Topical Cardiac Cooling by Recirculation: Comparison of a Closed System Using a Cooling Pad

Transcription

1 Topical Cardiac Cooling by Recirculation: Comparison of a Closed System Using a Cooling Pad with an Open System Using a Topical Spray Franklin L. Rosenfeldt, M.D., F.R.C.S.E., and Malcolm Arnold, M.B., M.S., F.R. A.C.S. ABSTRACT Topical cardiac cooling plays an important part in maintaining myocardial hypothermia during cardiac operations under hypothermic cardioplegic arrest. e have compared two systems of topical cooling in which the cooling fluid is recirculated. In one system the cold fluid circulates through a sealed plastic cooling pad wrapped around the heart; in the other it is sprayed over the surface of the heart. In dogs undergoing 6 minutes of hypothermic cardioplegic arrest, the cooling pad was ineffective: it did not adequately cool the anterior left ventricular wall or the interventricular septum. Under the same conditions, the spray system produced myocardial temperatures 6" to 12 C lower than the cooling pad system. In limited clinical tests, the cooling pad was unable to sustain overall myocardial hypothermia and there was difficulty in maintaining contact between the heart and the pad during coronary operations. Since the topical spray can maintain profound global myocardial hypothermia regardless of the position of the heart, we conclude that it is superior to the cooling pad system. Hypothermic cardioplegia results in better protection against ischemic damage to the myocardium during open-heart operations than previous techniques of myocardial preservation [l, 21. Topical cardiac cooling is an important part of hypothermic cardioplegia, since it cools poorly perfused areas distal to obstructed coronary arteries, minimizes rewarming from environmental heat sources [3], and avoids the necessity for multiple reinfusions of cold cardioplegic solution. The standard From the Baker Medical Research Institute and C. J. Officer Brown Cardiac Surgery Unit, Alfred Hospital, Prahran, Victoria, Australia. Accepted for publication Nov 19, Address reprint requests to Dr. Rosenfeldt, Baker Medical Research Institute, Commercial Road, Prahran, Victoria, 3181, Australia. technique of topical cooling is that described by Shumway and colleagues [4] in which cold fluid is dripped into the pericardial cavity and then aspirated to waste. A disposable recirculating cooling circuit* developed at the Alfred Hospital [5] enables cooling fluid to be sprayed over the heart and then be collected and recirculated instead of discarded. Both Shumway's technique and sprayrecirculation produce a pool of cold fluid in the pericardial cavity that may interfere with surgical exposure. One method of overcoming this problem is to circulate the cold fluid through a flexible sealed pad enveloping the heart. Recent trials of a form of this closed system, the topical cooling device,+ have indicated that it is a useful addition to current techniques of topical cardiac cooling [6]. e have tested the sealed pad system in dogs and patients and compared it with the spray-circulation system. Materials and Methods Cooling Pad The topical cooling device is a closed loop comprising a sealed plastic cooling pad, a Silastic pump segment, a cooling coil, and an intravenous fluid bag containing normal saline solution (Fig 1). The pad is placed behind the left ventricle and then wrapped as closely ss possible around the heart. The cooling coil and the intravenous bag are placed in ice. A roller pump draws normal saline solution from the bag through the pad and then pumps it through the cooling coil and back into the bag. Topical Spray The Alfred Hospital recirculating cooling circuit comprises a thin-walled coil of plastic tubing "Kal Life Systems, 82 Glenhuntly Rd, Caulfield, 3162, Victoria, Australia. Tobe Laboratories, Lakewood, CO by The Society of Thoracic Surgeons

2 139 Rosenfeldt and Arnold: Topical Cardiac Cooling by Recirculation L 'P 4- C St 4 ' I immersed in an ice-salt mixture at -1O"C, a roller pump, a delivery line, and a return line (Fig 2). 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 a multiholed catheter as a spray over the heart. This system can deliver fluid at 4 C at flow rates up to 1, ml per minute. The fluid temperature is monitored in the delivery line and can be varied by moving the coil in or out of the freezing mixture. Fig 1. Cooling pad system. (P = cooling pad wrapped around heart; C = connecting tubing; R = roller pump; B = ice bath containing the cooling coil and the bag of normal saline solution.) Fig 2. Topical spray recirculating cooling circuit. (PVC = polyvinyl chloride; SR = silicone rubber.) Evaluation of Cooling Pad Six greyhounds (weighing 21 to 3 kg) were anesthetized with pentobarbital (3 mg per kilogram of body weight), intubated, and mechanically ventilated. A median sternotomy and pericardiotomy were performed, and the heart was suspended in a cradle of pericardium. Pressure-monitoring catheters were placed in the left atrium and the femoral artery. Heparin (3 units per kilogram) was given, and the ascending aorta and the venae cavae were cannulated for cardiopulmonary bypass. Bypass was commenced at 37 C using a bubble oxygenator primed with Hartmann's solution. Bypass flow rate was maintained at 9 ml per kilogram per minute and arterial pressure at 8 to 1 mm Hg. Left atrial pressure was kept below 5 mm Hg by a left atrial vent. Thermistor temperature SIUCONE RUBBER IRRlG4TK)N CATHETER THIN COVERING

