Surgery: light – thermal – and electrical application – Light – thermal – and electrical application
Reexamination Certificate
1996-09-04
2001-09-25
Kamm, William E. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Reexamination Certificate
active
06293962
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods for maintaining environmental conditions for an electrotherapy medical device in order to allow immediate use in an emergency situation and to improve the shelf life and reliability of the device. More specifically, this invention relates to maintaining the storage temperature for an automatic or semi-automatic external defibrillator (AED) within a specified temperature range.
2. Description of the Prior Art
Over the last several years medical devices have become more portable and have begun moving into the hands of first responders. As a result, equipment that was once available only in a hospital, having a temperature controlled environment, is now being transported and stored in environments that are not predictable or controlled. AEDs are frequently stored in emergency vehicles and are transported by firefighters, police officers and ambulances directly to the scene of a sudden cardiac arrest. Although the size and portability of AEDs have made them more readily transportable and increased their availability in the field, they are still manufactured from many temperature sensitive components.
For example, an important component of an AED is the power source. Typically, portable AEDs rely on a battery pack to provide power to the unit. Fluctuations in storage temperature of the device can compromise the performance, reliability and life of the battery. Both alkaline and lithium batteries are affected by variations in storage temperature. Although lithium batteries typically have a longer shelf life than alkaline batteries, they are more susceptible to fluctuations in storage temperature. As a result a device that operates using a lithium or alkaline battery may have reduced reliability if the storage temperature is not maintained within a desired range. Additionally, as the temperature of the battery decreases the capacity of the battery decreases. Low battery capacity lowers the number of shocks that a defibrillator can deliver with that battery.
Another temperature sensitive component of an AED are the hydrogel coated electrode pads. These pads are applied to the skin of the person thought to be in need of defibrillation. The pads allow the AED to receive electrocardiogram (ECG) information from the patient so that the AED can analyze the ECG and determine whether a shock is advisable. Once the AED determines that a shock is advisable, the hydrogel pads conduct the shock through the patient's skin and to the heart. In view of the high amount of water contained in the hydrogel layer of the pads it is important that the water content of the hydrogel is not changed as a result of the storage temperature. For example, when storage temperatures are above 55° C., the hydrogel layer begins to dry out, changing the conductive and cohesive characteristics of the gel. Alternatively, as the temperature is lowered below 10° C., the hydrogel could begin to freeze, again changing the conductive and cohesive characteristics of the gel.
Another temperature sensitive component of an AED is the liquid crystal display (LCD). When an LCD becomes too cold, the crystals begin to freeze and the response of the display is slowed considerably. For example, when the temperature reaches −20° C. to −10° C. (−4° F. to 14° F.) it can take several seconds for the screen to register the display command. Since every second in an emergency response situation is critical, a delay of even a few seconds in the screen response per prompt could cumulatively be very significant; particularly since the survival rate of patients receiving a shock from an AED substantially improves the sooner after the incident the shock is applied. When an LCD becomes too hot, the LCD is unable to selectively polarize the crystals and the display will appear dark because voltage across the LCD is applied to all crystals. Loss of the LCD could be critical especially for an AED that displays an ECG or where the AED is being used by a hearing impaired person who is relying on the LCD for prompting.
Digital electronic circuits and components are also temperature sensitive. It is well known that transistor offset and bias currents are affected by both high and low temperatures. For example, a circuit with a range of 2-20 pA could increase to 0.5 nA if the temperature exceeds the T
max
for that circuit. The effects of temperature on the offset and bias current for a given circuit are typically published in the specifications for that circuit.
The prior art has recognized the temperature sensitivity of certain electronic devices. For example, video equipment is sensitive to storage temperature. Video tapes may become damaged from exposure to either high or low temperatures. Additionally, the video camera itself may become damaged if used after storage at a low temperature. Cruisers, Inc. (Brighton, Mich.) manufactures a carrier for video equipment for use in police vehicles. This carrier is capable of warming or cooling the temperature around the video equipment, as necessary.
As discussed above, batteries are affected by storage temperature. As a result of a number of methods and devices have been developed for controlling the storage temperature of the battery. Several patents have issued directed to thermal management systems for lead acid batteries used in cars. For example, U.S. Pat. No. 5,449,571 (Longardner et al.) teaches using a phase change material (PCM) filled housing to control the temperature around the battery. The PCMs can absorb heat generated by batteries during operation and can assist in maintaining a higher battery temperature in cold conditions.
Another example of a temperature insulator for a battery is shown in U.S. Pat. No. 5,516,600 (Cherng). The Cherng patent teaches the use of barrier film with expandable chambers to provide insulation. Chambers are filled with a fluid and may be fabricated as a blanket or bellows.
An alternative system for cooling and heating batteries is shown in U.S. Pat. No. 5,215,834 (Reher et al.) and U.S. Pat. No. 4,840,855 (Foti et al.). Reher et al. heats or cools a battery depending upon the storage temperature and the state of the charge by using a reversible fan. Conduits access either heated or unheated air which is then pumped over the battery, depending upon the temperature needs. Foti et al. provide a housing having inner and outer layers fabricated from waterproof material with an insulative layer therebetween. An electric heater coil provides additional heat, as necessary.
U.S. Pat. No. 5,229,702 (Boehling et al.) is directed to a device for protecting rechargeable batteries in a lighting unit. Boehling et al. uses a thermoelectric pump to cool the battery enclosure and heat sinks to warm the enclosure. Boehling et al. also teach a device for maintaining the life of a vehicle battery by keeping the temperature within optimal ranges.
What is needed is a way to maintain the storage environment for electrotherapy devices so that the effects of heat and cold do not prevent the equipment from being ready for immediate use in an emergency or compromise the performance of the components when the device is in use. No apparatus or methods currently known meets the specialized storage needs of electrotherapy devices such as AEDs.
None of the devices or methods taught in the prior art address the need to maintain the environmental conditions for a medical device or take into consideration the sensitivity of various components of a device.
SUMMARY OF THE INVENTION
In a preferred embodiment, this invention relates to a method for maintaining an electrotherapy device wherein the air temperature of the electrotherapy device environment is monitored and then adjusted based on the results of the monitoring step. The method may be accomplished by using an environmental carrying case. Additionally, adjusting the air temperature may be accomplished by circulating the air in the electrotherapy device environment. Monitoring the temperature may be accomplished by using a thermostat one
Agilent Technologie,s Inc.
Kamm William E.
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