Transcript Heart Functions: the MEA and the Frank Starling Law of the heart 2/14

Heart Functions: the ECG and the MEA
Feb 10
• How does one calculate heart rate, RR interval, PR
interval and ST interval from an ECG?
• How do these values change pathologically?
• 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?
Lets calculate pulse rate, heart rate, RR interval, PR
interval, ST interval from this recording.
1) Determine paper speed from X-axis
2) Determine distance (mm or cm) between events
3) Remember to average at least ECG 3 events (5 is best)
4) Re-check math to ensure your units are correct
Lets calculate pulse rate, heart rate, RR interval, PR
interval, ST interval from this recording.
1) Determine paper speed from X-axis
2) Determine distance (mm or cm) between events
3) Remember to average at least ECG 3 events (5 is best)
4) Re-check math to ensure your units are correct
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…
MEAN ELECTRICAL AXIS (MEA) GIVES A MORE ADVANCED
CARDIAC DIAGNOSIS OF FUNCTION/PATHOLOGY.
• The mean electrical axis is the average direction of depolarization
in the heart.
• Einthoven’s 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
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. (It’s Non-Invasive and
Cheap!) MEAs that are not down and to left are AXIS DEVIATIONS.
One Way to Measure MEA:
Easiest Way to Measure MEA to Assess Cardiac Function?
Normal MEA: 0o to +90o
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.
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?
How to measure MEA, another example. MEA can tell you the “size” of L/R
ventricles, the “location” of the heart in the epicardial sac, the “conduction
pathway” and the “location” of your cardiac infarct. For about $1!!
Mean Electrical Axis Calculations:
Lead I: Right Arm (-) Left Arm (+) Net deflection (mm) =_________
Lead II: Right Arm (-) Left Leg (+) Net Deflection (mm) =_________
Lead III: Left Leg (+) Left Arm (-) Net Deflection (mm) =_________
How to Measure MEA: (Method used in Dr Wilson’s Lecture)
WHY ECG? Remember: For 75 cents you can record some ECGs and measure an
MEA to determine if a heart is “probably” functioning correctly or incorrectly. This is a lot
“cheaper” and a lot “less invasive” than immediately cutting a persons chest open for a
$50,000 triple cardiac-bypass surgery because they have “chest pain”.
Remember Proper Leads:
Lead I-Right Arm (-) and Left Arm(+)
Lead II: Right Arm (-) and Left Leg (+)
Lead III: Left Leg (+) and Left Arm (-)
1) Measure the net amplitude of the QRS complex for leads I, II and III. Net QRS Amplitude:
Look at the two largest waves, if the R-amplitude is +9mm and the S-amplitude is -3mm, the
net amplitude would be (+9mm) – 3mm = +6mm
2) From the center of each lead line on Einthoven’s Triangle (I, II and III), move that number of
mm in the positive or negative direction from the center and place a “mark” at this spot on each
lead line.
3) Draw perpendicular lines to each “mark” and circle the place where the three perpendicular
lines intersect (or best estimate of the intersection point).
4) Draw a line from the center of the triangle to the center of the intersection.
5) Look and see how many degrees this line is at. This is the average vector taken during the
depolarization of the heart that you are evaluating.
6) A Normal heart depolarizes down and towards the left (0o to +90o ). Right Axis Deviation is
anything from (+90o to +180o ). Left Axis Deviation is anything from (0o to -90o ). Extreme
Right: if MEA is between 180o and -90o recheck the leads, they are probably mixed-up