lec 9 cardiovascular electrical pump

key components of cardiac electrical system

sinoatrial node, atrioventricular node, bundle of His, right and left bundle branches

sinoatrial node

“natural pacemaker” of the heart

in RA, initiates electrical impulse that starts each heartbeat, sets heart rate

atrioventricular node

located between atria and ventricles, delays signal slightly to allow atria to contract and fill ventricles

bundle of His

carries signal from the AV node to ventricular septum

right and left bundle branches

split from bundle of His, direct impulse down each side of ventricular walls

purkinje fibers

spread impulse throughout the ventricles, cause ventricles to contract

resting membrane potential

inside of a cardiac cell, at rest, is about -90 mV in contractile cells, -60 mV in pacemaker cells

in depolarization, ion channels open and what?

positive ions (Na+, Ca2+) rush into cell, making cell less negative, changing voltage. this triggers action potential

pacemaker cells depolarize when

depolarize primarily through calcium influx

contractile cells depolarize when

depolarization happens when sodium floods in during first phase

why is depolarization important

it starts the electrical signal that spreads through the heart; leads to contraction

electrical order in heart

sa node generates signal, signal spreads through heart, cardiac cells depolarize, heart contracts, blood pumped

myocyte

muscle cell

pacemaker cells key purposes

start heartbeat

found in sa node

generate electrical impulse

contractile cells main purpose

produce pumping force

found in myocardium

respond to impulse

pacemaker myocytes generate ___ that initiate and regulate heartbeat

spontaneous action potentials

clinical relevance: pacemaker cells

sa node dysfunction can result in bradycardia; pacemakers are devices that replace/support failing cells

process of electrical impulse to contraction of heart

impulse arrives at myocyte, triggers Ca2+ entry into cell, causes Ca2+ release from internal stores, Ca2+ binds to troponin which allows muscle fibers to slide and contract

excitation contraction coupling

action potential reaches ventricular myocyte

L type Ca2+ channels open, Ca2+ enters cell

Ca2+ triggers sarcoplasmic reticulum to release more Ca2+

Ca2+ binds to troponin, moves tropomyosin out of way

actin and myosin interact, contraction happens

after contraction, Ca2+ pumped back into SR → relaxation

in short, electrical signal causes what that makes contraction occur?

electrical signal → Ca2+ enters cell → muscle fibers interact → heart contracts

Ca2+ channel blockers reduce contractility by

limiting Ca2+ influx

ischemia/electrolyte imbalances can disrupt…

action potential, lead to arrhythmias

heart failure often involves impaired ___ of these cells

impaired contractile function (of contractile cells)

diastole consists of what contracting/relaxing?

atria contract

ventricles relax

end diastole

systole consists of what relaxing/contracting

atria relax

ventricles contract

end systole

when atria contract, valves do what

tricuspid and mitral valves open, push blood into ventricles

when ventricles relax, valves do what

ventricles fill and pulmonary/aortic valves close

end diastole is time of…

max ventricular filling

when atria relax, blood fills from…

svc and ivc on the right and pulmonary veins on the left (atria)

when ventricles contract, valves do what

pulmonary and aortic valves open, blood is pushed to lungs and body

end systole is time of…

max ventricular emptying

electrical and mechanical process of heart summarized

sa node depolarizes, atrial contraction

signal goes to av node, then ventricles

ventricular depolarization, systole (blood pumped out)

repolarization → diastole (heart relaxes, fills)

when sa node fires, the atria

contract

when ventricles depolarize, the

ventricles contract

when ventricles repolarize,

heart relaxes

three phases of cardiac cycle

atrial systole, ventricular systole, diastole

atrial systole occurs right after

sa node fires

ventricular systole means vent. contract, causing

av valves closure, semilunar valves opening

in diastole, both atria and ventricles

are relaxed, chambers fill, semilunar valves close and av valves open

electrocardiogram (ECG, EKG)

detects electrical signals from pacemaker and contractile myocytes as they repolarize/depolarize

p wave initated by

initiated by sa node firing

atria begin to contract

p wave electrical event

depolarization spread through atrial contractile myocytes; atria contract

pr interval

signal slows at av node to allow ventricles to fill

atria finish contracting, ventricles still relaxed

qrs complex

signal spreads through His-Purkinje system, ventricles depolarize (systole)

ventricles contract and blood ejcted

t wave

ventricles rest, relax and heart refills w blood

stroke volume

amount of blood ejected by left vent w each heartbeat

normal HR is

60-100 mL/beat

ejection fraction (EF)

% of blood ejected during systole from LV

normal EF range

50-70%

EF commonly measured through what NM scans

MUGA, SPECT

how to calculate EF

EF = ( SV / EDV ) x 100

HR is regulated by

autonomic nervous system, hormones, temp, emotions, fitness level

stroke volume is influenced by

preload

afterload

contractability

preload

vol in ventricles end-diastole

afterload

resistance heart must pump against

contractability

strength of vent contraction

what autonomic NS effect is HR regulated by

sympathetic increases, parasympathetic decreases

stress testing importance

HR and SV responses to exercise are monitored; abnormal responses indicate ischemia, cardiomyopathy

Gated blood pool imaging (MUGA) scans

measures EF, related to SV; assesses vent function in pts undergoing chemo/heart failure

heart failure often involves reduced ___ and increased ___

reduced SV, increased HR

cardiac output

vol of blood heart pumps per min

CO normal output in healthy adult

4-8 L/min

CO =

CO = HR X SV

why does CO matter

CO determines how much o2/nutrients are delivered; low CO is hypoperfusion, leads to organ dysfunction

decreased CO may indicate

HF, cardiomyopathy, valve disease

increased CO may indicate

sepsis, anemia, hyperthyroidism

CO guides treatment decisions in that

helps to decide on fluid resuscitation, use of inotropes or vasopressors, mechanical support

SPECT, PET MPI ___ eval CO

indirectly

beta blockers ___ CO

decrease

exercise ___ CO

increases

shock ___ CO

increases or decreases, depending on type