Controlling cardiac chaos.

The extreme sensitivity to initial conditions that chaotic systems display makes them unstable and unpredictable. Yet that same sensitivity also makes them highly susceptible to control, provided that the developing chaos can be analyzed in real time and that analysis is then used to make small control interventions. This strategy has been used here to stabilize cardiac arrhythmias induced by the drug ouabain in rabbit ventricle. By administering electrical stimuli to the heart at irregular times determined by chaos theory, the arrhythmia was converted to periodic beating.

Spatiotemporal complexity of ventricular fibrillation revealed by tissue mass reduction in isolated swine right ventricle. Further evidence for the quasiperiodic route to chaos hypothesis.

We have presented evidence that ventricular fibrillation is deterministic chaos arising from quasiperiodicity. The purpose of this study was to determine whether the transition from chaos (ventricular fibrillation, VF) to periodicity (ventricular tachycardia) through quasiperiodicity could be produced by the progressive reduction of tissue mass. In isolated and perfused swine right ventricular free wall, recording of single cell transmembrane potentials and simultaneous mapping (477 bipolar electrodes, 1.6 mm resolution) were performed. The tissue mass was then progressively reduced by sequential cutting. All isolated tissues fibrillated spontaneously. The critical mass to sustain VF was 19.9 +/- 4.2 g. As tissue mass was decreased, the number of wave fronts decreased, the life-span of reentrant wave fronts increased, and the cycle length, the diastolic interval, and the duration of action potential lengthened. There was a parallel decrease in the dynamical complexity of VF as measured by Kolmogorov entropy and Poincaré plots. A period of quasiperiodicity became more evident before the conversion from VF (chaos) to a more regular arrhythmia (periodicity). In conclusion, a decrease in the number of wave fronts in ventricular fibrillation by tissue mass reduction causes a transition from chaotic to periodic dynamics via the quasiperiodic route.

Quasiperiodicity and chaos in cardiac fibrillation.

In cardiac fibrillation, disorganized waves of electrical activity meander through the heart, and coherent contractile function is lost. We studied fibrillation in three stationary forms: in human chronic atrial fibrillation, in a stabilized form of canine ventricular fibrillation, and in fibrillation-like activity in thin sheets of canine and human ventricular tissue in vitro. We also created a computer model of fibrillation. In all four studies, evidence indicated that fibrillation arose through a quasiperiodic stage of period and amplitude modulation, thus exemplifying the "quasiperiodic transition to chaos" first suggested by Ruelle and Takens. This suggests that fibrillation is a form of spatio-temporal chaos, a finding that implies new therapeutic approaches.

Ventricular fibrillation: how do we stop the waves from breaking?

Combined experimental and theoretical developments have demonstrated that in addition to preexisting electrophysiological heterogeneities, cardiac electrical restitution properties contribute to breakup of reentrant wavefronts during cardiac fibrillation. Developing therapies that favorably alter electrical restitution properties have promise as a new paradigm for preventing fibrillation.

Hepatic lipase. Purification and characterization.

Hepatic lipase has been purified to homogeneity from rat liver homogenates. The purified enzyme exhibits a single band on SDS-polyacrylamide gel electrophoresis. The molecular size of the native hepatic lipase is 200 000, while on SDS-polyacrylamide gel electrophoresis the apparent minimum molecular weight of the enzyme is 53 000, suggesting that the active enzyme is composed of four subunits. The relationship between triacylglycerol, monoacylglycerol and phospholipid hydrolyzing activities of the purified rat liver enzyme was studied. All three activities had a pH optimum of 8.5. The maximal reaction rates obtained with triolein, monoolein and dipalmitoylphosphatidylcholine were 55 000, 66 000 and 2600 mumol fatty acid/mg per h with apparent Michaelis constant (Km) values of 0.4, 0.25 and 1.0 mM, respectively. Hydrolysis of triolein and monoolein probably takes place at the same site on the enzyme molecule, since competitive inhibition between these two substrates was observed, and a similar loss of hydrolytic activity occurred in the presence of diisopropylfluorophosphate. Addition of apolipoproteins C-II and C-I had no effect on the hydrolytic activity of the enzyme with the three substrates tested. However, the triacylglycerol hydrolyzing activity was inhibited by the addition of apolipoprotein C-III. Monospecific antiserum to the pure hepatic lipase has been raised in a rabbit.

