Response to myocardial ischemia as a regulated process

Instantaneous detection of acute myocardial infarction and ischaemia from a single carotid pressure waveform in rats

Aims

Myocardial infarction (MI) is one of the leading causes of death worldwide. It is well accepted that early diagnosis followed by early reperfusion therapy significantly increases the MI survival. Diagnosis of acute MI is traditionally based on the presence of chest pain and electrocardiogram (ECG) criteria. However, around 50% of the MIs are without chest pain, and ECG is neither completely specific nor definitive. Therefore, there is an unmet need for methods that allow detection of acute MI or ischaemia without using ECG. Our hypothesis is that a hybrid physics-based machine learning (ML) method can detect the occurrence of acute MI or ischaemia from a single carotid pressure waveform.

Methods and results

We used a standard occlusion/reperfusion rat model. Physics-based ML classifiers were developed using intrinsic frequency parameters extracted from carotid pressure waveforms. ML models were trained, validated, and generalized using data from 32 rats. The final ML models were tested on an external stratified blind dataset from additional 13 rats. When tested on blind data, the best ML model showed specificity = 0.92 and sensitivity = 0.92 for detecting acute MI. The best model's specificity and sensitivity for ischaemia detection were 0.85 and 0.92, respectively.

Conclusion

We demonstrated that a hybrid physics-based ML approach can detect the occurrence of acute MI and ischaemia from carotid pressure waveform in rats. Since carotid pressure waveforms can be measured non-invasively, this proof-of-concept pre-clinical study can potentially be expanded in future studies for non-invasive detection of MI or myocardial ischaemia.

Myocardial stunning and hibernation revisited

Unlike acute myocardial infarction with reperfusion, in which infarct size is the end point reflecting irreversible injury, myocardial stunning and hibernation result from reversible myocardial ischaemia-reperfusion injury, and contractile dysfunction is the obvious end point. Stunned myocardium is characterized by a disproportionately long-lasting, yet fully reversible, contractile dysfunction that follows brief bouts of myocardial ischaemia. Reperfusion precipitates a burst of reactive oxygen species formation and alterations in excitation-contraction coupling, which interact and cause the contractile dysfunction. Hibernating myocardium is characterized by reduced regional contractile function and blood flow, which both recover after reperfusion or revascularization. Short-term myocardial hibernation is an adaptation of contractile function to the reduced blood flow such that energy and substrate metabolism recover during the ongoing ischaemia. Chronic myocardial hibernation is characterized by severe morphological alterations and altered expression of metabolic and pro-survival proteins. Myocardial stunning is observed clinically and must be recognized but is rarely haemodynamically compromising and does not require treatment. Myocardial hibernation is clinically identified with the use of imaging techniques, and the myocardium recovers after revascularization. Several trials in the past two decades have challenged the superiority of revascularization over medical therapy for symptomatic relief and prognosis in patients with chronic coronary syndromes. A better understanding of the pathophysiology of myocardial stunning and hibernation is important for a more precise indication of revascularization and its consequences. Therefore, this Review summarizes the current knowledge of the pathophysiology of these characteristic reperfusion phenomena and highlights their clinical implications.

Regional myocardial perfusion defects during exercise, as assessed by three dimensional integration of morphology and function, in relation to abnormal endothelium dependent vasoreactivity of the coronary microcirculation

OBJECTIVE

To test the hypothesis that scintigraphic regional myocardial perfusion defects during exercise in patients with normal coronary angiography may be related to abnormal endothelium dependent vasoreactivity of the corresponding myocardial territory in response to cold pressor testing.

METHODS

38 patients were classified into two groups according to the presence or absence of exercise induced scintigraphic myocardial perfusion defects. A cold pressor test was done in all patients during routine coronary angiography, followed by dynamic positron emission tomography to establish coronary blood flow mediated vasoreactivity of the epicardial coronary artery and the myocardial territories supplied by the left anterior descending, left circumflex, and right coronary arteries.

RESULTS

28 patients had regional myocardial perfusion defects while 10 had normal scintigraphic imaging. The three dimensional scintigraphic fusion image revealed 49 regional myocardial perfusion defects with a mean (SD) reversibility of the original stress defect of 20 (3)%. In patients with exercise induced regional myocardial perfusion defects, the responses of epicardial luminal area and regional myocardial blood flow (RMBF) to cold pressor testing were reduced compared with patients with normal perfusion imaging (epicardial luminal area: 5.2 (1.2) to 4.2 (0.86) mm2 v 4.7 (0.5) to 5.8 (0.5) mm2; RMBF: 0.75 (0.16) to 0.78 (0.20) ml/g/min v 0.75 (0.15) to 1.38 (0.26) ml/g/min; p < or = 0.03, respectively). In patients with regional abnormal scintigraphic perfusion, the corresponding RMBF response to cold pressor testing was more severely impaired than the mean myocardial blood flow in the remaining two vascular territories, but the difference was not significant (0.75 (0.16) to 0.78 (0.20) ml/g/min v 0.75 (0.10) to 0.87 (0.12) ml/g/min; NS). The endothelium independent increase in RMBF induced by glyceryl trinitrate did not differ between patients with exercise induced myocardial perfusion defects and those with normal perfusion images (0.75 (0.16) to 0.94 (0.09) ml/g/min v 0.75 (0.15) to 0.94 (0.09) ml/g/min; NS). There was a highly significant correlation between the endothelium dependent responses of RMBF to cold pressor testing and the severity of exercise induced scintigraphic regional myocardial perfusion defects (r = 0.95, p = 0.001).

