RTC Magazine

Over the past decade, there has been a growing trend in the health care field to send hospital patients home as quickly as possible. Because of this, there is often a need for these patients to “bring the hospital home” with them. This has precipitated a rapid rise in the need for portable and highly reliable medical treatment, monitoring and data logging equipment.

Because this medical equipment is being installed and operated in patients’ homes, key requirements include compact size, very low power consumption and extreme dependability. Critical to meeting these criteria is highly reliable, non-volatile semiconductor memory to ensure that neither physician-prescribed equipment settings, nor stored patient information, such as vital signs, are accidentally lost.

From Hospital to Home

One piece of medical equipment that frequently moves with the patient from the hospital to the home is the portable infusion pump. Infusion pumps are used to introduce fluids into the patient’s circulatory system for hydration, pain management, chemotherapy and total parenteral nutrition (TPN), a treatment where all of the body’s nutrients are provided intravenously, bypassing the normal process of eating and digestion.

In addition to the mechanical components such as the pump and its DC motor, a portable fusion pump is a complex embedded system with a CPU at its core (Figure 1). The user, who is often the patient, controls operation of the system via a keypad mounted on the front of the device. Alternately, the infusion pump can be remotely programmed and operated by health-care providers using a telephone line and built-in modem. The system has a digital LCD display that shows the unit’s status, including programmed medication quantities and infusion times. In addition to the requisite beeps and buzzes, some modern systems actually include pre-recorded spoken messages that remind the patient of important tasks to perform. Operation of the mechanical components, including the pump and valves, are performed by a digital IC controlled by the CPU. Likewise, another IC interfaces to sensors and provides conditioning of signals throughout the system.

Last but not least is the system’s memory and real-time clock. In addition to routine scratchpad RAM, a portion of the memory space is dedicated to non-volatile memory. This non-volatile memory is critical to the operation of the infusion pump since it retains the calibration parameters, dynamic algorithm coefficients, stepper motor control data tables, intravenous therapy schedule, system state information and the data log, which is a detailed history of the patient’s physical parameters, including blood pressure, temperature and respiration rate. The real-time clock provides an accurate time and calendar source to administer the intravenous therapy schedule and time-stamp the data log.

As is common in many systems, but absolutely crucial to infusion pumps, the system must recover from any unexpected loss of power. If power is lost during the infusion of fluid, a battery pack takes over and ensures that normal operation continues. However, in the event that the battery fails, even for a very brief time, the unit not only must be able to resume the infusion once power is restored, but it must also have access to all programming data, including dates and times of medication delivery, as well as to the data log. Needless to say, loss of programming data can result in potentially fatal consequences, since the details of the patient’s medication delivery would be unavailable, resulting in possible under- or overdoses of medications being administered upon system restart. Similarly, losing the data log can result in important patient data being lost—data that may be critical to the physician learning of medical conditions previously not exhibited by the patient. In short, the contents of the infusion pump’s memory are often a matter of life-and-death.

In the event of a power-related disturbance of an infusion pump, the unit must detect the loss of power, then store vital system state information and the data log to non-volatile memory prior to the complete loss of power. Once power is restored to the system, this state information is used as a recovery point, and the system resumes operation at the exact point it was prior to the power loss.