Intra- and extracellular forms of lipoprotein lipase in adipose tissue.

The location of lipoprotein lipase activity in rat adipose tissue was studied using intact epididymal fat pads, isolated adipocytes, and lipoprotein lipase activity secreted from adipocytes as enzyme sources. The enzyme activities of these preparations were characterized by gel filtration. The method used for isolation of adipocytes had been modified to minimize activation of lipoprotein lipase during the procedures. Extracts of intact adipose tissue separated into two major lipoprotein lipase activity peaks, designated "a" and "b", the "a" fraction representing about 30 (fasted rats) to 50% (fed rats) of the total enzyme activity. An intermediate fraction (designated "i") was frequently observed. Extracts of isolated adipocytes from fed rats contained about 35% and those from fasted rats about 65% of the lipoprotein lipase activity present in intact tissue. The "b" fraction constituted 80--97% of the adipocyte lipoprotein lipase activity. In contrast, the enzyme activity secreted from the adipocytes contained only the "a" and "i" fractions. These data implicate the existance of one intracellular form of lipoprotein lipase (corresponding to the "b" fraction), different from extracellular forms of the enzyme (corresponding to fractions "a" and "i"). A transformation of the intracellular to the extracellular forms appears to occur in conjunction with secretion of enzyme from the fat cell.

Regulation of lipoprotein lipase. Induction by insulin.

Lipoprotein lipase activity in intact epididymal adipose tissue of fasted rats increased rapidly after treatment with insulin in vivo. In contrast, lipoprotein lipase activity in adipocytes isolated from the contralateral fat pads remained essentially unchanged. When adipocytes were incubated for 30 min at ambient temperature in vitro, about 2 times more lipoprotein lipase activity was found in the medium of cells from insulin-treated rats than in medium from cells of control animals. Following insulin treatment, extracts of tissue acetone powders separated by gel chromatography showed increases in both enzyme activity fractions obtained (designated lipoprotein lipase a and b). However, no consistent differences were observed between fractions derived from adipocyte acetone powders of insulin-treated and control animals. All the observed effects of insulin on lipoprotein lipase activity were abolished by cycloheximide treatment in vivo. These data indicate that following insulin treatment, increased lipoprotein lipase activity in adipose tissue results from enhanced enzyme secretion by the fat cell and subsequent accumulation in the tissue, thus implicating the adipocyte secretory mechanism as a major site of regulation of lipoprotein lipase activity in adipose tissue.

Regulation of lipoprotein lipase immunological study of adipose tissue.

An antibody to purified rat heart lipoprotein lipase was used to determine the relative specific activities of adipose tissue lipoprotein lipase from fed and fasted rats. The antibody was immobilized by coupling it to a Sepharose gel. This antibody bound approx. 80% of the lipoprotein lipase activity of extracts of rat adipose tissue. When the extracts were separated by gel chromatography into two lipase activity fractions (lipoprotein lipase "a" and lipoprotein lipase "b") and these fractions incubated with the antibody, only 10% of the lipoprotein lipase "a" activity was bound by the highest antibody concentration employed, whereas 93% of the lipoprotein lipase "b" was bound by the same amount of antibody. Increasing amounts of antibody incubated with extracts of adipose tissue of fed or fasted rats yielded similar titration curves. When a constant amount of antibody was incubated with increasing amounts of the adipose extracts, no significant difference was noted between extracts from fed and fasted animals. The data indicate that the high lipoprotein lipase activity of adipose tissue of fed rats, compared with that of rats fasted overnight, results from the presence of more lipoprotein lipase protein.

Antibodies to lipoprotein lipase. Application to perfused heart.

