Cardiovascular System: The Heart (pt 4)

• The lecture is about Cardiovascular II: The Heart (pt 4)
• Three goals include knowing the events in the cardiac cycle, understanding heart sounds and murmurs
• Understanding heart sounds involves causes and when
• A cardiac cycle includes all events associated blood flow through heart during one complete heartbeat

Systole and Diastole

• Systole is the period of contraction
• Diastole is the period of relaxation

Mid-to-late Diastole

• Atria and ventricles are relaxed
• Pressure is low
• AV valves are open, and aortic and pulmonary SL valves are closed
• Blood flows passively through atria and the open AV valves into ventricles
• Ventricles fill 80%
• P wave means atria depolarize and contract
• End Diastolic volume (EDV) occurs

Ventricular Systole

• The QRS complex means ventricles are depolarizing and contracting
• AV valves close
• The 1st heart sound occurs
• Isovolumic contraction phase occurs
• Aortic and pulmonary SL valves open
• Ventricular pressure rises
• Aortic pressure rises

Early Diastole

• The T wave means ventricles repolarize
• Ventricles relax
• Pulmonary and aortic SL valves close
• The 2nd heart sound occurs
• Dicrotic notch occurs
• Atria fill with blood
• AV valves open
• End systolic volume (ESV) occurs
• Electrical events (ECG) precede mechanical ones

Heart Sounds: "Lub"

• Turbulent bloodflow occurs from the closure of AV valves
• Onset of systole occurs

Heart Sounds: "Dub"

• Turbulent bloodflow occurs from the closure of aortic and pulmonary valves
• Onset of diastole occurs

Heart Murmurs

• Abnormal heart sounds can be common in children
• Adults can experience irregular turbulent flow through valves
• Incompetent/insufficient valve fails to close completely
• Backflow of blood regurgitates through partial openings, causing a swishing sound
• A stenotic valves does not open completely
• Narrow valve opening restricts blood flow, creating a high-pitched sound or click

Electrocardiogram (ECG)

• This composite includes all action potentials generated by nodal and contractile cells
• The ECG is not a tracing of a single action potential
• Electrical events precede mechanical events of contraction or relaxation

ECG Waves

• The P wave means atrial depolarization
• The QRS complex means ventricular depolarization
• The T wave means ventricular repolarization
• P-R internal measures time 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 describes the period from the beginning of ventricular depolarization, through ventricular repolarization

ECG Rhythms: Sinus

• A normal ECG trace

ECG Rhythms: Junctional

• This arrhythmia is caused by the SA node being damaged
• The AV node takes on the role of pacing the heart
• Can present with a slower than normal heartbeat
• SA node usually paces the heart at 75 BPM
• AV node paces the heart at 40-60 BPM
• A junctional ECG rhythm has no P wave
• Ventricles show greater contractility

ECG Rhythms: Heart Block

• This arrhythmia represents a blockage of the conductive pathway
• The AV node fails to conduct some SA node impulses
• Leads to slower heartbeat
• Presents with more P waves than QRS waves

ECG Rhythms: Ventricular Fibrillation

• This arrhythmia is caused by a continuous disorganized action potential pattern in the ventricles
• Creates chaotic and abnormal ECG deflections
• Can be cured by defibrillation using high voltage to cause simultaneous refractory period

Extrasystole

• This arrhythmia happens when the SA node fires early, leading to an extra heartbeat
• This might be followed by a pause, and be felt as a "thud"

Cardiac Muscle Cells:

• The heart is a muscle that contracts and pumps blood
• Two main types include intrinsically generated action potentials and contractile cells for pumping
• 1% perform this electrical generation
• 99% responsible for contractile pumping

Conduction Pathway

• Cardiac cells generate and conduct action potentials
• Include autorhythmic cells
• SA node
• AV node
• AV bundle (of His)
• Bundle branches
• Perkinje fibers

Excitation Sequence

• Impulses are generated via pacemaker cells
• Spread to and through surrounding cardiac myocytes to initiate muscle contraction
• The following pathway occurs in the heart
• SA node -> AV node -> AV bundle -> Bundle branches -> Perkinje fibers
• Atrial contraction is followed by ventricular contraction

Modifying Basic Rhythm of the Heart Rate

• There are fluctuations
• Cardiac nerves and norepinephrine act as the "accelerator"
• The vagus nerve and acetylcholine act as "brakes"

Pacemaker Cells

• These cells generate action potentials
• Spread throughout the heart
• Possess an unstable resting membrane potential called pacemaker potentials

Pacemaker Potentials: Depolarization

• Slow depolarization is due to the opening of sodium channels
• Sodium influx begins
• Membrane potential does not flatline

Pacemaker Potentials: Depolarization

• Pacemaker potential reaches a certain threshold
• Causes calcium channels to open
• Creating Calcium influx

Pacemaker Potentials: Repolarization

• Calcium channels inactivate
• Potassium channels open
• Potassium efflux brings the membrane potential back to the most negative voltage

Contractile Cardiac Myocytes: Depolarization

• Sodium channels opening
• Creation of a strong sodium influx

Contractile Cardiac Myocytes: Plateau

• Some potassium channels open
• Potassium efflux causes a slight dip
• Calcium channels open
• Calcium influx is necessary for muscle contraction

Contractile Cardiac Myocytes: Repolarization

• Calcium channels inactivate at this stage
• More potassium channels open
• Potassium Efflux brings the membrane potential back down

Heart Anatomy Basics

• Heart enclosed in pericardium filled with fluid
• Fluid protects and anchors the heart in place
• Composed of 3 main layers:
• Epicardium
• Myocardium
• Endocardium

Heart Valves

• Main function is to ensure unidirectional flow of blood
• Atrioventricular (AV) valves exist at each atrial-ventricular junction
• Tricuspid valve separates right atria and right ventricle
• Left AV valve separates left atria and left ventricle (bicuspid)
• AV valves prevent backflow of blood into the atria

AV Valves

• Valves close in response to ventricular contraction, stopping blood from going back into atrium

Semilunar Valves

• Exist between each ventricle/artery connection
• Pulmonic separates right ventricle from the pulmonary artery
• Aortic separate the left ventricle and aorta
• Semilunar valves prevent backflow of blood into the ventricle

Coronary Circulation

• Supplies blood to the heart muscle (myocardium)
• Coronary arteries arise from the base of the aorta
• Venous blood empties into the right atrium

Coronary Diseases

• Caused by blocked coronary arterial circulation
• Can be caused by fleeting halt in blood delivery
• Myocardial Infarction: blockage of coronary artery
• Leads to cell death forming non-contractile tissue