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Complete Guide to Reading an ECG and CT Scan

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PART 1: HOW TO READ AN ECG (ELECTROCARDIOGRAM)

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What is an ECG?

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The Basics: ECG Waveforms and Components

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Step-by-Step ECG Interpretation System

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STEP 1 - RATE

• 1 large box = 300 bpm
• 2 large boxes = 150 bpm
• 3 large boxes = 100 bpm
• 4 large boxes = 75 bpm
• 5 large boxes = 60 bpm
• 6 large boxes = 50 bpm

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STEP 2 - RHYTHM

1. Are there P waves before every QRS?
2. Are the R-R intervals regular?
3. Is the QRS narrow (< 0.12 sec = supraventricular) or wide (>= 0.12 sec = ventricular origin or aberrant conduction)?

• Sinus bradycardia: Normal P-QRS-T, rate < 60
• Sinus tachycardia: Normal P-QRS-T, rate > 100
• Atrial fibrillation (AF): No visible P waves, irregularly irregular R-R intervals, "fibrillatory baseline"
• Atrial flutter: Sawtooth P waves (flutter waves) at ~300/min, typically 2:1 or 4:1 block giving ventricular rate of 150 or 75
• Ventricular tachycardia (VT): Wide, bizarre QRS > 0.12 sec, no P waves, rate > 100 - life threatening
• Ventricular fibrillation (VF): Chaotic, no recognizable complexes - cardiac arrest

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STEP 3 - AXIS

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STEP 4 - ELECTRODE PLACEMENT AND TERRITORIES

• Lead I: Right arm (-) to Left arm (+)
• Lead II: Right arm (-) to Left leg (+)
• Lead III: Left arm (-) to Left leg (+)
• aVR: Augmented - looks at the heart from the right shoulder
• aVL: Augmented - looks from the left shoulder
• aVF: Augmented - looks from the feet (inferior)

• V1: Right side of sternum, 4th intercostal space
• V2: Left side of sternum, 4th intercostal space
• V3: Between V2 and V4
• V4: Midclavicular line, 5th intercostal space
• V5: Anterior axillary line (same level as V4)
• V6: Midaxillary line (same level as V4-V5)

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STEP 5 - P WAVES AND PR INTERVAL (AV Conduction)

• Right atrial enlargement (RAE): Tall peaked P wave > 2.5 mm in lead II ("P pulmonale")
• Left atrial enlargement (LAE): Broad notched P wave > 0.12 sec in lead II ("P mitrale"), biphasic P in V1

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STEP 6 - QRS COMPLEX (Bundle Branches)

• LBBB (Left BBB): W-shape in V1, M-shape in V6 - "WiLLiaM"
• RBBB (Right BBB): M-shape in V1 (RSR' pattern = "rabbit ears"), W-shape in V6 - "MaRRoW"

• LVH (Left Ventricular Hypertrophy): S in V1 + R in V5 or V6 > 35 mm (Sokolow-Lyon criteria); or R in aVL > 11 mm
• RVH (Right Ventricular Hypertrophy): Dominant R in V1 (R > S), RAD, right "strain" pattern (T inversion V1-V3)

• Width > 1 small box (0.04 sec) OR depth > 1/3 of QRS height
• Represent prior myocardial infarction (transmural necrosis)
• Location tells you which territory was affected

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STEP 7 - ST SEGMENT AND T WAVES (Ischemia)

• New ST elevation ≥ 1 mm (0.1 mV) in 2 or more contiguous limb leads
• New ST elevation ≥ 2 mm (0.2 mV) in 2 or more contiguous precordial leads
• In men < 40: ≥ 2.5 mm in V2-V3
• ST elevation is convex upward ("tombstone" or "domed")

• Horizontal or downsloping ST depression ≥ 0.5-1 mm in 2+ contiguous leads
• Upsloping depression is less specific

• Peaked/hyperacute T waves: Very early ischemia (before ST elevation develops)
• T wave inversion: Ischemia, ventricular strain, CNS events, PE (lead III + V1)
• Biphasic T waves: Wellens syndrome (critical LAD stenosis - T waves go positive-negative or negative-positive in V2-V3)