3 14 The Annals of Thoracic Surgery Vol 34 No 2 August 1982 probes were inserted into the anterior and posterior left ventricular walls at midmyocardial level (1 mm) and into the interventricular septum anteriorly at a depth of 2 mm. Systemic cooling was begun and the aorta crossclamped when myocardial temperatures had reached 3 C. The St. Thomas' Hospital POtassium-magnesium-procaine cardioplegic solution [l] (15 ml per kilogram) was infused into the aortic root over 2 minutes. The solution was at a temperature of 4 C in the bottle, but had risen to 8 C before reaching the aorta. hen the infusion was complete, topical cooling was begun using either the topical cooling device or the topical spray circuit. The device was used as specified by the manufacturer, with the reservoir bag and cooling coil immersed in ice-water without added salt and with saline solution circulating at 35 ml per minute. The pad was closely applied to the posterior and lateral aspects of the ventricles. The topical spray circuit was recirculated at 5 ml per minute. The aorta was cross-clamped for 6 minutes. Body core temperature (rectal) was maintained at 3 C for 5 minutes; then systemic rewarming was commenced. Both topical cooling methods were used in turn in each animal, and the order of use was alternated in sequential animals. To assess the effect of more profound systemic cooling on the performance of the topical cooling device, in 2 additional dogs core temperature was lowered to 25 C instead of 3 C and temperature measurements were made as before during two 6-minute cross-clamping periods in each dog. Statistical analyses were performed using the paired t test. Evaluation of Cooling Pad Supplemented by Cold Cardioplegic Reinfusions In another 4 dogs the cooling effect of the topical cooling device was supplemented by reinfusions of cardioplegic solution at 8 C given through the aortic root every 15 minutes during the 6-minute period of ischemia. The dose used was 7.5 ml per kilogram, i.e., half the initial dose. In these dogs, to simulate the effect of atherosclerotic coronary occlusion and poor collateralization on cooling, one major coronary branch was acutely occluded. In 2 dogs the proximal circumflex coronary artery (CCA) was snared; in the other 2 dogs the proximal left anterior descending coronary artery (LAD) was snared, and in addition, any easily visible epicardial collaterals were ligated. The major coronary was snared immediately before systemic cooling began and released just before removal of the aortic cross-clamp. The protocol followed for these experiments was otherwise similar to that used for the first 6 dogs. Three separate cooling methods were tested in each dog: 1. The topical cooling device with the addition of reinfusions of cardioplegic solution every 15 minutes 2. Reinfusions of cardioplegic solution every 15 minutes without topical cooling 3. Topical spray without reinfusions Experience with Cooling Pad in Patients The topical cooling device was used by different surgeons in 3 patients who underwent either mitral valve replacement, aortic valve replacement, or double coronary bypass grafting. There were 2 men and 1 woman aged between 41 and 64 years. After bypass was commenced, blood temperature was lowered rapidly to approximately 1O"C, and after 2 to 7 minutes of perfusion cooling, the aorta was cross-clamped. A single infusion of the St. Thomas' Hospital cardioplegic solution at 4 C was given at a dose of 1 ml per kilogram. Myocardial temperature was monitored in the interventricular septum anteriorly and posteriorly at a depth of 25 mm using diode-tipped myocardial temperature probes., After the initial cardioplegic infusion, the heart was cooled by the topical cooling device as previously described. Body core temperature was maintained at 28 C throughout the period of cross-clamping. Results Evaluation of Cooling Pad and Topical Spray in Dogs Figure 3 shows the mean values of temperature readings obtained in 6 dogs with patent coronary arteries. The topical cooling device cooled *Royal Melbourne Institute of Technology, Swanston St, Melbourne, Australia.

