heartbeat and the ecg Flashcards
where is the sino-atrial node?
in the wall of the right atrium wall
near the entrance of the superior vena cava
what is the primary pacemaker of the heart?
SAN
what does the SAN do?
initiates and controls the heart beat
what are pacemaker cells?
modified cardiac myocytes
they’ve lost the ability to contract but have become specialised for initiating and conducting APs
what joins pacemaker cells together?
gap junctions
what blood vessel supplies the SAN?
right coronary artery
what medical problem can the occlusion of the right coronary artery cause?
ischaemia in the SAN
where is the AVN located?
inter-atrial septum
near the tricuspid valve
explain how action potentials are fired off in a pacemaker cell?
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
which nerves innervate the SAN?
parasympathetic and the sympathetic
what effect do parasympathetic nerves have on the SAN?
inhibit K+ decay
slow the heart beat
what receptors of the SAN do parasympathetic nerves act on?
muscarinic receptors
what effect do sympathetic nerves have on the SAN?
accelerate K+ decay
speed up the heart beat
what receptors of the SAN do sympathetic nerves act on?
beta-1 adrenoreceptors
what receptors does blood-borne adrenaline act on and what does it do?
beta-1 receptors
increases force of contraction
how is an impulse transmitted through the heart?
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
what are Purkinje fibres?
muscle fibres specialised for speed of conduction
what can damage Purkinje fibres?
ischaemia
what is ischaemia?
inadequate blood supply to an organ
how do pacemaker cells depolarise?
spontaneously
what is the pacemaker potential?
not stable
around -60mV
what is the threshold of a pacemaker cell for an action potential?
-40mV
what is the size of a depolarisation of a pacemaker cell?
+10mV
explain how an action potential occurs in a ventricular cell
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
what causes the plateau of a ventricular AP?
slow and prolonged entry of calcium into the cell
calcium in is balanced by the potassium out
what is the plateau of a ventricular AP?
a prolonged depolarisation
what is the benefit of ventricular cells having a long refractory period?
they dont contract prematurely
ensures contraction is synchronous
how does a defibrillator make muscles contract synchronously?
forces all the heart muscle cells into refractory period
describe the phases of a ventricular action potential
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
what is the purpose of an ECG?
screen for possible cardiac ischaemia
what is an ECG lead?
voltage recorded between two points on the body
how many leads does an ECG have and what are they?
12
6 limb leads (3 standard and 3 augmented)
6 chest leads
how many electrodes does an ECG have?
10
are the standard limb leads unipolar or bipolar?
bipolar
what parts of the body do the standard limb leads measure between?
lead 1 - left and right axillae
lead 2 - right axilla and leg
lead 3 - left axilla and leg
which lead does a standard ECG record from?
lead 2
are the augmented limb leads unipolar or bipolar and why?
unipolar
measure between a virtual point on the chest and a physical point
only require a positive electrode
are the chest leads unipolar or bipolar?
unipolar
explain where each chest lead is placed
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
which leads show the inferior view of the heart?
2, 3 and aVF
which leads show the lateral view of the heart?
1, aVL, V%, V^
which leads show the anterior view of the heart?
V3, V4
which leads show the septal view of the heart?
V1, V2
explain what the different waves of a PQRST wave show
P wave - atrial depolarisation
QRS - ventricular depolarisation
ST segment - when ventricular muscles are contracting
T wave - ventricular repolarisation
what condition can changes in the ST segment show?
Acute Myocardial Infarction
explain the changes in the heart in atrial fibrillation
irregular and abnormally fast heart beat
abnormal electrical impulses from many areas of the atria
how can atrial fibrillation be identified from an ECG?
no P wave
explain the changes in the heart in atrial flutter?
coordinated but very rapid contraction of the atria
too rapid for the impulse to be conducted through the EVN
how can atrial flutter be identified from an ECG?
extra P waves
saw-tooth flutter waves
what causes atrial flutter?
underlying heart problems, such as
coronary heart disease
heart valve disease
high blood pressure
what condition causes ST elevation?
acute ischaemia in the viewed part of the heart
what condition causes ST depression?
chronic ischaemia
how can ventricular fibrillation be identified by an ECG?
no clear QRS complex
