•appropriate T wave discordance: in BBB, T wave deflection should be opposite to that of the terminal

QRS deflection (i.e.T wave negative if ends with R or R’; positive if ends with S)

• inappropriate T wave concordance suggests ischemia or infarction

Q-T INTERVAL

•duration of ventricular depolarization plus repolarization;often difficult to interpret

•corrected QT (QTc) corrects for the repolarization duration (since QT interval normally shortens with

increased HR)

QTc

=

QT + VRR (Bazett'

s formula. NOTE:Bazett formula is inaccurate at rapid rates (e.g. > 100/

min))

• normal QTc is 360-450 msec for males and 360-460 msec for females

• increased (>450 msec for males and >460 msec for females): risk of Torsades de Pointes (lethal

tachyarrhythmia;rare if <520 msec)

genetic long QT syndrome (often a channelopathy)

drugs: antiarrhythmics(classes I and 111), antipsychotics (haloperidol, ziprasidone),

antidepressants(dtalopram), antibiotics (erythromycin, azithromycin)

electrolytes:low Ca 2

'

, low K'

, low Mg2*

others:hypothyroidism, hypothermia, cardiomyopathy

• decreased (<360 msec): risk of Vl-

'

ib (very rare)

electrolytes: high Ca 2

'

drugs:digoxin

others:hyperthyroidism

Differential Diagnosis of ST Segment

Changes

ST Elevation-I HELP A PAL

Ischemia with reciprocal changes

Hypothermia (Osbornewaves)

Early repolarization (normal variant,

need old ECGsto confirm)

LBBB

Post-Ml

Acute STEMI

Prinzmetal’s (Vasospastic) angina

Acute pericarditis (diffuse changes)

Left/right ventricular aneurysm

ST Depression - WAR SHIP

WPW syndrome

Acute NSTEMI

RBBB/LBBB

STEMI with reciprocal changes

Hypertrophy (LVH or RVH) with strain

Ischemia

Post-Ml

r

^ LJ

U WAVE

•origin unclear but may be repolarization of Purkinje fibres or delayed/prolonged repolarization of the

myocardium

•more visible at slower heart rates

•deflection follows T wave with <25% of the amplitude

•variationsfrom norm could indicate pathologic conditions

prominent (>25% of f wave):electrolyte (low K+), drugs (digoxin, antiarrhythmics)

• inverted (from T wave):ischemia, volume overload

+

C13Cardiology and Cardiac Surgery Toronto Notes 2023

Cardiac Biomarkers

• provide diagnostic and prognostic information in acute coronary syndromes and in HI- —Troponin - T(early repartition!

5

Table 4. Cardiac Enzymes

f

Z

4troponin i Enzyme Peak Duration Elevated DDx ot Elevation

5

Oj“35 Troponin I,Troponin ? 12-24 h Upto 2 wk Ml.CHF,AFib, acute PE.aortic dissection,

myocarditis, pericarditis, endocarditis, cardiac defibrillation,

myocardial damage, infiltrative cardiomyopathy,ischemic

stroke, intracranial hemorrhage,acute hypotension, chronic

renal insufficiency,sepsis, hypovolemia, acute respiratory

distresssyndrome, chronic hypertension, diabetes mellitus,

hypothyroidism,rhabdomyotysis

Ml.myocarditis, pericarditis,muscular dystrophy,cardiac

defibrillation, chronic renal insufficiency

I

”

iTTnTaflfRAst'

icBtn'iirtuisteffifi!' ' '

Time after acute myocardial infarction|h)

Creatine Kinase-MB 3d

(CK -MB)

1d

Figure 16. Cardiac enzymes

• timing for troponin measurements is dependent on the assay used

high-sensitivity troponin I and T assays detect elevationsin cardiac troponin earlier than

traditional assays

• new CK-MBelevation can be used to diagnose re-infarction

• other biomarkers of cardiac disease

• CK-MB, AST,and lactate dehydrogenase (LDH ) also increases in Ml (low specificity)

