US 7211066 B1
Catheters for treating body fluids, particularly blood, include one or more tubes through which a working fluid is circulated. The fluid characteristics, e.g., temperature or drug content, interact with the body fluid by transfer through the tube walls. For enhancing the efficiency of such transfer, energy is added to the body fluid where it contacts the tube outer surfaces for reducing the thickness of thermal and concentration boundary layers at the tube surfaces. The energy adding is accomplished by causing pulsations in the walls of the tubes, or in the dimensions of a balloon parallel to the tubes, by means of pressure pulsations in the fluids circulated through the tubes and/or the balloon. Systems are disclosed for providing and controlling the circulation of various fluids and drugs through the catheter.
1. A heat transfer device comprising:
a multi-lumen catheter including a working fluid transfer pathway;
a first manifold for connecting the multi-lumen catheter to a return pathway;
a heterogeneous and compliant heat exchange portion connected to said first manifold, the heat exchange portion having a plurality of dynamic surface regions that provide internal and external boundary layer disruption;
a second manifold connected to the heat exchange portion that distributes the working fluid through the heat exchange portion; and
a high frequency working fluid pressure oscillator adapted to create surface dynamics in the heat exchange portion.
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3. The heat transfer device of
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19. A heat transfer device comprising:
a single lumen catheter including a working fluid pathway;
a compliant connective element connecting the single lumen catheter to a first manifold, the first manifold connecting said compliant connective element to a compliant heat exchange surface;
wherein during operation of the heat transfer device, the compliant heat exchange surface moves normal to and tangential to the longitudinal axis of the heat transfer device; and
a second manifold connected to said compliant heat exchange surface adapted to collect working fluid that exits the compliant heat exchange surface; and
a delivery device adapted to deliver working fluid to the inflatable distal manifold and remove working fluid from the inflatable distal manifold, thereby creating heat exchange surface dynamics.
20. The heat transfer device of
21. The heat transfer device of
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25. A device for enabling a heat exchange process comprising:
a multi-lumen catheter for providing a working fluid pathway;
a proximal manifold for connecting said multi-lumen catheter, distributing the working fluid, and driving rotary motion of a compliant heat exchange surface;
a compliant heat exchange surface that spins about the longitudinal axis of the heat transfer device;
a distal manifold connected to said heat exchange surface adapted to collect the working fluid inside the said heat exchange surface; and
a control system adapted to circulate the working fluid and drive the rotary motion of the proximal manifold;
whereby the heat exchange process is augmented by creating a circumferential heat exchange surface motion.
26. The device of
27. The device of
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30. A method of altering organ temperature comprising:
inserting a guide wire and guide catheter into a patient;
inserting a distal protection filter along the guide catheter;
inserting a heat transfer device along a shaft of the distal protection filter;
locating the heat transfer device upstream of organ of interest;
positioning said heat transfer device concentric within an artery leading to the organ of interest;
circulating a gas soluble working fluid through said heat transfer device;
infusing said working fluid into external fluid to deliver rapid cooling in stage one of organ temperature alteration;
monitoring said infusion by applying mass balance about said working fluid and controlling internal working fluid pressure; and
reducing or eliminating infusion of said working fluid to maintain organ temperature in stage two of organ temperature alteration.
This application is entitled to the benefit of Provisional Patent Application Ser. Nr. 60/395,842, filed Jul. 15, 2002.
I. Field of the invention
This invention relates to a medical device used to reduce tissue injury resulting from ischemia, occurring naturally, through trauma or from surgery.
II. Description of Related Art
Tissue in the human body is regulated at a constant temperature of approximately 37 C. An essential part of this regulation is achieved by adequate perfusion of body fluids. Blood perfusion carries out many functions in addition to heat exchange, namely oxygenation of tissue. Without blood perfusion and therefore oxygen delivery, tissue becomes ischemic. For example, normal flow to the brain ranges from 46 to 62 ml/min per 100 grams of brain matter. An accepted critical ischemic threshold is 20 ml/min per 100 grams of brain matter [Reference Cardiovascular Physiology, W. Milnor, pg. 395, 1990]. Unfortunately this threshold is reached during acute ischemic injury, such as stroke, heart attack, or spinal injury. Ischemic thresholds also take place during the course of an initial injury, such as brain swelling after trauma, or reperfusion of occluded coronary/cerebral arteries.
