Pumps – Processes
Reexamination Certificate
2000-06-16
2002-06-18
Freay, Charles G. (Department: 3746)
Pumps
Processes
C047S050000
Reexamination Certificate
active
06406267
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to pumps used in the circulation of blood from the body for treatment and return back into the body, a process called extracorporeal perfusion or circulation, and more particularly to peristaltic pumps of the flexible wall type deformed by a rolling or sliding engagement member.
BACKGROUND OF THE INVENTION
Extracorporeal Circulation
Cardiopulmonary Bypass and Cardiac Assist
Extracorporeal perfusion is used for the most part in cardiac bypass surgery. In a total bypass, all the patient's systemic venous return blood is diverted from the right side of the heart into an extracorporeal circuit, emptying the chambers of the heart. The circuit includes a heart-lung machine that comprises a pumping function and an oxygenation function, completely taking over cardiopulmonary function for the patient, returning oxygenated blood to the aorta. In a partial bypass only a portion of the blood is diverted to the extracorporeal circuit, the remaining flow passing to the lungs and from the lungs through the coronary and systemic arterial circulation. Partial bypass usually is temporarily used following total bypass surgery to slowly give the heart work to do, measurely decreasing flow through the heart-lung machine, until the heart is weaned from assist and can fully take over its pumping role.
Some procedures using blood pumps in extracorporeal circulation do not include an oxygenation function. These include cardiac assist procedures. In these, the blood pump provides higher systemic blood pressure and more blood flow than can be provided by a failing heart. A “fem-fem” (femoral vein to femoral artery) circuit is commonly used. Cardiac assist is also sometimes used if, after open heart surgery, the left side of the heart responsible for pumping to the body oxygenated blood returned from the lungs does not resume its pumping role despite attempts at weaning. If other assist circulatory devices are unsuccessful, the left heart may be bypassed to the aorta by cannulation of the left atrium, with the blood that has been oxygenated by the lungs being withdrawn through the cannula and pumped to the aorta extracorporeally without extracorporeal oxygenation.
Pulmonary Bypass
Another use of extracorporeal circulation is “extracorporeal life support” also called “extracorporeal membrane oxygenation” known by their respective acronyms of“ECLS” or “ECMO”, for simplicity herein called only ECMO. As opposed to the more conventional extracorporeal circulation in substitution or assist of the cardiac function, ECMO connotes the application of such support to supply oxygenation where the native lungs are compromised. This is especially useful for neonates, including premature birth babies, whose life is threatened because their immature lungs cannot provide adequate gas exchange. Another use is resuscitated drowning victims whose lungs are damaged and unable to supply adequate oxygenation without restorative healing. The extracorporeal circulation provides oxygenated blood to the patient's lungs under the impetus of the patient's native heart and gives time to allow healing of the lungs to occur until the lungs can take over oxygenation. In excess of 1,000 ECMO procedures are conducted annually in the United States.
Extracorporeal Circuit Components and Priming Volume
The basic components of the extracorporeal circuit for a conventional heart-lung machine are one or more venous cannulas, a venous reservoir, an oxygenator and heat exchanger, a pump, an arterial line filter, an arterial cannula, and a control module. The ECMO system includes a blood pump, a membrane oxygenator, a countercurrent heat exchanger to warm the blood, and a control module. In the typical extracorporeal circuit, deoxygenated blood drains by gravity into the circuit and flows into the venous reservoir, usually placed 25 to 30 inches below the plane of the great veins. If the oxygenator is a bubble type, the reservoir is incorporated into a oxygen-blood mixing chamber. In any case, the reservoir is placed upstream to the pump, for reasons amplified below, to prevent negative pressure in the inlet line. A water heat exchanger is used for the perfusate to control body temperature. Blood filters are used to trap particulate and gaseous emboli. The arterial cannula is usually placed in the ascending aorta but can be placed downstream in the arterial system where the vessel is large enough to accommodate the necessary flow.
Extracorporeal Circuit Pumps
The blood pump is the “heart” of the extracorporeal perfusion circuit. In general, extracorporeal circulation systems use either an occlusive compression peristaltic roller pump or a non-compressive centrifugal pump. Both produce flow rates on the order of several liters per minute, thus apply well to adult usage requirements.
Roller Pump
The basic roller pump consists of two rollers, 180 degrees apart, that rotate in a circle through a half circular raceway. A length of flexible tubing between ¼ and ⅝ inch inner diameter is placed between the rollers and the raceway. The rollers rotating in a circular movement compress the tubing against the raceway, squeezing the blood ahead of the rollers. The rollers are set to almost completely occlude the tubing, and operate essentially as a positive displacement pump, each passage of a roller through the raceway pumping the entire volume of the fluid contained in the tubing segment between the rollers. As a positive displacement pump, high positive pressures can be generated at the pump outlet and high suction (negative) pressures can be generated at the pump inlet. Roller pumps are typically driven by a constant speed motor which draws blood at a substantially constant rate. As a constant speed positive displacement pump, if a line downstream of the pump becomes occluded, the pump can over pressurize and rupture the downstream vessel, producing a “blowout”, with perfusate loss from the circuit and cessation of blood flow to the patient. If a line upstream of the pump becomes occluded, the pump can generate dangerously low negative pressures that can hemolyze the blood, and can empty the tissue vessel of the patient causing a collapse of the vessel resulting in damage to the tissue at the drainage catheter tip. Cavitation, or drawing of dissolved gasses from the blood, can occur when strong negative pressures develop.
Inlet suction risk is reduced through the use of a venous reservoir (which may be part of an oxygenator). The venous reservoir gives capacitance to the suction line and prevents imbalance between suction and discharge volumes, although at a cost of additional priming volume. However, when venous return to the extracorporeal apparatus is by gravity into an open reservoir, a decrease in venous return to the oxygenator without a corresponding output reduction can lead to accidental emptying of the oxygenator and infusion of air. The rate of blood inflow to the oxygenator is basically controlled by the position and size of the venous cannula and by the height difference between the venous cannulation site and the oxygenator. The blood output of the pump is directly controlled by the rate of blood pumping.
No matter what method is used to accomplish venous return, when venous volume depletion starts, fluctuations in blood flow rates also occur. When the great veins are emptied of their contents and venous collapse occurs, further increases in suction do not accomplish more blood flow but only further aggrevate venous collapse.
Another risk with the roller pump is that the work applied on the tubing creates a friction that causes abrasion and wear of the flexible tubing. This wear can cause accidental rupture. The abrasion can cause “spallation” or interior shedding of small particles that if returned to the native circulatory system can cause emboli formation.
Centrifugal Pump
Numerous surgical services limit the complications derived from roller pumps by using non-compressive pump centrifugal pumps. Centrifugal pumps rapidly rotate an impeller in a stationary blood co
Burgess Tim L.
Freay Charles G.
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