CONCLUSIONS

Exercise induced scintigraphic regional myocardial perfusion defects in patients with angina but normal coronary angiography may be related to abnormal endothelium dependent vasoreactivity of the corresponding myocardial territory.

Circulatory benefits of diastolic counterpulsation in an ischemic heart failure model after aortomyoplasty

OBJECTIVE

Aortomyoplasty is an experimental surgical procedure in which the latissimus dorsi muscle is wrapped around the thoracic aorta and stimulated to contract during diastole, providing diastolic counterpulsation. We hypothesized that aortomyoplasty could improve cardiac function in a chronic ischemic heart failure model, similar to the improvement seen with the intra-aortic balloon pump.

METHODS

Six dogs (25-30 kg) successfully underwent aortomyoplasty followed by serial coronary microembolization. Ejection fraction decreased from 63.5% to 36.5%. Two weeks after the final microembolization, the muscle was conditioned for 4 months to achieve fatigue resistance. One year after aortomyoplasty, hemodynamic studies during 1 hour of aortomyoplasty and 1 hour of intra-aortic balloon counterpulsation determined mean diastolic aortic pressure, peak left ventricular pressure, and endocardial viability ratio for assisted and unassisted beats. Cardiac output, stroke volume, and parameters of cardiac function were also measured.

RESULTS

Endocardial viability ratio increased by 23.8% +/- 7.9% (P =.001) with aortomyoplasty counterpulsation and by 22.7% +/- 12.9% (P =.021) with the intra-aortic balloon pump. Both aortomyoplasty and the intra-aortic balloon pump significantly increased mean diastolic aortic pressure and reduced peak left ventricular pressure. Improvements in cardiac function with aortomyoplasty and the intra-aortic balloon pump were similar. Cardiac output increased from 2.61 +/- 0.88 to 3.07 +/- 1.06 L/min (P =.006), and index of afterload decreased from 5.4 +/- 1.4 to 4.8 +/- 1.4 mm Hg/mL (P =.02) during 1 hour of aortomyoplasty counterpulsation.

CONCLUSION

One year after the procedure, aortomyoplasty counterpulsation provided diastolic augmentation and improved cardiac performance similar to the improvement provided by the intra-aortic balloon pump in a chronic ischemic heart failure model. Aortomyoplasty has the potential to benefit patients with ischemic heart disease refractory to current therapies.

Preserved myocardial blood flow and oxygen supply-demand balance with active coronary perfusion during simulated off-pump coronary artery bypass grafting

BACKGROUND

During off-pump coronary artery bypass surgery, concern remains about the possible myocardial injury associated with the transient occlusion and stabilization of the target vessels. Although intraluminal shunts are used to avoid ischemia during graft anastomosis, blood flow through the shunts can be affected by upstream pressure and inherent resistance, resulting in reduced blood flow during hypotension or severe proximal stenosis.

METHODS

In anesthetized dogs regional myocardial blood flow (microspheres), oxygen consumption, lactate extraction, and systolic shortening (sonomicrometry) were measured in the myocardium served by the left anterior descending coronary artery with native perfusion after interposition of a 2.25-mm shunt (> or = 90% of left anterior descending diameter) and during active coronary perfusion with a constant flow pump. Measurements were made under normotension and hypotension produced by partial caval occlusion to reduce arterial pressure by 50%.

RESULTS

Interposition of the shunt reduced blood flow by 67.8%, regional oxygen delivery by 59.8%, and systolic shortening by 45.6% relative to baseline, but lactate extraction (31.0% vs 31.2%) and oxygen supply-consumption (O(2)S/myocardial oxygen consumption ratio, 2.7 +/- 0.5 vs 2.6 +/- 0.5) were comparable with baseline values. Hypotension further decreased these physiologic values and was associated with local lactate production (-67.4% extraction) and decreased O(2)S/myocardial oxygen consumption ratio (1.3 +/- 0.1). Active coronary perfusion was associated with regional blood flow, oxygen delivery, systolic shortening, and lactate extraction comparable with baseline values. In contrast to the shunt, active perfusion maintained myocardial flow, oxygen delivery, and lactate extraction during hypotension and normalized the O(2)S/myocardial oxygen consumption ratio, although systolic shortening decreased as a result of ventricular unloading.

CONCLUSION

Intraluminal shunts may impede oxygen delivery to the target myocardium, which precipitates regional ischemia during transient hypotension. Active coronary perfusion provides adequate oxygen supply independent of systemic blood pressure.

