MED213 Clinical Insights: Understanding Electrophysiology

Risk Stratification & Exercise

• Risk Stratification:

  • Pre-participation screening is a part of risk assessment and risk management, based on the ACSM guidelines.

  • Screen for signs/symptoms of disease, known cardiovascular (CV), metabolic, or renal disease, and CVD risk factors.

  • Identify individuals at risk for acute exercise-related CV events, including sudden cardiac death and myocardial infarction.

  • Determine the need for medical clearance.

  • Old ACSM guidelines are referenced.

• Clinical Populations:

  • More likely to have known disease without symptoms or any signs/symptoms suggestive of disease.

• Screening Process (ACSM):

  • Medical clearance is needed.

  • If cleared, proceed with light to moderate exercise.

• Supervision Levels:

  • Low risk: unsupervised (usually safe to do alone).

  • Medium risk: HCP with experience in clinical exercise testing.

  • High risk: HCP with experience in clinical exercise testing, AND a trained doctor immediately available if required.

  • ILS-trained personnel (at least) for drug administration in case of a CV event.

• Safety Considerations:

  • Safety for both the healthcare provider (YOU) and the participant is key.

Exercise Contraindications

• Absolute Contraindications (Refer to GP):

  • Unstable angina.

  • Unstable or acute heart failure.

  • Unstable diabetes.

  • New or uncontrolled arrhythmias.

  • Resting or uncontrolled tachycardia.

  • Resting SBP > 180/200 mmHg or Resting DBP > 100/110 mmHg.

  • Symptomatic hypotension.

  • Febrile illness/Acute infections.

• Assessments Before Exercise:

  • Ask questions and obtain an ECG.

  • Check glucose levels.

  • Measure BP.

Exercise Termination Criteria

• Target HR achieved.

• Chronotropic incompetence.

• Hypertensive responses (>$230 mmHg and/or >110 mmHg).

• Drop in SBP (20 mmHg) or failure to increase with intensity.

• Verbal and/or physical manifestations of fatigue.

• ECG shows arrhythmias or ST depression >1 mm.

• Onset of angina/angina-like symptoms.

• Participant's request to stop.

• Equipment failure.

ECG Components

• P wave: Atrial depolarization.

• QRS complex: Ventricular depolarization.

• PR interval: Atrioventricular conduction time.

• T wave: Ventricular repolarization.

• Heart Rate Determination.

• Rhythm strip: Lead used for rhythm analysis.

• Key Intervals & Segments:

  • PR interval: 0.12-0.22 s

  • QRS duration: <<0.12 s

  • Corrected QT duration:

    • Men: ≤ 0.45 s

    • Women: ≤ 0.47 s

  • Reference level for ST-segment deviation: PR segment (isoelectric or baseline level).

ECG Waveform Durations & Values

• P wave: Max 0.12s

• PR interval: 0.12 -0.20 s

• QRS interval: 0.06-0.1s (max 0.12s)

• P wave amplitude: Max 2.5mm

• QRS amplitude: Max 30mm

• ST segment sets isoelectric line.

• T wave review for ischemia.

• QTc: < 0.44s (Male); < 0.46s (Female)

Cardiac Action Potentials

• Phase 0: Rapid influx of Na^+

• Phase 1: K^+ influx

• Phase 2: Influx of Ca^{2+ }balanced by K^+ efflux (plateau)

• Phase 3: Ca^{2+} channels close, K^+ channels stay open

• Phase 4: Na^+ & Ca^{2+} close, K^+ open

• Cardiac excitation-contraction coupling.

ECG Waveforms - Depolarization

• Wave of depolarization travels toward the positive electrode = positive deflection.

• Wave of depolarization travels away from the positive electrode = negative deflection.

Coronary Blood Flow

• Coronary arteries branch off the aorta to supply blood and oxygen to the myocardial cells (cardiomyocytes).

• Right Coronary Artery (RCA): Supplies RA, RV, SAN, AVN and posterior LV.

• Left Coronary Artery: Splits into LAD and CA, supplying LA and LV.