4 141 Rosenfeldt and Arnold: Topical Cardiac Cooling by Recirculation a 3 c s n E I- 4 r 14 1 I, I l o D---o TOPICAL COOLING DEVICE - RECIRCULATING COOLER I P. I POSTERIOR LV o a 7 ISCHAEMIC TIME (MINUTES) Fig 3. Temperature-time curves for three regions of the left ventricle in 6 dogs with patent coronary arteries comparing cooling by the pad (topical coding device) and by the topical spray (recirculating cooler). Values given represent mean k standard error of the mean. (LV = left ventricle.) the posterior left ventricular wall below 2 C within 9 minutes of cross-clamping the aorta and maintained it at 14" to 15 C after 3 minutes of clamping. In the septum and anterior wall, there was little further cooling after the completion of the cold cardioplegic infusion. Septa1 temperature remained at 22 C throughout the period of topical cooling. Anterior left ven- ('I I tricular temperature reached 21 C after 35 minutes and remained there until the end of the cooling period. In the 2 dogs in which core temperature was maintained at 25 C during cross-clamping, the temperature-time curves produced by the topical cooling device were similar to those in dogs maintained at 3"C, except that values were 2 C lower on average. The topical spray circuit showed high cooling efficiency at all sites of temperature measurement. Between 3 and 6 minutes after crossclamping, temperatures remained at 9" to 1 C in the anterior and posterior left ventricular walls and at 11 C in the septum (see Fig 3). From 3 minutes onward when cooled by the topical spray, the anterior left ventricle was 12 C colder, the septum 1 C colder, and the posterior left ventricle 6 C colder than with the topical cooling device ( p always <.1). Evaluation of Cooling Pad Supplemented by Cold Cardioplegic Reinfusions In hearts cooled by the topical cooling device, the effect of supplementary infusions of cold cardioplegic solution was studied in regions supplied by patent coronary arteries and in regions supplied by occluded coronary arteries. The 2 dogs with LAD occlusion produced similar cooling patterns; so did the 2 dogs with CCA occlusion. Figures 4 and 5 illustrate one experiment of each type. Also shown for comparison is the mean curve from the group of 6 dogs with patent coronary arteries in which the topical cooling device was used without reinfusions. The anterior left ventricular wall and the interventricular septum were not cooled effectively by the device alone. Reinfusions of cold cardioplegic solution partially compensated for this if the regional coronary artery (LAD) was patent (see Fig 5), but not if the LAD was occluded (see Fig 4). However, the topical cooling device was effective in cooling the posterior left ventricular wall even in the presence of an occluded CCA, and reinfusions did not add greatly to this (see Fig 5). The topical spray without reinfusions produced more profound cooling than the topical cooling device in all areas studied, whether supplied by patent coronary arteries or obstructed coronary arteries.

5 142 The Annals of Thoracic Surgery Vol 34 No 2 August r 14 ANTERIOR L V *Or -a 4 ANTERIOR LV _.._..._._._ 'O- c-a a 3 I- s n 3 c nreinfus1n ALONE c---otcd * REINFUSION c-.recirculatinq COOLER... TCD IN DOQS a I 1 9 a 3 I- a w ik w I-... Y REINFUSION ALONE ---o TCD + REINFUSION - REClRCULATlNQ COOLER... TCD IN DOQS I I I I I I I I, 4 r POSTERIOR L V lot ISCHAEMIC TIME (MINUTES) Fig 4. Temperature-time curves for three regions of the left ventricle from a dog with occlusion of the left anterior descending coronary artery. Also shown for comparison is the mean curve obtained with the topical cooling device (TCD) from the 6 dogs with patent coronary arteries. The vertical shaded areas represent infusions of cold cardioplegic solution. (LV = left ventricle.) C ISCHAEMIC TIME (MINUTES) Fig 5. Temperature-time curves from a dog with occlusion of the circumflex coronary artery and the mean curve obtained with the topical coding device (TCD) from the 6 dogs with patent coronary arteries. The vertical shaded areas represent infusions of cold cardioplegic solution. (LV = left ventricle.) Evaluation of Cooling Pad in Patients The temperature recordings from the mitral valve replacement procedure are illustrated in Figure 6. After perfusion cooling and the infusion of the cardioplegic solution, anterior and posterior septal temperatures were between 9" spite of the use of the topical cooling device, there was steady rewarming of both the anterior and posterior regions of the interventricular septum at a rate of 2 C every 1 minutes. After 48 minutes of ischemia when anterior and posterior septal temperatures were annroximatelv 2 C. a mot check was made of and 1 C. Over the subsequent 47 minutes, in -.~ ~. --J -I I

