Mechanoelectrical feedback: independent role of preload and contractility in modulation of canine ventricular excitability

Occurrence of Dysrhythmias During Ventilatory Weaning and Its Association With Length of Mechanical Ventilation and In-hospital Complications

BACKGROUND

The occurrence of cardiac dysrhythmias during ventilatory weaning and its impact on the length of ventilation and occurrence of complications have not been systematically investigated.

OBJECTIVES

The aim of this study was to evaluate the association between cardiac dysrhythmias during weaning and the duration of ventilation and complications during ventilator weaning.

METHOD

Data on the length of ventilation and complications were collected from the medical records of 30 mechanically ventilated patients. Continuous electrocardiographic recordings were collected at baseline and during the initial weaning trial. Multiple regression analysis was used to evaluate the association between dysrhythmias and length of ventilation. The relationship between prolonged ventilation and complications was assessed using the χ2 analysis.

RESULTS

Supraventricular ectopic beats during weaning positively predicted the length of ventilation (P < .01). Prolonged ventilation (>7 days) is associated with the occurrence of renal insufficiency and septicemia.

CONCLUSION

The association between the occurrence of supraventricular ectopic beats during ventilator weaning and the length of ventilation requires further evaluation and tailored management to improve patient outcomes.

Differences in Mechanical, Electrical and Calcium Transient Performance of the Isolated Right Atrial and Ventricular Myocardium of Guinea Pigs at Different Preloads (Lengths)

There are only a few studies devoted to the comparative and simultaneous study of the mechanisms of the length-dependent regulation of atrial and ventricular contractility. Therefore, an isometric force-length protocol was applied to isolated guinea pig right atrial (RA) strips and ventricular (RV) trabeculae, with a simultaneous measurement of force (Frank-Starling mechanism) and Ca2+ transients (CaT) or transmembrane action potentials (AP). Over the entire length-range studied, the duration of isometric contraction, CaT and AP, were shorter in the RA myocardium than in the RV myocardium. The RA myocardium was stiffer than the RV myocardium. With the increasing length of the RA and RV myocardium, the amplitude and duration of isometric contraction and CaT increased, as well as the amplitude and area of the "CaT difference curves" (shown for the first time). However, the rates of the tension development and relaxation decreased. No contribution of AP duration to the heterometric regulation of isometric tension was found in either the RA or RV myocardium of the guinea pig. Changes in the degree of overlap of the contractile proteins of the guinea pig RA and RV myocardium mainly affect CaT kinetics but not AP duration.

Cardiac Mechano-Electric Coupling: Acute Effects of Mechanical Stimulation on Heart Rate and Rhythm

The heart is vital for biological function in almost all chordates, including humans. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops, so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer term changes in gene expression and functional or structural remodeling). This process is known as mechano-electric coupling (MEC). In the current review, we present evidence for, and implications of, MEC in health and disease in human; summarize our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modeling in developing a more comprehensive understanding of ‟what makes the heart tick.ˮ.

Pathophysiology of Hypertensive Heart Disease: Beyond Left Ventricular Hypertrophy

PURPOSE OF REVIEW

Given that the life expectancy and the burden of hypertension are projected to increase over the next decade, hypertensive heart disease (HHD) may be expected to play an even more central role in the pathophysiology of cardiovascular disease (CVD). A broader understanding of the features and underlying mechanisms that constitute HHD therefore is of paramount importance.

RECENT FINDINGS

HHD is a condition that arises as a result of elevated blood pressure and constitutes a key underlying mechanism for cardiovascular morbidity and mortality. Historically, studies investigating HHD have primarily focused on left ventricular (LV) hypertrophy (LVH), but it is increasingly apparent that HHD encompasses a range of target-organ damage beyond LVH, including other cardiovascular structural and functional adaptations that may occur separately or concomitantly. HHD is characterized by micro- and macroscopic myocardial alterations, structural phenotypic adaptations, and functional changes that include cardiac fibrosis, and the remodeling of the atria and ventricles and the arterial system. In this review, we summarize the structural and functional alterations in the cardiac and vascular system that constitute HHD and underscore their underlying pathophysiology.

Effects of gadolinium on cardiac mechanosensitivity in whole isolated swine hearts

Mechanical stimulation can elicit electrical activation of the heart. This mechanosensitivity can start life-threatening arrhythmias (commotio cordis) or terminate them (precordial thump). Mechanosensitivity may also be involved in arrhythmogenesis in other settings. Stretch-activated ion channels (SACs) are thought to be important in mechanosensitivity and a number of agents that block them have been identified. Such agents could potentially be used as tools in experimental investigation of mechanosensitivity. However, studies using them in intact-heart preparations have yielded inconsistent results. In the present study, we used isolated, perfused hearts from 25-35 kg pigs and a computer-controlled device that repeatably delivered focal mechanical stimuli. The concentration-dependent ability of the SAC blocker gadolinium to suppress mechanical activation was assessed by the success rate of mechanical stimulation and by the delay between successful mechanical stimulation and electrical activation. In six hearts, perfusate was recirculated. In an additional six hearts, perfusate was not recirculated to prevent gadolinium from forming complexes with metabolic waste and possibly precipitating. Gadolinium did not suppress mechanically-induced activation. Although gadolinium has been shown to be an effective SAC blocker in isolated cells, using it to probe the role of mechanical stimulation in whole heart preparations should be done with great caution.

Measurement variability of right atrial and ventricular monophasic action potential and refractory period measurements in the standing non-sedated horse

BACKGROUND

In human and veterinary medicine, monophasic action potential (MAP) analysis and determination of local refractory periods by contact electrode technique gives valuable information about local cardiac electrophysiological properties. It is used to investigate dysrhythmias and the impact of drugs on the myocardium. Precise measurement of total MAP duration is difficult, therefore the MAP duration is usually determined at a repolarization level of 90% (APD90). Until now, no studies are published about the feasibility of this technique in the standing non-sedated horse. In 6 healthy Warmblood horses, on two different days, an 8F quadripolar contact catheter was passed through a jugular introducer sheath and placed under ultrasound guidance at the level of the intervenous tubercle or right atrial free wall (RA), and in the right ventricular apex (RV) to record the MAP. The MAP amplitude and APD90 were measured at a resting sinus rhythm (heart rate of 30-42 bpm) and at pacing cycle lengths (PCL) of 1000 and 600 ms. The effective refractory period (ERP) was determined at PCL of 1000 and 600 ms.

RESULTS

The overall mean (±SD) APD90 (rest), APD90 (1000) and APD90 (600) were 263 ± 39 ms, 262 ± 41 ms, 236 ± 47 ms for the RA and 467 ± 23 ms, 412 ± 38 ms, 322 ± 29 ms for the RV. The mean ERP1000 and ERP600 were 273 ± 24 ms and 256 ± 22 ms for the RA and 386 ± 40 ms and 293 ± 30 ms for the RV. The measurement variability for the amplitude, APD90 and ERP measurements in the RA ranged between 36 and 44, 9-22 and 7-8%, respectively. The measurement variability for the amplitude, APD90 and ERP measurements in the RV ranged between 49 and 66, 6-7 and 10-12%, respectively.

CONCLUSIONS

RA and RV MAP duration and ERP can be obtained by a contact electrode in standing non-sedated horses. The measurement variability varies with catheter location.

Intensive Exercise Training Improves Cardiac Electrical Stability in Myocardial-Infarcted Rats

Background

Moderate exercise training has been shown to decrease sudden cardiac death post myocardial infarction. However, the effects of intensive exercise are still controversial.

Methods and Results

Fourteen myocardial‐infarcted rats were divided into sedentary (n=8) and intensive training groups (n=6) and 18 sham control rats to sedentary (n=10) and intensive training groups (n=8). Heart rate variability was obtained at weeks 1 and 8. The inducibility of ventricular tachycardia/fibrillation was assessed in a Langendorff system. Fast Fourier transforms were applied on the recorded ventricular tachycardia/fibrillations. Training reduces low to high frequency ratio of heart rate variability at week 8 compared with that at week 1 (P<0.05). In isolated hearts, the probability for ventricular tachycardia/fibrillation was decreased from 4.5±0.8% in sedentary controls to 1.56±0.2% in intensive training controls (P<0.05) and from 13.5±2.1% in the sedentary group to 5.4±1.2% in the intensive training group (P<0.01). Moreover, the pacing current required for ventricular fibrillation induction in the trained groups was increased following exercise (P<0.05). Fast Fourier transform analysis of ECG findings revealed an exercise‐induced ventricular fibrillation transition from a narrow, single‐peak spectrum at 17 Hz in sedentary controls to a broader range of peaks ranging from 13 to 22 Hz in the intensive training controls.

