Electrical conduction in the Heart

1
Electrical conduction in the Heart

• The Sinoatrial node (SA node), is a group of
  autorhythmic cells (main pacemaker of the heart)
  in the right atrium near the entry of the
  superior vena cava.
• An internodal pathway connects the SA node to the
  atrioventricular node (AV node), a group of
  autorhythmic cells found near the floor of the
  right atrium.
• From the AV node action potentials move into
  fiber known as the bundles of his or
  atrioventricular bundle. The bundle passes from
  the AV node into the wall of the septum between
  the ventricles.
• A short way down the septum the bundle divides
  into left and right bundle branches.
• These fibers continue downward to the apex where
  they divide into many small purkinje fibers that
  spread outward among the contractile cells.

2
(No Transcript)
3

• If the electrical signals from the atria were
  conducted directly into the ventricles, the
  ventricles would start to contraction at the top.
  Then the blood would be squeezed downward and
  trapped at the bottom of the ventricle.
• The apex to base contraction squeezes blood
  toward the arterial opening at the base of the
  heart.
• The AV node also delays the transmission of
  action potentials slightly, allowing the atria to
  complete their contraction before the ventricles
  begin their contraction. This AV node delay is
  accomplished by slowing conduction through the AV
  node cells.

4
(No Transcript)
5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
Electrocardiogram (ECG)

• Composite of all action potentials of nodal and
  myocardial cells detected, amplified and recorded
  by electrodes on arms, legs and chest

11
ECG

• P wave
• SA node fires, atrial depolarization
• atrial systole
• QRS complex
• atrial repolarization and diastole (signal
  obscured)
• AV node fires, ventricular depolarization
• ventricular systole
• T wave
• ventricular repolarization

12
Normal Electrocardiogram (ECG)
13
Electrical Activity of Myocardium

• 1)atria begin to depolarize
• 2) atria depolarize
• 3)ventricles begin to depolarize at apexatria
  repolarize
• 4)ventricles depolarize
• 5) ventricles begin to repolarize at apex
• 6) ventricles repolarize

14
Diagnostic Value of ECG

• Invaluable for diagnosing abnormalities in
  conduction pathways, MI, heart enlargement and
  electrolyte and hormone imbalances

15
ECGs, Normal Abnormal
No P waves
16
ECGs, Abnormal
Arrhythmia conduction failure at AV node
No pumping action occurs
17
Cardiac Cycle

• One complete contraction and relaxation of heart
• Atrial systole
• Atrial diastole
• Ventricle systole
• Ventricle diastole
• Quiescent period

18
Principles of Pressure and Flow

• Measurement compared to force generated by
  column of mercury (mmHg) - sphygmomanometer

• Change in volume creates a pressure gradient

• Opposing pressures
• always positive blood pressure in aorta, holds
  aortic valve closed
• ventricular pressure must rise above aortic
  pressure forcing open the valve

19
Heart Sounds

• Auscultation - listening to sounds made by body
• First heart sound (S1), louder and longer lubb,
  occurs with closure of AV valves
• Second heart sound (S2), softer and sharper
  dupp occurs with closure of semilunar valves
• S3 - rarely heard in people gt 30

20
Phases of Cardiac Cycle

• Quiescent period
• all chambers relaxed
• AV valves open
• blood flowing into ventricles
• Atrial systole
• SA node fires, atria depolarize
• P wave appears on ECG
• atria contract, force additional blood into
  ventricles
• ventricles now contain end-diastolic volume (EDV)
  of about 130 ml of blood

21
Isovolumetric Contraction of Ventricles

• Atria repolarize and relax
• Ventricles depolarize
• QRS complex appears in ECG
• Ventricles contract
• Rising pressure closes AV valves
• Heart sound S1 occurs
• No ejection of blood yet (no change in volume)

22
Ventricular Ejection

• Rising pressure opens semilunar valves
• Rapid ejection of blood
• Reduced ejection of blood (less pressure)
• Stroke volume amount ejected, about 70 ml
• SV/EDV ejection fraction, at rest 54, during
  vigorous exercise as high as 90, diseased heart
  lt 50
• End-systolic volume amount left in heart

