introduction to the ECG

Question 1

what is syncytium?

Answer 1

one large cell having many nuclei that are not separated by cell membrane

Question 2

what is a function syncytium?

Answer 2

many cells functioning as one

Question 3

what are the 3 types of cardiac monocyte & their functions?

Answer 3

• pacemaker cells = for setting heart rhythms
• conducting cells = for transmitting rhythms throughout the body
• contractile cells = for contracting to that rhythm (most numerous)

Question 4

at what speed do the atrial and ventricular myocytes usually contract?

Answer 4

0.3-0.5 m/s

Question 5

what speed do the Purkinje fibres contract at?

Answer 5

5m/s

Question 6

what speed does the AV node contract at?

Answer 6

0.05m/s

Question 7

how are cardiomyocytes linked together?

Answer 7

linked together by low resistance pathways associated with gap junctions at the intercalated disks

Question 8

what ensures that the atria contract at the same time?

Answer 8

internodal bundles conduct the impulse from the SA node to the AV node

Question 9

what do the 4 specialised bundles in the atria consist of?

Answer 9

contain Purkinje-like cells and the cardiomyocytes are modified to conduct

*these bundles are also in direct contact with the atrial muscle

Question 10

what is the only point where the wave of depolarisation passes from atria to ventricles?

Answer 10

AV nodes

Question 11

what are ventricles insulated by?

Answer 11

insulated by connective tissue from atria

Question 12

how many seconds does the AV node delay the excitation from atria to ventricles by?

Answer 12

0.1-0.2s

Question 13

why is there a delay in the electrical conduction from atria to ventricles?

Answer 13

to permit longer and more effective ventricular filling

Question 14

what is the AV node composed of?

Answer 14

small modified myocytes

Question 15

what order does the depolarisation occur?

Answer 15

first part of ventricular wall is depolarised in the septum, then the apex and then finally the atrioventricular groove

Question 16

what is an ECG?

Answer 16

a gross electrical measurement of the heart

Question 17

why is it possible to measure the ECG from the skin surface?

Answer 17

due to the heart being a functional syncytium which large groups of cells all make electrical changes simultaneously

Question 18

what are 2 positives about ECGs?

Answer 18

they are very fast and affordable

Question 19

what is a lead?

Answer 19

a configuration of electrodes

Question 20

where is lead II placed?

Answer 20

• positive electrode is on the left leg
• negative electrode is on the right arm
• ground electrode is on the right leg

Question 21

what are the 12 standard leads?

Answer 21

• 3 bipolar leads (I, II, III) frontal plane
• 3 augmented leads
• 6 precordial leads

Question 22

what does the P-wave on an ECG show?

Answer 22

depolarisation of atria in response to SA node triggering

Question 23

what does the PR segment show on an ECG?

Answer 23

the delay of the AV node allowing the ventricles to fill

Question 24

what does the QRS complex show on an ECG?

Answer 24

depolarisation of the ventricles, triggering the main pumping contractions

Question 25

what does the ST segment show on an ECG?

Answer 25

the beginning of the ventricle repolarisation

Question 26

what does the T wave show on an ECG?

Answer 26

ventricular repolarisation

Question 27

what is present on an ECG if the ventricular conduction is abnormal?

Answer 27

the QRS is wide

Question 28

what is a sign of dead tissue on an ECG?

Answer 28

large (deep) Q waves

Question 29

what generates the heart rhythm?

Answer 29

the SA node

Question 30

what is sinus tachycardia?

Answer 30

a tachycardia driven by the SA node beating too quickly: it has normal PR waves and each P is matched with QRS

Question 31

what is the normal duration of a PR interval on an ECG?

Answer 31

3-5 boxes (120-200ms)

Question 32

what is the normal duration of a QRS complex on an ECG?

Answer 32

2-3 boxes (80-120ms)

Question 33

what is the normal duration of a QT interval on an ECG?

Answer 33

9-11.5 boxes (360-460ms)

Question 34

what does the parasympathetic muscarinic stimulus lead to?

Answer 34

decreased heart rate, decrease in contractility and decrease in conduction velocity

Question 35

what does parasympathetic withdrawal lead to?

Answer 35

increased heart rate, increased contractility and increase in conduction velocity

Question 36

what is atropine?

Answer 36

a muscarinic antagonist

Question 37

what is not innervated by the parasympathetic system?

Answer 37

vasculature

Question 38

what does sympathetic stimulation lead to?

Answer 38

increased heart rate, increased contractility and increased conductance velocity

Question 39

what nerve is involved with parasympathetic input?

Answer 39

vagus nerve

Question 40

what nerve is involved with sympathetic input?

Answer 40

stellate nerve

Question 41

what effect do beta-agonists have on the heart rate?

Answer 41

increase rate

Question 42

what effect to beta-blockers have on the heart rate?

Answer 42

decreased rate

Question 43

what is heart block?

Answer 43

a type of dysrhythmia caused by any kind of impulse contraction block of the heart

Question 44

what causes a heart block?

Answer 44

- ischaemia of AV node or AV bundle - compression of AV bundle by scar or calcified tissue - inflammation of the AV node or bundle

Question 45

what is an AV heart block?

Answer 45

a delay or failure of atrial signal stimulating ventricle

Question 46

what are the symptoms of heart block?

Answer 46

- can be asymptomatic - palpitations - hypotension-like: dizziness, malaise, syncope - risk of sudden death

Question 47

what is first degree heart block?

Answer 47

- when PR interval >5 little boxes - but all Ps followed by QRS - always almost asymptomatic - often young people - delayed AV node transmission - rarely treated

Question 48

what is mobitz type 1 second degree heart block?

Answer 48

- PR interval gets longer until QRS wave fails to follow P wave - likely cause is AV node damage - usually no treatment given

Question 49

what is second degree heart block?

Answer 49

some P waves are blocked and are not followed by QRS so some QRS complexes are missing

Question 50

what is mobitz type II second degree heart block?

Answer 50

- some P waves are blocked and are not followed by QRS - PR interval remains the same - likely problem in bundle of His - high risk: can progress to 3rd degree heart block - treatment: implant pacemaker

Question 51

what triggers premature (early) beats?

Answer 51

irritable tissue

Question 52

what triggers escape (late) beats?

Answer 52

natural rhythmicity of non-atrial tissue

Question 53

what are the characteristics of premature ventricular contractions?

Answer 53

- usually wide and weird looking ventricular electrical activity - no S wave, instead wide negative dip where the T wave should be - often beat triggered in middle of myocardium - the 2 ventricles will be electorally unsynchronized - width is determined by slowed conduction velocity

Question 54

what is atrial fibrillation?

Answer 54

- no P wave instead flat or wiggly line - ventricular rate is fast and irregular so many signals reach the AV node - it is very common in the elderly

Question 55

what can atrial fibrillation lead to?

Answer 55

thrombus formation in atrium due to slow flow of blood leads to stroke risk

Question 56

what is respiratory sinus arrhythmia?

Answer 56

- heart beat is slightly faster during inspiration, slightly slower during expiration - sign of a healthy heart - usually only present in children and athletes - caused by respiratory centres in brains medulla - observe ventricular rate: opposite of RR interval

Question 57

what is an interval?

Answer 57

duration from start of one mark to the end of another mark

Question 58

what is a segment?

Answer 58

the wave form between 2 other waves

Question 59

what is the ST segment elevation a sign of?

Answer 59

an acute MI

Question 60

what is an iso-electric baseline?

Answer 60

from end of T to next P