Studies of the effects of hypothermia on regional myocardial blood flow and metabolism during cardiopulmonary bypass

Keeping donor hearts in completely beating status with normothermic blood perfusion for transplants

BACKGROUND

Previously, we reported the preservation method of donor hearts in an empty beating status with mild hypothermic perfusion. To completely avoid cardiac arrest and myocardial ischemia, we performed the beating preservation technique from procurement of hearts to transplants and assessed its efficacy for long-term preservation and feasibility for heart transplantation.

METHODS

Thirty-two swine donor hearts were preserved in beating status (group A, n = 8 pairs, perfused continuously with normothermic blood) or in static cold storage (group B, n = 8 pairs, stored in 4°C histidine-tryptophan-ketoglutarate solutions) for 8 hours. Then the donor hearts were implanted either in beating or static status. During transplantation, the incidence of arrhythmia, duration of anastomosis and cardiopulmonary bypass, and dosage of inotropic drugs were recorded. Hemodynamics of left ventricle and serum level of creatine kinase-MB were measured during transplantation. Myocardial ultrastructure was observed.

RESULTS

Compared with group B, in group A the anastomotic time was significantly longer, the cardiopulmonary bypass time was significantly shorter, the cardiac output was larger, and the incidence of arrhythmia, dosage of cardiovascular-active drugs, and serum level of creatine kinase-MB were lower. After declamping for 2 hours and 3.5 hours, the left ventricular hemodynamics of group A was significantly better than that of group B. The myocardial ultrastructure of group A was superior to that of group B.

CONCLUSIONS

Preservation of donor hearts in beating status with continuous, normothermic, blood perfusion is an effective approach for long-term preservation and is appropriate for heart transplantation.

Effects of temperature and sodium on myocardial and hepatocellular fatty acid uptake

Fatty acid influx into human myocardium was studied in 15 patients during the cooling phase of cardiopulmonary bypass at myocardial temperatures of 37 degrees to 25 degrees C. The fitting of the data to a functional relation, developed in this study, revealed fatty acid influx to be a temperature-dependent saturable process corresponding to a Michaelis-Menten constant (Km) at 37 degrees C of 0.26 +/- 0.084 mumol/g, a maximal fatty acid influx velocity (Vmax) at 37 degrees C of 0.28 +/- 0.045 mumol/g per minute, activation energy for fatty acid binding to the putative carrier (E) of 23.8 +/- 5.6 kcal/mol, and a free energy for conformational change of the carrier (U) of 10.9 +/- 8.0 kcal/mol. In short-term cultured hepatocytes, Km increased in the absence of Na+ from 171 +/- 48 to 301 +/- 71 nmol/L, and Vmax of [3H]oleate decreased from 1063 +/- 69 to 847 +/- 68 pmol/min per milligram protein. The fitting of these data to a functional relation revealed a transmembrane potential-dependent component of parameters E and U to be -0.479 and -0.374 kcal/mol, respectively. It is proposed that for fatty acid influx a protonated fatty acid form is preferred that consists of a Na+ complex with the mesomeric form of nondissociated fatty acid from which Na+ and H+ are released during collision with the carrier.

Myocardial tissue pCO2 and calcium content during ventricular fibrillation and reperfusion periods

Forty-one patients who underwent cardiac surgery under conditions of systemic hypothermia and intermittent cold crystalloid potassium cardioplegia were studied, in order to elucidate the effects of ventricular fibrillation and reperfusion on the myocardium, by using the intramyocardial pCO2 and temperature sensor. All patients were assigned to 2 groups, namely; group A (21 cases), in which the time between the aorta declamping and defibrillation was under 10 minutes, and group B (20 cases) in which the time was over 10 minutes. In both groups A and B, myocardial pCO2 increased at the rate of 3.58 +/- 1.70 and 2.16 +/- 0.62 mmHg/min (p less than 0.05) after aorta declamping, respectively and the myocardial pCO2 decreased at the rate of 5.59 +/- 0.60 and 4.18 +/- 0.76 mmHg/min (p less than 0.05) after defibrillation, respectively. In group A, the myocardial calcium content, pre-CPB (cardio pulmonary bypass) was 10.98 +/- 1.62 nmol/mg/dry weight and at the time of aorta declamping it was 15.90 +/- 1.81 nmol/mg/dry weight (p less than 0.05). In group B, the myocardial calcium content, pre-CPB, was 14.62 +/- 2.15 nmol/mg/dry weight and at the time of aorta declamping it was 18.23 +/- 4.36 nmol/mg/dry weight (p less than 0.05). At both three and six hours after the operation, the left ventricular work index per minute (LVWI) in group A showed better cardiac pump function than that in group B. We therefore conclude that when reperfusion is encountered, acidosis can be minimized by prompt defibrillation.

