Method and Apparatus for Sampling Aortic Plaque

The present invention relates generally to surgical instruments for aspirating non-liquid body materials, and more particularly to methods and apparatus for detecting friable atheromatous deposits in or near the aortic arch.

Myocardial revascularization is a surgical procedure in which saphenous vein bypass grafting to obstructed coronary arteries is performed to help in relieving angina and in reducing the possibility of heart attack. During the surgery a cardiopulmonary bypass is routinely performed in which a heart-lung machine temporarily takes over the function of the heart and lungs. The machine is normally connected to the ascending aorta by a cannula through which blood is perfused thus replacing the natural action of the heart. However, ever since it was first implemented, an uncommon but persistent perioperative complication of neurologic injury, appearing many times as stroke, could occur leading to serious disability or death. Initially, the complication was ascribed to air embolizing either by accident or from what would be unreliable equipment by today's standards.

Prolonged perfusion time exacerbated the neuro injury patterns probably from particulate debris of thrombus and platelets generated within the machine's oxygenator. With cardiopulmonary bypass equipment now improved to almost perfection, these etiological factors are much less a problem, yet neurologic injuries still persisted. Advanced age of the patient, duration of cardiopulmonary bypass, cerebral vascular disease, and atherosclerosis in the vicinity of the aortic arch appeared to be strong predisposing factors along with intraoperative hypotension, carotid disease, air embolus, and postoperative arrest. Of these factors, atherosclerosis of the carotid artery, the ascending aorta, and the transverse aortic arch, singly or combined, have been implicated as among the major risk factors.

Tomography scans and aortograms of patients sustaining massive perioperative embolic strokes revealed large built-up areas of friable atheromatous plaques on the inner curvature of the aortic arch and the descending aorta just distal to the left subclavian artery. This condition appears to be present more often than previously suspected and might be etiologically explained by Bernoulli's theorem. More specifically, "lift" within the vessel wall appears to provide a hemodynamic factor which accelerates the accumulation of atherosclerotic plaque at the inner curvature and three major bifurcations of the arterial tree. Finding no other causes for the neurological injury in observed patients, it is postulated that the velocity of blood flow from the perfusion cannula in the vicinity of the plaques creates a "sandblast" effect, and turbulence which dislodges the plaque from the aortic walls to form particulate debris, allows it to travel through the blood system, and causes significant brain emboli.

This led to a procedure for exploring the aortic arch and descending aorta for friable plaque before inserting the perfusion cannula in the ascending aorta. The procedure is performed prior to revascularization by direct visual inspection through a long incision in the inner curvature of the aortic arch using deep hypothermic circulatory arrest and a clamp on the innominate artery. Hazardous friable debris is then scraped and wiped out, and the arch irrigated clean. See Nishida H., Grooters R. K., Yeager A. A., Soltanzadeh H., Thieman K. C., and Schneider R. F., "Carotid and Aortic Arch Endarterectomy Using Hypothermic Arrest with Coronary Bypass." Ann. Thorac. Surg. 1989; 48:865-6.

This procedure is not only extremely complex and time-consuming, but considerably risky. Moreover, it would have been completely unnecessary if no friable plaque were found.

Accordingly, it is an object of the present invention to reduce the risk of perioperative neurologic injury associated with bypass surgery by detection of friable atheromatous plaque in the aortic wall.

Another object is to provide an aortic sampling apparatus which is suitable for insertion through an arteriotomy made for a perfusion cannula, which is simple in design, and which can be made sufficiently inexpensive to be considered disposable.

Still another object is to provide a sampling apparatus for use in surgery which is capable of collecting friable atherosclerotic plaques and/or cholesterol lodged on the walls of the aortic arch.

A further object is to provide an aortic sampling apparatus which can be readily manipulated to reach the inner curvature of the aortic arch and the descending aorta distal to the left subclavian from an arteriotomy in the ascending aorta near the innominate artery.

A still further object is to provide a method for using an aortic sampling apparatus in the detection of atheromatous debris in the aortic arch prior to initiating cardiopulmonary bypass surgery.

