Evaluation of a trans ‐oesophageal Doppler probe in awake subjects

The manipulation of cardiac output and fluid status has been shown to improve outcome in the high risk surgical patient [1–4]. Studies looking at cardiac output measurement have, so far, used invasive methods and have been based in critical care environments. Trans-oesophageal Doppler (TOD) is a well validated, minimally invasive method of assessing cardiac output and fluid status but, due to the design of currently used probes, its use has been restricted to anaesthetised or sedated subjects [3, 5]. We present the results of a volunteer study using a new probe specifically designed for nasal insertion in awake subjects. Following approval from the Local Research Ethics Committee, we studied 20 healthy volunteers having obtained written, informed consent. Lidocaine spray was administered to the nose and the pharynx and lubricating gel applied to the probe and nares to be used for insertion. The probe (12 h Awake Doppler ProbeTM, Deltex Medical, Chichester, UK) was then inserted through the nose in the same way as a nasogastric tube. Once inserted the probe was manipulated to produce the optimum aortic flow waveform on a CardioQTM monitor (Deltex). Once the optimum waveform was obtained, the time taken to achieve this was noted, as were physiological data from the monitor. The probe was then disconnected from the monitor but left in place, secured by a feeding tube attachment device (Hollister Inc., Libertyville, IL) and the subjects allowed to continue with activities of daily living, except for eating. After an hour the subjects returned and the probe was reconnected, readjusted to obtain an optimal waveform and further recordings made of the physiological parameters. The probe was removed and the subjects completed visual analogue scores for discomfort (0 – no discomfort, 100 – intolerable) during probe insertion, probe manipulation and maintaining the probe in situ. There was a relatively narrow range in the times for insertion of the probe: median (interquartile range [range]) time to locate signal 7.5 (3–8 [2–18]) min. Once inserted and left in situ, the probe remained relatively stable and only a very short period of time was required to reposition the probe: 1.0 (0–4 [0–5]) min. The median visual analogue scores for insertion and manipulation were 22 and 20, respectively, with interquartile ranges of 11–29 and 6–30. There was a bimodal distribution with four subjects giving a VAS of greater than 33 for both procedures. Two subjects were unable to tolerate insertion of the probe. The volunteers' tolerance of the probe appears similar to that of routinely placed nasogatric tubes, which only a few patients find completely intolerable. Compared with other methods of fluid and cardiac output monitoring, the time from decision to collecting data is favourable (60 min for a pulmonary artery flotation catheter and 15 min for the LiDCO) [6]. TOD does not require a sterile environment and has its own self-contained monitor. A tolerable, rapidly useable monitor will be of use to the anaesthetist in a variety of situations where fluid status and cardiac monitoring are not yet realistic options. Obvious patient groups include the frail elderly undergoing urgent orthopaedic and colorectal surgery, and patients presenting to the emergency department with hypotension. We would like to thank Deltex Medical for providing the probes used in this study.