Mechanisms causing coronary microvascular dysfunction following crystalloid cardioplegia and reperfusion.

OBJECTIVE: The aim was to examine the mechanisms of coronary microvascular dysfunction during cardiopulmonary bypass and ischaemic arrest using a crystalloid cardioplegic solution. METHODS: Porcine hearts were arrested with cold hyperkalaemic (K+ = 25 mmol.litre-1) cardioplegic solution for 1 h during cardiopulmonary bypass and then reperfused for 1 h. Selected hearts were arrested but not reperfused. Coronary vessels of non-instrumented pigs were used as controls. In vitro vascular responses of subepicardial and subendocardial arterioles were examined in a pressurised (40 mm Hg) no flow state with video microscopy. RESULTS: Following 1 h of ischaemic cardioplegia, endothelium dependent relaxations of epicardial arterioles to the receptor mediated agent ADP and the non-receptor-mediated agent calcium ionophore A23187 were moderately reduced, and the contractile responses to KCl or the thromboxane A2 analogue U46619 were reduced compared to responses of vessels from control animals. After 1 h of reperfusion, U46619 caused contraction greater than control values, while contraction to KCl and endothelium dependent relaxations to ADP or A23187 were further reduced. Responses of endocardial microvessels to serotonin were slightly more affected by cardioplegia and reperfusion than were epicardial vessels, while the effect on responses of epicardial and endocardial vessels to bradykinin or A23187 were similar. Endothelium independent relaxation to sodium nitroprusside was not altered in any of the experimental groups. The addition of manganese superoxide dismutase to the cardioplegic solution markedly preserved endothelium dependent responses to ADP and A23187 and contractile response to U46619, compared to the responses of vessels from the plain crystalloid cardioplegia group, but had no effect on relaxation to sodium nitroprusside or on contraction to KCl. Five hours of normokalaemic hypothermia (5-10 degrees C) in Krebs buffer had minimal effect on vasodilator responses. Electron microscopy revealed preserved endothelial and smooth muscle cell structure, and focal mononuclear leucocyte-endothelium adherence following cardioplegic arrest and reperfusion. CONCLUSIONS: Ischaemic cardioplegia-reperfusion induced endothelium dependent and direct smooth muscle microvascular dysfunction is at least partially mediated by prolonged exposure of vessels to hyperkalaemia and to the generation of oxygen derived free radicals. Leucocytes probably mediate injury during reperfusion, while hypothermia has minimal effect on recovery of vasomotor function.

Impaired endothelium-dependent coronary microvascular relaxation after cold potassium cardioplegia and reperfusion.

Myocardial dysfunction after cardiac operations might be influenced by altered myocardial perfusion in the postoperative period. To investigate possible alterations in vascular reactivity, in vitro coronary microvascular responses were examined after ischemic cardioplegia with use of a porcine model of cardiopulmonary bypass. Since myocardial perfusion is primarily regulated by arteries less than 200 microns in diameter, these vascular segments were examined. After 1 hour of ischemic arrest with cold crystalloid cardioplegia and 1 hour of reperfusion, microvessels (100 to 190 microns in diameter) were pressurized in a no-flow state, preconstricted by 30% to 60% of the baseline diameter with acetylcholine, and examined with video microscopic imaging and electronic dimension analysis. Endothelium-dependent relaxations to bradykinin (55% +/- 13% versus 99% +/- 1% = maximum relaxation of the preconstricted diameter in cardioplegia-reperfusion vessels versus control vessels, respectively; p < 0.05) and the calcium ionophore A 23187 (33% +/- 6% versus 90% +/- 4%; p < 0.05) were markedly impaired while endothelium-independent relaxation to sodium nitroprusside was similar to control value. After 1 hour of ischemic cardioplegia without reperfusion, endothelium-dependent relaxation was only slightly affected. Transmission electron microscopy showed minimal endothelial damage after ischemic cardioplegia and reperfusion. These findings have important implications regarding coronary spasm and cardiac dysfunction after cardiac operations.

Altered pulmonary microvascular reactivity after total cardiopulmonary bypass.

