Neuraxial Anaesthesia: Does BMI Relate to Ease of Neuraxial Anaesthesia?

Overview

Neuraxial anaesthesia can be more difficult and associated with more complications if the patient's bony landmarks are difficult to palpate. They are more likely to be difficult to palpate if a patient has a high Body Mass Index (BMI), (\>30kg/m2). The depth that the spinal or epidural needle must be inserted is usually longer in these patients with high BMIs. We wish to palpate the backs of at least 100 such patients to see how many of them have impalpable bony landmarks. We then wish to use ultrasound to measure the distance from skin to the posterior epidural complex to discover if this length is longer than the standard needle length. If it is longer in the majority of people we study, we will recommend changing standard practice to start using a longer needle for all first attempts at neuraxial anaesthesia in this patient population.

Complications rates including failure are higher in obese patients undergoing anaesthesia procedures. Anaesthetists have adopted the use of ultrasound to assist in overcoming these difficulties. Neuraxial ultrasound is proving to be beneficial in those patients in whom identification of interspinous spaces is difficult and most of these patients are obese. In our institution, there is a 'standard practice' for neuraxial procedures, with a 'standard' needle used for first attempts (Whitacre 25G 90mm for spinal, Tuohy 18G 80mm for epidural). If the operator has not reached the desired space (spinal or epidural) with the needle inserted to 8cm, a longer needle is then considered for further attempts. This exposes the patient to at least one extra neuraxial needle insertion. Studies have shown that increased needle insertions and redirections are associated with increased complications. We hypothesised that a large number of patients with BMI\>30kg/m2 would have a depth of spinal/epidural space that is greater than the length of the 'standard' needle used. Therefore we suggest that practice should change to use the longer needle or a combined spinal-epidural on the first attempt in these patients. To test our hypothesis, we will assess the ease of palpation of the following anatomical landmarks: anterior and posterior iliac crests; lumbar spinous processes; scapulae; and sacral cornua. We will then perform neuraxial sonography of the lumbar spine, measuring depth to epidural space. Finally we will measure waist circumference in those patients with BMI \<30kg/m2.