2.2 - Heart Physiology Part I

Depolarization

A cell's shift from a negative towards a positive charge, triggering an action potential

Repolarization

The return of a cell's membrane potential from a positive back to a negative charge after the action potential

Cardiac conducting cell resting potential

Unlike skeletal muscle and neurons, cardiac conducting cells do not have a stable resting potential

Prepotential depolarization (pacemaker potential)

A gradual spontaneous depolarization in conducting cells caused by a steady, slow influx of sodium ions that raises the membrane potential from -60 mV to about -40 mV

Conducting cell depolarization sequence

At threshold (-40 mV), calcium ion channels open and Ca2+ enters the cell, rapidly depolarizing it to approximately +15 mV

Conducting cell repolarization

Occurs when calcium ion channels close and K+ channels open, allowing K+ outflux; when membrane potential reaches about -60 mV, K+ channels close and Na+ channels open, restarting the prepotential phase

Intrinsic SA node heart rate

The pattern of prepotential depolarization in the SA node establishes an intrinsic heart rate of 72 beats per minute

Cardiac contractile cell electrical pattern

Rapid depolarization, followed by a plateau phase of sustained depolarization, followed by repolarization; accounts for long refractory periods

Refractory period (cardiac, general purpose)

The period during which a cardiac cell is unable to undergo another action potential, required so cardiac muscle can pump blood effectively before firing again

Contractile cell impulse initiation

Cardiac myocytes normally do not initiate their own electrical potential but wait for an impulse to reach them

Contractile cell resting membrane potential (atria)

Approximately -80 mV

Contractile cell resting membrane potential (ventricles)

Approximately -90 mV

Contractile cell rapid depolarization

Triggered by an action potential, voltage-gated Na+ channels rapidly open, raising membrane potential to about +30 mV; lasts 3-5 ms before sodium channels close

Contractile cell plateau phase

A relatively slow decline in membrane potential caused by slow Ca2+ channels opening (allowing Ca2+ entry) while few K+ channels are open; lasts approximately 175 ms

Contractile cell repolarization

Occurs once membrane potential reaches about zero: Ca2+ channels close and K+ channels open, allowing K+ to exit; lasts approximately 75 ms

Total contractile cell action potential duration

Between 250 and 300 ms

Absolute refractory period (cardiac contractile cell)

Lasts approximately 200 ms; the phase during which the cell cannot respond to any stimulus regardless of strength

Relative refractory period (cardiac contractile cell)

Lasts approximately 50 ms; the phase during which the cell is repolarizing and can respond to a strong enough stimulus, though a greater stimulus strength than usual is required

Total refractory period (cardiac contractile cell)

Approximately 250 ms (absolute + relative)

Purpose of extended absolute refractory period

Ensures the heart muscle contracts and the contraction follows the electrical events; prevents premature contractions that would not be compatible with life

Intrinsic SA node firing rate (without nervous/endocrine control)

Approximately 60-100 times per minute

Intrinsic AV node firing rate (if SA node absent)

40-60 beats per minute

Intrinsic atrioventricular bundle firing rate (if AV node blocked)

Approximately 30-40 impulses per minute

Intrinsic bundle branch firing rate

20-30 impulses per minute

Intrinsic Purkinje fiber firing rate

15-20 impulses per minute

Bradycardia

A condition indicated by a resting heart rate lower than 50 beats per minute for most individuals (though trained aerobic athletes may have resting rates of 30-40 bpm normally)

Electrocardiogram (ECG/EKG)

A tracing of the heart's electrical activity captured by surface electrodes, providing insights into normal and abnormal heart function as a diagnostic tool

ECG leads

An ECG can use 3, 5, or 12 leads, with more leads offering more information; each lead measures the voltage difference between two electrodes

Holter monitor

A portable, battery-powered device that tracks heart electrical activity continuously, usually for 24-48 hours during regular activities

P wave

Represents depolarization of the atria on an ECG

Q wave

Represents the beginning of ventricular depolarization on an ECG

QRS complex

Represents depolarization of the ventricles on an ECG; requires a stronger electrical signal due to the larger size of ventricular cardiac muscle; ventricles begin to contract at the peak of the R wave

T wave

Represents repolarization of the ventricles on an ECG

Atrial repolarization on ECG

Occurs during the QRS complex, which masks it on the tracing

ECG interval

Includes one segment plus one or more waves

P-R interval

Measures the duration from the beginning of atrial depolarization (P wave) to the initiation of the QRS complex; a delay in impulse passage from the SA node to the AV node would appear here

Cardiac cycle

A series of pressure changes that occur within the heart from the beginning of one heartbeat to the beginning of the next

Systole

The period of contraction during which the heart pumps blood into circulation

Diastole

The period of relaxation during which heart chambers fill with blood

Pressure gradient and blood flow

Fluids flow according to their pressure gradient, moving from areas of high pressure to areas of low pressure

Passive ventricular filling

Occurs when blood moves from the atria into the ventricles through open AV valves during diastole; accounts for approximately 70-80 percent of ventricular filling

Tricuspid valve

The right atrioventricular valve; open during diastole to allow blood flow from the right atrium to the right ventricle

Mitral (bicuspid) valve

The left atrioventricular valve; open during diastole to allow blood flow from the left atrium to the left ventricle

Semilunar valves (pulmonary and aortic)

Closed during diastole to prevent backflow of blood into the ventricles from the pulmonary trunk and aorta

Atrial systole

Contraction of the atria following the P wave; pushes blood into the ventricles through open AV valves, contributing the remaining 20-30 percent of ventricular filling; lasts approximately 100 ms

End diastolic volume (EDV) / preload

The volume of blood in the ventricles just prior to ventricular contraction, approximately 130 mL in a resting adult standing

Ventricular systole

Contraction of the ventricles following the QRS complex, lasting a total of 270 ms

Ventricular ejection phase

The phase during which ventricular pressure exceeds pressure in the pulmonary trunk and aorta, pushing open the semilunar valves and ejecting blood from the heart

End systolic volume (ESV)

The volume of blood remaining in the ventricle after contraction, approximately 50-60 mL

Stroke volume (SV)

The amount of blood pumped from the ventricles, calculated as EDV minus ESV; normally in the range of 70-80 mL

Ventricular diastole duration

Approximately 430 ms; follows repolarization of the ventricles and is represented by the T wave; divided into two distinct phases

Dicrotic notch

A small dip seen in blood pressure tracings, produced when pressure in the ventricles drops below pressure in the pulmonary trunk and aorta, causing blood to flow back and close the semilunar valves

Ventricular diastole phase 1

As ventricular muscle relaxes, pressure drops below pulmonary trunk/aortic pressure, semilunar valves close, and AV valves remain closed, so ventricular volume does not change

Ventricular diastole phase 2

Pressure in the ventricles drops further, below atrial pressure, causing the AV valves to open and blood to flow from the atria into the ventricles, completing the cardiac cycle

Auscultation

A diagnostic technique that involves listening to heart sounds using a stethoscope

S1 heart sound

The "lub" sound created by the closing of the atrioventricular valves during ventricular contraction

S2 heart sound

The "dub" sound created by the closing of the semilunar valves during ventricular diastole

S3 heart sound

A rarely heard sound caused by blood flowing into the atria, blood sloshing in the ventricle, or tensing of the chordae tendineae; may be normal in youth, athletes, and pregnancy, but can indicate congestive heart failure if heard later in life

S4 heart sound

Results from contraction of the atria pushing blood into a stiff or hypertrophic ventricle, indicating left ventricular failure; occurs prior to S1