An antibody was prepared against purified rat heart lipoprotein lipase. 1. This antibody showed marked species specificity. It inhibited almost totally the lipoprotein lipase activity from all rat tissues examined (i.e., heart, adipose, postheparin plasma, and mammary gland), while having no effect on the activity of lipoprotein lipase partially purified from rabbit, guinea pig and bovine heart and from bovine milk. The antibody also had no effect on the hepatic lipase activity of rat postheparin plasma. 2. After antibody to rat heart lipoprotein lipase was recirculated for 5 min through isolated rat hearts, little or no lipoprotein lipase activity could be detected in the perfusate during 0-20 s of a subsequent non-recirculating perfusion with buffer containing 1 unit heparin/ml. 3. Following recirculation of antibody to lipoprotein lipase for 10 min and a non-recirculating perfusion with buffer for 2 min, the hearts no longer oxidized any significant amounts of 14C-labelled palmitate chylomicron triacylglycerol fatty acid to 14CO2 during a 15-min perfusion. The data give compelling evidence that the functional fraction of lipoprotein lipase in hearts is at the endothelial cell surface accessible to lipoprotein lipase antibody.

Mechanisms of discordant alternans and induction of reentry in simulated cardiac tissue.

BACKGROUND: T-wave alternans, which is associated with the genesis of cardiac fibrillation, has recently been related to discordant action potential duration (APD) alternans. However, the cellular electrophysiological mechanisms responsible for discordant alternans are poorly understood. METHODS AND RESULTS: We simulated a 2D sheet of cardiac tissue using phase 1 of the Luo-Rudy cardiac action potential model. A steep (slope >1) APD restitution curve promoted concordant APD alternans and T-wave alternans without QRS alternans. When pacing was from a single site, discordant APD alternans occurred only when the pacing rate was fast enough to engage conduction velocity (CV) restitution, producing both QRS and T-wave alternans. Tissue heterogeneity was not required for this effect. Discordant alternans markedly increases dispersion of refractoriness and increases the ability of a premature stimulus to cause localized wavebreak and induce reentry. In the absence of steep APD restitution and of CV restitution, sustained discordant alternans did not occur, but reentry could be induced if there was marked electrophysiological heterogeneity. Both discordant APD alternans and preexisting APD heterogeneity facilitate reentry by causing the waveback to propagate slowly. CONCLUSION: Discordant alternans arises dynamically from APD and CV restitution properties and markedly increases dispersion of refractoriness. Preexisting and dynamically induced (via restitution) dispersion of refractoriness independently increase vulnerability to reentrant arrhythmias. Reduction of dynamically induced dispersion by appropriate alteration of electrical restitution has promise as an antiarrhythmic strategy.

Chaos and the transition to ventricular fibrillation: a new approach to antiarrhythmic drug evaluation.

Sudden cardiac death resulting from ventricular fibrillation can be separated into 2 components: initiation of tachycardia and degeneration of tachycardia to fibrillation. Clinical drug studies such as CAST and SWORD demonstrated that focusing exclusively on the first component is inadequate as a therapeutic modality. The hope for developing effective pharmacological therapy rests on a comprehensive understanding of the second component, the transition from tachycardia to fibrillation. We summarize evidence that the transition from tachycardia to fibrillation is a transition to spatiotemporal chaos, with similarities to the quasiperiodic transition to chaos seen in fluid turbulence. In this scenario, chaos results from the interaction of multiple causally independent oscillatory motions. Simulations in 2-dimensional cardiac tissue suggest that the destabilizing oscillatory motions during spiral-wave reentry arise from restitution properties of action potential duration and conduction velocity. The process of spiral-wave breakup in simulated cardiac tissue predicts remarkably well the sequence by which tachycardia degenerates to fibrillation in real cardiac tissue. Modifying action potential duration and conduction velocity restitution characteristics can prevent spiral-wave breakup in simulated cardiac tissue, suggesting that drugs with similar effects in real cardiac tissue may have antifibrillatory efficacy (the Restitution Hypothesis). If valid for the real heart, the Restitution Hypothesis will support a new paradigm for antiarrhythmic drug classification, incorporating an antifibrillatory profile based on effects on cardiac restitution and the traditional antitachycardia profile (classes 1 through 4).

Mechanisms of ventricular fibrillation induction by 60-Hz alternating current in isolated swine right ventricle.