Briefly, these and other objects are achieved by a surgical probe assembly for detecting the presence of friable atheromatous debris deposited in or near the aortic arch. A cannula having a caged tip formed at one end is mounted on a handle at the other end. A suction tube connected to the handle causes fluid to be aspirated through the cannula when a manually operated valve in the handle is opened. A filter trap in the handle captures any particulate carried with the fluid and a check valve prevents reverse flow. The handle can be quickly disassembled to allow the trap to be removed for examining any particulate collected. The distal end of the cannula is arched lengthwise with the tip offset from the longitudinal axis of the handle.

Prior to initiating cardiopulmonary bypass surgery involving aortic perfusion, the cannula of the probe assembly according to the invention is inserted through an arteriotomy made for the perfusion cannula. As suction is applied, the tip of the cannula is passed along the walls of inner curvature of the transverse arch and the descending aorta distal to the left subclavian. The tip is then pulled off the walls, the suction stopped, and the cannula and trap removed and inspected for atherosclerotic debris. If there is none, the bypass surgery may proceed using routine aortic cannulation and perfusion. However, if debris is found, other perfusion sites such as the femoral artery, or other treatment modalities in the protocol, should be considered.

For a better understanding of these and other objects and aspects of the invention, reference will be made to the following detailed description taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic representation of a sampling apparatus according to the invention in the transverse arch of an aorta, a portion of the arch being cut away to show placement of the apparatus for collecting atherosclerotic debris;

FIG. 2 is an exploded view in longitudinal cross section of the apparatus of FIG. 1;

FIG. 3 is an enlarged side view of a caged tip at the end of the apparatus of FIG. 2;

FIG. 4 is an end view of the caged tip of FIG. 2;

FIG. 5 is an enlarged side view in longitudinal cross section of the apparatus of FIG. 2 in the vicinity of a check valve;

FIG. 6 is an end view of the check valve as view from the bottom of FIG. 5; and

FIG. 7 is an enlarged cross sectional view of the apparatus taken along the line 7--7 of FIG. 2.

Referring now to the drawings wherein like characters designate like or corresponding parts throughout the several views, there is shown in FIG. 1 an aortic sampling apparatus 10 inserted through an arteriotomy a made for arterial perfusion at a site next to the takeoff of the innominate artery b on the ascending aorta. The inserted end of the apparatus is shown in solid outline in contact with atherosclerotic deposits c on the inner curvature of the transverse arch of the aorta, and in dotted outline in contact with such deposits d, if any, on the descending aorta distal to the left subclavian artery e.

Referring to FIG. 2, apparatus 10 includes a probe or cannula 12 having a flanged end 12a threadingly engaging one end of a hollow elongate handle section 14 along a longitudinal axis A--A. This allows cannula 12 to be removed from handle section 14 for separate sterilizing or disposal of parts. Alternatively, the proximal end of cannula 12 may be cemented in place to ensure a permanent seal. The length and curvature of cannula 12 provide for optimum accessibility from the site of the aortic incision to either of the aforesaid aortic areas of plaque buildup. A typical cannula configuration for aortic sampling has an overall length of approximately 165 mm measured from handle section 14. The cannula is defined by a straight section 12b elongated along axis an A--A, and a curved section 12c which terminates about 13 mm of axis A--A with a four-strutted cage 12d. The cage insures that the tip of the cannula is free to draw any dislodged plaque quickly into the instrument while preventing large fragments from blocking the cannula passage. It also prevents the opening at the cannula tip from becoming completely blocked by engagement with the interior wall of the aorta.

Referring to FIGS. 5 and 6, the passage of cannula 12 communicates with a cylindrical chamber 14a in handle section 14a which contains an elastomeric check valve 16 having a hollow cylindrical section 16a with a flange 16b at the inlet seated on a shoulder 14b at the inlet of chamber 14a. The valve outlet includes resilient lips 16c tapering together to a normally closed state to prevent backflow of fluid to the cannula 12. This check valve helps to insure against flow of air into the cannula, which could result in the generation of an air embolism, with possibly disastrous consequences.