Pulmonary vascular resistance is frequently elevated after cardiac operations in which cardiopulmonary bypass is used. In our study of the possible contribution of altered pulmonary microvascular reactivity to this condition, sheep were heparinized, cannulated via the aorta and right atrium, and placed on total cardiopulmonary bypass. After 90 minutes of total cardiopulmonary bypass and pulmonary arterial occlusion, the sheep were removed from cardiopulmonary bypass, and their lungs were perfused normally for 60 minutes. Noninstrumented animals were used as controls. To evaluate the effect of 90 minutes of extracorporeal circulation without reduced pulmonary perfusion, we studied additional sheep after they underwent right heart bypass with a pump-oxygenator. Pulmonary microarterial vessels (130 to 230 microns in diameter) from each group were examined in vitro in a pressurized (20 mm Hg), no-flow state with video microscopic imaging and electronic dimension analysis. After preconstriction of vessels with the thromboxane A2 analog U46619 by 30% to 40% of the baseline diameter, vasoactive drugs were applied extraluminally. Serotonin caused control microvessels to dilate. In the presence of the nitric oxide synthetase inhibitor NG-methyl-L-arginine, this was converted to a significant contractile response. Acetylcholine alone had minimal effect on control vessels. However, in the presence of the cyclooxygenase inhibitor indomethacin, acetylcholine caused a significant relaxation response. After total cardiopulmonary bypass and pulmonary reperfusion, pulmonary microvessels contracted significantly when exposed to acetylcholine and serotonin, compared with respective control responses. Both these contractile responses were inhibited in the presence of indomethacin. Endothelium-independent responses to sodium nitroprusside and U46619 and dilation responses to adenosine were not altered after cardiopulmonary bypass. Extracorporeal circulation with continued pulmonary arterial perfusion (right heart bypass group) had no effect on microvascular responses. In conclusion, total cardiopulmonary bypass with associated reduced pulmonary perfusion causes significant alterations of endothelium-dependent pulmonary microvascular responses because of the increased release of a constrictor prostanoid substance and possibly because of reduced release of endothelium-derived relaxing factor.

Cardiopulmonary bypass and pulmonary thromboxane generation.

Sporadic cases of inexplicable noncardiogenic pulmonary edema occur after operations requiring total cardiopulmonary bypass (CPB). Prostaglandins, such as thromboxane (Tx) A2, have been implicated in this form of pulmonary pathology in many clinical and experimental settings. Because Tx generation has been demonstrated in association with ischemia of solid organs, we postulated that total CPB, which decreases pulmonary tissue perfusion and oxygenation, would stimulate local Tx synthesis. Total CPB was examined in 7 sheep. The level of TxB2 (a stable metabolite of the active, unstable A2) was measured in the left and right atria before, during, and after 105 minutes of total CPB. Significant increases in TxB2 concentrations occurred in the left atrium compared with the right (p < 0.05) during CPB. Immediately after reperfusion, both the left atrial and right atrial TxB2 concentrations increased significantly over the baseline values (p < 0.05), but this increase and the atrial gradient were rapidly abolished with continued pulmonary perfusion. To determine the effect of extracorporeal circulation without significant (< 30%) alteration in pulmonary perfusion, we evaluated the effect of partial CPB in 5 sheep. Increased TxB2 concentrations were noted at 15 and 30 minutes after the onset of partial CPB (left atrium increased significantly over baseline; p < 0.05), but this elevation spontaneously diminished to insignificance after 15 and 30 additional minutes of extracorporeal circulation. These data establish that total CPB stimulates Tx generation in the lung, and although the effect of partial CPB is transient, that of total CPB is progressive and abolished by reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood and albumin cardioplegia preserve endothelium-dependent microvascular responses.

Alterations of vascular reactivity may be a cause of reduced myocardial perfusion after cardioplegic arrest. The effects of blood and albumin cardioplegia on endothelium-dependent coronary microvascular function and ultrastructure were examined after cardiopulmonary bypass, ischemic arrest, and reperfusion. During cardiopulmonary bypass, porcine hearts were arrested with either blood, albumin-crystalloid, or crystalloid cardioplegia for 1 hour, followed with reperfusion for 1 hour. Noninstrumented pigs were used as controls. Coronary microarterial vessels (90 to 190 microns in diameter) were studied in a pressurized, no-flow state with video microscopic imaging and electronic dimension analysis. Ischemic arrest with crystalloid cardioplegia markedly reduced endothelium-dependent relaxations to the adenine nucleotide adenosine diphosphate and the calcium ionophore A23187. Enhanced contractile responses were observed to the platelet-derived vasoactive substance serotonin and to the thromboxane A2 analogue U46619. Indomethacin corrected the enhanced contractile responses to serotonin, indicating the enhanced release of a constrictor prostanoid substance. Indomethacin had no effect on the impaired relaxations to adenosine diphosphate or A23187. Endothelium-dependent relaxations to adenosine diphosphate, serotonin, and A23187 were significantly preserved with either blood or albumin-crystalloid cardioplegia, whereas contractile responses to U46619 were normal. Endothelium-independent relaxation to nitroprusside was similar in all groups, indicating normal smooth muscle responsiveness. Electron microscopy revealed minimal alterations of vascular morphology of vessels in both crystalloid and blood cardioplegia groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Coronary endothelial injury after cardiopulmonary bypass and ischemic cardioplegia is mediated by oxygen-derived free radicals.

