Fogging of desflurane vaporiser external sight glass: An interesting observation.

Sir,

Desflurane vaporisers are unique and different from others as they require an external heat source, which compensates for the significant heat loss associated with desflurane vaporisation, and they add agent directly to the gas stream.[1] Desflurane is heated to 39°C (degrees Celsius), which increases the vapour pressure in the sump to 1460 mmHg, thereby preventing the possibility of boiling in warm Operation theatres (OTs).[1] The boiling point of desflurane is 22.8°C with a low blood-gas co-efficient of 0.42.[23]

All vaporisers have external, graduated transparent indicators for showing the level of the agent inside them. Clouding or non-visibility of this indicator level can be a great hindrance in efficient desflurane utilisation as well as can affect the economics of this expensive volatile agent. In addition, if agent exhaustion is not visible to the anaesthesiologists, then it can result in inadvertent reduction in depth of anaesthesia and possibility of awareness in the patient. A unique case of fogging of external indicator of desflurane vaporiser is being reported here.

An elective neurosurgical procedure was scheduled as the first case during a cold, winter morning in our OT. Standard intravenous induction along with inhalational sevoflurane was done and the patient was put on Volume Control mode of ventilation after intubation, using Drager Primus anaesthesia machine. A decision to use desflurane for maintenance of anaesthesia was made and a D-Vapor vaporiser was mounted for this purpose. Upon checking the pre-existing level of desflurane agent (through the external sight glass), the level was not visible because of the intense fogging of the level indicator – sight glass [Figure 1]. Cleaning of the external surface of the sight glass did not help as the fogging was on the inner surface. The case proceeded with sevoflurane-oxygen-nitrous oxide for maintenance. The disconnected vaporiser was kept separately in a warmer area of the OT complex and was observed from time to time. The fogging partially resolved after 6-8 hours. The vaporiser was then sent to the service centre.

Figure 1

Intense fogging of the internal surface of level indicator – sight glass

The D-Vapor is a heated, calibrated anaesthetic vaporiser. The agent level indicator (sight glass) is located just beside the filling system on the front panel itself. There are three distinct horizontal marks on the level indicator. The top mark indicates that the vaporiser is full. The second mark indicates that one complete bottle containing 240 mL of desflurane is needed to completely fill the vaporiser. Also, the bottom mark indicates that only 30 mL of agent is remaining in the vaporiser. The total capacity of vaporiser is 300 mL.[4]

The D-Vapor vaporiser has its battery and heating apparatus. A shut-off valve prevents the reservoir from delivering vapour when the control dial is at zero setting, during heating phase, or in the event of vaporiser failure. The control dial setting determines the relationship between bypass resistance and flow control cone, which ultimately decides the delivered concentration of desflurane, with the help of a proportioning valve and a regulating sensor.[5]

We extensively searched the literature, but could not find any reported case of fogging of the external sight glass of any vaporiser. After a detailed discussion with the service engineer and an examination of the D-Vapor vaporiser at the workshop, it was found that there could be loosening of the sight glass which would allow inadvertent seepage of moisture inside the level indicator and malfunctioning of the 'O-rings'. The level indicator is bound, at both the top and bottom, by two washers, known as O-rings [Figure 2]. Any loosening of either of these O-rings would allow the desflurane vapours to seep into the level indicator and the temperature gradient would cause them to condense on the internal surface of the sight glass, leading to its fogging. Extensive seepage of desflurane vapours may also lead to a decrease in anaesthetic depth of the patient (risk of awareness).

Figure 2

Labelled diagram of desflurane vaporiser showing the “O” rings

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