Sudden cardiac death overview

# Overview
Sudden cardiac death (SCD) is defined as the unexpected death within an hour of onset of symptoms that is attributed to cardiac problems and usually confirmed on post-mortem examination. SCD is one the leading causes of death in the western countries, accounting to 15-20% of all deaths. Coronary artery disease (CAD) is the most common pathology related to SCD. With the advancement in medical treatment and prevention of CAD, the incidence of CAD and SCD have declined. Further investigation is needed to improve understanding of the underlying mechanisms involved in this condition.

# Historical perspective
Sudden cardiac death (SCD) occurs abruptly and unexpectedly in a person who is not known to have cardiac disease. It usually happens within one hour of onset of symptoms. About twenty-five percent of human deaths can be attributed to SCD, with the most common mechanism being the ventricular fibrillation (VF). This concept was first proposed by MacWilliam more than 120 years ago, during the time when the electrocardiogram was not yet invented.

# Classification
There are some definitions related to sudden death including Sudden cardiac death which is defined as sudden and unexpected death within one hour of being symptomatic such as palpitation, chest pain, shrtness of breath or within 24 hours in an asymptomatic patient due to arrhythmia or hemodynamic instability.Sudden cardiac arrest is suddenly cessation of cardiac activity,unresponsive patient with gasping respiration or no respiratory movement and unpalpable pulses due to cardiac etiology such as arrhythmia, pump failure. Aborted cardiac arrest is explained as unexpected circulatory collapse within one hour of being symptomatic, which is reversible after successful cardiopulmonary resuscitation. SIDS (sudden infant death syndrome) is sudden death  when there is normal structural heart without any specific findings in autopsy or toxicology.

# Pathophysiology
The pathogenesis of cardiac arrest is characterized by the myocardial inflammatory process in the setting of atherosclerosis, structural heart disease, genetic disorders, and environmental factors. The SCN5A, KCNH2, KCNQ1, RYR2, MYBPC3, PKP2, DSP genes mutation are associated with the development of inherited causes of cardiac arrest and  sudden cardiac death.

# Causes
Sudden cardiac arrest  may be caused by coronary artery abnormality such as coronary atherosclerosis, acute MI, coronary artery embolism, coronary arteritis
, hypertrophy of myocardium such as HCM, hypertensive heart disease, primary or secondary pulmonary hypertension
, myocardial disease such as ischemic cardiomyopathy, non-ischemic cardiomyopathy, myocarditis
,valvular heart disease such as aortic stenosis, aortic insufficiency, mitral valve prolapse, endocarditis 
, congenital heart disease such as congenital septal defect with eisenmenger physiology
, abnormality in conducting system such as Wolf-Parkinson-White syndrome
, electrical instability such as (CPVT, LQTS).

# Definition and Diagnosis
The diagnosis of sudden cardiac arrest is made when  the following diagnostic criteria are met: the absence of a palpable pulse of the heart due to abrupt cessation of pump function , absent carotid pulse,gasping respiration or NO respiration, loss of consciousness due to cerebral hypoperfusion.Following an initial diagnosis of cardiac arrest, healthcare professionals further categorize the diagnosis based on the ECG rhythm.
There are 4 rhythms that result in a cardiac arrest. Ventricular fibrillation (VF) and Pulseless Ventricular tachycardia (VT) are both responsive to a defibrillator and so are colloquially referred to as Shockable rhythms, whereas Asystole and Pulseless Electrical Activity (PEA) are non-shockable.
The nature of the presenting heart rhythm suggests different causes and treatment and is used to guide the rescuer as to what treatment may be appropriate.

# Epidemiology and Dermographics
Sudden cardiac death (SCD) comprises almost 50% of all mortality due to cardiovascular problems  Incidence of SCD increases proportionally with age. Men are more commonly affected with sudden cardiac death than women in all age groups. Racial backgrounds have large effects.

