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.

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.

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 QRS complex T 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 apex; atria 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 > 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 < 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

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) ml
Stoke volume (SV) ejected by ventricular systole ml
End-systolic volume (ESV) 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.