BACKGROUND: Cardiopulmonary bypass and crystalloid cardioplegia may lead to endothelial dysfunction in the coronary microcirculation. The aim of the present study was to examine whether the alteration of endothelium-dependent microvascular responses may be related to the generation of oxygen-derived free radicals. METHODS AND RESULTS: Pigs (30 kg) were heparinized and placed on cardiopulmonary bypass. The hearts were arrested for 1 hour with either plain hypothermic, hyperkalemic (25 mEq/L) crystalloid cardioplegic solution (n = 10) or crystalloid cardioplegic solution containing either deferoxamine (n = 8) or manganese superoxide dismutase (n = 6). Hearts were then reperfused for 1 hour while the pigs were separated from cardiopulmonary bypass. Noninstrumented pigs were used as controls (n = 8). Coronary microarteries (120 to 190 microns in diameter) were studied in vitro in a pressurized (40 mm Hg), no-flow state with videomicroscopy and electronic dimension analysis. After precontraction of microvessels, the endothelium-dependent and -independent agents were applied extraluminally. Serotonin caused a slight dilation of control vessels (percent dilation of acetylcholine-induced preconstriction at 10 mumol/L drug concentration, 5 +/- 8%; P < .05 versus crystalloid cardioplegia group) and a significant contractile response after crystalloid cardioplegia (-28 +/- 10%). Bradykinin elicited near complete relaxation of control vessels (96 +/- 3%, P < .05), whereas it caused considerably less relaxation after cardioplegia (33 +/- 9%). The addition of either deferoxamine or superoxide dismutase to the cardioplegic solution significantly (but not completely) preserved vasomotor responses of coronary microvessels to serotonin (9 +/- 6% and 11 +/- 4%, respectively; P < .05) or bradykinin (72 +/- 4% and 87 +/- 3%, respectively; P < .05). Endothelium-independent relaxations of vessels in response to sodium nitroprusside were similar in all groups. CONCLUSIONS: Either the hydroxyl radical synthesis inhibitor deferoxamine or manganese superoxide dismutase preserves endothelium-dependent relaxation during crystalloid cardioplegia-reperfusion. Therefore, ischemic cardioplegia-reperfusion-induced endothelial dysfunction is at least partially mediated by the generation of oxygen-derived free radicals.

Epicardial and endocardial coronary microvascular responses: effects of ischemia-reperfusion.

To examine whether endocardial microvascular function is preferentially impaired by ischemia and reperfusion, we studied endothelium-dependent responses of epicardial and endocardial coronary microvessels (130-220 microns) from control pigs and from pigs subjected to 1-h regional myocardial ischemia (circumflex occlusion) followed by 1-h reperfusion (n = 8) in vitro using videomicroscopy. In control animals (n = 8), no significant transmural differences were apparent in microvascular responses to the endothelium-dependent agents bradykinin or the calcium ionophore A23187, to the endothelium-independent agent sodium nitroprusside (SNP), or to adenosine. Serotonin caused a slight but statistically insignificant greater relaxation of endocardial than of epicardial microvessels. After ischemia-reperfusion, relaxations to all endothelium-dependent agents (serotonin, bradykinin, A23187) and to adenosine were significantly reduced (p < 0.05 for all agents) as compared with the respective control responses. There were no significant differences between epicardial and endocardial responses in the ischemia-reperfusion group for any of the vasoactive agents. Endothelium-independent responses to SNP were not affected by ischemia-reperfusion, indicating no alteration in the ability of vascular smooth muscle to relax through guanylate cyclase-mediated mechanisms. Control epicardial microvascular responses were examined after endothelial denudation and after pretreatment with NG-monomethyl-L-arginine (L-NMMA), indomethacin, or glibenclamide.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium concentrations and ventricular ectopy: a prospective, observational study in post-cardiac surgery patients.

