for Improved Topical Car & ac Hypothermia

Transcription

1 A Recirtrulating Cooling S stem for Improved Topical Car & ac Hypothermia F. L. Rosenfeldt, F.R.C.S.E., A. Fambiatos, B.Sc., J. PastorizaPinol, C.C.P., and G. R. Stirling, F.R.A.C.S. ABSTRACT A simple system is described that recirculates cooling fluid for topical cardiac hypothermia. This disposable system can produce a flow of 1,500 mumin at 2" to 4 C. The recirculating cooler produced significantly lower myocardial temperatures than a conventional fluiddiscard system in 22 patients having coronary operation. This system has been used as part of the technique of hypothermic cardioplegia in more than 600 patients. During various cardiac procedures, septa1 temperatures were maintained well below 20 C for 60 minutes or more without the need to reinfuse the cardioplegic solution. Optimal myocardial protection by hypothermic cardioplegia is achieved by maintaining profound local cardiac hypothermia throughout the period of aortic crossclamping. After initial cooling induced by a cold cardioplegic infusion, the heart progressively rewarms due to heat gain from the environment and the entry of warm blood into the cardiac chambers and into the coronary circulation [l]. This tendency for cardiac rewarming can be countered by efficient topical hypothermia. We have demonstrated previously [2] that the efficiency of topical hypothermia is improved by increasing the flow of irrigating fluid to 350 mllmin or more and by maintaining fluid temperature at 1" to 4 C. This report describes a recirculating cooling system that provides a rapid flow of lowtemperature fluid for topical hypothermia. When compared with a nonrecirculating technique, the recirculating system produced better From the Cardiac Surgical Research Unit of the Baker Institute and the C. J. Officer Brown Cardiac Surgical Unit of the Alfred Hospital, Melbourne, Australia. Supported in part by the Edward Wilson Memorial Fellowship. Accepted for publication Feb 5, Address reprint requests to Dr. Rosenfeldt, Baker Medical Research Institute, Commercial Rd, Prahran, 3181, Victoria, Australia. cardiac cooling in patients having coronary bypass operation. Methods Description and Use of Cooling System This system was modeled on one described by Wheeldon and associates [3] who used a water refrigeration unit connected to a reusable metal heat exchanger to recool the irrigating fluid. We simplified the circuit and made it disposable. The circuit (Fig 1) comprises a thinwalled plastic cooling coil immersed in an icesalt mixture at 1O"C, a roller pump, a delivery line, and a return line. Fluid is aspirated from behind the heart through the suction catheter, pumped through the cooling coil where it is cooled to 2" to 4"C, and returned through the multiholed catheter as a spray over the heart. This system can deliver fluid as cold as 2 C at flow rates up to 1,500 mllmin. The fluid temperature is monitored in the delivery line and can be varied by moving the cooling coil in or out of the freezing mixture. While the cardioplegic solution is being infused, the roller pump is turned on and the surgeon floods the pericardial cavity with cold Hartmann's solution to prime the cooling circuit. The cooling coil is immersed in the icesalt mixture when fluid is circulating satisfactorily. A porous spacer [21 is placed behind the left ventricle to ensure an adequate layer of cooling fluid between the left ventricle and the posterior pericardium. The table is tilted to the left and inclined with the patient's feet downward to improve coverage of the left ventricle by cold fluid. The delivery catheter is positioned so that as much as possible of the front of the heart is sprayed with cold fluid without interfering with surgical exposure. Evaluation of Cooler The recirculating system was compared with a conventional technique of topical hypothermia by The Society of Thoracic Surgeons