BNP and N-terminal prohormone of BNP (NT-proBNP):secreted by ventricles in response to

increased end-diastolic pressure and volume

DDx of elevated BNP:CHF,AFib, PH, pulmonary HTN

Ambulatory ECG

• description

• provides a view of two or three leads of electrocardiographic data over an extended period of time

permits evaluation of changing dynamic cardiac electrical phenomena

• the choice of monitor depends on the patient’s reported symptom frequency

if daily symptoms, use a 24 h or 48 h continuous EGG (Holter) monitor

if less frequent (i.e. weekly or monthly), use prolonged continuous monitoring (1-2 wk) or an

event monitor

• continuous ambulatory monitor: a small, lightweight, battery-operated recorder (box or patch)

which recordstwo or three channels of electrocardiographic data

patient activated event markers

minimum of 24-72 h, up to 14 d

• implantable loop recorder (ILR):subcutaneous monitoring device for the detection of cardiac

arrhythmias

typically implanted in the left pectoral region and stores events when the device is activated

automatically according to programmed criteria or manually with magnet application

generally used for monthsto years

note: devices implanted for bradyarrhythmias (pacemakers) or tachyarrhythmias

(defibrillators) also record rhythm continuously and have algorithms for automatic rhythm

detection and storage

• external event monitor

post-event monitoring device: placed on patient’s chest aftersymptom onset and records

“real-time” rhythm for a specified period (e.g. 30-150 s)

event/loop recorder: constantly records patient’s rhythm for a period of time but only saves

the data when the patient experiencessymptoms and presses the event button (usually 30-60

s recall)

auto-triggered event recorder: uses programmed algorithms to auto-detect, capture, and save

asymptomatic arrhythmias in addition to patient-triggered events

• patient administered single lead E(Xi

wrist or finger electrodes, commercially available direct to consumer (e.g. Apple Watch,

Kardia Mobile)

• indications

evaluation of cardiac rhythm abnormalities, especially asthey correlate with symptoms and

provoking factors

has also been used for assessing pacemaker and implantable cardioverter-defibrillator function,

evidence of myocardial ischemia, late potentials, and HR variability LJ

+

CM Cardiology and Cardiac Surgery Toronto Notes 2023

Echocardiography

Transthoracic Echocardiography

• description

non-invasive ultrasound directed across the chest wall to obtain real time images of the heart

• indications

evaluation of cardiac anatomy and functioningincluding:chamber size,wall thickness,wall

motion,valve morphology,proximal great vessel morphology,and LVE1-

evaluation of clinical cardiac abnormalities including: chest pain with hemodynamic instability,

peripheral edema with elevated )VP,murmurs,unexplainedhypotension,and syncope with

suspected structural cardiac cause

evaluation of suspected cardiac diseases including:aortic dissection,congenital heart disease,LV

thrombus,Ml,pericardial effusion,and pericardial tamponade

Transesophageal Echocardiography

• description

ultrasound probe insertedinto the esophagus to allow for better resolution of the heart and

structures

better visualization ofposterior structures,includingLA,mitral,aortic valves,and inter-atrial

septum

• indications

» initial test incertainlife-threatening situations (e.g.aortic dissection) when other tests

contraindicated (e.g.CT angiography in patient with renal failure or when TTEis technically

inadequate

key indicationis to evaluate valvular morphology,vegetation (e.g.infective endocarditis),and

function (e.g. stenosis and regurgitation) especially of the aortic,mitral,and prosthetic valves if

present

evaluate cardiac disease including:aortic dissection,aortic atheromas,intracardiac thrombi,

tumours,and shunts

evaluation forleft atrial thrombus/left atrial appendage thrombus in a patient with Al'

ib/atrial

flutter to facilitate clinical decision making regarding electrical cardioversion or ablation

. risks

• serious complications are extremely rare (<1 in 5000)

esophageal perforation

• gastrointestinal bleeding

pharyngeal hematoma

Stress Echocardiography

• description

echocardiography using exercise (treadmill or bicycle) or pharmacologic agents (dobutamine or

adenosine) as physiological stressor

• indications

when other stress imaging modalities are unequivocal or whenECG is non-diagnostic

intermediate pre-test probability with normal/equivocal exercise HCG

• post-ACS to decide on potential efficacy ofrevascularization (i.e. myocardial viability)

evaluate the clinical significance of valvular heart disease: AS,MS,MR,or AK

evaluation of cardiac disease:LV systolic dysfunction ofunclear etiology,latent or established

pulmonary HTN,LVOT obstruction in HCM,and syncope ofunclear etiology

• dobutamine

pharmacologic stress for patients physically unable to exercise;same indications as exercise

stress echo

« low dose dobutamine stress echo can be used to assess myocardial viability and to assess AS

with LV systolic dysfunction

• contraindications

absolute contraindications to exercise testing (see below)