23
Isovolumetric Relaxation of Ventricles

• T wave appears in ECG
• Ventricles repolarize and relax (begin to expand)
• Semilunar valves close (dicrotic notch of aortic
  press. curve)
• AV valves remain closed
• Ventricles expand but do not fill
• Heart sound S2 occurs

24
Ventricular Filling

• AV valves open
• Ventricles fill with blood - 3 phases
• rapid ventricular filling - high pressure
• diastasis - sustained lower pressure
• filling completed by atrial systole
• Heart sound S3 may occur

25
Major Events of Cardiac Cycle

• Quiescent period
• Atrial systole
• Isovolumetric contraction
• Ventricular ejection
• Isovolumetric relaxation
• Ventricular filling

26
(No Transcript)
27
Rate of Cardiac Cycle

• Atrial systole, 0.1 sec
• Ventricular systole, 0.3 sec
• Quiescent period, 0.4 sec
• Total 0.8 sec, heart rate 75 bpm

28
Overview of Volume Changes

• End-systolic volume (ESV) 60 ml
• Passively added to ventricle during atrial
  diastole 30 ml
• Added by atrial systole 40 ml
• Total end-diastolic volume (EDV) 130 ml
• Stoke volume (SV) ejected by ventricular
  systole -70 ml
• End-systolic volume (ESV) 60 ml
• Both ventricles must eject same amount of blood

29
Unbalanced Ventricular Output
30
Unbalanced Ventricular Output
31
Cardiac Output (CO)

• Amount ejected by each ventricle in 1 minute
• CO HR x SV
• Resting values, CO 75 beats/min x70 ml/beat
  5,250 ml/min, usually about 4 to 6L/min
• Vigorous exercise ? CO to 21 L/min for fit person
  and up to 35 L/min for world class athlete
• Cardiac reserve difference between maximum and
  resting CO

32
Diastole and Systole

• Diastole - the time during which cardiac muscle
  relaxes.
• Systole - the time in which cardiac muscle is
  contracting.
• I - The Heart at Rest Atrial and Ventricular
  Diastole
• While both atria and ventricles are relaxing, the
  atria begin filing with blood from the veins
  while the ventricles have just completed a
  contraction
• As the ventricles relax the AV valves between the
  atria and ventricles open, and blood flows from
  the atria to the ventricles.

33

• II - Completion of Ventricular Filling Atrial
  Systole
• The last 20 of the filling of the ventricles is
  accomplished when the atria contract. Atrial
  systole begins following depolarization of the SA
  node.
• Atrial contraction can aid filling of the
  ventricles in stenosis of the AV valves.
• The force of atrial contraction can also push
  blood back into the vein. This can be observed
  by the pulse in jugular vein of a normal person
  lying w/ the head and chest elevated about 30
  degrees. If there is an observable jugular pulse
  higher on the neck of a person sitting upright,
  it is indication that the pressure in the atria
  is higher than normal.
• III- Early Ventricular Contraction and the 1st
  Heart Sound
• Ventricular Systole begins at the apex of the
  heart as spiral bands of muscle squeeze the blood
  upward toward the base. Blood pushing upward on
  the underside of the AV valve forces them closed
  so that blood cannot flow back into the atria.
• Vibrations following closure of the AV valves
  creates the 1st heart sound, the lub of
  lub-dup.

34

• IV - The heart pumps Ventricular Ejection
• As the ventricles contract, they generate enough
  pressure to open the semilunar valves and the
  blood is pushed into the arteries.
• The pressure created by ventricular contraction
  becomes the driving force for blood flow.
• V - Ventricular Relaxation and the 2nd Heart
  Sound
• As the ventricles begin to relax, ventricular
  pressure decreases.
• Once ventricular pressure falls below the
  pressure in the arteries blood starts to flow
  backward into the heart. This backflow fills the
  cusps of the semilunar valves, forcing them
  together into the closed position.
• The vibrations of the semilunar valve closure is
  the 2nd heart sound, the dup of lub-dup.
• The AV valves open once the pressure in the
  ventricles falls below the pressure in the atria
  and the cycle starts again.

35
(No Transcript)