Conclusions

Intensive exercise in infarcted rats leads to reduced ventricular fibrillation propensity and is associated with normalization of refractoriness and intrinsic spatiotemporal electrical variations.

Heart rate variability as a predictor of cardiac dysrhythmias during weaning from mechanical ventilation

BACKGROUND

Weaning from mechanical ventilation to spontaneous breathing is associated with changes in the hemodynamic and autonomic nervous systems that are reflected by heart rate variability. Although cardiac dysrhythmias are an important manifestation of hemodynamic alterations, the impact of heart rate variability on the occurrence of dysrhythmias during weaning has not been specifically studied.

OBJECTIVES

To describe differences in heart rate variability spectral power and occurrence of cardiac dysrhythmias at baseline and during the initial trial of weaning from mechanical ventilation and to evaluate the impact of heart rate variability during weaning on occurrence of dysrhythmias.

METHOD

Continuous 3-lead electrocardiographic recordings were collected from 35 patients receiving mechanical ventilation for 24 hours at baseline and during the initial weaning trial. Heart rate variability was evaluated by using spectral power analysis.

RESULTS

Low-frequency power increased (P = .04) and high-frequency and very-low-frequency power did not change during weaning. The mean number of supraventricular ectopic beats per hour during weaning was higher than the mean at baseline (P < .001); the mean of ventricular ectopic beats did not change. Low-frequency power was a predictor of ventricular and supraventricular ectopic beats during weaning (P < .001). High-frequency power was predictive of ventricular and supraventricular (P = .02) ectopic beats during weaning. Very-low-frequency power was predictive of ventricular ectopic beats (P < .001) only.

CONCLUSION

Heart rate variability power spectra during weaning were predictive of dysrhythmias.

Living cardiac tissue slices: an organotypic pseudo two-dimensional model for cardiac biophysics research

Living cardiac tissue slices, a pseudo two-dimensional (2D) preparation, have received less attention than isolated single cells, cell cultures, or Langendorff-perfused hearts in cardiac biophysics research. This is, in part, due to difficulties associated with sectioning cardiac tissue to obtain live slices. With moderate complexity, native cell-types, and well-preserved cell-cell electrical and mechanical interconnections, cardiac tissue slices have several advantages for studying cardiac electrophysiology. The trans-membrane potential (Vm) has, thus far, mainly been explored using multi-electrode arrays. Here, we combine tissue slices with optical mapping to monitor Vm and intracellular Ca(2+) concentration ([Ca(2+)]i). This combination opens up the possibility of studying the effects of experimental interventions upon action potential (AP) and calcium transient (CaT) dynamics in 2D, and with relatively high spatio-temporal resolution. As an intervention, we conducted proof-of-principle application of stretch. Mechanical stimulation of cardiac preparations is well-established for membrane patches, single cells and whole heart preparations. For cardiac tissue slices, it is possible to apply stretch perpendicular or parallel to the dominant orientation of cells, while keeping the preparation in a constant focal plane for fluorescent imaging of in-slice functional dynamics. Slice-to-slice comparison furthermore allows one to assess transmural differences in ventricular tissue responses to mechanical challenges. We developed and tested application of axial stretch to cardiac tissue slices, using a manually-controlled stretching device, and recorded Vm and [Ca(2+)]i by optical mapping before, during, and after application of stretch. Living cardiac tissue slices, exposed to axial stretch, show an initial shortening in both AP and CaT duration upon stretch application, followed in most cases by a gradual prolongation of AP and CaT duration during stretch maintained for up to 50 min. After release of sustained stretch, AP duration (APD) and CaT duration reverted to shorter values. Living cardiac tissue slices are a promising experimental model for the study of cardiac mechano-electric interactions. The methodology described here can be refined to achieve more accurate control over stretch amplitude and timing (e.g. using a computer-controlled motorised stage, or by synchronising electrical and mechanical events) and through monitoring of regional tissue deformation (e.g. by adding motion tracking).

Relevance of cardiomyocyte mechano-electric coupling to stretch-induced arrhythmias: optical voltage/calcium measurement in mechanically stimulated cells, tissues and organs

Stretch-induced arrhythmias are multi-scale phenomena in which alterations in channel activities and/or calcium handling lead to the organ level derangement of the heart rhythm. To understand how cellular mechano-electric coupling (MEC) leads to stretch-induced arrhythmias at the organ level, we developed stretching devices and optical voltage/calcium measurement techniques optimized to each cardiac level. This review introduces these experimental techniques of (1) optical voltage measurement coupled with a carbon-fiber technique for single isolated cardiomyocytes, (2) optical voltage mapping combined with motion tracking technique for myocardial tissue/whole heart preparations and (3) real-time calcium imaging coupled with a laser optical trap technique for cardiomyocytes. Following the overview of each methodology, results are presented. We conclude that individual MEC in cardiomyocytes can be heterogeneous at the ventricular level, especially when moderate amplitude mechanical stretches are applied to the heart, and that this heterogeneous MEC can evoke focal excitation that develops into re-entrant arrhythmias.

The Relationship between Left Atrial Volume and Ventricular Arrhythmias in the Patients with Dilated Cardiomyopathy

BACKGROUND

The present study aimed to investigate the relationship between Left Atrial Volume (LAV), a marker of diastolic dysfunction, and the frequency of malignant ventricular arrhythmia in the patients with left ventricular dysfunction and a previously implanted Implantable Cardioverter Defibrillator (ICD) device.

METHODS

This cross-sectional study was conducted on 32 patients with ischemic or idiopathic dilated cardiomyopathy, each having had an ICD device implanted at least 1 year beforehand. The ventricular arrhythmia episodes which were detected and stored by the device were retrieved and evaluated. In addition to routine echocardiographic measurements, all the patients had their LAV and LAV indexes calculated. After all, student's t-test, Mann-Whitney U test, and Pearson correlation were used to analyze the data. Besides, P value < 0.05 was considered as statistically significant.

RESULTS

This study was conducted on 4 female and 28 male patients with the mean age of 58.41 ± 9.97 years. Among the study patients, 21 had at least one previous myocardial infarction. In addition, 17 patients had experienced sustained VT or VF within the last year. No significant difference was found between the patients with and without malignant ventricular arrhythmias (sustained VT or VF) regarding LAV (17 patients with arrhythmia (68 + 23.39 mL) vs. 15 patients without arrhythmia (55.13 ± 20.41 mL); P = 0.100). However, the LAV index was significantly higher in the patients with arrhythmia compared to those without arrhythmia (39.27 ± 12.19 mL / m2 vs. 25.18 ± 7.45 mL / m2; P = 0.004). Both LAV (73.33 ± 17.64 mL and 57.52 ± 23.15 mL, respectively; P = 0.040) and LAV index (40.86 ± 8.47 mL / m2 and 28.20 ± 11.77 mL / m2, respectively; P = 0.010) were significantly greater in the patients with ICD shock therapy within the last year compared to the others. However, both groups were similar regarding Left Ventricular Volume (LVV), LVV index, and ejection fraction.

CONCLUSIONS

The study findings demonstrated that LAV and LAV index could be used in detecting the patients who are at high risk of malignant ventricular arrhythmias.

Dissociation of hemodynamic and electrocardiographic indexes of myocardial ischemia in pigs with hibernating myocardium and sudden cardiac death

Many survivors of sudden cardiac death (SCD) have normal global ventricular function and severe coronary artery disease but no evidence of symptomatic ischemia or infarction before the development of lethal ventricular arrhythmias, and the trigger for ventricular tachycardia (VT)/ventricular fibrillation (VF) remains unclear. We sought to identify the role of spontaneous ischemia and temporal hemodynamic factors preceding SCD using continuous telemetry of left ventricular (LV) pressure and the ECG for periods up to 5 mo in swine (n = 37) with hibernating myocardium who experience spontaneous VT/VF in the absence of heart failure or infarction. Hemodynamics and ST deviation at the time of VT/VF were compared with survivors with hibernating myocardium as well as sham controls. All episodes of VT/VF occurred during sympathetic activation and were initiated by single premature ventricular contractions, and the VT degenerated into VF in ∼ 30 s. ECG evidence of ischemia was infrequent and no different from those that survived. Baseline hemodynamics were no different among groups, but LV end-diastolic pressure during sympathetic activation was higher at the time of SCD (37 ± 4 vs. 26 ± 4 mmHg, P < 0.05) and the ECG demonstrated QT shortening (155 ± 4 vs. 173 ± 5 ms, P < 0.05). The week before SCD, both parameters were no different from survivors. These data indicate that there are no differences in the degree of sympathetic activation or hemodynamic stress when VT/VF develops in swine with hibernating myocardium. The transiently elevated LV end-diastolic pressure and QT shortening preceding VT/VF raises the possibility that electrocardiographically silent subendocardial ischemia and/or mechanoelectrical feedback serve as a trigger for the development of SCD in chronic ischemic heart disease.