• Coronary blood flow: 250 ml min ^{-1} (0.8 ml min^{-1} g^{-1} of heart muscle) = 5% of resting Q.

• Deoxygenated blood leaves myocardial tissue of the LV via veins and collects in the coronary sinus à drains into RA.

• High capillary-to-cardiomyocyte ratio & short diffusion distances ensure adequate O2 delivery to cardiomyocytes and removal of metabolic waste (CO2 & H^+).

• Very high VO2 and highest a-vO2 difference (extracts 70-80% of O2 from blood).

• Sympathetic activation (exercise) à coronary vasodilation via β1-adreno-receptor activation à increased coronary blood flow.

• 2.5 times more blood flows in coronary vessels during diastole than systole.

• Atherosclerosis disrupts/impairs coronary blood flow.

Coronary Blood Flow and ECG

• Myocardial ischemia: Lack of blood flow to heart muscle.

• Relationship between artery occlusion, ECG leads, and myocardial region:

  • Inferior: RCA, leads II, III, avF.

  • Antero-septal or Antero-apical: LAD, leads V1-4.

  • Antero-lateral: CA, leads V3-6.

  • Posterior: RCA, no specific leads.

• Most common MIs are related to occlusions within the mid-lower section of the LAD.

• MI = myocardial infarction.

Common Arrhythmias

• Sinus tachycardia: Increased HR.

• Sinus bradycardia: Decreased HR.

• Sinus arrhythmia: Irregular rhythm (often normal).

• Sinus = Regular.

Premature Ventricular Contractions (PVCs)

• Unifocal PVCs: From RV outflow.

• Multifocal PVCs: More concerning.

• Bigeminy or Trigeminy.

• PVC = premature beat arising from an ectopic focus within the ventricles.

Ventricular Tachycardia (VT) & Fibrillation

• More than 3 PVCs = Ventricular Tachycardia (non-sustained).

• Sustained VT = impairs Q, reduces myocardial perfusion and could lead to VF.

• Ventricular Fibrillation (VF): Chaotic, irregular, varying amplitude, no identifiable waves, Rate 150 to 500/min = DEFIB.

Atrial Fibrillation (AF)

• 5-10% older adults.

• “Irregularly irregular” ventricular rhythm: rapid and chaotic depolarization within atria à erratic transmission of impulses at AV node.

• No P waves seen.

• QRS rate 170/min (variable).

• Fibrillatory waves (often V1): fine <<0.5mm, coarse >0.5mm.

• Increases risk of thrombosis/ embolism i.e. 20% of all strokes.

Wolf-Parkinson-White (WPW) Syndrome

• Very short PR intervals (0.08s).

• 2nd A-V connection in addition to normal conduction via AV node.

• Accessory pathway conducts quicker than AV node (pre-excitation).

• Sooner depolarization shown by a delta wave.

• Can be seen in athletes.

Atrioventricular (AV) Blocks

• 1st, 2nd, 3rd degree.

• Prolonged PR interval on the ECG.

• 3rd degree AV block – A (100bpm), V (40bpm).

Bundle Branch Blocks

• Right Bundle Branch Block (RBBB):

  • Delayed electrical activation of RV as must be depolarized by the LBB.

  • Distorted QRS shape due to abnormal depolarization pathway.

  • Wide QRS >3mm (120ms).

  • RSR’ pattern in V1-3 (‘M-shaped’ QRS complex).

  • Wide, slurred S wave in the lateral leads (I, aVL, V5-6).

  • Sometimes ST depression/T wave inversion in the right leads (V1-3).

  • RBBB is quite common, often seen in athletes and has no major symptoms.

  • Morro w QRS = M in V1. QRS = W in V6

• Left Bundle Branch Block (LBBB)

  • Do not do an exercise ECG test on someone with LBBB.

  • RV depolarizes normally and first via RBB.

  • Delayed electrical activation of LV as must be depolarized by the RBB with septal depolarization from R to L.

  • Distorted (notched/M) QRS shape due to abnormal depolarization pathway i.e. sequentially R-L and not simultaneously.

