Comprehensive Cardiac Conduction and EKG Interpretation Notes

Basic Cardiac Anatomy and Electrical Activity

  • The heart is supplied and monitored via electrical impulses that coordinate pumping.
  • Arteries carry oxygen-rich blood from the heart to body tissues; the first vessel described is arteries.
  • Veins carry blood back to the heart; they return often less oxygenated blood.
  • The heart’s activity can be observed noninvasively with an EKG/ECG, which records electrical activity of contractions of the heart muscle.
  • Each event on an EKG has a distinctive waveform representing depolarization and repolarization of myocardial cells.
  • Repolarization = relaxation; Depolarization = pumping (contraction).
  • The cardiac conduction system includes specialized myocardial cells with these properties: automaticity, excitability, conductivity, contractility, and refractoriness.
    • Automaticity: pacemaker cells spontaneously initiate an electrical impulse (action potential).
    • Excitability: myocardial tissue can be depolarized by a stimulus.
    • Conductivity: ability to transport an impulse from cell to cell.
    • Contractility: myocardial fibers shorten in response to the impulse.
    • Refractoriness: period after depolarization when the cell cannot respond to another stimulus.
  • The conduction system is crucial for properly timed phases of the heartbeat and becomes clinically important in recognizing dysrhythmias.

Ion Movements That Drive Conduction, Contraction, and Repolarization

  • Three key ions drive conduction and muscle activity:
    • Sodium (Na⁺) movement into the cell causes the upstroke (depolarization).
    • Calcium (Ca²⁺) entering the cell causes the plateau (phases of action potential prolongation).
    • Potassium (K⁺) moving out of the cell causes repolarization (restoration of the resting state).
  • Depolarized resting cell state: inside of the cell is negative relative to outside.
  • Repolarization involves ions returning to the polarized state more slowly.
  • Anti-dysrhythmic medications target various phases of the action potential (e.g., calcium and sodium channels).

The Cardiac Conduction Pathway (Anatomy to Rhythm)

  • The impulse starts in the Sinoatrial (SA) node, the heart’s primary pacemaker; typical rate: 60extto100extbeatsperminute60 ext{ to } 100 ext{ beats per minute}.
  • The impulse travels across the atria to the Atrioventricular (AV) node (junctional tissue) and the bundle of His.
  • The AV node slows conduction slightly, creating a delay before reaching the ventricles.
  • The bundle of His splits into the right and left bundle branches, delivering impulse to both ventricles via the Purkinje fibers.
  • The sequence yields atrial contraction (SA to AV), ventricular contraction (via bundle branches and Purkinje fibers).
  • Visual: SA → AV node → bundle of His → right/left bundle branches → Purkinje fibers → ventricular squeeze.

Cardiac Monitoring in Practice

  • In hospitals, many patients are on 3–5 lead monitors; a 12-lead ECG gives the best global view of the heart’s electrical activity.
  • 12-lead ECG provides multiple angles (inferior, anterior, etc.) and can detect regional ischemia or infarction (e.g., left/right bundle branch block, etc.).
  • Five-lead monitoring is common at the bedside; Lead II is often preferred for readability; 12 leads offer broader anatomical views.
  • Central monitoring staff may oversee many patients’ monitors in large facilities; electrode placement and skin contact are critical for accurate readings.
  • The standard color and placement convention (white-right, clouds over grass; black over red; green over brown) helps ensure correct electrode positioning.
  • Common electrode placements (simplified):
    • White: right arm (above the right clavicle)
    • Black: left arm
    • Red: left leg
    • Green: right leg (ground)
    • Brown: chest/abdomen area when needed; sometimes used in extended leads.
  • Electrode quality matters: clean, hairless skin, adequate moisture on gel, and proper placement ensure good conduction; movement and poor contact generate artifacts.
  • A central monitoring approach is used to provide continuous, broad surveillance; patches and wires may move with patient activity (e.g., PT, sleeping, meals) and affect readings.

Interpreting an EKG Strip: Basic Elements

  • Baseline (isoelectric line) is the reference where no current flows.
  • P wave: atrial depolarization (atrial contraction).
  • PR interval: time from start of P wave to start of QRS, representing atrial depolarization and conduction through the AV node and Purkinje network. Normal range: PR ext{ interval}
    \approx 0.12 ext{s} ext{ to } 0.20 ext{s}. (i.e., 1.5–5 small boxes)
  • QRS complex: ventricular depolarization (ventricular contraction); includes Q, R, and S waves.
  • Ventricular repolarization is represented by the T wave (ventricular relaxation).
  • U wave may appear rare and is often not seen; if present, may reflect slow repolarization of ventricles or Purkinje fibers.
  • ST segment: flat region between S and T; elevation or depression suggests ischemia or infarction (STEMI/NSTEMI).
  • Summary labeling: P before every QRS in normal sinus rhythm; QRS portrays ventricular activity; T reflects ventricular recovery.