Clinical evidence for myocardial derecruitment downstream from severe stenosis: pressure-flow control interaction

To verify the interaction between coronary pressure (CP) and blood flow (CBF) control, we studied nine candidates for angioplasty of an isolated lesion of the left anterior descending coronary artery [i.e. , percutaneous transluminal coronary angioplasty (PTCA)]. CBF (i.e., flow velocity x coronary cross-sectional area at the Doppler tip) and CP were monitored during washout of 2-5 mCi of (133)Xe after bolus injection into the left main artery before and after PTCA. Xe mean transit time (MTT) was calculated as the area under the time-activity curve, acquired by a gamma camera, divided by the dose obtained from a model fit of the Xe curve in the anterior wall. CBF response to intracoronary adenosine (2 mg) was also assessed. PTCA increased baseline CBF (from 14.5 +/- 9.4 to 20 +/- 8 ml/min, P < 0.01), coronary flow reserve (from 1.52 +/- 0.24 to 2.33 +/- 0.8, P < 0.01), and CP (from 64 +/- 9 to 100 +/- 10 mmHg, P < 0.05). MTT decreased from 89 +/- 32 to 70 +/- 19 s (P < 0.05) after PTCA; however, MTT and CBF changes were not correlated (r = -0.09, not significant). Inasmuch as MTT is the ratio of distribution volume to CBF, MTT x CBF was used as an index of perfused myocardial volume. Volume increased after PTCA from 23 +/- 18 to 56 +/- 30 ml. A direct correlation was observed between the percent increase in distal CP and percent increase in perfused volume (r = 0.91, P < 0.01). Thus low CP was not associated with exhaustion of flow reserve but, rather, with reduction of perfused myocardial volume. These data suggest that, in the presence of a severe coronary stenosis, derecruitment of vascular units occurs that is proportional to the decrease in driving pressure. Residual perfused units maintain a vasomotor tone, thus explaining the paradoxical persistence of coronary reserve.

Adenosine and cardioprotection during ischaemia and reperfusion--an overview

Adenosine is a local hormone, with numerous tissue-specific biological functions. In the myocardium, adenosine is released in small amounts at constant basal rate during normoxia. During ischaemia the production of adenosine increases several fold due to breakdown of adenosine triphosphate (ATP). Increased production of adenosine causes coronary vasodilatation. Thus, adenosine couples myocardial metabolism and flow during ischaemia and is called a homeostatic or "retaliatory metabolite". Furthermore, adenosine has electrophysiological effects in supraventricular tissue, causing a decrease in heart rate. In 1985 it was discovered that adenosine also exerts cardioprotective effects directly on cardiomyocytes. The aim of this review is to give an overview of the role of adenosine as a directly cytoprotective agent during myocardial ischaemia and reperfusion. We will focus on its effects on the myocytes, elicited by stimulation of adenosine receptors in sarcolemma, which triggers intracellular signalling systems. We will also address the new aspect that adenosine can influence regulation of gene expression. There is evidence that the myocardium is capable of endogenous adaptation in response to ischaemia, namely "hibernation" and early and late phases of "preconditioning". Endogenous substances produced during ischaemia probably trigger these responses. We will discuss the role of adenosine in these different settings. Adenosine can be given exogenously through intravasal routes; however, this review will also focus on the effects of endogenously produced adenosine. We will discuss pharmacological ways to increase endogenous levels of adenosine, and the effects of such interventions during ischaemia and reperfusion. Finally, we will review results from studies in humans together with relevant experimental studies, and indicate potential therapeutic implications of adenosine.

Experimental hyperlipidemia prevents the protective effect of ischemic preconditioning on the contractility and responsiveness to phenylephrine of rat-isolated stunned papillary muscle

This study was designed to establish a hyperlipidemic diet (significant increase in the cholesterol and triglycerides blood levels, but without atherogenic changes in heart muscle and coronary vessels) and to investigate the influence of experimental hyperlipidemia on the effects of ischemic preconditioning (PC) of rat-isolated papillary muscle on the time course of contractility during simulated ischemia and reperfusion and responsiveness to phenylephrine under such a condition. The animals were divided in four experimental groups: standard diet-fed control group (SD), SD underwent ischemic preconditioning (SD-PC), hyperlipidemic diet-fed group (HLD) and HLD underOFFt PC (HLD-PC). Force of contraction (Fc), velocity of contraction (+dF/dt), and velocity of relaxation (-dF/dt) were measured. HLD preparations were more sensitive to ischemia then SD ones. PC, performed by 5-min perfusion with no-substrate solution gassing with 95% N2/5% CO2 in the presence of fast electrical stimulation, and 10 min of reperfusion with normal solution and rate of stimulation, significantly increased the resistance of isolated cardiac tissues to simulated ischemia in SD-PC group, but not in HLD-PC group. Negative inotropic action of phenylephrine occured in SD group of preparations after simulated-ischemia/reperfusion period was also prevented by PC. Therefore, we conclude that experimental hyperlipidemia significantly influenced the function of rat heart muscle including the higher sensitivity to ischemia and different reaction to the same PC procedure.