Factors influencing coronary vascular resistance during hypothermia

Factors influencing total coronary vascular resistance (CVR) during hypothermia were studied in 30 mongrel dogs. Complete isolation of the heart in situ was achieved by transection of all cardiac neural and vascular connections in 15 dogs (denervated, Group I). Cardiac innervation was maintained in the other 15 dogs using systemic normothermic cardiopulmonary bypass (innervated, Group II). The aortic root was perfused with heparinized oxygenated blood at a constant flow rate at variable myocardial temperatures. Electromechanical arrest was achieved using potassium chloride (KCl) (25 meq/liter) added to the coronary perfusate. In each group, 5 dogs were maintained at a flow rate of 10 cc/kg/min without KCl and allowed to beat spontaneously, another 5 at a flow rate of 10 cc/kg/min and arrested with KCl, and the remaining 5 at a flow rate of 5 cc/kg/min with KCl. Total coronary vascular resistance was calculated from aortic root pressure, right atrial pressure, and flow rate and expressed in units per 100 grams of cardiac tissue. At 37 degrees C, resistance was lower in the denervated arrested (1.3 +/- 0.2) than in the innervated arrested hearts (2.1 +/- 0.2) (P less than 0.001). Preservation of spontaneous electromechanical activity in the innervated hearts resulted in a lower resistance (0.4 +/- 0.1) (P less than 0.001). A progressive decrease in myocardial temperature to 15 degrees C resulted in a corresponding decrease in coronary vascular resistance to a plateau value (0.5 to 0.7 U) in all arrested groups. The data suggest that at normothermia, innervation increases vascular tone in the coronary vascular bed, while electromechanical activity decreases it.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between perfusion pressure and myocardial microcirculation in the beating empty or spontaneously fibrillating heart

The influence of graded perfusion pressure (30, 60, and 90 mmHg) at cardiopulmonary bypass were studied on beating empty hearts (BEH) or spontaneously fibrillating hearts (SFH) in the normothermic state. The adequacy and distribution of coronary flow and the myocardial oxygen consumption (MVO2) were examined using the tracer microsphere technique in twelve mongrel dogs. In the SFH, the left ventricular (LV) endocardium (ENDO)/epicardium (EPI) flow ratio indicated significant decrease at 30 and 60 mmHg (0.83 +/- 0.05 and 0.86 +/- 0.06, p less than 0.005, respectively), but was recovered to control value at 90 mmHg (1.01 +/- 0.13). In the BEH, these low perfusion pressures did not result in an abnormal flow distribution in the LV (1.03 +/- 0.03 at 30 mmHg). The flow distribution to the right ventricle (RV) relatively increased in both the BEH and the SFH (p less than 0.001). The ENDO/EPI ratio of the RV did not decrease at 30 mmHg in both groups (1.11 +/- 0.03 in BEH and 1.16 +/- 0.08 in SFH). Coronary blood flow and MVO2 were significantly higher in the SFH than in the BEH. Coronary blood flow increased significantly with increase in the perfusion pressure, in both groups. The MVO2 was constant in the BEH, regardless of the perfusion pressure, but increased in the SFH at increasing pressure. These results show that in the SFH, subendocardial underperfusion of the LV is induced at the perfusion pressure of 30 and 60 mmHg.

An experimental circulatory arrest model in the rat to evaluate calcium antagonists in cerebral resuscitation

A circulatory arrest model in the rat was developed for use in cerebral and cardiac resuscitation studies. Whole-body ischemia was produced for 8 to 18 minutes by arresting the heart with a cold potassium chloride cardioplegic solution. Following cardiopulmonary resuscitation, minimal, standardized intensive care was provided. As the duration of ischemia was increased from 8 to 18 minutes, survival immediately following resuscitation decreased from 100% to 25%, and survival at 48 hours after ischemia decreased from 60% to 0%. Thirty per cent of the rats recovering from 11 minutes of ischemia suffered motor seizures. Survival and the incidence of motor seizures appear to be good measures of outcome following ischemic circulatory arrest. These measures can be used to test the possible anti-ischemic actions of calcium antagonists or other drugs.

