Case 1: Usha L. - Cardiac Cycle

Pericardium

Fibroserous pounch around heart

Layers:

• Fibrous layer

• Parietal serous layer

• Pericardial space

• Visceral serous layer (epicardium)

• Myocardium

• Endocardium

Pericardium: Pericardial Space

Between parietal and visceral layers

Contain fluid

Heart Chambers

4 (2 left, 2 right)

Right:

• Right atrium and ventricle

Left:

• Left atrium and ventricle

Heart Muscles

Cardiomyocytes

Papillary muscles

Cardiomyocytes

Striated

Actin and myosin filaments

Intercalated discs (cell membrane) forming gap junctions

Papillary Muscles

Attached to chordae tendineae (connective tissue)

Heart Valves

Attached to fibrous collagen rings → Fibrous heart skeleton

• Separate atrial and ventricular myocardia

Atrioventricular (AV) and semilunar (SL) valves

AV Valves

Right: Tricuspid

• Between right atrium and ventricle

• 3 leaflets

Left: Mitral (Bicuspid)

• Between left atrium and ventricle

• 2 leaflets

SL Valves

Right: Pulmonary

• Between right ventricle and pulmonary trunk

• 3 leaflets

Left: Aortic

• Between left ventricle and aorta

• 3 leaflets

Heart Chamber Physiology: Left Atrium

Receive oxygenated blood from pulmonary circulation (pulmonary veins)

Pump to left ventricle

Heart Chamber Physiology: Left Ventricle

Receive blood from left atrium

Pump to systemic circulation (aorta)

• Thicker muscles than right

Aorta Branching

Aortic Arch: 3 branches

1. Left subclavian artery

2. Left common carotid artery

3. Brachiocephalic trunk

Brachiocephalic Trunk: 2 branches

1. Right subclavian artery

2. Right common carotid artery

Heart Chambers Physiology: Right Atrium

Receive deoxygenated blood from systemic circulation (superior and inferior vena cava, coronary sinus)

Pump to right ventricle

Heart Chambers Physiology: Right Ventricle

Receive blood from right atrium

Pump to pulmonary circulation (pulmonary arteries)

Myocardial Cell Physiology

Prolonged skeletal muscle-like contractions

Gap Junctions: Fast ion diffusion = Action potential travel between cells

Syncytium Contraction: Rapid action potential = Muscle cells contract together

• Atrial and ventricular syncytia

• Atria contract befor ventricle (AV node delay)

Papillary Muscle Physiology

Attach to AV valves by chordae tendineae

• Pull and align valves

• Prevent eversion during systole

Contract with ventricles

Heart Valves Physiology

Prevent blood backflow

Passive opening and closing from pressure differences during systole and diastole

AV: Prevent ventricle → Atria back flow during systole

SL: Prevent aorta/pulmonary artery → Ventricle back flow during diastole

Cardiac Electrical Conduction System: Nodes

Specialized cardiac muscle cells for electrical conduction

Sinoatrial (SA) and atrioventricular (AV) nodes

SA Node

In right atrium

• Near superior vena cava (SVC) opening

Bachmann Bundle: Extend to left side

Intranodal Fibres: Connect to AV node

AV Node

In right atrium

• Between coronary sinus and tricuspid valve

Cardiac Electrical Conduction System: Bundle of His

Down interventricular septum

Split into right and left bundle branches

Cardiac Electrical Conduction System: Subendocardial (Purkinje) Fibres

In ventricles (subendocardial)

SA Node Physiology

Pacemaker

• Initial action potential

For atrial contraction

AV Node Physiology

Receive impulses from SA node

Delay signal conduction to bundle branches

Bundle of His Physiology

Receive impulses from AV node

Conduct delayed electrical impulse from atria to ventricles

• Less intercalated discs and gap junctions

Purkinje Fibres Physiology

Conduct electrical impulse to ventricles for contraction

Electrical Circuit

SA node → AV node → Bundle of His → Purkinje fibres

Excitation-Contraction Coupling

Electrical depolarization initiates muscle contraction (tension)

1. Cardiac action potential initiated in myocardial cell membrane

• Ca2+ flow into cell through L-type Ca2+ channels (ECF → ICF)

2. Ca2+-induced Ca2+ release

• Increased Ca2+ in cell triggers more Ca2+ release from sarcoplasmic reticulum

• Plateau phase

3. Ca2+ bind troponin C

• Move tropomyosin for myosin + actin cross-bridging

4. Cross-bridge cycling

• Thin + thick filaments overlap = Tension

  • Continue until Ca2+ releases troponin C

• Use ATP

5. Tension

• Tension magnitude proportional to intracellular Ca2+ concentration

• Increase Ca2+ = Increase contraction force

6. Ca2+ removal

• Ca2+ ATPase:

  • Ca2+ back into sarcoplasmic reticulum

  • Remove plateau Ca2+ into ECF

• Ca2+-Na+ Exchange:

  • Remove plateau Ca2+ into ECF

• Ca2+ dissociates from troponin C = No actin-myosin interaction = Muscles relax

Cardiac Cycle Duration

Systole + diastole = 1/HR

Cardiac Cycle: Systole

Heart contraction

1. Atrial Systole

   1. Atrial depolarization

   2. LA contracts = Increase pressure = Mitral valve opens

   3. Large blood flow from LA → LV = Increase LV volume

2. Isovolumic Ventricular Contraction

   1. LV depolarization

   2. LV contracts = Increase pressure
      (LV pressure > LA pressure = Mitral valve closes)

   3. Constant LV volume (closed valves)

3. Rapid Ventricular Ejection

   1. LV continues contracting = Increase pressure
      (LV pressure > Aortic pressure = Aortic valve opens)

   2. Blood enters aorta = Increase aortic volume + pressure

   3. Decrease LV volume

Cardiac Cycle: Diastole

1. Reduced Ventricular Ejection

   1. LV repolarize (no contraction)

   2. Decrease LV pressure = Decrease blood flow rate
      (Aortic valve open)

   3. Decrease aortic pressure

   4. LA pressure increases from blood return

2. Isovolumic Ventricular Relaxation

   1. LV finishes repolarizing

   2. Relaxed LV = Decrease LV pressure
      (LV pressure < Aortic pressure = Aortic valve closes)

3. Rapid Ventricular Filling

   1. LV pressure < LA pressure = Mitral valve opens

   2. LA blood into LV = Increase LV volume

4. Reduced Ventricular Filling (Diastasis)

   1. Slow LV filling
      (Until end-diastolic volume)

   2. Atrial systole (cycle starts again)

Pressure-Volume Loop

Relationship between LV volume and pressure during diastole and systole

1. Diastolic filling

2. Isovolumic contraction

3. Period of ejection

4. Isovolumic relaxation

PV Loop Phase 1: Diastolic Filling

Mitral valve open

• Passive LV filling

Volume: Increase

• AV valves open

• End-Systolic Volume (ESV): Blood left from previous heartbeat (50mL)

• End-Diastolic Volume (EDV, Preload): Blood entering LV from LA (120 mL)

Pressure: Low

• 2-3 mmHg

• Small increase from decreased compliance and increased volume (5-7 mmHg)

  • Increased pressure

PV Loop Phase 2: Isovolumic Contraction

LV pressure > LA pressure = Mitral valve closes

• LV begins contraction

Volume: No change

• Valves closed

Pressure: Increase

• LV contraction increase pressure to = Aortic pressure (afterload) (80 mmHg)

PV Loop Phase 3: Period of Ejection

LV pressure > Aortic pressure = Aortic valve opens

• LV contraction forces blood into aorta (high pressure)

Volume: Decrease

• Aortic valve open = Blood from from LV → Aorta (distension)

Pressure: Increase

• LV contraction

• Small decrease from rapid myocyte shortening + aortic elastic recoil

PV Loop Phase 4: Isovolumic Relaxation

LV pressure < Aortic pressure = Aortic valve closes

• LV begins relaxation

Volume: No change

• Valves closed (50 mL)

Pressure: Decrease

• LV relax (2-3 mmHg)

Heart Sounds

From valves closing

S1-4

Sound 1

“Lub”

AV valves closing

Beginning of systole

Sound 2

“Dub”

SL valves closing

End of systole

Sound 3

Weak

Blood flow from atria → stiff ventricles

Middle of diastole (AV valves open)

Ken-tuc-key

• S1-S2-S3

Sound 4

Weak

Atrial contraction

Forced atrial contraction from decreased ventricular compliance and increased resistance (stiff/hypertrophic ventricle)

End of diastole

Ten-nes-see

• S4-S1-S2

Cardiac Auscultation

Listening for valve sounds

Not directly on valves

• Move stethoscope to differentiate sounds (closer → louder)

S1: AV Valves

• Mitral: Left 5th intercostal space (lateral)

• Tricuspid: Left 4/5th intercostal space

S2: SL Valves

• Aortic: Right 2nd intercostal space

• Pulmonic: Left 2nd intercostal space

Wiggers Diagram

Pressure and volumes in left heart during cardiac cycle

Wiggers Diagram: Atrial Pressure

a wave: Atrial contraction = Increase pressure

• Left > Right

c wave: Ventricular contraction = Increase pressure pushing into atria

• Blood back flow + AV valve bulging into atria

v wave: Venous blood → Atria

• Closed AV valves

• End of ventricular contraction

• AV valves open = Atrial blood → Ventricle = v wave ends

Wiggers Diagram: Venous Pulse

JVP

Good estimate of atrial pressure (noninvasive)

Wiggers Diagram: Ventricular Pressure

Large Increasing Curve:

• Beginning of systole = Increase pressure

  • AV valves close

• Middle of systole = Increased pressure opens aortic valve

  • Blood ejection

• End of systole = Decreasing pressure

  • Aortic valve closes

Flat Portion:

• Diastole = Open AV valves = Passive blood flow into ventricle

Wiggers Diagram: Ventricular Volume

Increasing Curve:

• Diastole = Open AV valves = Blood fills ventricle

• Diastole end = Slower blood flow into ventricle (small increase)

Decreasing Curve:

• Systole = Open aortic valve = Blood from ventricle → Aorta

• Diastole closes aortic valve

Wiggers Diagram: ECG

Measure electrical voltages from heart → Body surface

P wave

QRS wave

T wave

ECG: P Wave

Atrial depolarization

1. SA node initiate action potential

2. SA node conduct electric signal to AV node (delay)

3. Atrial contraction

ECG: QRS Wave

Ventricular depolarization

4. Electric signal sent down Bundle of His and Purkinje fibres

Q: Left to right bundle branch conduction

R: Purkinje fibres send signals outwards

S: Before start of ventricular systole

ECG: T Wave

Ventricular repolarization

• Muscles relax

Ventricular systole → Diastole

Phonocardiogram: S1

AV valves closing

• Beginning of systole

Phonocardiogram: S2

Semilunar valves closing

• End of systole

Phonocardiogram: S3

Blood flow into ventricle

• Middle of diastole (AV valve open)

Ken-Tuc-Key (S1-S2-S3)

Phonocardiogram: S4

Atrial contraction

• Forced contraction pushing blood into stiff/hypertrophic ventricle

• End of diastole

Ten-nes-see (S4-S1-S2)