# Risk factors
Common risk factors related to underlying coronary artery disease and inherited causes in the development of sudden cardiac arrest are hypertension, male gender
,Diabetes mellitus, hyperlipidemia, obesity, smoking, older age, obstructive sleep apnea due to hypoxia, early VF (within 48 hours of ACS increasing in-hospital mortality five times), early repolarization patten in early phase of MI, family history of sudden death.

# Screening
Screening should be done based on the presentation of risk factors associated with sudden cardiac death.

# Natural history, Complications, Prognosis
Sudden cardiac arrest occurs due to sudden disturbance in cardiac electrical propagation or failure of the heart to pumping the blood into vital organs. 
Early clinical features include abrupt palpitation, presyncope, syncope, chest pain, dyspnea, hypotension within one hour before terminal event.
Patients  may progress to develop cardiac arrest , sudden collapse, loss of effective circulation, loss of consciousness.
If left untreated or failed resuscitation, biological death may occur within minutes to weeks. Common complications in survivors of cardiac arrest include pneumonia, gastrointestinal bleeding, injuries related to resuscitation, liver function test disturbance, acure renal failure, electrolytes disturbances, seizure.Two-thirds of patients with out-of-hospital cardiac arrest admitted in intensive care unit die of neurological complications.Most of the in-hospital cardiac death occur due to multiorgans dysfunction and one forth death is due to neurological complications. Factors associated poor prognosis after in-hospital cardiac arrest include age > 70 years old, concomitant underlying disorders such as pneumonia, hypotension, renal dysfunction, hepatic dysfunction,non shockable rhythm such as asystole or pulseless electrical activity. Factors associated with better prognosis after in-hospital cardiac arrest include early detection of cardiac arrest or being witnessed during arrest,shockable rhythm such as VF, VT, women between 15-45 years old. Prognosis of in-hospital cardiac arrest is generally better than out-of- hospital cardiac arrest and the 1-year survival rate of patients who survived to hospital discharge was approximately 25% in the GWTG-R registry. Survival after out-of-hospital cardiac arrest and in-hospital cardiac arrest has continued to improve over time according to the guideline.

# History and symptoms
Symptoms related to arrhythmia or underlying heart disease within one hour before cardiac arrest may include
, palpitation
, lightheadedness
, syncope
, Dyspnea at rest or on exertion
, orthopnea
, paroxysmal nocturnal dyspnea
, chest pain, edema.

# Physical Examination
Patients with cardiac arrest usually appear cyanotic. Physical examination maybe remarkable for heart rate and regularity, blood pressure, Jugular venous pressure, Murmurs
, Pulses, bruits
, Edema
, Sternotomy scars.

# Laboratory Findings
An elevated concentration of  brain natriuretic peptide (BNP) has been shown as the predictor of ventricular arrhythmia and sudden cardiac death.

# Electrocardiogram
An ECG may be helpful in the diagnosis of Sudden cardiac death. Findings on ECG associated with sudden cardiac arrest include Sinus tachycardia (39%), abnormal T-wave inversions (30%), prolonged QT interval (26%), left/right atrial abnormality (22%), left ventricular hypertrophy (17%), abnormal frontal QRS axis (17%), delayed QRS-transition zone in precordial leads (13%), pathological Q waves (13%), intraventricular conduction delays (9%), multiple premature ventricular contractions (9%), normal ECG (9%).

# X-Ray
A  chest x-ray may be helpful in the diagnosis of the underlying cause of cardiac arrest such as cardiomegaly, pulmonary  edema, massive pericardial effusion, widening aorta silhouette.

# Echocardiography
Echocardiography may be helpful in the diagnosis the cause of lethal arrhythmia and sudden cardiac arrest by assessment of ,Regional wall motion abnormality,systolic function  of left ventricle, evidence of myocardial infarction, valvular heart disease such as aortic stenosis,right ventricular cardiomyopathy
,pericardial effusion, Tamponade, aorta dissection.

# CT scan
Cardiac CT scan may be helpful in the diagnosis of the causes of cardiac arrest by evaluation of
left ventricular volumes, Ejection fraction,Cardiac mass
, anomalous origin of coronary arteries
, coronary arteries calcification
, pulmonary embolism
,aorta dissection.