In addition to steps taken to rapidly restore blood perfusion levels above ischemic thresholds, research shows induced hypothermia holds promise for protecting organ tissue from ischemic injury. Among other mechanisms, hypothermia decreases tissue metabolism, concentrations of toxic metabolic byproducts, and suppresses the inflammatory response in the aftermath of ischemic tissue injury. Depending on the time of initiation, hypothermia can be intra-ischemic, post-ischemic, or both. Hypothermic ischemic protection is preventive if tissue metabolism can be closed down. Hypothermia may also enhance recovery by ameliorating secondary tissue injury or decreasing ischemic edema formation. Since the metabolic reduction is less than 10% per degree Celsius, only deep hypothermia, targeting 20–25 degrees Celsius, conceivably provides adequate tissue protection via metabolic slowdown. Secondary tissue injury, thought to be mainly caused by enzymatic activity, is greatly diminished by mild to moderate hypothermia targeting 32–35 degrees Centigrade. As early as 24 hours after onset of ischemia, secondary tissue injury can set off a mass effect with damaging effects on viable surrounding tissues. Late post-ischemic hypothermia decreases edema formation, protecting tissue at risk.
With this in mind, physicians have tried to harness the benefits of hypothermia using a variety of cooling techniques. These techniques vary depending on clinical circumstances. For example, in preventive cooling the goal may be locally applied deep hypothermia, whereas in acute ischemic syndromes, systemic mild to moderate hypothermia may be preferred. The primary focus to this point has been systemic body surface or vascular cooling, only a few concepts have embarked on local, organ specific or cerebrospinal fluid cooling. Systemic cooling has specific limitations and drawbacks related to its inherent unselective nature. For example, research has shown that systemic or whole body cooling may lead to cardiovascular irregularities like reduced cardiac output and ventricular fibrillation, an increased risk of infection, and blood chemistry alterations. Local cooling approaches have been limited by the technological challenges related to developing tiny heat exchangers for small arterial vessels. These vessel inner diameters are 6 mm or smaller.
As a result, the challenge of organ specific cooling is the development of heat exchange catheters that meet the cooling requirements without causing significant physiological disruption. Fortunately, heat exchanger design is not a new science and numerous enhancement techniques have been employed for decades to performance. Two obvious examples include the condenser coil of your home air conditioning system and the radiator of your car. Regardless of the enhancement technique, each approach attempts to achieve one or a combination of the following objectives: 1) reduce the size of the heat exchange device, 2) increase the UA (U, the overall transport coefficient and A, the exchange surface area) to increase the heat exchange rate or reduce the required temperature differences used to drive the exchange process, and 3) reduce the pumping power required to meet a heat exchange target value. (Reference: Principles of Enhanced Heat Transfer, R. Webb, pg. 2, 1994).
For the cooling catheters used to cool organs and not the entire body, the objective is clear: reduce the size of the heat exchange catheter so that in can be placed safely inside an artery while maintaining a adequate heat transfer rate. To understand the design process, consider the following idea. An analogy can be made between electrical behavior and heat transfer; that is the total heat exchange, Q, is proportional to the ratio of the temperature difference (delta T) to the total sum of the heat transfer resistances (Rtotal), Q=(delta T/Rtotal). This is similar to Ohm's law where the current, I, is proportional to the ratio of the voltage difference to the sum of the electrical resistances.
To select the correct heat transfer enhancement approach for cooling catheters requires balancing two goals: (1) the goal to maximize heat transfer rates to enable sufficient device size reductions and (2) the goal to minimize fluid pressure drop increases to sustain safe organ blood perfusion levels.
Consider the first goal: heat transfer augmentation to minimize size. The first step is to determine the dominant heat transfer resistance and reducing it as much as possible.
Now consider the second cooling catheter design goal: minimizing pressure drop. Despite the reviewed specifications described for existing cooling catheter patents, heat transfer enhancement is only part of the cooling catheter design challenge. For example if a catheter designer attempts to minimize the dominant heat transfer resistance one may add surface area or rough surfaces to produce turbulent mixing near the heat transfer surface. These attempts likely will boost the heat transfer performance significantly, but they will also likely reduce perfusion rates because of an increased pressure drop levels across the enhanced surfaces.
