Intramural optical mapping of V(m) and Ca(i)2+ during long-duration ventricular fibrillation in canine hearts

Recurrence of ventricular fibrillation in out-of-hospital cardiac arrest: Clinical evidence and underlying ionic mechanisms

Defibrillation remains the optimal therapy for terminating ventricular fibrillation (VF) in out-of-hospital cardiac arrest (OHCA) patients, with reported shock success rates of ∼90%. A key persistent challenge, however, is the high rate of VF recurrence (∼50-80%) seen during post-shock cardiopulmonary resuscitation (CPR). Studies have shown that the incidence and time spent in recurrent VF are negatively associated with neurologically-intact survival. Recurrent VF also results in the administration of extra shocks at escalating energy levels, which can cause cardiac dysfunction. Unfortunately, the mechanisms underlying recurrent VF remain poorly understood. In particular, the role of chest-compressions (CC) administered during CPR in mediating recurrent VF remains controversial. In this review, we first summarize the available clinical evidence for refibrillation occurring during CPR in OHCA patients, including the postulated contribution of CC and non-CC related pathways. Next, we examine experimental studies highlighting how CC can re-induce VF via direct mechano-electric feedback. We postulate the ionic mechanisms involved by comparison with similar phenomena seen in commotio cordis. Subsequently, the hypothesized contribution of partial cardiac reperfusion (either as a result of CC or CC independent organized rhythm) in re-initiating VF in a globally ischaemic heart is examined. An overview of the proposed ionic mechanisms contributing to VF recurrence in OHCA during CPR from a cellular level to the whole heart is outlined. Possible therapeutic implications of the proposed mechanistic theories for VF recurrence in OHCA are briefly discussed.

Quantifying Propagation Velocity from Engineered Cardiac Tissues with High-Speed Fluorescence Microscopy and Automated Analysis Software

Many acquired or inherited forms of heart disease as well as drugs are known to increase the susceptibility of patients to arrhythmias. To predict arrhythmogenic events and discover new therapeutic strategies to mitigate them, approaches to efficiently quantify the velocity of propagation in engineered cardiac tissues are important research tools. In this chapter, we describe how to collect videos of propagating calcium waves in engineered cardiac tissues with a high-speed camera mounted on an inverted fluorescence microscope. We also provide instructions for downloading and using our software package to analyze these videos and calculate propagation velocity. These techniques should be compatible with a variety of voltage- or calcium-sensitive fluorescent dyes or genetically encoded sensors. Although these approaches were originally developed for engineered neonatal rat cardiac tissues, the same procedures can likely be used with human-induced pluripotent stem cell-derived cardiac myocytes, paving the way for patient-specific analysis of propagation due to features such as tissue architecture, substrate rigidity, genetic mutations, or drug treatments.

Organized Structure of Ventricular Fibrillation during Prolonged Heart Perfusion in Dogs

The aim of the study was to identify the organized ventricular fibrillation (VF) activity in the dog heart and characterize its quantitative parameters during prolonged heart perfusion.

Microenvironmental Modulation of Calcium Wave Propagation Velocity in Engineered Cardiac Tissues

Introduction

In the myocardium, rapid propagation of action potentials and subsequent calcium waves is critical for synchronizing the contraction of cardiac myocytes and maximizing cardiac output. In many pathological settings, diverse remodeling of the tissue microenvironment is correlated with arrhythmias and decreased cardiac output, but the precise impact of tissue remodeling on propagation is not completely understood. Our objective was to delineate how multiple features within the cardiac tissue microenvironment modulate propagation velocity.

Methods

To recapitulate diverse myocardial tissue microenvironments, we engineered substrates with tunable elasticity, patterning, composition, and topography using two formulations of polydimethylsiloxane (PDMS) micropatterned with fibronectin and gelatin hydrogels with flat or micromolded features. We cultured neonatal rat ventricular myocytes on these substrates and quantified cell density, tissue alignment, and cell shape. We used a fluorescent calcium indicator, high-speed microscopy, and newly-developed analysis software to record and quantify calcium wave propagation velocity (CPV).

Results

For all substrates, tissue alignment and cell aspect ratio were higher in aligned compared to isotropic tissues. Isotropic CPV and longitudinal CPV were similar across conditions, but transverse CPV was lower on micromolded gelatin hydrogels compared to micropatterned soft and stiff PDMS. In aligned tissues, the anisotropy ratio of CPV (longitudinal CPV/transverse CPV) was lower on micropatterned soft PDMS compared to micropatterned stiff PDMS and micromolded gelatin hydrogels.

