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#1
04-05-2013
 USMLE Forums Scout Steps History: Not yet Posts: 40 Threads: 22 Thanked 3 Times in 3 Posts Reputation: 13
Decreasing the slope of phase 4?

Propanalol, esmolol, metoprolol, atenolol, and timolol slow abnormal pacemakers by "decreasing the slope of phase 4"
What does "decreasing the slope of phase 4" do exactly?

#2
04-05-2013
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 465 Times in 298 Posts Reputation: 475

thats the phase of Inward Na funny current which brings membrane potential to threshold in pacemakers
this drugs decrease If Na current and increases time required to reach threshold=decreased slope or decreased heart rate
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 The above post was thanked by: melboghdady (06-23-2013), paperplane (11-12-2016)
#3
04-05-2013
 USMLE Forums Veteran Steps History: CS Only Posts: 219 Threads: 44 Thanked 44 Times in 33 Posts Reputation: 54

I've reached a conclusion: phase 4 = SA node =heart rate. Phase 0 = AV node = PR interval /conduction

Correct me if any doesn't agree

#4
04-05-2013
 USMLE Forums Scout Steps History: 1 + CK Posts: 40 Threads: 4 Thanked 22 Times in 14 Posts Reputation: 32

Quote:
 Originally Posted by ` Faith ` I've reached a conclusion: phase 4 = SA node =heart rate. Phase 0 = AV node = PR interval /conduction Correct me if any doesn't agree
I guess u xactly need the meaning of slope.
In any graph,slope =y-intercept/x-inetrcept (The higher the y-axis value,the higher the slope).This implies increased slope is closer to y-axis(more standing) and decreased slope is closer to x-axis (more sleeping). Try to recollect ur school mathematics..

When plotting on graph, phase 4 of the specialized heart cells with automaticity(SA,AV nodes) is caused by pacemaker potential(It is special and is not stable unlike other stable muscle potential).This pacemaker potential is a slow,gradual depolarisation towards thershold.

Beta blockers decrease heart rate by decreasing slope of phase 4.This means they increase the time of automatic cells to reach threshold thus decrease their rate of firing.
So wat I meant to say is phase 4 can be for SA node,AV node,Purkinje fibers or anyother abnormal pacemakers.
Phase 0 in nodal cells is bcoz of influx of calcium thru L-type Calcium channels seen in SA and AV node.
 The above post was thanked by: Faith-USMLE (04-05-2013), OMGStep (04-05-2013)

 Tags Pharmacology-

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