Can Blood Flow from Left to Right Through a PFO?
Yes, blood flow through a patent foramen ovale (PFO) is predominantly left-to-right under normal physiologic conditions, though the direction can reverse based on changing atrial pressure gradients throughout the cardiac cycle and with respiration. 1
Understanding Bidirectional Flow Dynamics
Normal Physiologic Flow Pattern
Under typical hemodynamic conditions, shunting through a PFO is predominantly left-to-right because left atrial pressure normally exceeds right atrial pressure 1. This pressure gradient forces the septum primum against the foramen, functionally closing it during most of the cardiac cycle 2.
Dynamic Pressure-Dependent Reversal
The critical concept is that flow direction through a PFO is not fixed—it changes dynamically based on instantaneous pressure gradients between the atria 1. The guidelines explicitly state: "Although shunting usually is predominantly left to right, there is some right to left shunting as the relative pressures in the two chambers change during the cardiac cycle and with respiration" 1.
Clinical Conditions Causing Flow Reversal
When Right-to-Left Shunting Occurs
Right-to-left shunting becomes clinically significant when right atrial pressure transiently or persistently exceeds left atrial pressure:
Valsalva maneuver: Transiently increases right atrial pressure, forcing the septal flap open and allowing right-to-left flow 1. This is why Valsalva is used diagnostically to increase sensitivity of bubble studies.
Cardiac surgery: During off-pump coronary bypass with heart manipulation, CVP can increase dramatically (15 to 30 mmHg), reversing a left-to-right shunt (307 mL/min) to right-to-left (161 mL/min) 3.
Pulmonary hypertension or right ventricular dysfunction: Any condition elevating right-sided pressures can promote right-to-left shunting 4.
Diagnostic Implications
Echocardiographic Detection
The 2018 BMJ guidelines specify that microbubbles entering the right atrium will pass into the left atrium within a few beats if a PFO is present 1. This diagnostic approach exploits the fact that even with predominantly left-to-right flow, there are moments during the cardiac cycle when right-to-left passage occurs.
Spectral Doppler interrogation of PFO flow can directly measure the instantaneous pressure gradient between atria, providing hemodynamic information about left heart pressures 5.
Clinical Significance
Paradoxical Embolism Risk
The pathologic concern with PFO is paradoxical embolism—when the shunt direction reverses, allowing venous thrombi or right atrial thrombi to travel directly into the arterial circulation 1. This mechanism is implicated in 25-50% of cryptogenic strokes in patients under 60 years 6.
Key Anatomic Risk Factors
- Atrial septal aneurysm: Associated with septal fenestrations and increased embolic risk 1
- Large shunt size: Correlates with higher stroke risk
- Prominent Eustachian valve: Can direct venous flow toward the PFO 7
Common Pitfalls
Do not assume PFO flow is unidirectional. The fetal structure exists precisely because it allowed right-to-left flow in utero (bypassing pulmonary circulation) 8. After birth, while the pressure gradient reverses making left-to-right flow predominant, the anatomic communication remains bidirectional and pressure-dependent.
Estimating PFO size based on contrast amount in the left atrium may be unreliable 1, as the amount of right-to-left shunting depends on both anatomic size and the instantaneous pressure gradient during the examination.