The trouble with PFOs
by Bob Cole
THE US SPACE AGENCY NASA nearly lost a mission because of a PFO, and there have been five NASA cases of decompression illness in space! That’s right, not UFOs, PFOs. Many doctors have yet to make the connection between PFOs and people under pressure – and that means not only astronauts but divers.
PFO stands for Patent Foramen Ovale. Translated from Latin, patent = open; foramen = aperture and ovale = oval, so a PFO is an oval hole, with a flap, between the right and left top chambers of the heart, or atria.
Which is right and which is left? The human body is usually described from the patient’s point of view.
A PFO is a remnant from our time in the womb. Before birth, before our lungs are used for breathing, oxygenated blood supplied by mum bypasses the lungs by flowing from the left to the right atrium.
At birth, the foramen ovale should close and seal, but in a number of people it doesn’t seal fully. Some of these PFOs require surgery; most do not. Some 25-30% of people are thought to have a PFO, which of course includes divers. In normal life a minor PFO causes no serious problems, but for some divers under certain conditions, large PFOs can become problematic. During any ascent from 10m and below on air, bubbles are formed. These are washed with the venous blood into the alveoli of the lungs.
They become trapped here, then, almost instantly, dissolve out, releasing their excess nitrogen and other gases to be breathed out in the normal way. It is the supply of micro-bubbles (MBs) that is important. MBs arriving at the lungs reach their peak in numbers within 15-20 minutes of surfacing, and then diminish in three broad reducing waves over the next 180-200 minutes (see graph, above right).
Poor ascent control can overwhelm the pulmonary system with micro-bubbles, leading to DCI. The lungs, which catch MBs, are sometimes referred to as the “pulmonary filter”. Bubbles are seen by the immune system as alien, and this may activate a response that can lead to tissue damage, wherever they are in the body. By controlling our ascents, stops and surface intervals properly, we reduce MB generation to a minimum.
A PFO is not just a hole in the heart between the right and left atrium. It is more like a short tube, up to 7mm long. The flap, in the left top atrium, acts as a one-way valve that, when open, allows blood to flow from right to left only (see diagram, left). For about 95% of the time in the cardiac cycle, the pressure in the left side of the heart is higher than that on the right. This tends to keep the flap valve closed.However, there are times during normal living, such as during a Valsalva manoeuvre, when the right-side heart pressure can exceed that of the left. Blood and any debris (clots, bubbles etc) may then flow through this opening, by-passing the filter of the lungs and entering the arterial circulation. Crossing blood clots can lead to a stroke; crossing bubbles can cause DCI.
ANTONIO-MARIA VALSALVA (1666-1723) was an Italian physician and anatomist who studied the ears. He coined the term “Eustachian Tube”, and described the aortic sinuses of Valsalva in his writings, published posthumously in 1740.
In a Valsalva Manoeuvre (VM), a person tries to exhale forcibly with a closed windpipe so that no air exits through the mouth or nose – as, for example, in strenuous coughing, straining during a bowel movement, or lifting a heavy weight.
Divers use VMs to equalise middle-ear pressures during descent, by pinching the nose and blowing to open the Eustachian tube. Valsalva actually described the technique as a method of expelling pus through a perforated eardrum.
For 18 years I’ve been trying to persuade divers to clear their ears without using VMs. I first published this advice in a book called Decompression And Computer Assisted Diving.
A number of divers, including trainees, say that they can clear their ears only by using VMs, to which I say, keep practising. If used, VM ear-clearing must be “gentle”, but how many times have you seen someone hanging on a line, pinching their nose with the other hand and blowing like hell?
Usually, this approach simply causes the Eustachian tube to lock up but, more seriously, it could well open any lurking PFO. This is not an issue on the day’s first dive, but may be important on dive two and three, if residual micro-bubbles are present!
