Cardiac Physiology

Components of Cardiac Conduction System and their Role

SA Node

• Natural pacemaker of the heart

• Starts the action potential

AV Node

• “Backup pacemaker” in a sense

• SA node AP hits AV node, but there is a bit of a pause before the AV node passes the signal along

Bundle of His (Atrioventricular bundle)

• A tunnel through the fibrous skeleton

• Contains right and left branches

• Sends signals from the AV node to the purkinje fibers

Purkinje Fibers (Subendocardial Conducting Network)

• Transfer signals to the myocardium

Cardiomyocytes structure and nuclei and Interclated Discs, Desmosomes and Gap Junctions

• Cardiomyocytes are striated, branched and contain 1-2 nuclei

• The sarcolemma of cardiomyocytes contains irregular thickenings called intercalated discs. These fit the myocytes together like puzzle pieces

• The intercalated discs contain desmosomes (half on each neighboring cell) which help the myocyte stay connected and gap junctions which transfer AP's like a straw.

• The muscle fibers connect to the fibrous skeleton of the heart, and swirl diagonally around the heart. This makes each contraction look like the heart is being "wringed"

Action Potential in Individual Cardiomocytes (3 Phases)

• Depolarization Phase: Sodium entry via fast voltage gated Na+ channels

• Plateau Phase: Na+ gates close. Ca+ starts to influx via slow voltage gated calcium channels while potassium effluxes

• Repolarization Phase: Closure of Ca+ channels. More K+ channels open leading to more K+ outflow.

Signifficance of Refractory Period

• Refractory period lasts longer than muscle contraction, preventing tetany

P Wave, P-Q interval, QRS Complex, S-T segment, T Wave

• P Wave: Depolarization of atrial contractile fibers

• P-Q Interval: Atrial Systole

• QRS Complex: Depolarization of ventricular contractile fibers

• ST Segment: Ventricular Systole (including isovolumetric contraction)

• T Wave: Repolarization of Ventricular Contractile Fibers

• After T Wave: Ventricular diastole (Including isovolumetric relaxation)

• Note: Atrial repolarization overshadowed by QRS complex

Phases of Cardiac Cycle (5 Physical Phases)

1. Atrial Contraction: Atria Contracts

2. Isovolumetric Ventricular Contraction: Ventricle contracts to try to be higher than aortic pressure

3. Ventricular ejection: Ventricle wins! Blood ejected out of ventricle

4. Isovolumetric Relaxation: Aorta closes and ventricle relaxes without being filled

5. Ventricular Filling: Ventricular pressure drops below atrial and ventricle begins to fill

Cardiac AP, Mechanical Events (Cycle), Heart Sounds, Ventricular P & Vol., Aortic P

1. SA node fires > P wave > Atrial Contraction > Dub Sound > Ventricular volume at peak and pressure increases

2. AV node fires and sends it down the subendocardial conducting network > QRS Complex> Isovolumetric ventricular contraction followed by contraction > Lub Sound > Aortic and Ventricular Pressure peaks and falls > Ventricular volume falls

3. T wave > Ventricular repolarization > No sound > Isovolumetric ventricular relaxation followed by ventricular filling > Ventricular volume increasing while aortic and ventricular pressure decrease

Stroke volume Equation

SV = EDV – ESV

EDV Normal Values

• 120-130. This is the peak ventricular volume

ESV Normal Values

• 50-60. This is the lowest ventricular volume

What is Stroke Volume

• SV = mL/beat. How much blood the heart moves in one beat

Cardiac Output Equation w/ values

CO (5.25L/min) = HR (70 bpm) x SV (70mL/beat)

3 Factors Affecting SV

• Preload: The degree of stretch of the heart before contraction. Proportional to EDV.

• Frank-Starling Law (More Stretch = More Contractility. Works within limits)

• Factors affecting Preload (HR, Venous Return)

• Contractility: The strength of a contraction at any given preload. Directly related to cytosolic calcium levels. Positive inotropes (epi, thyroxine) increase contractility. Negative inotropes (hypoxia, acidosis) decrease contractility

• Afterload (Arterial BP): The pressure the ventricles must overcome before the semilunar valves open. Affected by BP and vessel structure

Factors Affecting HR