• contraindications to dobutamine stress echocardiography:tachyarrhythmias and systemic

hypertension

relative contraindications toboth exercise and dobutamine stress echocardiography:AAA,

electrolyte abnormalities,left main CAD,and moderate stenotic valvular disease

Contrast Echocardiography with Agitated Saline Contrast

• description

improves visualization and provides real-time assessment ofintracardiac blood flow

conventional agent is agitated saline (contains microbubbles of air)

visualization ofright heart and intracardiac shunts,most commonly PFO and intrapulmonary

shunt

• in a normalheart,microbubbles are still seen but only in the right heart and eventually diffuse

into lungs after travelling through pulmonary circulation

n

LJ

+

AL GRAWANY

CISCardiology and Cardiac Surgery Toronto Notes 2023

• indications

detection of right-to-left shunts the presence of microbubbles in the left heart chambersindicates

a right-to-left intracardiac or extracardiac shunt

• Doppler signal enhancement agitated saline enhances tricuspid Doppler signals; this could be

used to assess transvalvular velocity and to estimate right ventricularsystolic pressure

diagnosis of persistent left superior vena cava if contrast injected in left arm vein appears in the

coronary sinus before the RA

Contrast Echocardiography with Transpulmonary Contrast Agents

• description

newer contrast agentssuch as Definity* contrast can crossthe pulmonary bed and achieve left

heart opacification following intravenous injection; these contrast agents improve visualization of

endocardial borders and enhance evaluation of LVEF and wall motion abnormalities(in patients

with technically inadequate echocardiograms) and intracardiac mass(e.g. LV thrombus)

• risks

• major complications (e.g. risk of non-fatal M1 and death are rare)

• ultrasound contrast agents may cause back pain, headache, urticaria, and anaphylaxis

• caution in patients with significant intra-cardiac shunts

Stress Testing

Most Commonly Used Treadmill Stress

Test Protocols

• The Bruce Protocol:7 stage test

with each stage lasting 3min.With

each successive stage, the treadmill

increases In both speed (2.7 km/h to

9.6 km/h) and grade (10% with a 2%

increase perstage up to 22%)

. The Modified Bruce. Modified

Naughton Protocol:for older

individuals or those with limited

exercise capacity

EXERCISE TESTING

• description

• cardiovascularstress test that uses treadmill or bicycle exercise with electrocardiographic and

blood pressure monitoring for the detection of inducible myocardial ischemia, exercise related

symptoms (e.g. arrhythmias), or objective measures of exercise tolerance

exercise test resultsstratify

- patients into 3 risk groups:

1. low-risk:can treat medically without invasive testing

2, intermediate-risk: may need additional testing in the form of exercise imaging studies or

cardiac catheterization

3. high-risk: refer for cardiac catheterization

• indications

patients with intermediate (10-90%) pretest probability of myocardial ischemia (usually due to

CAD) based on age, gender, and symptoms

•

ST depression < 1 mm at rest, LBBB, digoxin or estrogen use make the ST changes difficult to

interpret however, graded exercise stress test can still be valuable

important prognostic and diagnostic information (beyond ST changes) is obtained from

symptoms, total exercise time, HR, and BF response to exercise, if arrhythmia is provoked

note:this is a diagnostic test with false positives and false negatives. Management needs to

take into accountsymptoms and exercise tolerance

• absolute contraindications

acute Ml (within 2 d) or unstable angina pectoris

uncontrolled arrhythmias causing symptoms of hemodynamic compromise

• symptomatic severe valvular stenosis

uncontrolled symptomatic HF

active endocarditis, acute myocarditis, or pericarditis

• acute aortic dissection

acute pulmonary or systemic embolism

acute non-cardiac disorders that may affect exercise performance or may be aggravated by

exercise

termination of exercise testing

• target HR achieved

patient’

s desire to stop

drop in sBF of >10 mmHg from baseline despite an increase in workload, when accompanied

by other evidence of ischemia

• moderate to severe angina

• ST elevation (>1 mm) in leads without diagnostic Q-waves (other than VI oraVR)