2 402 The Annals of Thoracic Surgery Vol 32 No 4 October 1981 SILICONE RUBBER IRRIGATION CATHETEF I SUCTlON SILKONE RUBBER CATHETER COOUNG COIL t THIN COVERING FWlD FILM PLmC MESH SPACER Fig 7. Recirculating cooling circuit. (PVC = polyvinyl chloride; SR = silicone rubber.) in patients having coronary bypass operation. Patients were assigned alternately to one of two groups: Group 1, those for whom the recirculating system was used, and Group 2, those having conventional cooling. In both groups, systemic cooling was begun on bypass before the aorta was crossclamped. Perfusate temperature was reduced to 25 C for 6 to 8 minutes. The St. Thomas' Hospital cardioplegic solution L4J (10 ml per kilogram of body weight) was infused over 2 to 5 minutes into the aortic root at a temperature of approximately 4 C. In Group 1, the cooler was used as previously described, with the pump set at 500 mymin. In Group 2, Hartmann's solution was delivered over the heart as recommended in the Shumway technique [5], through a standard adult intravenous infusion set, from bottles which had been stored in a refrigerator at 4 C. The flow regulator on the set was opened fully and the bottle hung 1 meter above the operating table. Fluid temperature was measured by a needle sensor inserted through a side port in the infusion set. In addition, immediately after crossclamping and every 20 to 30 minutes thereafter, the sur geon flooded the operative field with 1 liter of cold fluid. The discardsuction system in the operating room was used to remove excess fluid from the pericardial well. Myocardial temperature was measured in the ventricular septum using a newly designed myocardial temperature probe.* This probe consists of a diode sensor 250 p in diameter, mounted in the tip of a 21 gauge needle. The needle protrudes 15 mm beyond a moulded button, which ensures a reproducible depth of insertion (Fig 2). Temperature was recorded in the ventricular septum every 2 minutes until the crossclamp was released or the cardioplegic solution was reinfused. For statistical analysis, the t test was used. Results The mean temperature of the irrigating fluid in Group 1 (recirculating system) for the 40 minute test period was 3 C (range, 0.3" to 4.7"C) and the flow rate, 500 mllmin. In Group 2 (conventional cooling) the mean fluid temperature was 6.5"C (range, 1.5" to 10.9"C) and the mean flow rate, 133 mllmin. *By courtesy of the Royal Melbourne Institute of Technology, Swanston St, Melbourne, 3000, Victoria, Australia.

3 403 Rosenfeldt et al: Recirculating Cooling System for Topical Hypothermia Fig 2. Myocardial temperature probe. The button format ensures minimum intrusion of the cable into the operative field. f 1.5"C and at 40 minutes, 12.7" k 0.9"C (range, 7.3" to 18.0"C) (p = not significant). Analyzing the data in another way: 8 minutes after crossclamping there was no significant difference in mean septal temperature between the two groups, but after 40 minutes Group 1 was 5.3"C colder than Group 2 ( p < 0.01). All 22 patients survived, and no clinical difference was observed between the two groups in the postoperative period. The only patient requiring inotropic support was from Group 1. No patients in either group showed new electrocardiographic Q waves postoperatively. Over an eighteenmonth period in our unit, the recirculating cooler has been used in more than 600 adult and pediatric patients operated on for a variety of acquired and congenital heart diseases. Figure 4 illustrates septal temperature measurements obtained in three groups of patients having coronary artery bypass operation, mitral valve replacement, or aortic valve replacement with only one infusion of cardioplegic solution during the measurement period of 40 to 60 minutes. It can be seen that the recirculating cooler prevented rewarming of the heart, septal temperature being maintained at 10" to 13"C, on average, for the duration of crossc~amp~ng in all groups. Widely different septal temperatures were observed at the completion of the cardioplegic infusion before topical cooling was begun. Eleven patients in each group were selected for comparison so that the mean septal temperatures after infusion for the two groups were similar. A 40minute test period was used because this was the minimum period before reinfusion of cardioplegic solution in both groups. The results are shown in Figure 3. In Group 2, a minimum temperature of 13.4" f 12 C (mean * standard error of the mean) occurred 8 minutes after crossclamping. Subsequently, septal temperature increased steadily and by 40 minutes had reached 18.0" k 1.2"C (range, 11.6" to 23.5"C), which was significantly warmer than at 8 minutes (p < 0.01). In contrast, in Group 1, mean septal temperature at 8 minutes was 13.9" Comment During cardioplegic arrest, the lower the myocardial temperature down to 4" to 5"C, the greater is the protection against ischemia. However, below about 20 C the additional protective effect conferred by each degree of cooling becomes less [6, 71. We found that an intracoronary infusion of 1 liter of cardioplegic solution at 4 C in 22 adults with coronary artery disease produced a septal temperature between 7" and 21 C. The addition of highefficiency topical cooling then led to two beneficial effects. In the outer myocardial layers it produced profound hypothermia (2" to 7 C) and thus ensured a high degree of protection. More importantly, we believe it also improved cooling of the vulnerable septal and subendocardial areas, which may be much warmer than the epicardium especially when supplied by obstructed coronary arteries.

