Minimum sample frequency for multichannel intraluminal impedance measurement of the oesophagus A. J. BREDENOORD,* B. L. A. M. WEUSTEN,* R. TIMMER* & A. J. P. M. SMOUT  *Department of Gastroenterology, Saint Antonius Hospital, Nieuwegein, The Netherlands  Gastrointestinal Research Unit, University Medical Center, Utrecht, The Netherlands Abstract In all systems for impedance monitoring signals are stored in digital format after analog- to-digital conversion at a predefined rate, the sample frequency. We aimed to find the minimum sample frequency required to evaluate oesophageal transit and gastro-oesophageal reflux studies using imped- ance monitoring. In 10 healthy subjects and five patients with gastro-oesophageal reflux disease impedance signals were acquired during a 90-min postprandial period, using a sample frequency of 1000 Hz. Additionally, 10 liquid swallows were recorded in the healthy subjects. A dedicated com- puter program was used to derive, from the original 1000-Hz files, series of new data files with sample frequencies of 500, 200, 100, 50, 20, 10, 8, 5 and 4 Hz. In all of these files, liquid and gas reflux events were identified. In the analysis of the swallows, bolus head advance time (BHAT) and total bolus transit time were measured. Reflux events were detectable down to a sample frequency of 50 Hz. In transit analysis errors for BHAT at frequencies below 8 Hz errors exceeded 2.5%. Therefore, in impedance monitoring the minimum sample frequency is 50 Hz for the detection of reflux and 8 Hz for the evaluation of oesophageal transit. Keywords electrical impedance, gastro-oesophageal reflux, oesophageal motility, oesophageal transit testing. INTRODUCTION Recently, intraluminal electrical impedance monitor- ing was introduced as a novel method to study bolus transport in oral and aboral direction in luminal organs. 1,2 Impedance measurement provides informa- tion about bolus transport during oesophageal peristalsis and has already been incorporated in routine function testing in some tertiary medical centres. 3,4 Studies combining impedance measurements with pH-metry provide new insights into pathophysiology of gastro- oesophageal reflux disease (GORD) and oesophageal acid clearance. 5–7 Furthermore, intraluminal impedance can be used to study aerophagia and belching. 8 Impedance measurements are also promising in the evaluation of reflux and regurgitation in paediatric practice. 9,10 In all systems for intraluminal impedance monitor- ing the signals are stored in digital format after analog- to-digital (A/D) conversion. A/D conversion is done at a predefined rate, the sample frequency. In order to be able to recognize events like bolus transit, liquid reflux, gas reflux and belches in intralu- minal impedance signals a sufficiently high sample frequency is necessary. The disadvantage of a high sample frequency is that the size of the data files becomes huge, especially when measuring for 24 h. In previous studies using oesophageal impedance monit- oring sample frequencies ranging from 30 to 1000 Hz were used, as no systematic study has been done to investigate the minimum sample frequency. 8,11,12 In this study, we aimed to find the minimum sample frequency required to evaluate oesophageal transit, gastro-oesophageal reflux and belching using oesopha- geal impedance monitoring. METHODS Subjects We recorded oesophageal intraluminal impedance in 10 healthy volunteers (four males and six females; mean Address for correspondence A. J. Bredenoord MD, Department of Gastroenterology, Saint Antonius Hospital, P.O. Box 2500, 3430 EM Nieuwegein, The Netherlands. Tel: +31 30 6093246; fax: +31 30 6056357; e-mail: a.bredenoord@antonius.net Received: 17 March 2004 Accepted for publication: 6 May 2004 Neurogastroenterol Motil (2004) 16, 713–719 doi: 10.1111/j.1365-2982.2004.00575.x Ó 2004 Blackwell Publishing Ltd 713