At the outlet of valve 16, chamber 14a reduces to a narrow bore 14d containing a manually operated slide valve 18 in a cross bore 18a. Referring to FIG. 7, a helical spring 20 and O-ring 22 cooperate to hold a spindle 18b in a position in which it substantially closes off bore 14d. A clearance between bore 18a and spindle 18b insures there will always be a slight amount of flow, further insuring against the passage of air into the blood vessels before the heart pump acts in a vacuum mode. Depressing a knurled end 18c of valve 18 against the force of spring 20 allows fluid to flow freely through passage 14d into a hollow elongated handle section 24 which is threadingly engaged in coaxial alignment with handle section 14 at the end opposite of cannula 12. The exteriors of handle sections 14 and 24 are uniformly circular in cross-section along their combined length but for oppositely disposed flat sides 14e (FIG. 7) which are normal to the length of valve 18 and provide a positive hand grip surface.

The interior of handle section 24 defines a cylindrical chamber 24a containing a removable filter trap 26 with a porosity sufficient to permit blood to flow freely while straining any particulate debris. Trap 26 includes a mesh pouch 26a secured around the inlet to a collar 26b which fits snugly on a boss 14f extending beyond the threaded end of section 14 and which seats against a shoulder 24b at the inlet of chamber 24a. Pouch 26a is substantially square in cross section but for a portion adjacent collar 26b thus providing clearance adjacent the side of chamber 24a for fluid to flow through the sides as well as through the bottom of pouch 26a.

Chamber 24 narrows into a bore 24c formed in a neck end 24d of section 24. A series of concentric ridges about the neck provide a positive grip and seal for an elastomeric suction tube 30 from the heart-lung machine. Handle section 24 is preferably fabricated of transparent plastic to enable visual inspection of fluid flowing through the apparatus.

A method of sampling for aortic plaque with the above-described apparatus according to the invention prior to initiating cardiopulmonary bypass surgery is as follows. Following exposure of the aorta and the heart for contemplated bypass surgery, a double purse string is placed around the site selected for arterial perfusion, usually next to the takeoff of the innominate artery on the ascending aorta, and an arteriotomy is made within the site area. With valve 18 of sampling apparatus 10 closed but for the slight clearance in cross bore 18a, a perfusion pump operating in the suction mode creates a vacuum in the cannula 12 through check valve 16. Cannula 12 is then inserted and the purse string tightened for hemostasis. Sampling starts by pressing the knurled valve 18 inward to increase suction, preferably at a flow rate of 40 cc/kg/min. or approximately one-half of cardiac output. As suction begins, the cage 12d is passed posteriorly along the inner curvature of the aortic arch gently rubbing the wall for about two to three seconds. The handle of the instrument is then rotated to bring the tip of the cannula into engagement with the wall of the descending aorta at a location distal to the left subclavian artery. The procedure continues for another two to three seconds over the descending aorta an estimated four to six centimeters distal to the left subclavian, bringing the cage 12d toward the left subclavian artery. The cage is then pulled off the wall of the descending aorta with suction continuing for another two to three seconds to allow the uptake of any loosened debris. The time required to test both areas of the aorta is approximately ten seconds. Valve 18 is then allowed to close while the tip of cannula 12 is still within the descending aorta. After it is removed, pump suction may be stopped and trap 26 removed for inspection of any yellow particulates suggestive of atherosclerosis. If no particulate or only intima is found in the filter, a perfusion cannula may be installed through the same arteriotomy in order to proceed with the bypass surgery. On the other hand, if the sample reveals particulate of yellow plaque and/or cholesterol crystals, there is risk that such debris will further loosen and cause brain emboli if aortic perfusion were to be used during the bypass surgery. Other arterial perfusion sites, such as a femoral artery, or other alternative treatment modalities in the protocol should then be considered.

Thus, it is apparent that serious complications of stroke can be averted by first performing a diagnostic sampling for atherosclerosis in the vicinity of the aortic arch before cardiopulmonary bypass procedure is initiated. The same arteriotomy contemplated for perfusion cannulation with the heart-lung machine is used for the probe of the present invention. The detection of friable atheromatous plaque harbored in or near the aortic arch and a protocol to avoid perfusing at or in the potentially harmful atherosclerotic pathology is possible. If friable plaque is detected, a different perfusion site or other options may be preferred to avoid dislodging atheromatous debris during perfusion and thereby lower the incidence of stroke and/or the severity of neurologic injury.

It will be understood, of course, that various other changes in the details, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.