Ann Thorac Surg 2001;72:804-809
© 2001 The Society of Thoracic Surgeons
Original article: cardiovascular
Regional topical hypothermia of the beating heart: preservation of function and tissue
Daniel S. Schwartz, MDa,
Ross M. Bremner, MDa,
Craig J. Baker, MDa,
Kanti M. Uppal, MDa,
Mark L. Barr, MDa,
Robbin G. Cohen, MDa,
Vaughn A. Starnes, MDa
a Department of Cardiothoracic Surgery, University of Southern California Keck School of Medicine, Los Angeles, California, USA
Address reprint requests to Dr Schwartz, Department of Cardiothoracic Surgery, University of Southern California School of Medicine, 1510 San Pablo St, Suite 415, Los Angeles, CA 90033
e-mail: dschwartz{at}surgery.usc.edu
Presented at the Poster Session of the Thirty-seventh Annual Meeting of The Society of Thoracic Surgeons, New Orleans, LA, Jan 29–31, 2001.
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Abstract
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Background. Protection of the myocardium during beating heart operations is paramount. The goal of this study is to determine if regional topical hypothermia (RTH) preserves myocardial viability and function during periods of temporary coronary artery occlusion.
Methods. Sixteen pigs were divided into two groups (RTH and control). Each group received 40 minutes of midleft anterior descending coronary occlusion followed by 3 hours of reperfusion. The RTH group (n = 10) received RTH and the control group (n = 6) received no cooling. Myocardial and core temperatures were measured with thermistors. Sonomicrometers and micromonameters were used to determine load independent indices of myocardial function. These indices were measured at base line, during coronary occlusion, and at 3 hours of reperfusion. The myocardium at risk and the infarct area were determined with monastral blue dye and triphenyl tetrazolium chloride staining.
Results. The mean myocardial temperature in the risk zone during coronary occlusion was significantly less in the RTH group (29.4°C ± 5.6°C versus 35.7°C ± 1.1°C, p < 0.05). After 40 minutes of coronary occlusion, both the RTH group and control had a significant reduction in regional elastance (9.38 ± 3.54 and 11.05 ± 1.67 mm Hg/mm) compared with base line measurements (14.70 ± 2.42 and 16.80 ± 4.79 mm Hg/mm), p < 0.05. However, after 3 hours of reperfusion, the elastance returned to base line levels in the RTH group (15.83 ± 3.06 mm Hg/mm) but remained significantly depressed in the control group (9.97 ± 3.63 mm Hg/mm, p < 0.04). Myocardial necrosis as a percentage of the risk zone was significantly less in the hypothermia group (25% ± 2% versus 62% ± 5%, p < 0.001).
Conclusions. Regional topical hypothermia during isolated temporary coronary occlusion provides regional myocardial protection expressed as a return of function and decreased necrosis. Regional topical hypothermia may be clinically applicable to myocardial preservation during beating heart operations.
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Introduction
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Protection of the regional myocardium at risk during temporary coronary artery occlusion remains an important consideration in this era of beating heart operations. In fact, several studies have recently demonstrated that the myocardial infarct rate associated with off-pump techniques is higher than that with conventional on-pump coronary bypass grafting [1, 2]. One explanation for this increased rate of injury may be the ischemic and reperfusion damage caused by temporary occlusion of the native coronary while the anastomosis is performed.
The use of systemic and global myocardial cooling to protect the heart is well known [3–7]. Myocardial cooling is effective because it slows cellular metabolism, decreases energy utilization, maintains ion homeostasis, and possibly (although controversial) decreases oxygen demand [8, 9]. Systemic cooling requires either a perfusion circuit with heat exchanger and or an extended period to cool and then rewarm the subject by application of a heating or cooling blanket. These requirements minimize the benefits realized by off-pump coronary operations because off-pump techniques avoid the cardiopulmonary bypass circuit and avoid systemically cooling the patient. We propose that regional cooling of the heart may protect the myocardium at risk of ischemia without the problems associated with the bypass circuit or whole body hypothermia.
The purpose of this study was to test the hypothesis that regional topical hypothermia (RTH) can be used as a therapeutic measure to preserve heart function and limit myocardial tissue damage during temporary coronary occlusion of the beating heart. We used a method to induce regional hypothermia that involved cooling the anterior surface of the porcine heart by the application of an ice-filled bag.
