Why do I have sudden fast and pounding heartbeats?

Why are some heartbeats louder than others?

• I have been working on a project for a physiology class for a while now. It has to do with heart disease. I am testing men between 18 and 60. The tests involve taking vitals, an ekg and extensive auscultation of the heart for murmurs, splits, gallops, rhythm, etc. One thing I cannot figure out nor find any information on is why does the volume of the heartbeat vary from person to person. I have tested over 120 men and I have noticed that about 50% have an "average" heartbeat volume, about 25% have a loud heartbeat, 15% have a very loud heartbeat and 10% have a very quiet, difficult to hear heartbeat. I have already looked at my data to find the reason for this. I figured that the higher the blood pressure the louder the heartbeat, but this is not true, people with high blood pressure had quiet and loud heartbeats. So then I thought about the thickness of the chest wall. But there were many very muscular and overweight men whose heartbeats were very loud and audible and thin guys whose heartbeats very quiet. Smoking didn't matter. Sexual orientation didn't matter. I also check for differences on the ekgs between loud and quiet hearts but could find no differences. The only two factors that made a slight difference was age and race. Men in their 20's tended to have a louder heartbeat and men in their late 40's and up tended to have a quieter heartbeat. But this was not related to their blood pressure or size at all. Also, black men tended to have a much clearer and louder heartbeat as well, regardless of their blood pressure or size. So why do some people have very quiet difficult to hear hearts and others are clear as a bell? If race and age are factors, why? What is the physiological reason? Is the heart deeper in the chest or close to the surface? Are the muscles on the heart less flexible as you get older and therefore make less of a noise? I have tried to search for the answer online using terms like "volume, auscultation, heart sounds" but I cannot not find anything at all. Any input would be great appreciated.

• Answer:

  It's perfectly simple. The loudness is proportional to the pulse-pressure, Pp, (that is, systolic minus diastolic) and pulse pressure is proportional to VS , stroke volume. Therefore Loudness is proportional (almost linearly) to Stroke Volume. Because cardiac output is the product of stroke volume and the Number of beats per minute, you can write CO = N x SV. But the relaxed ratio of N to SV differs from person to person for elementary, trivial physiological reasons (the same as eye-colour, height, etc) and so for the average of 5000mls/min cardiac output, you COULD get (say, - for instance) a pulse rate of 50 bpm, with an SV of 100 mls. OR you could get a pulse rate of 100 and an SV of 50... Both combinations give a CO of 5000mls/min. However, because of the SV: Pp relationship, the 100 SV would give a much greater loudness. If you re-visit your clinical test results on a 'blind' basis (that is, regardless of age, race, etc) and just plot "loudness" in arbitrary units against pulse pressure, then I think you'll see some correlation appear. Of course, your results will be clouded by scattering caused by physical parameters, because your "loudness" measuring scale is perhaps not strictly accurate? A high pulse pressure will be muffled by a large body mass, for instance... the pressure itself, within the artery, is not the sound, .. the sound is produced by the meatus surrounding it, and its transmissibility. To investigate this aspect you must imagine trying to create a loudspeaker with the arterial pressure as input signal, and the muscle, fat, etc, acting as the sounding-board.. If you wish, do feel free to email me on this. It's a fascinating topic and you've only just begun to scratch the surface !