Heat transfer enhancement techniques can be broken down into two broad categories: (1) passive techniques and (2) active techniques. Related endovascular cooling catheter patents to this point have relied solely on passive transport enhancement techniques, where a fixed or static cooling catheter is placed inside a stagnant or moving body fluid.
Consequently, passive techniques are transport enhancement approaches that do not add mixing energy to the fluid system of interest. The energy used to create enhanced performance is taken from or drawn from the stored hydraulic energy involved in the exchange process. They are particularly effective when fluid pumping power or hydraulic energy is not limited or prohibitive in cost. The approach involves adding surface area and or inducing turbulence adjacent to the effective exchange surface area (
U.S. Pat. No. 6,096,068, by Dobak and Lasheras (Aug. 1, 2000) describes a metallic cooling catheter used to exchange heat inside the body. To enhance heat transfer, articulated segments are used to increase surface area and induce turbulence, a classic passive enhancement technique. Clock-wise and counter clock-wise segments mix the thermal boundary layer in highly pulsatile flow inside the carotid artery, creating a turbulence intensity, I, of greater than 0.05 (1 equals the RMS velocity fluctuation/mean velocity). This patent, however, does not address the critical issue of device blood-side pressure drop, the inherent cost resulting from a heat transfer enhancement technique. In fact the turbulence generated with devices like these does not come without some fluid energy cost (
There are two more concerns with this patent (U.S. Pat. No. 6,096,068, Dobak and Lasheras (Aug. 1, 2000)): a) the coolant side pressure drop is large, equal to five atmospheres or 3800 mmHg; a catheter fracture failure would lead to a high pressure liquid jet impinging upon the vascular wall and b) the articulated segments are prone to thrombus generation due to blood flow stagnation zones that will inherently be established along the catheter surface.
Two other patents by Gobin et al., U.S. Pat. No. 6,126,684 (Oct. 3, 2000) and Dae, U.S. Pat. No. 6,231,594 (May 5, 2001) address endovascular cooling with inflatable static balloons. By static balloons we mean the balloon walls that remain motionless once they are inflated and the heat exchange process has begun. This approach is useful for device insertion and for increased exchange surface area when the cooling action is begun. Gobin et al. uses multiple balloon chambers with a coolant distribution technique to ensure a maximum temperature difference between coolant and blood flow. The multiple chambers help maintain catheter flexibility. The Dae patent, similar to Gobin et al., uses multiple balloons twisted into a helix with separate cooling pathways. In both of these designs heat exchange effectiveness is driven by two factors: 1) the increased heat exchange surface area (a passive enhancement approach) and 2) the carefully circulated coolant pathways. This is in contrast with the Dobak and Lasheras patent (U.S. Pat. No. 6,096,068 Aug. 1, 2000) discussed above that relied primarily on turbulence intensity boosts for improved heat transfer. Furthermore, in both of these balloon catheter patents information is not provided on the impact of device inflation on normal blood flow nor is there information provided on the internal coolant pressure. In addition, both patents do not address the issue of blood hemodynamics leading to flow stagnation zones on the catheter that are likely sites for thrombus generation.
In yet another patent by Keller et al., U.S. Pat. No. 6,264,679 (Jul. 24, 2001) a third approach is taken for endovascular cooling. Using small hollow fibers, coolant is passed from one manifold to the next. The arrangement is similar to a tube and shell heat exchanger used throughout large cooling systems such as chillers. This approach attempts to maximize the available heat exchange surface area and maintain a well-distributed coolant to ensure the maximum temperature difference between the coolant and the blood. This patent also does not address the fundamental design challenge of avoiding increased fluid pressure drops nor does it address the concern of blood stagnation zones inside the hollow fiber bundle.
In summary these endovascular cooling techniques mentioned above have one or more of these disadvantages.
