Heart Function: What can the ECG tell us about heart function 26

1
Heart Function What can the ECG tell us about
heart function? 2/6

• Goals
• How fast do the regions of the heart conduct a
  depolarization?
• How does a tissue depolarization create a
  voltimeter deflection?
• What is the sequence of depolarization within the
  heart?
• How do heart depolarizations create ECG
  deflections?
• What does an ECG really detect and why does it
  look the way it does?
• What are the diagnostic parts of an ECG?
• What does the ECG look like with arrhythmia?
• What do heart sounds tell us about heart
  function?
• Please work on the ECG assignment, it is due next
  Friday!

2
SA Nodal cells are non-contractile and
rhythmically depolarize themselves and adjacent
cells. Nodal cells eventually depolarize
adjacent contractile cells via gap junctions. As
a result, the depolarizations of contractile and
non-contractile cells are very different!
Non-Contractile Cells!
Remember this is depolarization in single cells,
not the whole heart!
3
The heart delays AP passage through the AV node
(slow conduction velocity) to ensure that the
atria completely contract, loading blood into the
ventricles. Then the AP quickly moves through
the conduction system to the ends of the Purkinje
fibers. Ventricular myocardium to contract the
septum then the ventricles from the apex towards
the exit point at the semilunar valves located at
the base (top) of the heart.
4
Electrocardiograms (ECGs) lets us measure
electrical activity in the heart and measure
heart health 7 Electrocardiogram Rules

• ECG is a record of voltage deflections observed
  in time
• 1) Depol towards a electrode Deflection
• 2) Depol away from a electrode - Deflection
• 3) Depol perpendicular to electrodes No
  Deflection
• 4) More tissue Larger depolarization
• 5) Body fluids contain salts, salts conduct
  electricity, so body conducts electrical changes
  originating from heart
• 6) Voltimeter detects electric changes from heart
• Voltage changes once/contraction or 70
  times/minute
• 7) An ECG/EKG/Electroc(K)ardiogram is an
  expensive voltimeter but easy to use to look at
  the heart.

5
EKG/ ECG/ELECTROCARDIOGRAM IS A RECORD OF WHAT
HAPPENS WHEN MANY CELLS DEPOLARIZE OVER TIME
(Sort of an average action potential for all the
cell in the whole heart).

• Membrane depolarization creates changing electric
  field for a cell!
• Wave of many cells depolarizing changes electric
  potential across the whole body on a larger
  scale!
• Voltimeter detects how voltage across heart/body
  changes!
• ECG measures voltage changes over time!
• Four Rules for Making an ECG
• 1) Wave moving in heart towards pole?
  deflection
• 2) Wave moving in heart towards pole? -
  deflection
• 3) More cell mass? larger sized depolarization
• 4) Perpendicular Wave? Causes of neutral/canceled
  deflections
• Perpendicular is called Isoelectric

6
What will the ECG deflections look like if strips
of myocardium with different sizes/shapes were
placed between two electrodes and depolarized in
the direction indicated by the arrow? (If the
wave of cell-cell Depolarization moves Towards
the () electrode it creates a Positive
Deflection on voltimeter over time)
(-)
()
()
()
(-)
(-)
()
()
(-)
(-)
Small Large Small Inverted
Isoelectric Isoelectric/Small
7
What is the normal spread of this wave or
depolarization across the heart that normally
originates in the SA Node? You need to
appreciate this prior to understanding why an ECG
looks the way it does!
Depolarization starts at SA Node
Depolarization ends at basal end of ventricle
8
ECG What happens as the entire heart
depolarizes relative to a and -? First Atrial
Depol (P wave) Second Septal Depol (Q) Third
Depol of Vent Walls (RS waves) Fourth
Repolarize(T-wave)
(-)
()
9
In this ECG, depolarizing tissue is shown as a
dark wave on the heart starting at the right
atria. The wave of depolarization moves toward
the electrode in the lower right, creating a
positive mV ECG deflection. This is the basic
P-QRS-T appearance of an ECG, caused by
depolarization of the atria then ventricles,
followed by ventricular repolarization (T-wave)
(-)
()
10
ECGs measure voltage differences between two
locations over time. In this case is left
front paw and is on the back right paw. A
wave of depolarization moving towards the
positive electrode creates a deflection and a
wave of depolarization moving towards the
electrode creates a negative deflection, and the
more mass depolarizing the larger the deflection.

11
ECG Diagnostics Look for these on the ECGs of
your loved ones who are in the hospital, is the
T.V. ECG Real or NOT?.Identification of ECG
deflections Definitions for Test/Clinic

• What doe we call the five parts?
• P-wave deflection caused by atrial
  depoalrization
• Q-Wave First NEGATIVE deflection caused by
  ventricular depolarization
• R-Wave First POSITIVE deflection caused by
  ventricular depolarization
• S-Wave First NEGATIVE deflection observed AFTER
  a R-wave
• T-Wave Any deflection that is caused by
  ventricular REolarization
• Any of the above may or may not be observed on
  your ECG depending upon where your positive and
  negative electrodes were placed on your
  body.Example You could have an ECG with just a P
  and R wave and no observable Q-, S- or T-waves.
  It is only a matter of how the heart is oriented
  across the two electrodes of interest.

12
ECG parts are identified below as intervals and
segments (time needed to move between two points
on the ECG). Amplitude is always (by convention)
measured in millivolts (More tissue depolarizing
? Wave Amplitude)
If the RR interval took 1.0 seconds, or it was
one second between beats Then (1
beat/second)(60sec/minute)60 beats/min
1.0 mV
Amplitude millivolts (mV)
0 mV
-0.5 mV
Time in Units of Milliseconds or Seconds
Intervals and segments can tell you about the
health of the heart. Example Is the heart
beating too quickly or too slowly? Is the AV node
delaying the depolarization too much or is the
delay not long enough?
13
The normal Sinus rhythm is about 70
beats/minuteThe sinus rate can be also be fast
or slow, the ECG lets you measure
this.Tachycardia too fast
Bradycardia too slow
14
ECGs let you evaluate arrhythmias and abnormal
conduction. Heart Block Depolarization cant
get through the conduction system to the
ventricles (Potentially Bad). There are Three
Types of AV Block 1st (slow passage), 2nd
(occasional passage) and 3rd degree (no passage)
15
Review WHAT ARE THE PHASES OF THE CARDIAC
CYCLERemember blood only moves if it goes down
a pressure gradient.

• RestQuiescence PASSIVE Ventricular filling-
• Atrial systole ACTIVE ventricular filling
• Ventricular loading from atria
• PRELOAD is the ventricular End Diastolic
  Volume
• Isovolumetric Ventricular Contraction pressure
  building so it is greater than that of the
  aorta/pulmonary artery
• Ventricular Ejection? Terms SV, Ejection
  Fraction and ESV
• SVEDV- ESV
• Blood pumped into aorta or pulmonary trunk
• Isovolumetric Ventricular Relaxation-
• Start over again Quiescence/ Passive Ventricular
  filling