Role of Na(+)-H(+) exchange in the modulation of L-type Ca(2+) current during fluid pressure in rat ventricular myocytes

Application of fluid pressure (FP) using pressurized fluid flow suppresses the L-type Ca(2+) current through both enhancement of Ca(2+) release and intracellular acidosis in ventricular myocytes. As FP-induced intracellular acidosis is more severe during the inhibition of Na(+)-H(+) exchange (NHE), we examined the possible role of NHE in the regulation of I(Ca) during FP exposure using HOE642 (cariporide), a specific NHE inhibitor. A flow of pressurized (~16 dyn/cm(2)) fluid was applied onto single rat ventricular myocytes, and the I(Ca) was monitored using a whole-cell patch-clamp under HEPES-buffered conditions. In cells pre-exposed to FP, additional treatment with HOE642 dose-dependently suppressed the I(Ca) (IC(50) = 0.97 ± 0.12 μM) without altering current-voltage relationships and inactivation time constants. In contrast, the I(Ca) in control cells was not altered by HOE642. The HOE642 induced a left shift in the steady-state inactivation curve. The suppressive effect of HOE642 on the I(Ca) under FP was not altered by intracellular high Ca(2+) buffering. Replacement of external Cl(-) with aspartate to inhibit the Cl(-)-dependent acid loader eliminated the inhibitory effect of HOE642 on I(Ca). These results suggest that NHE may attenuate FP-induced I(Ca) suppression by preventing intracellular H(+) accumulation in rat ventricular myocytes and that NHE activity may not be involved in the Ca(2+)-dependent inhibition of the I(Ca) during FP exposure.

Distension of the ischemic region predicts increased ventricular fibrillation inducibility following coronary occlusion in swine

INTRODUCTION AND OBJECTIVES

Distension of the ischemic region has been related to an increased incidence of spontaneous ventricular arrhythmias following coronary occlusion. This study analyzed whether regional ischemic distension predicts increased ventricular fibrillation inducibility after coronary occlusion in swine.

METHODS

In 18 anesthetized, open-chest pigs, the left anterior descending coronary artery was ligated for 60 min. Myocardial segment length in the ischemic region was monitored by means of ultrasonic crystals. Programmed stimulation was applied at baseline and then continuously between 10 and 60 min after coronary occlusion.

RESULTS

Coronary occlusion induced a rapid increase in end-diastolic length in the ischemic region, which reached 109.4% (0.9%) of baseline values 10 min after occlusion (P<.001). On average, 6.6 (0.5) stimulation protocols were completed and 5.4 (0.6) ventricular fibrillation episodes induced between 10 and 60 min of coronary occlusion. Neither baseline serum potassium levels nor the size of the ischemic region were significantly related to ventricular fibrillation inducibility. In contrast, the increase in end-diastolic length 10 min after coronary occlusion was associated directly (r=0.67; P=.002) with the number of induced ventricular fibrillation episodes and inversely (r=-0.55; P=.018) with the number of extrastimuli needed for ventricular fibrillation induction.

CONCLUSIONS

Regional ischemic expansion predicts increased ventricular fibrillation inducibility following coronary occlusion. These results highlight the potential influence of mechanical factors, acting not only on the triggers but also on the substrate, in the genesis of malignant ventricular arrhythmias during acute ischemia.

Ranolazine decreases mechanosensitivity of the voltage-gated sodium ion channel Na(v)1.5: a novel mechanism of drug action

BACKGROUND

Na(V)1.5 is a mechanosensitive voltage-gated sodium-selective ion channel responsible for the depolarizing current and maintenance of the action potential plateau in the heart. Ranolazine is a Na(V)1.5 antagonist with antianginal and antiarrhythmic properties.

METHODS AND RESULTS

Mechanosensitivity of Na(V)1.5 was tested in voltage-clamped whole cells and cell-attached patches by bath flow and patch pressure, respectively. In whole cells, bath flow increased peak inward current in both murine ventricular cardiac myocytes (24±8%) and human embryonic kidney 293 cells heterologously expressing Na(V)1.5 (18±3%). The flow-induced increases in peak current were blocked by ranolazine. In cell-attached patches from cardiac myocytes and Na(V)1.5-expressing human embryonic kidney 293 cells, negative pressure increased Na(V) peak currents by 27±18% and 18±4% and hyperpolarized voltage dependence of activation by -11 mV and -10 mV, respectively. In human embryonic kidney 293 cells, negative pressure also increased the window current (250%) and increased late open channel events (250%). Ranolazine decreased pressure-induced shift in the voltage dependence (IC(50) 54 μmol/L) and eliminated the pressure-induced increases in window current and late current event numbers. Block of Na(V)1.5 mechanosensitivity by ranolazine was not due to the known binding site on DIVS6 (F1760). The effect of ranolazine on mechanosensitivity of Na(V)1.5 was approximated by lidocaine. However, ionized ranolazine and charged lidocaine analog (QX-314) failed to block mechanosensitivity.

CONCLUSIONS

Ranolazine effectively inhibits mechanosensitivity of Na(V)1.5. The block of Na(V)1.5 mechanosensitivity by ranolazine does not utilize the established binding site and may require bilayer partitioning. Ranolazine block of Na(V)1.5 mechanosensitivity may be relevant in disorders of mechanoelectric dysfunction.

Heterogeneity of ventricular repolarization in newborns with severe aortic coarctation

Sudden death is a possible occurrence for newborns younger than 1 year with severe aortic coarctation (CoA) before surgical correction. Basic research and animal experiments have shown electrophysiologic changes during mechanical ventricular pressure overload. The current study aimed to evaluate the effect of severe CoA on the heterogeneity of ventricular repolarization by examining corrected QT and JT interval dispersion (respectively, QTc-D and JTc-D) and electrocardiographic parameters of spatial heterogeneity of ventricular repolarization in newborns with no associated congenital cardiac malformations. The study enrolled 30 isolated severe CoA neonates (age, 45 ± 15 days; 17 males) with normal size and wall thickness of the left ventricle before surgical correction and 30 age- and sex-matched healthy newborns used as control subjects. Heart rate, QRS duration, maximum and minimum QT and JT intervals, and QTc-D and JTc-D measurements were performed. The healthy control group did not significantly differ from the CoA group in terms of heart rate, weight, height, and echocardiographic parameters. Compared with the healthy control group, the CoA group presented significantly increased values of QTc-D (109.7 ± 43.4 vs. 23 ± 15 ms; P = 0.03) and JTc-D (99.1 ± 43.3 vs. 65.8 ± 24.1 ms; P = 0.04). A statistically significant correlation was found between the Doppler peak pressure gradient across the coarctation site and the values of QTc-D (r = 0.48; P = 0.03) and JTc-D (r = 0.42; P = 0.04). Our study showed significantly increased QTc-D and JTc-D in isolated CoA newborns with normal left ventricular geometry.

Temporal associations between thoracic volume overload and malignant ventricular arrhythmias: a study of intrathoracic impedance

BACKGROUND

Acute exacerbations of heart failure (HF) are believed to trigger malignant ventricular arrhythmias, but the temporal association between fluid accumulation and ventricular arrhythmias has not been evaluated in an objective manner. We hypothesized that increased intrathoracic fluid accumulation in patients with HF, as measured by an index of intrathoracic impedance, is associated with an increased risk of ventricular arrhythmias.

METHODS AND RESULTS

We analyzed interrogations in a cohort of 96 patients with left ventricular dysfunction (EF ≤ 35%) with devices that monitor intrathoracic impedance (OptiVol fluid index). Treated episodes of ventricular tachycardia or fibrillation (VT/VF) were adjudicated and stratified according to predetermined fluid index thresholds (OptiVol indices of 15, 30, 45, 60 Ω-days). VT/VF episodes occurred in 16 patients (17%). VT/VF was more common on days when the fluid index was elevated using threshold values of 15, 30, and 45 Ω-days (P = 0.006, 0.04, 0.02, respectively). There were no differences in the percent of time above any threshold between patients with and without VT/VF.

