Step I B1 blocker & PV Loops

[resiliens]

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Which figure shows the change in the PV loop associated with blocking B1 receptors on ventricular myocytes? I thought it'd be B, but apparently thats not the correct answer..

 

[BurghStudent]

Which figure shows the change in the PV loop associated with blocking B1 receptors on ventricular myocytes? I thought it'd be B, but apparently thats not the correct answer..

I think it's A. B1 blockade means negative ionotropy, which should correspond to a decreased contractile curve. The only answer that fits is A. Also, with negative chronotropy, you would expect increased LVEDV, preload. This also corresponds to A. Finally, B1 blockade decreases RAAS, so you'd expect decreased afterload.

Anyone correct me if I'm wrong.

 

[resiliens]

I think it's A. B1 blockade means negative ionotropy, which should correspond to a decreased contractile curve. The only answer that fits is A. Also, with negative chronotropy, you would expect increased LVEDV, preload. This also corresponds to A.

A is the right answer. The reason I thought it was B was because I thought -ve ionotropy meant that SV should be lower, so the new PV loop should be narrower. Why isn't this the case?

 

[BurghStudent]

A is the right answer. The reason I thought it was B was because I thought -ve ionotropy meant that SV should be lower, so the new PV loop should be narrower. Why isn't this the case?

I see why you'd think that. Discounting organs, the circuit has to be in series, blood has to move forward. SV should correspond to preload. My hunch is that Starling forces allow all that preload to move forward, hence a wide SV that doesn't result in backup.

Perhaps ionotropy should be thought of as power, rather than absolute force. Power, i.e. rate of work is lower, but the jobs still gets done. Someone else could chime in, I'm not as sure as I'd like to be.