5A. 3: Cardiac + ECG cycle

Cardiac muscle

* striated
* involuntary

\
Cardiomyocytes contain the same contractile filaments as skeletal muscle (sarcomere)

\
Cells are joined end-to-end and are connected via intercalated discs

\
Nuclei are centrally located, sometimes there are 2

2 types of cardiomyocytes

1. __**pacemaker**__
* Autorhythmic cells (“automaticity“)
* spontaneously contract
* SA node, some fibers in AV node, bundle of His, purkinje fibers

\
2. __**non-pacemaker cells**__
* bulk of heart
* basic contractile myocytes
* depolarization is induced by adjacent cells depolarizing

Syncytia

The heart can be divided into 2 syncytia:

* the atria syncytium
* ventricular syncytium

\
* allows for the atria to contract prior to ventricular contraction
* syncytium = network of cardiomyocytes connected via intercalated discs

Action potentials

neuronal vs cardiomyocytes

\
__**neuronal**__ = rapid depolarization (1ms)

* depolarization caused by fast sodium channels

\
__**Cardiomyocytes**__ = 200-400 milliseconds

* pacemaker cells = slow response
* Myocytes = faster response
* depolarization caused by sodium + calcium

Refractory period

phases 0-3

* can’t be re-excited
* limits firing rate

\

conduction system in the heart

__**SA node = pacemaker**__

* origin of cardiac impulse
* rate of depolarization is greatest here - which means it drives everything else

\
__**AV node**__

* located at the centre of the heart, in the floor of the right atrium, between the atria and ventricles
* electrically connects atria and ventricles via bundle of His
* slows the signal from the SA node
* allows for atrial blood to empty into ventricles

ECG

measures changes in electrical signal across cardiomyocytes

\
* as cardiomyocytes depolarize/ repolarize, electrical currents pass across the body
* electrical impulse picked up by electrodes
* Voltage measured as a difference between 2 electrodes
* toward +ve deflection, away from +ve deflection



multiple lead arrangements = many signals and characteristic patterns

ECG configuration examples

ECG recording

P wave = atrial depolarization

QRS = ventricular depolarization

T wave = ventricular repolarization

the different components of an ECG

* should be able to identify each component

The cardiac cycle

The sequence of events that occur and repeat with every heart beat

* systole = ventricular contraction
* Diastole = ventricular relaxation

\
7 phases

**Recordings:**

* Aortic pressure (AP)
* Left ventricular pressure (LVP)
* Left Arterial pressure (LAP)
* Left ventricular volume (LV)
* ECG wave
* Heart sounds

basic principles

conduction → contraction → flow

\
blood flows from higher to lower pressure

* contraction increases pressure
* relaxation/emptying decreases pressure

\
valves open/ close based on pressure gradients

* atria are always filling

(no valves in vena cava or pulmonary veins)

\
\

heart sounds

S1 = mitral valve (left AV valve)

S2 = semilunar valve (aortic)

1. Atrial systole

* atria contract
* ventricles in diastole
* LVEDV = left ventricle and diastolic volume
* Pushes last bit of blood into ventricles
* increased pressure in ventricles closes AV-valve
* heart sound S1 (mitral valve)
* electrical activity precedes contraction
* QRS complex starts (ventricular depolarization)

2. Isovolumetric contraction

* atria relax
* ventricles contract (systole)
* no blood is ejected = isovolumetric
* electrical activity precedes contraction - QRS complex starts

3. Rapid Ejection

* aortic + pulmonary valves open
* blood rushes into aorta + pulmonary trunk
* volume falls in ventricles rapidly
* pressure in ventricles continues to increase

4. Reduced ejection

* pressure begins to decrease in aorta as the last bit of blood leaves the ventricles
* pressure in atria continues to rise as atria passively fill with blood

\- T-wave = ventricular

* repolarization

5. Isovolumetric relaxation

* semilunar valves close
* heart sound #2
* ventricles enter diastole
* AV valves are still closed thus volume of blood doesn’t change = isovolumetric relaxation
* LVESV = left ventricle end systolic volume

6. Rapid filling

* pressure in atria exceed pressure in ventricles and AV valves open
* Blood dumps into ventricles from atria “rapidly filling” them
* atrial volume + pressure drops
* ventricular volume + pressure rises

7. Reduced filing

* blood passively flows into heart from vena cava + pulmonary arteries
* AV valves are open, so it flows directly into ventricles
* ventricular volume (and pressure) slowly rises
* P-wave starts = atrial contraction triggered

summary