Cardiac electrophysiology and ECG1 Flashcards
What are the main characteristics of cardiac electrical activation?
- Automaticity: cardiac muscle tissue contracts without neural stimulation
- Absence of tetany: after contracting, the heart must relax to fill up again. Sustained contraction of the heart without relaxation would be fatal, and this is prevented by a temporary inactivation of certain ion channels
- Substantial atrial to ventricular delay: this allows the atria to completely empty their contents into the ventricles; simultaneous contraction would cause inefficient filling and backflow.
- Coordinated contraction of ventricular cells: the ventricles must maximize systolic pressure to force blood through the circulation, so all the ventricular cells must work together
- Rapid depolarization propagation: cells of the ventricles contract nearly simultaneously
- Long action potential. This prevents premature relaxation, maintaining initial contraction until the entire myocardium has had time to depolarize and contract
What does the cardiac conduction system enable?
A fast and co-ordinated contraction of the heart.
What pathway does the normal electrical conduction through the heart muscle take?
A predicted pathway. It travels from the Sinoatrial node (SA node) to the Atrioventricular node (AV node) to the Bundle of His and then onto the left and right bundle branches (usually in a left to right pattern), ultimately ending up in the Purkinje fibers. that reach the ventricular wall. The cardiac pacemaker cells in the sinoatrial node are characterised by spontaneous depolarisation during the resting phase 4 mediated by Na+ and K+ entry into the cells via the If current through the hyperpolarisation-activated cyclic nucleotide gated (HCN) channels.
What is the cardiac conduction system composed of?
It is composed of specialized cardiac structures that are responsible for impulse formation and propagation.
The cardiac electrical activation is the result of what?
Voltage shifts across the cell membrane coordinated by multiple transmembrane proteins (ion channels), each regulating inward or outward electrical currents that determine the duration and electrophysiological properties of the cardiac action potential (AP).
The AP reflects the processes of depolarisation (linked to myocardial excitation) and repolarisation (linked to subsequent relaxation), which are reflected on the surface ECG, and is divided into phases.
What does the cardiac action potential (AP) reflect?
The processes of depolarisation (linked to myocardial excitation) and repolarisation (linked to subsequent relaxation), which are reflected on the surface ECG, and is divided into phases.
What is fundamental to maintain the cells’ negative resting potential at approximately -90mV?
The Na+/K+-ATPase pump
What is involved in the excitation-contraction coupling and relaxation of cardiac cells?
The sodium/calcium (Na+/Ca++) exchanger
What channels regulate the cardiac AP phases?
The voltage-gated (Na+), potassium (K+) and L-Type calcium (Ca++)
Define phase 0 and 1 of the cardiac conduction system.
The ventricular myocytes show a rapid upstroke of the AP (phase 0) determined by a large positive inward current through voltage-gated Na+ channels (INa), triggered by a threshold voltage of about -75 mV (e.g. caused by an adjacent cell depolarisation AP), and is followed by a transient repolarisation (phase 1) reflecting the Na+ channel inactivation, together with the activation of the voltage-gated transient outward K+ currents (Ito).
This transient repolarisation “notch” influences the height and duration of the plateau phase (phase 2), which is modulated by the balance between inward Ca++ (ICa), predominant, and Na +currents, and outward K+ currents.
Define phase 2 of the cardiac conduction system.
This transient repolarisation “notch” influences the height and duration of the plateau phase (phase 2), which is modulated by the balance between inward Ca++ (ICa), predominant, and Na +currents, and outward K+ currents.
Define phase 3 of the cardiac conduction system.
When the L-type Ca++ channels undergo voltage-dependent inactivation, the K+ currents prevail (IKr, IKs) and a second repolarisation phase (phase 3) occurs.
During phases 0-3 and early phase 4, no new AP can be initiated as the fast Na+ channels are still inactive (absolute refractory period, ARP).
True or false
True
After the absolute refractory period (ARP) only…
Impulses that exceed the threshold can cause re-excitation (relative refractory period, RRR), until the cell is completely repolarised and ready for a new excitation.
What does the the absolute refractory period (ARP) prevent?