# MRI
Cardiac MRI is an accurate modality for diagnosis of structural and functional causes of cardiac arrest by the evaluation of 
chamber volumes, left ventricular mass 
, left ventricular size and function
, right ventricular size and function
, regional wall motion abnormality

# Other Diagnostic Studies
For survivors of sudden cardiac death due to lethal arrhythmia from ischemic heart disease, coronary angiography and probable revascularization is recommended. Electrophysiology study is recommended for induction of bradyarrhythmia , ventricular tachyarrhythmia, determination the indication for ICD implantation  in dilated cardiomyopathy,ARVC, HCM. Electrophysiology study is not recommended in long QT syndrome (LQTS), cathecolaminergic polymorphic ventricular tachycardia (CPVT), short QT syndrome (SQTS).

# Urgent Treatment
The mainstay of therapy for patients with cardiac arrest  is starting cardiopulmonary resuscitation with minimizing interruption in chest compression.The rhythm should be reassessed. If the rhythm is VF or pulseless VT, the shock should be delivered immediately. If the rhythm is asystole or pulseless electrical activity (PEA), CPR should be resumed. Advanced life support (ALS) should be kept with minimizing interruption in chest compression including:
advanced airway, continuous chest compressions, capnography, IV/IO access, vasopressors, antiarrhythmics therapy, correcting  reversible causes including hypoxia, hypovolemia,hypothermia, hyperkalemia, hypokalemia,acidosis, tension pneumothorax, tamponade, toxins (benzodiazepines, alcohol, opiates, tricyclics, barbiturates, betablockers, calcium channel blockers). The followings should be considered immediately in post cardiac arrest patients: 12lead ECG ,Perfusion/reperfusion in patients with acute myocardial infarction, Oxygenation and ventilation,temperature controlling, treatment of  reversible causes.Management  of patients in  post-cardiac arrest status include treatment of the underlying disorder, hemodynamic stability, respiratory support, controlling the neurologic complications.

# Post Arrest Care and Prevention
Effective measures for the primary prevention of sudden cardiac death in individuals who are at risk of SCD but have not yet experienced an aborted cardiac arrest or life-threatening arrhythmias include ICD implantation based on the guideline. Secondary prevention strategy following aborted sudden cardiac death include revascularization in patients with ischemic heart disease and  ICD implantation in patients with reduced left ventricular ejection fraction who had an experience of lethal arrhythmia.
The optimal approach to prevention of SCD following ST-elevation MI (STEMI) has been evaluated in multiple randomized trials. In general, post-STEMI patients should be treated with evidence-based therapies that have been associated with a reduction in SCD including beta-blockers, ACE-inhibitors (or ARBs in patients who are ACEI intolerant) and statins. In patients who have symptomatic congestive heart failure (CHF), an aldosterone antagonist may be a reasonable additional therapy. Despite the intuitive benefits of antiarrhythmics, amiodarone and sotalol have not been shown to reduce all-cause mortality following STEMI, although amiodarone may be useful in reducing the frequency of shocks in patients with ICDs who have unacceptably high rates of shock. In general terms, ICD placement is indicated in those patients with a reduced left ventricular ejection fraction at 40 days post-MI and/or 3 months following revascularization (PCI or CABG) for STEMI given the survival benefits in this population.

# Ethical Issues
Cardiopulmonary resuscitation and advanced cardiac life support are not always in a person's best interest. This is particularly true in the case of terminal illnesses when resuscitation will not alter the outcome of the disease. Properly performed CPR often fractures the rib cage, especially in older patients or those suffering from osteoporosis. Defibrillation, especially repeated several times as called for by ACLS protocols, may also cause electrical burns.
Some people with a terminal illness choose to avoid such measures and die peacefully. People with views on the treatment they wish to receive in the event of a cardiac arrest should discuss these views with both their doctor and with their family. A patient may ask their doctor to record a do not resuscitate (DNR) order in the medical record. Alternatively, in many jurisdictions, a person may formally state the wishes in an advance directive or advance health directive.