Measurement errors caused by incompatibility of apparatus

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Editor,We present two examples in which a combination of two items of equipment, made by different manufacturers, caused unforeseen errors. Both involved capnography, which is a routine monitoring requirement in many countries.Standard connections for anaesthetic equipment allow products from different manufacturers to be used together. However, some combinations may have important and unexpected functional consequences. We noted errors from using an integral gas sampling tube provided in a coaxial breathing circuit, in combination with the D-Lite flow sensor (Part #733950, GE Healthcare, Helsinki, Finland), which is widely used. It is used with the Datex Ohmeda respiration module (type M-CAIOV-X, Datex Ohmeda S/5; Datex-Ohmeda Division, Instrumentarium Corp., Helsinki, Finland) to measure pressure, volume and flow close to the patient's airway. In a review of erroneous capnograph traces, we found several frequently reported abnormalities, but none caused by combining two unrelated pieces of equipment.During mechanical ventilation of a healthy patient undergoing pelvic surgery, we noted that the capnograph plateau was sloping downwards (Fig. 1). This was caused by incorrect connection of the exhaust gas returning from the analyser. The circle system had been used during the previous operating session for low-flow anaesthesia. The sampled gas was returned from the analyser to the circle system. When checking the apparatus, we did not notice that the return connection had been set up by connecting the exhaust gas flow to the coaxial circle system (Coaxial breathing system type 2902, Intersurgical, Wokingham, UK) (Fig. 2).This connection was possible because the tubing assembly is fitted with a narrow-bore integral coaxial tube that allows gas to be sampled from near the patient connection and avoids having a separate gas sampling tube outside the tubing assembly. However, a D-Lite sensor was in place at the patient connection, and this sensor has a port suitable for sampling gas for analysis. The sample tube had been connected externally from the gas analyser (Datex CAiOV) to the sample port on the D-Lite connector using the standard Datex sampling tubing. This left the Luer port on the disposable circle connection unused, and the exhaust gas tube had been connected to this port, which represents the machine end of the integral sampling tube of the coaxial breathing system.Consequently, the gas from the analyser was being delivered back into the breathing system very close to the sampling point within the D-Lite connector. In this healthy patient, the ventilator had been set to deliver a respiratory frequency of 10 breaths min−1, and expiratory flow was virtually complete after 2 s. Thus, for a further 2 s, there was hardly any gas flow in the D-Lite connector, and the composition of the sampled gas was affected by the returning flow. When we disconnected the return flow, the end-tidal CO2 fraction indicated by the monitor increased immediately from 4.3 to 4.6%.During a routine anaesthetic procedure, gas was sampled for analysis by sampling via the integrated gas sampling tube in the coaxial breathing system. At the same time, the D-Lite device was used to measure respired volumes. The flowmeter trace was displaced significantly from zero, and the inspired and exhaled volumes displayed by the monitor were considerably different. Close inspection showed that the tip of the gas sampling tube was close to the proximal manometer opening within the D-Lite connector. Aspiration of sample gas through the sampling tube reduced the pressure at this site and affected the flow signal.

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