Transcript electrical conduction in the heart

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.
• 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.
Electrocardiogram (ECG)
• Composite of all action potentials of
nodal and myocardial cells detected,
amplified and recorded by electrodes on
arms, legs and chest
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
Normal Electrocardiogram (ECG)
Electrical Activity of Myocardium
1)atria begin to
depolarize
2) atria depolarize
3)ventricles begin to
depolarize at apex;
atria repolarize
4)ventricles depolarize
5) ventricles begin to
repolarize at apex
6) ventricles repolarize
Diagnostic Value of ECG
• Invaluable for diagnosing abnormalities in
conduction pathways, MI, heart
enlargement and electrolyte and hormone
imbalances
ECGs, Normal & Abnormal
No P waves
ECGs, Abnormal
Arrhythmia: conduction failure at AV node
No pumping action occurs
Cardiac Cycle
• One complete contraction and relaxation
of heart
• Atrial systole
• Atrial diastole
• Ventricle systole
• Ventricle diastole
• Quiescent period
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
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 > 30
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
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)
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 < 50%
• End-systolic volume: amount left in
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
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
Major Events of Cardiac Cycle
• Quiescent period
• Atrial systole
• Isovolumetric
contraction
• Ventricular
ejection
• Isovolumetric
relaxation
• Ventricular filling
Rate of Cardiac Cycle
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•
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Atrial systole, 0.1 sec
Ventricular systole, 0.3 sec
Quiescent period, 0.4 sec
Total 0.8 sec, heart rate 75 bpm
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
Unbalanced Ventricular
Output
Unbalanced Ventricular
Output
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
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.
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”.
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.