Non-invasive hypothermia-inducing device mimics water immersion
—by Marie Pompili
The induction of mild hypothermia (lowering a patient’s temperature from 37°C to between 32° and 35°C) after cardiac arrest was proposed in the 1950s in an effort to protect the brain against global ischemia, which Webster’s defines as: “the localized tissue anemia due to obstruction of the inflow of arterial blood.” However, the idea wallowed in obscurity until recently because of the many uncertainties involved in deliberately inducing hypothermia in a resuscitated patient. Since then, various medical journals —
The New England Journal of Medicine (NEJM) and Resuscitation, among them — have included numerous studies on animals and humans demonstrating the usefulness of this technique. The February 21, 2002 issue of NEJM features two studies that suggest therapeutic hypothermia is beneficial to the neurological outcome of the patient when he has been resuscitated after cardiac arrest due to ventricular fibrillation. These studies and their favorable results have led to an endorsement of mild hypothermia therapy by the American Heart Association as well as to the development of a sophisticated system that is designed to perform the entire task simply and accurately.
The Arctic Sun 2000 non-invasive patient cooling device, by Medivance, Inc, Louisville, CO, precisely controls, monitors and maintains core body temperature to induce mild hypothermia. The system consists of the Arctic Sun Control Module and the patented Arctic Sun Energy Transfer Pads. Using standard temperature probes, the clinician can control a patient’s temperature within a range of 33° to 37°C. The system takes the patient’s current temperature and compares it to the preset target temperature programmed before the procedure begins. In turn, the system adjusts the temperature of circulating water through the energy transfer pads using a proprietary thermoregulatory algorithm. According to Gary Carson, vice president of engineering at Medivance, cooling takes place within 60 to 90 minutes as opposed to conventional techniques that may take six to eight hours.
Energy transfer pads
The key to the success of the Arctic Sun 2000 lies in the energy transfer pads, which, according to Medivance, are more capable of rapidly lowering a patient’s body temperature than the cooling blankets most often used in hospitals. The cooling blankets offer incidental contact with the patient’s body — that is, they rarely stick close enough to the body to achieve “maximum transfer”. The energy transfer pads, however, are designed to remain attached closely to the body until removed. “[We are] mimicking water immersion using an adhesive to achieve direct conduction into the skin,” says Carson.
Consisting of three layers, the energy transfer pads cover 40% of the patient’s body surface area. Inlet and outlet manifolds are heat-welded onto each pad. The inner hydrogel layer is made of a proprietary hypoallergenic formulation. It is radiation cross-linked, with no additives or catalysts that can cause biocompatibility issues. In addition, unlike conventional adhesives, hydrogel adhesives do not increase in adhesion over time. The pads can be left in place for up to 72 hours, removed and then reapplied if necessary without causing the patient pain.
The middle thin film layer is a polyolefin film that is two mils thick. It is a coextrusion of five layers that allow the pad to be flexible and to be stretched without tearing. When developing this section, the engineering team looked to thermally bond the middle layer to the foam layer with an extremely thin material for high heat transfer. “It took us quite awhile to identify a material with the perfect properties,” says Carson. They evaluated polyesters, polyurethanes and nylons before settling on the polyolefin for its strength and thermal properties. It can withstand the full range of designated water temperatures from 4° to 42°C.
The foam layer is made from compression-molded EVA foam. A perfectly flat heat transfer surface is required so that the pads conform wholly to the patient’s body. There are approximately 15,000 molded dimples on the surface of each pad. These dimples evenly support the film — any corrugations in the heat transfer surface would lead the pad to lift away from the skin, leading to a loss in heat transfer. This foam layer also acts as an insulator, preventing parasitic heat transfer into the environment.
Circulating water flow
Water is circulated through the pads, simulating immersion of the body into cool water. This is done using negative pressure to prevent leaks. Typically, the pump is placed on the inlet side of a product (pads in this case) when water flow is desired. High pressure is used to push the water through the pad with water pressure higher than the air pressure around it. Unfortunately, when a break in the circuit occurs, water leaks out. When negative pressure is used, a positive displacement pump is placed at the outlet side of the pad and the water is pulled through the device. The system is controlled to operate at -7 psi within the device, assuring the water pressure is lower than the air pressure. Any break in the circuit will cause air to be drawn into the circuit rather than allowing the water to leak out. The air is then removed in the reservoir. The pads can tolerate a reasonably high air leak without compromising heat transfer. If there is a tear or opening that is too big, the defective pad can be disconnected and replaced without disturbing the other pads.
