The Electrical System

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The Electrical System
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The Electrical System

• The pumping of the heart muscle generates a
  pulse, or heartbeat.
• The normal pattern of muscle contraction begins
  in the upper chambers (atria), which pump blood
  into the lower chambers (ventricles).

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The Electrical System

• The ventricles pump blood to the body and lungs.
• This coordinated action occurs because the heart
  is "wired" to send electrical signals that tell
  the chambers of the heart when to contract.

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The Electrical System

• Your heartbeat is able to speed up and slow down
  because it is wired with electrical tissue.
• Your heart also has built-in "pacemakers" that
  are like electrical outlets.

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The Electrical System

• The electrical system of the heart triggers the
  heartbeat.
• The pacemakers and the wiring that run through
  your heart also coordinate contractions in the
  upper chambers and lower chambers, which makes
  the heartbeat more powerful so it can do its job
  effectively.

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How is the Heart Wired?

• We normally have our own pacemakers that tell the
  heart when to beat.
• The master pacemaker is located in the atrium
  (upper chamber).
• It acts like a spark plug that fires in a
  regular, rhythmic pattern to regulate the heart's
  rhythm.

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How is the Heart Wired?

• This "spark plug" is called the sinoatrial (SA),
  or sinus node.
• It sends signals to the rest of the heart so the
  muscles will contract.
• First, the atrium contracts.
• The electrical signal from the sinus node spreads
  through the atria.

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How is the Heart Wired?

• Next, the electrical signal travels to the area
  that connects the atria with the ventricles.
• This electrical connection is critical.
• Without it, the signal would never reach the
  ventricles, the major pumping chambers of the
  heart.

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How is the Heart Wired?

• The first structure it reaches is another natural
  pacemaker called the atrioventricular (AV node).
• A structure called the bundle of His emerges from
  the AV node and divides into thin, wire-like
  structures called bundle branches that extend
  into the right and left ventricles.

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How is the Heart Wired?

• The electrical signal travels down the bundle
  branches to thin filaments known as Purkinje
  fibers.
• These fibers distribute the electrical impulse to
  the muscles of the ventricles, causing them to
  contract and pump blood into the arteries.

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When Your Heart Doesn't Work as It Should

• The S-A node doesn't produce the right number of
  signals.
• Another part of the heart takes over as the
  natural pacemaker.
• The electrical pathways are interrupted.

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When Your Heart Doesn't Work as It Should

• Slow Arrhythmias When the heart beats too
  slowly it's called bradycardia (brady slow,
  cardia heart).
• Slow arrhythmias can be a problem because they
  cause the oxygen- and nutrient-rich blood to
  travel more slowly to your organs and other
  tissues.
• Your body may not receive enough oxygen and
  nutrients to function properly.

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When Your Heart Doesn't Work as It Should

• Fast Arrhythmias When the heart beats too fast
  it's called tachycardia (tachy fast, cardia
  heart).
• During tachycardia the heart isn't able to pump
  blood to the body as well as it should.
• Fast rhythms in the upper chambers may not be
  life-threatening in themselves.
• But they may contribute to other problems that
  are serious.
• Fast arrhythmias in the lower chambers, the
  ventricles, can be dangerous and even fatal.

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What Causes These Problems?

• Heart disease causes changes in the heart tissue.
  
• Aging of the heart muscle can also change the
  heart tissue.
• Physical problems, such as diabetes, smoking, and
  excessive alcohol or drug use, can affect the
  heart tissue.
• There could be an inherited heart problem.
• There is evidence of heart failure or a heart
  attack.

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Tests for the Conduction System

• ECG or EKG (Electrocardiogram)
• Measure heart rate
• Look for arrhythmias
• Identify enlargement of the heart's chambers
• Help diagnose whether you've had a heart attack

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• The ECG or EKG, directly measures microvoltages
  in the heart muscle (myocardium) occurring over
  specific periods of time in a heartbeat,
  otherwise known as a cardiac impulse.
• With each heartbeat, electrical currents called
  action potentials, measured in millivolts (mV),
  travel through a conducting system in the heart.

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• The potentials originate in a sinoatrial (SA)
  node which lies in the entrance chamber of the
  heart, called the right atrium.
• These currents also diffuse through tissues
  surrounding the heart whereby they reach the
  skin.
• There they are picked up by external electrodes
  which are placed at specific positions on the
  skin.

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• They are in turn sent through leads to an
  electrocardiograph.
• A pen records the transduced electrical events
  onto special paper.
• The paper is ruled into mV against time and it
  provides the reader with a so-called rhythm
  strip.