Preconditioning prevents the negative inotropic action of phenylephrine in rat isolated stunned papillary muscle

The aim of the present study was to establish a model of ischemic preconditioning of rat isolated papillary muscle and to investigate its effect on the simulated ischemia-induced disturbances in contractility and responsiveness to isoproterenol and phenylephrine. Experiments were performed in rat left ventricle papillary muscle. The following parameters were measured: force of contraction (Fc), velocity of contraction (+dF/dt), velocity of relaxation (-dF/dt), time to peak contraction (ttp), and relaxation time at the level of 10% of total amplitude (tt10). After 60 min of simulated ischemia induced by the perfusion of isolated tissues with no-substrate solution aerated by 95% N2/5% CO2, all of the measured parameters were markedly decreased. There was not complete recovery of Fc, +dF/dt and -dF/dt after 60 min of reperfusion. Positive inotropic action of isoproterenol does not differ before and after simulated ischemia. In contrast, phenylephrine induces a positive inotropic action in non-treated, but a significant negative one in simulated ischaemia/reperfusion treated preparations. The latter effect of phenylephrine was reversed by chloroethylclonidine (CEC), a selective blocker of alpha1b-adrenoceptor, but not by WB-4101, a selective blocker of alpha1a-adrenoceptor. Ischemic preconditioning of rat isolated cardiac tissue induced by the 5 min perfusion with no-substrate solution, aerated by 95% N2/ 5% CO2, in the presence of fast electrical pacing (BCL shortened from 2000 ms to 700 ms) and 10 min reperfusion, significantly improves a recovery of the contractility and prevents phenylephrine negative inotropic action.

Downregulation of ventricular contractile function during early ischemia is flow but not pressure dependent

The mechanism responsible for the abrupt fall in myocardial contractile function following coronary artery obstruction is unknown. The "vascular collapse theory" hypothesizes that the fall in coronary perfusion pressure after coronary artery obstruction is responsible for contractile failure during early ischemia. To test the role of vascular collapse in downregulating myocardial contractile force at the onset of ischemia, coronary flow of isolated rat hearts was abruptly decreased by 50, 70, 85, and 100% of baseline, and subsequent changes in coronary perfusion pressure and ventricular function were recorded at 0.5-s intervals. At 1.5 s after flow reductions ranging from 50 to 100%, decreases in contractile function did not differ, although perfusion pressure varied significantly from 45 +/- 1 to 20 +/- 2 mmHg. When function fell to 50% of baseline, perfusion pressures ranged from 35 +/- 0.5 to 2.5 +/- 1 mmHg for flow reductions ranging from 50 to 100%. Identical contractile function at widely differing coronary perfusion pressures is incompatible with the vascular collapse theory.

Effects of critical coronary stenosis on global systolic left ventricular function quantified by pressure-volume relations during dobutamine stress in the canine heart

OBJECTIVES

In this study we quantified the effects of a critical coronary stenosis on global systolic function using pressure-volume relations at baseline and during incremental dobutamine stress.

BACKGROUND

The effects of coronary stenosis have previously been analyzed mainly in terms of regional (dys)function. Global hemodynamics are generally considered normal until coronary flow is substantially reduced. However, pressure-volume analysis might reveal mechanisms not fully exposed by potentially load-dependent single-beat parameters. Moreover, no systematic analysis by pressure-volume relations of the effects of dobutamine over a wide dose range has previously been presented.

METHODS

In 14 dogs left ventricular volume and pressure were measured by conductance and micromanometer catheters, and left circumflex coronary flow by Doppler probes. Measurements in control and with left circumflex stenosis were performed at baseline and at five levels of dobutamine (2.5 to 20 microg/kg/min). The end-systolic pressure-volume relation (ESPVR) dP/dtMAX vs. end-diastolic volume (dP/dtMAX - V(ED)) and the relation between stroke work and end-diastolic volume (preload recruitable stroke work [PRSW]) were derived from data obtained during gradual caval occlusion.

RESULTS

In control, dobutamine gradually increased heart rate up to 20 microg/kg/min, the inotropic effect blunted at 15 microg/kg/min. With stenosis, the chronotropic effect was similar, however, contractile state was optimal at approximately 10 microg/kg/min and tended to go down at higher levels. At baseline, the positions of ESPVR and PRSW, but not of dP/dtMAX - V(ED), showed a significant decrease in function with stenosis. No differences between control and stenosis were present at 2.5 microg/kg/min; the differences were largest at 15 microg/kg/min.

CONCLUSIONS

Pressure-volume relations and incremental dobutamine may be used to quantify the effects of critical coronary stenosis. The positions of these relations are more consistent and more useful indices than the slopes. The positions of the ESPVR and PRSW show a reduced systolic function at baseline, normalization at 2.5 microg/kg/min and a consistent significant difference between control and stenosis at dobutamine levels of 5 microg/kg/min and higher.

