title: Cadaveric feasibility study on the use of ultrasound contrast to assess spread of injectate in the serratus anterior muscle plane.
abstract: BACKGROUND
  The Serratus anterior muscle plane (SAP) block has recently been described for the purpose of perioperative pain management following cases of trauma and breast surgery. It might prove a safer alternative to the other regional thoracic paravertebral and central neuraxial blockade techniques. There are no descriptive cadaveric studies in the pre-existing literature to delineate the anatomical plane for this novel technique. The main objectives for our study were to examine the location of the Serratus anterior muscle belly, assess the efficacy of achieving adequate delineation of the muscle plane utilising ultrasound imaging with agitated water as the contrast agent, and finally, to observe the extent of the cepahlo-caudal spread of the injectate in the SAP.
  
  
  METHODS
  Seven cadavers were studied. 20 mls of saline was injected into posterior axillary line (PAL) at the level of the 4-5(th) rib under ultrasound guidance. This was followed by injection of 10 mls of water with air (8 mls water and 2 mls of air). The presence of hyperechoic air bubbles in the fluid distended SAP (hypoechoic) area demonstrated the spread of water and air.
  
  
  RESULTS
  In 36% of cadavers, fully formed Serratus Anterior muscle belly was identified at the midaxillary line (MAL), 14% in PAL, and remaining 50% between PAL and MAL. The lower most limit of air-water spread was identified at the subcostal margin. Cephalad spread of contrast was noted in 2(nd) intercostal space ICS (7%), 3(rd) ICS (71%), and 4(th) ICS (22%).
  
  
  CONCLUSIONS
  This study describes that the serratus anterior muscle is well-formed near the PAL and the injectate spread can be determined with the help of agitated water contrast on ultrasound. Furthermore, there was variability in the cephalad spread of the injectate.
authors.affiliation: Specialist Registrar Anaesthetics, University Hospitals of Leicester NHS Trust, Leicester, UK.
authors.forename: V
authors.initials: V
authors.lastname: Daga
authors.affiliation: Consultant Anaesthetics, Frimley Park Hospital NHS Foundation Trust, Frimley, Surrey, UK.
authors.forename: M K
authors.initials: MK
authors.lastname: Narayanan
authors.affiliation: Consultant Anaesthetics, United Lincolnshire Hospitals NHS Trust, Lincoln, UK.
authors.forename: J D
authors.initials: JD
authors.lastname: Dedhia
authors.affiliation: Medical student, University College London, London, UK.
authors.forename: P
authors.initials: P
authors.lastname: Gaur
authors.affiliation: Medical and Social Care Education, University of Leicester, Leicester, UK.
authors.forename: H
authors.initials: H
authors.lastname: Crick
authors.affiliation: Consultant Anaesthetics, University Hospitals of Leicester NHS Trust, Leicester, UK.
authors.forename: A
authors.initials: A
authors.lastname: Gaur
keywords: Cadaver; serratus anterior plane; ultrasound