The present invention relates generally to medical devices useful for externally warming fluid or blood products prior to infusion into a patient""s body cavity or vessel. More particularly, the devices minimize air embolization by removing gaseous material generated during the warming process.
Patients undergoing blood or fluid processing, e.g., hemofiltration, hemodialysis, hemodiafiltration, ultrafiltration, peritoneal dialysis, or infusion of saline, drugs, or nutritional fluid, are at risk for hypothermia in the absence of warming the fluids infused into the patient""s blood stream or peritoneum. Hypothermia, defined as body temperature significantly below normal, typically at 98.6xc2x0 F. (37xc2x0 C.), is of considerable importance because it can represent a medical emergency requiring aggressive treatment. Causes of hypothermia are usually classified into three categories: (1) accidental hypothermia, (2) hypothermia due to acute illness, and (3) immersion hypothermia.
Accidental hypothermia usually occurs in elderly or inebriated individuals after prolonged exposure to low external temperature, e.g., during winter months. Patients with body temperature below 85 and 90xc2x0 F., usually appear pale and cold with stiff musculature. Patients having body temperature below 80xc2x0 F. are usually unconscious with shallow and slow respiration, bradycardia, and hypotension. Patients having body temperature below 77xc2x0 F. are usually comatose and areflexic. Hemoconcentration, azotemia, metabolic acidosis, and cardiac arrythmias can occur in these patients. Moderate hypothermia in association with acute illnesses including congestive heart failure, uremia, diabetes mellitus, drug overdose, acute respiratory failure, and hypoglycemia, is usually found in elderly and hospitalized patients. These patients usually have metabolic acidosis and cardiac arrythmia. Most of them are comatose. In immersion hypothermia, there is great variability in each individual""s ability to tolerate heat loss in cold water. A lean person generally is less able to tolerate a fall in temperature than an obese swimmer. In hypersensitive individuals, immersion in cold water may cause vascular spasm, vomiting, and syncope.
In addition to maintaining adequate airway and cardiovascular support in hypothermia patients, the main treatment for each type of hypothermia constitutes rewarming the body. In treating patients with mild hypothermia, external rewarming using a warm blanket or placing the patient in a warm room is usually sufficient. However, patients with moderate to severe hypothermia require reestablishment of body core temperature. This is usually achieved by placing the patient in a warm bath or a Hubbard tank at 104 to 108xc2x0 F. Unfortunately, external warming tends to dilate constricted peripheral blood vessels, thereby shunting blood away from the internal organs. In patients with severe hypothermia, external warming may lead to rewarming shock and may not be sufficient to warm the myocardium to allow antiarrythmic agents to take effect. In this situation, hemodialysis, peritoneal dialysis, or ultrafiltration, where blood or dialysate is warmed externally, can be used. Warmed intravenous fluid, such as glucose and saline, low molecular-weight dextran, or albumin, can be used to maintain blood volume and facilitate warming of core temperature. Unfortunately, current external devices for warming intravenous fluid suffer a significant drawback in that air or gaseous material that arises as a result of warming can reach the patient""s blood stream and cause air embolization, resulting in organ ischemia or infarction.
Therefore, devices and methods useful for treating and/or preventing hypothermia, or to maintain patient comfort, are needed that allow warming of fluid which can be infused intravenously or be used in conjunction with hemodialysis, peritoneal dialysis, or ultrafiltration, during which risk of air embolization is eliminated or minimized.
The present invention provides devices and methods for pre-infusion external warming of fluids, such as saline, lactated Ringer""s solution, dialysate, or blood, for infusion therapy, including hemofiltration, hemodialysis, ultrafiltration, hemodiafiltration, or peritoneal dialysis. It will be understood that the devices and methods disclosed herein can also be used in treating patients with hypothermia or in critically ill patients to prevent hypothermia. The devices remove air or gaseous material generated during the warming process, thereby minimizing risk of air embolization that causes organ ischemia or infarction, e.g., in the lungs.
In a first embodiment, the device includes an air separation chamber that is oriented above a fluid warming chamber. The fluid warming chamber has an inlet that communicates with a fluid pathway adapted for transfer of heat from a heating mechanism to the fluid. In certain embodiments, the fluid inlet is at a bottom of the fluid warming chamber. The air separation chamber, in communication with the fluid pathway, includes a fluid outlet and a gas outlet. The air separation chamber typically lies above the heating mechanism so that air or gaseous material generated during the warming process rises to the top of the air separation chamber. In certain embodiments, the air separation chamber also includes a partition that separates the fluid outlet from the gas outlet to minimize escape of gas into the infusion line. In other embodiments it does not.
In another embodiment, the device includes a heating mechanism comprising first and second heat plates. The first heat plate is disposed adjacent to the fluid pathway. The second heat plate is disposed adjacent to the fluid pathway and opposite the first heat plate so that the fluid pathway is sandwiched between the first and second heat plates. Fluid in the pathway is warmed by heat generated from the heat plates. In other embodiments a support wall is disposed adjacent to the fluid pathway and opposite the first heat plate so that the fluid pathway is sandwiched between the first heat plate and the support wall.
In other embodiments, the fluid flows through the fluid pathway of the fluid warming chamber at a substantially higher rate than the fluid flows through the air separation chamber. This difference in flow rate can be accomplished simply by providing a fluid pathway with a smaller diameter channel than the channel of the air separation chamber. The fluid will therefore slow substantially upon exiting the fluid pathway of the fluid warming chamber and entering the air separation chamber. A diminished rate of flow through the air separation chamber facilitates de-gassing of the fluid, and ensures that small bubbles are not swept into the fluid outlet.
It will be understood that the devices and methods of the invention provide a safety mechanism to guard against overheating when fluid flow stops momentarily. When fluid in the fluid pathway stops, it will heat above the set point. The air separation chamber, however, is typically independent of the heating source, and therefore extra fluid in the air separation chamber remains at a lower temperature. When flow resumes, hot fluid from the fluid pathway mixes with low temperature fluid in the air separation chamber. The low temperature fluid in the air separation chamber thereby buffers the temperature of fluid exiting the warming system. This feature serves to reduce the extent of overheating.
In use, a fluid source is connected to the fluid inlet of the warming bag, and an output device, such as a hemofiltration, hemodialysis, hemodiafiltration, ultrafiltration, or peritoneal dialysis machine, is connected to the fluid outlet. Fluid flows from the source through the fluid inlet and through the fluid pathway of the fluid warming chamber. One or more heat plates surrounding the fluid pathway heat the fluid as it flows through the fluid pathway. The temperature of the heat plates or other heat source and the flow rate are adjusted by an operator comfort setting to obtain an appropriate temperature. Fluid passes through the fluid pathway into the air separation chamber where, in certain embodiments, the flow rate slows. Gas bubbles rise in the air separation chamber and collect at the top of the air separation chamber. Gas is periodically vented through a gas outlet. The fluid, while de-gassing, flows through the lower part of the air separation chamber and through the fluid outlet to an output device.
It will further be understood that there are several advantages to using the fluid warming devices and methods described herein. For example, the devices and methods (1) improve patient comfort during infusion therapies by providing external warming of fluid and blood products prior to their administration to a patient, (2) protect against hypothermia in patients receiving a large volume of intravenous fluid, e.g., patients undergoing hemofiltration, hemodialysis, hemodiafiltration, or ultrafiltration, (3) provide a treatment alternative for patients with hypothermia, and (4) avoids the need for a separate drip chamber.