Inorganic phosphate content and free energy change of ATP hydrolysis in regional short-term hibernating myocardium

OBJECTIVE

Short-term myocardial hibernation is characterized by an adaptation of contractile function to the reduced blood flow, the recovery of creatine phosphate content and lactate balance back towards normal, whereas ATP content remains reduced at a constant level. We examined the hypothesis that, despite the absence of ATP recovery, the short-term hibernating myocardium regains an energetic balance.

METHODS

An enzymatic method was modified for the measurement of inorganic phosphate (Pi) in transmural myocardial drill biopsies (about 5 mg). In 12 anaesthetized swine, moderate ischemia was induced by reduction of coronary inflow into the cannulated left anterior descending coronary artery to decrease regional myocardial function (sonomicrometry) by 50%.

RESULTS

The development of short-term hibernation was verified by the recovery of creatine phosphate content, the persistence of inotropic reserve in response to dobutamine and the absence of necrosis (triphenyl tetrazolium chloride). At 5-min ischemia, Pi was increased from 3.6 +/- 0.3 (SD) to 8.1 +/- 1.1 mumol/gwet wt (p < 0.05). The free energy of ATP hydrolysis (delta GATP) was decreased from -57.8 +/- 0.8 to -52.2 +/- 1.4 kJ/mol (p < 0.05). The relationships between function and Pi (r = -0.81) and delta GATP (r = -0.83), respectively, during control and at 5-min ischemia became invalid at 90-min ischemia, as myocardial blood flow and function remained reduced at a constant level, but Pi decreased back to 4.9 +/- 0.9 mumol/g (p < 0.05 vs. control and 5-min ischemia), and delta GATP fully recovered back to -57.2 +/- 1.3 kJ/mol (p < 0.05 vs. 5-min ischemia).

CONCLUSIONS

In short-term hibernating myocardium, myocardial inorganic phosphate content recovers partially and the free energy change of ATP hydrolysis returns to control values. Contractile function remains reduced by mechanisms other than an energetic deficit.

Expression of calcium regulatory proteins in short-term hibernation and stunning in the in situ porcine heart

BACKGROUND

Myocardial hibernation and stunning are characterised by a reversible contractile dysfunction during and after ischaemia, respectively. Calcium homeostasis might be disturbed in hibernation and stunning due to altered expression of cardiac proteins involved in calcium handling.

METHODS

In enflurane-anaesthetised swine the coronary blood flow through the left anterior descending coronary artery was decreased to reduce regional contractile function (microsonometry) by approximately 50%. In transmural biopsies obtained during ischaemia and reperfusion creatine phosphate as well as the expression of sarcoplasmic reticulum calcium ATPase (SERCA), phospholamban (PLB), calsequestrin (CSQ), and troponin inhibitor (TnI) were determined.

RESULTS

During ischaemia creatine phosphate, after an initial reduction, recovered back to control values, and necrosis was absent (hibernation). After 90 min of ischaemia the myocardium was reperfused for 120 min but regional contractile function continued to be depressed (stunning). PLB, SERCA, CSQ, and TnI proteins were unchanged during ischaemia as well as reperfusion. Likewise, levels of PLB and SERCA mRNAs were unchanged.

CONCLUSION

It is concluded that other mechanisms than altered expression of these regulating proteins underlie the contractile dysfunction observed during acute ischaemia, short-term hibernation and stunning.

Proton-decoupled myocardial 31P NMR spectroscopy reveals decreased PCr/Pi in patients with severe hypertrophic cardiomyopathy

Disturbed myocardial energy metabolism may occur in patients with primary hypertrophic cardiomyopathy (HCM). A noninvasive way to gain insight into cardiac energy metabolism is provided by in vivo 31P nuclear magnetic resonance (NMR) spectroscopy. 31P NMR spectroscopy with proton decoupling was performed in 13 patients aged 13-36 years with HCM on a 1.5 T Magnetom with a double resonant surface coil. A 2D chemical shift imaging (CSI) sequence in combination with slice selective excitation was used to acquire spectra of the anteroseptal region of the left ventricle (volume element: 38 mL). The chemical shifts of the phosphorus metabolites, intracellular pHi, and coupling constants J(alphabeta) and J(gammabeta) were calculated. Peak areas of 2,3-diphosphoglycerate (DPG), Pi, and adenosine triphosphate (ATP) were determined and corrected for blood contamination, saturation, and differences in nuclear Overhauser enhancements (NOE). The maximum thickness of the interventricular septum (IVSmax) was determined from tomographic long-axis images and expressed as number of standard deviations above the mean of the normal population (Z score). The patients were then divided into 2 groups: 6 patients with moderate HCM (HCMm, Z score < or = 5) and 7 patients with severe HCM (HCMs, Z score > 5). No differences between both groups and a control group of healthy volunteers (n = 16) were found with respect to phosphocreatine (PCr)/gamma-ATP ratio, pHi, or the coupling constants. Only the PCr/Pi ratio differed significantly from the control group (HCM(all), alpha < 0.05, HCMs, alpha < 0.02, 2-sided U test). The decrease of the PCr/Pi ratio in patients with HCM is probably caused by ischemically decreased oxygen supply in the severely hypertrophied myocardium.