Normal Sinus Rhythm (NSR) and Variants

  • NSR basics:
    • Rate: 60extto100extbpm60 ext{ to } 100 ext{ bpm}
    • Rhythm: regular; P waves present and upright before every QRS; each QRS is preceded by a P wave.
    • PR interval: 0.12ext–0.20exts0.12 ext{–}0.20 ext{ s}; QRS duration: < 0.12 ext{ s}; ST segment normal.
  • Sinus arrhythmia:
    • All features of NSR, but the rhythm is irregular.
    • Rate remains 60$-$100$ bpm.
    • Common causes include athletic conditioning, sleep, stimulants, inadequate sleep, inferior MI, age, digoxin, sleep-related factors.
    • No treatment unless symptomatic.
  • Sinus bradycardia:
    • Rate < 60 bpm; rhythm regular; P waves and QRS normal; ST not elevated.
    • Causes: well-conditioned athletes, increased vagal tone, sleep, medications (e.g., beta blockers, other rate-slowing drugs), hypothyroidism, electrolyte disturbances, sleep apnea, certain cardiac conditions.
    • Treatments when symptomatic: atropine, IV fluids, oxygen, addressing underlying cause; chronic cases may require a pacemaker.
    • Warning: chronic bradycardia can predispose to VT/VF in some patients.
  • Sinus tachycardia:
    • Rate > 100 bpm; rhythm regular; P waves present; QRS narrow; ST normal.
    • Causes: physical activity, nicotine, pain, fever, sepsis, anxiety, dehydration, metabolic disorders, anemia, shocK, exercise, etc.
    • Treatment focuses on treating the underlying cause; medications may slow rate (e.g., beta blockers, calcium channel blockers, digoxin).

Atrial Arrhythmias

  • Atrial rhythms originate in the atria but outside the SA node; may involve ectopic pacemakers or reentry circuits.
  • Atrial fibrillation (AFib):
    • Loss of synchronized atrial contraction; P waves are replaced by fibrillatory or wavy baseline; irregularly irregular ventricular response.
    • No discrete P waves before each QRS; QRS typically narrow unless conduction abnormality exists.
    • Risk factors: age, hypertension, prior ischemia, TIAs/strokes, CAD, heart failure, diabetes, obesity, hypothyroidism, CKD, excessive alcohol use, sleep apnea, valvular disease, obesity, and other cardiac conditions.
    • Clinical considerations: blood-thinning (anticoagulation) to reduce stroke risk before attempting rhythm control; rate control with diltiazem, amiodarone, beta blockers, digoxin; rhythm control with cardioversion after stabilization and/or anticoagulation; possible ablation in recurrent cases.
    • Symptoms: palpitations, shortness of breath, tachycardia, potential hemodynamic compromise.
  • Atrial flutter:
    • Sawtooth-shaped flutter waves (F waves); typically a ratio of F waves to QRS (e.g., 2:1, 3:1).
    • Atrial rate often 240–360 bpm; ventricular rate depends on the AV conduction ratio (e.g., 150 bpm with 2:1 conduction).
    • Similar causes to AFib; management includes rate control, anticoagulation, and possible cardioversion or ablation.
  • Supraventricular tachycardia (SVT):
    • Rapid rhythm arising above the ventricles; usually rate > 150 bpm; regular rhythm; P waves may be hidden in T waves; QRS narrow.
    • Common causes: stimulants, fever, anxiety, pregnancy, hyperthyroidism, sepsis.
    • Treatments: vagal maneuvers (e.g., adenosine), adenosine as a temporary reset (brief pause in rhythm); calcium channel blockers or beta blockers for rate control; carotid sinus massage generally reserved for cardiology; synchronized cardioversion if unstable.
    • Adenosine: rapid IV push with a brief pause in rhythm; may cause a transient sensation of chest pressure; must be used with caution and in appropriate settings.
  • Wolff-Parkinson-White (WPW) syndrome: a conduction abnormality that can promote tachyarrhythmias; often familial; associated with fast heart rate and potential pauses.
  • General SVT management notes:
    • If patient is unstable or pulse absent with VT/VF: immediate defibrillation; ACLS protocol applies.
    • If a stable SVT: consider adenosine first; if ineffective or contraindicated, rate control with calcium channel blockers/beta blockers; consider synchronized cardioversion if symptoms worsen.

