Year : 2007 | Volume
: 10 | Issue : 1 | Page : 34--41
Treating metabolic impairment and myocardial stunning with phosphodiesterase inhibitor type III, milrinone, administered prior to coronary artery occlusion in the presence of calcium channel blockade in pigs
Avner Sidi, Jochen D Muehschlegel, David S Kirby, Emilio B Lobato
Department of Anesthesiology, University of Florida College of Medicine and Anesthesia Service, Malcom Randall Veterans Administration Medical Center, Gainesville, Florida., USA
Department of Anesthesiology, University of Florida College of Medicine, Box 100254, JHMHSC, 1600 SW Archer Road, Gainesville, Florida.
This study examined milrinone effects on ischaemic myocardial metabolism and function with calcium blockade. We studied 15 pigs in 3 groups: group C received no drugs; group D received diltiazem 5 mg bolus followed by infusion; group D+M received diltiazem and milrinone (50 µg/Kg). The left anterior descending (LAD) artery was then occluded for 15 minutes. Left ventricular (LV) function data obtained included rate, pressures, output, Emax, and dP/dT. Blood lactate was obtained from the LAD and circumflex vessels at baseline, end of occlusion, early (15 min) and late (1 hour) reperfusion. In group D+M, less depression of LV function occurred during ischaemia and early reperfusion. Lactate extraction in the LAD region was less negative in D+M group than in the group without milrinone during ischaemia and late reperfusion. We conclude the preemptive administration of milrinone prior to ischaemia added to calcium blockade improved myocardial function and ischaemic metabolic effects.
|How to cite this article:|
Sidi A, Muehschlegel JD, Kirby DS, Lobato EB. Treating metabolic impairment and myocardial stunning with phosphodiesterase inhibitor type III, milrinone, administered prior to coronary artery occlusion in the presence of calcium channel blockade in pigs.Ann Card Anaesth 2007;10:34-41
|How to cite this URL:|
Sidi A, Muehschlegel JD, Kirby DS, Lobato EB. Treating metabolic impairment and myocardial stunning with phosphodiesterase inhibitor type III, milrinone, administered prior to coronary artery occlusion in the presence of calcium channel blockade in pigs. Ann Card Anaesth [serial online] 2007 [cited 2019 Sep 16 ];10:34-41
Available from: http://www.annals.in/text.asp?2007/10/1/34/37922
Transient coronary artery occlusion is associated with metabolic derangement that can result in prolonged myocardial dysfunction. , Myocardial dysfunction can occur in different clinical settings and interventions. Milrinone has both inotropic and vasodilatory properties that, like other phosphodiesterase (PDE) inhibitors, improve myocardial function without increasing oxygen consumption. , This protective effect is probably based on its ability to improve left ventricular (LV) performance and increase blood flow to the ischaemic myocardium.  However, milrinone in the presence of myocardial ischaemia or depression, and other frequently used "protective" drugs such as beta- and calcium channel-blockers, has been investigated infrequently. , Patients with ischaemic cardiovascular disease are often treated with betaadrenergic and calcium channel blockers, and the implications of incorporating a PDE inhibitor in their treatment could be crucial.
Calcium channel blockers are frequently used in patients with coronary artery disease who are undergoing surgery. However, the presence of severe myocardial ischaemia in patients treated with calcium channel blockers may prolong myocardial dysfunction compounded by the negative inotropic effects of calcium channel blockade. PDE inhibitors, such as milrinone have been shown to have inotropic and vasodilatory properties with a protective metabolic effect, even in the presence of calcium channel blockade,  most likely based on the reversal of myocardial depression.
This study examined the effects of PDE type III (PDEIII) inhibition on regional myocardial metabolism and global LV function in a porcine model of acute coronary occlusion with calcium channel- or beta-blockade. We hypothesized that the PDEIII inhibitor with calcium blockade, would improve myocardial function without increasing metabolic consumption. This effect may be possible because of increased myocardial supply or reversed myocardial depression, and decreased consumption or demand.
The protocol was approved by the Institutional Animal Care and Use Committee. Animals were handled in accordance with guidelines established by the National Institutes of Health (NIH publication 85-23, revised 1985).
