Multiplace Delivery System

Monthly Hyperbaric Safety Notice: August 2005

Patient Carbon Dioxide Hood Monitoring

In this notice, the importance of monitoring the patient hood carbon dioxide levels is discussed.

Background

In the July notice, Owen Doar discussed the need for monitoring oxygen concentration in oxygen delivery systems. While this is essential to ensure that the patient is receiving the required amount of oxygen, we have found monitoring of the hood CO₂ levels to be a more sensitive indicator of problems with the delivery system. While monitoring hood O₂ and CO₂ levels during some assessment trials we were deliberately disconnecting inlet and outlet hoses, running low oxygen flows and under inflating the hood. In almost all cases, a change in CO₂ concentration was the first non-visual indication that there was a problem.

Method

We continually monitored O₂ and CO₂ from an AMRON hood exhaust by installing a "T" piece on the hood neck ring connector and then connecting the hose to the "T". The sample line was led from the "T" to a through hull penetrator and then to a multi-port ball valve which supplies the gas analyser. We used a paramagnetic oxygen analyser and an infra-red CO₂ analyser. After stabilising the subject's hood atmosphere we reduced the flow, disconnected the exhaust hose at the overboard dump, adjusted the overboard dump to deflate the hood, and partially obstructed both the supply and exhaust hoses.

Results

These are the results from one subject on just one chamber exposure, but all others showed similar changes in gas concentrations.

Hood Condition	O₂%	CO₂%
Stable	99.9	0.5
Low Flow (25lpm)	99.6	0.77
Disconnect exhaust hose at overboard dump	99.9	0.5
Deflation	99.0	0.3
Obstructed O_{2 hose}	99.9	0.76
Obstructed exhaust hose	99.9	0.79

All of these changes occurred within 45 seconds

As can be seen from these results carbon dioxide percentage changed by 20-25% while oxygen percentage changes were minimal. The only mishap that the analyser failed to pick up was the exhaust hose disconnect; this makes sense as the flow through the hood is not altered. Also of note and, once again, quite logical is the drop in both O₂ and CO₂ during a steady deflation, in the other cases, the rise in CO₂ is considerably more rapid than any change in oxygen percentage.

The Safety Issue

As we have an experienced, well trained attendant inside the chamber, why do we need to monitor for these mishaps? Surely the attendant should pick them up before the analyser does? Well, sometimes the inside attendant can be distracted by other events in the chamber, such as putting hoods on 6 or 7 patients or a patient having a hypoglycaemic episode. Of 1,131 incidents reported to the Hyperbaric Incident Monitoring Study through 2003, 33% involved equipment and of those, 32% involved; hoods, flowmeter or overboard dumps. They included incidences of hose disconnects and misconnects; low oxygen flows and overboard dump malfunctions causing hood deflation. Hose disconnects were frequently only picked up by the technician noticing a rise in the chamber's environmental oxygen concentration, a potential fire risk. Many of the low flows and deflations could possibly have been noticed earlier if there was continuous monitoring of all hoods.

Key Operational Issues

For in-hood monitoring to be effective in alerting us to mishaps it needs to be continuous. For some of us this means monitoring up to 6 hoods simultaneously or possibly having an air actuated multi-port ball valve that cycles through the hoods one at a time for a set period.

Bottom Line

Continuous in-hood CO₂ monitoring of up to 6 hoods can be a useful safety and QA tool. However, it can be costly to set up (if you purchase 6 analysers) or complicated (the ball valve option). In none of the mishaps recorded, neither in the HIMS data nor in any observed personally, has there been any harm to a patient. However, there is potential to cause harm and the decision on whether to carry out continuous in hood monitoring is up to each individual unit's risk management process.

Contributing Author: Stephen John Goble

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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