Cardiac Fibrillation-Defibrillation

The following sections are included:

• CONTENTS

• PREFACE

• ACKNOWLEDGMENTS

• INTRODUCTION

The arrhythmia called fibrillation is introduced as a generalized disordered activity of the cardiac fibers with no actual output of blood from the chambers. Its mechanisms of maintenance are tentatively explained by the reentry, the multiple ectopic foci, or the rotor theories, which are supported only partially by experimental and/or theoretical evidence. By and large, a disturbance starts up the arrhythmia, often in a sensitized heart, but not necessarily always so. Thus, the setting of the phenomenon and the type of disturbance become relevant, leading to the so-called clinical fibrillation (due to disease), industrial or domestic fibrillation (due to electric accidental shock), and surgical fibrillation (due probably to handling). Since the mammalian heart is structurally and electrophysiologically inhomogeneous, a disturbance can be viewed as some added extra-inhomogeneity, which increases the probability of triggering the phenomenon. Thus, electrical, mechanical, chemical, and thermal energies are the different disturbances to add inhomogeneity. Cardiac fibrillation can be classified on the basis of its etiology (primary or secondary), localization (atrial or ventricular), or characteristics (strong or weak). Its clinical and surgical significance is underlined while the fibrillation variables, divided into electrophysiologic and metabolic, are discussed. Since the arrhythmia is a group phenomenon, the concept of minimal mass appears as a pertinent quantity yet to be numerically determined. The minimum magnitude (amplitude in adequate units, of whatever kind) required to trigger fibrillation is called fibrillation threshold. When electrically induced, it is called electrical fibrillation threshold (EFT), as a parameter to evaluate and test the sensitivity of the myocardium to the arrhythmia. Single rectangular pulses or train of pulses, either synchronized or not with the QRS complex, can be used as testing signal, wherefrom a time window of weakness called vulnerable period is found, which roughly coincides with the duration of the T-wave. Some numerical illustrative values taken from the literatures are offered.

This chapter is devoted to the process of arresting the incoordinate and hemodynamically useless cardiac fibrillary movements and, by so doing, offer them the chance to return to the normal beat. In case of ventricular fibrillation, it means a resuscitatory act. The historical development is reviewed, since its inception back in the middle 1700s until the twentieth century pioneers, as example of knowledge acquisition. Thereafter, and even though there are many drawbacks and controversial aspects, chemical defibrillation is also described since it might become the ultimate procedure when the arrhythmia is refractory to the electric shock. Most of this chapter deals with electrical defibrillation, which, so far, is the only available life-saving procedure, either in its direct or indirect forms, depending on the emergency circumstances. The core of this chapter appears in the section dealing with the variables affecting defibrillation, threshold definition, and the concept of dose. For convenience, variables are divided in two groups: physiopathological and technological. The former includes the critical defibrillatory mass, heart size, K⁺, Ca⁺⁺, H⁺, oxygen concentrations, and temperature, all influential variables in the probability of a successful defibrillation. When myocardial injury is present and/or pharmacological agents have been administered, the situation changes radically, bringing into the forefront the so-called risk patient. The last added factor is fibrillation time: the longer, the lower the probability of getting out, because damage to the tissues increases. The technological electrical variables constitute determinant factors in the success of a defibrillating discharge. The manufacturer, the physician, the biomedical engineer, and the intervening paramedics must have a clear understanding of them and of the estimated values to apply. Hence, voltage V, current I, resistance R, power P, and energy E are briefly reviewed, particularly underlining the resistive hindrance imposed either by the myocardium or the thorax, which usually is referred to as cardiac or thoracic impedance. Thereafter, the concepts of defibrillation threshold and dose are defined, emphasizing that current, in amperes, should be the parameter to be used because it directly relates to membrane behavior and, if possible, it should be scaled to body weight. For this matter, old basic laws, such as Weiss' and Lapicque's, are recalled. The conceptual and numerical distinctions between defibrillatory threshold and electric dose for defibrillation are underlined. The former is defined as the minimum value (either expressed as current or energy) needed to defibrillate or, in other words, the electric value with a 50% probability of success. The latter, instead, corresponds to the value necessary to defibrillate with at least 90% probability of success and minimum or no damage to the myocardium. Threshold is a reference value while dose is a working quantity related to the previous one by a safety factor. To end, the reader will find cardiopulmonary resuscitation (CPR), cardioversion, and electrical ablation of re-entrant circuits as other techniques to deal with the lethal or very serious arrhthymic arrhythmias.