Conclusion

Propagation velocity in engineered cardiac tissues is sensitive to features in the tissue microenvironment, such as alignment, matrix elasticity, and matrix topography, which may underlie arrhythmias in conditions with pathological tissue remodeling.

Evaluation of excitation propagation in the rabbit heart: optical mapping and transmural microelectrode recordings

Background

Because of the optical features of heart tissue, optical and electrical action potentials are only moderately associated, especially when near-infrared dyes are used in optical mapping (OM) studies.

Objective

By simultaneously recording transmural electrical action potentials (APs) and optical action potentials (OAPs), we aimed to evaluate the contributions of both electrical and optical influences to the shape of the OAP upstroke.

Methods and Results

A standard glass microelectrode and OM, using an near-infrared fluorescent dye (di-4-ANBDQBS), were used to simultaneously record transmural APs and OAPs in a Langendorff-perfused rabbit heart during atrial, endocardial, and epicardial pacing. The actual profile of the transmural AP upstroke across the LV wall, together with the OAP upstroke, allowed for calculations of the probing-depth constant (k ~2.1 mm, n = 24) of the fluorescence measurements. In addition, the transmural AP recordings aided the quantitative evaluation of the influences of depth-weighted and lateral-scattering components on the OAP upstroke. These components correspond to the components of the propagating electrical wave that are transmural and parallel to the epicardium. The calculated mean values for the depth-weighted and lateral-scattering components, whose sum comprises the OAP upstroke, were (in ms) 10.18 ± 0.62 and 0.0 ± 0.56 for atrial stimulation, 9.37 ± 1.12 and 3.01 ± 1.30 for endocardial stimulation, and 6.09 ± 0.79 and 8.16 ± 0.98 for epicardial stimulation; (n = 8 for each). For this dye, 90% of the collected fluorescence originated up to 4.83 ± 0.18 mm (n = 24) from the epicardium.

Conclusions

The co-registration of OM and transmural microelectrode APs enabled the probing depth of fluorescence measurements to be calculated and the OAP upstroke to be divided into two components (depth-weighted and lateral-scattering), and it also allowed the relative strengths of their effects on the shape of the OAP upstroke to be evaluated.

Toward microendoscopy-inspired cardiac optogenetics in vivo: technical overview and perspective

The ability to perform precise, spatially localized actuation and measurements of electrical activity in the heart is crucial in understanding cardiac electrophysiology and devising new therapeutic solutions for control of cardiac arrhythmias. Current cardiac imaging techniques (i.e. optical mapping) employ voltage- or calcium-sensitive fluorescent dyes to visualize the electrical signal propagation through cardiac syncytium in vitro or in situ with very high-spatiotemporal resolution. The extension of optogenetics into the cardiac field, where cardiac tissue is genetically altered to express light-sensitive ion channels allowing electrical activity to be elicited or suppressed in a precise cell-specific way, has opened the possibility for all-optical interrogation of cardiac electrophysiology. In vivo application of cardiac optogenetics faces multiple challenges and necessitates suitable optical systems employing fiber optics to actuate and sense electrical signals. In this technical perspective, we present a compendium of clinically relevant access routes to different parts of the cardiac electrical conduction system based on currently employed catheter imaging systems and determine the quantitative size constraints for endoscopic cardiac optogenetics. We discuss the relevant technical advancements in microendoscopy, cardiac imaging, and optogenetics and outline the strategies for combining them to create a portable, miniaturized fiber-based system for all-optical interrogation of cardiac electrophysiology in vivo.

Dantrolene improves survival after ventricular fibrillation by mitigating impaired calcium handling in animal models

BACKGROUND

Resistant ventricular fibrillation, refibrillation. and diminished myocardial contractility are important factors leading to poor survival after cardiac arrest. We hypothesized that dantrolene improves survival after ventricular fibrillation (VF) by rectifying the calcium dysregulation caused by VF.