While endovascular heat exchangers have not employed active mixing to augment transport, some other applications are noted. Reeder et al. U.S. Pat. No. 6,217,826 (Apr. 17, 2001) and Borovetz et al. U.S. Pat. No. 6,348,175 (Feb. 19, 2002) each describe the value of active mixing enhancement for a blood pump design. Furthermore, Hattler et al. U.S. Pat. No. 5,501,663 (Mar. 26, 1996), describes an endovascular mass exchanger that uses active mixing. It uses a balloon with porous fibers to enhance mass transfer between a low viscosity blood gas flowing within the fibers and the blood flow surrounding the fibers. Krantz et al. U.S. Pat. No. 5,626,759 (May 6, 1997) also uses active mixing for blood oxygenation outside the body. In this patent, blood flows inside hollow fibers that are vibrated in an axial direction held inside a bedside container.
A catheter for insertion into a body cavity for contact with a body fluid comprises one or more tubes making surface contact with the body fluid. A working fluid, or fluids, is circulated internally through the tubes for interaction with the body fluid through the tube surfaces. For enhancing such interaction, a working fluid drive system provides various means for adding energy to the tube surface and or the surrounding body fluid. These enhancement measures induce boundary layer disruption or secondary flows at the tube surfaces, reducing the thickness of the thermal and concentration boundary layers in the fluid space adjacent to the tube surfaces, resulting in improved exchange rates In many embodiments the working fluid has two purposes: 1) to act as a vehicle to carry energy and mass through the drive system, and 2) to add energy to exchange surfaces and reduce fluid boundary layer thicknesses.
In one embodiment, an elongated tube or several elongated tubes with nonuniform compliance and a working fluid drive system are used to cause boundary layer disruption at the tube surface. The working fluid drive system creates periodic changes in internal working fluid pressure resulting in tube surface deformation and boundary layer disruption. Surface designs for nonuniform compliance tubes may be chosen to augment body fluid motion in an axial direction using a screw-like action.
In another embodiment, a directional cycling or shuttle working fluid drive system is used to dispense and remove working fluid from a bundle of tubes in a periodic fashion. The directional cycling characteristic of the drive system has the effect of creating tube surface vibrations in both an axial and a radial direction. Again, these vibrations are used to disrupt and reduce adjacent fluid boundary layer thicknesses.
In another embodiment, a small turbine is used to rotate a bundle of tubes. Rotation in this embodiment creates boundary layer disruption near the tube bundle surfaces. The turbine converts the hydraulic energy provided by the working fluid drive system into tube bundle rotational energy.
In another embodiment, an elongated balloon is provided in parallel with an encircling bundle of tubes. The working fluid drive system in this embodiment pulsates the inner balloon and circulates the working fluid through the tube bundle. Balloon pulsation is used to disrupt and reduce adjacent boundary layer thicknesses.
In still another embodiment, the tube surfaces or other portions of the exchange catheter surfaces are porous. Together with the working fluid drive system, this porous characteristic enables two separate and controllable modes of heat and mass transfer: 1) direct infusion of the working fluid into the body fluid, and 2) surface heat and mass exchange found with nonporous exchange surfaces. Furthermore, a sealed penetration point at end of the catheter is described that allows interconnection between an exchange catheter and a distal protection filter catheter, enabling correct alignment of the exchange catheter inside a blood vessel.
In the drawings, closely related figures have the same number but different alphabetic suffixes.
When a physician faces a patient that is in endanger of ischemic injury, a fundamental objective is the safe return to acceptable blood perfusion levels. While the core infarcted tissue that is immediately affected by perfusion reduction may not be salvageable there exists a region called the ischemic penumbra that is salvageable. This may involve altering cardiac function and or removing a thromboembolic occlusion that prevents sufficient perfusion. Drugs called thrombolytics are used to chemically remove the occlusion.
Two additional approaches for treating ischemic injuries also show promise. The first approach involves more chemical intervention with neuropreservative drugs. These drugs address the biochemical process behind cell death resulting from ischemic perfusion levels. The second approach is the use of induced hypothermia to alter metabolism and biochemical transport behavior. This invention application describes methods to induce localized or organ specific cooling.
The previous patents described above, with catheters having typical sizes ranging from 3 to 4.5 mm in diameter, attempt to provide rapid endovascular cooling using only passive heat transfer enhancement techniques. While these devices may cool effectively, they do not address the central problem of the device's impact on perfusion, particularly in small vessels like the common carotid artery. The invention we describe here addresses that critical problem and others.
Consider the common carotid artery (
Before describing our embodiments and the methods that address this fundamental design optimization challenge, a review of heat transfer physics surrounding an endovascular cooling process is helpful.
