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Physiology > Conducting system of the heart > Flashcards

Conducting system of the heart Flashcards

1
Q

Describe the conduction system of the heart from SA node to purkinje fibres?

A

SA node-> 3 antrial internodal pathways -> AV node-> bundle of HIS -> R and L bundle branches -> Purkinje fibres

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2
Q

Which are the only fibres to traverse fibrous cardiac skeleton?

A

Bundle of His

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3
Q

Why are purkinje fibres necessary?

A

Convey electrical activities to the muscles-> normal myocardial cells cannot themselves spontaneously generate an action potential

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4
Q

Where is the SA node located?

A

Wall of RA, below SVC

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5
Q

How fast is depolarisation through the SA node, muscle and internodal pathway?

A

Depolarisation through node is 0.05m/s
Atrial muscle 0.3m/s
Through internodal pathway at 1m/s
AV node 0,05m/s

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6
Q

Why is conduction through the AV node so slow?

A

AV node is in posterior interatrial septum, behind tricuspid valve
Delay of 0.1 to 0.13 seconds
This delay allows completion of atrial systole before ventricles begin to contract

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7
Q

Which direction does the interventricular septum get activated?

A

From left to right

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8
Q

Which are the last parts of the heart to be depolarised?

A

Spreads from apex to base of heart
From endocardium to epicardium
Finally; pulmonary conus, top part of interventricular septum, basal posterior parts of LV

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9
Q

Describe how parasympathetic activation affects HR

A

Vagus nerve-> releases Ach and SA and AV node
Ach binds to M2 muscarinic receptors-> +G protein -> opens special K+ channels -> hyperpolarisation of membrane -> so it takes longer for cell to reach threshold -> reduces HR

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10
Q

Describe how sympathetic activation affects HR via SA node

A

Noradrenaline binds to B1-> increased cellular cAMP -> increased permeability of sarcolemma to Na + Ca ->depolarisation +steeper prepotential -> cells reach threshold more quickly

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11
Q

How does contraction of atria compare to contraction of ventricles from left to right?

A

RA systole precedes LA systole

LV contracts before RV

RV ejection begins before LV ejection (as pulmonary arterial pressure is lower than aortic pressure)

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12
Q

How does the timing of valve closer vary with respiration?

A

Inspiration: aortic valve closes before pulmonary valve

Expiration: aortic + pulmonary valve close simultaneously

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13
Q

How does the cardiac cycle correspond to an ECG?

A

P wave: ventricular diastole
R wave: ventricular contraction
QTinterval: period of ventricular depolarisation and repolarisation
T wave: ventricular relaxation with falling ventricular pressure

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14
Q

What are the key features of atrial flutter?

A

Sawtooth pattern
HR 200-350
There is a large counterclockwise circus movement in RA
Almost always associated with 2:1 or greater AV block because the AV cannot conduct >230/min
Ventricular rate can be slowed by carotid sinus pressure

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15
Q

Where in the heart is the issue in WPW syndrome?

A

Aberrant connection between atria and ventricles
(bundle of Kent)
Circus movement tachycardia is usually initiated by an atrial premature beat

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16
Q

What are the ECG manifestations of WPW syndrome?

A

Short PR interval
Wide slurred QRS complex (J wave)
Normal PJ interval
Paroxysmal atrial tachycardia

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17
Q

Why is AF life threatening in WPW syndrome?

A

Frequently degenerates into VT or VF

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18
Q

What is a normal QT interval?

A

0.35-0.42 seconds

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19
Q

What are the key facts about QT interval?

A

Corresponds with ventricular action potential
Varies inversely with HR
NOT prolonged with hypokalemia

20
Q

What is normal EF, EDV and SV?

A

EF: 65%
EDV: 130mL
SV: 70-90mL
50mls remains in ventricle at end of systole

21
Q

What are systolic and diastolic pressures?

A

Systolic: peak pressure in ventricles during contraction
Diastolic: lowest pressure in ventricles during relaxation
Pulse pressure=systolic-diastolic

22
Q

Which section of the cardiac cycle has the most rapid change in pressure per unit time?

A

Isometric contraction of left ventricle

23
Q

How do the pressures in the ventricles differ?

