Zurich—Since the first reported cases of SARS, epidemiologists and medical practitioners have been grabbing most of the headlines. But early on in the outbreak, a group of design engineers at Hamilton Medical (www.hamilton-medical.com), a Swiss maker of ventilator equipment, was busy working behind the scenes to help stop the spread of the disease.
Difficulty in breathing brought on by SARS requires the sickest patients to be placed on mechanical ventilators, which deliver a blend of air and supplemental oxygen to the airways. According to Hamilton Medical's estimates, that's 10 to 20% of the 8,450 total cases of SARS reported by the World Health Organization through June 27, 2003.
Essentially taking over for the respiratory muscles, mechanical ventilators inflate the lungs by delivering a pressurized flow of gas into the patient's trachea. Flow sensors monitor pressure levels, triggering the opening and closing of an expiratory valve in a proportional manner to maintain constant pressure at the patient's airway.
Exhalation Gases a Concern
The handling of a patient's exhalation gases is of particular concern since airborne droplets are thought to be a primary route of infection. In many ventilator designs, the gases simply are returned back to the machine for pressure measurements and then released to the surrounding atmosphere. This strategy poses both the potential for machine contamination and a risk to hospital workers in intensive care units (ICUs).
Breathing easy: Arrows show pathway of air and supplemental oxygen from the Galileo ventilation supply to the patient and to the environment. Note that the exhaled gas never enters the ventilator After the SARS outbreak, engineers added a tubing system to transport exhalation gases directly to the hospital's central vacuum system.
One way to protect against machine contamination is through the use of filters, but they can impose an unduly high breathing resistance. Hamilton Medical, whose sales account for approximately 15% of the 15,000 ventilator machines purchased each year, avoided the machine contamination issue altogether, thanks to the unique design of its Galileo ventilator.
"The resistance created by filters wasn't an acceptable trade-off for us," says Managing Director Josef Brunner. Instead, Hamilton Medical engineers located the expiratory valve and flow sensor at the airway opening so that exhaled gases never enter the ventilator.
The expiratory valve allows positive pressure to build up in the patient's lungs during inhalation and maintains a baseline pressure during exhalation. In the Galileo design, engineers employed an electromechanical valve that incorporates a piston connected to a coil— functioning much like a dynamic speaker. During inhalation, the piston exerts a force on a flexible membrane (which also doubles as a barrier) that fits over the valve, forcing it closed over the exhalation port. To maintain baseline pressure during exhalation, current to the coil is varied proportionally.
To prevent any possibility of contamination of the ventilator through the flow sensor, a rinse is continuously applied through the tubing. The valve assembly can also be steam autoclaved.
Even though machine contamination was not an issue for Hamilton Medical's ventilators, doctors weren't completely satisfied. They wanted engineers to find a way to contain the exhaled gases. "They had concern over the fact that medical personnel in ICUs were breathing these gases. Given that SARS was rapidly spreading at the time, we knew we needed to find a quick fix," says Brunner.
The challenge for engineers: Quickly identify a strategy to deal with the gases—one that could be incorporated easily into the existing machine design. Also, any proposed modifications had to be retrofittable to as many as 150,000 units estimated to be in use out in the field. No one knew at the time how extensive the outbreak would be or how many ventilators would be called into service for SARS patients.
Engineers determined that the best way to deal with exhalation gases would be to take advantage of a hospital's central vacuum system, which is designed to handle a wide array of infectious materials and biological waste. Scavenging systems (as they are known) already exist for handling anesthetic gases, but Brunner says that these systems were too expensive to be considered. "These type of reusable systems typically incorporate an expensive steel tank to contain the gases," says Brunner.
Engineers hit upon the idea of a low-cost, disposable tubing system that consists of plastic (PVC) tubes designed to easily hook directly up to a hospital's central vacuum system. According to Bruner, this economical design is ten times less expensive than other systems—affordable enough to justify replacing it after use with a single patient. And it simply plugs in and is ready to go.
In early May, Hamilton Medical released the new scavenging system, which it sells either as an option on new machines or as a retrofit part for existing machines. As of early July, the company has sold 1,200 units.
Now that SARS appears to be under control, Brunner isn't too worried about future sales. "It's designed for use not only for all SARS patients, but also anyone with a highly infectious disease," he says.
Managing the Unpredictable
One of the most difficult challenges for ventilator manufacturers like Hamilton Medical was meeting the sharp increase in demand for their machines when SARS broke out. Production planners were flummoxed, to say the least. "Demand for our machines was up. No one knew how many people would get sick, whether or not they would require ventilation, who would get sick where, or how quickly the disease would be contained," recalls Brunner.
Hamilton Medical experienced an increase of 150% over the projected forecast for its ventilator machines, and Brunner says that inventories were completely depleted in a matter of four weeks. Most of the units were purchased by hospitals in Hong Kong and China.
Hamilton Medical had to scramble to meet the unexpected demand. Although delivery lead times grew from three weeks to two months, Brunner says that things were not as bad as they could have been. The company did have some difficulties in procuring parts, particularly single-sourced items like motors and compressors. But Brunner credits the company's parts ordering strategy for keeping lead times to a minimum.
"Typically we work with frame-orders, which means that we order quantities of parts for a period of, say, 12 months. In essence, we guarantee that we will purchase a certain number of parts from a supplier," he explains. "That strategy gives us the option to call for more parts earlier in the cycle than planned for. Ultimately, it saved us."
As of mid-July, Hamilton Medical had worked off the backlog in orders and production has returned to normal. On July 5, 2003, the World Health Organization declared that SARS had been contained around the world.