6 143 Rosenfeldt and Arnold: Topical Cardiac Cooling by TIME (MINUTES) Fig 6. Temperature-time curves from a patient undergoing mitral valve replacement during cooling by the topical cooling device (TCD) and later by the topical spray (RCC). The represents a spot check of endocardia1 temperature made after 48 minutes of ischemia. (LV = left ventricle, CP = cardioplegic infusion.) subendocardial temperature by inserting a probe through the mitral valve and into the posterior wall of the left ventricle. hen a reading of 27 C was recorded, the topical cooling device was abandoned and cooling by the topical spray was begun. This resulted in rapid recooling of the anterior region of the septum and slowing of the rate of rewarming of the posterior septa1 region in spite of the fact that by this time systemic rewarming was in progress. In the other two procedures in which the topical cooling device was used, the surgeons found difficulty in maintaining adequate contact between the pad and the ventricles. In the aortic valve replacement procedure, the pad was not large enough to surround the hypertrophic left ventricle. In the coronary bypass graft procedure, when the heart was retracted anteriorly to perform the distal circumflex anastomosis, contact between pad and heart was lost. Therefore in both instances, the surgeons elected to supplement the topical cooling device with topical irrigation. Comment The level of hypothermia required for adequate myocardial protection for a particular procedure depends on the duration of cross-clamping, the amount of collateral flow, and the frequency and nature of reinfusions given. During cardioplegic arrest, the lower the myocardial temperature, down to 4" to 5"C, the greater is the protection against ischemia. However, below about 2"C, the additional protective effect conferred by each degree of cooling becomes less [7, 81. Lowering temperature in all areas of the myocardium to 2 C during cardioplegic arrest gives good myocardial preservation for two hours of ischemia in a normal dog's heart [9] and can probably be regarded as the minimum degree of hypothermia necessary for uncomplicated cardiac operations. Temperature gra-

7 144 The Annals of Thoracic Surgery Vol 34 No 2 August 1982 dients in the myocardium, such as those illustrated in Figure 5, are well recognized during cold cardioplegic arrest, especially in patients with coronary artery disease [lo] or left ventricular hypertrophy. By lowering epicardial temperatures to 4" to 1O"C, an efficient technique of topical cooling can minimize the effect of these gradients and ensure that no area of the myocardium is warmer than 2 C. This study showed that the topical cooling device could not maintain the temperature of the anterior left ventricular wall and the interventricular septum below 2 C even in dogs with normal ventricles and unobstructed coronary arteries. As might be expected, the device was effective in cooling only the area of the left ventricle in contact with the pad, namely, the posterior wall. The need for local cardiac cooling is reduced if profound systemic hypothermia is employed. Most surgeons use systemic cooling to 28" to 3 C for routine procedures in adults. This necessitates using an efficient technique of topical cooling or frequent reinfusions of cold cardioplegic solution to maintain myocardial temperature below 2 C. hen core temperature is lowered to 2O"C, little if any topical cooling is required. The use of such profound levels of total-body hypothermia in adults is inconvenient due to the long bypass times required for thorough rewarming of the whole body. e chose a core temperature of 3 C to simulate the usual clinical conditions. The pilot clinical study was not encouraging. In the coronary artery graft procedure, contact between the heart and the pad could not conveniently be maintained while the anastomosis to the posterior coronary branch was done. During the mitral valve replacement procedure, despite satisfactory contact between the pad and the heart, progressive rewarming of the subendocardium and of the anterior and posterior regions of the interventricular septum occurred. In view of the poor performance of the topical cooling device and the efficacy of the topical spray proven in more than 6 patients [5], it was thought that further trial of the device in patients was not justified. The efficiency of topical cooling depends on the flow rate, temperature, and area of distribution of cold fluid over the heart [ll]. The major problem with the cooling pad concept is that of achieving a wide enough area of contact between the pad and the heart. If the heart could be completely enveloped by the pad, then cooling would be effective but surgical access would be compromised. Hence in practice, direct cooling by the pad is restricted to the posterior and lateral aspects of the ventricular portion of the heart. This study has shown that the posterior wall of the left ventricle was adequately cooled whereas the anterior wall and interventricular septum were not. In contrast, the topical spray is able to cool a wide area of the heart including the atria, and the delivery catheter can be easily repositioned during the operation so as not to interfere with surgical exposure. In the spray system, the fluid is delivered over the heart at 4"C, whereas our tests showed that the pad of the topical cooling device is cooled only to 7" to 1 C. This temperature difference occurs because in the spray circuit the cooling coil is packed in an ice-salt mixture at -1O"C, which produces a more profound cooling effect than the ice-water mixture at C around the coil of the cooling device. Our laboratory studies have shown myocardial necrosis due to freezing, but no evidence of damage in isolated dogs' hearts after two hours of ischemic arrest at 4 C [12]. Damage to the heart is avoided in the topical spray system by continuously monitoring temperature in the fluid inflow line and maintaining it at 4 C or higher. Undoubtedly, the cooling effect of the pad could be improved somewhat by immersing the cooling coil in an ice-salt mixture as is done with the topical spray system. However, this modification could result in pad temperature falling below O"C, thus running the risk of inducing freezing damage to the myocardium. To avoid such excessive cooling, a further modification to the system would then be necessary, namely, installing a temperature-monitoring probe in the inflow line to the pad. Pad temperature could then be controlled by moving the coil in or out of the ice-salt mixture. However, since the main problem with the pad system is an inadequate