Clinical application of coronary flow reserve using an intracoronary Doppler guide wire

Coronary flow reserve (CFR) is a critical measurement in the assessment of the coronary circulation. The development of this physiologic variable in animal and human studies is reviewed. Human studies documenting the limitations of coronary angiography, especially in the setting of severe diffuse coronary artery disease, are analyzed. Furthermore, the important variables that must be accounted for when CFR is measured are examined. With this background, the application of CFR in a variety of clinical settings and the development and use of the Doppler FloWire for its measurement are discussed.

Requirement of glycolytic substrate for metabolic recovery during moderate low flow ischemia

Low flow ischemia with stable hemodynamic function can result in partial metabolic recovery characterized by an increase in phosphocreatine (PCr). Prior data suggest that glycolytic production of adenosine triphosphate (ATP) may be critical for this recovery and that the ATP produced by oxidative phosphorylation alone may be insufficient. This study tested the hypotheses that, during moderate low flow ischemia, (a) metabolic recovery is dependent on glycolytic production of ATP, and, therefore, (b) a mitochondrial substrate such as pyruvate alone is inadequate to allow metabolic recovery. High energy phosphates, pH, and lactate release were measured during 2 h of moderate low flow ischemia. Hearts were perfused with either a glycolytic plus mitochondrial substrate (glucose, insulin and pyruvate) or a mitochondrial substrate alone (pyruvate). Flow reductions required to reduce PCr by approximately 8% resulted in stable and equal reductions of rate-pressure product in each group. PCr recovered fully during the ischemic period in control hearts with glycolytic substrate, associated with preservation of normal end-diastolic pressure, and increased lactate release during the first hour of ischemia. Reperfusion of these hearts restored hemodynamic function and increased PCr above baseline values. In contrast, the use of pyruvate alone as a substrate resulted in a progressive fall of PCr during ischemia, increased end-diastolic pressure, and no significant increase in lactate release. Reperfusion in these hearts restored hemodynamic function, but did not result in normalization of PCr. Both groups had significant reductions in ATP during ischemia. Recovery of PCr during ongoing moderate low flow ischemia is observed in the presence of mixed glycolytic and mitochondrial substrates (glucose, insulin and pyruvate) but is not observed with pyruvate as a sole mitochondrial substrate. These data support a critical role for glycolytic flux under these conditions, suggesting that ATP generated solely by oxidative phosphorylation is not sufficient to promote metabolic recovery or maintain diastolic function during moderate low flow ischemia.

Microvascular dysfunction in collateral-dependent myocardium

OBJECTIVES

The aim of this study was to evaluate myocardial blood flow regulation in collateral-dependent myocardium of patients with coronary artery disease.

BACKGROUND

Despite great clinical relevance, perfusion correlates of collateral circulation in humans have rarely been estimated by quantitative methods at rest and during stress.

METHODS

Nineteen patients with angina and isolated occlusion of the left anterior descending (n = 14) or left circumflex (n = 5) coronary artery were evaluated. Using positron emission tomography and nitrogen-13 ammonia, we obtained flow measurements at baseline, during atrial pacing-induced tachycardia and after intravenous administration of dipyridamole (0.56 mg/kg body weight over 4 min). Flow values in collateral-dependent and remote areas were compared with values in 13 normal subjects.

RESULTS

Flow at rest was similar in collateralized and remote myocardium (0.61 +/- 0.11 vs. 0.63 +/- 0.17 ml/min per g, mean +/- 1 SD), and both values were lower than normal (1.00 +/- 0.20 ml/min per g, p < 0.01). During pacing, blood flow increased to 0.83 +/- 0.25 and 1.11 +/- 0.39 ml/min per g in collateral-dependent and remote areas, respectively (p < 0.05 vs. baseline); both values were lower than normal (1.86 +/- 0.61 ml/min per g, p < 0.01). Dipyridamole induced a further increase in perfusion in remote areas (1.36 +/- 0.57 ml/min per g, p < 0.01 vs. pacing) but not in collateral-dependent regions (0.93 +/- 0.37 ml/min per g, p = NS vs. pacing); again, both values were lower (p < 0.01) than normal (3.46 +/- 0.78 ml/min per g). Dipyridamole flow in collateral-dependent myocardium was slightly lower in patients with poorly developed than in those with well developed collateral channels (0.75 +/- 0.29 vs. 1.06 +/- 0.38 ml/min per g, respectively, p = 0.06); however, the former showed higher flow inhomogeneity (collateral/control flow ratio 0.58 +/- 0.10 vs. 0.81 +/- 0.22, respectively, p < 0.02). A linear direct correlation was observed between flow reserve of collateral-dependent and remote regions (r = 0.83, p < 0.01).