OBJECTIVE: To determine whether a correlation exists between concentrations of intracellular and extracellular potassium and to determine the frequency of ventricular ectopy in patients after cardiac operations. DESIGN: Prospective, observational clinical evaluation. SETTING: Surgical-respiratory intensive care unit of a university-affiliated tertiary care center. PATIENTS: Continuous 24-hr electrocardiographic monitoring was performed, and serum (extracellular) and erythrocyte (intracellular) potassium concentrations ([K+]e and [K+]i) were determined, before cardiopulmonary bypass, immediately postoperatively, and at 2, 4, 12, and 20 hrs after elective coronary bypass grafting in 31 patients. INTERVENTIONS: None. Potassium replacement was left to the discretion of the attending physicians. MEASUREMENTS AND MAIN RESULTS: Although the mean [K+]e varied significantly during the postoperative 24-hr period (p<.0001), the [K+]i did not (p = .953). No significant correlations were found between premature ventricular beats and [K+]i, [K+]e, or [K+]i/[K+]e (all p>.05). However, among the few patients who had one or more episodes of ventricular tachycardia (VT) within 30 mins of a study K+ sample, the mean [K+]e was significantly lower during the episode(s) of VT compared with the mean [K+]e in the absence of VT (p<.01). CONCLUSIONS: Although it is clear that over the clinically acceptable range of [K+]e and [K+]i concentrations seen in this population, there is no correlation between potassium concentrations and the occurrence of premature ventricular beats, the infrequent association of more serious ventricular ectopy, VT, with lower [K+]e concentrations supports the practice of using serum potassium to guide potassium replacement in patients after cardiac operations.

Endothelial modulation of porcine coronary microcirculation perfused via immature collaterals.

Porcine hearts have relatively few native collateral vessels and lack the propensity to develop normal perfusion to the collateral-dependent myocardium. To examine microvascular responses in the collateral-dependent region, collateral vessels were stimulated in pigs by the Ameroid constrictor technique. After 4-7 wk, isolated microarterial vessels (90-170 microns ID) were studied in a pressurized (40 mmHg), no-flow state. Microvessels from noninstrumented pigs were used as controls for vascular studies. Although myocardium in the collateral-dependent region showed minimal evidence of infarction, percent systolic shortening was reduced at rest and after pacing compared with myocardium in the normally perfused region. Relaxations to the receptor-mediated endothelium-dependent agents ADP and bradykinin were impaired in collateral-dependent coronary microvessels. Relaxations to the calcium ionophore A23187, which acts through a non-receptor-mediated mechanism, were similar in control and Ameroid microvessels. Relaxations to the endothelium-independent agent sodium nitroprusside were markedly enhanced in microvessels from the collateral-dependent region compared with microvessels from control hearts. In conclusion, receptor-mediated endothelium-dependent relaxation is impaired and endothelium-independent relaxation to sodium nitroprusside is enhanced in microvessels from myocardium perfused by immature collateral vessels.

Relaxation responses of the coronary microcirculation after cardiopulmonary bypass and ischemic arrest with cardioplegia: implications for the treatment of postoperative coronary spasm.

Coronary arteriolar spasm may occur following cardiopulmonary bypass and ischemic arrest, resulting in impaired cardiac function. Because myocardial perfusion is principally regulated by the microcirculation, the in vitro effects of various clinically used vasodilating drugs on porcine coronary microvessels less than 200 microns in diameter were examined following cardioplegic arrest and reperfusion. After 1 hour of ischemic arrest using either crystalloid or blood cardioplegia solutions followed by 1 hour of reperfusion, microvessels were studied in a pressurized (40 mmHg), no-flow state, and imaged with a video tracking device. Vessels were preconstricted by 30% to 60% of their resting diameter using acetylcholine, and various vasodilatory agents were applied extraluminally. Responses to the beta-adrenergic receptor agonist isoproterenol and the nitrovasodilator sodium nitroprusside were minimally altered by either cardioplegia solution as compared to control. In contrast, relaxation responses to both the calcium channel antagonist nifedipine and nitroglycerin were diminished after ischemic arrest and reperfusion. Relaxation responses were similar with crystalloid or blood cardioplegia for all drugs tested. Despite its somewhat attenuated response, nifedipine remained the most potent vasodilator of those studied. It is concluded that (1) following ischemic arrest with either crystalloid or blood cardioplegia solutions, responses to sodium nitroprusside and isoproterenol were minimally altered, while responses to nifedipine and nitroglycerin were attenuated; (2) relaxation responses of coronary arterioles were not significantly different with either blood or crystalloid cardioplegia; and (3) despite a slightly decreased response after cardioplegia, nifedipine was the most potent vasodilator of coronary arterioles, and may be the best choice for treating postoperative coronary arteriolar spasm.