CONCLUSIONS

In patients with HF who develop VT/VF, volume overload, as detected by an index incorporating changes in intrathoracic impedance, was temporally associated with malignant ventricular tachyarrhythmias.

Pharmacological modifications of the stretch-induced effects on ventricular fibrillation in perfused rabbit hearts

Stretch induces modifications in myocardial electrical and mechanical activity. Besides the effects of substances that block the stretch-activated channels, other substances could modulate the effects of stretch through different mechanisms that affect Ca2+ handling by myocytes. Thirty-six Langendorff-perfused rabbit hearts were used to analyze the effects of the Na+/Ca2+ exchanger blocker KB-R7943, propranolol, and the adenosine A2 receptor antagonist SCH-58261 on the acceleration of ventricular fibrillation (VF) produced by acute myocardial stretching. VF recordings were obtained with two epicardial multiple electrodes before, during, and after local stretching in four experimental series: control ( n = 9), KB-R7943 (1 μM, n = 9), propranolol (1 μM, n = 9), and SCH-58261 (1 μM, n = 9). Both the Na+/Ca2+ exchanger blocker KB-R7943 and propranolol induced a significant reduction ( P < 0.001 and P < 0.05, respectively) in the dominant frequency increments produced by stretching with respect to the control and SCH-58261 series (control = 49.9%, SCH-58261 = 52.1%, KB-R7943 = 9.5%, and propranolol = 12.5%). The median of the activation intervals, the functional refractory period, and the wavelength of the activation process during VF decreased significantly under stretch in the control and SCH-58261 series, whereas no significant variations were observed in the propranolol and KB-R7943 series, with the exception of a slight but significant decrease in the median of the fibrillation intervals in the KB-R7943 series. KB-R7943 and propranolol induced a significant reduction in the activation maps complexity increment produced by stretch with respect to the control and SCH-58261 series. In conclusion, the electrophysiological effects responsible for stretch-induced VF acceleration in the rabbit heart are reduced by the Na+/Ca2+ exchanger blocker KB-R7943 and by propranolol but not by the adenosine A2 receptor antagonist SCH-58261.

Myofilament Ca2+ sensitization causes susceptibility to cardiac arrhythmia in mice

In human cardiomyopathy, anatomical abnormalities such as hypertrophy and fibrosis contribute to the risk of ventricular arrhythmias and sudden death. Here we have shown that increased myofilament Ca2+ sensitivity, also a common feature in both inherited and acquired human cardiomyopathies, created arrhythmia susceptibility in mice, even in the absence of anatomical abnormalities. In mice expressing troponin T mutants that cause hypertrophic cardiomyopathy in humans, the risk of developing ventricular tachycardia was directly proportional to the degree of Ca2+ sensitization caused by the troponin T mutation. Arrhythmia susceptibility was reproduced with the Ca2+-sensitizing agent EMD 57033 and prevented by myofilament Ca2+ desensitization with blebbistatin. Ca2+ sensitization markedly changed the shape of ventricular action potentials, resulting in shorter effective refractory periods, greater beat-to-beat variability of action potential durations, and increased dispersion of ventricular conduction velocities at fast heart rates. Together these effects created an arrhythmogenic substrate. Thus, myofilament Ca2+ sensitization represents a heretofore unrecognized arrhythmia mechanism. The protective effect of blebbistatin provides what we believe to be the first direct evidence that reduction of Ca2+ sensitivity in myofilaments is antiarrhythmic and might be beneficial to individuals with hypertrophic cardiomyopathy.

Fluid pressure modulates L-type Ca2+ channel via enhancement of Ca2+-induced Ca2+ release in rat ventricular myocytes

This study examines whether fluid pressure (FP) modulates the L-type Ca2+ channel in cardiomyocytes and investigates the underlying cellular mechanism(s) involved. A flow of pressurized (∼16 dyn/cm2) fluid, identical to that bathing the myocytes, was applied onto single rat ventricular myocytes using a microperfusion method. The Ca2+ current ( ICa) and cytosolic Ca2+ signals were measured using a whole cell patch-clamp and confocal imaging, respectively. It was found that the FP reversibly suppressed ICa (by 25%) without altering the current-voltage relationships, and it accelerated the inactivation of ICa. The level of ICa suppression by FP depended on the level and duration of pressure. The Ba2+ current through the Ca2+ channel was only slightly decreased by the FP (5%), suggesting an indirect inhibition of the Ca2+ channel during FP stimulation. The cytosolic Ca2+ transients and the basal Ca2+ in field-stimulated ventricular myocytes were significantly increased by the FP. The effects of the FP on the ICa and on the Ca2+ transient were resistant to the stretch-activated channel inhibitors, GsMTx-4 and streptomycin. Dialysis of myocytes with high concentrations of BAPTA, the Ca2+ buffer, eliminated the FP-induced acceleration of ICa inactivation and reduced the inhibitory effect of the FP on ICa by ≈80%. Ryanodine and thapsigargin, abolishing sarcoplasmic reticulum Ca2+ release, eliminated the accelerating effect of FP on the ICa inactivation, and they reduced the inhibitory effect of FP on the ICa. These results suggest that the fluid pressure indirectly suppresses the Ca2+ channel by enhancing the Ca2+-induced intracellular Ca2+ release in rat ventricular myocytes.

The role of mechanoelectric feedback in vulnerability to electric shock

Experimental and clinical studies have shown that ventricular dilatation is associated with increased arrhythmogenesis and elevated defibrillation threshold; however, the underlying mechanisms remain poorly understood. The goal of the present study was to test the hypothesis that (1) stretch-activated channel (SAC) recruitment and (2) geometrical deformations in organ shape and fiber architecture lead to increased arrhythmogenesis by electric shocks following acute ventricular dilatation. To elucidate the contribution of these two factors, the study employed, for the first time, a combined electro-mechanical simulation approach. Acute dilatation was simulated in a model of rabbit ventricular mechanics by raising the LV end-diastolic pressure from 0.6 (control) to 4.2 kPa (dilated). The output of the mechanics model was used in the electrophysiological model. Vulnerability to shocks was examined in the control, the dilated ventricles, and in the dilated ventricles that also incorporated currents through SAC as a function of local strain, by constructing vulnerability grids. Results showed that dilatation-induced deformation alone decreased upper limit of vulnerability (ULV) slightly and did not result in increased vulnerability. With SAC recruitment in the dilated ventricles, the number of shock-induced arrhythmia episodes increased by 37% (from 41 to 56) and the lower limit of vulnerability (LLV) decreased from 9 to 7 V/cm, while ULV did not change. The heterogeneous activation of SAC caused by the heterogeneous fiber strain in the ventricular walls was the main reason for increased vulnerability to electric shocks since it caused dispersion of electrophysiological properties in the tissue, resulting in postshock unidirectional block and establishment of reentry.

Mechanosensitive-mediated interaction, integration, and cardiac control

This review covers aspects of the cardiac mechanotransduction field at different levels, and advocates the possibility that mechanoelectro-chemical transduction forms part of a network of mechanically linked integration in heart-mechanically mediated integration (MMI). It assembles evidence and observations in the literature to promote this hypothesis. Mechanical components can provide the bond between interactions at molecular, cellular, and macro levels to enable the integration. Stretch-activated channels (SACs) exist in the heart, but stresses and strains can affect other membrane channels or receptors. A cellular mechanical change can thus promote several ionic or downstream changes. Cell signal cascades have been implicated and can affect membrane electrophysiology. MMI could shape intracellular and downstream signals using the cytoskeleton and intracellular Ca(2+). MMI also spans other regulatory systems and processes such as the autonomic nervous system (ANS) and operates throughout the whole heart as an integrative system. Finally, supporting the hypothesis, if elements of the normal integration become deranged it contributes to cardiovascular disease and, potentially, lethal arrhythmia.

Prominent positive U waves appearing with high-dose intravenous phenylephrine

A 71-year-old woman who had critical cerebral ischemia secondary to a carotid artery occlusion was receiving high-dose intravenous phenylephrine for a trial of hypertensive therapy. While on a maximal dose of phenylephrine she developed prominent positive U waves, which disappeared with the cessation of the drug. The possible underlying electrophysiological mechanisms of this phenomenon are discussed in this paper.

