Heart Rate Drops During Training... and Cardiac output/Stroke Volume

Dont know if it was this podcast or another where there was discussion of heart rate drops during training…but the answer to whether or not a dropping heart rate is good or bad is …yes. If during training there is a gradually lowering or stable heart rate with increasing performance…this is generally a good adaptation showing improving perfusion/efficiency and/or cardiac output and/or muscle efficiency allowing more work with less perfusion…if however there is a dropping heart rate with a plateau or drop in the ability to do work/train or dropping power…training should be suspended and the person immediately referred for a cardiac evaluation…if its associated with increasing shortness of breath, chest, arm or jaw pain or pressure a trip to the ER would be a good thing. This is because coronary insufficiency…i.e. not enough blood flow in the coronary arteries to keep the heart muscle supplied with blood can cause a reflexive drop in heart rate. The coronaries fill during diastole, i.e. between contractions of the heart/ left ventricle so the slower the heart rate the better the filling of the coronaries as there is more time for blood flow. But if there is a critical coronary stenosis or narrowing you can see cardiac muscle ischemia (a prelude to a heart attack) causing a lowering of the heart rate as a silent symptom. Also regarding cardiac output… the more time the heart has to fill the left ventricle typically the higher the stroke volume (amount of blood pumped out per cardiac beat) is, which allows a higher cardiac output. (some patients with heart failure are given small dose of beta blockers to allow the heart to fill more to improve their cardiac output) . Cardiac output = stroke volume x heart rate. But the limitation of just jacking up your heart rate to get a higher cardiac output is that there is less fill time with a higher heart rate so the stroke volume decreases. Bottom line with training improving power with the same or slightly lower heart rate is generally a good adaptation to the training.

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On the pharmacological side, beta-blockers drastically reduce your maximum heart rate, but in a strange twist your stroke-volume increases markedly and ultimately your cardiac output stays about the same. WADA bans beta-blockers for a few select sports that involve precision-aiming because it reduces tremors from adrenaline – but cycling allows it (*last I checked…).

Beta blockers are tricky for athletes…they do reduce heart rate but they can exacerbate bronchospasm in asthmatics, and there are different subclasses of these drugs with different effect profiles. So like most drugs best to avoid unless specifically directed to use them and worth a discussion with the prescribing provider if like most people on this forum you are an endurance athletes. Thanks for the comment.