* increasing nervoussystem symptoms (e.g. ataxia, dizziness, or nearsyncope)

signs of poor perfusion (cyanosis or pallor)

technical difficultiesin monitoring ECG or sBF

• sustained VT

Important Contraindications to

Exercise Testing

• Acute Ml. aortic dissection.

pericarditis, myocarditis, PE

. Severe AS. arterial HTN

• Inability to exercise adequately

Important Prognostic Factor

Duke Treadmill Score (DTS)

Weighted Index Score

. Treadmill exercise time using

standard Bruce protocol

• Maximum net ST segment deviation

(depression or elevation)

• Exercise-induced angina provides

diagnostic and prognostic

information (such as1yr mortality)

DTS •

exercise time -(5 x MaxST) -

(4 x angina index)

Angina index:0 (no angina),1(angina

but not exercise-limiting), 2 (exerciselimiting angina)

DTS z5:0.25%1 yr mortality

DTS 4 to -10:1.25%1yr mortality

DTS < -11:5.25%1yr mortality

Aim Mem Med1987:106:193-800

• risks L J

death,Ml,arrhythmia, hemodynamic instability,and orthopaedic injury (<1-5/10000 superv ised

tests)

Patients with normal imaging (nuclear

perfusion orstress echo) studies at peak

stress have a <1%/yr incidence of death

or nonfata! Ml and are thus often spared

further invasive evaluation +

C16Cardiology and Cardiac Surgery Toronto Notes 2023

NUCLEAR CARDIOLOGY

• description

• MP1 with ECG-gated single photon emission computed tomography (SPECT), using radiolabelled

tracer

evaluates myocardial viability, detects ischemia, and assesses perfusion and LV function

simultaneously

predictsthe likelihood of future cardiac event ratesindependent of the patients history',

examination, resting ECG,and stress ECG

often denoted as MIBI scan with reference to radiolabelled tracer (sestamibi)

stress with either treadmill or IV vasodilator stress(e.g.dipyridamole, adenosine,regadenoson)

• images of the heart obtained during stress and at rest 3-4 h later

tracers

Thallium-201 (

20,

n. a K‘

analogue)

Technetium-99 (

^Ici-labeled tracer (sestamibi/Cardiolite* or hexamibi/Myoview*)

• indications

to diagnose CAD in possible ACS patients with non-diagnostic ECG and negative serum

biomarker

exercise MP1

when ECG cannot be interpreted appropriately due to LBBB or abnormal baseline ECG

intermediate pre-test probability with normal/equivocal exercise ECG

in patients with previous imaging whose symptoms have changed

to diagnose ischemia

dipyridamole/adenosine MPI

exercise testing is always preferred

pharmacologicalstressimaging test for patients who cannot exercise or do not want to hold

cardiac medications((5-blockers/CCBs)

same indication as exercise MPI

• contraindications

vasodilators (i.e. adenosine,regadenoson, and dipyridamole) are contraindicated in patients with

hypotension,sick sinussyndrome, high-degree AV block (in the absence of backup pacemaker

capability), and reactive airways disease

• pregnancy

• risks

radiation exposure

STRESS ECHOCARDIOGRAPHY

• see Echocardiography,C14

Cardiac Catheterization and Angiography

Right Heart Catheterization (Swan-Ganz Catheter)

• description

also known as pulmonary artery catheterization

• obtain direct measurements of central venous,right-sided intracardiac, pulmonary artery, and

pulmonary artery occlusion pressures

can estimateCO,SVR,and PVR as well as mixed venous oxyhemoglobin saturation,oxygen

delivery, and oxygen uptake

right atrial, right ventricular,and pulmonary artery pressures are recorded

• can also be used to measure the Cardiac Index (Cl),a measure of cardiac function

Cl = CO/body surface area

2.6-4.2 L/min/m -is considered normal while <1.8 L/min/m2usually means cardiogenic

shock

- PCWP

obtained by advancing the catheter to wedge in the distal pulmonary artery'

records pressure measured from the pulmonary venoussystem

in the absence of pulmonary venous disease, reflectsleft atrial pressure

• indications

unexplained or unknown volume status in shock

severe cardiogenic shock (e.g. acute valvular disease,suspected pericardial tamponade)

• suspected or known pulmonary artery HTN

severe underlying cardiopulmonary'disease (e.g.congenital heart disease,left-to-rightshunt,

severe valvular disease,pulmonary HTN) and undergoing surgery (e.g.corrective)

