Ann Thorac Surg 2001;71:872-876
© 2001 The Society of Thoracic Surgeons
Original article: cardiovascular
Proper timing of blood cardioplegia in infant lambs: superiority of a multiple-dose regimen
Kenneth G. Warner, MDa,
Malachi G. Sheahan, MDa,
Sameh M. Arebi, MDa,
Anirban Banerjee, MDb,
Judith M. Deiss-Shrem, CCPa,
Kamal R. Khabbaz, MDa
a Division of Cardiothoracic Surgery, New England Medical Center and Tufts University School of Medicine, Boston, Massachusetts, USA
b Division of Pediatric Cardiology, New England Medical Center and Tufts University School of Medicine, Boston, Massachusetts, USA
Accepted for publication October 18, 2000.
Address reprint requests to Dr Warner, Division of Cardiothoracic Surgery, Box 266, New England Medical Center, 750 Washington St, Boston, MA 02111
e-mail: kwarner{at}lifespan.org
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Abstract
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Background. In the pediatric and infant age groups, it is unclear whether repeated infusions of blood cardioplegia solution during ischemic arrest are beneficial or detrimental when compared with a single-dose regimen.
Methods. Twenty lambs (aged 6 to 7 weeks) were placed on cardiopulmonary bypass. A miniature glass-tip electrode measured myocardial pH and hydrogen ion concentration, [H+], in the anterior wall. The aorta was clamped for 2 hours. Group S (n = 10) received a single dose of blood cardioplegia solution. Group M (n = 10) received multiple doses of blood cardioplegia solution at 20-minute intervals.
Results. The amount of [H+] generated during the cross-clamp period was greater in group S than in group M (39.2 ± 10.1 nmol/L versus 0.4 ± 1.4 nmol/L, p < 0.008). The percent increase in the time constant, tau, an index of diastolic relaxation, was more prolonged after cardiopulmonary bypass in group S when compared with group M (51.4% ± 2.8% versus 6.4% ± 3.0%, p < 0.0001). Similarly, the percent decrease in end systolic elastance, a measure of systolic contractility, was greater in group S after cardiopulmonary bypass when compared with group M (29.5% ± 1.4% versus 7.3% ± 1.3%, p < 0.0001).
Conclusions. In this infant lamb model, multiple doses of blood cardioplegia solution provided superior metabolic preservation and hemodynamic support after 2 hours of aortic clamping when compared with a single-dose regimen.
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Introduction
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A wide variety of cardioplegia delivery techniques are currently used to facilitate intracardiac and extracardiac repairs of congenital heart defects [1]. The lack of a uniformly accepted policy regarding the administration of cardioplegia solution may, in part, reflect the confounding and inconsistent experimental data derived from animal models [2–8]. An important unresolved issue is whether or not multiple doses of cardioplegia solution provide an incremental benefit over an initial dose in nascent hearts. A multiple-dose regimen of cardioplegia solution has been associated with either improved or impaired recovery of ventricular function depending on the experimental protocol [2–4]. Several other studies have shown that repeated dosing of cardioplegia solution is actually detrimental when compared with a single-dose regimen [5–8]. The primary purpose of this study was to assess whether repeated dosing of blood cardioplegia solution is beneficial or deleterious to the preservation of myocardial metabolism and subsequent recovery of ventricular function in infant lambs.
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Material and methods
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Experimental model
Twenty infant lambs (11 to 13 kg) aged 6 to 7 weeks were housed and handled according to the "Guide for the Care and Use of Laboratory Animals" of the National Institutes of Health. The anesthetic agents included intramuscular ketamine (10 mg/kg), intramuscular xylazine (2 mg/kg), intravenous pancuronium (5 mg), and inhalational isoflurane (1% to 2%). Arterial pH was maintained between 7.30 to 7.40 by the appropriate adjustments of the ventilator and the administration of sodium bicarbonate when necessary. Systemic temperature was measured with a 9F rectal probe (model 24031, Medtronic, Minneapolis, MN).
A median sternotomy was performed. An 8- or 10-mm flow probe was placed around the innominate artery (Transonic Systems Inc, Ithaca, NY). A high-fidelity micromanometer 5F catheter (Millar Instruments, Inc, Houston, TX) was placed in the left ventricular (LV) apex for continual measurements of LV pressure and the maximum rate of increase of LV pressure (dP/dT). Two pairs of piezoelectric ultrasonic crystals were placed in the LV for orthogonal measurements of minor and major LV diameters (Sonometrics Inc, London, Ontario, Canada). The micromanometer and sonomicrometer signals were continuously recorded on a model TRXSenS digital ultrasonic measurement system. Myocardial pH and temperature were measured in the anterior wall of the LV with a miniature glass-tip electrode (Vascular Technology Inc, North Chelmsford, MA). A reference electrode was placed in the subcutaneous tissue [9, 10]. Myocardial pH and temperature data were continuously recorded on a computer. A pair of electrical pacing wires were placed on the right atrium and connected to a pacemaker (model 5530, Medtronic Inc). A snare was placed around the inferior vena cava and loosely threaded through a tourniquet. The ascending aorta was cannulated with a 12F cannula (DLP Inc, Grand Rapids, MI). The right atrium was cannulated with a 26F single-stage cannula. Figure 1 illustrates the experimental preparation.
