welcome redditors!to snoo-finity ... and beyond!

NBME 20 Answers

nbme20/Block 4/Question#25

A 52-year-old man is brought to the emergency ...

Increased total peripheral resistance and decreased cardiac output

Login to comment/vote.

Propanolol is a non-selective Beta blocker. So your HR will decrease (B1), which will cause a compensatory increase in TPR.

home_run_ball  ^ Above is partially right: Propranolol is non-selective Beta blocker: Beta1 stimulation causes inc HR, therefore blocking it will dec HR and dec Cardiac output Beta 2 stimulation causes vasodilation, therefore blocking it will CAUSE UNOPPOSED alpha1 activation --> therefore increasing total peripheral resistance. +16  
amarousis  so why tf do we give beta blockers for hypertension -.- +4  
dr_jan_itor  I would also add that the patient was previously on an a2 inhibitor (clonidine), which he ran out of. So he is rebounding on that with upregulated a1 receptor activity. Adding labetalol would cause a greater degree of unopposed alpha, increasing tpr +1  
llamastep1  @amarousis They are used for hypertension because the hypotensive effect of the reduced CO is greater than that of the effect of the increase of TPR. Cheers. +1  
hungrybox  @dr_jan_itor Adding labetalol would not cause unopposed α1 because labetalol and carvedilol are α1 blockers in addition to being nonspecific β blockers (great name btw, I love scrubs haha) +1  

 +0  upvote downvote
submitted by hungrybox(318),

So... theoretically an isolated decrease in HR would increase CO due to inc. preload, right?

But CO decreases in this case b/c the effect of inc. TPR is more powerful?

kernicterusthefrog  @hungrybox: No. Isolating HR, you would look at CO like this: CO=HR*SV so if HR or stroke volume go down, CO goes down. The change in preload wouldn't affect the CO as much as the change in rate of flow. So, the decrease in CO is solely due to the beta1 blocking effect on the AV node to decrease HR. +  

Where does the role of B1 stimulation of RAAS come into this? Wouldn't the B1 action cause decrease RAAS? That being said, I can also understand if that's a long term thing and this is a question about the immediate effects...?