sudden cardiac death - epidemiology

Defibrillation electrode positions

Source: Wikipedia

Sudden cardiac death statistics are complicated by a lack of uniformity of definition. Some researchers utilise the World Health Organisation (WHO) definition of death occurring within 24 hours of the onset of acute ‘cardiac’ symptoms, whilst others utilise the Framingham Heart Study cut-off of death due to cardiovascular collapse less than 1 hour from the onset of symptoms.

An added complicating factor is the 25% of people who have clinically silent myocardial infarctions, and those who suffer from ‘atypical angina’, presenting with abdominal pain or hiccoughs etc (Zipes and Wellens 1998; Leor et al 1996; Marks and Greene 1995; Myers and Dewar 1975; Norris 2000).

In the USA, sudden cardiac death accounts for between 200,000 – 600,000 deaths per year depending upon definitions used. 80-90% of those deaths occur on a background of significant coronary artery disease (atherosclerosis) (Zipes and Wellens 1998; Marks and Greene 1995; Hirsch and Adams 1993).

In England, the rate of sudden unexpected adult deaths due to ischaemic heart disease is estimated to be 9.1 per 100,000 per annum. This is equivalent to 3338 cardiac deaths in adults aged 16-64 years. The term Sudden Adult Death Syndrome (SADS) has been proposed for these deaths (Bowker et al 2003, Davies 1992). 

Sudden death is often the first presentation of cardiac disease, estimated as occurring between 33% and 70% of cases (Zipes and Wellens 1998; Fornes 1993; Chi and Kloner 2003; Davies 2000). Over 80% occur at home, and at rest, whilst 30% are thought to occur in sleep (Fornes 1993; Zipes and Wellens 1998).

Most researchers agree that the common pathway leading to sudden cardiac death is overwhelmingly ventricular fibrillation (VF), with varying numbers having ‘structural abnormalities’.

3D model of the heart in ventricular fibrillation - note the uncoordinated waves of electrical energy travelling around the heart (download 3Dpdf document of this model)

For example, up to 80% have significant stenoses, whilst between 15% and 75% have evidence of thrombus within a coronary artery, depending upon the inclusion or exclusion criteria of the various published series (Sheppard and Davies 1998). Davies (1992 and 2000) and Kolodgie (2004) are amongst those researchers favouring thrombi as a common occurrence in sudden cardiac deaths. It should also be noted that 30% of the ‘structural abnormalities’ relate to ‘non-specific’ myocardial fibrosis (Davies and Popple 1979; Chugh et al 2000).

Those who have structurally normal hearts (between 5% and 20%) are said to die of ‘idiopathic VF’, although underlying genetic abnormalities predisposing an individual to fatal arrhythmias including VF are being elucidated, such as in the ‘long QT syndromes’, Brugada syndrome, and others. (Brugada 2004; Zipes and Wellens 1998; Vatta et al 2002; Behr et al 2003; Gaita et al 2003; Priori et al 2001).

Some of the syndromes may be inherited, and symptoms including syncope, cardiac arrest, palpitations, dizziness and atrial fibrillation may be found amongst surviving family members. (Gaita et al 2003).

pinterest - natural disease

mechanisms

The anatomical substrates for sudden cardiac death include coronary artery atherosclerosis, myocardial fibrosis and necrosis and left ventricular hypertrophy. However, many individuals have such a substrate, but are able to carry on their daily activities.

The prevailing hypothesis regarding the mechanism of sudden cardiac death involves the superimposition of a ‘trigger’ or ‘transient risk factor’ acting in concert with the anatomical substrate to perturb cardiovascular function and produce the final common pathway of a fatal arrhythmia, usually VT deteriorating into VF. (Willich 1993; Hirsch and Adams 1993; Chugh et al 2000; Marks and Greene 1995).

The external ‘trigger’ may be associated with any of the following effects;

• Increased sympathetic activity
• Increased circulating catecholamines
• Raised heart rate and blood pressure
• Alpha sympathetic receptors stimulation increasing vascular tone
• Alterations of sheer stresses within coronary vessels
• Vulnerable atherosclerotic plaque erosion/ rupture, and/ or increased platelet aggregation, and/ or direct arrhythmogenesis

Pump failure is an additional mechanism, whereby chronic ischaemia causes global hypoxia, giving rise to electrical instability and ectopic beats, leading to the generation of fatal arrhythmias.