After a very brief historical review, this chapter deals with the defibrillator as an electrical–electronic machine without entering into technical details because technology advances so rapidly that, when published, soon it is superseded by newer one. Thus, only general principles are discussed. A defibrillator is an instrument designed to deliver an electric discharge able to terminate the arrhythmia. It can be characterized technologically and functionally, i.e. first, by defining energy and output current ranges, power source, and other specifications of its hardware and software (block diagram and general equations are given), and, second, by the circumstances of its intended use, such as place and operational settings, which lead to the external units, nowadays mostly automated, and internal models, exclusively for implantation and obviously also automated. Thereafter, the pulsed waveforms are presented: simple capacitor discharge, damped sinusoidal discharge, fractional sinusoidal shocks, truncated and rectangular pulses, and biphasic and triphasic discharges. Because of its better efficacy, in addition to biphasic waveforms, virtually all new units have added some type of impedance compensation that changes the shock based on the measured value of the patient impedance. This chapter ends with a very short discussion of cardiac pacemakers, which are devices akin of defibrillators and sometimes even combined with them.

In automatic defibrillation, early detection of the arrhythmia constitutes an essential and extremely sensitive task. Its failure means no shock delivery and, hence, no possible reversal leading to the patient's death. Besides, as Golden Rule, a shock should not be delivered to a collapsed patient not in cardiac arrest and a successfully defibrillated patient should not be defibrillated again. After defining basic evaluating parameters (sensitivity, specificity, receiver operating curve, positive predictivity, and accuracy), several algorithms are reviewed comparing them at the end of the chapter in an attempt to help the designing engineer in his/her selection. Acronyms are used along the text for the sake of space knowing the risk of confusion, although frequently their full identification is repeated and realizing that occasionally the same algorithm shares two abbreviations. To make navigation in this chapter easier, find here listed the seven algorithms treated plus other nine mentioned in the discussion, including also two algorithms for QRS complex detection, and calling attention to some overlapping between the two sets, that is: Probability Density Function (PDF), Threshold Crossing Intervals (TCI), Cardiac Frequency (CF), Signal Morphology (SM) or Correlation Waveform Analysis (CWA), Time–Frequency Analysis (TFA), Wavelet Transform (WT), and Phase Space Analysis (PSA), in the first group, followed by Threshold Crossing Intervals (TCI), described in Sect. 4.2.2, AutoCorrelation Fischer (ACF₉₅) algorithm, based on Correlation Waveform Analysis (CWA), explained in Sect. 4.2.4, VF Filter algorithm, after Kuo and Dillman (1978); Spectral (SPEC) algorithm based on Fourier Transform analysis, described in Sect. 4.2.5, Complexity (CPLX) algorithm, the Standard Exponential (STE) algorithm, the Modified Exponential Algorithm (MEA), an STE akin, the Signal Comparison Algorithm (SCA), the Wavelet (WVL) Algorithm, also explained in Sect. 4.2.6. Likewise, two QRS complex detection algorithms are considered: Tompkins (TOMP, see Sect. 4.2.3) and LI algorithm (see Sect. 4.2.6).

The main objective here is to develop the overall concept of the impedance faced by the defibrillator clearly separating out its components: the electrode–electrolyte interface impedance and the biological load impedance as main actor. A second objective aims at the defibrillating electrodes (both internal and external) along with contact impedance decreasing agents (pastes), which are only employed in the latter situation. Finally, since the connecting system (wiring) represents a weak link in the overall chain, possible failures are discussed, including also failures of other origins. A better tuning of the subject leads us to a single main idea: the biological load impedance, which is mostly resistive and represented either by the myocardial mass or the thorax, must be as low as possible in order to guarantee that all current efficiently traverses it to depolarize a maximum number of cardiac fibers. This requires the electrode contacts offer minimum hindrance to current flow.

