Why does the heart beat? The discovery of the electrical system of the heart.

Why does the heart beat? This question--known as the myogenic versus neurogenic theory--dominated cardiac research in the 19th century. In 1839, Jan Evangelista Purkinje discovered gelatinous fibers in the ventricular subendocardium that he thought were muscular. Walter Gaskell, in 1886, demonstrated specialized muscle fibers joining the atria and ventricles that caused "block" when cut and found that the sinus venosus was the area of first excitation of the heart. By examining serial embryologic sections, Wilhelm His, Jr, showed that a connective tissue sheet became a bundle connecting the upper and lower cardiac chambers, the bundle of His. Sunao Tawara traced the atrioventricular (AV) bundle of His backward to find a compact node of fibers at the base of the atrial septum and forward where it connected with the bundles of cells discovered by Purkinje in 1839. Tawara concluded that this "AV connecting system" originated in the AV node, penetrated the septum as the His bundle, and then divided into left and right bundle branches that terminated in the Purkinje fibers. Martin Flack and Arthur Keith studied the conduction system of a mole and found a structure in the sinoauricular junction that histologically resembled the AV node. They felt that this was where "the dominating rhythm of the heart normally begins" and named it the sinoauricular node in 1907. The ECG of Einthoven soon brought a new understanding to the complex electrical system that makes the heart beat. In 2006 and 2007, we celebrate the 100th anniversaries of the publication of the exciting discovery of the AV and sinus nodes, truly landmarks in our understanding of cardiac structure and physiology.

Extracardiac manifestations of infective endocarditis and their historical descriptions.

In his landmark "Gulstonian Lectures on Malignant Endocarditis," published in 1885, William Osler commented, "Few diseases present greater difficulties in the way of diagnosis than malignant endocarditis, difficulties which in many cases are practically insurmountable." At that time, the fields of microbiology and blood cultures were in their infancy, and the diagnosis was made premortem in just half the patients with the condition. After Osler's report, extracardiac physical findings became essential clues to earlier diagnosis. Today, infective endocarditis is most commonly suggested from the history and often clinched by an echocardiogram and blood cultures. Although prized physical manifestations are much less frequent now, they still do occur and may be an invaluable clue that leads to earlier, more effective treatment. The investigators review these extracardiac findings along with their historical descriptions: splinter hemorrhages, emboli, Osler's nodes, Janeway and Bowman lesions of the eye, Roth spots, petechiae, and clubbing.

Woldemar Mobitz and His 1924 classification of second-degree atrioventricular block.

Woldemar Mobitz, an early 20th century German internist, analyzed arrhythmias by graphing the relationship of changing atrial rates and premature beats to AV conduction. Through an astute mathematical approach, he was able to classify second-degree atrioventricular block into 2 types, subsequently referred to as Mobitz type I (Wenckebach) and Mobitz type II (Hay). Type I AV block was most often due to digitalis and was reversible. There were no associated pathological findings. Type II AV block frequently progressed to complete AV block and was associated with seizures, death, and pathological findings.

Walter Gaskell and the understanding of atrioventricular conduction and block.

Walter Holbrook Gaskell was a nineteenth-century British physiologist whose investigations from 1874 until 1889 became central to our current understanding of cardiac physiology. His many cardiac contributions include the following: 1) the recognition of certain inherent properties of cardiac muscle; 2) the experimental proof that led to the acceptance of the myogenic theory of the origin of the heartbeat; 3) the mapping of the anatomy of the sympathetic nervous system; 4) the understanding of the dual autonomic control of the heart; 5) the discovery of the vasodilating effect of sympathetic stimulation on blood flow through skeletal muscle arteries; and 6) the introduction of the concept of heart block. Gaskell's elucidation of the sequence of cardiac contraction and atrioventricular block and his concepts of rhythmicity, excitability, contractility, conductivity and tonicity provided the physiologic explanation necessary for the future understanding of cardiac rhythm disturbances.

Reduced prevalence of atrial fibrillation in black patients compared with white patients attending an urban hospital: an electrocardiographic study.

Electrocardiographic differences occur between African-American and white persons. The cardiac rhythms of 2123 African-American and white hospital patients from 20 through 99 years of age were studied in a consecutive manner. The prevalence of atrial fibrillation increases dramatically with advancing age in both African-American and white patients. The prevalence of atrial fibrillation begins to increase at age 60 years and continues to increase through the 10th decade of life, although the rate of rise of the prevalence of atrial fibrillation is less in African-American patients compared with white patients. The cause of the reduced prevalence of atrial fibrillation in African-American patients remains unexplained. Atrial fibrillation occurs in 2.5% of African-American patients compared with 7.8% of white patients attending an urban hospital. There is little difference in the prevalence of atrial fibrillation between men and women. Atrial fibrillation occurs nearly seven times more often than does atrial flutter.

Lower prevalence of intraventricular block in African-American patients compared with Caucasian patients: an electrocardiographic study II.

BACKGROUND: Electrocardiographic (ECG) differences occur between African-American and white persons. METHODS: Intraventricular conduction abnormalities of ECGs of 2,123 African-American and white hospital patients ages 20-99 years were studied in a consecutive manner. RESULTS: Intraventricular conduction abnormalities develop later in life and are less common in African-American patients, compared with white patients. The prevalence of conduction abnormalities increases with advancing age in both races. Left- and right ventricular conduction abnormalities begin to rise at age 50 for white patients but begin to rise at age 70 for African-American patients. The prevalence of left ventricular conduction abnormalities peaks in the ninth decade of life in both races but declines in both races in the tenth decade of life. The prevalence of right ventricular conduction abnormalities gradually increases and peaks in the tenth decade of life in both races. CONCLUSIONS: The prevalence of intraventricular block is significantly less in African-American patients, compared with white patients--occurring in 8.6% of African-American patients and in 15.2% of white patients. The prevalence of intraventricular block is lowest in African-American women at 6.5% and highest in white men at 16.8%.

Comparison of the prevalence of first-degree atrioventricular block in African-American and in Caucasian patients: an electrocardiographic study III.

BACKGROUND: Electrocardiographic (ECG) differences occur between African-American and Caucasian patients. METHODS: The study includes ECGs of 2,123 patients, ages 20-99 years attending an urban hospital. RESULTS: First-degree atrioventricular (AV) block was more prevalent in African-American patients compared with Caucasian patients in all age groups of the study except for those patients in the eighth decade of life. The prevalence of first-degree AV block began to increase at age 50 years in both ethnic groups and gradually increased with advancing age, peaking in African-American patients in the 10th decade of life, and in Caucasian patients in the ninth decade of life. The continuing increase in first-degree AV block in African-American patients in the 10th decade of life suggests increasing impairment but greater durability of the AV conduction system in African-American compared with Caucasian patients. The dramatic decline of the prevalence of first-degree AV block in Caucasian patients in the 10th decade of life suggests more frequent failure of the AV conduction system in this group of patients at ages 90-99 years, compared with African-American patients in the same age group. In population-based surveys, first-degree AV block was more prevalent in African-American subjects compared with Caucasian subjects.