Ventricular Arrhythmias

  • Ventricular tachycardia (VT):
    • QRS complexes are wide and bizarre; rate often fast; may have a pulse or be pulseless.
    • If VT with a pulse: assess consciousness and blood pressure; synchronized cardioversion is indicated if unstable; oxygen and ACLS meds may be used.
    • If VT without a pulse: treat as VT with no pulse—immediate CPR and defibrillation; follow ACLS for medication and shocks.
    • Torsades de Pointes: a form of VT with twisting of points; may be precipitated by electrolyte disturbances or drugs (e.g., cocaine in some cases); treat with magnesium sulfate and address electrolytes; may require defibrillation if no pulse.
  • Ventricular fibrillation (V-fib):
    • Chaotic, irregular electrical activity with no recognizable QRS complexes; no usable waveform; pulseless and unresponsive; most lethal arrest rhythm.
    • Management: immediate defibrillation, CPR, ACLS medications; treat underlying causes.
  • Pulseless electrical activity (PEA):
    • Rhythm on monitor but no palpable pulse; electrical activity present but inadequate perfusion.
    • Management: CPR and treat reversible causes; do not rely on rhythm alone for prognosis.
  • Asystole: flatlining; no electrical activity>
    • No shock indicated; manage with CPR and ACLS protocols focused on reversible causes.
  • Non-sustained VT and PVCs (premature ventricular contractions):
    • Isolated PVCs can occur; runs may progress toward sustained VT; monitor and assess underlying cause (ischemia, electrolyte disturbance, drugs, ischemia).

Interpretation and Systematic Approach to Reading EKGs

  • Systematic steps to read an EKG:
    • Rate assessment: normal rate 60–100 bpm; bradycardia < 60 bpm; tachycardia > 100 bpm.
    • Rhythm regularity: check whether R-R intervals are regular; determine if the rhythm is regular or irregular.
    • P waves: presence, upright configuration, and presence before every QRS; consistency across beats.
    • PR interval: measure from the start of P to start of QRS; normal range PR ext{ interval}
      \approx 0.12 ext{s} ext{ to } 0.20 ext{s}.
    • QRS duration: measure from start to end of QRS; normal width < 0.12 ext{s}; a wider QRS suggests ventricular conduction or aberrant pathways.
    • ST segment: evaluate for elevations or depressions; ST elevation suggests STEMI; ST depression suggests NSTEMI or ischemia.
    • T wave: assess for normal repolarization; U waves may appear rarely.
  • Common measurement aids:
    • Small box duration: 0.04 ext{s} per small box.
    • Large box duration: 0.20 ext{s} per large box (5 small boxes).
    • One second equals 5 large boxes; a 6-second strip can be used for a quick rate estimate by counting complexes and multiplying by 10: HR ext{ (bpm)} \approx N_{QRS} imes 10 over a 6-second interval.
    • Alternative (less preferred in modern practice): the 300-rule: HR \approx \frac{300}{RR\text{ interval in large boxes}}, useful for regular rhythms but less reliable when irregular.
  • Normal NSR checklist (summary):
    • Rate: 60–100 bpm; Rhythm: regular; P waves: present and upright before every QRS; PR interval: 0.12–0.20 s; QRS: < 0.12 s; ST segment normal; no abnormal waves.

