heartbeat and the ecg

Question 1

where is the sino-atrial node?

Answer 1

in the wall of the right atrium wall

near the entrance of the superior vena cava

Question 2

what is the primary pacemaker of the heart?

Answer 2

SAN

Question 3

what does the SAN do?

Answer 3

initiates and controls the heart beat

Question 4

what are pacemaker cells?

Answer 4

modified cardiac myocytes

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

Question 5

what joins pacemaker cells together?

Answer 5

gap junctions

Question 6

what blood vessel supplies the SAN?

Answer 6

right coronary artery

Question 7

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

Answer 7

ischaemia in the SAN

Question 8

where is the AVN located?

Answer 8

inter-atrial septum

near the tricuspid valve

Question 9

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

Answer 9

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

Question 10

which nerves innervate the SAN?

Answer 10

parasympathetic and the sympathetic

Question 11

what effect do parasympathetic nerves have on the SAN?

Answer 11

inhibit K+ decay

slow the heart beat

Question 12

what receptors of the SAN do parasympathetic nerves act on?

Answer 12

muscarinic receptors

Question 13

what effect do sympathetic nerves have on the SAN?

Answer 13

accelerate K+ decay

speed up the heart beat

Question 14

what receptors of the SAN do sympathetic nerves act on?

Answer 14

beta-1 adrenoreceptors

Question 15

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

Answer 15

beta-1 receptors

increases force of contraction

Question 16

how is an impulse transmitted through the heart?

Answer 16

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

Question 17

what are Purkinje fibres?

Answer 17

muscle fibres specialised for speed of conduction

Question 18

what can damage Purkinje fibres?

Answer 18

ischaemia

Question 19

what is ischaemia?

Answer 19

inadequate blood supply to an organ

Question 20

how do pacemaker cells depolarise?

Answer 20

spontaneously

Question 21

what is the pacemaker potential?

Answer 21

not stable

around -60mV

Question 22

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

Answer 22

-40mV

Question 23

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

Answer 23

+10mV

Question 24

explain how an action potential occurs in a ventricular cell

Answer 24

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

Question 25

what causes the plateau of a ventricular AP?

Answer 25

slow and prolonged entry of calcium into the cell | calcium in is balanced by the potassium out

Question 26

what is the plateau of a ventricular AP?

Answer 26

a prolonged depolarisation

Question 27

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

Answer 27

they dont contract prematurely | ensures contraction is synchronous

Question 28

how does a defibrillator make muscles contract synchronously?

Answer 28

forces all the heart muscle cells into refractory period

Question 29

describe the phases of a ventricular action potential

Answer 29

``` 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 ```

Question 30

what is the purpose of an ECG?

Answer 30

screen for possible cardiac ischaemia

Question 31

what is an ECG lead?

Answer 31

voltage recorded between two points on the body

Question 32

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

Answer 32

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

Question 33

how many electrodes does an ECG have?

Answer 33

10

Question 34

are the standard limb leads unipolar or bipolar?

Answer 34

bipolar

Question 35

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

Answer 35

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

Question 36

which lead does a standard ECG record from?

Answer 36

lead 2

Question 37

are the augmented limb leads unipolar or bipolar and why?

Answer 37

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

Question 38

are the chest leads unipolar or bipolar?

Answer 38

unipolar

Question 39

explain where each chest lead is placed

Answer 39

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

Question 40

which leads show the inferior view of the heart?

Answer 40

2, 3 and aVF

Question 41

which leads show the lateral view of the heart?

Answer 41

1, aVL, V%, V^

Question 42

which leads show the anterior view of the heart?

Answer 42

V3, V4

Question 43

which leads show the septal view of the heart?

Answer 43

V1, V2

Question 44

explain what the different waves of a PQRST wave show

Answer 44

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

Question 45

what condition can changes in the ST segment show?

Answer 45

Acute Myocardial Infarction

Question 46

explain the changes in the heart in atrial fibrillation

Answer 46

irregular and abnormally fast heart beat | abnormal electrical impulses from many areas of the atria

Question 47

how can atrial fibrillation be identified from an ECG?

Answer 47

no P wave

Question 48

explain the changes in the heart in atrial flutter?

Answer 48

coordinated but very rapid contraction of the atria | too rapid for the impulse to be conducted through the EVN

Question 49

how can atrial flutter be identified from an ECG?

Answer 49

extra P waves | saw-tooth flutter waves

Question 50

what causes atrial flutter?

Answer 50

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

Question 51

what condition causes ST elevation?

Answer 51

acute ischaemia in the viewed part of the heart

Question 52

what condition causes ST depression?

Answer 52

chronic ischaemia

Question 53

how can ventricular fibrillation be identified by an ECG?

Answer 53

no clear QRS complex