Bradycardia

Definition

ECG of Sinus Bradycardia

• Normal P wave morphology (upright in leads I, II, aVF)
• P wave precedes every QRS complex
• Constant PR interval > 120 ms
• Rate < 60 beats/min (< 50 beats/min by Braunwald's criteria for sinus bradycardia)
• Sinus arrhythmia often coexists

Types / Classification

1. Sinus Bradycardia

2. Sick Sinus Syndrome (SSS)

3. AV Conduction Blocks

Causes

Physiologic

• Athletes: Increased stroke volume, downregulation of SA node "funny current" (If channels), and enhanced vagal tone produce resting bradycardia - Guyton & Hall Medical Physiology, p. 166
• High resting vagal tone in young adults

Pathologic

• Vagal stimulation: Carotid sinus syndrome, vomiting, vasovagal episodes, hemoperitoneum
• Cardiac ischemia: Inferior wall MI (involves SA node blood supply - usually RCA)
• Intrinsic SA node disease: Sick sinus syndrome
• Drugs: Beta-blockers, calcium channel blockers, digoxin, dexmedetomidine, amiodarone
• Metabolic: Hypothermia, hypothyroidism, hypoxia, hyperkalemia
• Autonomic: High cervical spinal cord injury (loss of sympathetic tone with intact vagal activity)
• Severe sepsis / shock: Bradycardia as a sign of profound cardiac dysfunction

Clinical Presentation

• Fatigue, dizziness, lightheadedness
• Syncope or presyncope
• Dyspnea, exercise intolerance
• Hypotension, chest pain (if ischemia)
• Altered mental status (severe cases)

Management

General principle

Pharmacologic

Special case - Dexmedetomidine-induced bradycardia: Atropine and epinephrine are largely ineffective due to alpha-2 agonism. Use vasopressors and pacing if needed. - Sabiston Textbook of Surgery, p. 2826

Special case - High cervical spinal cord injury: Treat with atropine plus vasopressors (with chronotropic effect) for concurrent neurogenic shock.

Special case - Infranodal (junctional/ventricular) block: Atropine is ineffective - acts on the AV node which is already bypassed. Pacing is required. - Sabiston Textbook of Surgery, p. 2828

Electrical (Pacing)

• Transcutaneous pacing: Emergency bridge therapy
• Transvenous temporary pacing: For hemodynamically unstable patients not responding to drugs
• Permanent pacemaker: For symptomatic SSS (required before nodal blocking agents can be safely used for concurrent atrial fibrillation); complete (3rd degree) AV block

Long-Term Management

• Sick sinus syndrome with symptomatic bradycardia: Permanent pacemaker - Rosen's Emergency Medicine, p. 1046
• Complete heart block: Permanent pacemaker in most cases
• 2nd degree Mobitz II: Usually requires permanent pacemaker given risk of progression to complete block
• 2nd degree Wenckebach (Mobitz I): Pacing indications depend on context and hemodynamic stability - Sabiston Textbook of Surgery, p. 2842
• Review and withdraw causative drugs where possible

ECG Interpretation

Systematic Analysis

Conclusion

• AV block (PR normal, 1:1 conduction)
• Bundle branch block (QRS narrow)
• Acute ischemia or infarction (no ST changes, no pathological Q waves)
• Significant arrhythmia

Important caveat: This is a 6-lead limb tracing only (I, II, III, aVR, aVL, aVF). The precordial leads (V1-V6) are not visible. A full 12-lead ECG is needed to evaluate the anterior, septal, and lateral walls for ischemia, hypertrophy, or conduction abnormalities.

⚠️ Critical Re-interpretation: This ECG Represents Neonatal Bradycardia

Normal Heart Rate for a Newborn (0-24 hours of life)

Revised ECG Interpretation for a 6-Hour Neonate

Differential Diagnosis for Neonatal Bradycardia at 6 Hours of Life

Immediate Management (NRP Protocol)

1. Assess airway, breathing, color - is the baby responsive? Breathing?
2. Stimulate and dry the neonate
3. Provide supplemental oxygen / positive pressure ventilation (PPV) if not breathing adequately
4. If HR remains < 60 bpm despite adequate ventilation with 100% O₂:
   • Begin chest compressions (3:1 ratio with ventilations)
   • Obtain vascular access
   • Epinephrine 0.01-0.03 mg/kg IV (or 0.05-0.1 mg/kg ET if no IV access)
   • Consider volume expansion: Normal saline 10 mL/kg IV over 5-10 min if hypovolemia suspected

If HR is between 60-100 bpm, PPV alone is the priority while further assessment continues.

Additional Workup to Consider

• Blood gas (to assess hypoxia/acidosis)
• Blood glucose
• Full sepsis screen (CBC, blood culture, CRP)
• Maternal history: SLE, anti-Ro/La antibodies, medications
• Echocardiogram if structural heart disease or persistent congenital heart block suspected
• Thyroid function (hypothyroidism can cause bradycardia)
• Full 12-lead ECG - this tracing only shows limb leads; chest leads needed to assess for complete AV block pattern

ECG Block Analysis - Detailed Answer

AV Block Checklist

However - Two Points Warrant Attention

• Congenital heart disease (e.g., tricuspid atresia, AV septal defect, AVSD)
• Left ventricular hypertrophy from structural lesion

Summary

Echocardiographic Views of the Newborn Heart

The Four Acoustic Windows

Basic image planes in transthoracic echocardiography - Textbook of Clinical Echocardiography

Window 1: Parasternal Window

Parasternal Long Axis (PLAX)

• Structures seen: LV, RV outflow, mitral valve, aortic valve, LA, ascending aorta, descending aorta (in cross section)
• Assesses: LV size and function, MV and AV morphology, aortic root, pericardial effusion
• In neonates: evaluate ventricular septal defect (VSD) in perimembranous region, aortic root size, PDA

Parasternal Short Axis (PSAX) - Multiple Levels

Neonatal tip: The ductus arteriosus is best seen from the parasternal short axis at the base - critical in first hours of life for PDA assessment.