Practical Considerations and Clinical Pearls

  • Lead placement matters: wrong placement can mimic or mask real arrhythmias; ensure correct limb and chest lead positions; Lead II is commonly used for rhythm interpretation due to its readability.
  • 12-lead EKGs provide different anatomical perspectives (infarct localization: inferior, anterior, lateral, etc.).
  • 12-lead vs continuous monitoring: 12-lead is a snapshot; continuous monitoring provides ongoing rhythm assessment but may require interpretation by skilled clinicians (e.g., ER physicians, cardiologists, paramedics).
  • Priority in chest pain or MI: rapid identification via 12-lead; “priority one” calls ensure cath lab activation and reduced myocardial tissue death due to ischemia.
  • Rhythm evaluation in practice often involves balancing diagnostic clarity with patient safety; some rhythm patterns fall outside textbook boxes, and clinicians must reason with imperfect data.
  • Arrhythmia etiologies span noncardiac and cardiac causes:
    • Noncardiac: caffeine, sleep disorders (sleep apnea), stress, electrolyte disturbances, hypothermia, acidosis, certain drugs, etc.
    • Cardiac: ischemia, infarction, valve disease, structural heart disease, age-related degeneration, prior surgeries, and conduction system disease (e.g., WPW).
  • Treatments and interventions (high-level):
    • Rate control: calcium channel blockers (e.g., diltiazem), beta blockers, digoxin, amiodarone.
    • Anticoagulation in AFib to prevent stroke.
    • Cardioversion: synchronized shock for certain stable rhythm disturbances (AFib, flutter, SVT, VT with pulse).
    • Defibrillation: unsynchronized shock for VFib or pulseless VT.
    • Ablation: catheter-based destruction of aberrant pathways (e.g., for AV nodal reentrant tachycardia or accessory pathways like WPW).
    • Adenosine: acute reset for SVT (brief rhythm pause; rapid IV push, followed by saline flush).
    • Supplemental oxygen, IV fluids, correction of electrolytes, and addressing underlying ischemia or hemodynamic instability.
  • Important clinical caveats:
    • Do not shock asystole; synchronize shocks only when appropriate (e.g., VT with pulse or AFib with instability requesting synchronized cardioversion).
    • In AFib, stroke risk necessitates anticoagulation before cardioversion when feasible; risk assessment depends on time in AFib and patient-specific factors.
    • Pacing or pacing wires may be left in place after certain surgeries; monitor placement and electrode contact are critical to prevent misreads and misinterpretation.

Quick Reference: Key Numbers and Formulas

  • Normal NSR rate: 60 ext{–} 100 ext{ bpm}
  • PR interval: 0.12 ext{ s} ext{ to } 0.20 ext{ s}
  • QRS duration: < 0.12 ext{ s}
  • One large box: 0.20 ext{ s};onesmallbox:; one small box:0.04 ext{ s}
  • 6-second strip rule for HR: HR \approx N_{QRS} imes 10
  • 300-rule (alternative): HR \approx \frac{300}{RR\text{ interval in large boxes}}$$
  • Ventricular arrhythmias: wide QRS in VT; V-fib is chaotic with no recognizable waves; PEA shows electrical activity without a pulse; asystole is a flat line.
  • ST segment elevations/depressions indicate ischemia or infarction; ST elevation often indicates STEMI; ST depression indicates NSTEMI or ischemia.
  • Atrial arrhythmias: AFib (irregularly irregular, no discrete P waves), flutter (sawtooth F waves; atrial rate ~240–360 bpm), PACs (premature atrial contractions).

Connections to Practice and Real-World Relevance

  • Timely recognition of myocardial ischemia via 12-lead ECG can reduce tissue death; “priority one” activation of cath labs mirrors real-world protocols.
  • Understanding conduction physiology informs why certain drugs affect rhythm (e.g., Na⁺/Ca²⁺ channel blockers slow conduction or prolong refractoriness).
  • Differentiating between NSR, sinus arrhythmia, bradycardia, and tachycardia is essential before pursuing invasive interventions.
  • Knowledge of rhythm terminology (e.g., irregularly irregular, sawtooth waves, wide QRS) guides appropriate interventions and when to escalate care.

Ethical and Practical Implications (brief)

  • Cardiac rhythm management must balance rapid intervention with patient safety, including sedation risks during procedures like cardioversion.
  • Anticoagulation decisions in AFib require weighing stroke risk against bleeding risk, patient comorbidities, and timing relative to rhythm-control strategies.
  • Patient education about symptoms and when to seek care is essential, given the potentially rapid progression from manageable rhythms to life-threatening arrests.
  • In real settings, avoid over-reliance on TV portrayals of resuscitation; follow ACLS protocols and institutional guidelines for patient safety and outcomes.

Quick Summary to Aid Memory

  • NSR features: regular rhythm, P before every QRS, PR ~ 0.12–0.20 s, QRS < 0.12 s, rate 60–100.
  • Atrial arrhythmias disrupt normal atrial activity (AFib, flutter, PACs).
  • SVT is rapid, regular, above 150 bpm; adenosine is a common initial diagnostic/therapeutic tool.
  • VT/VF are life-threatening; VT with a pulse may be cardioverted if stable; without a pulse, CPR + defibrillation.
  • AFib requires anticoagulation consideration and potential cardioversion after stabilization.
  • Always check lead placement, contact quality, and artifacts to avoid misinterpretation.