BACKGROUND: The mechanisms by which 60-Hz alternating current (AC) can induce ventricular fibrillation (VF) are unknown. METHODS AND RESULTS: We studied 7 isolated perfused swine right ventricles in vitro. The action potential duration restitution curve was determined. Optical mapping techniques were used to determine the patterns of activation on the epicardium during 5-second 60-Hz AC stimulation (10 to 999 microA). AC captured the right ventricles at 100+/-65 microA, which is significantly lower than the direct current pacing threshold (0.77+/-0.45 mA, P:<0.05). AC induced ventricular tachycardia or VF at 477+/-266 microA, when the stimulated responses to AC had (1) short activation CLs (128+/-14 ms), (2) short diastolic intervals (16+/-9 ms), and (3) short diastolic intervals associated with a steep action potential duration restitution curve. Optical mapping studies showed that during rapid ventricular stimulation by AC, a wave front might encounter the refractory tail of an earlier wave front, resulting in the formation of a wave break and VF. Computer simulations reproduced these results. CONCLUSIONS: AC at strengths less than the regular pacing threshold can capture the ventricle at fast rates. Accidental AC leak to the ventricles could precipitate VF and sudden death if AC results in a fast ventricular rate coupled with a steep restitution curve and a nonuniform recovery of excitability of the myocardium.

Role of papillary muscle in the generation and maintenance of reentry during ventricular tachycardia and fibrillation in isolated swine right ventricle.

BACKGROUND: The role of papillary muscle (PM) in the generation and maintenance of reentry is unclear. METHODS AND RESULTS: Computerized mapping (477 bipolar electrodes, 1.6-mm resolution) was performed in fibrillating right ventricles (RVs) of swine in vitro. During ventricular fibrillation (VF), reentrant wave fronts often transiently anchored to the PM. Tissue mass reduction was then performed in 10 RVs until VF converted to ventricular tachycardia (VT). In an additional 6 RVs, procainamide infusion converted VF to VT. Maps showed that 77% (34 of 44) of all VT episodes were associated with a single reentrant wave front anchored to the PM. Purkinje fiber potentials preceded the local myocardial activation, and these potentials were recorded mostly around the PM. When PM was trimmed to the level of endocardium (n = 4), sustained VT was no longer inducible. Transmembrane potential recordings (n = 5) at the PM revealed full action potential during pacing, without evidence of ischemia. Computer simulation studies confirmed the role of PM as a spiral wave anchoring site that stabilized wave conduction. CONCLUSIONS: We conclude that PM is important in the generation and maintenance of reentry during VT and VF.

Prognostic value of dobutamine stress echocardiography in predicting cardiac events in patients with known or suspected coronary artery disease.

OBJECTIVES: The study sought to determine the utility of dobutamine stress echocardiography (DSE) in predicting cardiac events in the year after testing. BACKGROUND: Increasingly, DSE has been applied to risk stratification of patients. METHODS: Medical records of 1,183 consecutive patients who underwent DSE were reviewed. The cardiac events that occurred during the 12 months after DSE were tabulated: myocardial infarction (MI), cardiac death, percutaneous transluminal coronary angioplasty (PTCA), and coronary artery bypass surgery (CABG). Patient exclusions included organ transplant receipt or evaluation, recent PTCA, noncardiac death, and lack of follow-up. A positive stress echocardiogram (SE) was defined as new or worsened wall-motion abnormalities (WMAs) consistent with ischemia during DSE. Classification and regression tree (CART) analysis identified variables that best predicted future cardiac events. RESULTS: The average age was 68+/-12 years, with 338 women and 220 men. The overall cardiac event rate was 34% if SE was positive, and 10% if it was negative. The event rates for MI and death were 10% and 8%, respectively, if SE was positive, and 3% and 3%, respectively, if SE was negative. If an ischemic electrocardiogram (ECG) and a positive SE were present, the overall event rate was 42%, versus a 7% rate when ECG and SE were negative for ischemia. Rest WMA was the most useful variable in predicting future cardiac events using CART: 25% of patients with and 6% without a rest WMA had an event. Other important variables were a dobutamine EF <52.5%, a positive SE, an ischemic ECG response, history of hypertension and age. CONCLUSIONS: A positive SE provides useful prognostic information that is enhanced by also considering rest-wall motion, stress ECG response, and dobutamine EF.

Wave propagation in cardiac tissue and effects of intracellular calcium dynamics (computer simulation study).

Computer simulation using Luo-Rudy I1 model of ventricular myocyte showed that intracellular calcium dynamics become irregular in case of high rate stimulation. This causes the transition from stationary to nonstationary spiral wave and its breakup in 2D model of cardiac tissue. Obtained results suggest how ventricular fibrillation may occur due to the abnormalities of intracellular calcium dynamics. The short review of existing cardiac cell models with calcium dynamics is presented.