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Fig 1. Illustration of the experimental preparation. (RA = right atrium; LA = left atrium; IVC = inferior vena cava.)
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After a 20-minute period of stabilization, baseline data were recorded. The lambs were then placed on cardiopulmonary bypass (CPB) at a flow rate of 150 mL/kg and cooled to a systemic temperature of 25°C. Arterial blood gases were managed according to the 
-stat principle. An LV vent was placed through the base of the left atrium.
Experimental protocol
After 20 minutes of systemic cooling the aorta was clamped for a period of 2 hours. The heart was arrested with an initial dose of blood cardioplegia solution (20 mL/kg) administered into the aortic root. The cardioplegia solution was composed of blood and a tromethamine-containing crystalloid solution (pH 7.45) mixed at a ratio of 4:1. The cardioplegia solution was delivered at a pressure of 80 to 100-mm Hg and a temperature of 6°C to 8°C. Cold saline solution was topically applied in the pericardial well during the entire cross-clamp period.
The animals were divided into two groups. Group S (n = 10) received a single dose of cardioplegia solution. Group M (n = 10) received multiple doses of cardioplegia solution (10 mL/kg) at 20-minute intervals during the remainder of the cross-clamp period. The initial dose of cardioplegia solution contained 24 mEq/L of KCI; additional doses consisted of 12 mEq/L of KCI. Systemic rewarming was commenced 30 minutes before release of the aortic clamp. The lambs were successfully weaned from CPB at a systemic temperature of 37°C. Inotropic support was not required. Repeat hemodynamic measurements were obtained 20 minutes after weaning from CPB.
Data collection and analysis
Myocardial pH was derived from electrode measurements of the electrical potential and a modification of the Nernst equation [9]. The pH electrodes were calibrated in pH buffers of 4.00 and 7.00 before and after each experiment. In each case the electrode drift was less than 0.05 pH units. Hydrogen ion concentration, [H+], was derived from the formula: [H+] = antilog (-pH). Accumulation of [H+] during the cross-clamp period was calculated by the difference between [H+] measured at the end of cross-clamp and [H+] measured at the beginning of cross-clamp.
Hemodynamic measurements were obtained before and after CPB. In each instance the heart rate was kept constant at 130 beats/min with atrial pacing. Measurements were obtained at a left atrial pressure of 5 cm H2O. The maximal rate of increase of LV pressure (dP/dT) was determined by averaging data between 10 and 20 beats.
Tau, the time constant of relaxation, was derived from the micromanometer measurements of LV diastolic pressure during isovolumetric relaxation by the logarithmic method [11, 12].
Left ventricular pressure-volume loops were generated from the simultaneous sonomicrometer signals and the micromanometer readings of LV pressure. Different loading conditions were produced by tightening the inferior vena cava snare. Left ventricular volumes were estimated according to the ellipsoid formula [13].
The end-systolic elastance, Ees, was determined by calculating the slope of the pressure-volume loops at end-systole generated during inferior vena cava occlusion [13, 14].
The data are reported as the mean ± standard error of the mean. Repeated measures analysis of variance was used to identify differences between the various time periods and the two groups. Unpaired Student’s t tests were used to compare differences between the two groups. The Bonferroni method was used to adjust for multiple comparisons. Statistical significance was achieved at a p less than 0.05.
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Results
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Myocardial temperature data are shown in Table 1. Myocardial temperatures were nearly identical before, during, and after the cross-clamp period between the two groups.
Myocardial pH data for the two groups during the various stages of the experimental preparation are shown in Figure 2. The baseline pH data were identical in both groups: 7.18 ± 0.01. After the initiation of CPB and during the initial 90 minutes of aortic cross-clamping, there was a small increase in myocardial pH largely caused by systemic and myocardial cooling. Myocardial pH subsequently fell during the remainder of the cross-clamp period and the rewarming phase. However the fall in tissue pH was considerably greater during this time period in group S. At the time of weaning from CPB, myocardial pH had recovered in group S and was identical to group M: 7.18 ± 0.01.
