INFECTION CONTROL Isolation Room Controls: Ensuring Safety with the Right Technology Facility Care • July/August 2002
Stephen J. Davis is president of Laboratory Control Systems, Inc., 2259 Scranton-Carbondale Hwy.,Scranton, PA 18508
Pressurization holds the key to proper operation of an isolation room. However, being aware of appropriate technology and pertinent environmental parameters is crucial to ensuring the safety of everyone involved.
Control and monitoring of the environment is critical for the operation of an isolation room as well as for the safety and security of everyone involved in its use. To achieve these characteristics, key environmental parameters in the room must be controlled precisely and with predictable repeatability. These parameters include air temperature, airflow, humidity and pressurization. Pressurization is the most critical because without it precise and accurate control of the other parameters is virtually impossible .
Unlike other confined spaces that require special environmental control and monitoring systems, such as clean rooms or pharmaceutical processing areas, which may not need pressurization control, hospital isolation rooms must always be pressurized. The very nature of dealing with airborne transmitted diseases requires pressurization and airflow control: airflow entering the room must be accurately controlled to protect severely immuno-compromised patients, and airborne transmitted diseases must not migrate to other spaces in the hospital. Therefore, pressurization control, combined with high air change rates for dilution, is the most practical method of dealing with this application A variety of conditions must be maintained to achieve reliable airflow control. Positive or negative air pressure in the room must be maintained to prevent contamination, and the air distribution system must provide maximum dilution while preventing localized, high concentration areas of airborne contaminates. In addition, the temperature gradient throughout the room must be maintained at a uniform rate while the high air change rate should not create drafts.
Airflow Tracking vs. Differential Pressure
While there are a number of methods to accomplish isolation room pressurization, the two most popular are airflow tracking and differential pressure control. Controlling isolation room pressurization using airflow tracking methods is based on the principle of measuring and controlling air flow in and out of a confined space. This method maintains desired cubic feet per minute (cfm) differential, or offset, between supply and exhaust air and permits precise airflow control resulting in either positive or negative pressure, depending upon the requirements.
Differential pressure is a technique that directly measures the pressure difference from the enclosed workspace to a reference space. This is usually an anteroom and/or an adjacent corridor. Variable airflow control into the pressurized isolation room maintains a fixed level of differential pressure between the controlled space and adjacent area.
Advanced Instrumentation Means Precise Control
As with most technology, each of these methods offers certain advantages and disadvantages, depending upon the circumstances. The good news is that both are practical and easily achievable thanks in large part to the development of sophisticated instrumentation capable of measuring extraordinarily low levels of differential pressure, typically in the region of .001 inch water column (wc). To a large extent, lack of accurate instrumentation in the past prevented the possibility of achieving isolation room pressurization at desired levels, which is typically about .02 inch to .05 inch wc. In the past, pressurization levels were almost an order of magnitude higher because of limitations in both measurement devices and display instruments. Lower levels of differential pressure are desirable for several reasons. Safety considerations dictate reasonable levels of differential pressure be maintained so doors can be operated properly and safely. For example, if a door opens inward and the space is at a high positive pressure, it may not be possible to open the door. Conversely, if that space were under a high negative pressure, releasing the latch may cause the door to fling open possibly causing injury to the person attempting to exit. Furthermore, depending upon construction, it is possible that the pressurized space could implode—blasting ceiling panels downward causing injury or equipment damage. Finally, to maintain accurate temperature control, it is desirable to minimize the quantity of air being infiltrated to or exfiltrated from the space. Basically, from a technology standpoint, both airflow tracking and differential pressure control will maintain the desired direction of airflow into or out of a space. However, there are factors that must be considered to help determine which method is best for the application. The most important of these are access to and from the isolation room and architectural details.
Traffic Flow and Building Architecture
The first consideration is access and traffic flow of the isolation room. Determine how accessible the room is to corridors and means of egress. Is the room located in a high or low traffic area? Will there be limited access to the room? Will air locks be utilized? How many people will be in the room? Answers to these questions will help determine the most suitable technology.
For example, if the isolation room is in a high traffic area open to active movement of many people, pressurization control would not be desirable because of the multiple upsets to the airflow in the room causing too many pressurization variations.
Remember, every change in the room or reference pressure causes the control system to respond and vary the airflow to or from the isolation room. The more isolated the controlled space, the easier it will be to successfully implement differential pressure control methods vs. airflow tracking. (See Table1).
To sum up considerations with regard to methodology, the two main issues in determining which method is most suitable are building design and/or construction and people movement and traffic considerations as opposed to discrete methods for controlling pressurization.
