Step I beta blockers and stroke volume

[dorian24]

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kind of confused about this point -- there is a usmle-rx question that says that metoprolol reduces contractility but does not reduce stroke volume - rather, it decreases heart rate and allows more diastolic filling time to increase SV.

however, in FA (p.273) it says that stroke volume is affected by contractility (??)

so i'm a tad confused since the logic can seem to go either way. why doesn't a decrease in contractility directly decrease SV? Thanks!

 

[sahell]

It can go both ways, confuses me too. Decreased contractility increases EDV (preload) and so SV may be maintained.

 

[PugsAndHugs]

Confused me too but think of it like this

1. B-blockers would decrease HR = increased EDV —> leads to increase SV
2. B-blockers decrease contractility —> leads to decreased SV

I THINK why it ultimately results in increased SV is because the increase in EDV outweights the decreased contracility.