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• This is a non-invasive method to used evaluate
  the electrical counterparts of the myocardial
  activity in any series of heart beats.
• Careful observation of the records for any
  deviations in the expected times, shapes, and
  voltages of the impulses in the cycles gives the
  observer information that is of significant
  diagnostic value, especially for human medicine.

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• The normal rhythm is called a sinus rhythm if the
  potentials begin in the sinoatrial (SA) node.

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• A cardiac cycle has a phase of activity called
  systole followed by a resting phase called
  diastole.
• In systole, the muscle cell membranes, each
  called a sarcolemma, allow charged sodium
  particles to enter the cells while charged
  potassium particles exit.

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• These processes of membrane transfer in systole
  are defined as polarization.
• Electrical signals are generated and this is the
  phase of excitability.
• The currents travel immediately to all cardiac
  cells through the mediation of end-to-end
  high-conduction connectors termed intercalated
  disks.

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• The potentials last for 200 to 300 milliseconds.
• In the subsequent diastolic phase, repolarization
  occurs.
• This is a period of oxidative restoration of
  energy sources needed to drive the processes.
• Sodium is actively pumped out of the fiber while
  potassium diffuses in.
• Calcium, which is needed to energize the force of
  the heart, is transported back to canals called
  endoplasmic reticula in the cell cytoplasm.

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• The action potentials travel from the superior
  part of the heart called the base to the inferior
  part called the apex.
• In the human four-chambered heart, a pacemaker,
  the SA node, is the first cardiac area to be
  excited because sodium and potassium interchange
  and energize both right and left atria.

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• The impulses then pass downward to an
  atrioventricular (AV) node in the lower right
  atrium where their velocity is slowed, whereupon
  they are transmitted to a conducting system
  called the bundle of His.
• The bundle contains Purkinje fibers that transmit
  the impulses to the outer aspects of the right
  and left ventricular myocardium.

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• In turn, they travel into the entire ventricular
  muscles by a slow process of diffusion.
• Repolarization of the myocardial cells takes
  place in a reverse direction to that of
  depolarization, but does not utilize the bundle
  of His.

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Cardiac Cycle

• The events that occur from the beginning of one
  heartbeat to the beginning of the next.
• Consists of ventricular diastole (relxation) and
  ventricular systole (contraction)

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Diastole

• During diastole, blood flows from the atria
  through the open tricuspid and mitral valves into
  the relaxed ventricles.
• The aortic and pulmonic valves are closed.
• 75 of blood flow is passive

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Systole

• During ventricular systole the mitral and
  tricuspid valves are closed.
• The relaxed atria fill with blood. A ventricular
  pressure rises, the aortic and pulmonic valves
  open.
• The ventricle contract, and blood is ejected into
  the pulmonic and systemic circulation.

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Cardiac Output

• Is the amount of blood the left ventricle pumps
  into the aorta per minute.
• Is measured by multiplying heart rate times
  stroke volume.
• Stroke volume refers to the amount of blood
  ejected with each ventricular contraction and is
  usually about 70 mls.

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Cardiac Output

• Normal CO is 4 to 8 L/min.
• The heart pumps only as much blood as the body
  requires, based on metabolic requirements.

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Cardiac Output

• Three factors determines stroke volume
• Preload
• Afterload
• Myocardial contractility.

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Preload

• Is the degree of stretch or tension on the muscle
  fibers when they begin to contract..
• Its usually considered to be the end-diastolic
  pressure when the ventricl has filled

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Afterload

• Is the load (or amount of pressure) the left
  ventricle must work against to eject blood during
  systole.
• It corresponds to systolic pressure.
• The greater this resistance is, the greater the
  hearts workload.

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Myocardial contractility

• Is the ventricles ability to contract, which is
  determined by the degree of muscle fiber stretch
  at the end of diastoe.
• The more the muscle fibers stretch during
  ventricular filling, up to an optimal length, the
  more forecful the contration.

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Autonomic innervations of the heart

• Two branches of the autonomic nervous system
  supply the heart.
• Sympathetic nervous system
• Parasympathetic nervous system

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Sympathetic

• Innervate all the areas of the heart
• Nerve stiulation causes the release of
  norepinephrine, which increases the heart rate by
  increasing SA node discharge, accelerates AV noe
  conduction time, and increases the force of
  myocardial contraction and cardiac output.

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Parasympathetic

• Vagal stimulation causes the release of
  acetylcholine, which produces the opposite
  effects.
• The rate of SA node dischare is decrease, thus
  slowing hear rate and conduction through the AV
  node and reducing cardiac output.

