POROUS VS HOLLOW MONITORING
Porous level monitoring:
The original test pack was developed in England by Dr. J. Bowie and Mr. J. Dick, and was first published in the 'Lancet' journal in 1963. Since that time, the 'Bowie-Dick' type test pack has been widely used and is recognized as a valuable means of monitoring the air removal efficiency of pre-vacuum and vacuum pressure pulse Steam sterilizers. The original test consisted of 29-36 huckaback type towels, each folded and stacked to a height of 10 -11 inches. A sheet of paper with chemical indicator tape (applied in the pattern of a St. Andrew's cross) was placed in the center of the towel stack. The towel stack was then placed inside a metal dressing casket or equivalent container.
Today this technique has been modernised into smaller pack type tests and in most dental practices has been incorporated into their B-Class bench top sterilisers. AS4815 (2006) have removed the term "Bowie-Dick" test as to save the confusion for most operators. Although the technology has changed over the years, the concept of monitoring to a Porous level has remained. This means that the B-Class sterilisers with the built in air detector can still only detect down to a porous level which is only good when sterilising linen and some solid instruments.
The measurement for porous level monitoring is >150ml of air detected inside a chamber for a failed result. This means that if the build in air detector on the steriliser detects 149.99ml of air then it should not abort the cycle, and it will allow processing of instruments to continue.
Hollow level Monitoring
With the introduction to keyhole surgery and the innovation of Minimal Invasive Surgical (MIS) instruments at the turn of the new millennium, the call for higher levels of monitoring within sterilisation was imminent within healthcare facilities. Because of the complexity of MIS instruments with their hollow canulated structures, hollow monitoring helix devices, also called 'Process Challenge Devices (PCD's), were introduced to replicate a greater challenge than the most complex MIS instrument used in surgery.
PCD's were created to monitor the presence of Non Condensible Gases (NCG's) down to less than 1ml in the sterilising chamber, in order to eliminate any spore population. The principle stands that; if the chamber contains >1ml of NCG's then the PCD chemical indicator should fail, even at a result of 1.1ml of air detected as this is sufficient for survival of a spore population.