Cardiac electrical activity

Na+ channel Site 2 Rat brain Cardiac arrhythmia, epilepsy, Alzheimer s disease, pain, gastrointestinal, stroke, glaucoma Control of neuronal and cardiac electrical activity, neuroprotection... 

When the cardiac electrical activity is maintained, but there is no mechanical output (pulseless electrical activity, electromechanical dissociation), then hypovolaemia, tension pneumothorax, pulmonary embolism, cardiac tamponade, and various forms of metabolic or pharmacological disturbance may be responsible. In asystole or pulseless electrical activity (with an underlying rate of less than 60 beats per minute) a single intravenous bolus of 3 mg atropine is recommended. 

Schematic representation of the heart and normal cardiac electrical activity (intracellular recordings from areas indicated and ECG). Sinoatrial (SA) node, atrioventricular (AV) node, and Purkinje cells display pacemaker activity (phase 4 depolarization). The ECG is the body surface manifestation of the depolarization and repolarization waves of the heart. The P wave is generated by atrial depolarization, the QRS by ventricular muscle depolarization, and the T wave by ventricular repolarization. Thus, the PR interval is a measure of conduction time from atrium to ventricle, and the QRS duration indicates the time required for all of the ventricular cells to be activated (ie, the intraventricular conduction time). The QT interval reflects the duration of the ventricular action potential.

Undoubtedly, the most promising modehng of the cardiac dynamics is associated with the study of the spatial evolution of the cardiac electrical activity. The cardiac tissue is considered to be an excitable medium whose the electrical activity is described both in time and space by reaction-diffusion partial differential equations [519]. This kind of system is able to produce spiral waves, which are the precursors of chaotic behavior. This consideration explains the transition from normal heart rate to tachycardia, which corresponds to the appearance of spiral waves, and the fohowing transition to fibrillation, which corresponds to the chaotic regime after the breaking up of the spiral waves, Figure 11.17. The transition from the spiral waves to chaos is often characterized as electrical turbulence due to its resemblance to the equivalent hydrodynamic phenomenon. 

Repolarization of the ventricle leads to the T wave. The T wave usually goes in the same direction as the QRS complex. The normal axis of the ECG is 30 degrees (above the horizontal) to 4-110 degrees (away from the horizontal) (Fig. 11-5). The six frontal plane (A) and the six horizontal plane (B) leads provide a three-dimensional representation of cardiac electrical activity. 

FIGURE 11-5. The six frontal plane (A) and six horizontal plane (6) leads provide a three-dimensional representation of cardiac electrical activity. (Redrawn from ref. 7 with permission.)... 

Most studies attempting to define the effects of calcium inhibitory compounds upon cardiac electrical activity have... 

Figure 10-21. Schematic of cardiac electrical activity. (0-4 represents phases. ERP = effective refractory period. See text for discussion.)...

DiFrancesco, D. D. Noble. 1985. A model of cardiac electrical activity... 

DiFrancesco, D. and D. Noble (1985). A model of cardiac electrical activity incorporating ionic pumps and concentration changes. Phil. Trans. R. Soc. Land. B 307,353-398. 

The heart is an electromechanical system, in which mechanical pumping function is initiated and coordinated by automatic rhythmic cardiac electrical activity. Transient electrical disturbances m this activity (cardiac arrhythmias) can immediately cause death. The implantable cardioverter defibrillator (ICD) is a medical device that can be implanted in a human body and will automatically detect and treat cardiac arrhythmias. Mirowski et al. [1] first reported on a demonstration of a functioning ICD in 1970. In his demonstration, he induced a fatal ventricular tachyarrhythmia in a dog. The dog was then successfully and dramatically rescued by the automatic operation of a previously implanted ICD. The first report of successful ICD implantation in humans soon followed [2]. In subsequent decades, advances in ICD technology have reduced ICD size from more than 200 cm, originally, to as httle as 25 cm, while markedly improving functionality, reliability, and longevity. ICDs are now considered as standard of care for patients at risk for ventricular arrhythmia and are implanted in more than 150,000 patients per year in the United States alone [3]. 

Sudden cardiac death (SCD) is defined as abrupt loss of consciousness caused by failure of cardiac pumping function, generally because of an electrical problem. In certain cases, cardiac activity ceases completely because cardiac electrical impulse generation fails (bradycardic sudden death). Most commonly, however, cardiac electrical activity is present, but is either too rapid for effective mechanical response or too disorganized for coordinated mechanical response (tachycardic sudden death). Despite the term, resuscitation from SCD is possible, but lack of prompt treatment is almost invariably fatal. SCD is responsible for at least 160,000 deaths each year in the United States alone [4]. 

The function of the heart is the outcome of a complex combination of the different processes which relate to cardiac mechanics. The basic laws which determine the function of the heart are influenced by the cardiac electrical activity, the coronary perfusion supplying the energy demands of the system, the pathways of metabolism involved in the energetic processes and the general control mechanisms which are aimed to adjust cardiac function to the body needs. Each of the above-mentioned fields has been a subject for intensive investigation for many years. In spite of that, the rules which determine the function of the system are not always clear, and are actually sometimes ambiguous. Furthermore, the interrelationship between different processes have been studied, and quantified, only to a limited extent. 

Simulation of the cardiac electrical activity sequence using 3-D stochastically distributed parameters... 

If the pacemaker is too sensitive, it can misinterpret muscle movement or extracardiac events as intrinsic cardiac electrical activity and pacing won t occur when it s needed. 

The recorder-transmitter converts cardiac electrical activity to acoustic waves and sends them through a telephone line to a receiver, which converts the waves and records them on ECG paper. 

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