The negative U wave: a pathogenetic enigma but a useful, often overlooked bedside diagnostic and prognostic clue in ischemic heart disease

The pathogenesis of U-wave inversion and its clinical value are still not clear, although the U wave was described by Einthoven together with the other electrocardiographic (ECG) waves. Not considered a useful diagnostic clue, it is not usually mentioned in ECG reports. In recent years, stimulated by the long QT syndromes and by the discovery of U-wave changes in some pathologic, mostly cardiac states, this neglected wave has attracted new interest. This review focuses on the negativity of the U wave in ischemic heart disease. The discovery of M cells and their electrophysiology has established the cellular basis for repolarization and has contributed to our knowledge of U-wave genesis. Hemodynamic changes during diastole in acute ischemia also furnish interesting elements for the interpretation of U-wave changes, and some experimental and clinical studies, besides designating stretch as a cause of U-wave changes, have also proved their value for more accurate bedside diagnosis and prognosis. They may indicate the extent of myocardial ischemia, the presence of collateral circulation, and the possible territory and vessel involved. When U-wave changes are the first and only sign of ischemia, they may contribute to a decision regarding the hospital admission of a patient without typical ischemic symptoms. Furthermore, U-wave changes during exercise tests increase their sensitivity.

Enhanced external counterpulsation therapy: significant clinical improvement without electrophysiologic remodeling

BACKGROUND

Enhanced external counterpulsation therapy (EECP), in addition to improving coronary flow and increasing the time to ischemia, noninvasively alters hemodynamics in patients with severe coronary artery disease (CAD). Other treatments that alter hemodynamics, for example, balloon valvuloplasty, left ventricular assist devices, and pharmacologic antagonism of the rennin-angiotensin system, promote electrophysiologic remodeling, as evidenced by alterations in the QT interval.

METHODS

We studied 28 patients who completed a 7-week, 35-hour session of EECP to assess whether such therapy would also result in electrophysiologic remodeling.

RESULTS

All patients had class II-III angina, imaging-proven ischemia, and severe, near-inoperable CAD. Of 28 patients, with a mean age 62 +/- 13 years (mean +/- SD), 78% were male, 46% diabetic, 82% hypertensive, 60% had undergone angioplasty, and 67% had undergone bypass surgery. The mean ejection fraction was 44% (range 25-60%). Following EECP, most patients (82%) had at least a one full class improvement in their anginal pattern. In most patients, there was substantial baseline conduction system disease present: a mean QRS of 105 +/- 19 ms. It is to be noted that there was no significant change in heart rate (HR), PR, QRS, or QT(c) intervals before and after EECP in either clinical responders or nonresponders. When analyzed by response to EECP, ejection fraction, or history of revascularization, there were still no detectable changes in ECG parameters (all P = NS).

CONCLUSIONS

While EECP remains an effective treatment for severe CAD, it does not prompt early electrical remodeling of the heart.

Stretch-induced regional mechanoelectric dispersion and arrhythmia in the right ventricle of anesthetized lambs

Regional mechanical and electrophysiological changes accompany most ventricular arrhythmias and, it has been suggested, by mechanoelectric feedback. We hypothesized that an intervention producing regional mechanical dispersion was associated with regional, proarrhythmic electrical dispersion and studied the regional mechanoelectric feedback in the right ventricle (RV) of anesthetized lambs. Ten lambs were deeply anesthetized, and their hearts were exposed. Three tripodal devices, each incorporating three monophasic action potential electrodes and an integrated strain-gauge system, were placed on the RV apex outflow and inflow regions. Measurements were made before, during, and after 10-s pulmonary arterial occlusion. Pulmonary arterial occlusion increased RV pressure and overall regional segment length. Length excursion became out of phase with RV pressure beats immediately after occlusion, and the strain patterns were different in the three regions at the peak of occlusion. The occlusion resulted in different alterations in regional monophasic action potential morphology, including reduction in monophasic action potential amplitude and duration by different amounts and early afterdepolarizations that were unevenly distributed in the monophasic action potential recordings. This was associated with dispersion of repolarization and recovery time. The combination of electromechanical events precipitated a variety of arrhythmias. Acute RV distension is proarrhythmic, possibly through a causal relationship among mechanically induced afterdepolarizations, dispersion (heterogeneity) of mechanical strain, and dispersion of electrical recovery. The relationship among the different wall motions, the dispersion of repolarization, and arrhythmia underscored mechanoelectric feedback as an important part of arrhythmogenesis in pulmonary embolism and commotio cordis.

Atrial Refractoriness and Action Potential Duration After Sudden Reversal of Atrioventricular Sequence

To address the potential of atrioventricular (AV) asynchrony to provoke cardiac arrhythmias, atrial electrophysiology was examined during normal and reversed AV interval in anesthetized pigs. A new automatic stimulation technique was adapted to monitor rapid changes in the effective refractory period (ERP), using continuous AV sequential pacing, incremental extrastimulus interval scanning, and automatic detection of capture. Right atrial ERP using 2-8 ms stimulus interval increments and right atrial and ventricular monophasic action potential (MAP) duration were determined simultaneously when the AV interval was changed from normal (+80 ms) to reversed (-40 ms) and back. During reversed AV interval the peak right atrial pressure increased from 8 +/- 3 to 14 +/- 4 mmHg (P < 0.001) and mean arterial pressure decreased from 86 +/- 18 to 65 +/- 21 mmHg (P < 0.001). At new steady state, atrial ERP and MAP duration at 90% level of repolarization were lengthened by 22 +/- 16 and 42 +/- 12 ms respectively (P < 0.001). Ventricular MAP duration did not change. A statistically significant lengthening in atrial ERP could be demonstrated in 5-10 seconds. After reversion of the AV sequence, the ratio of atrial ERP to MAP duration decreased from 1.27 to 0.94 (P < 0.001) on average for 15 seconds, the change being thought to favor reentry. Thus atrial wall stress from contraction during ventricular systole even for a short period of time modifies atrial electrophysiology. Deficient AV synchrony may immediately contribute to the development of atrial arrhythmias.

Electrophysiological changes following balloon valvuloplasty and angioplasty for aortic stenosis and coartaction of aorta: clinical evidence for mechano-electrical feedback in humans

BACKGROUND

Basic research and animal experiments have shown electrophysiological changes during or after changes in mechanical loading. Electrical instability following mechanical stretch has been observed as development of after-depolarisation and dispersion of refractoriness and repolarisation. The aim of the present study was to evaluate the presence of the mechano-electrical feedback in humans, assessing the ventricular repolarisation changes following acute changes in left ventricular pressure.

MATERIAL AND METHODS

The study group comprised 30 consecutive patients (22 M and 8 F, aged 2 days-24 years) affected by severe congenital aortic stenosis and 30 patients (20 M and 10 F, aged 6 months-16 years) affected by severe coartaction of aorta. Ventricular repolarisation was evaluated before and after percutaneous balloon valvuloplasty and angioplasty in terms of absolute measures (JT, JTc, QT, QTc) and in terms of dispersion across the myocardium: QT and QTc dispersion (QTD, QTcD), JT and JTc dispersion (JTD and JTcD) and T-peak to T-end interval (Tp-Te).

RESULTS

Patients with severe aortic stenosis and patients with aortic coartaction showed a significant decrease in dispersion of ventricular repolarisation time indexes (QTD, QTcD, JTD, JTcD and Tp-Te) following valvuloplasty and angioplasty.

CONCLUSIONS

Changes in hemodynamic loading can also produce electrophysiological effects in humans. Acute reduction in left ventricular pressure overload following balloon valvuloplasty and angioplasty, decreases electrical instability, as expressed by the reduction across the myocardium of the dispersion of ventricular repolarisation.

Mechano-electrical feedback underlying arrhythmias: the atrial fibrillation case

Mechanoelectrical feedback (MEF) has become firmly established as a mechanism in which mechanical forces experienced by myocardial tissue or cell membranes convey alterations in electrophysiologic characteristics of such tissue. Observations to date mainly concern mechanically induced changes in action potential duration, resting and active potential amplitude, enhanced pacemaker frequency, or afterdepolarizations. While some of these changes (i.e. after depolarizations) may give rise to premature beats, a role of MEF in explaining sustained ventricular tachyarrhythmias has so far been elusive. Here, we review recent findings showing that acute atrial dilatation facilitates atrial fibrillation (AF) and that two stretch-activated channel (SAC) blockers (gadolinium and GsMTx-4) are able to suppress stretch-facilitated AF. These findings strongly support a role of MEF and SACs in promoting sustained arrhythmias and point to a new class of antiarrhythmic drugs.

Origin of ischemia-induced phase 1b ventricular arrhythmias in pig hearts

OBJECTIVES

The goal of this study was to establish the role of ventricular filling on the 1b phase of ischemia-induced arrhythmias.

BACKGROUND

Ischemia-induced ventricular arrhythmias occur in two phases. The mechanism of the initiation of delayed (1b) arrhythmias is unknown. The 1b arrhythmias (15 to 60 min of ischemia) are abundant in in situ hearts but scarce in isolated perfused hearts (with drained ventricles).

