Principles of infection control and ventilator management taken from the SARS epidemic may serve during future viral outbreaks

The new millennium has seen the emergence of new strains of viruses around the world. Notable worldwide viral outbreaks in the last 5 years have been the severe acute respiratory syndrome (SARS) due to the corona virus and the avian H5N¹ influenza. Health care professionals and health care authorities have to adapt quickly to the rapidly changing epidemiology in order to contain the viral outbreaks and prevent nosocomial transmission. Viral pneumonia may progress quickly and often results in severe acute respiratory distress syndrome (ARDS).¹ When this happens, mechanical ventilation is needed to support the respiratory system. As airway management is associated with exposure to respiratory droplets and secretions, it is imperative that appropriate precautions be undertaken to prevent transmission to the health care workers.

Severe Acute Respiratory Syndrome (SARS)
In November 2002, SARS was first reported in Guangzhou, China.² SARS became a global threat that has resulted in 8,098 SARS cases in 26 countries and 774 deaths by July 2003.² As the SARS coronavirus is transmitted from human to human, the lessons learned about preventing nosocomial transmission can guide us in the management of future similar viral outbreaks.

Ventilator Management
Twenty-five percent of patients who are diagnosed with SARS develop acute respiratory distress syndrome and will need mechanical ventilation. The ventilatory management of patients with SARS does not differ much from that for patients with ARDS alone.^3-5 A low tidal volume lung protective strategy should be adopted, with the target tidal volumes at 6 mL per ideal body weight. Plateau pressures are preferably kept less than 30 cm H2O. This is modeled after the recommendations from the National Institutes of Health ARDS Network.⁶ Either volume- or pressure-control ventilation can be used. High levels of positive end-expiratory pressure (PEEP) may be used to maintain a PaO₂ greater than 55 mm Hg. PEEP helps to increase end-expiratory lung volume, recruits unventilated alveoli, decreases intrapulmonary shunt, and improves V/Q (ventilation perfusion) matching. Recruitment maneuvers can also be carried out to improve oxygenation. Permissive hypercapnia is allowed, and bicarbonate may be given to maintain the pH. Short-term neuromuscular blockade can be given when indicated.

For refractory hypoxemia, ventilatory options include inverse ratio ventilation, prone positioning, or other unique modes of ventilation, such as airway pressure release ventilation (APRV). Rescue therapy with inhaled nitric oxide may be attempted, but its efficacy has not been demonstrated.³

Unfortunately, a high rate of barotrauma (34%) has been reported.⁴ This highlights the importance of a low tidal volume lung protective strategy. Other complications of mechanical ventilation reported include ventilator-associated pneumonia, acute renal failure, deep-vein thrombosis, and pulmonary embolism.⁴ Further research is necessary to provide early identification of the subgroup of patients with SARS who may progress to severe ARDS.

Although high-frequency oscillatory ventilation (HFOV) has been shown to improve oxygenation for patients with severe ARDS, the inability to filter the exhaled air to the environment is of extreme concern. The oscillator circuit requires a special filter that is able to withstand the high airway pressures that the oscillator maintains. In response to this concern, the manufacturer of the high-frequency oscillator has developed prototypes of oscillator circuits with specially attached filters. This filtered circuit is currently undergoing clinical trials. For now, though, much caution should be observed when using HFOV for patients with SARS or any suspected infectious virus.

Noninvasive ventilation (NIV) has been used in early acute respiratory failure to aid in ventilation and reduce intubation rates. During the SARS period, there were many concerns regarding the unfiltered aerosolized exhalation from NIV. This droplet dispersal ventilation may pose an increased exposure risk to health care workers. No case reports of nosocomial infection from NIV have been reported when proper precautions are taken.⁵ In addition to the usual isolation precautions, further care can be taken to reduce aerosol generation. A “swivel” type exhalation port can be used instead of a jet flow exhalation port.⁵ A bacterial-viral filter can be placed between the mask and exhalation port to reduce aerosol contamination in the room. The use of NIV can be helpful when adequate precaution measures are taken.