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Depolarization and Repolarization

• As impulses are transmitted, cardiac cells
  undergo cycles of depolarization and
  repolarization.
• Cardiac cells at rest are considered polarized,
  meaning that no electrical activity takes place.

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• Cell membranes separate different concentrations
  of ion, such as sodium and potassium and create a
  more negative charge inside the cell.
• This is called resting potential
• After a stimulus occurs, ions cross the cell
  membrane and cause an action potential or cell
  depolarization.
• When a cells fully depolarized it attempts to
  return to its resting state in a process called
  repolarization.
• Electrical charges in the cell reverse and return
  to normal.

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Electrical Activity

• Of the heart is represented on an ECG.
• ECG represent only electrical activity not the
  mechanical activity or actual pumping of the
  heart.

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How to Read an EKG Strip

• EKG paper is a grid where time is measured along
  the horizontal axis.
• Each small square is 1 mm in length and
  represents 0.04 seconds.
• Each larger square is 5 mm in length and
  represents 0.2 seconds.

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How to Read an EKG Strip

• Voltage is measured along the vertical axis.
• 10 mm is equal to 1mV in voltage.
• The diagram on the next slide illustrates the
  configuration of EKG graph paper and where to
  measure the components of the EKG wave form

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Heart rate can be easily calculated from the EKG
strip

• When the rhythm is regular, the heart rate is 300
  divided by the number of large squares between
  the QRS complexes.
• For example, if there are 4 large squares between
  regular QRS complexes, the heart rate is 75
  (300/475).

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Heart rate can be easily calculated from the EKG
strip

• The second method can be used with an irregular
  rhythm to estimate the rate. Count the number of
  R waves in a 6 second strip and multiply by 10.
• For example, if there are 7 R waves in a 6 second
  strip, the heart rate is 70 (7x1070).

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P wave

•   Indicates atrial depolarization, or
  contraction of the atrium.
• Location precedes the QRS complex
• Amplitude (height) is no more than 3 mm
• Duration 0.06 to 0.12 seconds
• Usually rounded and smooth

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PR Interval

• Tracks the atrial impulse from the atria through
  the AV node, bundle of His and right and left
  bundle branches.
• Location from the beginning of the P wave to the
  beginning of the QRS complex.
• Duration 0.12 seconds to 0.20 seconds
• Short intervals indicate that the impulse
  originated somewhere other than the SA node.

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QRS complex

• Indicates ventricular depolarization, or
  contraction of the ventricles.
• Location follows the PR interval
• Amplitude 5 to 30 mm high, but differs with each
  lead used.
• Duration 0.06 to 0.10 seconds or half of the PR
  interval measured from the beginning of the Q
  wave to the end of the S wave
• R waves are deflected positively and the Q and S
  waves are negative

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ST Segment

• Represents the end of ventricular conduction or
  depolarization and the beginning of ventricular
  recover or repolariztion.
• Location extends from the S wave to the
  beginning of the T wave.
• Deflection usually on the baseline may vary from
  -0.5 to 1.0 mm.
• A change in the ST segment may indicate
  myocardial injury or ischemia.

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T wave

• Indicates ventricular repolarization
• Location follows the ST segment
• Amplitude 0.5 mm in standard leads and 10 mm in
  precordial leads
• Rounded and smooth

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ST segment

• Represents the end of ventricular conduction or
  depolarization and the beginning of ventricular
  recovery or repolariztion.
• Normally not depressed more than 0.5 mm

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QT interval

• Indicates repolarization time
• Location extends from the beginning of the QRS
  complex to the end of the T wave
• General rule duration is less than half the
  preceding R-R interval.
• Duration varies according to age, gender and
  heart rate. From 0.36 to 0.44 seconds

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Paper and Pencil method of measuring Rhythm

• Place the ECG strip on a flat surface.
• Position the straight edge of a piece of paper
  along the strips baseline.
• Move the paper up slightly so that the straight
  edge is near the peak of the R wave.
• Mark the paper at the R waves of 2 consecutive
  QRS complexes. This distance is the R-R interval.

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• Move the paper across the strip lining up the two
  marks with succeeding R-R intervals. If the
  distance for each R-R interval is the same, the
  ventricular rhythm is REGULAR.
• If the distances varies the RHYTHIM ins
  irregular.
• The same can be done using the distance between
  the P waves to determine atrial rhythm

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• Building 1 suite313
• Dr. Sneider
• Dec 2nd at 215

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• http//usasam.amedd.army.mil/_fm_course/Study/Unde
  rstandingECG.pdf