Measurement errors caused by incompatibility of apparatus

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).

Fig. 1

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. As the reduced pressure generated by the gas sampling was constant, the effect was to displace the flowmeter trace from baseline, suggesting a continuous expiratory flow. Flow caused by respiration was superimposed on this offset signal, and integration of the flow trace gave incorrect results. When the tube connection between the circle assembly and the D-Lite sensor was slightly rotated, the error was reduced, and after the sample tubing was shortened, the error was abolished (Fig. 3).

Fig. 3

Combining equipment from two independent manufacturers caused both of these events. Incorrect assembly of the gas sampling system was possible because the coaxial circle system and the D-Lite flowmeter offer alternative connections for gas sampling. As the exact sample site of the integral coaxial tube is not clearly visible, this problem can be easily overlooked. Previous problems have been reported with integral sampling tubes. A crack in the sample system allowed exhaled gas to mix with the sample gas and increase the apparent inspired carbon dioxide concentration ^[1].

If these systems are to be used together, we suggest that a sample return point should be formed in the expiratory limb of the circle assembly. This can be done by using a breathing system filter fitted with a gas sample port. If this is attached to the expiratory limb at the machine end, gas can be returned safely and unobtrusively. However, this introduces an additional component, perhaps with its own attendant hazards, if the user wishes to conserve gas to allow low-flow anaesthesia.

A wide range of abnormalities of capnogram shape have been reported associated with faults with side-stream capnography. The most widely reported is the effect of a leak in the gas sampling line combined with positive pressure ventilation, creating a characteristic waveform with a long low plateau ending in a peak ^[2–4]. This waveform is caused by entrainment of ambient air. Its height is inversely proportional to the size of the leak (Fig. 4a) ^[5]. Because the leak is related to the pressure inside the sample tube, different waveforms can be generated such as a two-tailed pattern (Fig. 4b) ^[6] and a ‘cleft’ appearance (Fig. 4c) ^[7]. A less usual ‘dromedary’ capnograph (Fig. 4d) ^[8] has also been reported, again caused by leaks in the sampling apparatus. This was caused by a leak in tandem with equipment modification, in which a pressure manometer tube had been used to extend the sample tubing. In the published reports, the commonest cause of a leak is a loose Luer-lock connection, but other causes including damage of the sampling line causing leaks that may be completely invisible ^[1], cracks on the sampling port ^[9] and a leak within the housing of the capnometer unit ^[3]. A leak allows the entrainment of air into the sample line during expiration and may not occur when the pressure in the breathing system is greater than atmospheric pressure. The initial sample gas to be analysed is, therefore, a mixture of gas exhaled by the patient and entrained room air. The effects on the capnograph depend on the leak size and pressure within the circuit (and therefore the sample line). The pattern of airway pressure changes affects the abnormality. The pressure causing the leak depends on the type of ventilator being used. A falling bellows generates a pressure less than atmospheric pressure early in expiration. Airway pressure then abruptly increases when the bellows becomes full. Such pressure changes within the system alter the rate of gas entering the leak. Airway pressure changes also affect the rate of gas sampling by sidestream analysers and result in waveform distortion ^[10]. In cases with a leak, the greatest value in the capnogram is likely to be the closest to the real value.

Fig. 4

A biphasic capnogram can occur in some circumstances and has been suggested as a way of detecting accidental bronchial intubation ^[11].

The combination of two unrelated devices caused the errors we describe. Vigilance should detect such problems and prevent clinical errors. Fortunately, when the flowmeter baseline is displaced from zero, the volume data provided are unrealistic and unlikely to be accepted. An erroneously low end-tidal CO₂ reading caused by the downsloping capnograph trace might only result in slight hypoventilation, which is unlikely to cause serious harm to patients with normal intracranial pressure. However, the end-tidal anaesthetic agent concentration will be artefactually high, and this could lead to underdosage.

In summary, we suggest that concealed gas sampling tubes can have unintended effects on associated airway measurement equipment and should be checked for interactions.