Infection Control
SARS appears to spread primarily through respiratory droplets.⁶ The virus-laden droplets are produced by infected people when they cough or sneeze. The disease is spread when the droplets are propelled a short distance through the air and enter the mouth, nose, or eyes of anyone in the vicinity. Transmission can also occur via fomites, such as a dish or article of clothing that might be contaminated. For example, when a person touches contaminated objects or surfaces and thereafter touches his mouth, nose, or eyes, viral transmission can occur. It is still unclear as to how long the virus remains viable outside the host body.

In the early stages of the SARS outbreak, knowledge about the mode of transmission of the virus and the necessary precautions and measures to take was unavailable. Health care workers were infected from patients that they treated. The number of infected health care workers dropped dramatically after adequate isolation, contact and droplet precautions, and compulsory hand-washing practice were implemented. It is clear that nosocomial infection can be reduced with the necessary infection control measures in place.

SARS patients should be nursed in private rooms with a negative pressure system in the intensive care unit. Health care workers attending to the patients wear N95 masks, protective eye wear, full face shields, caps, long-sleeved waterproof gowns, surgical gloves, and shoe covers before entering the patient’s room. Thorough hand washing is done after contact with patients or contaminated objects and after taking off protective garments. In situations where there is intimate airway involvement, for example endotracheal intubation and disconnection of the ventilator circuit, a powered air purification respirator (PAPR) hood should be worn in addition to the normal barrier precautions.

Protection Strategies to Adopt
A major concern for the management of SARS patients on mechanical ventilators is the protection of health care workers from the droplet transmission of the virus. High-risk procedures such as bronchoscopy should be avoided when possible. Health care workers are shown to be at a substantially increased risk of infection when they are involved in the early critical care period and endotracheal intubation of patients.⁶ Extra precautions should, therefore, be observed during endotracheal intubations. First, the health care worker should wear the PAPR in addition to the usual barrier protection. A bacterial-viral filter can be placed between the manual resuscitator and the mask to filter out the exhaled air. A good fitting mask is essential, as a tight seal should be maintained to prevent leakage. Manual ventilation should take the shortest time possible. The patient should be well sedated or even paralyzed during the procedure so that he does not cough excessively. The ventilator should be switched on only after the placement of the endotracheal tube is confirmed by the doctor. A closed-system, in-line suction catheter should be used for the suctioning of the endotracheal tube to reduce disconnection of the circuit.

Heated humidification is commonly used for patients on a ventilator, but during an infectious virus outbreak, a viral-bacteria heat moisture exchanger with filter properties (HMEF) should be used for humidification purposes instead. An HMEF not only provides humidification, but also can filter the exhaled air from the patient before it reaches the ventilator. Changing the HMEF is necessary only when it is blocked with excessive water condensation or a mucus plug or according to hospital protocol. When changing the HMEF, it is advisable for health care workers to preoxygenate the patients and temporarily switch the ventilator to standby as there is droplet spray to the surroundings. The health care worker should be wearing the PAPR prior to the anticipated disconnection.

It is best to have two filters per ventilator. One filter is placed between the inspiratory port and the ventilator circuit and the other between the expiratory port and the ventilator circuit. A third filter can be placed at the exhalation outlet of the ventilator if needed. The filters provide additional protection from the exhaust gases emitted to the health care workers and minimize ventilator contamination. A scavenger system for the exhalation port of the ventilator is optional if the room has no negative pressure system.

Conclusion
The strategies presented herein have been effective in containing infection of health care workers during the SARS epidemic. Although future virus outbreaks may be unpredictable and different, lessons drawn from SARS will help us better prepare and manage our health care resources. We cannot be complacent about virus outbreaks. Ongoing research, improved management guidelines for ARDS, and technological development would lead us in conquering future virus outbreaks.

Nancy Lew, RRT, is a respiratory therapist; Constance Lo, MBBS, MRCP, is a consultant respiratory physician; and and Philip Eng, MBBS, MMed (Int Med) is head, Department of Respiratory and Critical Care Medicine, Singapore General Hospital.

References
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2. World Health Organization. WHO guidelines for the global surveillance of severe acute respiratory syndrome (SARS). October 2004. Available at: www.who.int/csr/resources/publications/WHO_CDS_CSR_ARO_2004_1/en.   Accessed January 26, 2006.
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