Multiple, compounded action potentials from occurring (i.e., it limits the frequency of depolarisation and therefore the heart rate), allowing adequate cardiac filling and reducing the risk of arrythmia occurring.
What does the surface ECG reflect?
The phases of the cardiac AP in the different regions of the heart.
The P wave represents the atrial depolarisation; the PR interval reflects the conduction velocity through the A-V node; the QRS complex reflects the ventricular depolarisation, while the QT interval represents the duration of the ventricular AP. The J point, the end of the QRS complex, occurs during phase 1 of the AP i.e. the notch at the beginning of the AP plateau.
What is an ECG?
a graph of voltage versus time of the electrical activity of the heart, obtained using electrodes placed on the skin that can detect the sum of all cardiac cells depolarizations and repolarizations.
What does an ECG convey?
The ECG conveys a large amount of information about the structure of the heart and the function of its electrical conduction system.
Among other things, an ECG can be used to measure what?
The rate and rhythm of heartbeats, the size and position of the heart chambers, the presence of any damage to the heart’s muscle cells or conduction system, the effects of heart drugs, and the function of implanted pacemakers
When can ECGs be recorded?
At rest, during exercise, for one day or several days.
Why and how is an EP study performed?
To study more in detail the conduction system of the heart.
It’s performed via a right-sided cardiac catheterization: a wire with an electrode at its tip is inserted into the right heart chambers from a peripheral vein, and placed in various positions in close proximity to the conduction system
What does the intracardiac Electrograms (EGM) record?
The electrical activity at a localized area between two electrodes on a catheter
What are the benefits and disadvantages of an ECG?
Benefits:
Accurate measurement of cardiac conduction
Can locate area of disease
Can help diagnose arrhythmias that cant be identified noninvasively
Can be used to estimate risk of future cardiac arrest in certain diseased
Disadvantages:
Invasive
Risk of complications
What is presented on a normal ECG?
Sinus rhythm
Heart rate between 60 - 100 beats per minute
P wave positive in lead II, negative in aVR , ≤ 0.11 s
PR interval between 0.12 and 0.20 s
QRS complex ≤ 0.12 s, amplitude greater than 0.5 mV in at least one standard lead, and greater than 1.0 mV in at least one precordial lead.
Proceeding from V1 to V6, the R waves get taller while the S waves get smaller
ST segment isoelectric, can be slightly elevated (up to 2.0 mm in some precordial leads BUT never normally depressed greater than 0.5 mm
T wave same direction as the QRS complex
QT interval ≤ 0.40 s in males, 0.44 s in females
What are conduction disorders?
Abnormalities in the electrical system affecting the heart rate/rhythm:
the cardiac electrical signal either does not generate properly, or it does not travel properly through the heart, or both.
They can occur in both structurally normal hearts and abnormal hearts (i.e. partial or total absence of structures, gradual replacement by fatty and/or fibrous infiltration and calcification)
What are acquired causes of conduction disorders?
electrolyte problems hypothermia drugs ischemic heart disease degenerative heart disease infiltrative heart disease infections
What are congenital causes of conduction disorders?
damages during foetal development(i.e. infections, toxic agents)
autoimmune disease (myocarditis, vasculitis)
inheritable genetic defects
What are sites in which conduction disorders occur?
Sino-atrial node results in Sick sinus syndrome
Atrio-ventricular node results in A-V block
Bundle branches results in Left or right bundle branch block
Ion channel disease, all sites can have issues
What are ECG manifestations of Sino-atrial node?
irregular P waves cycle that can determine sinus bradycardia, sino-atrial block, brady-tachy alternans, sinus arrest
What are ECG manifestations of Atrio-ventricular node?
A-V block which can be characterised by increased PR interval or no correspondence between P and QRS
What are ECG manifestations of Bundle branches?
Left or right bundle branch block. This is characterised by broad QRS and axis deviation.
Hyperkalaemia is an important acquired cause of conduction disease.
How is it defined?
Increased extracellular potassium reduces myocardial excitability, with depression of pacemaker and conducting activity. From mild repolarization abnormalities (peaked T waves), to progressive paralysis of the atria, conduction abnormalities and bradycardia, cardiac arrest
Ischemia is an important acquired cause of conduction disease.