When a cooling catheter is placed inside a blood vessel heat is first transferred by convection from the warm blood to the cool catheter exchanger surfaces. These exchanger surfaces then transfer heat by conduction across the material thickness from the outer surface in contact with the blood to the inner surface in contact with the cool working fluid. Finally heat is transferred from the inner surface of the catheter to the working fluid, again using convective heat transfer. An in-depth analysis of this heat transfer process shows that in most cases the dominant resistance for heat transfer is blood side convection, not the other resistance terms, coolant side convection and conduction through the catheter wall (
To reduce this dominant resistance it is helpful to understand the convective resistance relationship where convective resistance, Rconv=1/hbloodAblood, where hconv is the blood side convective heat transfer coefficient and A is the blood side exchange surface area. Therefore, to reduce this resistance or in other words provide heat transfer enhancement we can either increase hblood or Ablood or both.
To gauge the effectiveness of a particular enhancement a term called an efficiency index, e, is used where e=(j/js)/(f/fs) and j=Colburn j factor, a dimensionless heat transfer coefficient, and f=Fanning friction factor, a dimensionless pressure drop term. The subscript s indicates the value for a smooth or non-enhanced surface.
Most passively enhanced surfaces like those found in earlier described patents and like those used in cooling equipment like air conditioners have efficiency indices between 0.9 and 0.8 (Reference: Principles of Enhanced Heat Transfer, R. L. Webb, p. 50, 1994). In other words, for well-performing passive enhancements there is nearly a one to one relationship between the increases in heat transfer rates and the resulting increases in pressure drops. This does not pose a problem with most heat exchanger applications, but it does pose a problem with endovascular heat exchange. In fact, the efficiency index is closer to 0.5 for laminar flow conditions. Consequently for every percentage increase in heat transfer, the pressure drop increases at twice that percentage. Therefore, while passive enhancement approaches alone may increase heat transfer by inducing localized turbulent mixing near a heat exchange surface, the viscous dissipation of these turbulent mixing eddies usually leads to substantial pressure drops and flow reductions if pumping power can not be increased.
To address this fundamental design problem, we are applying a hybrid enhancement approach, utilizing both active and passive enhancement techniques. The active enhancement concepts used with our embodiments discussed here add energy to either the fluid—exchanger surface interface or the fluid itself, disrupting thermal and concentration boundary layers that lie adjacent to the exchange catheter surfaces. By disrupting these boundary layers the boundary layer thicknesses are reduced and since the convective heat and mass transfer is inversely proportional to boundary layer thickness, higher rates of transport occur. Since the energy used to reduce these boundary layer thicknesses is primarily drawn from the active enhancement technique and not drawn solely from the hydraulic energy of the body fluid (as is the case with passive enhancement techniques), devices can be made sufficiently small to fit inside arteries, while achieving sufficient heat exchange rates and safe perfusion levels.
To achieve this outcome, two devices are needed: a working fluid drive system that provides the energy to drive active enhancement and an exchange catheter that provides the interface between the working fluid and the body fluid.
The control system, together with user inputs, operates the entire system, including primary and secondary circulation loops. The control system automatically adjusts active mixing parameters such as the frequency of pulsation and the amplitude of pulsation.
The multi-lumen delivery catheter, 13, 18, is sized to allow sufficiently rapid circulation of the working fluid and therefore maximum internal heat and mass transfer coefficients without excessive working fluid pressure drops like those found in earlier described patents. The multi-lumen delivery catheter enters the proximal manifold, 14, and extends the length of the exchange catheter to the distal manifold, 15. It is made of a polymer with low thermal conductivity to minimize heat transfer between the incoming and outgoing working fluid. Multi-lumen catheters like these are readily made with existing polymer extrusion techniques. They can be easily purchased from manufacturers like Zeus Industrial Products Inc., in Orangeburg, S.C.