Window 2: Apical Window

Apical 4-Chamber View (A4C)

• Structures seen: All 4 chambers simultaneously - RV, LV, RA, LA; mitral valve, tricuspid valve, atrial septum, ventricular septum, pulmonary veins
• Key assessments:
  • AV valve morphology and level of insertion (tricuspid inserts more apically than mitral)
  • Atrial septal defect (foramen ovale)
  • AV septal defect (AVSD) - common in Down syndrome
  • Ventricular size comparison (RV dominance is normal in neonates)
  • Pulmonary venous return (look for total anomalous pulmonary venous return - TAPVR)

Apical 5-Chamber View

• Anterior tilt from A4C reveals the LVOT and aortic valve (5th "chamber")
• Assess subaortic obstruction, VSD extending to LVOT

Apical 2-Chamber View

• 60° counterclockwise rotation from A4C
• Shows LV and LA only - inferior and anterior walls of LV

Apical Long Axis View (3-Chamber)

• Shows LV, LVOT, aortic valve, LA - similar to PLAX but from apex

Window 3: Subcostal Window ⭐ (Most important in neonates)

In neonates, this window is often the best - thin abdominal wall, horizontal heart position, no interfering ribs.

Subcostal 4-Chamber View

• Excellent view of atrial septum (perpendicular beam = best for ASD/PFO)
• Assess: ASD, sinus venosus defect, total/partial APVR, IVC, hepatic veins

Subcostal Short Axis (Coronal)

• Excellent view of the right ventricular outflow tract, pulmonary valve, main PA and branches
• Assess: pulmonary stenosis, branch PA stenosis, PA size in duct-dependent lesions

Subcostal Sagittal View

• Shows IVC, SVC entering RA, hepatic veins
• Critical for systemic venous drainage (heterotaxy, interrupted IVC, SVC-RA junction)

Window 4: Suprasternal Notch Window

Suprasternal Long Axis

• Shows: Aortic arch, ascending aorta, descending aorta, left/right branch head vessels (innominate, left carotid, left subclavian)
• Critical for: Coarctation of the aorta (narrowing at the isthmus), interrupted aortic arch, right aortic arch (associated with ToF, truncus)

Suprasternal Short Axis

• Shows: Right pulmonary artery in long axis (running behind the ascending aorta)
• Assess: RPA size, branch PA stenosis

Transducer Movements (How to Navigate Between Views)

Transducer motions: Tilt (rocks within same plane), Angle (changes image plane), Rotate (switches between long/short axis) - Textbook of Clinical Echocardiography

Neonatal-Specific Structures to Assess

Note on this baby's ECG: The unexplained left axis deviation (unusual in a neonate) should raise suspicion for structural heart disease (AVSD, tricuspid atresia, heterotaxy). The subcostal 4-chamber and apical 4-chamber views will be the most informative starting points on echo.

🫀 Echo Views of a Newborn Heart — Explained Simply

First, Why is Newborn Echo Special?

🚪 The 4 Windows

Window 1 — Parasternal (Left side of the chest)

View A: Parasternal Long Axis (PLAX)

• LV (left ventricle) — the main pumping chamber
• Mitral valve — the door between left atrium and left ventricle
• Aortic valve — where blood exits to the body
• Left atrium behind it
• RV (right ventricle) as a thin strip at the front

View B: Parasternal Short Axis (PSAX)

In a newborn: The ductus arteriosus (a fetal blood vessel connecting aorta to pulmonary artery) is seen beautifully here. In the first hours of life it may still be open — this is the best view to check it.

Window 2 — Apical (Tip of the heart)

View: Apical 4-Chamber (A4C) — The "Money View"

    RA  |  LA
    ——————————
    RV  |  LV

• Holes in the wall between the two upper chambers? → ASD
• Holes in the wall between the two lower chambers? → VSD
• Are all 4 pulmonary veins draining normally into the left atrium? → Rule out TAPVR (total anomalous pulmonary venous return)
• Is the RV bigger than the LV? → Normal in newborns (fetal circulation makes RV dominant)

Window 3 — Subcostal ⭐ (Below the ribs)

This is the BEST window in newborns because:

• The heart sits more horizontally in a baby
• The abdominal wall is thin
• You get a perfect beam angle onto the atrial septum

View: Subcostal 4-Chamber

The ultrasound beam must hit a structure at 90° to "see" it properly. For the atrial septum, subcostal is the perfect angle — the apical view can give false-negative results (the beam runs parallel to the septum and makes thin tissue "disappear").

View: Subcostal Short Axis (Coronal)

• Right ventricular outflow tract (RVOT)
• Pulmonary valve
• Main pulmonary artery splitting into left and right branches

View: Subcostal Sagittal

Window 4 — Suprasternal Notch (Top of the chest)

View: Suprasternal Long Axis

• Ascending aorta going up
• Arch curving over (with 3 branches coming off — innominate, left carotid, left subclavian)
• Descending aorta going down

View: Suprasternal Short Axis

🗺️ Summary Map

         SUPRASTERNAL
              ↑
              |  (aortic arch, coarctation)
              |
PARASTERNAL ←-♥-→ APICAL
(LV, valves,  |   (all 4 chambers,
 PLAX, PSAX)  |    septal defects)
              ↓
          SUBCOSTAL
    (atrial septum — BEST VIEW,
     pulmonary artery, IVC, situs)

What Every First-Day Newborn Echo Should Answer

One Practical Tip for Neonates