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)

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

Electrical Activity of Myocardium

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

• Opposing pressures– always positive blood

pressure in aorta, holds aortic valve closed

– ventricular pressure must rise above aortic pressure forcing open the valve

• Change in volume creates a pressure gradient

Principles of Pressure and Flow• Measurement: compared to force

generated by column of mercury (mmHg) - sphygmomanometer

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 periodQuiescent period

– all chambers relaxedall chambers relaxed– AV valves openAV valves open– blood flowing into ventriclesblood flowing into ventricles

• Atrial systoleAtrial systole– SA node fires, atria depolarizeSA node fires, atria depolarize– P wave appears on ECGP wave appears on ECG– atria contract, force additional blood into atria contract, force additional blood into

ventriclesventricles– ventricles now contain end-diastolic ventricles now contain end-diastolic

volume (EDV) of about 130 ml of bloodvolume (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 heart

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

• 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 Amount ejected by each ventricle in 1 minuteminute

• CO = HR x SVCO = HR x SV

• Resting values, CO = 75 beats/min x70 Resting values, CO = 75 beats/min x70 ml/beat = 5,250 ml/min, usually about 4 to ml/beat = 5,250 ml/min, usually about 4 to 6L/min6L/min

• Vigorous exercise Vigorous exercise CO to 21 L/min for fit CO to 21 L/min for fit person and up to 35 L/min for world class person and up to 35 L/min for world class athleteathlete

• Cardiac reserve: difference between Cardiac reserve: difference between maximum and resting COmaximum 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.