- Left to right
- Right to left
- Can be in both directions
- No significant flow at all !
Answer : Every response can be correct
The patent foramen ovale is a physiological orifice , which becomes pathological if persist into adult hood .The incidence is estimated to be about 20 % of the population (Amounts to 100 crore PFOs roaming in our planet!). It makes no sense to believe just spotting a PFO in routine echocardiography be termed as pathological . But recently (Adding much to interventionist’s delight ! ) the presence of which is being linked with migraine and stroke in young.
The size of the orifice can be from a single millimeter to one centimeter* . The direction of blood flow in PFO is determined by the mean gradient across the orifice. It has to be left to right as the LA pressure is generally higher by few mm mercury ,hence there is a small tide of flow entering into RA with each left atrial filling or contractile wave .(v and a ). This quantum is miniscule and has no hemodynamic significance in most life situations.
* Some call( Wrongly ) 1cm PFO as small ASD.
When can Right to left to flow occur ?
When the right atrial pressure increase more than LA pressure it is obvious blood can enter LA . It is well-known this occurs in any pathological situations like RVOT obstruction severe PHT , tricuspid valve obstructions etc.
Physiological Right to Left flow :
Forced expiration (Valsalva) can cause transient right to left flow. This may happen in many real life situations like straining, heavy isometric exercise, blowers, muscians etc.
Which is clinically significant ?
Left to right or right to left ?
Left to right shunting is rarely an issue as there is no systemic desaturation.
Right to left shunting can be important for two reasons
- Arterial desaturation( transient )
- Shifting of venous debris into arterial side can result in potential paradoxical embolism .(This can be air, clot fat , amniotic fluid etc) This is the reason stroke in young is closely linked to presence of PFO.
PFOs during positive pressure ventilation
PEEP is a classical example where a right atrial positive pressure , shunts the blood in pulsatile manner into left atrium .
Platyponea hypoxia syndrome .
This is postural right to left shunting across PFO .It is a less recognised (but a common entity) where -in , when the patient lies down there is a right to left PFO shunt and transient hypoxia .This is often corrected as the patient sits up. The reason being the valve of PFO , the door like flap which guards the orifice , is aligned in such a fashion , it opens up in a lying posture(Aided by gravity ?) , shuts down in sitting posture .It should be noted The PFO valve is not a constant feature . The size of this valve , the stiffness , the hinge points , ability to float are highly variable .Hence the clinical variation in PFO syndrome.
The IAS septal aneurysm is an important variation where the valve of PFO balloons out into left atrium may become a nidus for thrombus or a focus for atrial arrhythmias .
Stroke in young and PFO :This topic deserves a separate article
Excellent PFO images from Yale university library ( http://www.yale.edu/imaging/chd/e_pfo/index.html)
Excellent PFO images from Yale university library