The distal manifold, 15, evenly distributes the inlet working fluid to the annulus created between the delivery catheter and the exchange surfaces (
The tube material, 16, is polypropylene, polyurethane or similar flexible polymer. These nonuniform complaince tubes may be manufactured easily using timed dipping of polish metallic mandrels into and out of containers of liquid polymer. Over time these dipping steps accumulate tube wall material on the mandrel. When the wall thickness is about 0.1 to 1.0 mm thick, the polymer may be removed from the mandrel with warm water, soap, and perhaps a glycerol water mixture. Variation in dipping procedures and mandrel design allow the alteration of tube wall thickness and the resulting tube compliance properties. As an alternative, tubes like these could be purchased from companies like Advanced Polymers Inc. in Salem, N.H. Advanced Polymers Inc. makes custom tubes and balloons for medical applications and they have proprietary methods for producing nonuniform compliance tubes.
The tube surface, like all of blood contacting surfaces, are coated to avoid thrombus generation. Although, due to the inherent fluid mixing generated with each active enhancement technique described below, blood stagnation zones are minimized significantly. Coatings like covalently bonded heparin are used. In addition, outer surface roughnesses, a passive enhancement technique, may also be added to promote outer boundary layer disruption and subsequent transport enhancement. Sand grains together with a polymer adhesive may be used to achieve transport enhancement
During operation the preferred embodiment (
Before leaving this preferred embodiment description it should be noted that other methods for surface movement can be used. Embodiments shown in
In another embodiment shown in
The embodiment shown in
Common additional properties to all embodiments are two characteristics not yet discussed: porous heat exchange surfaces and distal manifold design options.
First porous surfaces are surfaces having holes that are measured in fractions of a micrometer. Both balloons and hollow fibers may be made porous. These balloons and fibers can be purchased from Celgard Inc. Charlotte, N.C. and Advanced Polymers Inc. Salem, N.H. These pores enable simultaneous heat and mass transfer. In terms of mass transfer, alterations in blood gases or drug levels are possible with porous heat exchange surfaces. For example it has been shown that partial pressure increases of carbon dioxide may cause significant increases in blood perfusion to the brain (Reference: Cardiovascular Physiology, W. Milnor, Pg. 397, 1990). Under normal blood pressure conditions, an increase in carbon dioxide partial pressure from 40 mmHg to 60 mmHg causes a 40% increase in cerebral blood perfusion. The body responds to increases in carbon dioxide partial pressure with vasodilatation, reducing cerebral vascular resistance and increasing perfusion to the brain. In terms of blood oxygen content it has be shown that increases in blood oxygen partial pressure may help patients recover more successfully from head injury (Reference, Results of a prospective randomized trail for treatment of severely brain-injured patients with hyperbaric oxygen, G. Rockswold, Journal of Neurosurgery, Vol. 76, Pg. 929–934, 1992). In summary, porous heat exchange surfaces enable blood composition modifications to go beyond the thermal arena.
Second, the distal manifold design may be altered to allow infusion of the working fluid and interconnection with distal protection filters (
An Clinical Application Example
The practice of the invention is shown in the following non-limiting example, Accordingly, the details of the procedure described below will change depending upon the application.
For this example we assume an ischemic stroke patient and we assume that there is a need to transfer heat and well as mass. There are two likely arterial locations for placement of the active mixing exchange catheters (
Using a standard angiography suite equipped with fluoroscopy, a guide catheter is placed in the desired common carotid artery. An appropriately sized exchange catheter is then filled with a perfluorocarbon emulsion. This size will typically range from 2–6 mm in diameter and 3–9 cm in length. The exchange catheter is then placed into the carotid artery via the guide catheter or over a guidewire and its placement is confirmed with fluoroscopy. Upstream of this placement a stint and or distal protection filter may be used to prevent further neurological complications from emboli or vessel wall movement. A stint may also be used to enable a larger cross sectional area for blood flow passage.
While the exchange catheter is being inserted, the primary working fluid circulation system and secondary conditioning fluid circulation system are readied for use. In this application the primary working fluid is a perfluorocarbon emulsion. The secondary circulation system has two components: a liquid system (saline or other coolant) to adjust primary working fluid temperature and a gas system (oxygen, carbon dioxide, nitrogen, etc.) to adjust the primary working fluid dissolved gas or therapeutic drug concentration. A hollow fiber device like a blood oxygenator may be used to adjust these parameters.
Assuming the exchange catheter has been placed in the common carotid artery of the ischemic stroke patient, cooling, gas delivery, and or drug delivery is begun by making the final connection between the exchange catheter and the working fluid drive system,
Throughout this and other clinical applications, the circulation or drive system described in