CONCLUSIONS

Despite rest hypoperfusion, collateral-dependent myocardium maintains a vasodilator reserve that is almost fully utilized during increases in oxygen consumption. A global microvascular disorder might hamper adaptation to chronic coronary occlusion.

From coronary artery disease to heart failure: role of the hibernating myocardium

Hibernating myocardium is defined as persistently impaired myocardial and left ventricular (LV) function at rest resulting from reduced myocardial blood flow. It may occur in unstable angina and chronic stable angina, acute myocardial infarction, and LV dysfunction and congestive heart failure. Recovery of the hibernating myocardium has clearly been shown to occur with the establishment of successful revascularization either by coronary bypass surgery or by percutaneous transluminal coronary angioplasty. It may be possible to show recovery of the viable myocardium by reducing myocardial oxygen demand and/or by increasing coronary blood flow with pharmaceutical agents.

Mechanism of impaired myocardial function during progressive coronary stenosis in conscious pigs. Hibernation versus stunning?

The major goal of this study was to determine whether impaired myocardial contractile function during the development of progressive coronary artery stenosis induced by ameroid constriction in conscious pigs reflected myocardial "hibernation" or "stunning." Minipigs were instrumented with a coronary ameroid constrictor and hydraulic occluder, regional wall thickness crystals, a left ventricular (LV) pressure gauge, and aortic and left atrial catheters. In the seven pigs in which it was measured, systolic wall thickening (WT) distal to the ameroid fell by a maximum of 56 +/- 6% at 20 +/- 3 days after ameroid implantation and then began to recover. At 1 day after ameroid implantation, brief complete coronary artery occlusion (CAO) resulted in wall thinning distal to the ameroid (-113 +/- 4%) and transmural decreases in myocardial blood flow in endocardial (from 0.82 +/- 0.08 to 0.02 +/- 0.01 mL/min per gram) and epicardial (from 0.73 +/- 0.13 to 0.03 +/- 0.02 mL/min per gram) layers. At 20 +/- 3 days, baseline myocardial blood flow was not altered either in endocardial (0.92 +/- 0.10 mL/min per gram) or epicardial (0.85 +/- 0.12 mL/min per gram) layers, whereas brief complete coronary artery occlusion still reduced WT (-83 +/- 12%) and myocardial blood flow in endocardial (to 0.21 +/- 0.03 mL/min per gram) and epicardial (to 0.43 +/- 0.12 mL/min per gram) layers, indicating that the coronary artery was not totally occluded. Pathology in four pigs demonstrated no gross necrotic myocardium shortly after this time point. Transient reductions in WT distal to the ameroid were observed during progressive coronary artery stenosis in response to spontaneous increases in activity. Beat-by-beat analysis of these episodes revealed that acute reductions in WT followed increases in LV dP/dt and heart rate and exhibited delayed recovery. These data suggest that the reduced function during ameroid-induced coronary stenosis reflected cumulative myocardial stunning rather than a primary deficit in coronary blood flow or "hibernating myocardium."

Patterns of systemic oxygen utilization in cardiac ischemic syndromes: oxygen utilization in cardiac ischemia

Cardiac ischemia can present as distinctive clinical syndromes such as acute myocardial infarction, cardiogenic shock, sudden cardiac arrest or chronic congestive heart failure. All of the clinical syndromes share common pathophysiological events including reduction of cardiac output and systemic oxygen delivery (DO2) and activation of neurohumoral stress response. The balance between systemic DO2 and oxygen consumption (VO2) is maintained by modification of systemic oxygen utilization and demands which are essential for tissue viability and survival in cardiac ischemic syndromes. Low blood flow and the neurohumoral response may influence cellular metabolism (e.g., acute ischemia preconditioning and chronic downregulation of aerobic metabolism) and microcirculatory perfusion patterns to decrease systemic oxygen demands and VO2 in harmony with low cardiac output and systemic DO2. The clinical relevance of these metabolic adaptations and their influence on the outcome in cardiac ischemic syndromes remains unknown.

Characterization of systemic oxygen transport in end-stage chronic congestive heart failure

Chronic congestive heart failure (CHF) is characterized by low cardiac index (CI) and low systemic oxygen delivery (DO2), which frequently are associated with lethal cardiogenic shock after acute myocardial infarction. Nevertheless, patients with severe CHF are able to survive with these low levels of systemic DO2 and CI. It was hypothesized that patients with CHF survive low CI and DO2 by downregulation of global metabolism and resting oxygen consumption (VO2) and a concomitant increase in systemic oxygen extraction ratio (O2ER). Therefore the objective of this study was to characterize the resting pattern of systemic oxygen transport (O2T) and utilization in patients with stable CHF. Seventeen patients with CHF (New York Heart Association functional class III or IV) for > or = 3 months and with left ventricular ejection fraction (EF) < 25% and whose condition was stable with conventional oral therapy were studied. The control group comprised 10 subjects (NYHA class I) who had coronary angiography and who were found to have normal left ventricular function and EF > 60%. Subjects underwent right-heart catheterization for measurement and calculation of hemodynamic and O2T variables (VO2, DO2, and O2ER). There were no significant differences in mean age (67 +/- 6 vs 64 +/- 17 years) or gender ratio (male:female 14:3 vs 7:3) between CHF and control groups, respectively. The cause of CHF was ischemic in 13 and idiopathic in 4 patients. There were 9 patients in NYHA class III and 8 in class IV.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation of coronary flow reserve and myocardial function after temporary subtotal coronary artery occlusion and increased myocardial oxygen demand in dogs

