ECG basics


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ECG basics

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Anatomy and physiology of the female reproductive system

Menstrual cycle

Contraception: Clinical (To be retired)

Vulvovaginitis: Clinical (To be retired)

Chlamydia trachomatis

Neisseria gonorrhoeae

Gardnerella vaginalis (Bacterial vaginosis)

Cervical cancer

Cervical cancer: Pathology review

Androgens and antiandrogens

Oxytocin and prolactin

Estrogen and progesterone


Amenorrhea: Clinical (To be retired)

Estrogens and antiestrogens

Progestins and antiprogestins


Ectopic pregnancy

Complications during pregnancy: Pathology review

Hypertensive disorders of pregnancy: Clinical (To be retired)


Placental abruption

Cell cycle

Mitosis and meiosis

Anatomy clinical correlates: Viscera of the gastrointestinal tract

Gastrointestinal hormones

Gastrointestinal system anatomy and physiology

Anatomy of the gastrointestinal organs of the pelvis and perineum

Abdominal pain: Clinical (To be retired)

Anatomy of the abdominal viscera: Innervation of the abdominal viscera

Appendicitis: Clinical (To be retired)


Appendicitis: Pathology review

Bowel obstruction


Diverticular disease: Pathology review

Peptic ulcer

Peptic ulcers and stomach cancer: Clinical (To be retired)

Gastric motility

Pancreatic neuroendocrine neoplasms

Helicobacter pylori

Cholinomimetics: Direct agonists

GERD, peptic ulcers, gastritis, and stomach cancer: Pathology review

Gastrointestinal bleeding: Pathology review

Acetaminophen (Paracetamol)

Non-steroidal anti-inflammatory drugs

Anatomy of the abdominal viscera: Liver, biliary ducts and gallbladder

Cirrhosis: Pathology review

Acute cholecystitis

Bile secretion and enterohepatic circulation

Jaundice: Pathology review

Jaundice: Clinical (To be retired)

Pancreatitis: Pathology review

Liver anatomy and physiology

Chronic cholecystitis

Diarrhea: Clinical (To be retired)

Irritable bowel syndrome

Vibrio cholerae (Cholera)

Lactose intolerance

Ulcerative colitis

Crohn disease

Inflammatory bowel disease: Clinical (To be retired)

Vitamin B12 deficiency

Anemia: Clinical (To be retired)

Anal conditions: Clinical (To be retired)

Colorectal cancer: Clinical (To be retired)

Innate immune system

B- and T-cell memory

MHC class I and MHC class II molecules


Cell-mediated immunity of natural killer and CD8 cells

Cell-mediated immunity of CD4 cells

Antibody classes

B-cell activation and differentiation


Body temperature regulation (thermoregulation)

Complement system

Nasal cavity and larynx histology

Anatomy of the nose and paranasal sinuses

Anatomy and physiology of the ear

Anatomy of the lymphatics of the neck

Anatomy of the larynx and trachea

Anatomy of the pharynx and esophagus

Anatomy of the external and middle ear

Anatomy and physiology of the eye

Respiratory syncytial virus

Streptococcus pyogenes (Group A Strep)

Bacterial epiglottitis

Epstein-Barr virus (Infectious mononucleosis)




Retropharyngeal and peritonsillar abscesses

Human parainfluenza viruses


Influenza virus

Pseudomonas aeruginosa

Haemophilus influenzae

Staphylococcus aureus

Microcirculation and Starling forces

Bone remodeling and repair

Bone histology

Fibrous, cartilage, and synovial joints

Muscles of the hand

Muscles of the forearm

Muscle contraction

Sliding filament model of muscle contraction

Development of the axial skeleton

Bone tumors

Bone tumors: Pathology review

Substance misuse and addiction: Clinical (To be retired)

Alcohol use disorder

Tobacco dependence

Cannabis dependence

Drug misuse, intoxication and withdrawal: Hallucinogens: Pathology review

Toxidromes: Clinical (To be retired)