METHODS

Left ventricular (LV) epicardial mapping (11 x 11 matrix, 5 mm interelectrode distance) of the initiation of delayed arrhythmias was performed in open-chested pigs (group A, n = 7) and isolated pig hearts without (group B, n = 8) and with a filled intraventricular balloon (group C, n = 5).

RESULTS

There were no differences in ischemic zone size between groups. The ischemia-induced rise in tissue impedance was similar in groups A and B. Arrhythmias were less frequent and less severe in group B than in groups A or C, with no differences between groups A and C. An epicardial focal origin was detected in 26% of all first beats, significantly more from the ischemic border than from elsewhere. During a pacing protocol with a long pause (a separate group of four isolated hearts with a balloon), more premature beats occurred in the first postpause interval than in any other interval.

CONCLUSIONS

In isolated hearts 1b arrhythmias were less frequent and less severe than in working preparations. Focal activity was documented in 26% of arrhythmias and emerged from the ischemic border. Postpause contractile potentiation was associated with more arrhythmias. Our study suggests that the initiation of ischemia-induced 1b arrhythmias is related to LV wall stress.

Mechanoelectrical feedback: role of beta-adrenergic receptor activation in mediating load-dependent shortening of ventricular action potential and refractoriness

BACKGROUND

Augmented preload increases myocardial excitability by shortening action potential duration (APD). The mechanism governing this phenomenon is unknown. Because myocardial stretch increases intracellular cAMP, we hypothesized that load-dependent changes in myocardial excitability are mediated by beta-adrenergic stimulation of a cAMP-sensitive K(+) current.

METHODS AND RESULTS

The effects of propranolol on load-induced changes in electrical excitability were studied in 7 isolated ejecting canine hearts. LV monophasic APD at 50% and 90% repolarization (MAPD(50) and MAPD(90)) and refractoriness were determined at low (9+/-3 mL) and high (39+/-4 mL) load before and after beta-adrenergic blockade. During control, the MAPD(50) decreased from 193+/-26 to 184+/-26 ms with increased load, as did the MAPD(90) (238+/-28 to 233+/-28 ms), P</=0.04. Similar changes were observed in ventricular refractoriness. Treatment with propranolol completely abolished these load-induced effects. Myocardial catecholamine depletion with reserpine in 2 hearts also abolished changes in MAPD and excitability in response to increased preload.

CONCLUSIONS

Increases in ventricular load mediate a decrease in ventricular APD and refractoriness through activation of the beta-adrenergic receptor. An increase in a cAMP-mediated K(+) current, possibly the slowly activating delayed rectifier I(Ks), may account in part for this form of mechanoelectrical coupling.

Analysis of T wave changes by activation recovery interval in patients with atrial septal defect

We examined the distributions of the activation recovery interval (ARI), which is correlated with the local action potential duration (APD), to clarify the origin of the repolarization changes in ASD. The ECGs, QRST isointegral maps and ARI isochronal maps of 21 children with ASD from 3 to 5 years old in age were studied in comparison with 21 age-matched normal children. A conventional and 87 unipolar body surface ECG were simultaneously recorded. The ARIs were determined from the first derivatives of the ECG waveforms. Abnormal ST-T patterns were observed in 11 of 21 ASD, but only in two normal children. The QRST maps of a split positive area pattern were seen in 15 of ASD but none of the normal. In the ARI maps, all the normal children exhibited a short-ARI area on the left and a long-ARI area on the right side of the chest. In 19 of ASD, the ARI distribution revealed a leftward extension of the long-ARI area on the anterior chest, a relative shortening on the right anterior chest, and a localized prolonged ARI on the left anterior chest. The results suggest that right ventricular (RV) volume overload in ASD produces a localized prolongation of the APD on the RV epicardium.

Gadolinium decreases stretch-induced vulnerability to atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is frequently associated with atrial dilatation caused by pressure or volume overload. Stretch-activated channels (SACs) have been found in myocardial cells and may promote AF in dilated atria. To prove this hypothesis, we investigated the effect of the SAC blocker gadolinium (Gd(3+)) on AF propensity in the isolated rabbit heart during atrial stretch.

METHODS AND RESULTS

In 16 isolated Langendorff-perfused rabbit hearts, the interatrial septum was perforated to equalize biatrial pressures. Caval and pulmonary veins were occluded. Intra-atrial pressure (IAP) was increased in steps of 2 to 3 cm H(2)O by increasing the pulmonary outflow fluid column. Vulnerability to AF was evaluated by 15-second burst pacing at each IAP level. At baseline, IAP needed to be raised to 8.8+/-0.2 cm H(2)O (mean+/-SEM) to induce AF. A dose-dependent decrease in AF vulnerability was observed after Gd(3+) 12.5, 25, and 50 micromol/L was added. AF threshold increased to 19.0+/-0.5 cm H(2)O with Gd(3+) 50 micromol/L (P<0.001 versus baseline). Spontaneous runs of AF occurred in 5 hearts on a rise of IAP to 13.8+/-3.3 cm H(2)O at baseline but never during Gd(3+). Atrial effective refractory period shortened progressively from 78+/-3 ms at 0.5 cm H(2)O to 52+/-3 ms at 20 cm H(2)O (P<0.05). Gd(3+) 50 micromol/L had no significant effect on effective refractory period.

CONCLUSIONS

Acute atrial stretch significantly enhances the vulnerability to AF. Gd(3+) reduces the stretch-induced vulnerability to AF in a dose-dependent manner. Block of SAC might represent a novel antiarrhythmic approach to AF under conditions of elevated atrial pressure or volume.

Effect of atrial pressure increase on effective refractory period and vulnerability to atrial fibrillation in patients with lone atrial fibrillation

BACKGROUND

There is evidence suggesting that atrial fibrillation (AF) may be induced by acute increase of atrial pressure. The aim of the present study was to investigate the effect of alterations in atrial pressure, induced by varying the atrioventricular (AV) interval, on atrial refractoriness, and on the frequency of induction of (AF), in patients with a history of lone atrial fibrillation (LAF).

METHODS AND RESULTS

Twenty-five patients were included in this study. The patients were divided in two groups: the LAF group, and the control group. None of the patients in either group had organic heart disease. Effective refractory period (ERP) and duration of atrial extrastimulus electrogram (A(2)) were measured at two right atrial sites (high lateral wall, atrial appendage) during AV pacing (cycle length: 500 msec) with different AV intervals. Peak, minimal and mean atrial pressure increased from 8.57 +/- 2.37 to 18.14 +/- 4.74 mm Hg, 2 +/- 2.23 to 5.14 +/- 2.60 mm Hg (p = 0.0001) and from 4.28 +/- 1.6 mm Hg to 9.77 +/- 2.9 mm Hg (p = 0.001), respectively during AV interval modification. During lateral and atrial appendage pacing, with a progressive decrease of AV interval to 160, 100, 80, 40, 0 msec, the ERP, the dispersion of ERP, functional refractory period (FRP), A2 and latency period (LP) did not change significantly, in both groups. The frequency of induction of AF was not statistically different in both lateral atrial wall and appendage, during pacing in different AV intervals.

CONCLUSIONS

This study demonstrates that alterations in the intraatrial pressure does not have important effects on atrial refractoriness and does not increase vulnerability to AF in patients with a history of LAF.

Role of the sarcoplasmic reticulum in the modulation of rat cardiac action potential by stretch

We have investigated the role of sarcoplasmic reticulum (SR) in the modulation on rat action potentials by stretch. The action potentials were recorded intracellularly from rat atrial myocytes in an isolated atrial preparation with small, physiological stretch produced by pressure (1-3 mmHg) inside the atria. The SR function was inhibited by pharmacological interventions, either with ryanodine (100 nmol L-1), thapsigargin (10 nmol L-1) or caffeine (1 mmol L-1). The duration of action potentials was increased by stretch from 1 to 3 mmHg. The repolarization indices APD30% (P < 0.05), APD60% (P < 0.01), and APD90% (P < 0.01) were all increased significantly (n=10). Ryanodine, thapsigargin, and caffeine inhibited this prolongation, or even reversed the effect with repolarization indices APD30% (P < 0.05) and APD60% (P < 0.05) which decreased in stretch with thapsigargin treatment. As a conclusion, we suggest that the SR and the intracellular calcium balance play an important role in the modulation of the shape of the rat atrial action potential during stretch.