How is it defined?
Conduction defects at any level of the conduction system:
RCA : Sinus bradycardia, A-V blocks (often reversible)
LCA : A-V blocks with wide QRS, cardiac arrest
Congenital/inheritable cardiac conditions can cause conduction defects in the heart. They are defined in what two categories?
- Cardiomyopathies
Hypertrophic cardiomyopathy, non-ischemic dilated cardiomyopathy, arrhythmogenic cardiomyopathy
Scarring within the myocardium can damage conduction tissue
Conduction abnormalities can be seen on the ECG before any structural changes are visible on cardiac imaging
- Primary arrhythmia syndromes : Brugada Syndrome
Autosomal dominant condition linked to loss-of-function SCN5A gene mutations
Significant cause of autopsy negative sudden death
Diagnostic ECG pattern (Type 1 Brugada pattern) – spontaneous or concealed (unmasked after drug provocation with Class 1 antiarrhythmic agents given intravenously)
Requires ECG placement in “high precordial position”
to better explore the right ventricular outflow tract
What is Ajmaline?
antiarrhythmic agent
What is Premature Cardiac Conduction Disease (PCCD)?
Primary arrhythmia syndrome
Conduction disease is common with advancing age and in the presence of cardiac structural abnormalities – when present in isolation an ion channel disorder should be suspected (Na++ channel disorders)
Sinus node disease – absence of P waves
Atrioventricular node disease - varying degrees of A-V block
Intraventricular conduction delay (BBB, axis deviation)
What is Long QT Sydrome (LQTS)?
Primary arrhythmia syndrome
Due to loss of function mutation in K+ channel (LQTS1-2)
Affects flow of K+ out of the cell (delay in action potential returning to baseline)
Rarer forms include gain of function mutation in Na++ channels and loss of function in Ca++ channels
Resting ECG may show normal QT interval
Exercise ECG often shows paradoxical prolongation
Increased risk of ventricular arrhythmias
What can the three Long QT Sydrome (LQTS) types be treated with?
LQTS1 and 2- beta blockers
LQTS3- Flecainide and Mexilitine
What is Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT)?
Primary arrhythmia syndrome
Most commonly results from mutations in two genes, RYR2 and CASQ2, causing disruption in the handling of calcium within myocytes.
Affected individuals develop life threatening arrhythmia in response to adrenaline - activity or stress
Can result in recurrent collapse during exercise
Surface ECG is normal
Exercise ECG or epinephrine provoked ECG are the testing modalities of choice. The former is more specific for CPVT than epinephrine
Bidirectional ventricular extra systole DURING exercise is a diagnostic feature
The cardiac conduction system is composed of structures that are responsible for impulse formation and propagation
True or false
False
The cardiac conduction system is composed of specialized cardiac structures that are responsible for impulse formation and propagation
The cardiac electrical activation is the result of voltage shifts across the cell membrane coordinated by multiple transmembrane proteins (ion channels)
True or false
True
Acquired/congenital cardiac conditions cannot affect the cardiac electrical activation at different sites of the conduction system and cause symptoms and/or increase susceptibility to life-threatening arrhythmias
True or false
False
Acquired/congenital cardiac conditions can affect the cardiac electrical activation at different sites of the conduction system and cause symptoms and/or increase susceptibility to life-threatening arrhythmias
The electrocardiogram (ECG) recording reflects the cardiac electrical activity and is fundamental to diagnose electrical or suspect structural disorders affecting the cardiac activation and conduction
True or false
True
What is the first part of the cardiac cycle is called?
Atrial depolarization
At what rate does the paper move through an ECG machine?
25 mm/s.
What distance across ECG paper represents 1.0 seconds?
5 large squares
What is the normal duration of a QRS complex?
0.12s (3 small squares)
If there were 3 large squares in an R-R interval what would the heart rate be?
Why?
100bpm
To calculate heart rate from an ECG you can count the number of large squares in an R-R interval then divide 300 by this number. Therefore 300/3 = 100 bpm.
