Cardiovascular II Full

Lecture Goals

• Know all the events in the cardiac cycle, understand each of the graphs from the lecture, and be aware of the timing of events
• Cause of heart sounds, and when sounds occur relative to cardiac cycle phases should be understood
• Heart murmurs and their causes should be understood
• The sequence of events in a regular ECG and what each wave, complex, or interval represents should be explained
• Irregular ECGs, events in each type of irregular ECGs, and their underlying physiology should be known
• Physiology that results in extrasystole should be explained

Cardiac Cycle: Systole and Diastole

• Systole: Periods of contraction
• Diastole: Periods of relaxation
• Cardiac cycle refers to all events that occur concerning blood flow through the heart during one complete heartbeat

Cardiac Cycle: Mid-to-Late Diastole

• Atria and ventricles are relaxed
• Pressure is low
• AV valves are open
• Aortic and pulmonary SL valves are closed
• Blood flows passively from the atria through the open atrioventricular (AV) valves into the ventricles
• Ventricles fill 80%
• The P wave causes the atria to depolarize and contract
• End Diastolic Volume (EDV) measures the volume of blood in the ventricles at the end of diastole

Cardiac Cycle: Ventricular Systole

• The QRS complex causes the ventricles to depolarize and contract
• AV valves close
• First heart sound heard
• Isovolumic contraction phase occurs
• Aortic and pulmonary SL valves open
• Ventricular pressure rises
• Aortic pressure rises

Cardiac Cycle: Early Diastole

• T wave results in ventricular repolarization
• Ventricles relax
• Pulmonary and aortic SL valves close
• 2nd heart sound heard
• Dicrotic notch occurs
• Atria fill with blood
• AV valves open
• End Systolic Volume (ESV) measures the volume of blood remaining in the ventricle after systole

Electrocardiogram (ECG) Basics

• ECG shows electrical activity of the heart
• Electrical events precede the mechanical events they cause
• Pressure changes will lead to volume changes if valves are open

Key Points on ECG Waves

• P wave: Indicates atrial depolarization and precedes atrial contraction
• QRS complex: Indicates ventricular depolarization and precedes ventricular contraction
• T wave: Indicates ventricular repolarization and precedes ventricular relaxation
• The volume of blood in the ventricle is greatest at the end of diastole, referred to as the End Diastolic Volume (EDV)
• The amount of blood ejected from each ventricle refers to the stroke volume (SV)
• The volume of blood in the ventricle is lowest at the end of systole: End Systolic Volume (ESV)

Heart Sounds: "Lub" and "Dub"

• "Lub": Turbulent blood flow from closure of AV valves, signifying onset of systole
• "Dub": Turbulent blood flow from closure of aortic and pulmonary valves, signifying onset of diastole

Heart Murmurs

• Heart murmurs are abnormal heart sounds
• Common in children
• Related to irregular turbulent blood flow through valves in adults
• Incompetent/Insufficient Valve: Fails to close completely, causing blood backflow (regurgitation) through a partial opening, resulting in a swishing sound
• Stenotic Valve: Fails to open fully, causing narrow valve opening and restricted blood flow. It results in a high-pitched sound or click

Summary of Heart Info

• Ion movement affects electrical activity
• ECG provides insight into electrical activity
• Pressure changes makes valves to open and close, affecting muscle contraction
• Valve closures create heart sounds

Electrocardiogram (ECG)

• ECG depicts total action potentials by nodal (pacemaker) and contractile cells over time and not of a single action potential
• Electrical events come before the mechanical events of contraction/relaxation
• P wave signifies atrial depolarization
• The QRS complex signifies Ventricular depolarization
• T wave signifies Ventricular repolarization
• The P-R interval measures from the beginning of atrial excitation to the beginning of ventricular excitation
• The S-T segment measures the time between ventricular depolarization and the start of ventricular repolarization
• The Q-T interval measures from the beginning of ventricular depolarization through ventricular repolarization

Heart Rhythms

• Sinus Rhythm: Normal ECG trace
• Arrhythmia: Junctional Rhythm is a damaged SA node, and the AV node takes over pacing. This results in slower heartbeat, no P wave, and ventricles with greater contractility.
  • SA node usually paces the heart at 75 BPM
  • AV node paces the heart at 40-60 BPM
• Arrhythmia: Heart block is a blockage of conductive pathway where the AV node fails to conduct SA node impulses resulting in slower heartbeat and more P waves than QRS waves
• Arrhythmia: Ventricular Fibrillation is continuous disorganized AP pattern in the ventricles causes chaotic abnormal ECG deflections that are cured with defibrillation to cause simultaneous refractory period
• Arrhythmia: Extrasystole occurs with the SA node firing early. An extra heartbeat is followed by a pause

Heart's circuits

• Pulmonary circuit is responsible for blood to and from lungs.
• Systemic circuit is repsonsible for blood to tissues.
• Arteries carry blood away from the heart.
• Veins carry blood towards the heart.