• Diffuse ST elevation in ALL leads (no territory)
• PR depression (pathognomonic)
• Saddle-shaped ST elevation
• No reciprocal changes

• Hyperkalemia: Peaked T waves → widened QRS → sine wave pattern
• Hypokalemia: Flattened T waves, prominent U waves, prolonged QT
• Hypercalcemia: Short QT interval
• Hypocalcemia: Long QT interval
• Pulmonary embolism: S1Q3T3 pattern (S wave in I, Q wave and T inversion in III) + sinus tachycardia + RBBB

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STEP 8 - QT INTERVAL

• Measure from start of QRS to end of T wave
• Must correct for heart rate: use Bazett's formula: QTc = QT / √(RR interval in seconds)
• Normal QTc: < 440 ms (men), < 460 ms (women)
• Prolonged QTc risk: Torsades de pointes → VF → sudden death
• Causes: Drugs (quinidine, amiodarone, haloperidol, methadone, some antibiotics), electrolyte disorders, congenital long QT syndromes

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Textbook ECG Strip Examples from Braunwald's Heart Disease

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PART 2: HOW TO READ A CT SCAN

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What is a CT Scan?

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STEP 1 - Understand Hounsfield Units (HU)

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STEP 2 - Understand CT Windowing

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STEP 3 - Reading a CT Head (Brain)

• Sulci and gyri symmetry
• Grey/white matter differentiation
• Ventricular system (lateral ventricles, 3rd ventricle, 4th ventricle) - should be symmetric
• Cisterns (basal cisterns around brainstem)
• Midline structures - no shift should be present

1. Blood - any hyperdense (white) areas in the parenchyma, subarachnoid space, ventricles, or extra-axial spaces?
2. Brain - grey/white differentiation preserved? Any areas of hypo- or hyperdensity?
3. CSF spaces - ventricles and sulci normal? Midline shift?
4. Bone - skull fractures, erosions?

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STEP 4 - Reading a CT Chest (Thorax)

1. Lung parenchyma: Any areas of increased density (consolidation, ground-glass opacity, nodules)?
2. Airways: Trachea central? Bronchi visible to 3rd/4th generation?
3. Pleura: Any pleural effusion (hypodense fluid tracking dependent), pneumothorax (hypodense rim with no lung markings)?
4. Fissures: Major (oblique) and minor (horizontal) fissures visible?

1. Mediastinum: Widened? (> 8 cm at aortic arch suggests aortic dissection)
2. Heart: Size, pericardial effusion?
3. Great vessels: Aorta - diameter, dissection flap?
4. Lymph nodes: Short axis > 10 mm is abnormal

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STEP 5 - Reading a CT Abdomen

• Non-contrast: Good for stones, calcifications, acute blood
• Arterial phase (25-35 sec): Arteries bright, organ arterial supply
• Portal venous phase (70-80 sec): Liver, spleen, kidneys at peak enhancement - MOST USEFUL phase
• Delayed phase (3-5 min): Renal collecting system, bladder

1. Liver (right side, largest organ): Homogeneous, moderate density. Look for focal lesions (hypodense = cysts/metastases; hyperdense = HCC with arterial enhancement; calcified = old infection). Measure HU - fatty liver < 40 HU (less than spleen).
2. Gallbladder: Thin-walled, fluid-filled (black). Stones = dense white (calcified) or may not be visible.
3. Spleen (left side): Homogeneous, slightly denser than liver. Look for lacerations (jagged hypodense lines), infarcts (wedge-shaped hypodense area).
4. Pancreas (transverse, anterior to IVC and aorta): Uniform, head on right, tail on left. Pancreatic duct (< 3 mm). Pancreatitis = swelling + peripancreatic stranding (haziness in fat).
5. Kidneys (retroperitoneal, bilateral): Look for stones (hyperdense), hydronephrosis (dilated collecting system), masses (hyperdense = RCC with enhancement; hypodense = cysts).
6. Aorta (midline, left of IVC): Normal < 3 cm diameter. > 3 cm = aneurysm. Dissection = intimal flap visible.
7. Appendix: Look in right iliac fossa. Normal < 6 mm, no fat stranding. Appendicitis: > 6 mm, wall thickening, periappendiceal fat stranding.
8. Bowel: Normal wall < 3-4 mm, no stranding. Thickened wall + mesenteric stranding = inflammation/ischemia.
9. Free fluid / free air: Fluid appears black in dependent spaces (perihepatic, perisplenic, pelvis). Free air (black) under diaphragm or anterior abdomen = bowel perforation.