AS WELL AS THE 25-30% OF PEOPLE with a PFO (Fawcett EF; 1901), Divers Alert Network (DAN) estimates that about 10% of folk have a right-to-left arteriovenous pulmonary malformation. Normally, the lungs filter out micro-bubbles, which get stuck in the very small capillaries of the alveolus, but in these people a section or sections of these alveolus blood vessels are big enough to allow normal MBs to pass through. In a recent study by DAN Europe, a group of divers was retested after a 6-8-year period and a number of changes were observed. Twelve per cent had a bigger PFO than before, a further 12% had acquired one where none had existed, and one had closed. No reason was found for the changes, but it is thought that the very small PFOs seen originally in some of the divers had enlarged over time. If you find all this alarming, remember that of the PFOs found in 25% of autopsies, only 3% are of the 10mm-plus size thought necessary to have a significant right-to-left shunt.We are not seeing high numbers of divers getting DCI from these causes. It is thought to occur only to those with the largest shunts, though this would still put around 5% of us at risk. Mother Nature, as always, is working to protect you. Three mechanisms limit the incidence of DCI cases from this cause: the fact that the PFO is a one-way valve restricting right-left blood flow; that the one-way valve is kept closed 95% of the time; and that returning blood mixes turbulently within the right atrium and is swept, with any micro-bubbles, away from the entry of any PFO. Fairly recently doctors have linked some “migraines with aura” with the presence of a PFO, though this has yet to be confirmed. If you suffer from these, check with your GP and then, if necessary, with your local UK Sports Diving Medical Council Referee. Unfortunately, there is no non-invasive PFO testing method, and doctors won’t routinely test for PFOs during normal diver medicals, because of the risks and cost involved. Most people with a PFO find out only after suffering an unearned DCI, particularly where a skin bend is involved. If a PFO is found it can be surgically closed, and the benefits are more than just diving-related, because patients with a PFO are also at greater risk of a stroke. However, some hyperbaric doctors feel that the risks of PFO DCI are too small to warrant concern or repair, particularly as repair has its own share of risks. Dr Richard Vann of DAN USA told me of a diver who got bent again after having a PFO repaired. Dr Phil Bryson, of Plymouth’s Diving Diseases Research Centre, says he also has such a patient, and two others who want their PFO closures removed. So having a PFO fixed doesn’t make you immune to DCI. No dive is risk-free, and closing a PFO reduces the risk only to that of a diver with a normal heart. As there is no medical consensus on DCI and PFOs, they are not considered an absolute bar (contra-indication) to diving. Better control of micro-bubbles by means of superior dive profiles, better ascent control, decompress/safety stops at the correct depth and diver behaviour as outlined in the panel below reduce the risk. No MBs = no DCI.
I’M SURE MANY EXPERIENCED DIVERS reading this article will shrug and think: I’ve been diving all these years, and I’m still all right. It was what happened to two of my friends that led me to write this. They had been diving for 30+ years (5000+ dives) and 12+ years (2000+ dives) respectively with no DCI problems – until recently. The first got a vestibular bend (ear/CNS – very serious) from a 24m dive that was much shorter than the allowable no-stop bottom time, with no ascent violations. The other had a very serious neurological hit from a normal, non-provocative dive profile, with extra time spent at the last stop.
Later, both were diagnosed with PFOs. Even after all their years of diving experience, they had no idea of what was in store for them. One has elected for surgery, the other has not. Both have moderately changed their behaviour before, during and after diving. The recommendations below will cost you very little but may save you a lot. Think of them as an insurance policy.
All surgical procedures involve risks, and the cardiologist should explain these before any decision to operate is taken. PFO closure is achieved without opening the chest or heart. A special wire with a button closure is fed into a vein in the groin up to the right top atrium and through the PFO. The device is released in the PFO to form what looks like a tight cuff-link (see diagram below left).The patient usually goes home the next day. A month later, a repeat echo cardiogram test verifies that the device is still in place, while a bubble test with Valsalva checks that the right-to-left shunt is sorted out.Two or three months later, the heart’s skin cells will have grown over and covered the device. After six to eight weeks the lining tissue will have filled the spaces in the device, making it invisible to ultrasound. Blood-thinning drugs are taken to reduce the risk of clotting and unwanted deposits building on the implant.The patient can dive again after three months or so, if the cardiologist agrees.
The rare, but possible, risks include:
# Incomplete sealing of the hole
# Dislodgement of the closure device
# Fracture of the implanted device
# Perforation of vessels or the muscle tissue of the heart (myocardium)
# Abnormal heart rhythms
Modify your behavior PFOs become less important for divers if there are fewer or, preferably, no free-gas (bubbles). Modest changes in diver behavior can help reduce the amount of free gas and limit PFO opening and micro-bubble wash-through. Divers can best protect themselves by learning to work with, rather than against, Nature. For example, some technical divers dekit in the water and let others do the lifting – sounds good to me!
They also use many of the other techniques mentioned here:
AS A MINIMUM:
1. Maintain your hydration. Drink plenty of water and check that your urine is no darker than a pale straw colour.
2. Avoid VMs for ear-clearing – use a procedure such as swallowing.
3. Skip-breathing causes CO2 retention. Breathe long and slowly, using your diaphragm, not your upper chest.
4. Don’t dwell at depth, and ascend at 10m/minute.
5. Use deep stops to help manage your ascents.
6. Have longer surface intervals – three hours or more, the length of time it generally takes for MBs to disperse.
7. Avoid hot baths/showers and sunbathing for three hours after diving – heat promotes bubbling.
8. Avoid unnecessary carrying or lifting of heavy items for at least three hours.
9. Avoid straining during a bowel movement after diving.
IF YOU HAVE A PFO, AND MEDICAL CLEARANCE TO DIVE:
10. Breathe pure oxygen for about 15 minutes with a nose-clip fitted before diving.
11. Use the richest nitrox possible for the planned depth.
12. Treat the nitrox as if it were air and decompress accordingly.
13. Breathe pure oxygen for about 30 minutes with a nose-clip fitted after diving.
14. For the first three hours get someone else to do your lifting and carrying.
15. Avoid deep or long dives, too many dives in one day, stage-stop diving and reverse dive profiles.