Severe acute respiratory syndrome (SARS): lessons learnt in Hong Kong
Many healthcare workers were infected while looking after the SARS patients on the medical wards in 2003. The high infectivity of the SARS coronavirus with peak viral load on day 10 of illness when patients were ill, overcrowding of the old medical wards with low air changes/hr (ACH), and aerosol-generating procedures while resuscitating the patients were the major factors. Procedures reported to present an increased risk of SARS transmission include tracheal intubation, non-invasive ventilation, tracheotomy and manual ventilation before intubation whereas oxygen therapy and bed distance <1 m were also implicated. Studies based on laser visualization technique with smoke particles as smokers in the human patient simulator has shown that oxygen therapy via Hudson mask and nasal cannula could disperse exhaled air of patients to 0.4 and 1 m respectively whereas jet nebulizer could disperse exhaled air >0.8 m from the patient. Bigger isolation rooms with 16 ACH are more effective than smaller isolation rooms with 12 ACH in removing exhaled air and preventing room contamination but at the expense of more noise and electricity consumption. Non-invasive ventilation via face masks and single circuit can disperse exhaled air from 0.4 to 1 m. Both higher inspiratory pressures and use of whisper swivel device (to facilitate carbon dioxide removal) could increase the exhaled air leakage and isolation room contamination during on-invasive ventilation. Addition of a viral-bacterial filter during manual ventilation by bagging may reduce the exhaled air leakage forward and yet increase the sideway leakage. N95 mask was more effective than surgical mask in preventing expelled air leakage during patient’s coughing but there was still significant sideway leakage to 15 cm. Clinicians should be aware of air leakage from the various face masks and adopt strict infection control measures during resuscitation of patients with severe respiratory infections. Carefully designed clinical trials are required to determine the optimal timing and dosage of any antiviral agents, convalescent plasma, and immuno-modulating agents in the treatment of the possibly immune-mediated lung injury in SARS and newly emerged infection such as the Middle East Respiratory Syndrome.