Importance of Initial Coronary Artery Flow After Heart Procurement to Assess Heart Viability Before Transplantation
Background The objective of this study was to evaluate different tests of heart viability in a pig model of warm ischemia.
Methods and Results Pig hearts (n=30) were submitted to 0 (= group I), 10 (group II), 20 (group III), 30 (group IV), and 60 (group V) minutes of in situ warm ischemia (animal exsanguination). Hearts were removed, then flushed with cardioplegic solution for 3 minutes at a fixed pressure of 60 cm H2O, and edema formation, initial coronary flow, and ionic composition (Na+, K+, and Ca++) of coronary sinus effluent were evaluated. Hearts were then stored for 2 hours in a cold (4°C) preservation solution. Myocardial biopsies (and evaluation of energetic index) were performed, then the hearts were reperfused for 30 minutes with whole blood with an in vitro functional testing system. No edema occurred during cardioplegic flush in the hearts in groups I through IV, but a 37±11% weight increase (P<.001) occurred in hearts in group V. There was a progressive decrease in initial coronary flow with the increase in the duration of warm ischemia (70±14 mL/min per 100 g of tissue in group I and 52±9, 41±16, 25±11, and 23±5 mL/min per 100 g, respectively, in groups II through V (P<.01 to P<.001 versus group I). Initial coronary flow was positively correlated with the energetic index (r=.84, P<.001), and the left ventricle developed pressure at reperfusion (r=.90, P<.001). Finally, there were significant differences between hearts in the control group and those in group V for calcium and sodium release (lower in the control group; P<.001 and P<.01, respectively) and for potassium removal (lower in group V, P<.05).
Conclusions These data suggest that early measurement of coronary flow after removal of the heart may help to assess heart viability before transplantation. This approach may provide a comprehensive clinical evaluation to increase the number of hearts available for transplantation among those that are rejected in the absence of accurate criteria of viability.
Although there is a tragic lack of hearts available for transplantation, some hearts are rejected because of the absence of objective criteria of viability. There is no simple in vitro test that can predict heart function after orthotopic transplantation. In fact, the only test available for assessment of myocardial viability for transplantation is transplantation itself. Thus, pretransplant investigation of graft viability appears to be a major challenge of transplantation.1 Proctor et al2 suggested that coronary artery resistance during hypothermic perfusion may be used to assess heart viability, and Suros and Woods3 showed that a 60% increase in coronary resistance was associated with irreversible myocardial damage. Watson et al4 reported a relation between coronary artery flow during hypothermic perfusion and graft viability after transplantation. Other studies have suggested that an increase in coronary artery resistance is related to platelet and vascular thromboxane production,5 adenosine,6 or free radical formation.7 The degree of myocardial edema was also proposed as a viability test. Guerraty et al8 suggested that a 25% gain in heart weight may be a criterion of irreversible damage after heart preservation. On the other hand, Wicomb et al9 and Cooper et al10 reported that myocardial edema was not deleterious for the heart during hypothermic preservation. Others reported that ventricular compliance11 12 13 or intramyocardial pressure14 could be useful for assessment of graft viability. However, these conflicting data need to be confirmed before they are used in a clinical setting. Any test to assess heart viability must be reliable, nontraumatic, and easy and rapid to perform under clinical conditions. The main objective of the present investigation was to study edema formation, early coronary artery flow, and electrolyte exchanges during harvesting to assess heart viability.
Human Animal Care
All animals were treated in accordance with the Principles of Laboratory Animal Care formulated by the National Society for Medical Research and the Guide for the Care and Use of Laboratory Animals prepared by the National Academy of Sciences and published by the NIH (NIH Publication No. 85-23, revised 1985).
Adult pigs of both sexes weighing 25 to 30 kg were anesthetized with ketamine hydrochloride (0.33 mg/kg) for induction and a mixture of fentanyl (0.04 mg/kg) and Pavulon (0.30 mg/kg) for maintenance, as described previously.15 Animals were artificially ventilated, and heparin was given immediately (1000 IU/kg IV). Pigs were exsanguinated and randomized into five groups: group I (n=6) received 0 minute of in situ warm ischemia; group II (n=6), 10 minutes warm ischemia; group III (n=6), 20 minutes warm ischemia; group IV (n=6), 30 minutes warm ischemia; and group V (n=6), 60 minutes warm ischemia. A left thoracotomy was performed quickly, and the hearts were removed and washed in St Thomas’s cardioplegic solution (Aguettant Laboratory) at 4°C.