Heart Design and Structure

• Three-dimensional hollow mass of muscle
• Two receiving chambers called atria
  • The left atrium receives blood returning from the pulmonary circuit
  • The right atrium receives blood returning from the systemic circuit
• Two pumping chambers: ventricles
  • The right ventricle pumps blood into the pulmonary circuit
  • The left ventricle pumps blood into the systemic circuit

Heart Anatomy

• The heart is enclosed in fluid filled sac
  • It protects and anchors the heart
• Heart wall layers
  • Epicardium
  • Myocardium
  • Endocardium

Heart Valves: Atrioventricular (AV) and Semilunar (SL)

• Heart valves ensure unidirectional blood flow
• Atrioventricular (AV) valves: One at each atrial-ventricular junction
  • Separates right atria and right ventricle with the right AV valve
  • Separates left atria and left ventricle with the left AV valve.
  • They prevent backflow of blood into the atria when the ventricles contract
• Semilunar (SL) valves: One between each ventricle/artery connection
  • Separates right ventricle and pulmonary arteries
  • Separates left ventricle and aorta
  • They prevent backflow of blood into the ventricle

Coronary Circulation and Heart Health

• The circulation delivers blood to the heart muscle (myocardium)
• Coronary arteries arise from base of aorta
• Venous blood empties into the right atrium
• Coronary disease conditions:
  • Blockage of coronary arterial circulation and a fleeting halt in blood delivery (Angina pectoris)
  • Coronary blockage (Myocardial infarction). Cell death or scar tissue resulting when it is non-contractile

Cardiac Muscle Cells and Function

• The heart is a muscle that contracts and pumps blood
• Two main types of cardiac muscle cells:
  • 1% is intrinsically generate action potentials
  • 99% are contractile cells responsible for heart's pumping ability
• Unlike skeletal muscle, the heart does not need neural input to initiate depolarization and contraction, but neural input can alter its basic rhythm

Electrical Conduction and Cells

• Electrical conduction in the heart uses electrical impulses that result from the change in ion flow across the cell membrane
• Pacemaker cells make 1% of cells and are autorhythmic cells
  • Autorhythmic Cell: Continually generates action potentials
• Myocytes makes up the myocardium

Conduction and Excitation

• Pacemaker cells generate and conduct action potentials
  • Can be called autorhythmic cells
  • Make up the conduction pathway
• Conduction Pathway:
  • SA node
  • AV node
  • AV bundle (Bundle of His)
  • Bundle branches provide ventricular contraction
  • Purkinje fibers

Excitation Info

• Excitation Sequence:
• Impulses from pacemaker cells spread surrounding cardiac myocytes (contractile cells) to initiate muscle contraction
• Order proceeds as SA node -> AV node -> AV bundle -> Bundle branches -> Purkinje fibers
• Atrial contraction occurs, followed by ventricular contraction

Heart Rate Rhythm

• Heart rate has fluctuations based on:
  • Cardiac nerves and Norepinephrine for an "accelerator" effect on heart rate
  • Vagus nerve and Acetylcholine for "brakes" effect on heart rate

Spreading throughout Heart

• Pacemaker cells have unstable resting membrane potential are referred to as having pacemaker potentials
  • 1: Slow depolarization due to opening of Na+ channels and Na+ influx
  • 2: Depolarization occurs when Pacemaker potential reaches threshold
  • 3: Ca2+ channels inactivate, K+ channels open in Repolarization, bringing the membrane potential back to most negative voltage

Myocyte Activity

• Opening of Na+ channels and a rapid Na+ influx causes Depolarization of surrounding contractile cardiac myocytes
• Maintained Some K+ channels open in Plateau, causing K+ efflux causes slight dip and muscle contraction!
• Ca2+ channels in Repolarization close, so more K+ channels open allowing K+ efflux to brings membrane potential back down

Refractory Period

• Refractory period is the the time following an action potential when a new action potential cannot be initiated
• Important so threshold voltage is reached before another contraction takes place