Cocaine dependence

Opioid antagonists

Opioid agonists, mixed agonist-antagonists and partial agonists

Psychomotor stimulants

Drug misuse, intoxication and withdrawal: Alcohol: Pathology review

Drug misuse, intoxication and withdrawal: Stimulants: Pathology review

Sympathetic nervous system

Parasympathetic nervous system

Nervous system anatomy and physiology


Adrenergic antagonists: Presynaptic

Atypical antidepressants

Tricyclic antidepressants

Monoamine oxidase inhibitors

Major depressive disorder

Adrenergic antagonists: Beta blockers

Pharmacodynamics: Desensitization and tolerance

Sympathomimetics: Direct agonists


Pharmacokinetics: Drug metabolism

Enzyme function

Pharmacokinetics: Drug elimination and clearance

Plasma anion gap

Metabolic and respiratory acidosis: Clinical (To be retired)

Acid-base disturbances: Pathology review

Graves disease

Hyperthyroidism: Pathology review

Hyperthyroidism: Clinical (To be retired)

Thyroid hormones

Thyroid and parathyroid gland histology

Thyroid storm

Hypothyroidism and thyroiditis: Clinical (To be retired)

Anatomy of the thyroid and parathyroid glands

Hypothyroidism: Pathology review


Atypical antipsychotics

Typical antipsychotics

Bipolar disorder

Mood disorders: Clinical (To be retired)

Mood disorders: Pathology review

Celiac disease

Respiratory system anatomy and physiology

Development of the respiratory system

Pediatric allergies: Clinical (To be retired)

Food allergy


Hypersensitivity skin reactions: Clinical (To be retired)


Vaccinations: Clinical (To be retired)

Neuromuscular junction and motor unit

Anatomy of the ascending spinal cord pathways

Anatomy of the descending spinal cord pathways


Migraine medications

Cranial nerves

Cranial nerves rap

Cranial nerve pathways

Introduction to the cranial nerves

Anatomy of the cranial meninges and dural venous sinuses

Uterine disorders: Pathology review

Uterine fibroid

Uterine stimulants and relaxants


Osteoporosis medications


Parathyroid conditions and calcium imbalance: Clinical (To be retired)

Endometrial cancer

Urinary incontinence

Urinary incontinence: Pathology review

Lower urinary tract infection

Urinary tract infections: Pathology review

Anatomy of the urinary organs of the pelvis

Neurogenic bladder

Elimination disorders: Clinical (To be retired)

Development of the renal system

Development of the reproductive system

Dyslipidemias: Pathology review


Cushing syndrome and Cushing disease: Pathology review

Hypertension: Clinical (To be retired)

Hypertension: Pathology review


Endocrine system anatomy and physiology

ECG basics

ECG axis

ECG intervals

ECG QRS transition

ECG rate and rhythm

ECG normal sinus rhythm

Diabetes mellitus: Clinical (To be retired)

Diabetes insipidus

Diabetes mellitus

Diabetes mellitus: Pathology review


Diabetic nephropathy

Citric acid cycle


Arterial disease

Peripheral artery disease: Pathology review


Atherosclerosis and arteriosclerosis: Pathology review

Ischemic stroke

Coagulation (secondary hemostasis)


Anticoagulants: Heparin

Anticoagulants: Warfarin

Anticoagulants: Direct factor inhibitors

Mixed platelet and coagulation disorders: Pathology review

Disseminated intravascular coagulation

Coagulation disorders: Pathology review

Atrial flutter

Atrial fibrillation

Endocarditis: Pathology review


Infective endocarditis: Clinical (To be retired)

Pneumonia: Pathology review


Pneumonia: Clinical (To be retired)

Anatomy of the leg

Anatomy clinical correlates: Leg and ankle

Anatomy clinical correlates: Hip, gluteal region and thigh

Anatomy of the anterior and medial thigh

Pediatric orthopedic conditions: Clinical (To be retired)

Pediatric musculoskeletal disorders: Pathology review

Leg ulcers: Clinical (To be retired)

Legg-Calve-Perthes disease

Peripheral vascular disease: Clinical (To be retired)

Peripheral artery disease

Coarctation of the aorta

Joints of the ankle and foot

Anatomy of the knee joint

Anatomy of the tibiofibular joints

Joint pain: Clinical (To be retired)

Anatomy of the hip joint

Ankylosing spondylitis

Lower back pain: Clinical (To be retired)

