You can hear the first and second heart sounds, S1 and S2, in a person with a pacemaker, but they often sound different than they would in a heart with a natural rhythm. The heart’s familiar “lub-dub” sound is created by the mechanical closing of the heart’s four valves. S1 (“lub”) results from the closure of the mitral and tricuspid valves at the start of ventricular contraction. S2 (“dub”) is caused by the closure of the aortic and pulmonic valves when the ventricles relax and fill with blood. A pacemaker is an implanted device that delivers electrical impulses to regulate the heart rate and timing of these contractions.
How Pacemakers Influence Heart Contraction
The changes in heart sounds are a direct result of how a pacemaker alters the heart’s electrical activation sequence. In a healthy heart, the electrical signal travels rapidly through a specialized pathway, the His-Purkinje system, to both ventricles simultaneously. This rapid, synchronized electrical spread ensures the left and right ventricles contract almost perfectly together, which is necessary for efficient blood pumping.
Standard pacemakers typically use a lead positioned in the right ventricle to deliver the electrical impulse. This artificial stimulation bypasses the natural, high-speed conduction system. The impulse spreads more slowly through the muscle tissue from the point of contact. This slower, less coordinated spread causes the right ventricle to contract before the left ventricle, creating ventricular dyssynchrony.
This electrical timing difference translates directly into a mechanical timing difference in the closing of the heart valves. The loss of synchronized contraction reduces the efficiency of the heart’s pump function. The mechanical delay in contraction prolongs the time it takes for the ventricles to build enough pressure to close the valves.
Specific Alterations to S1 and S2 Sounds
The altered sequence of contraction and valve closure significantly changes the auditory characteristics of the heart sounds. A common change is a notable softening or muffling of the first heart sound, S1. This reduced intensity is due to the asynchronous filling and contraction of the ventricles, which affects the force and timing of the mitral and tricuspid valve closures.
The altered timing can also cause an audible splitting of the heart sounds, where S1 or S2 is heard as two distinct sounds instead of one fused sound. S1 is normally a single sound because the mitral and tricuspid valves close very close together, but pacing can widen the interval between their closures. S2 may also exhibit a wider split between the aortic and pulmonic valve closures than normal, reflecting the delayed contraction of the left ventricle.
The intensity of S1 can also become variable, especially in single-chamber ventricular pacing. This variability occurs because the pacemaker’s fixed timing may not align optimally with the atrial contraction. When the atrium contracts against a closed mitral valve, it affects the subsequent ventricular contraction. This causes the S1 sound to change in loudness from one beat to the next.
Identifying Pacemaker Function Through Listening
Listening to the heart can offer clues about the pacemaker’s function and the resulting heart rhythm. In a pacemaker-dependent person, the rhythm will be perfectly regular, dictated by the programmed rate of the device. This fixed regularity is a strong indicator that the heart is being controlled by the pacemaker.
A distinct, high-frequency sound, sometimes called a “pacemaker click” or “pacing artifact,” may also be heard. This faint, sharp sound is the auditory manifestation of the electrical impulse delivered by the device, occurring just before the paced S1 sound. The intensity of this click can vary widely, sometimes being louder than the heart sounds themselves. It is usually best heard with the stethoscope’s diaphragm.
The relationship between the audible click and the subsequent heart sounds confirms the device is actively pacing the heart. If the heart rhythm becomes highly irregular despite the fixed rate programming, it may suggest the underlying native rhythm has temporarily taken over. Irregularity could also indicate a potential issue with the device’s function, such as a battery or lead problem.