The use of an acoustic device to identify the extradural space in standing horses

To determine the usefulness of an acoustic device to confirm correct placement of extradural needles in horses.

Prospective experimental study.

Twelve adult healthy horses weighing between 434 and 640 kg.

Horses were sedated, placed in stocks and subjected to caudal extradural anaesthesia using lidocaine 2%. For extradural puncture an 18 gauge Tuohy needle connected to an acoustic device via an electronic pressure transducer was used. This device allowed recording of the extradural pressure profile and the transformation of pressure changes into an audible signal. Extradural needle advancement was stopped and the local anaesthetic administered when either a sharp decrease in sound pitch occurred or a 'pop' sensation was felt. Correct needle placement was assessed clinically by the degree of extradural anaesthesia present using needle prick stimuli, loss of tail tone and occurrence of localized sweating. Descriptive statistical analysis was used for evaluation of the data.

Extradural anaesthesia was successful in nine of 12 horses. In these horses during extradural needle advancement the pitch of the audible signal first increased and abruptly changed to a constant low pitched sound. In the three horses with unsuccessful punctures only a slight decrease of the pitch was noted. A 'pop' was clearly distinguished in five of nine horses with successful extradural puncture. In four horses a 'pop' was not present despite correct extradural needle placement. In three horses with incorrect needle placement a 'pop' was present. In the horses with correct needle positioning the median extradural pressure after the puncture was -0.8 kPa (-6 mmHg).

The acoustic device is useful to assist in the identification of successful or unsuccessful extradural needle placement in the horse.