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Material and methods
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Study groups
Sixteen healthy swine (30 to 40 kg) were randomly divided into treatment (RTH) and control groups at the time of operation. Both groups underwent 40 minutes of midleft anterior descending coronary occlusion followed by 3 hours of reperfusion. In the RTH group (n = 10), the myocardium at risk was topically cooled during the period of coronary occlusion with the application of an iced-filled bag of slush to that region. In the control group (n = 6), the myocardium at risk was not cooled. All animals received humane care in compliance with the "Principles of Laboratory Animal Care" (National Society for Medical Research) and "Guide for the Care and Use of Laboratory Animals" (Institute of Laboratory Animal Resources, National Research Council; published by the National Academy Press, revised 1996). This study was approved by the University of Southern California Keck School of Medicine, Laboratory Research Animal Review committee and conducted according to the University of Southern California Keck School of Medicine policy.
Surgical preparation
All animals were premedicated with intramuscular atropine (0.05 mg/kg). Anesthesia was administered with xylazine (4 mg/kg) and Telazol (tiletamine/zolazepam; 4 mg/kg; Fort Dodge Animal Health, Fort Dodge, IA) also given as an intramuscular injection. After endotracheal intubation, maintenance of deep anesthesia consisted of 1% to 3% isoflurane. Muscle paralysis was obtained by administration of pancoronium (0.1 mg/kg). All animals received a continuous infusion of intravenous lidocaine (50 µg · kg-1 · min-1) and heparin (100 U/kg). The animals were monitored throughout the entire procedure with electrocardiography, femoral arterial pressure, and oxygen saturation. Arterial blood gases and pH were kept in the normal range by adjusting inspired oxygen concentrations and minute ventilation or by the administration of bicarbonate. Electrolyte and hematocrit levels were also monitored at appropriate intervals throughout the surgical procedure. No blood transfusions were given in any of the animals. A standard median sternotomy was performed and the pericardium was opened. A 16-gauge catheter was placed into the left atrial appendage for administration of dye and euthanasia at the end of the procedure. In addition, a 4-0 monofilament suture was placed around the midleft anterior descending artery and the ends were placed through flanged tubing to provide temporary vascular occlusion. A 22-gauge thermocouple probe (OmegaCorp, Stamford, CT) was inserted obliquely into the myocardium in the area supplied by the distal left anterior descending artery (the ischemic risk zone) and into the right ventricular outflow tract (nonrisk zone). Heart rate, blood pressure, and core body temperature were recorded throughout the study. Core temperature was maintained by placing a heating pad directly beneath the animal and by maintaining the ambient room air temperature at 25°C.
Left ventricular segment dimensions and pressure measurements
Instantaneous myocardial segment dimensions were measured with subepicardial pulse-transit ultrasonic dimension transducers (Sonometrics, Ontario, CA). A pair of ultrasonic crystals was sutured into the epicardium of the left ventricle and was oriented across the major axis of the left ventricle in the area at risk of ischemia. The crystals were then connected to a sonomicometer (Sonometrics, Ontario, CA) for calibration and recording of analog output. Left ventricular pressure was measured and recorded continuously with an 8 mm micromanometer (Millar Instruments, Houston, TX) placed directly into the apex of the left ventricle.
Myocardial contractile data acquisition and analysis
Left ventricular pressure and myocardial segment dimensions were measured just before coronary occlusion (base line), at the completion of 40 minutes of coronary occlusion, and at 3 hours after the reperfusion period. At each sampling time, left ventricular pressure and segment dimension data were collected over a range of end-diastolic volumes produced by transient inferior vena caval occlusion. Left ventricular pressure and segment dimensions were sampled and recorded at 5-millisecond intervals and digitally stored onto hard disk by a microprocessor. All data processing was performed by cardiac functional data software (CardioSOFT, Ontario, CA). Values for the maximal elastance of the end systolic pressure–length ratio at end systole (Emax), x-intercept of the end diastolic pressure–length relationship (Lw), and correlation coefficient were calculated for each study condition. The method for measuring cardiac function was based on algorithms developed by other investigators [10, 11].