OBJECTIVES

We examined whether subtotal coronary artery occlusion and reperfusion alter coronary flow reserve and regional myocardial function.

BACKGROUND

Total coronary artery occlusion followed by reperfusion results in decreased coronary flow reserve and regional myocardial dysfunction.

METHODS

Thirteen anesthetized dogs were subjected to subtotal occlusion of the left anterior descending coronary artery for 1 h, followed by reperfusion for 1 h. During subtotal left anterior descending occlusion, heart rate was increased by atrial pacing. After reperfusion, coronary flow reserve, indicated by reactive hyperemia, as well as coronary flow responses to acetylcholine and nitroglycerin, regional myocardial function and myocardial leukocyte accumulation were measured.

RESULTS

After reperfusion, coronary flow reserve was decreased in the ischemic left anterior descending but not the nonischemic circumflex coronary artery region. Myocardial function was also depressed in the left anterior descending coronary region and did not improve on reperfusion. Histologic study showed no leukocyte infiltration in the ischemic left anterior descending coronary region. Myeloperoxidase, an index of myocardial leukocyte accumulation, was similar in the left anterior descending and circumflex coronary regions. Sensitivity of epicardial left anterior descending coronary artery rings to the thromboxane A2 analog U46,619 was enhanced, and relaxation of these rings in response to endothelium-dependent relaxants was decreased.

CONCLUSIONS

Coronary flow reserve is reduced and regional myocardial function depressed after subtotal coronary artery occlusion and increased heart rate. A decreased synthesis or increased breakdown of endothelium-derived relaxing factor may be related to a decrease in coronary flow reserve. However, the reduction in coronary flow reserve appears to be unrelated to leukocyte accumulation in the reperfused region.

Regional myocardial blood flow in stable angina pectoris associated with isolated significant narrowing of either the left anterior descending or left circumflex coronary artery

Myocardial perfusion measurements were obtained with positron emission tomography under basal conditions and after intravenous dipyridamole infusion (0.56 mg/kg over 4 minutes) to determine if myocardial perfusion is maximized in areas of resting wall motion abnormalities in patients with stable angina. Thirty-three patients with no history of myocardial infarction, and with coronary stenosis > 50% involving the left anterior descending (n = 24) and left circumflex (n = 9) coronary arteries were evaluated. Quantitative perfusion images were recorded twice in each subject using nitrogen-13 ammonia at baseline and after intravenous administration of dipyridamole. Computer-assisted analysis of left ventriculograms showed abnormal wall motion in the stenosis-related regions in 16 patients (group 1), and normal regional function in 17 (group 2). The flow values in the anterior and posterolateral wall were considered to reflect left anterior and left circumflex coronary artery flow, respectively. Quantitative angiography showed that coronary stenosis severity was higher in group 1 than in group 2 (cross-sectional area reduction 94 +/- 7% vs 87 +/- 11%; p < 0.05). Resting blood flow in the stenosis-related areas was significantly lower than in contralateral regions in group 1 (0.66 +/- 0.19 vs 0.77 +/- 0.26 ml/min/g; p < 0.05), but not in group 2 (0.73 +/- 0.18 vs 0.78 +/- 0.21 ml/min/g; p = NS). Dipyridamole significantly (p < 0.01) increased myocardial blood flow in both stenotic and remote regions in both groups 1 (0.95 +/- 0.41 vs 1.57 +/- 0.70 ml/min/g) and 2 (1.54 +/- 0.53 vs 2.01 +/- 0.84 ml/min/g).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence in support of the spasm of resistance vessel concept of ischemic heart disease: an update in 1993

The goal of this communication is to provide more evidence for the ischemic heart disease (IHD) component of the spasm of resistance vessel (S-RV) concept of IHD and other ischemic diseases. The S-RV concept of IHD is considered to be an alternate paradigm which challenges the accepted understanding of this disorder. The theory asserts that primary S-RV directly induces symptoms in IHD, and this position is in opposition to the accepted view that symptoms are induced directly by primary occlusions of epicardial arteries by coronary artery disease, spasm, and thromboses. The theory, if valid, should be useful in reducing the impact of IHD, as it generally is accepted that the correct appreciation of basic pathophysiological mechanisms helps ensure the most appropriate prevention and treatment of disease.