1. Field of the Invention
This invention relates to intravenous infusion pumps, and in particular, to a high accuracy infusion pump for the delivery of drugs to patients and in which the liquid delivered is pumped through a disposable cassette.
2. Description of the Prior Art
In the operating rooms of hospitals, most of the interventions and maneuvers which anesthesiologists perform involve potent and dangerous drugs. From an anesthesiologist's point of view, the goal in any procedure carried out in an operating room is to take an awake patient and using specialized drugs and techniques make him unconscious and unresponsive so that surgery can safely performed. In planning anesthetic treatment, the anesthesiologist considers the drugs and techniques used, how these drugs alter the patient's physiology, the nature of the surgery, and concurrent medical illnesses unrelated to the surgery. For example, consider a patient who has had an intracerebral hemorrhage, and is scheduled for surgery to control an aneurism of one of the major blood vessels in the head. Concurrent medical conditions might include hypertension, hypovolemia (low blood volume), and increased intra-cranial pressure. Dangers related to the surgery include a risk of uncontrollable hemorrhage, very long surgery, the potential of air entering the blood stream and occluding blood flow to the lungs, heart, brain, or other organs, as well as cerebral edema or excess brain swelling.
The anesthesiologist administering the anesthetic for such a patient typically will use up to eight different drugs to safely induce the desired state of unconsciousness in the patient. At the same time, other drugs, often as many as ten, are used to keep the patient asleep and immobile for the duration of the surgery. Some of these drugs will be applied continuously. Using specialized monitors, the anesthesiologist will watch the effects of the interventions and titrate the doses of these drugs. As the anesthesiologist adjusts the medications that keep the patient asleep, he or she must also watch for changes caused by the surgery. During the hypothetical surgery described, the anesthesiologist will be called on to drastically reduce the blood pressure of the patient for up to 30 minutes to enable the surgeon to clip the fragile aneurism. The resulting hypotension can be difficult to control, and the drugs used to cause it must be adjusted as frequently as every two or three minutes. At the end of the procedure, the anesthesiologist must then awaken the patient, restore his breathing, continue to carefully control blood pressure, reverse the effects of the anesthetics and transport the patient to a suitable care unit.
There are now several different techniques for anesthesiologists to apply drugs to patients. Probably the most common method is to give the drugs intravenously as a bolus, in other words, doses of several milliliters at a time. Continuous infusions, in which very small quantities of drugs are delivered continuously to a patient over an extended period of time, are another common method of administering drugs. Many of the newer drugs and techniques, require drugs to be given as continuous infusions which are precisely metered. In the example just discussed, one of the drugs used for manipulation of blood pressure might be nitroprusside. The infusion rate for this potent drug must be carefully adjusted every few minutes based upon the patient's response.
There are presently two types of continuous infusion systems. The first is a gravity-dependent bag-and-tubing setup with flow controlled by a variable orifice. The alternative is an electrically-powered infusion pump having an external drug reservoir. Unfortunately, all of the systems developed to date have at least one of many disadvantages. For example, size is a major constraint upon equipment designed for an operating room. Because the anesthesiologist must attend to both the equipment and the patient, the equipment must be within arm's reach. Unfortunately, operating rooms are already overcrowded with carts, monitors, wires, tubing, blood warmers, humidifiers, and the like. There may also be significant special constraints caused by surgical equipment employed as well as by the layout of the room. Most infusion pumps now commercially available are much too large to be used efficiently in an operating room.
Present systems also involve a tangle of tubes, wires, hoses, and the like, which frequently encircle each other forming dangerous knots. If the intravenous lines from potent drug infusions become entangled, the wrong amount or the wrong drug might be delivered to the patient. The tangle also increases the chance of a disconnection, and becomes particularly dangerous when the patient is moved from the operating room table to the patient's bed, as well as when the patient or the table are moved during the course of an operation. Most existing infusion pumps require a power cord and two intravenous "IV" lines--one from the IV bag drug reservoir to the pump and one from the pump to the patient.
Another disadvantage of conventional systems is the time required to set them up before use. The setup procedure includes opening the drug vial, opening a syringe, drawing the drug into the syringe, opening a bag for diluting the drug, injecting the drug into the bag, inserting an IV tube set into the bag, labeling the bag, and then purging air from the tubing. The infusion pump must also be plugged in and the IV tubing connected to the pump mechanism to supply the drug.
Prior art continuous infusion pumps are also expensive. Because of the need for sterility, multiple disposable parts are required for each infusion. For example, a new drug vial, syringe and needle, IV bag, tubing set, and appropriate pump parts are required. Each part has an expense which is incurred during setup, even if the pump is not needed during the procedure.
Another disadvantage of prior art infusion pumps is that the infusion rate is susceptible to change due to temperature, table height, back pressure, partial occlusion, bag height, and the relative height and rates of other infusions going into the same IV line. (In most cases, several infusions will be "piggy-backed" into one IV line.) All of these systems require that the anesthesiologist verify the infusion rate frequently and make adjustments. While infusion pumps are less sensitive to these effects than other systems, corrections may still be required.
A further disadvantage of the prior art systems is their potential for error. Because all drugs look the same once they leave the vial, errors include mislabeling of the IV bag and calculation errors. The calculation errors can occur because drugs are delivered in various concentrations and usually adjusted by patient weight. For example, nitroprusside, discussed above, is supplied in vials. After dilution it is often delivered at a rate of 1 microgram per kilogram of body weight per minute. The anesthesiologist must determine the correct conversion, and this calculation must usually be made under stressful circumstances.