METHODS AND RESULTS

VF was induced in 26 Yorkshire pigs for 4 minutes. Cardiopulmonary resuscitation was then commenced for 3 minutes, and dantrolene or isotonic saline was infused at the onset of cardiopulmonary resuscitation. Animals were defibrillated and observed for 30 minutes. To study the effect of VF on calcium handling and its modulation by dantrolene, hearts from 14 New Zealand rabbits were Langendorff-perfused. The inducibility of VF after dantrolene administration was documented. Optical mapping was performed to evaluate diastolic spontaneous calcium elevations as a measure of cytosolic calcium leak. The sustained return of spontaneous circulation (systolic blood pressure ≥60 mm Hg) was achieved in 85% of the dantrolene group in comparison with 39% of controls (P=0.02). return of spontaneous circulation was achieved earlier in dantrolene-treated pigs after successful defibrillation (21 ± 6 s versus 181 ± 57 s in controls, P=0.005). The median number of refibrillation episodes was lower in the dantrolene group (0 versus 1, P=0.04). In isolated rabbit hearts, the successful induction of VF was achieved in 83% of attempts in controls versus 41% in dantrolene-treated hearts (P=0.007). VF caused diastolic calcium leaks in the form of spontaneous calcium elevations. Administration of 20 μmol/L dantrolene significantly decreased spontaneous calcium elevation amplitude versus controls. (0.024 ± 0.013 versus 0.12 ± 0.02 arbitrary unit [200-ms cycle length], P=0.001).

CONCLUSIONS

Dantrolene infusion during cardiopulmonary resuscitation facilitates successful defibrillation, improves hemodynamics postdefibrillation, decreases refibrillation, and thus improves survival after cardiac arrest. The effects are mediated through normalizing VF-induced dysfunctional calcium cycling.

A century of optocardiography

In the past decade, optical mapping provided crucial mechanistic insight into electromechanical function and the mechanism of ventricular fibrillation. Therefore, to date, optical mapping dominates experimental cardiac electrophysiology. The first cardiac measurements involving optics were done in the early 1900s using the fast cinematograph that later evolved into methods for high-resolution activation and repolarization mapping and stimulation of specific cardiac cell types. The field of "optocardiography," therefore, emerged as the use of light for recording or interfering with cardiac physiology. In this review, we discuss how optocardiography developed into the dominant research technique in experimental cardiology. Furthermore, we envision how optocardiographic methods can be used in clinical cardiology.

Cardiac optogenetics

Optogenetics is an emerging technology for optical interrogation and control of biological function with high specificity and high spatiotemporal resolution. Mammalian cells and tissues can be sensitized to respond to light by a relatively simple and well-tolerated genetic modification using microbial opsins (light-gated ion channels and pumps). These can achieve fast and specific excitatory or inhibitory response, offering distinct advantages over traditional pharmacological or electrical means of perturbation. Since the first demonstrations of utility in mammalian cells (neurons) in 2005, optogenetics has spurred immense research activity and has inspired numerous applications for dissection of neural circuitry and understanding of brain function in health and disease, applications ranging from in vitro to work in behaving animals. Only recently (since 2010), the field has extended to cardiac applications with less than a dozen publications to date. In consideration of the early phase of work on cardiac optogenetics and the impact of the technique in understanding another excitable tissue, the brain, this review is largely a perspective of possibilities in the heart. It covers the basic principles of operation of light-sensitive ion channels and pumps, the available tools and ongoing efforts in optimizing them, overview of neuroscience use, as well as cardiac-specific questions of implementation and ideas for best use of this emerging technology in the heart.

Mapping cardiac surface mechanics with structured light imaging

Cardiovascular disease often manifests as a combination of pathological electrical and structural heart remodeling. The relationship between mechanics and electrophysiology is crucial to our understanding of mechanisms of cardiac arrhythmias and the treatment of cardiac disease. While several technologies exist for describing whole heart electrophysiology, studies of cardiac mechanics are often limited to rhythmic patterns or small sections of tissue. Here, we present a comprehensive system based on ultrafast three-dimensional (3-D) structured light imaging to map surface dynamics of whole heart cardiac motion. Additionally, we introduce a novel nonrigid motion-tracking algorithm based on an isometry-maximizing optimization framework that forms correspondences between consecutive 3-D frames without the use of any fiducial markers. By combining our 3-D imaging system with nonrigid surface registration, we are able to measure cardiac surface mechanics at unprecedented spatial and temporal resolution. In conclusion, we demonstrate accurate cardiac deformation at over 200,000 surface points of a rabbit heart recorded at 200 frames/s and validate our results on highly contrasting heart motions during normal sinus rhythm, ventricular pacing, and ventricular fibrillation.