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STEP 6 - CT with Contrast: What to Look For

• Homogeneous enhancement: Benign (cyst, adenoma)
• Rim enhancement: Abscess or necrotic tumor
• Arterial phase enhancement: Hypervascular lesion (HCC, carcinoid, RCC, hemangioma)
• No enhancement: Cyst, necrosis, non-vascular structure
• Washout on delayed phase: Strongly suggests HCC

• Contrast allergy risk - check history
• Nephrotoxicity - check creatinine/GFR before IV contrast
• Metformin should be held before contrast (risk of lactic acidosis)
• Never give IV contrast intrathecally

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Quick Reference Summary Tables

ECG Interpretation Checklist

CT Scan Window Quick Reference

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Key Sources

• Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine - ECG chapters 61, 80
• Harrison's Principles of Internal Medicine, 22nd Edition (2025) - Chapter 297 (CT imaging, Hounsfield units)
• Rosen's Emergency Medicine - ECG and CT interpretation chapters
• ECGWaves.com - Normal ECG characteristics
• ECGWeekly.com - Intervals and segments reference
• Kenhub - Abdominal CT - Abdominal anatomy guide

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Clinical note: This guide is for educational purposes. ECG and CT interpretation in clinical practice requires direct training, supervised reading, and correlation with patient history and examination. Never make clinical decisions based on imaging alone without appropriate training and clinical context.

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Complete Guide to Reading X-Rays and CT Scans

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PART 1: HOW TO READ AN X-RAY (RADIOGRAPH)

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What is an X-Ray?

The 5 Densities on Any X-Ray

Golden rule: You can only see a border between two structures if they have DIFFERENT densities. The "silhouette sign" - loss of a normally visible border - indicates the two adjacent structures have become the same density (e.g., consolidation touching the heart border erases it).

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SECTION A: CHEST X-RAY (CXR)

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Step 1 - Technical Quality Assessment (RIPE)

• PA = patient stands, chest against detector, X-ray beam enters from behind. Best quality, minimizes cardiac magnification. Used in outpatient and standing patients.
• AP = portable/bedside, X-ray beam enters from front. Heart appears up to 15% larger. Used for sick, ICU, or trauma patients.

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Step 2 - Normal CXR Anatomy (ABCDE Approach)

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A - Airway

1. Trachea: Should be midline or slightly deviated to the right at the aortic arch (normal variant). Significant deviation suggests:
   • Deviation TOWARD pathology: Lung collapse, fibrosis (pulling)
   • Deviation AWAY from pathology: Pneumothorax, large pleural effusion, tension PTX, large mass (pushing)
2. Carina: Located at T4-T5 level. Normal angle < 70°. Splaying of the carina (> 70°) suggests left atrial enlargement.
3. Right and left main bronchi: Right is shorter, wider, more vertical (why foreign bodies more commonly lodge here).
4. Hilar regions: The hila contain pulmonary arteries, veins, and bronchi. Left hilum normally sits slightly higher than the right. Hilar size: right pulmonary artery should be < 16 mm on the PA view.
   • Hilar enlargement: Bilateral (sarcoidosis, lymphoma, TB, primary pulmonary hypertension) vs Unilateral (neoplasm, TB)
   • Hilar position: Pulled up (upper lobe collapse), pulled down (lower lobe collapse)

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B - Bones (and Soft Tissues)

