Pictures of the Future      Fall 2008

Seeing the Invisible

One of the principal causes of hospital-based infections is the presence of airborne germs during operations. Simulation tools now under development for the visualization of operating room airflows will help designers optimize the placement of vents so that surgical wounds are ideally ventilated. More advanced simulation systems will take body heat and lighting into account.

In an operating room at the Rechts der Isar hospital in Munich, Germa­ny, two doctors stand next to a patient. Behind them are tables for surgical instruments, and above the patient table is a large lamp. Sudden­ly, pieces of medical equipment begin to move as if by magic—as do strange lines that run through the operating room like colored cobwebs. "The lines show the currents; and the color is the speed of the air particles," says Dr. Gerta Köster of Siemens Corporate Technology in Munich. Köster moves people and machines around with a mouse; the operating room is only a simulation.

If a doctor stands between the air jets in the wall and the patient table, more airflow lines take a detour around the team of doctors, and less fresh air reaches the patient. That could be dangerous. The study "Krank im Kranken­haus" (Hospital Infections) from the German Society for Hospital Hygiene estimates that 15 % of all patients in intensive care units acquire an infection, and there are indications that the main cause of infection is the presence of airborne germs during operations. For the most part, the infectious agents come from the air breathed by operating room personnel, but they are also blown into the room from outside through bad filters. Studies carried out at Uppsala University Hospital in Sweden show that the risk of infection can be reduced by 95 % through controlled ventilation of wounds. One important advantage of more-effective ventilation is that patients can more easily be cooled in this way. In the past, however, this was little more than wishful thinking. "In the operating room, it’s often like Grand Central Station," says Köster. The constant comings and goings of medical staff change the air currents. Sometimes the doors are even open, particularly when emergency surgery is in progress.

Today, the planning of  operating room airflows is pretty much a matter of trial and error. There is usually a standard layout with vents for fresh air in the ceiling and air extraction vents in the walls. At Rechts der Isar hospital, how­ever, clean air is fed in from a side wall so that its path to the patient is not impeded by the large lamps above the operating room table. No one knows which arrangement is better. And that’s where the simulation comes in.

When they hear the term "airflow simulation" experts generally think of finite-volume methods with elaborate interconnections, whereby the actual space is divided into small virtual volumes in a computer, with air movements being calculated for each area. There are commercial software packages that perform this analysis, and the results are very good. The drawback is that such a calculation can take several weeks—much too long for Project Manager and mathematician Gerta Köster. "We need interactive simulations in real time," she says. In other words, when objects are moved on the display, the changes in airflow lines should be visible within seconds. Only then can operating room planners be satisfied that all of their questions have been answered.

Mathematical Adaptation. To develop such a tool, Köster and her partners at the Computation in Engineering department of Munich’s Technical University (TU) and the Fraunhofer Institute for Building Physics in Holzkirchen had to find a more efficient mathematical approach. They opted for the "thermal lattice Boltzmann" model, for which, however, no suitable commercial software package was available. The researchers had to adapt the model to suit their own purposes (Pictures of the Future, Spring 2006, Entering the Comfort Zone).

What they discovered was that the underlying airflow physics for wall- and ceiling-based vents were the same, says the TU’s Dr. Christoph van Treeck, a project manager in the Computation in Engineering department. What’s more, the Boltzmann method proved itself to be well-suited to efficient programming because it calculated three-dimensional lattices  in seconds. In an interactive simulation, this speed comes at the cost of precision, but the result is still good enough to generate reliable airflow lines.

The software for ComfSim—which is the name of a joint research project funded by the Bavarian Research Foundation—is still being run on a multi­million-euro high-performance computer where it is successfully calculating airflows for ICE rail cars and aircraft at the TU. The finished product for OR planning, which is expected to be ready in two years, and for which Köster is still seeking development partners at Siemens divisions, will be run on a PC cluster costing only a few thousand euros. However, the software could already be used today to plan a new operating room.

In principle, Köster believes, it would be possible to run a simulation during an operation and continuously monitor the positions of personnel and equipment, all the time dynamically controlling airflows. But this would not be very useful. It would be better, she says, to run through various scenarios during the layout of the operating room and to define general guidelines for behavior.

When an operation is planned, the doctor could run a simulation to optimize the placement of equipment and establish the workflow during the operation. "In the past, we’ve tried to position equipment and systems in the operating room so that they disrupt the airflow as little as possible," says Dr. Rainer Burgkart, an orthopedic surgeon at Rechts der Isar. "But we did this based on our intuition. So of course we couldn’t know for certain whether we were ultimately achieving an optimal inflow of fresh air."

The full potential of the simulation software has not yet been realized. In the next step, the ventilation should be adjusted to ensure not only the correct temperature but also a sense of comfort for the persons in the room. The patient should be cooled, but not below a certain threshold, and the doctors must not perspire beneath their sterile uniforms.

Another objective is to include light distribution models in the simulation. This is important because it makes a difference whether sunlight is flooding the OR, or whether it is foggy outside and lamps are turned on.

The time is ripe for the introduction of ComfSim to OR planning, Köster believes. More and more hospitals are being built, and many old operating rooms are being renovated. The simulation tool would give Siemens a clear competitive advantage in designing these facilities. Köster’s message: "No one planning an operating room should do so without our airflow simulation."

From Operating Rooms to Architects. Considering that air currents are important not only in the OR but, in principle, in all buildings, the simulation tool could also be developed into a universal tool for architects. Currently, buildings are primarily designed according to visual criteria. But armed with an airflow and temperature distribution planner, an architect would be able to design buildings in which people are always comfortable—buildings in which the heating and cooling take place at precisely the locations needed. This would save energy. For example, the ventilation of concert halls could be simulated to ensure that the climate control was always ideal, regardless of whether the hall is mostly empty or packed. Eventually, simulations will also include crowd flows, such as those encountered at airport terminals. "That would allow us to take body heat into account when simulating a climate control system," says Köster.
                                                                                                                Bernd Müller