Heart Functions: the MEA and the Frank Starling Law of the heart 29

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Heart Functions the MEA and the Frank Starling
Law of the heart 2/9

• What leads are used to evaluate the heart during
  intensive care?
• What does the MEA tell us about heart function?
• What happens to the MEA in left side or right
  side hypertrophy?
• What is the Frank-Starling Law of the Heart?
• How does EDV determine SV in a healthy heart?
• How does EDV determine SV in an unhealthy heart?
• Where does fluid accumulate when the right and
  left ventricles fail?
• Why does the fluid accumulate in these places?

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What are the three standard LEADS that folks
use to look at heart function? Lead I (RA- and
LA), Lead II (RA- and LL) and Lead III (LA- and
LL). These form a perfect triangle around the
heart that sits at the exact center of chest.
Sort of
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MEAN ELECTRICAL AXIS (MEA) AND A MORE ADVANCED
CARDIAC DIAGNOSIS OF HEART FUNCTION/PATHOLOGY.

• The mean electrical axis is the average direction
  of depolarization in the heart.
• Einthovens Triangle represents an equilateral
  triangle at the center of chest. (remember the
  heart is shifted slight to left side)
• Dead heart tissue does not depolarize
• ECG Changes Large R-wave becomes a small R-wave
• ECG leads can identify exact spot on heart where
  infarct or clot is located with out cutting you
  open to look.
• MEA if aortic BP is 180/130
• Left Ventricle works harder so it gets larger
• MEA shifts to Left Side Left Shift
• MEA if you have emphysema (hard to drive blood
  through lung)
• RightVentricle must work much harder
• Pulm arterial pressure 20/4?80/30
• Right side gets more muscular? Mass increases?ECG
  Changes

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Calculation of the Mean Electrical Axis (MEA)
with an ECG lets you evaluate how the heart is
oriented in the chest, Ventricular size (heart
attack risk), and where in heart infarct
occurred. (Its Non-Invasive and Cheap!) MEAs
that are not down and to left are AXIS DEVIATIONS.
One Way to Measure MEA
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Easiest Way to Measure MEA to Assess Cardiac
Function?

• What would causes axis deviations?
• What happens to MEA if you lose 20 of the mass
  of left ventricle?
• What happens to MEA if you had a rare right side
  heart attack and lost 20 of the right ventricle?
• What happens to size of left ventricle and MEA if
  aortic valve will not open properly?
• What happens to MEA if a tumor inside pericardial
  sac pushes heart to the right side?

• Normal MEA -30o to 110o
• Why Measure MEA
• 1) Non-invasive! Do you want them to crack your
  chest to look around for problems?
• 2) Its easy! Calculating MEA takes 5 minutes and
  can be done anywhere anytime.
• 3) Its cheap! 1 for a few electrodes and 10
  cents for paper.
• However, it is not flawless.you still have to
  back up what you see with more expensive
  diagnostics. Now you have a reason to spend the
  extra cash.

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Another easy way to easily express the MEA.
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FRANK STARLING LAW OF THE HEART Energy of
contraction is proportional to the initial length
of the cardiac muscle fiber. This explains how
a heart can regulate its own output based simply
on venous return to the heart.

• Normally the heart contracts at less than the
  ideal actin/myosin length and preload! Curve
  EDV (ml) vs Stroke Volume (ml)
• However as the EDV is increased, the myocardium
  is stretched and the actin/myosin orientation
  becomes closer to ideal (In healthy heart!)
• Each curve is unique to conditions around
  heart temp, CO2, etc.
• As EDV?Contactility?SV?.therefore CO goes
  upSO WHAT?
• Heart moves more blood/beat for about the same
  ATP cost
• Cardiac Output can increase with no change in
  rate
• Adaptation can occur independently of nervous
  system
• VIP Limits to System Exist
• With respect to cardiac reserve, why does your
  heart normally work/function in middle of
  ascending part of curve, and not at the top or
  back side of the curve?

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The key is to ask where you are on the curve
given the observed EDV for a given systole, this
lets you predict the SV for that systole
Often Folks call this the ventricles PRELOAD
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Frank-Starling curves are shifted by

• A Shift makes the heart pump a larger or
  smaller stroke volume.
• A Shift creates a new F-S curve with a new
  shape.
• Factors That Modify the Shape of F-S curves
  Make myocardium contract more/less forcefully
• Sympathetic stimulation
• Digitalis and calcium uptake inhibitors
• Hypercapnia/Hypoxia-
• Body temperature-
• Thin myocardial walls (alcoholic heart)-
• Many more factors exist
• What determines where your heart is Located on
  a F-S Curve
• Answer Preload and Stroke Volume

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The original F-S Curve is modified based changes
to myocardial function caused by temperature,
positive ionotropic drugs, myocardial
infarct,etc.
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WHAT HAPPENS TO THE HEART WHEN IT FAILS? In
terms of the FS-curve, why will SV and cardiac
output plummet?

• 1) Loss of myocardium due to infarct-
• Can you generate force if the myocytes are
  dead?
• 2) EDV gets too large- ejection fraction small
• 3) Actin/myosin are over stretched- no traction
• 4) Contractility is reduced and SV is reduced-
  This is bad!
• 5) Cardiac Output Plummets!
• Can you supply the heart with oxygen without
  CO?
• Other Classic Problems
• If heart hypertrophies (grows to big/thick)-
• If heart dilates (creates a large EDV, small SV
  and a thin wall)-
• During lung problems emphysema, core pulmonare,
  etc.
• ProblemLaw of Laplace radiustensionwork
• These examples can result in death from a loss in
  cardiac reserve and a set or neurological
  responses that make the original problem WORSE
  and lead to a heart attack.

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A failing heart results in the accumulation of
fluids (edema) in the lung or body. With respect
to Rt or Lt ventricular failure, where does the
pressurized fluid accumulate? Why? What could
cause Rt/Lt Failure? Why might your brain try to
make the heart work even harder? Can it work
harder?
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WHAT ARE SOME MECHANISMS FOR IMPROVING VENOUS
RETURN IN A LOW PRESSURE (VENOUS) LOOP?

• Simple pressure gradient-Systemic vs. Pulmonary
  circuits mmHg mmHg
• Thoracic (Respiratory) Pumping by ribs/sternum
• Valsalva Maneuver and preload changes heart
  attack risk
• Cardiac Suction Negative Pressure in Atria or
  Ventricles
• Venous Return to Heart Venous valves and
  skeletal muscle activity-
• Varicosities and Anal Hemorrhoids
• Gravity changes pressure 1.92 mmHg/inch of
  elevation or depression
• If the top of your head is 15 inches higher than
  your head the BP at the higher location would be
  decreased by 29 mmHg(15X1.92 -28.8mmHg)
• BP at brain is (120-29)/(80-29)91/51
• BP at foot is added! 40X1.9277 mmHg 199/157
• If you are dizzy, why does laying down prevent
  you from passing out?
• Why does your sprained ankle hurt and swell more
  if you are standing up?

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Gravitational effects on blood flow and pooling
are part of the reason we are asked to lay flat
when we feel dizzy or why our sprained ankles
throb and become swollen only when we are
standing up.Remember/- 1.92 mmHg pressure
for each Inch of elevation or depression!If the
brain has no pressure, there is no blood being
delivered!
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Venous pumping provides a very simple and very
effective way to increase the return of venous
blood to the heart? (If venous return to atria
causes the Preload to increase too!)