Fifteen domestic swine weighing 50 to 55 Kg were premedicated with intramuscular ketamine (35 mg/Kg) and anaesthetized with isoflurane (1% end-tidal) in 100% oxygen. A tracheostomy was then performed and the animals mechanically ventilated at 12 breaths/minute, with tidal volumes of 12 mL/Kg to maintain an end-tidal carbon dioxide (CO 2 ) between 32 and 36 mm Hg. Anaesthesia and mechanical ventilation were maintained with the use of a Narkomed 4 anaesthesia machine (North American Drager, Telford, PA). Pancuronium was used for muscle relaxation during the surgical preparation. A 7 French (Fr) pressure-tipped, flotation pulmonary artery catheter (Millar Instruments Inc, Houston, TX) was inserted via the right internal jugular vein into the main pulmonary artery through an 8 Fr Cordis introducer (Arrow International, Reading, PA). A 7 Fr triple-lumen, central venous catheter was placed through the left internal jugular vein. The left carotid artery was exposed, and a 5 Fr pressure-tipped catheter (Millar Instruments Inc, Houston, TX) was placed and advanced into the ascending aorta for continuous arterial pressure monitoring. A median sternotomy was then performed and the heart placed in a pericardial cradle. A 5 Fr pressure-tipped catheter (Millar Instruments Inc, Houston, TX) was inserted via a small stab wound in the apex into the LV cavity for measurement of LV pressure.
The left anterior descending (LAD) coronary artery was isolated proximal to the first major branch running off the LAD coronary artery diagonally and was loosely encircled with a cardiac pacing wire ligature. ,, A constrictor was placed in the LAD coronary artery between the first and second diagonal arteries. Twenty-four gauge Teflon catheters were placed at the following sites: the LAD coronary artery distal to the ligature, the adjacent LAD coronary vein, and the circumflex vein. Blood samples were taken through the catheters to measure and calculate arterial-venous lactate concentrations and difference. ,,,,,
Ischaemia was created by coronary occlusion by aneroid constrictor  for 15 minutes, and reperfusion was permitted for one hour. ,, Ischaemia was produced by reducing the flow in the LAD artery by >80%.  This is very comparable to previous and similar model studies of coronary stenosis, which created similar haemodynamic changes and metabolic changes in dogs ,,,,, and pigs. 
Two pairs of ultrasonic dimension transducers were placed in the subendocardium on the short axis proximal to a LAD artery occluder and the long axis of the LV. The inferior vena cava was encircled with an umbilical tape to produce acute reductions in preload.
Maintenance of intravascular volume was accomplished with lactated Ringer's solution administered by continuous infusion through a peripheral vein at a rate of 10 mL/Kg/hour Normothermia (pulmonary artery temperature of 37 o C) was maintained by the application of a warming blanket. All animals were allowed to stabilize for one hour following the surgical preparation prior to data collection. ,,,,, Also, according to previous studies using a similar transient ischaemia (15 minutes) and reperfusion (1 hour) model, haemodynamic and metabolic variables return to baseline values after one hour (= late reperfusion) of reperfusion. ,,,,,
Haemodynamic measurements included systemic arterial pressure, pulmonary artery pressure, LV pressure, central venous pressure, cardiac output (CO), Emax, and left ventricular pressure-firsttime-derivative (LV dP/dT). Electrocardiogram (standard lead II) and heart rate (HR) were recorded continuously, and CO was measured in triplicate using 5 mL of 4°C injectate with a thermodilution CO computer. Pulmonary and systemic vascular resistances were calculated using standard formulae (PVR=MPAP-PAOP/CO and SVR=MAP-CVP/CO; where PVR=pulmonary vascular resistance, MPAP=mean pulmonary artery pressure, PAOP=pulmonary artery occlusion pressure, CO=cardiac output; SVR=systemic vascular resistance, MAP=mean arterial pressure; CVP=central venous pressure). All transducers were connected to a biomedical amplifier (Grass model 7D, Grass Instruments Co, Quincy, MA). The signals were digitized and continuously recorded at 200 Hz on a personal computer for later analysis (Sonometrics Corp, London, Ontario, Canada).
LV volumes were derived automatically by changes in dimension determined by sonomicrometry in the long and short axes. The formula of an ellipsoid was assumed, using commercially available software (Sonometrics, Corp, London Ontario, Canada). The inferior vena cava was encircled with an umbilical tape to produce acute reductions in preload. During each period of data collection, the inferior vena cava was constricted with umbilical tape for approximately six to eight beats to construct a series of pressurevolume loops. The slope of the end-systolic pressure-volume relationship (Emax) was then determined using the Sonometrics commercial software. The slope of LV end-systolic pressurevolume relationship (Emax) was calculated. We referred to measurements of Emax and dP/dT as contractility parameters. However, Emax is a relatively load-independent index of contractility, whereas dP/dT is still known to be afterloaddependent.