  • Deep S waves in V1-V3 (dominant V1).

  • Broad R waves in lateral leads (I,aVL,V5- V6).

  • Wide QRS >3mm (>120ms).

  • Poor R-wave progression.

  • Needs attention!

  • Willia m QRS = W in V1. QRS = M in V6

Left Ventricular Hypertrophy (LVH)

• Can be “Normal” in athletes.

• Seen if people have thin chest walls.

• Seen with uncontrolled/long-lasting hypertension and aortic stenosis.

• LVH = S wave depth V1 + tallest R wave height in V5-V6 > 35 mm PLUS ST depression & T inversion in left-sided leads.

• Thickened walls leads to:

  • Prolonged depolarization (R) and delayed repolarization (ST & T) in lateral leads.

• Increased R in I, avL, V4-6.

• Increased S in III, avR, V1-3.

Ischemia & Myocardial Infarction

• ST Depression: ≥1mm$$, most common and useful indication of myocardial ischemia; likely to see during exercise test and in several leads (usually I, II, V4-V6).

• T-wave inversion (or flattening) can be seen but not always.

• ST Elevation: Most common and useful indication of myocardial infarction (STEMI); location of the ST elevation = region of infarct & likely artery occlusion (e.g., Anterior STEMI (V2-V5) is likely to be occluded in LAD artery).

ECG Use in Athletes

• Varies across countries.

• Uncommon in USA.

• Refer to the 2018 international recommendations (Sharma et al. 2018).

• Dhutia & MacLachlan (2018) reported that the new recommendations have:

  • Reduced false positive ECG rate to 3%.

  • Reduced cost of screening by 25%.

Causes of Sudden Cardiac Death (SCD) in Young Sportspeople

• Congenital/genetic pathology

  • Hypertrophic cardiomyopathy

  • Arrhythmogenic ventricular cardiomyopathy

  • Dilated cardiomyopathy

  • Congenital coronary artery anomalies

  • Premature atheromatous coronary artery disease

  • Wolff-Parkinson-White syndrome

  • Right ventricular outflow tachycardia

  • Mitral valve prolapse

  • Congenital aortic stenosis

  • Marfan syndrome

  • Congenital long QT syndrome

  • Catecholaminergic polymorphic ventricular tachycardia

  • Brugada syndrome

• Acquired causes

  • Infections (myocarditis)

  • Drugs (cocaine, amphetamine)

  • Electrolyte disturbances (hypokalemia or hyperkalemia)

  • Hypothermia

  • Hyperthermia

  • Trauma (commotio cordis)
    *Highlighted causes that can be detected via a resting ECG, per Dhutia & MacLachlan, 2018.

ECG Findings in Athletes

• Normal ECG Findings

  • Increased QRS voltage for LVH or RVH

  • Incomplete RBBB

  • Early repolarization/ST segment elevation

  • T wave inversion V1-V3 < age 16 years

  • ST elevation followed by T wave inversion V1-V4 in black athletes

  • Sinus bradycardia or arrhythmia

  • Ectopic atrial or junctional rhythm

  • 1° AV block

  • Mobitz Type 12° AV block

• Borderline ECG Findings

  • Left axis deviation

  • Left atrial enlargement

  • Right axis deviation

  • Right atrial enlargement

  • Complete RBBB

  • In isolation

  • 2 or more

  • No further evaluation required in asymptomatic athletes with no family history of inherited cardiac
    disease or SCD

• Abnormal ECG Findings

  • T wave inversion

  • ST segment depression

  • Pathologic Q waves

  • Complete LBBB

  • QRS ≥ 140 ms duration

  • Epsilon wave

  • Ventricular pre-excitation

  • Prolonged QT interval

  • Brugada Type 1 pattern

  • Profound sinus bradycardia < 30 bpm

  • PR interval ≥ 400 ms

  • Mobitz Type II 2° AV block

  • 3° AV block

  • ≥ 2 PVCs

  • Atrial tachyarrhythmias

  • Ventricular arrhythmias

• Further evaluation required to investigate for pathologic cardiovascular disorders associated with SCD in athletes