During a cardiac diastole the heart-cavities are all filling; just before the commencement of the cardiac systole the blood is forced from the lungs and the cavæ through the auricles into the ventricles, while the mitral and tricuspid valves are pressed against the walls of the ventricles, and no obstruction is offered to the blood-current. If, as the result of disease, any obstruction exists at either one of the auriculo-ventricular orifices, the blood as it passes through the opening will impinge on such obstruction and cause a presystolic murmur.
During a cardiac systole the filled ventricles contract; blood is thrown through the semi-lunar openings, the flaps of whose valves are pressed against the walls of the vessels, so that no obstruction is offered to the outgoing current. At the same instant the auriculo-ventricular valves close their orifices, so that blood may not be forced back into the auricles.
If, as a result of disease, the semi-lunar valves should obstruct the outgoing current, or the mitral or tricuspid valves should not wholly close the auriculo-ventricular orifices, then in the one case the direct blood-current, as it passes over the obstruction at the semi-lunar orifices, would produce a systolic murmur, and in the other the backward current through the abnormal opening at the auriculo-ventricular orifice would also produce a systolic murmur.
Again, if the lungs and the aortic system (when filled at the systole) have, back of them, a semi-lunar valve that does not completely close that end of the circuit, the blood will regurgitate into the ventricles during the period of cardiac rest, so that semi-lunar incompetence will cause a diastolic murmur.
ENDOCARDIAL MURMURS.
| Rhythm. | Situation. | Orifice. | Nature. | |
| Systolic | 1 | Basic, | Aortic, | Obstructive. |
| " | 2 | " | Pulmonary, | " |
| " | 3 | Apical, | Mitral, | Regurgitant. |
| " | 4 | " | Tricuspid, | " |
| Diastolic | 1 | Basic, | Aortic, | " |
| Presystolic | 1 | Apical, | Mitral, | Obstructive.5 |
5 Pulmonary regurgitant murmur (diastolic) and tricuspid obstructive murmur (presystolic) are so rarely met with that, clinically, they may be disregarded.
The following is the order of relative frequency of cardiac murmurs: 1. Mitral regurgitation; 2. Aortic obstruction; 3. Aortic regurgitation; 4. Mitral obstruction; 5. Tricuspid regurgitation; 6. Tricuspid obstruction; 7. Pulmonary obstruction; 8. Pulmonary regurgitation. The most frequent combinations of murmurs are—1. Aortic obstruction and regurgitation; 2. Mitral obstruction and regurgitation; 3. Mitral obstruction and tricuspid regurgitation; 4. Aortic obstruction and mitral regurgitation; 5. Double valvular disease at aortic and mitral orifices (four murmurs).
It is often difficult, after having satisfied ourselves of its existence, to determine the rhythm of an endocardial murmur. To resolve this difficulty it is necessary to determine which is the first and which the second sound of the heart.
The first sound of the heart is synchronous with the carotid pulse, the radial pulse, and the apex-beat. It may be wholly replaced by a systolic murmur, but the second sound is always heard following the apex-beat, for the pulmonic and the aortic valves are never diseased at the same time.