8 145 Rosenfeldt and Arnold: Topical Cardiac Cooling by Recirculation area of contact with the heart, it is unlikely that lowering the pad temperature in this way would greatly improve overall cardiac cooling. e conclude that with the heart in the normal position, the cooling pad satisfactorily cools the posterior left ventricular wall but not the anterior left ventricular wall or the interventricular septum. The cooling effect of the topical cooling device depends on maintaining close contact between the pad and the ventricles; this is difficult to achieve in practice. Cooling by the pad can be supplemented by frequent reinfusions of cold cardioplegic solution but this is ineffective in the presence of obstructed coronary arteries. In contrast, the topical spray provides good cooling regardless of the position of the heart or the state of the coronary arteries. The authors acknowledge the assistance given by Mr. B. B. Davis, Mr. G. Shardey, and Mr. G. R. Stirling who used the topical cooling device in their patients. Thanks are due also to Janet Ness, Jenny Griffiths, Michelle Bebbington, Jeanette Dixon, and Mandy Leaman for technical assistance and to Karen Kerr for secretarial help. References Braimbridge MV, Chayen J, Bitensky L, et al: Cold cardioplegia or continuous coronary perfusion? J Thorac Cardiovasc Surg 74:9, 1977 Dingerness SB: Cold cardioplegia versus hypothermia for myocardial protection: randomized clinical study. J Thorac Cardiovasc Surg 76: 577, 1978 Rosenfeldt FL, atson DA: 11. Interference with local myocardial cooling by heat gain during aortic cross-clamping. Ann Thorac Surg 27: 13, Griepp RB, Stinson EB, Shumway NE: Profound hypothermia for myocardial protection during open-heart surgery. J Thorac Cardiovasc Surg 66:731, Rosenfeldt FL, Fambiatos A, Pastoriza-Pinol J, Stirling GR: A recirculating cooling system for improved topical cardiac hypothermia. Ann Thorac Surg 32:41, Bruner JD: Interim report of clinical evaluation of topical cooling device. Report no. P/N27-999, Cobe Laboratories, Lakewood, CO, Hearse DJ, Stewart DA, Braimbridge MV: Cellular protection during myocardial ischemia. Circulation 54:193, 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: 114, Rosenfeldt FL, Hearse DJ, Cankovic-Darracott S, Braimbridge MV: The additive protective effects of hypothermia and chemical cardioplegia during ischemic cardiac arrest in the dog. J Thorac Cardiovasc Surg 7999, Chiu RCJ, Blundell PE, Scott HJ, Cain S: The im- portance of monitoring intramyocardial temperature during hypothermic myocardial protection. Ann Thorac Surg 28:317, Rosenfeldt FL, atson DA: 111. Local cardiac hypothermia: experimental comparison of Shumway's technique and perfusion cooling. Ann Thorac Surg 27:17, Rosenfeldt FL, Arnold A, Fambiotos A, Stirling GR: Myocardial damage due to profound local hypothermia-fact or fiction? J Thorac Cardiovasc Surg (in press)