Core and myocardial temperature
Core body temperature was recorded with a rectal probe and myocardial temperatures were measured by a 22-gauge thermistor needle placed in the anterior-apical left ventricle (area at risk distal to the occluded coronary artery) and a thermistor placed in the right ventricular outflow tract (area not at risk). All temperature probes were connected to an electronic thermometer (Omega Corp, Stamford, CT). The probe is specified to be accurate to within 0.1°C. The porcine left ventricular wall is approximately 8 to 10 mm thick and the probe was inserted midway within the wall. The temperature indicated by the probe reflects a temperature gradient across the wall from warm blood in contact with the endocardium to the iced saline bag in contact with the epicardium.
To ensure proper placement of the thermistor within the myocardium and not the ventricular cavity, cool saline was placed over the surface of the heart overlying the probe. An immediate reduction in temperature indicated proper placement. Measurements were recorded before left anterior descending artery ligation, every 5 minutes during coronary occlusion, and every 30 minutes during reperfusion.
Induction of topical hypothermia
A bag containing iced saline slush (0°C) was placed in direct contact with the epicardial surface, cooling the heart by conduction. The bag cooled the entire region of the left anterior ventricle that was expected to become ischemic during coronary occlusion.
Myocardial infarct size
After 3 hours of reperfusion the left anterior descending artery was reoccluded with the 4-0 monofilament suture snare. The hearts were injected with 25 mL of monastral blue dye (Ciba-Geigy, Hawthorne, NY) introduced through the left atrial catheter and allowed to circulate. The animals were then euthanized by an overdose of intravenous xylazine and potassium chloride. The hearts were excised and rinsed with saline and both ventricles were separated from the atria along the atrioventricular groove. The ventricles were then sectioned into eight equal portions transverse to the long axis of the heart. All sections were incubated in 1% triphenyl tetrazolium chloride preheated at 37°C for 10 minutes and then weighed [5, 7]. All sections were preserved in formalin.
Perfused viable myocardium was stained blue. Myocardium at risk of ischemia was not colored by the blue dye (Fig 1). Necrotic myocardium was identified as a pale nonstained area after incubation in the triphenyl tetrazolium chloride. All slices of myocardium were then photographed digitally. These images were planimetered using a graphic digitizer and analyzed. Areas at risk for ischemia and the necrotic areas were calculated as a proportion of the two ventricles as described in previous reports [5]. These areas were then multiplied by the weight of the slice and then summed. Prospective exclusion criteria were an ischemic zone of less than 10% of the left ventricular weight.
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Fig 1. Digital photograph from representative transverse left ventricular sections in both the control group (A) and the regional topical hypothermia group (B). Normal perfused myocardial tissue is stained blue (NL). The myocardium at risk of ischemia is not stained (outlined in yellow). The proportion of necrotic tissue (outlined in white) to myocardium at risk is greater in the control specimen than the specimen treated with regional topical hypothermia.
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End points and data analysis
The following end points were measured: core body temperature, myocardial risk zone temperature, mean arterial blood pressure, heart rate, myocardial regional elastance (end-systolic pressure–length relationship) and x-intercept (end-diastolic pressure–length relationship), area at risk, and infarct size. All data summary and statistical analysis were performed using SPSS (SPSS Inc, Chicago, IL). Core temperature, heart rate, and blood pressure were analyzed by repeated-measures analysis of variance. Statistical analysis of the effects of hypothermia on elastance, x-intercepts, infarct size and area at risk were compared by Student’s t test. Comparison between groups was by two-way analysis of variance. The Tukey test was used to test for differences among the individual means. Statistical significance was accepted at p less than 0.05. All data are expressed as mean ± standard error of the mean.
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Results
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Animal population
Eighteen pigs were studied. One pig, in the control group, died from fatal arrhythmias during coronary occlusion and was excluded from analysis. Two additional pigs, 1 in each group, developed ventricular fibrillation during reperfusion. Each responded immediately to direct electrical cardioversion (10 to 30 J) and data were included. There was 1 animal excluded based on prospective exclusion criteria of an area at risk of ischemia of less than 10%. Data are presented here for 10 pigs in the RTH group and for 6 pigs in the control group.
Hemodynamics
Heart rates were similar in both groups at base line and tended to be slightly higher in the control group throughout the protocol (Fig 2). This difference was not significant. Mean arterial blood pressure was also similar in both groups at base line and did not vary significantly.
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Fig 2. Changes in hemodynamics over time. There were no significant differences in heart rates and mean arterial pressure between the groups. (RTH = regional topical hypothermia group.)
