of the same volume of local anesthetic in 16 infants (Lundblad et al. 2012). This relatively large discrepancy between studies may be due to the difference in local anesthetic spread with patient age and the possible detection bias caused by ultrasound visibility of spinal structures. The local anesthetic within the epidural space is detected as a highly hypo-echoic pattern, and injected local anesthetics widen the epidural space, which displaces the dura mater anteriorly. Therefore, the accuracy and reliability of estimating local anesthetic spread with ultrasound can be said to be dependent on the visibility of spinal structures (Brenner et al., 2011, Lundblad et al., 2011, Marhofer et al., 2005, Tsui and Suresh, 2010).
We performed this prospective study in 80 small children between 2 and 54 mo of age, to assess ultrasound visibility of spinal structures that may affect the detection of local anesthetic spread within the epidural space during caudal block. The extent of cranial drug spread at different predetermined volumes was also observed with real-time ultrasound.
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Patients
After approval was obtained from the institutional review board of Yonsei University Health System and the study was registered at www.ClinicalTrials.gov (NCT01340313, Principal investigator: Hae Keum Kil, registered on April 19, 2010), 80 American Society of Anesthesiologists physical status I children, aged 2 to 54 mo, undergoing ambulatory urologic surgery were enrolled in this study. Written informed consent was obtained from the parents of all patients. Children with local infections on
Results
Demographic data and patient characteristics including vertebral levels of the conus medullaris and dural sac are listed in Table 1. On preblock ultrasound, the conus medullaris and dural sac were clearly or sufficiently identified in 78 (97.5%) children. The dura mater exhibited good visibility in 92.5% at the lower lumbar level and in 85.0% at the upper lumbar level. However, this decreased to only 55.0% at the lower thoracic level. Visibility of the ligamentum flavum also decreased
Discussion
It is well known that the posterior vertebral column of infants and small children is largely cartilaginous and, therefore, relatively favorable to ultrasound beam penetration (Roberts et al. 2005). The quality of detection of local anesthetic within the epidural space depends heavily on the ultrasound visibility of spinal structures that serve as major landmarks and indices, such as the dura mater and ligamentum flavum. The primary goal of the present study was to evaluate the ultrasound
Conclusions
Ultrasound visibility of spinal structures was found to decrease with increasing vertebral level. Although the dura mater was sufficiently identifiable up to the lower thoracic levels, visibility of the thoracic ligamentum flavum markedly decreased after 7 mo of age in an age-dependent manner and also in patients weighing more than 8.5 kg. Real-time ultrasound may also be helpful in estimating spread levels at drug volumes up to 1.25 mL/kg, considering that cranial spread observed with
Acknowledgments
Funding was provided solely by departmental sources.
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