Long QT syndrome, a condition characterized by an abnormally prolonged interval between the QRS complex and the T wave, has been identified in some individuals. This syndrome has various congenital forms, with two genes known to be associated with the condition: the HERG gene, located on chromosome 7, and the SCN5A gene, located on chromosome 3. Patients with congenital long QT syndrome may experience episodes of syncope (fainting), and approximately 10% of pediatric subjects with this disorder may die suddenly without preceding symptoms. In some individuals with silent ion channel mutations, long QT syndrome may not be evident until a drug that affects the involved ion channel is taken. Acquired long QT syndrome can be caused by several drugs, including certain antiarrhythmic agents.

Drugs that prolong the refractory period can convert atrial fibrillation to a normal sinus rhythm. However, if atrial fibrillation does not respond adequately to medication, electrical defibrillation may be necessary for correction.

Ventricular fibrillation requires immediate and dramatic therapy. The conversion to a normal sinus rhythm is achieved by applying a strong electrical current that temporarily renders the entire myocardium refractory. Techniques have been developed to safely administer this current through the intact chest wall. In successful cases, the sinoatrial (SA) node resumes its normal pacemaker function for the entire heart.

To prevent death in patients who develop either ventricular fibrillation or paroxysmal ventricular tachycardia, implantable cardioverter-defibrillator (ICD) devices have been developed. These devices are implanted subcutaneously in the left subclavicular region of the chest wall. Atrial and ventricular leads enable the recording of electrocardiograms and provide the ability for atrial or ventricular pacing, or both. The defibrillation coil in the right ventricle allows the application of a strong electrical current to the ventricle, which usually terminates the lethal arrhythmia.

Understanding the heart's important task involves considering the relationship between the structure and function of its components. Both skeletal muscle and cardiac muscle are striated due to the regular arrangement of the contractile proteins actin and myosin. Force generation and muscle fiber contraction occur through their interactions, known as the sliding filament mechanism.

The length-force relationship in cardiac muscle is similar to that of skeletal muscle. This relationship can be graphically represented by substituting ventricular systolic pressure for force and end-diastolic ventricular volume for resting myocardial fiber length. The lower curve represents the increment in pressure produced during diastole, while the upper curve represents the peak force or pressure developed during systole based on initial fiber length or diastolic filling pressure. This curve illustrates the Frank-Starling law of the heart.
Insomeindividuals,theintervalbetweentheQRScomplexandtheTwaveisabnormallyprolonged,aconditiontermedlong QT syndrome (Fig.16.35 ).SeveralcongenitalformsoflongQTsyndromehavebeenidentifiedinhumansubjects.TwoofthemanygenesthathavebeenidentifiedasabasisforthissyndromearetheHERG gene(aK+ channelgene),locatedonchromosome7,andtheSCN5A gene(aNa+ gene),locatedonchromosome3.PatientswithcongenitalformsoflongQTsyndromemayhaveperiodicepisodesofsyncope(fainting),andapproximately10%ofpediatricsubjectswiththisdisordermaydiesuddenly,withoutanyprecedingsymptoms.Insomeindividualswithsilentionchannelmutations,longQTsyndromemaynotbeevidentuntiladrugistakenthataffectstheionchannelinvolved.Manydrugs,includingseveralantiarrhythmicagents,havebeenidentifiedascausingacquiredlongQTsyndrome.
Atrial fibrillation may be changed to a normal sinus rhythm by drugs that prolong the refractory period. As the cardiac impulse completes the reentry loop, it may then encounter refractory myocardial fibers. When atrial fibrillation does not respond adequately to drugs, electrical defibrillation may be used to correct this condition.
Dramatic therapy is required for ventricular fibrillation. Conversion to a normal sinus rhythm is accomplished by means of a strong electrical current that places the entire myocardium briefly in a refractory state. Techniques have been developed to administer the current safely through the intact chest wall. In successful cases, the SA node again takes over the normal pacemaker function for the entire heart.
The Cardiac Pump
The great amount of work performed by the heart over an individual’s lifetime is impressive. A useful way to
Implantablecardioverter-defibrillator(ICD)deviceshavebeendevelopedtopreventdeathinpatientsinwhomeitherventricularfibrillationorparoxysmalventriculartachycardiahassuddenlydeveloped.Theformerislethalunlessitistreatedimmediately,andthelatteroftenleadstoventricularfibrillationandsuddendeath.TheICDdeviceisimplantedsubcutaneouslyintheleftsubclavicularregionofthechestwall.Atrialandventricularleadsenablerecordingoftherightatrialandrightventricularelectrocardiogramsandprovidetheabilityforrightatrialorrightventricularpacing,orboth.Thedefibrillationcoilintherightventricleenablestheapplicationofastrongelectricalcurrenttotheventricleandtherebyusuallyterminatesthelethalarrhythmia.
understand how the heart accomplishes its important task is to consider the relationships between the structure and function of its components.
Relationship of Heart Structure to Function
The Myocardial Cell ). Of importance is that both are striated as a result of regular arrangement of the contractile proteins actin and myosin, and generation of force and contraction of muscle fiber occur as a result of their interactions (i.e., sliding filament mechanism).
Skeletal muscle and cardiac muscle show similar length-force relationships. As shown in , this relationship for the heart may be expressed graphically, as in
Fig.
16.36, by substituting ventricular systolic pressure for force and end-diastolic ventricular volume for resting myocardial fiber (and hence sarcomere) length. The lower curve in
Fig.
16.36 represents the increment in pressure produced by each increment in volume when the heart is in diastole. The upper curve represents the peak force or pressure developed by the ventricle during systole as a function of initial fiber length (or diastolic filling pressure). This curve illustrates the Frank-Starling law of the heart.