Assessment of Heart Viability During Cardioplegia
The ascending aorta was immediately cannulated, and coronary arteries were flushed for 3 minutes with the same hypothermic solution at a pressure of 60 cm H2O. Initial coronary flow was expressed in milliliters per minute per 100 grams of myocardial tissue. To assess edema, the hearts were weighed before and after flushing. The ionic composition (Na+, Ca++, and K+) of the coronary sinus effluent was analyzed in hearts in groups I and V. Sodium and potassium were measured with an ion-selective electrode module (614 CIBA Corning), whereas calcium was measured spectrophotometrically at 550 nm, as described by Kessler and Wolfman.16
Left ventricular biopsies were performed, and myocardial samples were frozen in liquid nitrogen for subsequent adenine nucleotide measurements, as described.17 Briefly, frozen biopsies were lyophylized, weighed, crushed to powder, and homogenized with 0.6N perchloric acid (7 mL/g frozen tissue). The supernatant was neutralized with 1 mol/L KOH. The neutralized extract was freed of potassium perchlorate by centrifugation (15 minutes at 1000g and 4°C). Adenine nucleotides (ATP, ADP, and AMP) were separated by high-performance liquid chromatography, as described,17 and an energetic index was calculated according to the formula
After 2 hours of cold storage in St Thomas’s solution, a latex balloon was introduced into the left ventricle and connected to a pressure transducer with a recorder. Hearts were then reperfused with 600 mL conserved, heparinized whole blood at 38°C. The initial pH of the solution was 7.2 to 7.4 after equilibrium with a mixture of 95% O2/5% CO2 and 20 mL Na2CO3 at 4.2%. Perfusion pressure was gradually increased from 2.9 KPa (30 cm H2O) at the beginning of reperfusion to 4.9 KPa (50 cm H2O) at 20 to 30 minutes. At baseline and after 15 and 30 minutes of reperfusion, the functional state of the heart was evaluated by measuring left ventricular developed pressure (LVDP).
Results are expressed as the mean±SD. Groups of pigs were compared by ANOVA (Fisher’s test). Difference was considered significant when the P value was <.05. Correlation significance was obtained with the Fisher-Yates statistical tables.
Effect of In Situ Warm Ischemia on Myocardial Edema
Statistical comparisons were performed among all groups (F=70.89, P=.0001). During the cardioplegic flush, there was no weight increase in group I hearts. In hearts in groups II, III, and IV, there was a mild, nonsignificant weight increase (4.1±0.8%, 3.1±2.8%, and 3.2±1.5%, respectively). In contrast, there was a significant weight increase (37±11%, P<.001, Fig 1⇓), in hearts in group V.
Effect of In Situ Warm Ischemia on Initial Coronary Flow
During the cardioplegic flush, mean coronary artery flow in the control hearts (group I) was 70±14 mL/min per 100 grams myocardial tissue. Results in all groups were different (F=21.85, P=.0001). A decrease in coronary flow was observed in groups II through V: group II, 52±9 mL/min per 100 grams (P<.01 versus group I); group III, 41±16 mL/min per 100 grams; group IV, 25±11 mL/min per 100 grams; and group V, 23±5 mL/min per 100 grams (P<.001 versus group I). Differences in hearts in groups II and III were also statistically significant in those in groups IV and V (Fig 2⇓).
Effect of In Situ Warm Ischemia on the Ionic Composition of Coronary Sinus Effluent
During the cardioplegic flush, coronary sinus effluents of each heart in groups I and V were collected and their electrolyte contents (Na+, K+, and Ca++) were measured and compared (F=26.27, P=.0001). Ionic concentrations of the input medium (St Thomas’s solution) were substracted from those of the coronary sinus. Thus, when electrolytes were released from cardiomyocytes, the concentration was above the baseline, whereas when electrolytes were removed from the medium by cardiomyocytes, the concentration was below the baseline (Fig 3⇓). Significant removal of calcium and sodium was observed in hearts in group V compared with group I (−0.05±0.02 mmol/L versus +0.11±0.02, respectively, for calcium, P<.001, and −7.74±3 mmol/L versus +0.88±0.7, respectively, for sodium, P<.01), whereas we found no significant difference between groups with regard to potassium: −0.41±0.3 versus −1.15±0.09 mmol/L.
Relation With Initial Coronary Flow
The energetic index was positively correlated with the initial coronary flow during cardioplegia (r=.84, P<.001, Fig 4⇓). A significant correlation was also found between the initial coronary flow measured during cardioplegic flushing and the maximal LVDP (Fig 5⇓) assessed during reperfusion (r=.90, P<.001).
The only test now available for assessing myocardial viability for transplantation is transplantation itself. Thus, harvesting and preservation must be done as quickly as possible to avoid myocardial injury during cold ischemia. Surgical teams must work according to emergency procedures. Moreover, in the absence of accurate criteria of viability, it is accepted that the heart must not be transplanted beyond 4 to 6 hours of cold ischemia. Finally, despite these precautions, some hearts considered viable before harvesting show irreversible failure after transplantation. Obviously, there is a need for a reliable test of heart viability before transplantation to help to reduce the incidence of graft failure and to increase the number of hearts available for transplantation. Furthermore, the ability to evaluate graft viability would permit hearts to be preserved beyond the present limit of time.