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Fig 2. The time courses of myocardial pH for group S (solid circles) and group M (open circles) are shown. During the initial phase of cardiopulmonary bypass (CPB) and the first 90 minutes of cross-clamp (XC), myocardial pH values were similar between the two groups. However during the last 30 minutes of cross-clamp and during the reperfusion phase, myocardial pH values in group S were significantly lower than in group M. At the time of separation from bypass, myocardial pH values were identical in both groups. *p < 0.001.
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There was a minimal accumulation of [H+] during the cross-clamp period in group M (0.4 ± 1.4 nmol/L). In contrast there was a significant accumulation of [H+] during the cross-clamp period in group S (39.2 ± 10.1 nmol/L, p < 0.0001). These data are illustrated in Figure 3.
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Fig 3. The accumulation of hydrogen ion, [H+], during the cross-clamp interval was significantly greater in group S (closed bar) than in group M (open bar).
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Shown in Table 2 are the comparisons for tau, innominate artery flow, the maximum rate of increase of LV pressure, and Ees before and after CPB for the two groups. The rise in tau after CPB reached statistical significance in group S, but not in group M. There were statistically significant decreases in innominate flow, the maximum rate of increase of LV pressure, and Ees in group S. In comparison, the changes in group M were relatively mild.
The percent changes in tau and Ees for the two groups are illustrated in Figure 4. The percent increase in tau after CPB was significantly greater in group S (51.4% ± 2.8%) than in group M (6.4% ± 3.0%, p < 0.0001). Similarly the diminution in Ees after CPB was significantly greater in group S (29.5% ± 1.4%) than in group M (7.3% ± 1.3%, p < 0.0001).
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Fig 4. Shown are the percent changes in the post–cardiopulmonary bypass values of tau and the end-systolic elastance (Ees) when compared with the pre–cardiopulmonary bypass values. The percent changes in tau and Ees were significantly greater in group S (closed bars) than in group M (open bars).
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Comment
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Measurement of myocardial tissue pH has been shown to accurately detect and quantify the degree of ischemic damage during aortic cross-clamping in animal models and in humans [9, 10, 15–17]. However, with the exception of an isolated experimental study [18], the feasibility and potential benefits of pH monitoring have previously been limited to adult or mature hearts. The results of this study illustrate the importance of identifying and quantifying the degree of tissue acidosis in nascent myocardium. The multiple-dose regimen of blood cardioplegia solution minimized myocardial acid production during 2 hours of aortic clamping. This was associated with near complete recovery of both systolic and diastolic function.
Based on the assessment of myocardial pH, myocardial protective strategies that use only a single dose of cardioplegia solution may provide sufficient metabolic protection for up to 90 minutes, However, the data from this study indicates that a single dose failed to sustain adequate metabolic support beyond 90 minutes of aortic clamping, resulting in progressive myocardial acidosis during the remainder of the cross-clamp interval and during the initial reperfusion period. The deleterious effects of persistent acidosis after release of the cross-clamp has also been documented in a clinical study [19]. The progression of tissue acidosis in the group S animals portended relatively poor systolic and diastolic performance after weaning from CPB.
Of particular interest is that the rapid decline in myocardial pH in group S was associated with the commencement of systemic rewarming. The fall in myocardial pH was not the result of regional rewarming as myocardial temperatures measured in the anterior wall were maintained at less than 15°C in both groups during the entire cross-clamp interval (Table 1). However, it is certainly possible that heterogeneous rewarming of the heart occurred during the latter stages of the cross-clamp interval producing regional temperature variations and suboptimal protection in the single-dose cardioplegia solution group.
The selection of blood as the cardioplegic vehicle in this study was based on its efficacy in limiting tissue acidosis in adult hearts [20–22]. The virtual absence of acid accumulation in group M illustrates the importance of buffering acid metabolites at regular intervals during extended periods of aortic clamping in immature hearts.
An important effect of multiple-dose cardioplegia solution on limiting myocardial acidosis is the washout of acid metabolites [23]. The infusion of cardioplegia solution at regular intervals in group M animals intermittently washed out the products of ischemic metabolism including H+. In contrast, there was minimal, if any, washout of acid metabolites after the initial dose of cardioplegia solution in group S animals, resulting in the progressive buildup of H+ during the cross-clamp interval.
In summary this study demonstrates that on-line monitoring of tissue pH in infant hearts provides a continual assessment of the adequacy of myocardial protection. Techniques that limit the amount of acid accumulation during the cross-clamp period, such as the repeated infusions of cardioplegia solution or the use of blood as the cardioplegic vehicle, are associated with better recovery of ventricular function. On-line measurement of myocardial pH during complex congenital heart repairs may lead to advances in clinical preservation techniques, resulting in enhanced ventricular performance and ultimately improved outcomes. Additional clinical studies are indicated to validate this strategy.
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References
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Abstract
Introduction