In all pigs, before, during, and after LAD artery occlusion period, blood was sampled from the aorta, pulmonary artery, and the LAD artery and vein for measurements of arterial blood gas tensions, and concentrations of lactate, as described in previous publications using this ischaemic animal model. ,,,,,
Animal Groups and Drug Treatment
The animals were randomized to 3 groups: Group C (n=5) control or sham without drug treatment; Group D (n=5) received diltiazem 5 mg bolus followed by infusion of 5 mg/hour, administered and titrated to decrease heart rate by 10% for 30 minutes; Group D+M (n=5) received diltiazem followed by 50 µg/Kg intravenous milrinone. All drugs were administered prior to ischaemia, with a stabilization period after drug administration of 30 minutes. Milrinone was administered in a dose of 5 µg/Kg/min (50 µg/ Kg in 10 minutes). All drugs were administered intravenously via an infusion pump (Medfusion 2010, Medexinc, Dublin, OH), before and during ischaemia. The dose, rate and administration route of milrinone in a dog  or a swine,  is well established in the literature.
After the surgical preparation, catheters and monitoring insertion, and drug treatment (milrinone and/or diltiazem) ischaemia was created by the LAD constrictor for 15 min. Haemodynamic (invasive and noninvasive monitoring) and metabolic (blood sampling) data was collected at five different stages: baseline, after drug treatment, ischaemia (after 15 minutes), early (5 minutes), and late (60 minutes) reperfusion.
Data from each pig were collated to allow group comparisons. Analysis of variance (ANOVA) for repeated measure design and post-hoc test were used to determine significant variability within groups. The control group was used to test the effect of constriction alone (no drugs) on the variables. Data from the groups that received diltiazem were analysed to determine drugs effect. Then, data from the control and treatment groups were compared statistically by the unpaired t-test with the Bonferroni correction. A two-way ANOVA was used, followed by Student NewmanKeuls test for multiple comparisons. A one-way ANOVA was used for baseline measurements to determine whether the three groups were comparable prior to interventions; Alpha set at level of less than 0.05. (p decreased and LVEDP increased in group D during ischaemia, but CO and LVEDP were significantly different and unaffected (unchanged from baseline level) in group D+M during ischaemia and early reperfusion. The SVR increased only during ischaemia without milrinone (group D). Additional milrinone (group D+M) decreased SVR during ischaemia and prevented the increase in SVR during and after ischaemia. Thus, group D+M maintained better haemodynamic stability.
Contractility variables, dP/dT, and Emax, at different stages without drugs (group C), are presented in [Table 3]. The dP/dT and Emax decreased only during ischaemia in group C.
Contractility variables, dP/dT, and Emax , at different stages in the 2 treatment groups, are presented in [Table 4]. The dP/dT and Emax decreased during ischaemia and dP/dT also during early reperfusion in group D. During the late reperfusion stage in groups without milrinone (group D), no change was seen in these parameters from the baseline, implying that both were well maintained. However, treatment with milrinone maintained contractility variables better than without milrinone (compared to group D): dP/dT was stable and unaffected, and Emax was significantly different and higher (compared to group D) during ischaemia and early reperfusion. This maintenance in contractility with milrinone was expressed better for dP/dT with diltiazem treatment (in group D+M) during ischaemia and early reperfusion.
Following are the changes in all groups along the experiment time-line:
Baseline haemodynamic (HR, MAP, CO, LVEDP, and SVR in [Table 1],[Table 2],[Table 3],[Table 4] and metabolic (lactate extraction in [Figure 1],[Figure 2] data were similar between groups.
During ischaemia without drugs, occlusion produced significant depression in LV function (dP/dT, Emax in [Table 3]) and concomitant elevation of LVEDP that persisted over reperfusion [Table 1].
During ischaemia with drugs: left ventricular pressure-first-time derivative (dP/dT) was preserved during ischaemia with milrinone (D+M group) versus without milrinone (C, D groups), respectively [Table 3].
After ischaemia, in early and late reperfusion periods, LV function was better preserved with milrinone. In the diltiazem groups, less depression of LV function (CO, LVEDP, SVR in [Table 2]; and Emax, dP/dT, in [Table 4]) occurred during early reperfusion with milrinone (D+M) compared to without milrinone (D). The depression of LV function was similar in late reperfusion in the diltiazem group with milrinone or without milrinone [Table 2],[Table 4].
