Senior staff nurse Susan Parotte was in charge of a Guy's Hospital ward with 11 patients on ventilators suffering from breathing problems when the system went down.
Nurses rushed to disconnect the ventilators, which supply a mixture of air and pure oxygen, and replace them with hand operated pumps.
Moments later she had a phone call from the paedeatric ward where the ventilators were also down and there was a shortage of staff.
The staff nurse told the Old Bailey how the risks were much higher when manually ventilating babies because their lungs were more fragile than adults.
Mrs Parotte said: "When you have a baby with only a small lung you need to know how much oxygen or air to give them."
She was asked if lungs could burst and replied: "Yes."
Anthony McGrory, 34, is accused of sabotaging machinery controlling the supply of heat, air and hot water on 24 October 1994 in revenge for being made redundant from his job as a maintenance worker at the hospital. Mr McGrory, of Croydon, denies one charge of causing a nuisance to the public.
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