Any risk from PFO test?

[bluebanded goby]

One of the ways to test for the presence of a patent foramen ovale (PFO) is to inject bubbles into the patient's blood system, and then use ultrasound to watch to see if they pass across a shunt in the heart.

Question: If the patient has a PFO, is there any risk from these test bubbles getting into his/her arterial system (for example, going to the brain)? I can't imagine that the test would be given if it's risky, but it's not clear to me why these bubbles would be safe and bubbles from diving aren't. Is the size of the bubbles controlled in some way? (Which begs another interesting question -- How do they do that?) Thanks for any comment.

 

[Dr Deco]

Dear BB Goby:

This is one of those questions similar to “If exercise causes micronuclei, how can exercise be OK for washout of nitrogen?” It all lies in the “intensity factor.” That is, how many bubbles are we dealing with during the test?

Safety

The PFO test is safe because not a very great volume of gas is used, and the bubbles are very small. They pass through the capillaries of the brain and exit into the veins.

- - - - - - -

EXTRA STUFF - - :grad:

Saline Contrast Microbubbles

For the PFO test, microbubbles are formed in saline (= special salt solution), typically by drawing the solution aback and forth between two coupled syringes. The solution is then injected into a vein in the arm and the presence of gas bubbles detected ultrasonically by several methods. The most sensitive is either transesophageal echo imaging or transcranial Doppler flowmeter ultrasound. Both of these are quite good. The third method, and most common, is transthoracic echo imaging. This last does not really have very good sensitivity compared to the first two.

These ultrasound tests are performed without and with a Valsalva maneuver. The PFO is then identified as a “resting” or “augmented” PFO.

Hemodynamic Significance

It is at this point that the test becomes problematic. Physicians investigating the passage of gas bubbles or clots from the veins to the arteries have noted that all PFOs are not created equal with regard to hemodynamics (= blood flow). Thus a positive test does not necessarily mean that one is a risk for neurological DCS. It is necessary that quite a few bubbles be detected ultrasonically during the injection of the saline contrast. :doctor:

Bubble Generation

It is also necessary that an individual generate venous bubbles during decompression. The probability that bubbles are generated AND a hemodynamically significant PFO is present is not very high.

Added to this is the observation that fewer bubbles pass the PFO when the Valsalva maneuver is performed in the recumbent position than when upright. Thus, divers are protected to some degree because they are erect.

Raising Your Chances for DCS

The advantage of “erect” is lost is you are lying down and coughing (when you have bubbles and a PFO). You are also at risk if you perform a Valsalva-like maneuver when you have a PFO. This would include climbing up the boat ladder and holding and releasing your breath. (We seldom breathe in a rhythmic fashion when engaged in a strenuous activity.)=-)

Dr Deco :doctor:

 

[The Iceni]

If I can add to Dr Deco's excellent reply?

As I undersatnd it the bubbles are not harmful because they are small but also because they rapidly disappear and so do not cause a relatively permanent obstruction to blood flow in any part of the body when they reach the capilliaries, in which the larger bubbles would lodge.

As Dr Deco described they are generated artificially from a solution quite capable of dissolving that gas (That's where it came from after all!). Even when injected into the body the gas phase finds itself in a solution that is not oversaturated with that gas, thus even the nitrogen within these bubbles is rapidly taken back into solution within the body;-

This is all to do with surface tension. As Dr Deco frequently points out the pressure within a small bubble is greater than its surroundings because of surface tension. This pressure forces the gas into solution and the bubbles collapse, like a leaky rubber toy balloon. As you know, if the syringe of bubbles is left for only a minute or so all the bubbles will disappear into solution.

On the other hand, :egrin: if this test was done immediately after diving it could be very nasty indeed because some of the bubbles would grow by taking on gas from the surrounding supersaturated tissues which have a higher effective pressure than the interior of the bubbles , causing a form of iatrogenic DCI if a sufficient number found their way into the arterial circulation.

Hence the significance of micronuclei in the pathogenesis of DCI.

 

[bluebanded goby]

Thanks to Dr. Deco for the very helpful reply, and also to Paul for his additional comments.

A couple of statements prompted an extra question or two:

"The most sensitive is either transesophageal echo imaging or transcranial Doppler flowmeter ultrasound. Both of these are quite good. The third method, and most common, is transthoracic echo imaging. This last does not really have very good sensitivity compared to the first two."