The degree of coronary artery stenosis required, and the role of ‘vulnerable’ atherosclerotic plaques and coronary artery thrombi are debated, but all can result in acute physiological derangements, destabilising electrical activity in the heart.

Ventricular irritability can be mediated by alcohol and drugs, including caffeine and cocaine, whilst myocardial depression leading to delayed conduction and cardiac standstill can be mediated by parasympathetic activity or electrolyte disturbances. (Hirsch and Adams 1993).

The role of the CNS in cardiovascular physiology is also uncertain, but CNS influences can be strong enough to cause cardiac standstill, and trauma to the brain or spine can cause instantaneous cardiovascular collapse (Hirsch and Adams 1993).

references

• Behr E, Wood DA, Wright M et al. Cardiological assessment of first-degree relatives in sudden arrhythmic death syndrome. The Lancet 362: 1457-59
• Brugada R, Hong K, Dumaine R et al. Sudden death associated with Short-QT syndrome linked to mutations in HERG. Circulation 2004; 109:30-35
• Chi JS, Kloner RA. Stress and myocardial infarction. Heart. 2003; 89: 475-476
• Chugh SS, Kelly KL, Titus JL. Sudden cardiac death with apparently normal heart. Circulation. 2000 102(6):649-654
• Davies MJ. Anatomic features in victims of sudden coronary death. Coronary artery pathology. Circulation. 1992; 85 [suppl I]:I-19-I-24
• Davies MJ. Unexplained death in fit young people: a category of the sudden unexpected death syndrome is needed. Br Med J 1992; 305:538
• Davies MJ. The pathophysiology of acute coronary syndromes. Heart. 2000; 83:361-366
• Davies MJ, Popple A. Sudden unexpected cardiac death – a practical approach to the forensic problem. Histopathology. 1979; 3:255-277
• Fornes P, Lecomte D, Nicolas G. Sudden out-of-hospital coronary death in patients with no previous cardiac history. An analysis of 221 patients studied at autopsy. Journal of Forensic Sciences. 1993; 38(5):1084-1091
• Gaita F, Giustetto C, Bianchi F et al. Short QT Syndrome – A familial cause of sudden death. Circulation. 2003; 108:965-970
• Hirsch CS, Adams VI. Sudden and unexpected death from natural causes in adults. Chapters 5 and 6 in ‘Spitz and Fisher’s Medicolegal investigation of death – Guidelines for the application of pathology to crime investigation’, Spitzt WU (Ed) (3^rd Ed) 1993 Charles C Thomas Publishers, Springfield Illinois
• Kolodgie FD, Virmani R, Burke AP et al. Pathologic assessment of the vulnerable human coronary plaque. Heart. 2004; 90: 1385-1391
• Leor J, Poole WK, Kloner RA. Sudden cardiac death triggered by an earthquake. NEJM 1996; 334:413-419
• Marks ML, Greene HL. Sudden Cardiac Death. Chapter 42 in Cardiac Arrhythmias. Mandel WJ. 3^rd Ed 1995 JB Lippencott Co. Philadelphia USA.
• Myers A, Dewar HA. Circumstances attending 100 sudden deaths from coronary artery disease with coroner’s necropsies. British Heart Journal. 1975; 37:1133-1143
• Norris RM. The natural history of acute myocardial infarction. Heart. 2000; 83: 726-730
• Priori SG, Napolitano C, Tiso N, et al. Mutations in the cardiac Ryanodine receptor gene (hRyR2) underlie Catecholaminergic Polymorphic Ventricular Tachycardia. Circulation. 2001; 103:196-200
• Sheppard M, Davies MJ. Practical Cardiovascular Pathology. 1998 Arnold Publishing, London UK
• Vatta M, Dumaine R, Varghese G et al. Genetic and biophysical basis of sudden unexplained nocturnal death syndrome (SUNDS), a disease allelic to Brugada syndrome. Human Molecular Genetics 2002 11(3):337-345
• Willich SN, Maclure M, Mittleman M et al. Sudden cardiac death – support for a role of triggering in causation.  Circulation 1993 87(5):1442-1450
• Zipes DP, Wellens HJJ. Sudden cardiac death. Circulation. 1998; 98:2334-2351

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