Oxygen

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Published 7 April 2020

This weekend Watford General Hospital declared an emergency and told people to stay away due to a problem with its oxygen supply.

Pre-Covid-19 the supply of oxygen through “magic wall outlets” was not too much a concern for most hospitals as capacity significantly exceeded demand – even during peak use periods. Covid-19 hugely increases demand on oxygen.

For example, a 600-bed hospital with 100 using 5 litres of oxygen per minute (e.g. for respiratory illness) and 10 critical care beds using 15 litres of oxygen per minute would have a peak flow around 650 litres of oxygen per minute. Well within the limit of most systems, which are often between 1,500 and 3,000 litres per minute depending on their age.

Under Covid-19 demand projections from NHSIE, hospitals of this size have been told to expect demand that vastly exceed normal parameters – 200 beds with oxygen, up to 100 for critical care, and around 40 beds with high flow oxygen. This category, which sits between normal beds and critical care uses devices that are non-invasive and often much less efficient in their use of oxygen – sometimes consuming up to 60 litres per minute or more.

Under this type of scenario, the peak demand for oxygen would increase from 650 litres per minute to 4,900 litres per minute. This could exceed the limits some systems were designed to meet (many have not had their limits tested in practice). When this happens, the pressure flattens and the alarms on the oxygen supplying machines across a hospital go off*.

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* I understand that this happens due to the cooling effect of liquid oxygen being evaporated. When too much is evaporated at once, the temperature drops significantly (like when you spray an aerosol can too much in one go). When it goes beyond a certain point it causes the pressure in the supply system to drop quite suddenly.