How do I diagnose a large right to left shunt with my patent foramen ovale (PFO)?

Diagnosing a Large Right-to-Left Shunt with Patent Foramen Ovale (PFO)

Transesophageal echocardiography with contrast (bubble) study during Valsalva maneuver is the gold standard for diagnosing and quantifying a large right-to-left shunt through a PFO. 1, 2

Diagnostic Methods for PFO Shunt Assessment

Primary Diagnostic Method: Echocardiography

1. Transesophageal Echocardiography (TEE)

   • Higher sensitivity than transthoracic imaging 1, 2
   • Recommended for detailed visualization of atrial septal anatomy 1
   • Essential for accurate PFO detection in younger adults with unexplained cerebrovascular events 1

2. Contrast Study (Bubble Study) Technique

   • Involves injection of agitated saline solution (microbubbles)
   • Bubbles enter right atrium and, if PFO present, pass into left atrium within a few beats 1
   • Valsalva maneuver is crucial - increases right atrial pressure, enhancing sensitivity of detection 1, 2

3. Quantification of Shunt Size

   • Small: <10 microbubbles in left atrium
   • Moderate: 10-20 microbubbles
   • Large: >20 microbubbles 3

Alternative Diagnostic Methods

• Transthoracic Echocardiography (TTE) with contrast

  • More accessible but less sensitive than TEE
  • May actually have higher sensitivity and accuracy for detecting large right-to-left shunts associated with cryptogenic stroke 3

• Transcranial Doppler (TCD) with contrast

  • Quantifies right-to-left shunt in brain vessels
  • Useful for follow-up after PFO closure 4

Clinical Indicators of Large Right-to-Left Shunt

Physical Examination Findings

• Differential cyanosis (lower extremities more cyanotic than upper)
• Clubbing in lower extremities 1
• Positional desaturation (orthodeoxia) 5
• Shortness of breath that worsens in upright position (platypnea) 5

Laboratory and Imaging Findings

• Oxygen saturation differences between upper and lower extremities 1
• Arterial blood gas showing unexplained hypoxemia, especially with positional changes
• Chest X-ray may show normal heart size (unlike other shunt lesions) 1

Factors That Increase Right-to-Left Shunting

• Activities that increase right atrial pressure:

  • Valsalva maneuvers
  • Positive pressure ventilation
  • Coughing, straining 2

• Medical conditions:

  • Pulmonary hypertension
  • Right ventricular failure
  • Acute pulmonary embolism
  • Acute respiratory distress syndrome 2

Anatomical Risk Factors

• Atrial septal aneurysm - excessive bulging of atrial septal fossa ovalis

  • Often associated with septal fenestrations
  • Marker of increased embolic risk 1

• Tunnel-like PFO with spontaneous left-to-right shunt 1

Clinical Significance and Complications

Large right-to-left shunts through a PFO increase risk of:

• Paradoxical embolism leading to cryptogenic stroke
• Worsened hypoxemia in patients with pulmonary disease
• Decompression sickness in divers
• Platypnea-orthodeoxia syndrome 2, 6

Pitfalls in Diagnosis

• Estimating PFO size based solely on amount of contrast seen in left atrium may be unreliable 1
• Failure to perform adequate Valsalva maneuver during testing can lead to false negatives
• Confusing PFO with other causes of right-to-left shunting (ASD, pulmonary AV malformations) 1
• Overlooking PFO in patients with cryptogenic stroke 1

Remember that a large right-to-left shunt through a PFO may warrant consideration of closure, particularly if you've experienced a cryptogenic stroke or TIA, especially if you're between 18-60 years old 2.