Myocardial preservation during anoxic arrest. Experimental model ventricular fibrillation

An experimental model with anaesthetized healthy mongrel dogs on extracorporeal circulation is described. Anaesthesia and cardiopulmonary bypass are the same as used in clinical practice. Various methods of myocardial preservation were investigated and their protective effect was judged by cardiac performance after termination of 60 min of anoxic arrest. In this study, the first part of an experimental series, electrically-induced fibrillation during 60 min of normothermic and local hypothermic anoxic arrest was investigated. In group I, the hearts were fibrillated immediately after cross-clamping. In group II, which served as controls, the hearts were allowed to fibrillate spontaneously after aortic cross-clamping. All the hearts in group I went into an ischaemic contracture, whereas those in group II showed a 50% recovery, but with a strongly reduced cardiac performance after termination of anoxic arrest and cardiopulmonary bypass. Measurements of myocardial surface pH demonstrated a rapidly developed acidosis during the period of anoxic arrest. The most impressive finding by light microscopy was pronounced myocardial oedema. External cooling by 4 degrees C glucose 5.5% continuously flushed into the pericardial sac in combination with electrically-induced fibrillation proved to be ineffective as a protective method. None of the eight dogs in this group survived. External cooling combined with intraventricular injection of 4 degrees C glucose 5.5% seemed to protect the hearts against ischaemic damage, insofar that all six hearts in this group were able to take over the circulation after declamping. The working capacity was, however, impaired and a relatively long period of mechanical support and stimulation with inotropic drugs was necessary.

The efficacy of potassium-induced cardioplegia and topical hypothermia in the correction of congenital cardiac lesions

Myocardial metabolic, structural and functional preservation were evaluated in 44 patients who underwent open heart surgery under cardiopulmonary bypass, potassium cardioplegia and topical hypothermia. The duration of cardiac arrest varied from 14 to 87 minutes. Myocardial high-energy phosphates were not only adequately preserved but actually exceeded the control values during periods of cardiac arrest. Glycogen levels were decreased moderately and one to three fold increases in lactate/pyruvate ratios were detected. Myocardial ultrastructure was well preserved. Postoperative clinical recovery of the patients was excellent. The only patient who died from a low output syndrome showed no evidence of myocardial damage. The results of this study serve to illustrate the beneficial effects of potassium cardioplegia and topical hypothermia on myocardial preservation during prolonged periods of ischaemic cardiac arrest, as used for the correction of congenital cardiac lesions.

Improved oxygen delivery to the myocardium during hypothermia by perfusion with 2,3 DPG-enriched red blood cells

The oxygen affinity of red cells increases stepwise with temperature reductions below 37 degrees C. In vitro studies demonstrated that biochemically modified red cells with increased 2,3 diphosphoglycerate (2,3 DPG) (150% and 250% of normal) exhibited significantly less oxygen affinity at 24 degrees C than did unmodified cells. At 15 degrees C, significant attenuation of affinity was observed with 250%, but not 150%, of normal 2,3 DPG cells. Measurements made of isolated fibrillating dog hearts during perfusion at 24 degrees C alternately with unmodified (80% of normal 2,3 DPG) and modified (300% of normal 2,3 DPG) red cells demonstrated significantly greater oxygen consumption, higher coronary sinus partial pressures of oxygen and carbon dioxide, higher in vitro P50 values, and lower arterial and coronary sinus lactate levels during perfusion with modified as compared with unmodified cells. This evidence, indicating improved oxygen delivery to hypothermic dog hearts by red cells with 300% of normal 2,3 DPG activity, suggests that high 2,3 DPG cells might protect myocardial tissue in patients undergoing hypothermic cardiac operation.

Intraoperative changes in coronary resistance during aortic valve replacement

Coronary vascular resistance was investigated in 10 patients undergoing aortic valve replacement using continuous constant-pressure coronary perfusion at 32 degrees C. After coronary flow was initiated, resistance was low but increased steadily until it reached a certain resting level. The plateau was attained faster after a short period of anoxia than after a longer period. The initial postischemic resistance was dependent on the duration preceding anoxia, being of the same magnitude after short and moderate periods of anoxia but significantly higher after a long period. This resistance difference between the groups lasted for the whole perfusion. The total coronary resistance and flow reached a plateau in 30 minutes, while resistance increased threefold but flow decreased to half of the initial postanoxia flow. Our results indicate the importance of initiating coronary perfusion soon after aortic cross-clamping to avoid increase in the initial vascular resistance and subsequent inadequate myocardial flow.

Left ventricular subendocardial necrosis

The hearts of as many as 90% of patients who die after open-heart operations have left ventricular subendocardial necrosis. This form of myocardial infarction depresses myocardial performance postoperatively and may result in late myocardial fibrosis. It occurs without anatomical obstruction of the coronary arteries and is caused by a discrepancy between subendocardial oxygen supply and demand during the perioperative period. This review of subendocardial necrosis summarizes the author's current understanding of: (1) why the subendocardium is especially vulnerable to this injury; (2) how to predict which patients are most susceptible to it; (3) how interventions before, during, and after extracorporeal circulation can either contribute to it, minimize its severity, or prevent it; and (4) where future study of this problem should be directed.