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heartbeat and the ecg Flashcards

1
Q

where is the sino-atrial node?

A

in the wall of the right atrium wall

near the entrance of the superior vena cava

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2
Q

what is the primary pacemaker of the heart?

A

SAN

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3
Q

what does the SAN do?

A

initiates and controls the heart beat

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4
Q

what are pacemaker cells?

A

modified cardiac myocytes

they’ve lost the ability to contract but have become specialised for initiating and conducting APs

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5
Q

what joins pacemaker cells together?

A

gap junctions

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6
Q

what blood vessel supplies the SAN?

A

right coronary artery

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7
Q

what medical problem can the occlusion of the right coronary artery cause?

A

ischaemia in the SAN

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8
Q

where is the AVN located?

A

inter-atrial septum

near the tricuspid valve

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9
Q

explain how action potentials are fired off in a pacemaker cell?

A

after AP there’s a constant outward K+ current
constant Na+ influx
over time, K+ decays
membrane slowly depolarises
K+ reaches low enough for membrane to depolarise
K+ is reset

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10
Q

which nerves innervate the SAN?

A

parasympathetic and the sympathetic

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11
Q

what effect do parasympathetic nerves have on the SAN?

A

inhibit K+ decay

slow the heart beat

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12
Q

what receptors of the SAN do parasympathetic nerves act on?

A

muscarinic receptors

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13
Q

what effect do sympathetic nerves have on the SAN?

A

accelerate K+ decay

speed up the heart beat

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14
Q

what receptors of the SAN do sympathetic nerves act on?

A

beta-1 adrenoreceptors

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15
Q

what receptors does blood-borne adrenaline act on and what does it do?

A

beta-1 receptors

increases force of contraction

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16
Q

how is an impulse transmitted through the heart?

A 
SAN releases an AP
spreads over both atria by 60ms
delay of 60ms at the AVN
moves down the bundle of His
Through the left and right Purkinje fibres
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17
Q

what are Purkinje fibres?

A

muscle fibres specialised for speed of conduction

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18
Q

what can damage Purkinje fibres?

A

ischaemia

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19
Q

what is ischaemia?

A

inadequate blood supply to an organ

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20
Q

how do pacemaker cells depolarise?

A

spontaneously

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21
Q

what is the pacemaker potential?

A

not stable

around -60mV

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22
Q

what is the threshold of a pacemaker cell for an action potential?

A

-40mV

23
Q

what is the size of a depolarisation of a pacemaker cell?

A

+10mV

24
Q

explain how an action potential occurs in a ventricular cell

A

threshold is met
fast opening Na+ channels open –> Na+ influx
K+ and Ca2+ channels open
K+ out, Ca2+ in –> plateau
more K+ channels open and Ca2+ channels close –> repolarisation
cell goes into a really long refractory period
returns to resting membrane potential

25
Q

what causes the plateau of a ventricular AP?

A

slow and prolonged entry of calcium into the cell

calcium in is balanced by the potassium out

26
Q

what is the plateau of a ventricular AP?

A

a prolonged depolarisation

27
Q

what is the benefit of ventricular cells having a long refractory period?

A

they dont contract prematurely

ensures contraction is synchronous

28
Q

how does a defibrillator make muscles contract synchronously?

A

forces all the heart muscle cells into refractory period

29
Q

describe the phases of a ventricular action potential

A 
Phase 0 - depolarisation
Phase 1 - slight repolarisation because of the closing of Na+ channels
Phase 2 - plateau
Phase 3 - repolarisation
Phase 4 - resting membrane potential
30
Q

what is the purpose of an ECG?

A

screen for possible cardiac ischaemia

31
Q

what is an ECG lead?

A

voltage recorded between two points on the body

32
Q

how many leads does an ECG have and what are they?

A

12
6 limb leads (3 standard and 3 augmented)
6 chest leads

33
Q

how many electrodes does an ECG have?

A

10

34
Q

are the standard limb leads unipolar or bipolar?

A

bipolar

35
Q

what parts of the body do the standard limb leads measure between?

A

lead 1 - left and right axillae
lead 2 - right axilla and leg
lead 3 - left axilla and leg

36
Q

which lead does a standard ECG record from?

A

lead 2

37
Q

are the augmented limb leads unipolar or bipolar and why?

A

unipolar
measure between a virtual point on the chest and a physical point
only require a positive electrode

38
Q

are the chest leads unipolar or bipolar?

A

unipolar

39
Q

explain where each chest lead is placed

A

V1 - right of the sternum, 4th intercostal space
V2 - left of the sternum, 4th intercostal space
V3 - inbetween V2 and V4
V4 - midclavicular line of 5th intercostal space
V5 - anterior axillary line - same level as V4
V6 - midaxillary line, same level as V5

40
Q

which leads show the inferior view of the heart?

A

2, 3 and aVF

41
Q

which leads show the lateral view of the heart?

A

1, aVL, V%, V^

42
Q

which leads show the anterior view of the heart?

A

V3, V4

43
Q

which leads show the septal view of the heart?

A

V1, V2

44
Q

explain what the different waves of a PQRST wave show

A

P wave - atrial depolarisation
QRS - ventricular depolarisation
ST segment - when ventricular muscles are contracting
T wave - ventricular repolarisation

45
Q

what condition can changes in the ST segment show?

A

Acute Myocardial Infarction

46
Q

explain the changes in the heart in atrial fibrillation

A

irregular and abnormally fast heart beat

abnormal electrical impulses from many areas of the atria

47
Q

how can atrial fibrillation be identified from an ECG?

A

no P wave

48
Q

explain the changes in the heart in atrial flutter?

A

coordinated but very rapid contraction of the atria

too rapid for the impulse to be conducted through the EVN

49
Q

how can atrial flutter be identified from an ECG?

A

extra P waves

saw-tooth flutter waves

50
Q

what causes atrial flutter?

A

underlying heart problems, such as
coronary heart disease
heart valve disease
high blood pressure

51
Q

what condition causes ST elevation?

A

acute ischaemia in the viewed part of the heart

52
Q

what condition causes ST depression?

A

chronic ischaemia

53
Q

how can ventricular fibrillation be identified by an ECG?

A

no clear QRS complex

c (25 decks)

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