Lactate extraction was negative (equal to increased lactate production during ischaemia in all group [Figure 1] in the LAD coronary vessels region, but not in the circumflex (non-ischaemic) region [Figure 2] Lactate extraction in the circumflex (non-ischaemic) region was within the expected normal values in all groups, even if the variations between groups during and after ischaemia were borderline significant statistically (p=0.055 using two way ANOVA followed by Student t test for multiple comparisons). Lactate extraction in the LAD coronary vessels region was lower (less negative ratio) in the diltiazem with milrinone group (D+M) than in the group without milrinone (D, diltiazem alone) during ischaemia and late reperfusion [Figure 1]. Diltiazem without milrinone (group D) ischaemic lactate production was similar to group C during ischaemia, but different and worse than in group C after ischaemia (early and late reperfusion).
The results of this study demonstrated that during general anaesthesia, the pre-emptive administration of milrinone prior to ischaemia was associated in the diltiazem-milrinone group with less depression of LV function during ischaemia and early reperfusion. LV function changes during ischaemia were related also to Emax, which is a relatively loadindependent index of contractility. Ischaemia with or without the presence of existing calcium channel blockade (with or without milrinone pretreatment) produced significant ventricular dysfunction (2943% decrease in Emax). Pre-emptive milrinone with calcium blockade produced significantly less myocardial metabolic impairment during ischaemia. Thus, the combination of diltiazem and milrinone not only preserved LV function and normal haemodynamics, but also did not worsen metabolic impairment during ischaemia.
We found previously that milrinone improved myocardial function without increasing metabolic (lactate) consumption,  a finding that also correlates with other PDEIII inhibitors  probably a result of an increased myocardial supply. In the present study, however, milrinone in the presence of calcium blockade reversed myocardial depression with increased performance, probably related to supply and induced less ischaemic metabolic impairment, which may be related to reduced demand. However, this combination was not associated with less lactate production after ischaemia during early reperfusion. Also, when milrinone was applied in the presence of calcium blockade, myocardial depression was not decreased or reversed in the late reperfusion period, although there was some decrease in lactate production during the same period (late reperfusion), probably related to decreased demand. Decreased demand is an essential factor in many studies evaluating the protective effect of drugs or manipulations that demonstrate a positive effect on myocardial oxygen and lactate metabolism. ,, In this animal model, calcium channel blockade was not as effective as betablockade  with regard to lessening the metabolic impairment during reperfusion.
Calcium channel blockers are frequently used in patients with coronary artery disease undergoing surgery. The presence of severe myocardial ischaemia may prolong myocardial dysfunction, compounded by the negative inotropic effects of calcium channel blockade. PDE inhibitors like milrinone have the above mentioned inotropic and vasodilatory properties with a protective metabolic effect, even in the presence of calcium channel blockade, , probably as a result of the reversal of myocardial depression. , Thus, their effect is achieved by increasing myocardial supply. However, we could not demonstrate any advantage of diltiazem in reducing metabolic impairment, thus, reducing myocardial demand is probably not as effective, when compared to other drugs creating beta-blockade. ,
Our study is the first to address and evaluate separately the known protective effect of preoperative treatment with calcium blockers, in combination with the known protective effect of pre-ischaemic treatment with PDE inhibitors, in an animal model. The protective metabolic effect was demonstrated previously with amrinone, which has similar inotropic and vasodilatory properties, to have a protective metabolic effect, even in the presence of beta blockade with propranolol,  most likely based on reversing myocardial depression. ,,
The advantage of our study was that we could quantify and connect the haemodynamic effects primarily to improvement of function and metabolism in the ischaemic myocardium (LV dP/ dT, Emax, and lactate production). However, the main limitation of our study was that we could neither quantify myocardial supply, because of changes in myocardial blood flow, nor calculate regional myocardial metabolic supply and demand. Another limitation of the study is that the pigs in this model were anaesthetized with isoflurane, which is known to have potent ischaemic preconditioning properties. However the effect of isoflurane was similar across all the groups.
In conclusion, our study has potential clinical applications, such as during off-pump myocardial revascularization when temporary coronary artery occlusion is performed. Following appropriate clinical trials, milrinone administration should be considered in patients with acute ischaemic LV dysfunction and preexisting calcium blockade. Milrinone and perhaps other PDE type III inhibitors may be considered as first-line inotropes in patients with coronary artery disease who receive calcium blockers, because they improve ventricular function and protect against myocardial ischaemia. Pre-existing beta blockade, rather than calcium channel blockade, may have better protective properties when pretreated with milrinone because it simultaneously improves supply and decreases demand in the ischaemic myocardium, during ischaemia and reperfusion. Human studies are needed to further support the clinical relevance of our animal model.
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