• Fractures (acute: irregular cortical break; old/healed: smooth callus)
• Notching of inferior rib margins (coarctation of aorta - from dilated intercostal collaterals)
• Lytic lesions (metastases, myeloma)

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C - Cardiac (and Mediastinum)

• Measure the widest transverse diameter of the heart
• Divide by the widest internal thoracic diameter
• Normal CTR: ≤ 0.5 (50%) on a PA film
• CTR > 0.5 = cardiomegaly (causes: dilated cardiomyopathy, pericardial effusion, ventricular hypertrophy, valvular disease)
• Note: CTR is unreliable on AP films (heart always appears bigger)

• Right heart border: Right atrium (lower half) + Superior vena cava (upper half)
• Left heart border: Left ventricle (lower 2/3) + Aortic knob (top) + Left atrial appendage (middle, visible only when enlarged) + Pulmonary trunk

• Superior mediastinum width: Should not exceed 8 cm on PA (> 8 cm = widened mediastinum: consider aortic dissection, lymphoma, substernal goitre, thymoma)
• Aortic knob: Left lateral contour of aortic arch - should be clearly visible. Loss of outline = periaortic pathology
• Aortopulmonary window: Space between aortic arch and left pulmonary artery. Fills in with lymphadenopathy (sarcoidosis, lymphoma)
• Paratracheal stripe: Right side of trachea - normally < 4 mm wide. Widening = paratracheal lymphadenopathy, haemorrhage, right pleural effusion

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D - Diaphragm

• Right hemidiaphragm is normally higher than the left (by 1.5-2.5 cm) because of the liver underneath
• Both hemidiaphragms should form smooth, sharp domes
• Costophrenic (CP) angles should be sharp and acute (< 90°). Blunting of CP angle requires ~200-300 mL of pleural fluid to develop
• Check for subdiaphragmatic free gas (pneumoperitoneum) = dark crescent of gas below the diaphragm, above the liver

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E - Everything Else (Lung Fields)

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Step 3 - Key CXR Pathologies

Consolidation (Airspace Opacification)

• Appearance: White/grey opacity that obscures normal lung markings; air bronchogram sign (dark air-filled bronchi visible within white consolidation - confirms it is truly airspace disease)
• Silhouette sign: If consolidation touches the heart/diaphragm border, that border is erased
• Lobar distribution: Does not cross fissures
• Common causes: Pneumonia (bacterial, viral, fungal), pulmonary oedema, pulmonary haemorrhage, infarction (PE), aspiration

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Pleural Effusion

• Appearance: White opacity in the lower zones; meniscus shape at the lung base; blunting of the costophrenic angle
• Massive effusion: White-out of entire hemithorax; may push mediastinum to the OPPOSITE side
• Subpulmonary effusion: Fluid trapped between lung and diaphragm - appears as "elevated hemidiaphragm" with lateral peak
• Lateral decubitus view: Best way to confirm mobile pleural fluid
• Causes: Heart failure (bilateral), pneumonia/parapneumonic (unilateral), malignancy, PE, liver disease, TB

• Effusion: Homogeneous opacity, meniscus shape, no air bronchograms, may shift with position
• Consolidation: Air bronchograms present, does not shift, follows lobar/segmental pattern

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Pneumothorax

• Appearance: Black rim of air at the lung periphery with no lung markings beyond the pleural line; a thin white visceral pleural edge is visible
• Look at the apex first (most common site for spontaneous PTX in thin young males)
• On supine films: "Deep sulcus sign" - abnormally deep costophrenic angle on one side
• Tension pneumothorax: Mediastinal shift AWAY from the PTX side; depressed ipsilateral diaphragm - clinical emergency, do NOT wait for CXR if clinically suspected

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Pulmonary Oedema (Cardiac/Cardiogenic)

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Cardiomegaly

• CTR > 0.5 on PA film
• Causes: Dilated cardiomyopathy, biventricular failure, pericardial effusion ("globular/water-bottle" heart shape), left or right ventricular hypertrophy

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Lung Nodule / Mass

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Other Common CXR Findings

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Checking Tubes and Lines (Critical on ICU CXRs)

