What activities increase right-to-left shunting through a patent foramen ovale (PFO)?

Activities That Increase Right-to-Left Shunting Through a Patent Foramen Ovale

Activities that increase right atrial pressure above left atrial pressure are the primary drivers of right-to-left shunting through a patent foramen ovale (PFO). These include Valsalva maneuvers, positive pressure ventilation, and conditions that elevate pulmonary vascular resistance 1, 2.

Physiological Mechanisms of Right-to-Left Shunting

A PFO is a communication between the right and left atrium that typically has predominantly left-to-right flow. However, when right atrial pressure exceeds left atrial pressure, the shunt direction can reverse, causing right-to-left shunting 1. This reversal allows venous blood to bypass the lungs and enter the systemic circulation directly, potentially leading to paradoxical embolism.

Activities and Conditions That Promote Right-to-Left Shunting:

1. Valsalva Maneuver

   • Straining during defecation
   • Heavy lifting or weight training
   • Coughing forcefully
   • Bearing down during childbirth
   • Playing wind instruments
   • Diving equalization techniques 3

2. Respiratory Factors

   • Positive Pressure Ventilation: High positive end-expiratory pressure (PEEP) in mechanical ventilation significantly increases right-to-left shunting 1
   • Obstructive Sleep Apnea: Associated with increased prevalence of right-to-left shunting 4

3. Hemodynamic Changes

   • Pulmonary Hypertension: Increases right atrial pressure
   • Right Ventricular Failure: Leads to elevated right atrial pressure
   • Pulmonary Embolism: Acute increase in pulmonary vascular resistance 1

4. Body Position Changes

   • Orthodeoxia-Platypnea Syndrome: Upright posture can increase shunting in some patients 5
   • Knee bends with breath-holding (increases intrathoracic pressure by 172% compared to standard isometric effort) 3

5. Disease States

   • ARDS: Increased pulmonary vascular resistance and right ventricular afterload 1
   • Septic shock: Can cause right ventricular dysfunction

Clinical Implications

The clinical significance of right-to-left shunting depends on the size of the PFO, the magnitude of the pressure gradient, and the clinical context:

• Hypoxemia: Right-to-left shunting can worsen hypoxemia, particularly in patients with pulmonary disease 1
• Stroke Risk: Increased risk of paradoxical embolism and cryptogenic stroke 1
• Decompression Sickness: Divers with PFO have increased risk when performing maneuvers that cause rebound blood loading to the right atrium 3, 6

Diagnostic Considerations

• Transesophageal Echocardiography (TEE): Has higher sensitivity for PFO detection than transthoracic imaging 1, 2
• Bubble Study: Sensitivity is increased by having the patient perform a Valsalva maneuver 1, 2
• Transcranial Doppler: Can detect right-to-left shunting as a surrogate for PFO 4

Management Implications

For patients with symptomatic right-to-left shunting:

• Avoid Activities that increase right atrial pressure in patients with known PFO and history of paradoxical embolism 1
• Optimize Treatment of any underlying conditions causing elevated right atrial pressure 2
• Mechanical Ventilation Considerations: When mechanical ventilation is required, limit positive end-expiratory pressure and use low tidal volumes (approximately 6 ml/kg lean body weight) to minimize adverse hemodynamic effects 1
• PFO Closure: May be considered for patients with right-sided cardiac disease with elevated filling pressures or those with cryptogenic stroke 2

Understanding these mechanisms is crucial for managing patients with PFO, particularly those at risk for paradoxical embolism or significant hypoxemia due to right-to-left shunting.