Non-invasive Cardiac Diagnostics

ECG - Electric Heart Beats

• Records heart's electrical activity. Standard 12 leads: 6 limb (I, II, III, aVR, aVL, aVF), 6 precordial (V1-V6).
• Calibration: 1mV = 10mm. Paper speed: 25mm/s (1 small square = 0.04s; 1 large square = 0.2s).
• Key Waveforms & Intervals:
  • P wave (Atrial depol.): < 0.12s dur, < 2.5mm amp.
  • PR interval (AV conduction): 0.12-0.20s.
  • QRS (Ventricular depol.): < 0.12s.
  • ST segment: Isoelectric. ↑/↓ = ischemia/infarct.
  • T wave: Ventricular repolarization.
  • QT interval (Ventricular activity). QTc (Bazett's: $QTc = QT / \sqrt{RR}$): < 0.44s M, < 0.46s F.
• Systematic Interpretation:

• 

⭐ Wellens' syndrome: Deeply inverted/biphasic T waves in V2-V3, specific for critical LAD stenosis.

Echocardiography - Ultrasound Heart Peeks

Uses ultrasound waves for real-time imaging of heart anatomy, function, and hemodynamics.

• Modes & Utility:
  • 2D/M-mode: Structure, dimensions, LVEF (Normal: >55%).
  • Doppler (Color, PW, CW): Blood flow, valvular lesions, shunts. $P = 4v^2$ (Modified Bernoulli for pressure gradients).
  • TEE (Transesophageal Echo): Superior for posterior structures (LA/LAA, MV, aorta), endocarditis, aortic dissection.
  • Stress Echo: Ischemia detection (new Regional Wall Motion Abnormalities - RWMA with stress).
  • Contrast Echo: LV opacification, perfusion enhancement.
• Key Applications:
  • Valvular heart disease (stenosis/regurgitation severity).
  • Cardiomyopathies (HCM, DCM, RCM).
  • Ischemic Heart Disease (IHD): RWMA, complications (e.g., VSR, MR).
  • Pericardial effusion, tamponade.
  • Diastolic dysfunction (E/A ratio, E/e' ratio).

⭐ TEE is superior to TTE for visualizing small vegetations (<5mm) in infective endocarditis and for detecting left atrial appendage (LAA) thrombus prior to cardioversion for atrial fibrillation.

Stress Testing - Heart Under Pressure

Evaluates cardiac function and ischemic response to stress.

• Types & Modalities:
  • Exercise ECG (TMT): Bruce protocol. Preferred if able to exercise.
  • Pharmacological Stress:
    • Dobutamine Stress Echo (DSE): For wall motion.
    • Vasodilator Stress MPI (Adenosine, Dipyridamole, Regadenoson): Perfusion defects (coronary steal).
• Positive ECG: ST↓ ≥1mm (horizontal/downsloping); ST↑ ≥1mm (no prior Q-waves).
• Termination: Severe angina, ST↓ >2mm, ST↑ >1mm (no Q), ↓SBP >10mmHg, VT, target HR.
• Contraindications: Acute MI (<2 days), unstable angina, severe AS, uncontrolled HTN (>200/110 mmHg).

  ⭐ Duke Treadmill Score (DTS) predicts risk. High risk: score ≤ -11.

• DTS Formula: $Exercise\ time\ (min) - (5 \times ST\ deviation\ mm) - (4 \times Angina\ index)$

Ambulatory & Advanced Imaging - Beyond Clinic Walls

• Ambulatory ECG:
  • Holter: 24-48hr continuous; frequent symptoms (palpitations, syncope).
  • Event Recorders (Loop): Weeks-months; patient/auto-triggered for infrequent symptoms.
  • Implantable Loop Recorder (ILR): Subcutaneous, up to 3 years; rare, severe symptoms.
• Advanced Imaging:
  • Cardiac MRI (CMR): Gold standard for LV/RV volumes, function, tissue characterization (fibrosis, scar).
  • Cardiac CT (CCT): Coronary Artery Calcium (CAC) score for risk; CT Coronary Angiography (CTCA) for CAD anatomy.
  • Nuclear (SPECT/PET): Myocardial perfusion (ischemia/viability); PET for inflammation (e.g., sarcoidosis).

⭐ CMR is the gold standard for assessing right ventricular (RV) structure and function, key in ARVC.

High‑Yield Points - ⚡ Biggest Takeaways

• ECG is crucial for acute coronary syndromes (ACS) and arrhythmia diagnosis.
• Stress testing (TMT/Stress Echo) assesses for inducible ischemia; Dobutamine Stress Echo if unable to exercise.
• Echocardiography is key for structural heart disease, valvular function, and ejection fraction (EF).
• Holter monitoring for paroxysmal arrhythmias; Event recorders for infrequent symptoms.
• CT Coronary Angiography (CTCA) is excellent for ruling out CAD (high NPV).
• Cardiac MRI (CMR) is superior for myocardial tissue characterization, viability, and cardiomyopathies.