A mini blind study
Richard Purchon RODP (2017)
Introduction
There are currently two ways of testing Endo Tracheal Tube (ETT) cuffs prior to use. Positive and Negative pressure testing. The manufacturers recommend testing tubes before use, however there are no official guidelines. With the two different techniques being used, depending on individual practice and experiences of the Anaesthetic Practitioners involved.
There has recently been a number of Students questioning this practice in on-line forums in order to determine which is the correct method to use.
The responses so far have been mainly based on the personal experiences of the individual practitioners. For example some have had cuffs fail in use following a positive test, that either hadn’t identified the leak, or caused additional stress to the cuff which then caused a leak due to perforation.
Many practitioners stated that a negative pressure test did not always identify leaks, as the folds of the collapsed tube seal against each other creating a false seal.
The purpose of this mini study is to compare negative and positive pressure testing, in order to identify a tube cuff fail.
Breakdown
For the purposes of this mini study ten unopened, expired ETT’s size 9.0 were used. All ten ETTs where high volume low pressure tubes, produced by Smiths Portex.
A fully inflated cuff resting diameter is given to be 30mm. The volume needed to inflate the cuff to this diameter was determined using a separate (control) size 9.0 tube and is a minimum of 20mls of air.
Five tubes where selected at random and a BD Microlance 25G orange needle was used to make a hole through the central area of the cuff, in order to create a leak.
Control tube with cuff inflated with 20mls
All ten tubes where then tested using the negative pressure technique, which involves drawing back to extract all residual air from the cuff using a syringe. In theory if a hole was present the cuff would not stay deflated.
All ten tubes successfully passed the negative pressure test, and would therefore be deemed suitable for end use.
Ten passed negative test tubes
All ten tubes where then tested using a positive pressure technique, this simply involves inflating the cuff of each tube with 20mls of air as previously determined.
After inflating all ten tubes it was clear exactly which tubes had holes and which were intact, all five tubes that had been pierced, failed to maintain their volume.
Five passed positive test tubes
Five failed positive test tubes
Conclusion
Although it is commonly thought that negative pressure testing of ETT’s is a good enough method of tube testing prior to use, this small study has proved that negative testing could not identify which tubes had been pierced.
Positive testing identified all pierced tubes and is therefore the only accurate method of testing tubes as determined by this small limited study.
Further research should be conducted. It is possible that data on pressure testing may well be available from tube manufacturers; this data would give evidence from a larger pool of samples than ten.
