Sunday, February 25, 2007

Using Manual Ventilators in a Pandemic Crisis

Some of the current plans for dealing with the expected shortage of available ventilators in a pandemic crisis depend on stockpiling manual ventilators (ambu-bags). They would arrange for teams of friends and family members operate the ambu-bags during the crisis. There is some disagreement as to whether this is a viable option. Some experts dismiss this as being totally unworkable in a mass casualty situation as there are three basic problems with this plan:

1. The physical effort of squeezing a bag continuously is too exhausting for a person to manage very long.
2. Infection control issues related to having many volunteers man the bags in an ICU full of contagious flu victims. The number of deaths due to additional infections generated by such a plan could actually be greater than the number of people saved with the manual ventilators.
3. Lack of monitoring combined with minimally trained volunteers will probably result in a very high morbidity and mortality for the patients.

Now of course it is possible for a team of trained experts to keep someone alive on a manual ventilator for an extended time. In 1955, as a result of a polio epidemic, the demand for negative pressure ventilators exceeded the availability of negative pressure ventilators. There was such a shortage in Sweden that medical students, working in shifts, manually ventilated patients to keep them alive. Now a polio epidemic may sound bad, but the number of patients requiring mechanical ventilation in a flu pandemic will probably be many times higher.

Now instead of throwing our hands in the air in a panic and crying doom, suppose we looked for a way to solve these three problems. Number 1 is easy to solve. Just take an electric motor with some gear reduction and a cam arrangement, to make a pair of mechanical hands to squeeze the bag. The operator can now just turn a control knob to speed up or slow down the rate, or press a button for each cycle if you wanted to have some sort of assist mode going. If you solve problem #1, you also solve problem #2 because you do not need that large group of volunteers.

Solving problem #3 is a little harder. You would need a pressure sensor and an electronic controller to analyze the pressure conditions in the ventilator circuit to determine when alarms such a low rate, high pressure and disconnection occur and sound an alarm. A rudimentary but functional device could be constructed from any standard instrumentation pressure sensor and basic industrial programmable logic controller. If someone worked out a good program ahead of time, that was well tested, this could be freely shared to run thousands of devices across the country.

Incidentally, if you were fortunate enough to have stockpiled a quantity of pressure driven transport ventilators ahead of time, you do not get problems #1 and #2. The same monitoring system could be used to enhance safety of these devices for unattended operation.

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