Seronegative arthritis: Clinical (To be retired)

Back pain: Pathology review

Reactive arthritis

Cauda equina syndrome


ECG basics


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USMLE® Step 1 questions

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High Yield Notes

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ECG basics

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USMLE® Step 1 style questions USMLE

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A 55-year-old man is brought to the emergency department with severe midsternal chest pain. The patient reports 2 episodes of non-bloody emesis and diaphoresis since the start of the chest pain half an hour ago. Blood pressure is 110/75 and pulse is 89/min. Physical examination is unremarkable. An electrocardiogram (ECG) is performed based on which a provisional diagnosis of inferior wall myocardial infarction is made. Which of the following patterns of ECG changes are most likely seen in this patient?  

External References

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Electrocardiogram (ECG)

with pulmonary embolism p. 697

Electrocardiograms (ECGs) p. 312

acute pericarditis on p. 320

cardiac tamponade on p. 481

low-voltage p. 317, 481

MI diagnosis with p. 313

tracings of p. 300


electrocardiograms p. 312


Content Reviewers

Rishi Desai, MD, MPH


Charles Davis, MD

An electrocardiogram is also known as an ECG; the Dutch and German version of the word, elektrokardiogram, is shortened to EKG. It is a tool used to visualize, or “gram,” the electricity, or “electro,” that flows through the heart, or “cardio.” Specifically, a 12-lead ECG tracing shows how the depolarization wave, which is a wave of positive charge, moves during each heartbeat, by providing the perspectives of different sets of electrodes. This particular set of electrodes is called lead II; one electrode is placed on the right arm and the other on the left leg. Essentially, when the wave’s moving toward the left leg electrode, you get a positive deflection. This big, positive deflection corresponds to the wave moving down the septum.

To understand the basics, let’s start with an example of how we can look at the heart with only one pair of electrodes: a positive and a negative one. These electrodes detect the charge on the outside of the cell. Remember, at rest, cells are negatively charged relative to the slightly positive outside environment; let’s make these cells red. When they depolarize, the cells become positively charged, and leave a slightly negative charge in the outside environment; let’s make these cells green. Now, if we freeze this “wave of depolarization” as it’s moving through the cells, half of the cells are negative, or depolarized, and half are positive and resting; therefore, there’s a difference of charge across this set of cells. You can think of the charge difference as being a dipole, because there are two electrical poles. We can draw this dipole out as an arrow, or vector, pointing towards the positive charge. Remember, the electrodes detect charge on the outside of the cell, so this points toward where the positive charge is, outside.

Now, if there’s a dipole vector pointing toward the positive electrode, then the ECG tracing shows it as a positive deflection; the bigger the dipole is, the bigger the deflection is. If we unpause this, then everything becomes depolarized. Since there’s no difference in charge, there’s no dipole, and thus no deflection. Moments later, a wave of repolarization goes through, and the cells become negative once again. Pausing halfway through again, now the vector dipole goes in the opposite direction, and faces the negative electrode; this means that there will be a negative ECG tracing. Again, the bigger the dipole is, the bigger the negative deflection is. Even though it’d be nice if the depolarization wave lined up perfectly with the electrodes, usually that’s not the case. So, we simply look at the vector component that is parallel to that electrode. For example, let’s say that the depolarization happened this way, at an angle; then, we’d simply break the vector into two parts. The one we care about is the one that’s going towards the positive electrode, which causes a deflection, even though it’s a slightly smaller deflection than previously. In other words, the size of the deflection on the ECG tracing always corresponds to the magnitude, or size, of the dipole in the direction of the electrode. The perpendicular component isn’t pointing at the electrodes, so it doesn’t cause any deflection. In fact, if there’s a depolarization wave that goes straight up, perpendicular to the positive and negative electrodes, there would be no deflection!


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2017)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Screening for Cardiovascular Disease Risk With Electrocardiography" JAMA (2018)
  6. "Screening for Coronary Heart Disease With Electrocardiography: U.S. Preventive Services Task Force Recommendation Statement" Annals of Internal Medicine (2012)
  7. "Activation of the Interventricular Septum" Circulation Research (1955)

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