Heartbeat and ECG Flashcards
Describe how a cardiac pacemaker cell works
At rest there is constant sodium influx into the cell coupled with simultaneous potassium efflux (prevents depolarisation) –> potassium efflux decays with time –> membrane potential slowly depolarises –> action potential generated when outward potassium reaches critically low level and there’s transient increase in sodium influx –> potassium efflux then ‘reset’ to a high level again –> cycle repeats
What is happening biochemically in a cardiac pacemaker cell at rest?
There is a constant sodium influx (leak) into the cell at rest, and this would normally depolarise the cell but a simultaneous outward potassium current is preventing this.
Why are cardiac pacemaker cells myogenic?
Due to the constant inward sodium current, the pacemaker cells in the SAN are spontaneously active and therefore the rate of stimulation simply depends on the rate of decay of outward potassium current.
What is the voltage of the cardiac pacemaker cells at rest?
~ -70mV
What is the voltage of the cardiac pacemaker cells before generating an action potential?
~ -40mV
Where is the SAN found?
On the wall of the right atrium near the entrance of the superior vena cava
Describe the SAN
A group of modified cardiac muscle cells that generate impulses (pacemaker) that is sent to adjacent atrial cells by gap junctions
What is the blood supply to the SAN?
Right coronary artery
Where is the AVN located?
Located on the inter-atrial septum close to the tricuspid valve (right side of heart)
Describe the function of the AVN
Transmits the impulse from the SAN down to the ventricles down the bundles of His and then up the Purkinje fibres, to cause upwards contraction from the apex of the heart
What nervous stimulation is there to the SAN?
Sympathetic and parasympathetic
Describe the parasympathetic nervous innervation of the SAN
Parasympathetic nerves from the vagus act via interneurons within the node to inhibit the closure of the potassium channels via muscarinic receptors, making the pacemaker cells slow down.
Describe the sympathetic nervous innervation of the SAN
Sympathetic nerves at the SAN increase closure of the potassium channels by beta adrenoreceptor actions, which makes pacemaker cells ‘speed up’
How does adrenaline act to increase the force of contraction of the myocardium
Adrenaline in the blood acts on beta receptors throughout the myocardium as well as in the SAN, in order to produce ionotropic actions (increased force of contraction)
What nervous stimulation is there to the AVN?
Both parasympathetic and sympathetic nervous system (weaker inputs than into the SAN)
Describe the spread of the action potential from the SAN in the heart
SAN –> both atria within 60ms –> 60ms delay to AVN to allow time for atrial to contract and push blood into ventricles before ventricular contraction –> fast conduction muscle fibres leave AVN –> travel down IV septum –> down the left and right bundles of His to the apex of the heart –> activates the Purkinje fibres which then move the impulse up the walls of the ventricles to cause upwards contraction –> blood moves into the great vessels
What is a first degree atrioventricular block?
Where the PR interval is lengthened beyond 200ms
How may conduction through the bundles of His or Purkinje fibres be impaired?
Conduction can be damaged by ischaemia
What type of calcium ion channel is present in cardiac muscle?
slow, L type
Describe the action potential graph in ventricular muscle
Fast rise in voltage (depolarisation) due to opening of the sodium channel, this is then followed by a prolonged depolarisation phase known as the plateau (due to late and prolonged entry of calcium into the cell –> muscles can contract for much longer than skeletal muscle) – followed by a refractory period whereby potassium efflux occurs
What is the function of L-type calcium channel antagonists?
Drugs which block these slow calcium channels are used to decrease the force of ventricular contraction and thus the work (and oxygen demand) of the heart
How many leads are there in an ECG?
12
What produces an ECG trace?
Although an ECG is caused by the cardiac action potentials, it is not the same shape as the cardiac action potential graphs; generally the ECG is mainly generated at the start and the end of cardiac action potentials
What does the R wave signify in an ECG?
The start of ventricular depolarisation
What does an ECG lead do?
Records the voltage between two points on the body
What does Lead I in an ECG record?
Records the signal between the left and right axillae
What does Lead II in an ECG record?
Records the signal between the right axilla and leg
What does Lead III in an ECG record?
Records the signal between the left axilla and leg
Where is a standard ECG recorded from and why?
From lead II (between right axilla and leg) as this ordinarily gives the largest signal of the three limb leads
How long should the QRS complex be ordinarily?
Less than 100ms
How long should the PR interval be ordinarily?
120-200ms
What represents the first heart sound (Lubb - closure of AV valve) in an ECG?
QRS complex
In which limb leads is the P wave normally positive?
Leads I and II and sometimes also in III
How should the P wave appear in an ECG?
Smooth and rounded is normal
Is the Q wave positive or negative on an ECG trace?
Negative, no Q wave is present if QRS complex signal starts upwards
How does the Q wave appear on the limb leads in an ECG?
Normally small; but it’s absent on lead II (right axilla and leg)
How does the R wave appear on the limb leads in an ECG?
R wave is positive by definition and is usually present on all three leads
How does the S wave appear on the limb leads in an ECG?
S wave is negative by definition
What does the ST segment represent in an ECG?
When all of the ventricular muscles are contracting
What are ST segment changes important in the diagnosis of?
Acute myocardial infarction
How does the T wave appear on the limb leads in an ECG?
Normally orientated in the same direction as the preceding QRS complex
What are the augmented limb leads in an ECG?
Unipolar leads because the amplitude of the signal is calculated between one physical recording point and one virtual reference point on the centre of the chest; the ECG automatically calculates the values on these leads
How many limb leads are there in total?
6; I, II, III, aVR, aVL, aVF
Describe the recordings present on the aVR limb lead
Always has a large Q waave and small or non-existent R wave
Describe the recordings present on the aVL limb lead
Readings are often very small
How many chest leads are there in the production of an ECG?
6; V1-V6
What type of leads are the chest leads used in the production of an ECG trace?
Unipolar leads that reference the amplitude of signal to a virtual point in the centre of the chest
Where is the V1 chest lead placed?
In the 4th intercostal space next to the sternum on the patient’s right
Where is the V2 chest lead placed?
In the 4th intercostal space next to the sternum on the patient’s left
Describe the recordings present on the V1 chest lead
Mainly negative with a large S wave
Describe the recordings present on the V5 and V6 chest leads
Mainly positive with a large R wave
Why are 12 recordings collected for an ECG?
Electrodes are affected more by electrical activity in physically nearer parts of the body so different leads view different regions of the heart to pick up abnormalities in the areas
Which leads provide a view of the inferior heart?
II, III and aVF
Which leads provide a view of the lateral heart?
I, aVL, V5, V6
Which leads provide a view of the anterior heart?
V3, V4
Which leads provide a view of the septal heart?
V1 and V2
How may COPD or coronary heart failure appear on an ECG trace?
Notched or peaked P waves
What is the absence of a P wave in an ECG trace likely to represent?
Atrial fibrillation
What is the presence of extra P waves in an ECG trace likely to represent?
Atrial flutter
What is ST elevation?
Where, instead of the S portion returning to baseline prior to the T wave, it continues to increase and doesn’t reach the baseline until after the T wave
What is ST depression?
Where, instead of the S portion returning to baseline it remains beneath this point until the T wave
What is ST elevation indicative of?
Acute ischaemia in the viewed part of the heart such as acute myocardial infarction
What is a ST depression indicative of?
Sign of chronic ischaemia in the viewed region of the heart
