Multiplace Delivery System
Monthly Hyperbaric Safety Notice: December
Evaluation of Respiratory Support Devices for Use in the Hyperbaric Chamber: A Critical Review
Background:
Between the years 2000 and 2003 the US Navy tested 11 commercial off the shelf ventilators in a hyperbaric environment to determine their suitability for acceptance into service. This evaluation has been made available for public release as document: NEDU TR 03-18 November 2003. A copy of this document has finally made its way to my desk in Australia and seeing as our unit is in an eternal quest for the perfect hyperbaric ventilator it was suggested that I write this review as a monthly safety notice. As I am a technical officer and not a medical officer I elicited some specialist comment from my medical director who is a consultant anaesthetist (anaesthesiologist to you).
Evaluation Précis:
In accordance with NFPA Standards for hyperbaric medical devices each ventilator was evaluated for safety prior to its exposure to hyperbaric conditions. These evaluations included reviews of electrical and mechanical schematics as well as bench reviews of interior and exterior to identify components vulnerable to crushing or likely to cause sparking. Ventilators utilising batteries were tested separately according to NAVSEA, NFPA99 and NEDU standards.
Following these evaluations all ventilators were reassembled and pressurised to 220fsw for one hour, they were then disassembled and internal components examined. After being reassembled the ventilators were operated at the surface to ensure that they still functioned normally. Once satisfied that the ventilators were safe and functional they were all subject to a series of tests to 30, 60 and 165fsw.
A test lung was used to simulate a patient and data output was led through a hull penetrator into a computer outside the chamber to collect, store and analyse data from the test lung. Various pulmonary resistance and compliance settings were used to simulate patients with different lung pathologies. The test lung was calibrated at surface and at depth to assure accurate measurements. Each ventilator was operated for 5 minutes during this time the ventilator parameters were adjusted to achieve the required breathing frequency and minute volume. Ventilator parameters were recorded by the data acquisition software every 15 seconds during this time.
Of the eleven ventilators tested only four met all testing requirements, however, all bar one required considerable adjustment of settings to meet the required breathing frequency and minute volume. A few ventilators failed to make it to the pressurised testing and some failed to progress past the 30fsw pressure test. Rather than list all the ventilators tested and the results I’ll leave you to read the report yourselves, I will, however, provide some technical and anaesthesiologist comment on the report.
Anaesthesiologist Comment:
From an anaesthesiologists point of view this is a systematic, methodical and thorough evaluation of eleven ventilators comprehensively covering the range of issues an anaesthesiologist would want to know prior to use. It considers in sequence the safety of the device, how it performs under variable lung parameters and how the ventilator itself is affected by altered ambient pressure.
There are a few limitations to the study; it was completed in the year 2003 and so could only use a range of ventilators available at that time. Some of the devices are transport ventilators which tend to be crude but robust; they do not offer the range of options available on modern ventilators. These options are increasingly considered the minimum standard these days.
A survey of hyperbaric facilities around Australia reveals that we use a range of ventilators, suggesting that we don’t have the perfect solution either. Most are using adaptations of ventilators used for anaesthesia in operating theatres, most of these have required some modifications to make them chamber safe and many require some alteration of ventilator settings as ambient pressure changes. All seem to adequately ventilate a range of lung conditions.
Technician Comment (mine!):
First let me say that I agree with the above comment. However, I believe the US Navy should be applauded not only for undertaking the testing but also for making the report public. At the very least they have provided the rest of us with a template to follow for undertaking our own testing whenever we are asked to test a new ventilator.
However, this report didn’t really tell us anything that many of us knew already; namely that volume cycled pneumatic ventilators reliant on fluidics don’t operate well under hyperbaric conditions. Having tested a few over the years we could have told them to save their time. In fact the only volume cycled pneumatic ventilator that we have found which works in hyperbaric chambers is the Dräger Hyperlog which was designed specifically for hyperbaric conditions. It copes with the increased ambient pressure by having a reference line open to connected to a through hull penetrator which allows it to track the pressure differential between outside and inside the chamber. However, it only has basic functionality and is less than ideal for complicated patients.
My comment on there being nothing new in the conclusions of this report is not a criticism of the Navy, rather an observation of our industry. Until the advent of MedEdOnline the only forum for the distribution of information like this was to write a paper for the UHMS, not an undertaking all technicians wish to take on just to save other people from reinventing the wheel. Dick Clarke is to be applauded for setting up a system where we can pass on information such as this without having to go through the entire peer review process of a mainstream journal.
The hyperbaric community has for many years been searching for a ventilator capable of operating under hyperbaric conditions with no modification or adjustment of controls and which is capable of options such as SIMV, PEEP and variable inspiratory/expiratory rations. For many of us this has been a fruitless exercise and overtures to manufacturers about making a purpose built hyperbaric ventilator have been rejected due to the cost of research and development and the perceived limited market.
However, all is not lost, the Italian company Siare has released the Siaretron 1000 IPER which has been developed specifically for hyperbaric use. So far I have not been able to test one of these ventilators but have seen it in use at 1ATA and had a chance to read the manufacturer’s information. We are currently awaiting our turn to evaluate the only one in existence in our part of the world.
Bottom Line:
This report is definitely worth a read for the excellent testing template put together by the researchers. Also for the list of seven ventilators which failed the testing and the list of four which passed. Reading this can save you time by letting you know which ventilators to avoid and which ones may be worth trying out in your own unit.
If undertaking your own testing it is important to use the same standards as the Navy used here, follow the NFPA guidelines and the NEDU battery testing procedures and get your biomedical engineers to check out all the components for hyperbaric compatibility before allowing any equipment into the chamber.
Acknowledgement:
I would like to acknowledge the contribution of Dr David Wilkinson for supplying the Anaesthesiologist comment.
Contributing Author: Stephen John Goble
Steve has held the position of head hyperbaric technical officer at Royal
Adelaide Hospital, in Adelaide, Australia, since 1985. He arrived in Adelaide
by way of the British Royal Navy (Clearance Diver) and the offshore commercial
diving industry. Steve is a founding member of the Hyperbaric Technicians
and Nurses Association (HTNA) and edits their journal (‘Offgassing’).
Steve is closely associated with, and intimately involved in, HIMS (Hyperbaric
Incident Monitoring Study), and is extensively published on topics ranging
from technical and safety issues, standards, incident reporting and decompression
accidents.
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