4 404 The Annals of Thoracic Surgery Vol 32 No 4 October IV SET FUH) DISCARD 00 RECFICUATNG COOLER M 40" y 30 F s Y 2o W I $ w 10' ' ' I,\ '8 a. 30" 20 loo I 01 lo" ISCHEMIC TIME (MINI Fig 3. Temperaturetime curves for two groups of patients having coronary bypass operation with topical cooling, following a single infusion of cardioplegic solution (CP). The values are shown as the mean and the standard error of the mean. (IV = intravenous.) Fig 4. Mean temperaturetime curves for three groups of patients having topical cooling by recirculation following a single infusion of cardioplegic solution (CP). (CABG = coronary artery bypass graft; AVR = aortic valve replacement; MVR = mitral valve replacement.) 4or The advantages of the recirculating cooling system over conventional fluiddiscard systems are as follows: 1. There is better overall cardiac cooling by a high flow of lowtemperature fluid. 2. The need to refrigerate large volumes of fluid in the operating area is avoided, as is the repetitive task of replacing bottles of cold Hartmann's solution as they are used during operation. 3. The annoying noise produced by the en 40' * 30" 20" " I I Ol. I ISCHEMIC TIME (MINI. 10'

5 405 Rosenfeldt et al: Recirculating Cooling System for Topical Hypothermia 4. trainment of large volumes of air by the usual fluiddiscard system using highpower suction is eliminated since delivery and suction in the recirculating system are controlled by the same roller pump. The highflow spray system for delivering cooling fluid eliminates the need for a pool of cold fluid in the pericardium deep enough to cover the front of the left ventricle. More than half the cost of the disposable circuit now produced commercially* is covered by the saving in Hartmann s solution usually discarded during a 60minute crossclamp period with a nonrecirculating system. The recirculating cooling system for topical hypothermia has proved reliable and convenient in more than 600 patients and is now used in our unit as part of the routine technique of hypothermic cardioplegia. *Kal Life Systems, 27 Greeves St, St. Kilda, Victoria, 3182, Australia. References 1. Rosenfeldt FL, Watson DA: 11. Interference with local myocardial cooling by heat gain during aortic crossclamping. Ann Thorac Surg 27: 1316, Rosenfeldt FL, Watson DA: 111. Local cardiac hypothermia: experimental comparison of Shumway s technique and perfusion cooling. Ann Thorac Surg 27:1723, Wheeldon DR, Bethune DW, Gill RD, English TAH: A simple cooling circuit for topical cardiac hypothermia. Thorax 31:%5571, Braimbridge MV, Chayen J, Bitensky L, et al: Cold cardioplegia or continuous coronary perfusion? J Thorac Cardiovasc Surg 74:900906, Griepp RB, Stinson EB, Oyer PE, et al: The superiority of aortic crossclamping with profound local hypothermia for myocardial protection during aortacoronary bypass grafting. J Thorac Cardiovasc Surg 70: , Hearse DJ, Stewart DA, Braimbridge MV: Cellular protection during myocardial ischemia: the development and characterization of a procedure for the induction of reversible ischemic arrest. Circulation 54:193202, Harlan BJ, Ross D, Macmanus Q, et al: Cardioplegic solutions for myocardial preservation: analysis of hypothermic arrest, potassium arrest and procaine arrest. Circulation 58:Suppl 1: , 1978