A

Left V: 120 systole 80 diastole
Right V: 25 systole 15 diastole
When aortic valve closes the pressure in RV is 15mmHg

24
Q

What is Starling’s law of the heart?

A

Increased venous return, increased cardiac output

Up to a point. No change in contractility.

25
Q

What is the Frank-Starling curve?

A

Relationship between EDV and SV
Curve shifts up and to the left -> with increased contractility (inotropes)
Shifts down and to the right -> decreased contractility

26
Q

Describe the different parts of the jugular venous wave?

A

a: atrial systole
c: rebound of triscuspid valve into RA during isovolumetric contraction
v: rise in atrial preessure before tricuspid valve opens in diastole

27
Q

What are the 2 heart sounds?

A

1st: closure of mitral and tricuspid valves
2nd: closure of aortic/pulmonary valves
3rd: rapid ventricular filling
4th: pathological-stiff ventricle

28
Q

What is the general function of inotropes?

A

reduce afterload and preload as well as increasing cardiac output and ejection fraction (widening pressure volume loop)
Safe use requires monitoring to be available

29
Q

What is the mechanism of action by which adrenaline, dopamine and NA are inotropes?

A

Act on b1 receptors-> + adenylyl cylase -> increased cAMP -> influx of calcium-> stronger contraction

30
Q

What is the mechanism of action by which glucagons are inotropes?

A

Forms cAMP in cardiac muscles-> influx of calcium-> stronger contraction

31
Q

What is the mechanism of action by which caffeine and theophylline are inotropes?

A

Inhibit breakdown of cAMP-> influx of calcium->stronger contraction

32
Q

What is the mechanism of action by which digoxin is an inotrope?

A

Inhibitory effect on Na K ATPase in myocardium-> influx of calcium

33
Q

What are some negative inotropes?

A

Hyoercapnia
Heart failure
Beta blockers, calcium channel blockers and many anaesthetics

34
Q

Which conditions have no effect on cardiac output?

A

Sleep

moderate changes in environmental temperature

35
Q

Which conditions increase cardiac output?

A 
Anxiety/excitement
eating
exercise
high environmental temperature
pregnancy
adrenaline
36
Q

What conditions decrease cardiac output?

A

Sitting or standing from lying position
Rapid arrhythmias
Heart disease

37
Q

How does diastolic dysfunction and systolic dysfunction affect the volume pressure curve?

A

Systolic: shifts curve up and to the right, decreases SV
Diastolic: shifts curve up and to the left, decreases SV

38
Q

How is cardiac output maintained in exercise in dennervated vs normal hearts?

A

Denervated: Cardiac output increases by frank starling mechanism, ie SV increases

Normal: HR increases

39
Q

Why is angina more likely to increase with aortic stenosis than aortic regurgitation?

A

Myocardial O2 requirement increases more with increased pressure than volume

40
Q

Which factors increase and decrease end diastolic volume?

A

Decreases in MI, pericardial effusion and reduced venous return to heart (standing)

Increases with stronger atrial contraction, negative intrathoracic pressure during inspiration, increased venous return to the heart

41
Q

Which parameters are decreased in a failing LV?

A

Ejection fraction
Rate of rise of pressure at the commencement of systole
Stroke-volume at a given filling pressure
Systolic shortening of myocardial fibres

42
Q

What are the features of pressure overload seen in aortic stenosis?

A

Hypertrophy, fibrosis, vascular insufficiency and changes in myosin isoform expression
Surgery fixes cardiac status but not pressure changes

43
Q

What makes cardiac function worse in aortic stenosis?

A

Increased pressure gradient across the aortic valve
Increased reflux through the aortic valve
Increased aortic systolic pressure
Rapid HR

44
Q

When does backflow in the proximal aorta occur?

A

Transiently during the initial phase of diastole

45
Q

What happens to blood flow in the coronary arteries during systole?

A

Still flows to RA and RV
Flow to LV decreases
Flow to subendocardial portion of LV stops

46
Q

What is happening to the pressure in the aorta during the isometric contraction phase of the ventricles?

A

The aortic pressure is falling until the aortic valve opens again

Physiology (7 decks)

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