Development of a model for measurement of depth using ultrasound
Abstract Number: 39
Abstract Type: Original Research
Introduction: Ultrasound is becoming increasingly popular within obstetric anaesthesia to guide placement of epidurals, in particular, to measure the depth of the epidural space. Studies have shown that when measuring the depth of the epidural space during placement of labour epidurals ultrasound measured depth correlates well with needle measured depth.(1,2) This requires the use of a different ultrasound technique and probe from that with which most anaesthetists were familiar with. We set out to develop a model to aid teaching of these skills.
Method: A model was developed to allow measurement of the depth of several standard objects by multiple anaesthetists. The model comprised of a container of a gelatine solution with 10 standardised objects suspended within it. 10 anaesthetists of different grades performed an ultrasound scan of the gelatine solution. Each anaesthetist measured the depth of all 10 suspended objects. The gelatine solution was then removed from the container as a solid block. It was sliced and the depth of the objects was measured directly with a ruler. Bland-Altman analysis was carried out to assess the agreement between the ruler measured depth and the ultrasound measured depth.
Results: The ultrasound measured depth correlated well with the ruler measured depth. The difference between ruler measured depth and ultrasound measured depth ranged from -0.31 to 1.87 cm, with a mean difference of 0.68 cm (standard deviation 0.52). As can be seen from fig. 1, the majority of the measurements fall within the limits of agreement. These results are comparable to results published in the current literature obtained when measuring the depth of the epidural space during labour epidural placement.(1,2)
Discussion: This is a simple model that produces results equivalent to those produced when measuring the depth of the epidural space on patients. It is therefore a useful model for teaching and training anaesthetists in the use of ultrasound and, in particular, in the measurement of depth using ultrasound.
1. Arzola C, Davies S, Rofaeel A, Carvalho JCA. Ultrasound using the transverse approach to the lumbar spine provides reliable landmarks for labor epidurals. Anesth Analg 2007;104:1188-92.
2. Balki M, Lee Y, Halpern S, Carvalho JCA. Ultrasound imaging of the lumbar spine in the transverse plane: the correlation between estimated and actual depth to the epidural space in obese parturients. Anesth Analg 2009;108:1876-81.