Cardiac mechano-electric feedback in man: clinical relevance

Clinical conditions associated with sudden cardiac death due to arrhythmia are frequently accompanied by abnormalities of mechanical loading and wall stretch. These arrhythmias may result from several mechanisms including secondary depolarisations during or following the action potential or from a combination of conduction slowing and action potential shortening. Mechanical perturbations have been shown to reproduce these electrophysiological effects experimentally. However the effect of mechanical intervention is complex depending on the timing and intensity of the stimulus and the interplay between effects mediated via stretch activated channels and calcium cycling. Studies in patients during cardiac catheterisation or cardiac surgery using monophasic action potentials have shown alteration in the time course and shape of action potential repolarisation in response to changes in ventricular loading. Although stretch in experimental preparations has been shown to be arrhythmogenic, particularly in pathological conditions, the role of mechanically induced electrophysiological changes in important clinical ventricular arrhythmias remains to be established.

Mechanosensitivity as an integrative system in heart: an audit

This review examines a manifold of apparently loosely linked observations and mechanisms, from membrane to man, and assembles them to support the notion that mechanoelectric transduction is an integrative regulatory system in the heart. For this, the assemblage has to satisfy, at least to some extent, criteria that apply to other integrative regulatory systems such as the endocrine and nervous systems. The integrative effectors in the endocrine system are chemical linkages, circulating hormones: in the nervous system the linkage is a network of cables, nerve conduction and neurotransmitters. Mechanical integration is would be effected through mechanical machinery, cardiac contractile and hydraulic function with attendant stress and strain transmitted via "tensegrity". This can, through the cytoskeleton, begin with membrane integrins and transmit intracellularly for example via F actins to reach the rest of the membranous integrins. Further transmission to the organ is via cell-to-cell adhesion complexes and the extracellular matrix. This tensegrity facilitates integration of force and strain changes from area to area. In consequence, and analogous to the neurendocrine system, mechanoelectric transduction should, and does (1) operate at the molecular or membrane level--this would be via mechanotransducers affecting transmembrane ionic flow; (2) operate in the cell--to influence electrophysiology; (3) have a multicellular expression--e.g. mechanical distortion of one cell can raise intracellular calcium of an adjacent cell; (4) express in the intact organ--e.g. an increase in venous return hydraulically distends the sinoatrial node, steepening its pacemaker potential, thus increasing heart rate. It should also (5) demonstrate elements of a feedback system--"mechanoelectric feedback", and (6) interact with other systems--the cytoskeleton incorporates cell signalling complexes intersecting with other signal cascades. Finally, (7) it can malfunction to produce clinical abnormality--it contributes electrophysiologically to lethal cardiac arrhythmia. This anatomical and functional behaviour of mechanoelectric transduction could sanction the prospect of viewing it as analogous to the other integrative physiological systems.

Evolution of the organization of epicardial activation patterns during ventricular fibrillation

INTRODUCTION

This study quantified how the organization of epicardial activation changes during the first 40 seconds of ventricular fibrillation (VF).

METHODS AND RESULTS

Unipolar potentials were mapped from a 504 (24 x 21) electrode array (2-mm interelectrode spacing) on the anterior right ventricle (RV) and left ventricle (LV) epicardium. The array covered approximately 20% of the epicardial surface. In each of seven pigs, six episodes of VF were induced by premature stimulation. One-half second epochs of VF were analyzed, starting 0, 10, 20, 30, and 40 seconds post induction and using novel pattern analysis algorithms. Eight parameters were quantified: (1) the number of wavefronts; (2) the epicardial area activated by wavefronts; (3) the fraction of wavefronts arising from epicardial breakthrough or from a focus; (4) the fraction of wavefronts terminated by conduction block; (5) the multiplicity index (number of distinct activation pathways in the rhythm); (6) the repeatability index (number of times activation pathways are traversed); (7) the activation rate; and (8) the wavefront propagation velocity. The results showed that VF patterns were less organized at 10 than at 0 seconds, with more, smaller wavefronts traversing a larger variety of pathways for fewer repetitions. VF activation patterns then gradually reorganized up to 40 seconds, but by a different mechanism: the spatial size of subpatterns grew, but the dynamics otherwise appeared unchanged. During both transitions, both activation rate and propagation velocity slowed monotonically.

CONCLUSION

Thus, changes in organization during VF can occur by multiple mechanisms.

Mechanisms of stretch-induced changes in [Ca2+]i in rat atrial myocytes: role of increased troponin C affinity and stretch-activated ion channels

To study the effects of stretch on the function of rat left atrium, we recorded contraction force, calcium transients, and intracellular action potentials (APs) during stretch manipulations. The stretch of the atrium was controlled by intra-atrial pressure. The Frank-Starling behavior of the atrium was manifested as a biphasic increase of the contraction force after increasing the stretch level. The development of the contraction force after step increase of the stretch (intra-atrial pressure from 1 to 3 mm Hg) was accompanied by the increase in the amplitude of the calcium transients (P<0.05, n=4) and decrease in the time constant of the Ca2+ transient decay. The APs of the individual myocytes were also affected by stretch; the duration of the AP was decreased at positive voltages (AP duration at 15% repolarization level, P<0.001; n=13) and increased at negative voltages (AP duration at 90% repolarization level, P<0. 01; n=13). To study the mechanisms causing these changes we developed a mathematical model describing [Ca2+]i and electrical behavior of single rat atrial myocytes. Stretch was simulated in the model by increasing the troponin (TnC) sensitivity and/or applying a stretch-activated (SA) calcium influx. We mimicked the Ca2+ influx by introducing a nonselective cationic conductance, the SA channels, into the membrane. Neither of the 2 plausible mechanosensors (TnC or SA channels) alone could produce similar changes in the Ca2+ transients or APs as seen in the experiments. The model simulated the effects of stretch seen in experiments best when both the TnC affinity and the SA conductance activation were applied simultaneously. The SA channel activation led to gradual augmentation of Ca2+ transients, which modulated the APs through increased Na+/Ca2+-exchanger inward current. The role of TnC affinity change was to modulate the Ca2+ transients, stabilize the diastolic [Ca2+]i, and presumably to produce the immediate increase of the contraction force after stretch seen in experiments. Furthermore, we found that the same mechanism that caused the normal physiological responses to stretch could also generate arrhythmogenic afterpotentials at high stretch levels in the model.

U wave: facts, hypotheses, misconceptions, and misnomers

The clinical significance of U wave is limited to the occasional obfuscation of the end of T wave and an inadequately explained U wave inversion associated with myocardial ischemia, infarction, and ventricular hypertrophy and dilatation. Lengthening of QT interval often interferes with the recognition of U wave. The characteristics of U wave are not compatible with the Purkinje or ventricular muscle repolarization hypotheses. The timing of the U wave during ventricular relaxation and the links between U wave and mechanical events favor the mechanoelectrical hypothesis of U wave genesis. Unfortunately, little research has been done to test this hypothesis.

Divergent effect of acute ventricular dilatation on the electrophysiologic characteristics of d,l-sotalol and flecainide in the isolated rabbit heart

INTRODUCTION

The interaction between acute ventricular dilatation (AVD) as one aspect of ventricular dysfunction and Class I and III antiarrhythmic drugs is uncertain. We therefore investigated the effects of AVD on the electrophysiologic properties of d,l-sotalol and flecainide.

METHODS AND RESULTS

The isolated rabbit heart was used as a model of AVD. The ventricular size and, therefore, the diastolic pressure were modified by sudden volume changes of a fluid-filled balloon placed in the left ventricle. Pacing was performed alternately using epi- and endocardial monophasic action potential (MAP)-pacing catheters at cycle lengths from 1,000 to 300 msec. d,l-Sotalol (10 microM) resulted in a significant (P < 0.05) lengthening of refractoriness (+13.5% +/- 3.1%), MAP duration (+14.9% +/- 3.2%), and QT interval (+15.5% +/- 4.1%) (mean +/- SEM at 1,000 msec). These effects had a reverse rate-dependence. AVD to a diastolic pressure of 30 mmHg reduced refractoriness and left ventricular MAP duration. In comparison with the control group with the same extent of AVD, d,l-sotalol still led to a significant prolongation of repolarization for all cycle lengths except 300 msec, so that its effects were not absolutely but relatively preserved. In contrast, flecainide (2 microM) had no significant effects on refractoriness or MAP duration. It led to a significant, rate-dependent increase of pacing thresholds (+47.6% +/- 8.2%), prolongation of QRS (+48.8% +/- 5.6%), and conduction time (+78.6% +/- 8.6%) (mean +/- SEM at 300 msec). In the flecainide group, AVD significantly increased the normal rate-dependent prolongation of QRS (+16.7% +/- 5.5%) and conduction time (+17.1% +/- 4.3%).