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Myocardial temperature
Myocardial temperature (Fig 3) was similar in the RTH and control groups at base line (36.0°C ± 0.8°C and 36.9°C ± 0.6°C, respectively). Cooling initiated during coronary occlusion reduced the myocardial temperature in the risk zone from 36.0°C ± 0.8°C an average of 6°C within 5 minutes to an average of 29.4°C ± 5.6°C. Cooling remained in this range throughout the entire period of occlusion. This was significantly less than the control group temperature (35.7°C ± 1.1°C, p < 0.05) during the same time period. Upon reperfusion the average temperature increased in the RTH group to 36.3°C ± 0.9°C within 5 minutes of reperfusion. This change was likely a result of the reintroduction of warm blood to the ischemic area. The temperature in the RTH group remained at base line levels (36.4°C ± 1.0°C) for the remainder of the study period. In the control group the temperature decreased slightly after occlusion to 35.6°C ± 1.1°C. After reperfusion the myocardial temperature in this group increased 2°C to an average of 37.5°C ± 1.1°C. The control group reperfusion temperature trended above base line (37.5°C ± 1.1°C and 36.9°C ± 0.6°C); however, this difference was not significant. This increase in temperature may have been due to a response to reperfusion injury. There were no significant differences in the rectal temperature or the myocardial temperature outside the risk zone in both groups (Fig 3).
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Fig 3. Average rectal temperature (top), myocardial temperature (middle), and the myocardial risk region of the myocardium (bottom, area distal to the site of coronary occlusion) in the control group and regional topical hypothermia (RTH) group. The control group received no intervention. Cooling was started in the RTH group at time zero (coronary occlusion). There was a significant drop in myocardial risk zone temperature (average 6°C) within 5 minutes of topical ice application. *Differences determined by repeated-measures analysis of variance.
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Risk region and infarct size
Left ventricular weight, the myocardium at risk of ischemia, and necrotic myocardium (expressed as a weight and a percentage of left ventricle) were comparable between groups (Table 1). Myocardial necrosis as a percentage of the risk zone was significantly less in the RTH group (25% ± 2%) than in the control group (62% ± 5%, p < 0.001).
Figure 4 demonstrates the relationship between the weight of necrotic tissue and the weight of myocardium at risk in both groups. Analysis of variance for group effect revealed a significant difference (p < 0.05). The slope of the regression line of the RTH group was less than that of the control group, indicating that for the same ischemic zone at risk, the RTH group had a significantly smaller infarct size.
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Fig 4. Scatter plot of the quantity of necrotic myocardium plotted against the quantity of the myocardium at risk for both groups. The solid line represents the regression line for the regional topical hypothermia group (RTH) and the dashed line represents the regression line for the control. On average for any given size of risk zone, a significantly smaller infarct developed in the animals treated with topical hypothermia.
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Myocardial functional analysis
When the instantaneous left ventricular intracavitary pressure was plotted against the regional left ventricular segment length, pressure–length work loops resulted. At base line, regional myocardial function did not differ between the groups (Table 2). There were no significant differences in the x-intercepts, elastance, and regression coefficients. After 40 minutes of left anterior descending artery occlusion (Fig 5), there was a reduction in regional myocardial elastance in both the RTH group (9.38 ± 3.54 mm Hg/mm) and control group (11.05 ± 1.67 mm Hg/mm) when compared with base line values (14.70 ± 2.42 mm Hg/mm and 16.80 ± 4.79 mm Hg/mm, p < 0.05). After the 3-hour reperfusion, regional myocardial elastance returned to near base line levels in the RTH group (15.83 ± 3.06 mm Hg/mm) but was significantly depressed in the control group (9.97 ± 3.63 mm Hg/mm, p < 0.04).
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Table 2. Hemodynamics and Regional Myocardial Function in Swine Undergoing 40 Minutes of Coronary Occlusion and 3 Hours of Reperfusion
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Fig 5. Elastance of the end-systolic pressure–length relationship represented as bar graphs of the mean values ± standard error of the mean recorded at base line (before coronary occlusion), after 40 minutes of coronary occlusion, and at 3 hours of myocardial reperfusion. Both groups demonstrated a significant reduction in elastance after 40 minutes of ischemia; however, after 3 hours of reperfusion the regional topical hypothermia (RTH) group’s values returned to base line and the control groups value remained depressed.