This chapter tackles the problems faced by the defibrillation procedure regarding its safety and efficacy. First, safety — defined as the set of measures to prevent damage or injury of any kind — is introduced in its three levels of importance: the patient, the operator(s), and the equipment involved, the latter enclosed within a given environment. For that matter, electric isolation appears as an essential condition that must be met by the unit in order to minimize unwanted electrical connections or leaks. Thereafter, sources of electromagnetic interference have to be identified, as they can elicit malfunction. Another aspect refers to the standards regulating design, construction, and operation; however, this book is not intended to describe them, at most and after briefly mentioning them, it may discuss a few particular subjects. Accordingly, herein it merely gives the essential information that the clinical engineer, technician, or manufacturer can take as a beginning. One very peculiar, transient, and obviously undesired effect encompassed by the concept of safety refers to any pain or discomfort the electrical discharge may elicit. The phenomenon has been amply demonstrated and it can be traced back to improper defibrillator discharge, or to such an early ventricular fibrillation or tachyarrhythmia detection and shock delivery that the latter catches the subject still conscious or, in the case of cardioversion, which always captures him/her awake. Possible measures are described among the several proposed in the literature, such as injection of an anesthetic agent or neural inhibiting stimulation, always applied before the shock, or reduction of the energy level. Finally, the concept of efficacy is developed, which is defined as the power, capacity, or ability to produce the sought effect, that is, to successfully defibrillate. It recognizes several factors, all complex and still somewhat under discussion: lead system, pulse waveform (monophasic or polyphasic), pulse duration, and safety margin (in terms of energy or current). The latter tries to determine how much more energy or current is recommended above the minimum required value or threshold, always keeping in mind that a safety margin too large for the sake of efficacy may cause injury to the patient. Thus, an acceptable compromise must be reached.

This is, no doubt, the most difficult chapter of the book, written mostly by its first author, a specialist in non-linear dynamic systems, and seconded by a statistician that has been working on the connections between Chaos Theory and Statistics; the third co-author, responsible for the whole book, acted mainly as editor, adding, removing, or changing minor pieces. A good deal of exchange occupied considerable room and time in the cybernetic emailing space making the task enjoyable and fruitful. In the introduction, attention is called to the heart as paradigmatic case of high biological complexity reminding that interaction with several factors contributes often to further complicate its framework. No wonder, then, if complex theories become necessary to tackle pathological cardiac dynamics, in general, and ventricular fibrillation, in particular. Thereafter, the bottom-up and the top-down strategies are mentioned to start the modeling adventure. The Hodgkin–Huxley equations are presented as example of the first approach from which dynamic bifurcations can be derived. Linearization is also discussed as a possible simplification; however, a linear system might not show the same qualitative behavior as that of a non-linear one because it loses information. Alternatively, numerical integration becomes as a way out, but there are many and severe drawbacks. Thus, we expect better contributions from other topdown non-linear dynamic methods; they reduce microscopic bottom-up models to their minimal expression exhibiting also the main features of the complete model. Fibrillation is divided into two functional phases, its onset and as an already developed condition. The former can be considered as a bifurcation from a periodic condition of relatively low dimensionality; the latter, instead, implies rapid and irregular activity of the heart. From a medical point of view, the interest in modeling the onset of fibrillation relates to the possibility of avoiding the arrhythmia and/or diagnosing a propensity of falling into it. The interest in modeling established fibrillation lies in how to best intervene to revert the condition. The three sections after the introduction deal, respectively, with non-linear microscopic models, non-linear dynamics and the onset of fibrillation, and extended models and connection with sustained fibrillation, thus getting deeper into the subject. The relaxation oscillator of van der Pol appears as the starting mark up to more recent authors, as Winfree and others, going through a fascinating series of intermediate contributions. Several include experimental validations. For sustained fibrillation, the ventricular surface may be viewed as a sea of little reentry waves wandering on the ventricles; these waves arise from destabilization of one or a few greater spiral waves formed around an anatomical or dynamical singularity. The induced pathological dynamics can produce new singularities and also two singularities can merge and eventually annihilate or reduce to one. The statistical analysis relies on the reconstruction of the whole state space of the process (say, fibrillation) when just one or few time series components are available. Thereafter, Lyapunov exponents are discussed and followed by the concepts of complexity and entropy, all to better assess the cardiac signal under study. Heavy math is a characteristic of the section … and there is no other way. As in previous chapters, we end with a discussion, a few review questions and a good list of backing up references.

This last chapter attempts to round things out by looking back into what was said, tries to summarize the current situation of the subject by underlining those aspects that are still pending and, most difficult of all, wants to play the magician — the Nostradamus (Fig. 8.1) — by watching into the crystal ball to signal possible new avenues for research, development, and applications. During such endeavor, some references not given before will be brought up to better complete the list and partially compensate for ignorance and/or forgetfulness. No separate sections were considered necessary, rather following roughly the subject order of the chapters in a continuous easy discussion style.

The following sections are included:

• REFERENCES

• ABOUT THE AUTHORS

• INDEX