• contraindications

• infection at the insertion site

presence of a right VAD

• insertion during cardiopulmonary bypass

n

+

C17Cardiology and Cardiac Surgery Toronto Xotes 2023

• risks

• complications for diagnostic catheterization:<1%

inadequate diagnostic procedures occur in <1% of cases

complications of insertion:atrial and/or ventricular arrhythmias(~3% of patients)

catheter misplacement or knotting (uncommon)

perforation of a cardiac chamber and rupture of a cardiac valve or the pulmonary artery (rare)

complications of catheterization: pulmonary artery rupture, pulmonary infarction,

thromboembolic events, infection,and data misinterpretation

within 24 h of catheterization:death,Ml,orstroke (0.2% to 0.3% of patients)

Thermistor port

Right atrial port

Pulmonary arteryport

Inflation port Pulmonary

capillary

bed

Pulmonary

Balloon

is inflated er stor

lumen opening

tnc e -

Pulmonary Arteiy

(Balloon wedge)

Right Atrium Right Ventricle Pulmonary Artery

® Kateryna Procunier 2015

Figure17.Swan-Ganz catheter placement

Left Heart Catheterization

• description

accomplished by introducing a catheter into the radial, brachial,or femoral artery and advancing

it through the aorta, acrossthe aortic valve,and into the LV

evaluates mitral and aortic valvular defects and myocardial disease

systolic and end-diastolic pressure tracings recorded

LV size, wall motion, and ejection fraction can be assessed by injecting contrast into the LV (left

ventriculography) via femoral/radial artery catheterization

• indications

identification of the extent and severity of CAD and evaluation of left ventricular function

• assessment of the severity of valvular or myocardial disorders (e.g. AS or insufficiency,MSor

insufficiency, and various cardiomyopathies) to determine the need forsurgical correction

collection of data to confirm and complement non-invasive studies

investigating CAD in patients with confusing clinical features or chest pain of uncertain origin

• contraindications

severe uncontrolled HTN

ventricular arrhythmias

• acute stroke

severe anemia

active gastrointestinal bleeding

allergy to radiographic contrast

acute renal failure

ii

Cbanbers Pressate

(systolic; mmHg]

KsgtstatriumAentral 14

venous

fcg&tmrtiKle

Pitaoaary artery

Left atri.m pulmonary 4-12

capillary wedge

Leftrentrideend 4-12

diastolic

1-8115-30)

4-12 (15-30)

• uncompensated congestive failure (patient cannot lie flat)

unexplained febrile illness or untreated active infection

electrolyte abnormalities (e.g. hyperkalemia)

severe coagulopathy

• risks

major complications of diagnostic catheterization (Le.death,Ml,stroke): <3 in 1000

minor complications (e.g. vascular accessissue, kidney dysfunction): <1 in 100

• inadequate diagnostic procedures occur in <1% of cases

+

CISCardiology and Cardiac Surgery Toronto Notes 2023

Coronary Angiography

description

radiographic visualization of the coronary vessels after injection of radiopaque contrast media

coronary vasculature accessed via the coronary ostia

• indications

to define the coronary anatomy and the degree of luminal obstruction of the coronary arteries

to determine the presence and extent of obstructive CAD

to assess the feasibility and appropriateness of variousforms of therapy,

by percutaneous or surgical interventions

• can be used when the diagnosis of CAD is uncertain and cannot be excluded by non-invasive

techniques

• contraindications

severe renal failure due to contrast agent toxicity (must check patient’

s renal status)

ACC/AHA 2011 Recommended

Indicationsfor Coronary Angiography

• Disabling chronic stable angina (CCS

classesII and IV) despite medical

therapy

• High-risk criteria on clinical

assessment or non-invasive testing

• Serious ventricular anhythmia or CHF

• Uncertain diagnosis or prognosis

after non-invasive testing

• Inability to undergo non-invasive

testing

such as revascularization

• risks

major complications of diagnostic catheterization (Le.death, Ml,stroke);