CONCLUSION

Our data demonstrate that, during AVD, the Class III effect of d,l-sotalol is preserved, whereas flecainide's effect of slowing conduction is exaggerated. This may contribute to flecainide-related proarrhythmia in certain clinical situations.

Segmental wall motion abnormalities alter vulnerability to ventricular ectopic beats associated with acute increases in aortic pressure in patients with underlying coronary artery disease

OBJECTIVE

To evaluate whether patients with coronary artery disease are susceptible to pressure related ventricular arrhythmias, and if so to identify possible risk factors.

DESIGN

Interventional study.

METHODS

Metaraminol was given to 43 patients undergoing coronary arteriography for ischaemic heart disease to increase their aortic pressure, provided their systolic blood pressure was < 160 mm Hg and they were in sinus rhythm, without any ventricular ectopic activity (or with fewer than six ventricular ectopic beats a minute) during a five minute control period.

RESULTS

During the metaraminol infusion, systolic aortic pressure rose from 131 (15) to 199 (12) mm Hg (mean (SD)). Ventricular ectopy appeared (or ventricular ectopic beats increased by > 100%) in 13/43 patients. Ventricular ectopy was not related to age, sex, presence of hypertension, history of myocardial infarction, use of beta blockers, positive exercise test, number of vessels diseased, or heart rate change during metaraminol infusion. There was a strong relation between the appearance of ventricular arrhythmia and segmental wall motion abnormalities: 1/19 (5.3%, 95% confidence interval 0.1% to 26.0%) without abnormality; 2/12 (16.7%, 2.1% to 48.4%) with hypokinesia; and 10/12 (83.3%, 51.6% to 97.1%) with akinesia or dyskinesia, chi 2 = 22.7, p < 0.001). Ejection fraction was also a significant but not independent risk factor.

CONCLUSIONS

Patients with segmental wall motion abnormalities are predisposed to ventricular ectopic beats during an increase in systolic aortic pressure. This could be explained by associated electrophysiological inhomogeneity. The presence of mechanical inhomogeneity, as may occur in postinfarction akinesia or dyskinesia, may affect the aortic pressure above which ventricular arrhythmias appear.

Electrophysiological effects of acute dilatation in the isolated rabbit heart: cycle length-dependent effects on ventricular refractoriness and conduction velocity

BACKGROUND

Acute ventricular dilatation has important electrophysiological effects: Dilatation shortens action potential duration and refractoriness without an apparent effect on conduction velocity. These effects have been implicated as a potential mechanism of arrhythmias in patients with congestive failure. Because the influence of cycle length on these phenomena has not been studied, we examined the effects of dilatation during ventricular pacing at cycle lengths from 1000 to 150 ms.

METHODS AND RESULTS

Thin epicardial layers were created in isolated, perfused rabbit left ventricles (n=7). A fluid filled latex balloon was secured in the left ventricle to dilate the left ventricle. Mapping was performed with 248 epicardial electrodes. Longitudinal conduction velocity (76+/-1 cm/s; mean+/-SEM) and transverse conduction velocity (26+/-1 cm/s) were not influenced by dilatation at any cycle length. In contrast, the effects of dilatation in decreasing left ventricular effective refractory period (ERP) were significantly greater at shorter drive cycle lengths: The decrease in ERP was 2+/-2 ms (a 1% change) at a drive cycle length of 1000 ms and 18+/-4 ms (a 20% change) at a drive cycle length of 150 ms. In 10 additional intact, isolated perfused rabbit hearts, dilatation decreased ERP to a greater degree during 250 ms drive cycle length pacing than during pacing at 400 ms (25+/-4 versus 16+/-3 ms; P=.01).

CONCLUSIONS

Acute dilatation exaggerates the normal rate-dependent shortening of refractoriness but does not influence transverse or longitudinal conduction velocity. This observation suggests that the electrophysiological effects of acute dilatation may be greater during tachycardia than at slower cycle lengths. This may have implications for arrhythmias in patients with congestive heart failure.

Impact of volume loading and load reduction on ventricular refractoriness and conduction properties in canine congestive heart failure

OBJECTIVES

This investigations was undertaken to examine the alteration of electrophysiologic properties, including refractoriness, strength-interval relations and conduction, with the development of heart failure and to characterize the impact of volume loading on these indexes in the cardiomyopathic setting.

METHODS

Electrophysiologic properties in eight dogs with pacing-induced dilated cardiomyopathy were compared with those in six control dogs before and after rapid infusion of 800 ml of intravenous saline.

RESULTS

The right ventricular (RV) and left ventricular (LV) effective refractory period (ERP) and absolute refractory period (ARP) were significantly longer in dogs with pacing-induced cardiomyopathy than in control dogs: RV ERP 181 +/- 11 ms versus 138 +/- 7 ms (mean +/- SD) (p < 0.0001) and anterior LV ERP 177 +/- 13 ms versus 128 +/- 11 ms (p < 0.0001), respectively; ARP 159 +/- 14 ms versus 114 +/- 7 ms (p < 0.0001) at the RV site and 153 +/- 12 versus 117 +/- 5 ms (p < 0.0001) at the anterior LV site. After volume loading in cardiomyopathic animals, posterior and anterior LV ERPs became prolonged to 178 +/- 5 ms (p = 0.004) and 189 +/- 14 ms (p = 0.065), respectively, shifting the strength-interval relation in the direction of longer S1S2 coupling intervals. Anterior LV monophasic action potential durations at 90% repolarization also became prolonged from 192 +/- 10 ms to 222 +/- 23 ms (p < 0.012) with volume loading. These findings were not altered by subsequent sodium nitroprusside. Local conduction times parallel and perpendicular to fiber orientation were not altered by development of cardiomyopathy or volume alterations.

CONCLUSIONS

The development of dilated cardiomyopathy results in significant prolongation of refractoriness and repolarization that is increased further by volume augmentation but is not reversed by pharmacologic load reduction. Although these abnormalities may contribute to the environment needed for a non-reentrant, triggered or stretch-mediated arrhythmogenic process in cardiomyopathic states, additional studies will be required to demonstrate such a focal mechanism conclusively.

Defibrillation efficacy with endocardial electrodes is influenced by reductions in cardiac preload

Little is known about the effects of cardiac preload and cardiac geometry on defibrillation efficacy with endocardial electrodes. We studied nine pigs implanted with an endocardial lead system in the normal and reduced preload state. In the reduced preload state, a balloon catheter was inflated in the inferior vena cava (IVC) for 20 seconds prior to the induction of ventricular fibrillation (VF). Complete occlusion of the IVC and reductions in preload were confirmed by observing deformation of the contrast-filled balloon, a reduction in cardiac size by fluoroscopy, and reductions in ventricular pressures. Biphasic shocks were delivered after 10 seconds of VF using a recursive up-down protocol. VF was induced 20 times for each preload state, and the 50% effective doses (ED50) for energy, current, and voltage were estimated by averaging all shocks for that state. At reduced preloads, energy decreased from 12.1 +/- 3.0 J (+/- SD) to 10.5 +/- 2.9 J (p < 0.01), voltage decreased from 415 +/- 51 V to 390 +/- 51 V (p < 0.05), and current decreased from 8.6 +/- 1.5 A to 7.6 +/- 1.5 A (p < 0.01), while impedance rose from 49.2 +/- 3.8 omega to 52.8 +/- 4.4 omega (p < 0.001). We conclude that reducing cardiac preload and cardiac size significantly lowers ED50 defibrillation energy, current, and voltage. This outcome may be caused directly by the decrease in blood volume as evidenced by increased impedance and/or may be due to changes in heart geometry and stretch.

The cellular and molecular response of cardiac myocytes to mechanical stress

External load plays a critical role in determining muscle mass and its phenotype in cardiac myocytes. Cardiac myocytes have the ability to sense mechanical stretch and convert it into intracellular growth signals, which lead to hypertrophy. Mechanical stretch of cardiac myocytes in vitro causes activation of multiple second messenger systems that are very similar to growth factor-induced cell signaling systems. Stretch of neonatal rat cardiac myocytes stimulates a rapid secretion of angiotensin II which, together with other growth factors, mediates stretch-induced hypertrophic responses in vitro. In this review, various cell signaling mechanisms initiated by mechanical stress on cardiac myocytes are summarized with emphasis on potential mechanosensing mechanisms and the relationship between mechanical loading and the cardiac renin-angiotensin system.