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Comment
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The goal of our study was to produce regional myocardial cooling during coronary occlusion. The surface temperature of the heart was lowered by the application of an ice-filled bag. Using this method, we were able to quickly cool the portion of the heart at risk for ischemia by an average of 6°C within 5 minutes. By comparison, the mean temperature of the control group changed only 1°C during this time.
We found that topically applied regional hypothermia, during acute coronary occlusion, limited damage from ischemia. When the myocardial temperature in the risk zone was decreased 6°C during coronary occlusion, infarct size was reduced by 60%. Our data also showed that regional hypothermia preserved myocardial function by the return of myocardial elastance to near base line levels in the RTH group and not the control. This reversal may be an indication that the reperfusion injury after ischemia may also be ameliorated by temperature reduction. These data favor the potential use of topical hypothermia as a therapeutic measure to protect the regional myocardium during coronary occlusion of the beating heart.
Differences in base line characteristics or hemodynamics during this study cannot explain the differences in mean infarct size or myocardial functional recovery. Both groups had similar average heart rates and blood pressure. Myocardial temperature (not at risk) and core body temperature were also similar in both groups.
Porcine myocardium, as opposed to canine and human myocardium, has little collateral blood flow [8]. One limitation of this study is that we tested our hypothermia model on porcine hearts that did not have coronary artery disease and thus the hearts did not have the opportunity to develop collateral circulation. Swine were chosen to minimize the variability in infarct size, which results from interanimal differences in collateral blood flow, because this species has a negligible innate collateral circulation [8]. In humans, the development of infarction may occur more slowly because humans have more collateral blood flow. Collateral blood flow becomes even more pronounced in human hearts that have chronic coronary artery disease. Another limitation of this study is that we did not demonstrate that coronary snare occlusion was able to produce a regional reduction in myocardial blood flow. However, this point has been investigated in previous studies [7, 8], and coronary snare occlusion has been shown to substantially reduce blood flow to the area at risk in swine. It was for this very reason that we prospectively excluded animals that did not have greater than 10% of the myocardium at risk.
Porcine hearts are notoriously vulnerable to ventricular arrhythmias. To counteract the effects of hypothermia and ischemia on inducing arrhythmias, lidocaine was infused throughout the study in both groups. All porcine hearts were meticulously handled to avoid arrhythmia induction. Despite these measures, ventricular fibrillation occurred in 3 of 18 animals (16.7%). Two of these animals were converted back into sinus rhythm with direct electrical cardioversion, and 1 animal died (5.5%).
We chose 40 minutes of coronary occlusion in this porcine model based on a previous study [8], which demonstrated that 40 minutes was needed to produce a probable infarct in the ischemic zone. We realized that in humans an off-pump coronary anastomosis generally takes less time. However, our data demonstrate that even when coronary occlusion was extended to 40 minutes, we were able to significantly reduce the severity of injury by topically cooling the heart.
Studies in animals have shown a correlation between blood or myocardial temperature and infarct size [3–8]. Dunker and coworkers [8] studied the effect of temperature on infarct size in swine. In that study swine core body temperatures were cooled to 35°C by the application of iced towels to the skin. They were rewarmed by application of a heated pad and a warm blanket. There was a strong correlation between body core temperature and the proportion of the ischemic risk zone that became necrotic. Whole body hypothermia has also been shown to regionally protect ischemic myocardium [6]. Infarct size was reduced by 30% in hypothermic dogs that underwent 5 hours of coronary occlusion. Our study showed that regional hypothermia protects the myocardium to a significant extent without the need for core cooling and the time required to systemically rewarm.
The technique of regional cooling of the myocardium is easily translated to the clinical setting. Heart muscle at risk for ischemia can be cooled by application of a small plastic bag filled with iced saline or by application of a small cooling jacket placed distal to or incorporated into the stabilizing device.
In summary, regional myocardial cooling after isolated coronary occlusion in swine provides protection against progression of necrosis with a return of function. Topical regional hypothermia is a simple and effective technique that may be clinically applicable to preserve myocardium during beating heart operations.
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Acknowledgments
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This work was supported in part by an Educational and Research Grant from Medtronic, Inc, Minneapolis, MN. %We acknowledge the excellent technical assistance of Paul and Linda Kirkman.
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Footnotes
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Daniel S. Schwartz, MD, was supported in part by a seed grant from the University of Southern California Keck School of Medicine and the Hastings Foundation.
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References
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Abstract
Introduction