Role of lipids in osteoporosis.

Cardiovascular disease and osteoporosis together account for most of the morbidity and mortality in our aging population despite significant improvements in treatment. Recently, converging lines of evidence suggest that these 2 diseases share an etiologic factor--that hyperlipidemia contributes not only to atherosclerotic plaque formation, but also to osteoporosis, following a similar biologic mechanism involving lipid oxidation. In vitro studies indicate that lipid products of oxidation promote osteoblastic differentiation of vascular cells and inhibit such differentiation in bone cells. Ex vivo, in vivo, and clinical studies further suggest that lipid-lowering agents reduce both atherosclerotic calcification and osteoporosis. Whether lipid-lowering agents reduce osteoporosis directly or indirectly through lipid reduction remains controversial.

Chaos control of cardiac arrhythmias.

Chaos theory has shown that many disordered and erratic phenomena are in fact deterministic, and can be understood causally and controlled. The prospect that cardiac arrhythmias might be instances of deterministic chaos is therefore intriguing. We used a recently developed method of chaos control to stabilize a ouabain-induced arrhythmia in rabbit ventricular tissue in vitro. Extension of these results to clinically significant arrhythmias such as fibrillation will require overcoming the additional obstacles of spatiotemporal complexity.

Nonlinear analysis of EEG sleep states.

Recent advances in the field of nonlinear dynamics have provided new conceptual models as well as novel analytical techniques applicable to neuropsychopharmacologic studies. One measurement technique that has been recently developed in an attempt to characterize nonlinear systems in physics and biology is the estimation of dimension. Dimension may be seen as a measure of the information required to describe the current behavior of a system. We have applied these techniques to the analysis of the sleep EEG, and have found that the dimension of rapid eye movement (REM) sleep is significantly higher than non-rapid-eye-movement (NREM) sleep. These data support a preliminary hypothesis that EEG dimension may represent the number of nonlinear modes activated in the brain. Thus, sleep states of low arousal or low input would be envisioned as having low dimension (e.g., slow-wave sleep) whereas increased arousal (REM) would activate more nonlinear modes. Although more investigations will be needed to explore this hypothesis, these studies suggest that further development of nonlinear approaches to the analysis of brain systems are likely to generate new clinical measures as well as new ways of viewing brain electrical function.

Purification and characterization of lipoprotein lipase from human heart.

Human heart lipoprotein lipase was purified by affinity chromatography on heparin-Sepharose 4B. When crude extracts of heart acetone powder were applied to columsn, about 40% of total lipase activity was bound to the gel and then eluted with 1.5 M NaCl. At this stage the eluted enzyme was purified 1900-fold. Disc gel electrophoresis yielded a single protein band corresponding with lipolytic activity. Minimum molecular weight of the protein was 60,000 as determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The purified enzyme was highly unstable; however, its activity could be partially stabilized at --20C by bovine serum albumin, glycerol, or ethylene glycol. The activity of the purified enzyme (i) had a pH optimum between 7.8 and 8.0; (ii) required serum for full enzymatic activity; apoC-II could be substituted for serum; (iii) was inhibited by by apoC-I in the presence of activated substrate; (iv) was markedly inhibited by NaCl; and (v) was stimulated by heparin.

Rat heart lipoprotein lipase.

Rat heart lipoprotein lipase was highly purified by affinity chromatography using heparin-Sepharose 4B. When extracts of acetone powder were applied to columns, lipase activity was firmly bound to the gel matrix and was later eluted with 1.5 M NaCl. At this stage the eluted enzyme was purified 1500-fold. Disc gel electrophoresis yielded a single protein band corresponding with the enzyme activity. The apparent molecular weight was 60,000. The purified enzyme was highly unstable; however, its activity could be partially stabilized by glycerol or ethylene glycol. In studying the purified enzyme we observed: (i) a cofactor in serum was required for full enzymatic activity; ApoLp-Glu could be substituted for this cofactor; (ii) ApoLp-Ser was inhibitory to lipase activity; (iii) NaCl and protamine sulfate also markedly inhibited the lipase activity; (iv) heparin stimulated the enzymatic activity.