Evaluation of a trans‐oesophageal Doppler probe in awake subjects

Anaesthetists use a needle and syringe to deliver local anaesthetic agent to create skin anaesthesia prior to cannula insertion [1] and a needleless device may be useful on the grounds of patient desirability (for repeat injections, or needle phobic patients) and reducing needlestick injuries. Needleless injection systems have been widely used in preventative medicine [2]. Needleless injection depends on forcing fluid through a small orifice. If there is sufficient pressure across the orifice, the velocity of the fluid emerging is sufficient to breach the skin and enter the subcutaneous tissues. The Medical House Insulin Jet System 500 (Medical House Products Ltd, Sheffield, UK) appears to be one of the first needleless devices to use coiled spring technology to produce the pressure required for skin puncture. It has a disposable drug reservoir and injection heads. This device is used successfully for injection of subcutaneous insulin. The Medical House Company has developed a prototype which may be useful for administration of local anaesthetic. By adjusting the delivery pressure, the manufacturers suggest that the system may deliver local anaesthetic intradermally or subcutaneously. At present the depth of delivery is not known, but the time for delivery is 0.3 s through a 0.18 mm nozzle. The pressure produced is around 25–28 MPa. These details compare with the J-Tip single-use, carbon dioxide powered, needleless injection system (National Medical Products Inc., Irvine, CA) which has been used to administer local anaesthetic [3]. This device delivers injectate over 0.2 s to a depth of 5– 8 mm, at an unspecified pressure. The J-Tip has been shown to be less painful in administering local anaesthetic than the conventional needle and syringe method. However, of the two, more pain was experienced with the J-Tip when an intravenous cannula was subsequently inserted [3]. These findings led us to investigate the effectiveness of the new Medical House prototype. With Local Research Ethics Committee approval and informed consent we undertook a pilot study in 20 volunteers, aged 21–60, who received two separate deliveries of lidocaine 1% 0.1 ml, on the dorsum of the hand using the prototype delivery system. Site 1 was away from superficial veins, whereas site 2 was close to a superficial vein, as if numbing an area in readiness for intravenous cannula insertion. The pain on injection using a visual analogue scale (where 0 1⁄4 no pain and 100 1⁄4 worst pain imaginable) and the area of skin anaesthesia after 30 s and 60 s were assessed. The area of anaesthesia was measured with a sharp needle point applied to the area around the injection site while out of view to the subject. The area was measured by determining the length of the numb area in two directions at right angles; the area of the corresponding ellipse was then calculated. The VAS scores for pain were low (Table 1). Anaesthesia was produced in 39 of the 40 sites at 30 s. In one volunteer there was no anaesthesia at site 2 at 30 s, but there was anaesthesia at 60 s. This volunteer was reassessed 1 h later and a haematoma had developed, probably due to poor injection technique; it appeared the vein had been injected. These results are similar to those previously described for the J-Tip device [3]. We anticipated that older skin may be thinner, leading to increased spread of anaesthesia but although we expected some correlation between age and spread of anaesthesia, none was found in this small study. The volunteers were asked about personal preferences. Eleven said the needleless system would be their preference for skin anaesthesia, eight expressed no preference, and two preferred a needle and syringe. Eighteen would prefer the needleless system if self-injecting daily, while two preferred a needle and syringe. Further studies are investigating whether this system will be useful for the insertion of an intravenous cannula.