We previously developed an experimental cardiac hypothermic preservation and biophysical data recorder system19 that enables us to measure coronary artery flow, myocardial perfusion flow, and edema formation in a real-time manner. Our previous data suggested that coronary flow and edema may be good indicators of heart viability. In the present study, edema formation during cardioplegia was not correlated with duration of warm ischemia. Except in the hearts in group V, which underwent 60 minutes of in situ warm ischemia, no weight gain was observed among the groups, despite an evident myocardial injury. We conclude that edema evaluation during cardioplegic flush is not a reliable index of viability in this model. In contrast, the initial coronary flow correlated well and inversely with the increase in in situ warm ischemia. Coronary flow and coronary resistance are two correlated parameters, expressed as follows:
where CR is the coronary resistance; PP, the perfusion pressure; Wt, the heart weight; and CF, the coronary flow. Because the coronary perfusion pressure was fixed at 60 cm H2O and the heart weight was stable (except in group V), coronary flow reflected coronary artery resistance. If the whole coronary system is considered a unique blood vessel, then coronary resistance could be expressed with the Poisseuille law as follows:
where μ is the viscosity of the cardioplegic medium and l and r are the length and radius of the vessel, respectively. Two parameters could vary: the length could decrease when arteriovenous shunt occurred, and the radius could increase with arterial vasomotion. Therefore, the increase in coronary resistance involved a vasoconstriction phenomenon. We thus hypothesize that one of the main consequences of warm ischemia is coronary artery wall injury and that initial coronary flow measurement may be a good predictor of severity of heart injury. As a matter of fact, we found a strong positive correlation between LVDP during reperfusion and coronary flow during cardioplegic flush. However, warm ischemia may also induce depletion of high-energy compounds, and the myocardial energetic index was strongly correlated with initial coronary flow. Thus, initial coronary flow may be used as an indicator of myocardial energy–reserve status. Finally, electrolyte exchanges in myocardial tissue were shown to be altered by warm ischemia. A significant calcium accumulation occurred in the cardiomyocytes subjected to 60 minutes of ischemia (group V), which suggests formation of incipient stone heart.20 In addition, our data indicate significant sodium retention in the ischemic (group), which is in accordance with the measured weight increase that is probably due to water accumulation. Finally, there was also a significant difference among groups in potassium concentration, suggesting potassium loss and cardiomyocyte injury.
Any technique aimed to evaluate heart viability for human surgery must be simple, safe, rapid, reliable, and easy to perform in a clinical setting. In such a context, biochemical evaluations, such as energetic compound measurements, are very time consuming and therefore unsuitable. In contrast, measurement of coronary flow at the time of cardioplegic flush lasts 3 minutes but requires only aortic cannulation and careful weighing of the heart and coronary effluents. This test could be used without complex and costly apparatus. Obviously, these data have to be confirmed on an ischemic pig heart model to include orthotopic transplantation and evaluation of the in situ working heart. We also plan to evaluate the viability of cadaveric beating hearts that are not considered suitable donor candidates under the current rules of human grafting.
A 3-minute coronary artery flow measurement during cardioplegic flush after heart harvesting may be a reliable predictor of heart viability and may permit procurement and transplantation of hearts now considered unsuitable for transplantation. A better estimate of initial graft status may help to improve prognosis.
- Received September 15, 1994.
- Accepted November 25, 1994.
- Copyright © 1995 by American Heart Association
Fahy GM. Viability assessment. In: Karow AM, Pegg DE, eds. Organ Preservation for Transplantation. New York, NY: Dekker; 1981;53-73.
Proctor E, Matthews G, Archibald J. Acute orthotopic transplantation of hearts stored for 72 hours. Thorax. 1971;26:99-101.
Watson DC, Gregg DL, Rhodes GR. Survival after orthotopic transplantation of hearts preserved for 48 hours. Surg Forum. 1976;255-257.
Vanhaecke J, Flameng W, Bargers M, et al. Evidence for decreased coronary flow reserve in viable postischemic myocardium. Circ Res. 1990;67:1201-1210.
Magovern GJ, Bolling SF, Casale AS, Bulkley BH, Gardner TJ. The mechanism of mannitol in reducing ischemic injury: hyperosmolarity or hydroxyl scavenger. Circulation. 1984;70:91-94.
Wicomb W, Boyd ST, Cooper DKC, Rose AG, Barnard CN. Ex vivo functional evaluation of pig hearts subjected to 24 hours preservation by hypothermic perfusion. South Med J. 1981;60:245-248.
Cooper DKC, Wicomb WN, Barnard CN. Storage of the donor heart by a portable hypothermic perfusion system: experimental development and clinical experience. Heart Transplantation. 1983;2:104-110.
Kresh JY, Cobanoglu MA, Brockman SK. The intramyocardial pressure: a parametre of heart contractility. Heart Preservation. 1985;4:241-246.
Kessler G, Wolfman M. An automated procedure for the determination of calcium and phosphorus. Clin Chem. 1964;10:686-703.
Ferrera R, Larèse A, Marcsek P, Guidollet J, Verdys M, Dittmar A, Dureau G. Comparison of different techniques of hypothermic pig heart preservation. Ann Thorac Surg. 1994. In press.
Ferrera R, Marcsek P, Jossinet J, Delhomme G, Larèse A, Dureau G, Dittmar A. Tissular biophysic and thermic parameters study during cardiac graft hypothermic preservation. Innov Tech Biol Med. 1991;12:502-516.
Dureau G, Ferrera R. Principles of organ perfusion and reperfusion with special reference to the heart. In: Libbey J, ed. Research in Organ Transplantation. 1994. In press.