<3in 1000

minor complications (e.g. vascular access issue, kidney damage): <1 in 100

Coronary Angiography

Gold standard for localizing and

quantifying CAD

^^

arotid Hemodynamically significantstenosisis

defined as 70% or more narrowing of the

luminal diameter 1 - Inferobasal

2 - Inferoapical

3- Apical

4 - Anteroapical

5 - Anterobasal

Figure18. Coronary angiogram schematic

AM = acute marginal:LAD = left anterior descending; OM = obtuse marginal; RAO = right anterior oblique;RCA =

right coronary artery

Diagnostic Catheterization

• provocative pharmacological agents can be used to unmask pathology

fluid loading may unmask latent pericardial constriction

afterload reduction or inotropic stimulation may be used to increase the outflow tract gradient in

HCM

coronary vasoreactive agents(e.g. methylergonovine,acetylcholine)

a variety'of pulmonary vasoreactive agents in primary'pulmonary HTN (e.g.oxygen,CCBs,

adenosine, nitric oxide, prostacyclin)

Contrast-Enhanced CT Coronary Angiography

• description:fast ECG-synchronized multi-sliceCT image acquisition in the heart to enable noninvasive imaging of the coronary arterial tree

• indications:often used to assess coronary artery and previous graft stenosis/viability that could not

be seen during coronary angiography

• sensitivity = 85%,specificity = 90% for the diagnosis of obstructive coronary'disease with >50%

stenosis

• contraindications: allergy to contrast dye;severe renal dysfunction;irregular heart rhythm or

tachycardia which may impact image quality

• risks: radiation exposure; and contrast induced nephropathy

Magnetic Resonance Imaging

• description

offers high spatial resolution,eliminates the need for iodinated contrast, and does not involve

exposure to ionizing radiation

often used with gadolinium injection to assess myocardialscar

• indications

valuable in assessment of congenital cardiac anomalies, abnormalities of the aorta, assessment of

viable myocardium, and assessment of cardiomyopathies

most accurate measure of ejection fraction

especially valuable for assessing RV

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C19Cardiology and Cardiac Surgery Toronto Notes 2023

• contraindications

metallic foreign bodies/implants (e.g. pacemaker,1CD,CRT, cerebral aneurysm clips, metal

shrapnel, piercings)

kidney dysfunction due to gadolinium contrast medium

• risks

hazards posed by certain metallic devices inside patients

CARDIAC DISEASE

Arrhythmias

Mechanisms of Arrhythmias

Alterations in Impulse Formation :<

Sinus Arrhythmia

- Normal P waves, with variation of

the P-P interval, especially with

respiration,due to varying rate of SA

node depolarization

Respiratory SA

• Seen more often in young adults

• Normal, physiologic results from

changes in autonomic tone during

respiratory cycle

• Rate increases with inspiration,slows

with expiration

Non-Respiratory SA

• Seen more often in the elderly

• Can occur in the normal heart; if

marked may be due to sinus node

dysfunction (e.g.in heart disease or

after digitalistoxicity)

. Usually does not require treatment

A.Normal Automaticity

impulsesfrom the SA node,caused by spontaneous depolarization, result in the basic cardiac

pacemaker function. “Downstream"

cellsin the AY node and Purkinje fibres also depolarize

spontaneously, but at a slower rate; they serve asthe “backup"

pacemaking cells if the upstream

rate isslower than the more distal spontaneous rate

normal automaticity is influenced by:

• neurohormonal tone (sympathetic tone increases and parasympathetic tone decreases

spontaneousfiring rate and thus HR)

• myocardial ischemia/infarction or other cardiac pathology (e.g. HF) may alter HR via these

mechanisms

• abnormal metabolic conditions (e.g. hypoxia,acidosis, hypothermia)

• electrolyte abnormalities, especially hyperkalemia which slows HR

• drugs (e.g.digitalis, p-blockers, CCB)

• athletic training:endurance athletes often have sinus bradycardia

• age:elderly often have sinus bradycardia

• increased circulatory demand can result in sinustachycardia (e.g.pregnancy, anemia,

exercise)

B. Abnormal Automaticity due to Triggered Activity (due to Afterdepolarizations)

1.Early Afterdepolarizations

during the terminal plateau or repolarization phases of action potential

• consequence of the membrane potential transiently becoming more positive during

rcpolarization (depolarization interrupting repolarization)

• these are called EADs and DADs (early and delayed afterdepolarization, respectively)

may result in self-maintaining oscillations of depolarization,giving rise to action potentials

thereby generating a tachyarrhythmia (e.g. new baseline voltage is greater than threshold,which

automatically triggers a new action potential after the refractory period ends)