Cascaded Pressure Control
A variation that uses elements of both airflow tracking and differential pressure control is cascaded pressure control. This technique measures all supply and exhaust flows in and out of an isolation room and maintains a fixed flow differential between supply and exhaust air. Cascaded pressure control adds the element of measuring differential pressure in the space as well, using that measurement as a reset point to the flow offset. This allows the flow differential to be varied between minimum and maximum values to respond to any influences that might affect the pressure. The advantage of this technology is that it provides the stability of airflow tracking with the flexibility of allowing variable airflow differentials to meet temporary external conditions without sending the space out of control.
Differential Pressure Monitoring
Another variation that has application in certain areas is tracking with differential pressure monitoring. In this application, airflow tracking is used as the control method and differential pressure monitoring is overlaid to function as an alarm set point and as a maintenance management point through the isolation room’s ventilation system. For example, other than when a door is opened or closed, if a differential pressure measurement changes over time, it usually means that one of two events has occurred: Either airflow was degraded on one side of the system, thus eliminating desired differential pressure, or there has been a change in the envelope. Someone may have opened a hole in a confining wall to install new duct-work without properly sealing it. Or, perhaps a pipe was installed through a floor and the resultant gap was not sealed. Cascaded pressure control techniques can be handy in these applications because they do not particularly add complexity to an overall control scheme.
Each of these control methods offers distinct advantages and disadvantages depending upon the application, and each should be evaluated based on practicality for the circumstances, such as logical work flow/employee movement patterns, and cost-effectiveness. With regard to differential pressure control, consider the kind of measurements necessary and review the advantages and disadvantages of this technology.
Other Controlled Environmental Parameters
It is also important to look into overall environmental control. In the critical environment of a hospital isolation room, there is generally a sequential hierarchy for most control requirements that mandates pressurization and airflow and then includes temperature and ventilation, or air changes per hour (ach) control.
System Performance Considerations
The method of pressurization control selected has a direct bearing on managing these other parameters. For example, with airflow tracking one can achieve a precisely desired temperature in the isolation room. However, if differential pressure is being used, temperature control may be lost because the volume of air is being dictated by pressure requirements as opposed to temperature requirements. In essence, if pressurization control is the driving variable for the quantity of airflow into the isolation room, all other control parameters may suffer as a result, with temperature usually being the first.
For example, if you wanted to maintain a space under a negative pressure and were using differential pressure control, the supply airflow would vary to maintain pressurization. If a door were opened to that space, the first response from the system would be to reduce the amount of supply air into the room so as to maintain a negative pressure. The system response would likely include closing or reducing the supply airflow to zero in order to maintain the space at a negative pressure. When this occurs, temperature control is lost immediately.
If airflow tracking is the control method, the system will not respond to a door being opened. While pressurization is not maintained, the airflow differential remains the same. Consequently, you will still maintain direction of airflow for pressurization purposes while maintaining temperature and ventilation control, or overall environmental control. This is basically the only way to accomplish this level of control if those parameters are critical.
Temperature And Humidity Management
Tolerance levels for temperature and humidity management should also be discussed. In most hospital isolation rooms one can expect to control temperatures to tolerances of 1 to 2 degrees Fahrenheit. Humidity is normally controlled at the air handling units and usually has a wider tolerance parameter, depending upon environmental factors within the facility. Typically, a hospital would ideally maintain humidity at 50 percent relative humidity (rh) but that can easily range between 40 to 60 percent rh, which is usually an acceptable deviation.
Monitoring Parameters Is Critical
Control and monitoring of isolation room conditions should be simple and direct. For example, pressurization, air change rate and temperature must be displayed at each isolation room and possibly at the nurses’ station and also remotely at a facility manager’s office.
In essence, monitoring and controlling the critical environment inside an isolation room presents two key concerns: The safety of the patient and healthcare worker, and accountability. To verify that everyone in the room is adequately protected, it is critical that accurate records be maintained including daily hard copy printouts that record any alarm conditions as well as their duration and parameter values. Archival records that show hourly pressurization and air change rates are also important in case of litigation. By networking isolation rooms and coordinating performance data into a central computer, an archive record of all critical performance parameters can be maintained.
There are many products and systems designed to accomplish all of these objectives. The key to successful implementation is selecting a supplier that has a track record with the type of facility you are constructing or retrofitting; and, of course, utilizing equipment and instrumentation that has been designed for this application.
Stephen J. Davis is president of Laboratory Control Systems Inc. in Scranton, PA.; Davis has been responsible for hundreds of projects involving critical environmental control for the past two decades. He can be reached at (570) 487-2490; email: firstname.lastname@example.org.
Facility Care • July/August 2002