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Increased body mass index and ASA physical status IV are risk factors for block failure in ambulatory surgery - an analysis of 9,342 blocks

[Un indice de masse corporelle élevé et un état physique de classe IV selon l’ASA sont des facteurs de risque d’échec de l’anesthésie régionale ambulatoire – l’analyse de 9 342 cas]

Juliann T. Cotter, BS^*, Karen C. Nielsen, MD, Ulrich Guller, MD MHS, Susan M. Steele, MD, Stephen M. Klein, MD, Roy A. Greengrass, MD FRCP and Ricardo Pietrobon, MD

^* From the University of North Carolina School of Medicine,
the Departments of Anesthesiology, and
Surgery, Duke University Medical Center, Durham, North Carolina; and
the Department of Anesthesiology, Mayo Clinic, Jacksonville, Florida, USA.

Address correspondence to: Dr. Ricardo Pietrobon, Division of Ambulatory Anesthesia, Center for Excellence in Surgical Outcomes, DUMC P.O. Box 3094, Durham, NC 27710, USA. Phone: 919-668-2054; Fax: 919-668-2081; E-mail: rpietro{at}duke.edu

Purpose: Regional anesthesia can be the technique of choice for selected ambulatory surgery procedures, but in spite of its benefits, it has an inherent failure rate even in experienced hands. We examine the efficacy and factors associated with failure of ambulatory regional anesthesia techniques.

Methods: This study included 9,342 blocks performed on 7,160 patients at the Duke University Ambulatory Surgery Center. Blocks were classified as interscalene, supraclavicular, axillary, lumbar plexus, femoral, sciatic, ankle, paravertebral, spinal, and other (frequency less than 100). A block was considered surgical if a single attempt at placing the block resulted in a complete sensory, motor, and sympathetic nerve block. Multiple logistic regression analyses were used to assess the risk-adjusted association between patient characteristics and block failure.

Results: Paravertebral blocks and those considered in the "other" category had significantly higher failure rates (P < 0.001), while spinal and lumbar plexus blocks had lower than average rates of failure (P < 0.001 and P = 0.03, respectively).

In multiple logistic regression analyses excluding paravertebral blocks, body mass index (BMI) scores greater than 25 (P values: BMI 25–29: < 0.001; BMI 30–34: P < 0.001; BMI 35: P < 0.001) and ASA physical status IV (P < 0.001) were significantly associated with higher block failure rates.

Conclusion: High BMI and ASA IV are independent risk factors for block failure in ambulatory surgery patients.

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