Exercise - which increases first? SV or HR?

[Phloston]

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UWorld QID#6691 of UWSA2 says, in one of the explanations: "the first cardiac response to exercise is an increase in heart rate, which is mediated by decreased vagal tone on the heart as well as increased sympathetic stimulation. Stroke volume is increased during exercise as well."

P. 280 of FA2012 says: "during the early stages of exercise, CO is maintained by stroke volume; during the late stages of exercise, CO is maintained by HR."

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Could someone please help out here? I had always thought the latter was correct. Btw, this is what I was talking about in my other post about "freaking out" every time I encounter something contradictory in UWorld. I haven't declared it an error yet; I'm just asking people's thoughts first.

Thanks,

 

[Myxedema]

Check Figure 84-10 in Guyton Physiology if you have access to it. What FA means is this: By the time CO reaches half of its maximum, SV has already reached its maximum value. After that point, only the increased in heart rate can produce a further increase in CO.

From Guyton:

Figure 84-10 shows the approximate changes in stroke volume and heart rate as the cardiac output increases from its resting level of about 5.5 L/min to 30 L/min in the marathon runner. The stroke volume increases from 105 to 162 milliliters, an increase of about 50 percent, whereas the heart rate increases from 50 to 185 beats/min, an increase of 270 percent. Therefore, the heart rate increase accounts by far for a greater proportion of the increase in cardiac output than does the increase in stroke volume during strenuous exercise. The stroke volume normally reaches its maximum by the time the cardiac output has increased only halfway to its maximum. Any further increase in cardiac output must occur by increasing the heart rate.

 

[viper930]

Yeah in early exercise muscle contraction increases venous return to the heart and increases stroke volume. This accounts for the majority of the CO increase until it maxes out at which point CO only increases with increased HR.

 

[Old Style Nanny]

Check Figure 84-10 in Guyton Physiology if you have access to it. What FA means is this: By the time CO reaches half of its maximum, SV has already reached its maximum value. After that point, only the increased in heart rate can produce a further increase in CO.

From Guyton:

This has been my understanding as well. HR definitely is the major component and logicially, should also be the first to increase. But, does it really matter which comes first? I would think what constitutes the majority would be the main concept tested?

 

[Spartacus824]

Early stages of exercise: CO mainly changes based off of SV
Late stages of exercise: CO mainly changes based off of HR

Does the extra time from completely a block early get added onto the total break time during the exam?

 

[Myxedema]

This has been my understanding as well. HR definitely is the major component and logicially, should also be the first to increase. But, does it really matter which comes first? I would think what constitutes the majority would be the main concept tested?

Both SV and HR increase at the initial phase, but the change in SV is greater initially. When SV becomes maxed out, HR continues to increase. This may be asked in graph form.

 

[munchymanRX]

UWorld QID#6691 of UWSA2 says, in one of the explanations: "the first cardiac response to exercise is an increase in heart rate, which is mediated by decreased vagal tone on the heart as well as increased sympathetic stimulation. Stroke volume is increased during exercise as well."

P. 280 of FA2012 says: "during the early stages of exercise, CO is maintained by stroke volume; during the late stages of exercise, CO is maintained by HR."

-------

Could someone please help out here? I had always thought the latter was correct. Btw, this is what I was talking about in my other post about "freaking out" every time I encounter something contradictory in UWorld. I haven't declared it an error yet; I'm just asking people's thoughts first.

Thanks,

Cardiac output is a function of stroke volume and heart rate by CO=SVxHR

In order for CO to increase, one of those two variables has to increase first. If you're exercising, your HR increases, and SV can increase as well due to heightened contractility.

 

[cbrons]

I don't know if the review books talk about this, but I'm fairly certain this is the phenomenon of cardiac drift in exercise physiology.