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SECTION B: ABDOMINAL X-RAY (AXR / KUB)

• Suspected bowel obstruction
• Suspected perforated viscus (use erect CXR - more sensitive for free air)
• Renal/ureteric stones (though CT KUB is superior)
• Foreign bodies
• Assessment of colitis (toxic megacolon)

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Normal AXR Anatomy

• Stomach: Left upper quadrant, large gas bubble
• Small bowel: Central abdomen, should be < 3 cm in diameter, valvulae conniventes (mucosal folds cross the full width of the bowel)
• Large bowel: Peripheral, frames the abdomen, should be < 6 cm (> 9 cm in the caecum = toxic megacolon threshold); haustral folds do NOT cross the full width
• Rectum: Pelvic region, should contain some gas

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AXR: The BBC Approach

B - Bowel

• Dilated central loops of small bowel
• "Stacked coins" or "step-ladder" pattern of air-fluid levels on erect AXR
• No gas in large bowel (in complete obstruction)
• Causes: Adhesions (most common), hernia, volvulus, tumour

• Dilated peripheral large bowel loops
• Haustral pattern visible
• Caecal dilation (if ileocaecal valve competent)
• Causes: Colorectal cancer (most common), volvulus, diverticular disease, Ogilvie's syndrome

• "Coffee bean" or "bent inner tube" sign: massively dilated loop of sigmoid arising from the pelvis, pointing toward the right upper quadrant

• Best seen on erect CXR (patient must sit upright for 15-20 minutes)
• Dark crescent of gas below the right hemidiaphragm (above the liver)
• Rigler's sign (double wall sign) on supine AXR: both sides of the bowel wall visible = extraluminal gas present
• Causes: Perforated peptic ulcer, perforated bowel, post-laparoscopy (can persist up to 7 days)

B - Bones

• Lumbar spine: vertebral alignment, disc spaces, compression fractures
• Pelvis and hips: fractures, avascular necrosis, joint space
• Lower ribs: fractures, lesions

C - Calcifications (and solid organs)

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Section C: Bone X-Rays (Musculoskeletal)

Basic Approach: Always Request Two Views

• A - Alignment: Is the bone in normal anatomical position? Any displacement/angulation?
• B - Bone density: Generalized (osteoporosis) or focal lesion (lytic/sclerotic)?
• C - Cortex: Follow the entire cortical outline. Any break, step deformity, periosteal reaction?
• S - Soft tissue: Swelling, gas, calcification, foreign bodies?

Types of Fractures - Key X-Ray Patterns

Describing a Fracture Radiologically

1. Which bone and which part (diaphysis, metaphysis, epiphysis)
2. Complete vs incomplete
3. Type of fracture (see above)
4. Displacement (none/minimal/moderate/complete)
5. Angulation (direction and degrees)
6. Shortening (overriding)
7. Rotation
8. Open vs closed (clinical correlation)
9. Intra-articular involvement (step deformity at joint surface)
10. Associated dislocation

Common Bone X-Ray Examples

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PART 2: HOW TO READ A CT SCAN

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Quick CT Reference Card

Hounsfield Units (Tissue Density Scale)

CT Window Settings

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CT vs X-Ray: When to Use Which

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Key Sources

• Grainger & Allison's Diagnostic Radiology (chest X-ray, abdominal X-ray, bone X-ray chapters)
• Rosen's Emergency Medicine - Chest Radiography, Chapter 165
• Harrison's Principles of Internal Medicine, 22nd Edition (2025) - Imaging chapters
• Rockwood & Green's Fractures in Adults, 10th ed. (2025) - Plain radiograph interpretation
• Radiology Assistant - CXR interpretation
• Radiology Masterclass - AXR pathology
• IEM Student - How to read chest X-rays - ABCDEF approach
• Geeky Medics - Abdominal X-ray - BBC approach

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Clinical note: X-ray and CT interpretation requires hands-on training, supervised practice, and integration with clinical findings. This guide is a reference framework for learning. In clinical practice, radiology reports from qualified radiologists should guide management decisions.