Peculiarities of the first heart sound in bundle branch blocks. A new interpretation based on graphic analysis.

A phonocardiographic study in a medium frequency range was made over various areas of the precordium in 27 cases of right bundle branch block (RBBB), 28 cases of left bundle branch block (LBBB), and 30 normal subjects of the same age. The various components of the first heart sound plus the pulses at the suprasternal notch and the right carotid artery were studied in regard to timing, relationship with the ECG and the arterial pulses, intervals between components, and amplitude. The timing and intervals of the three components of the first sound were found normal in RBBB and so were the arterial pulses; apparent wide splitting was occasionally noted, due to recording of the first (a) and third (c) components, the latter being larger, as frequently observed in old age. All three components of the first sound were found small and delayed in LBBB; a delay of the arterial pulses was also noted. No additional component that might be attributed to the right heart preceded the delayed first sound. This study confirms that the first heart sound recorded on the chest wall originates only in the left heart and aorta.