I had a transthoracic without bubbles once, which I understood to be the least sensitive of all the tests. Nevertheless, I was given to understand that it (being negative) ruled out a "really really really big" PFO. Next month I'm going to be tested again, this time with a bubble study; I don't know the type of imaging, but I'm guessing that it'll be transthoracic again. I was given to understand that the addition of the bubbles would significantly boost the sensitivity of the test, at least to detect a significant (i.e. large) PFO. Is this the case? Given that I'm getting the test for free through my HMO, is the benefit of transcranial Doppler great enough that it would be worth paying out of pocket for it? (I'm not sure that I'd want transesophageal -- probes snaked down one's throat don't sound like a cakewalk.)

"Added to this is the observation that fewer bubbles pass the PFO when the Valsalva maneuver is performed in the recumbent position than when upright. Thus, divers are protected to some degree because they are erect."

They are? I have a lot of diver friends who subscribe to the DIR philosophy and try to stay as horizontal as possible throughout their dives, including the safety stops. Is this an argument for doing your safety stops in a more vertical position?

Also, I heard somewhere that offgassing of nitrogen peaks at about 20-40 minutes after the end of a dive. Would this be an argument for walking around the dive boat instead of sitting in the galley (or worse, hitting a bunk)? Should one observe a margin of safety before going to bed after a night dive? (Sorry to get off on a tangent here, I know that issues such as post-dive exercise have been discussed in other threads, and it would probably behoove me to look them up.)

 

[The Iceni]

bluebanded goby once bubbled...
. . ."Added to this is the observation that fewer bubbles pass the PFO when the Valsalva maneuver is performed in the recumbent position than when upright. Thus, divers are protected to some degree because they are erect."

They are? I have a lot of diver friends who subscribe to the DIR philosophy and try to stay as horizontal as possible throughout their dives, including the safety stops. Is this an argument for doing your safety stops in a more vertical position?

I suspect there are many reasons DIR afficionados recommend horizontal stops not least of which is that it should be the trimmed (natural) position and therefore the most comfortable. In addition all parts of the body will be at the stop depth.

I am not sure you read this part of Dr Deco's post correctly (or possibly he has transposed the words "erect" and "supine" in his final sentence) as the results seem to suggest that if you have a haemodynamically significant PFO a vertical stop would allow more bubbles to pass through the PFO into the arteries, thus actually increasing the risks of DCI. To my mind this adds more weight to the recommendation for a horizontal attitude, not less.

Also, I heard somewhere that offgassing of nitrogen peaks at about 20-40 minutes after the end of a dive. Would this be an argument for walking around the dive boat instead of sitting in the galley (or worse, hitting a bunk)? Should one observe a margin of safety before going to bed after a night dive? (Sorry to get off on a tangent here, I know that issues such as post-dive exercise have been discussed in other threads, and it would probably behoove me to look them up.)

I would be interested to learn the theory behind this 20 minutes.

There are several factors at work here.

First is the Haldanian theory, which predicts offgassing is solely determined by the partial pressure gradient and is maximum immediately after surfacing and then decays exponentially with time.

Then we have those bubbles, which if I have understood the RGBM theory correctly, take on gas from the tissues and grow in size until the overall tissue loading is sufficiently reduced, not until then do the bubbles (and affected tissues) start to offgass. Thus I suppose the body's total maximum offgassing may be delayed until that time. (This is purely conjecture, my brain working overtime and I may be quite wide of the mark)

From my reading it would seem any moderate, non weight-bearing exercise will increases offgassing by improving blood flow to the tissues.

By the way, if I may ask. Why the PFO test?

 

[Dr Deco]

Dear BB Goby:

Transposed Body Positions

During a Valsalva or Valsalva-like maneuver, there are more gas bubbles passed through the PFO in the RECUMBENT position than the SITTING or upright position. The proofreading of the part under “Bubble Generation” was not adequate. Thanks for catching that, Paul.

This revolves around the magnitude of the rebound flow (= slug of blood that enters the chest when the breath-hold is released) and the increase in atrial preload (= increase in volume of blood in the right atrium). These two processes result in a reversal of the normal left-to-right pressure gradient that would normally prevent blood (and bubbles) from directly passing from the right to the left atrium, and hence to the systemic arterial system.

The ATTACHMENT shows the original study of Schwartz involving contrast agents and Valsalva maneuvers. The red line shows, in this example, that 2.5 ml of contrast agent are need to arterialize 20 bubbles if recumbent whereas 8.0 ml are need for the same 20 bubbles if the subject is upright (green line).

Water Body Position during Stops

I suspect that this "erect vs. recumbent" business would not play a big role during stops as gas bubble formation is not really very high at this stage. Most bubbles appear a bit after surfacing in recreational divers.