The Arctic Sun 2000 system stores about five liters of water. A five-piston positive displacement pump from Aquatech, Irvine, CA, is used to move the water through the system and the pads. It is driven by a custom-designed brushless DC motor from the APW Motion Technologies, Aspen Group, Radford, VA, for small size and long life. Running at 1800 rpm maximum speed, the Aquatech pump is controlled to maintain -7 psi with a resulting flow rate of approximately four liters/minute with a full set of pads.
The system’s three valves, supplied by KIP, Inc, Farmington, CT, are screwed into a custom-machined manifold. Their metallic components are isolated from the circulating fluid by diaphragms. An internal bypass valve, for prewarming or cooling before the pads are applied, has a flow rate of 1.5 liters/min. The fill valve has a flow rate of 1.0 liters/min, while the purge valve, which empties the pads at the end of the procedure, flows at up to 4 liters/min.
The Arctic Sun 2000 has dual microprocessors for control and monitoring. At all times, the primary control is monitored by a secondary channel for safekeeping. Each unit monitors independent water and patient temperatures as well as other parameters determined by the clinician. Additionally, there are dual patient temperature sensor inputs. If a fault were to occur, each processor has full control to stop therapy delivery. The system monitors the patient temperature, ensuring that it is maintained between 33° and 37°C. If the temperature exceeds 38°C or falls below 32°C, and the system senses that the water temperature is not moving the correct the situation, the microprocessors will stop the therapy. Again, the water temperature is maintained between 4° and 42°C.
The automatic patient temperature control algorithm is based on data from hundreds of patients. Three years in the making, it takes real time patient temperature data, target temperature and the patient’s sedation level set by the clinician, and controls the water temperature to achieve maximum cooling rate with minimal overshoot for each patient, says Carson. And, although rapid cooling is essential in many applications such as stroke and head trauma, gradual, controlled rewarming is important for positive patient recovery. “Our controller can be programmed to control the rewarming phase to as slow as a 2°C rise over a 48-hour period,” Carson says. Carson also points out that the controller “repeats” the resistance of the patient sensor so that another instrument plugged into the controller can “sense” the temperature the system senses. This enables a single temperature probe in a patient to be used by more than one instrument.
The controller is placed in a housing that features a top that is a rotationally molded polyethylene tank from Bonar Plastics, Lindsay, ON. The top includes three separate integrally molded water reservoirs. In addition, the controller features a pressure-formed plastic grill from Plastics Design and Manufacturing, Englewood, CO.
Sensing devices & heating element
A pressure sensor, supplied by Honeywell’s Microswitch division, Morristown, NJ, is used to measure the level of negative pressure applied to the pads. It maintains a constant pressure regardless of the number of pads connected. The Arctic Sun 2000 also contains temperature sensors, or thermistors, that are placed throughout the system to accurately monitor water temperatures within ±0.2°C.
A turbine flowmeter sensor by GEMS Sensors, Inc, Plainville, CT, monitors the flow rate. The system circulates only 1 liter or quart of water at one time, allowing a rapid change in water temperature in response to a change in the patient’s temperature if necessary. The 750W, cartridge-type heater from Heatron Inc, Leavenworth, KS, is a high-dielectric breakdown version of a standard product offered by the company.
The Arctic Sun 2000 system was introduced at the Transcatheter Cardiovascular Therapeutics 2003 Conference in September. “Early initiation of cooling therapy is thought to be a critical parameter towards achieving any potential benefit [with this treatment],” says Robert A. Kline, president and CEO of Medivance. “The simple, non-invasive design of the Arctic Sun 2000 enables cooling early in the treatment cycle, making mild hypothermia a possible first-option treatment for these patients upon entry to the emergency room or in the future during transport from the field,” says Kline. He points out that his system may potentially save the health care system “hundreds of millions of dollars” each year. Let’s hope so — medical costs have sure skyrocketed since the therapy’s conception 50 years ago.
APW Motion Technologies, Aspen Group,
Plastics Design & Mfg,
Honeywell, Microswitch div,
GEM Sensor Inc,