EADs are the basisfor the arrhythmias associated with QT prolongation, either congenital or

acquired;termed “Torsades de Pointes”

2.Delayed Afterdepolarizations

occur after the action potential hasfully repolarized,but before the next usual action potential

commonly occursin situations of high intracellular calcium (e.g.digitalis intoxication, ischemia)

or during enhanced catecholamine stimulation (e.g.“twitchy” pacemaker cells)

Alterations in Impulse Conduction

A.Re-Entry Circuits

the presence ofself-sustaining re-entry circuit causes rapid repeated depolarizations in a

region of myocardium (see figure 27,C24,for an example in the context of AV nodal re-entrant

tachycardia)

the conditions necessary for re-entry include block of an impulse into a region of the heart

that is refractor)'(non-excitable tissue or because of local functional block, where the impulse

encounterstissue still in its refractor)'period),followed by “re-entry" of the impulse around a

region of block to the site of origin,forming a complete re-entry circuit

e.g.myocardium that isinfarcted/ischemic will consist of non-excitable and partially

excitable zones which will promote the formation of re-entry circuits

mostsustained tachyarrhythmias are due to re-entry

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C20 Cardiology and Cardiac Surgery Toronto Notes 2023

B.Conduction Block

ischemia, fibrosis,trauma, and drugs can cause transient or permanent, unidirectional or

bidirectional block

• most common cause of block is “functional block"due to refractory myocardium

(cardiomyocytes are in refractory period) or “anatomical block” (area of myocardium unexcitable

due to fibrosis);cells in the conduction system distal to the block can assume pacemaking control

if the block occurs along the specialized conduction system

the consequence of conduction block are reentry arrhythmias (tachyarrhythmias- see above) or

failure of impulses to conduct to ventricular cells (bradycardia)

• conduction block in the AV node or His Furkinje tissue can lead to bradycardia

C.Bypass Tracts

normally, the only electrical connection between atria (As) and ventricles (Vs) isthe AV node and

connected penetrating Bundle of His;

an accessory bypass tract is a direct connection between A and V, histologically similar to atrial

tissue, through the valve ring which is normally impenetrable to electrical impulses (hence

"accessory atrio-ventricular bypass tract”)

see Pre-Excitation Syndromes.C25

Arrhythmias

1

Bradyarrhythmias l<G0 bpm)

• Sinus bradycardia

• Sinoatrial block

• Sinus arrest

• AV block (2nd and 3rd degree)

• Junctional rhythm

• Idioventricular rhythm

Tachyarrhythmias (>100 bpm)

1

;

Regular Irregular

1

; ; :

Narrow QRS (SVTs)

• AFib

• Atrial flutter with variable

block

• Multifocal atrial

tachycardia

•Premature artrial

contraction

Narrow QRS (SVTs)

• Sinus tachycardia

• Atrial tachycardia

• Junctional tachycardia

• AVNRT

• AVRT (orthodromic)

• Atrial flutter

Wide QRS

• SVT with aberrancy/BBB

• Ventricular tachycardia

• AVRT (antidromic)

Wide QRS

• AFib with BBB

• Atrial flutter with BBB and

variable block

• Polymorphic VT

•Premature ventricular

contraction

Figure 19. Clinical approach to arrhythmias

Bradyarrhythmias

Table 5. Types of Bradyarrhythmias

1.SA NODAL DYSFUNCTION

• P axis normal ( P waves positive in I and aVF)

• Rate < 60 bpm;marked sinus bradycardia (<S0 bpm)

• May be seen in normal adults,particularly athletes, and in elderly individuals

• Increased vagal tone or vagalstimulation

• Drags (e.g. P-blockers.CCS)

• Iscbemiafinfarction

Atropine: pacing lor sick {

sinussyndrome

Figure 20. Sinus bradycardia

2.AVCONDUCTION BLOCKS

A.first Degree AV Block

. Prolonged PR interval (»220 msec)

• Frequently found among otherwise healthy adults

No treatment required

Figure 21.First degree AV block

B.Second Degree AV Block;Type I (Mobitz I)

. Gradual prolongation of the PR interval precedes the failure of conduction of

a P wave (Wenckebach phenomenon)

. AV block is usually in AV node (proximal) triggers (usually reversible):

increased vagal tone (eg.following surgery). RCAmediated ischemia

J c r 1

i

t

_ J

.