Off gassing

The twenty minutes refers, I believe, to the peak in Doppler bubble appearance. Off gassing is greatest when the gradients are highest, and they decrease with time. If tissue bubbles are present, dissolved gas will be lost but to the bubbles rather than to the capillaries. [This is not a big advantage to the diver.]

Chest Echo

Unless there is a good size PFO, this test will not detect it. However, passage of saline contrast bubbles in this case would indicate a “hemodynamically significant lesion.” There does not exist too much data on this subject.

What apparently was done before (without the bubbles) was a color flow Doppler ultrasound study. This detects bigger PFOs.

I would suspect that a transcranial Doppler study might be more trouble than it is worth (with an equivocal result for a recreational diver).

As Dr Thomas asks, is there a reason that you suspect a PFO problem? Otherwise this echo bubble contrast test is not done, either for US Navy divers, US Air Force personnel or for NASA astronauts.

Dr Deco :doctor:

 

[bluebanded goby]

Thanks to both Dr. Deco and Paul for their follow-up responses. I'll have to spend some time digesting them.

In response to the question posed by both of you -- Why am I having this test? -- I sometimes experience peripheral neuropathies, which is to say tingling, pains, etc, in my extremities. These have occasionally emerged after diving, and in some cases have been difficult to distinguish from DCS. I'm being seen by an excellent specialist in hyperbaric medicine in our area. He seems fairly certain that none of my symptoms are dive-related, but just to be on the safe side he suggested a 2-D echo with bubble study to rule out a significant PFO as a factor. I'm well aware from readings on the topic that PFO usually isn't an issue for recreational divers who stay within no-deco limits.

The dive doc has also suggested other tests to investigate possible causes of the symptoms (head/neck MRI; various blood tests for diabetes, thyroid, whatnot). So far these aren't turning anything up. I suspect I may be left in a situation where we're fairly confident that the sensations aren't dive-related, but there isn't anything that can be done to cure them per se. I would therefore have to decide whether it's possible to continue scuba without worrying about every pain and assorted sensation following a dive. But I'll deal with that when I can have a long talk with the doctor after all the test results are in. Thanks again for the help.

 

[Dr Deco]

Dear BBGoby:

Neuropathy

As the specialist stated, it is doubtful that these problems would be from embolization, since that is essentially a random process with respect to where the arterial bubbles will go in the central nervous system. Systemic arterial bubbles would be accompanied, most likely, with CNS problems (for example, weakness, paralysis, cognitive difficulties, vision problems, etc).

Nevertheless, since saline contrast echo ultrasonography is a simple test, the clinician wishes to give it a try to rule out this possibility – small though it is.

Dr Deco :doctor:

 

[The Iceni]

Hi BB Goby,

I hope your problems sort themselves out and you can continue diving.

Horizontal vs vertical?

Dr Deco once bubbled...
Dear BB Goby:

. . Added to this is the observation that MORE bubbles pass the PFO when the Valsalva maneuver is performed in the recumbent position than when upright. Thus, divers are protected to some degree because they are erect.
Dr Deco

Forgive my liberty editing your comment Dr Deco but I think this is what is observed, but does this really provide evidence that a vertical stop is safer than a horizontal stop? I too very much doubt it because this is a completely artificial set of circumstances and the incidence of type II DCI is so rare while PFOs are present in up to 30% of the diving population.

Firstly, the test is performed on the surface where the effects of gravity on venous return in the great vessels are not countered by bouyancy.

Secondly, a valsalva manoeuvre is required and few divers perform this during decompression.

Thirdly, gravity will encourage arterialised bubbles to migrate to the upper parts of the artery in which they find themselves, ending up in the positionally higher parts of the arterial tree to cause effects in those areas. In an erect diver this would of course be the brain.

So in my humble opinion the jury's still out.

 

[The Iceni]

bluebanded goby once bubbled...
I sometimes experience tingling, pains, etc, in my extremities. These have occasionally emerged after diving, and in some cases have been difficult to distinguish from DCS. I'm being seen by an excellent specialist in hyperbaric medicine in our area. He seems fairly certain that none of my symptoms are dive-related, but just to be on the safe side he suggested a 2-D echo with bubble study to rule out a significant PFO as a factor.

As a generalist we are taught to consider differential diagnoses and prioritise according to incidence (common things are more likely) and severity before we order tests or refer to an appropriate specialist.

Having not seen or examined you, least of all taken a history, I am not sure if the following will help. However, your symptoms may be due to problems with your peripheral circulation alone.

Your doctors may wish to exclude Raynaud's phenomenon.