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John,
As you point out, the people doing the test often don't know why you want
it done. GIGO. The real issue is whether it actually is a sensitive test:
Whether it measures what you want to know - do you have a PFO? And whether
it makes a damn bit of difference.
There are no studies that I can find dealing with the predictive value
of the test, because there is no gold standard. Every few years someone
develops an improved method and reports an "increased" incidence of
PFO.(See refs) The fact that the Navy and others use a given test, should
NOT be considered an endorsement of the test's validity. With all due
respect to the Navy, not everything it does, even as part of medical
evals, is based on sound medical logic.
I think you may have confused the post-dive doppler studies with the
PFO issue. Probably my fault as I mentioned them together in a previous
post. Post-dive doppler studies look for bubbles in venous blood (usually
in the vena cava or great arm veins). As you point out, "silent" bubbles
are found in some divers and the assumption is these predict who is at
risk of DCS. This assumption may not be true, may be true and related, or
(my opinion) be true but unrelated to how DCS occurs.
PFO studies examine if there is right (venous) to left (arterial)
side blood flow in the heart, through the hole that supposedly closes up
shortly after birth. The idea is that arterial gas embolus, the most
severe form of DCS, is associated in some cases with this cardiac defect.
It allows bubbles or foam to cross to the arterial side and do more damage
- like go straight to your brain or coronary arteries. AGE is pretty rare.
If 30% of everyone actually has a PFO, then PFOs are bound to show up in
(1:3) people who get AGE. Not everyone who has AGE has a PFO, even at
autopsy. So there's an association, but not proof that PFOs lead to AGE.
Would I dive if I knew I had a PFO? Depends on how much I had
invested in my gear ;-) Probably not, if you believe that gas/foam can
cross through a PFO and create an embolus to your brain. But suppose that
the gas/foam is created largely where it causes the damage. It actually
forms around tiny nuclei that have no problem passing through your lungs
and so are present (under the right/wrong circumstances) in *both* the
arterial and the venous circulation? Then it doesn't really matter if you
have a PFO, except for those very unusual cases where a bubble or foam can
actually pass from the arterial to the venous side.
So let's examine the data to see if that does happen:
Of the 15 deaths reported by DAN for 1994 that were autopsied and
listed arterial gas embolus or air embolus as either as a major or
the significant cause of death, *none* listed PFO as a contributing or
incidental finding. I would have expected PFO to turn up as an incidental
finding in at least 5 of the deaths, just by coincidence.
So what is the real incidence of PFO in an otherwise normal population?
Bennet & Elliot's book, The Physiology and Medicine of Diving refers to
the study by Moon, Camporesi and Kisslo in Lancet 1, 513-514 (1989) which
started all this. In 30 divers with DCI, they found PFO with shunting
during Valsalva in 61% of a subset of 18 who had "severe" DCI; they
compare this with an incidence of 3% in 174 "normal" controls. Open and
shut case. Right? No.
In the same book and in other published papers, the incidence of PFO is
10-30% in otherwise normal (no DCI events) people, depending which test
you use. So maybe what Moon et al found was a true factoid, but unrelated
to the DCI events.
So, what can we say?
(1) PFO is probably a normal finding in a lot of people if you
increase the right heart pressure enough. (Cursing George -
yes; farting at 1 ATM - no)
(2) There is no evidence that PFO is found excessively (or at
all) in people who die of AGE.
(3) No test for PFO appears to physiologically approximate the
kind of increased right heart pressures that may be seen in
divers.
(4) While blood clots rarely pass from the
right to the left side of the heart through a PFO, there is no
evidence that this is the mechanism of propagation for AGE or
other DCI.
As Francis and Gorman summarize in Bennett & Elliots' book, "However,
further studies will be necessary before the association between atrial
septal defects (PFO) and decompression illness can be considered causal."
So after all this, if you've decided that whatever, you won't dive if you
have a PFO, doesn't it make sense to *know* if you actually have a PFO?
(Yes George, if you've read this far, I know you're saying "Right!
Right!") BUT, I don't think the current tests do that reliably. A bad test
may be worse than no test because it may give you a false sense of
security. Much better to do all those things you can to reduce your chance
of getting DCS.
-ph
Some references on PFO prevalence and diagnosis:
Title: Paradoxical embolism. An underrecognized problem.
Author: Ward-R. Jones-D. Haponik-E-F.
Source: CHEST, (1995 Aug) vol. 108(2): 549-58
Abstract: Despite reports of the clinical presentations and
devastating consequences of paradoxical embolus (PDE) for
more than a century, this diagnosis continues to be
frequently missed. Because the prevalence of patent foramen
ovale (PFO) is 27 to 35% in the normal population and the
presence of deep vein thrombosis or pulmonary embolus may
not be clinically obvious, a high suspicion for PDE is
needed in the event of unexplained arterial occlusion.
While contrast echocardiography and transcranial Doppler
ultrasound have facilitated clinical recognition of PDE, the
optimum approach to diagnosis requires clarification.
Primary therapy for patients with PDE is anticoagulation,
with thrombolytics considered in carefully selected
individuals, but there is little published information
regarding long-term treatment and outcomes. Prevention
remains essential whenever possible. It is not yet defined
whether prophylactic treatment of persons with recognized
predispositions to PDE (eg, PFO and pulmonary hypertension)
is beneficial.
> Title: Paradoxical embolism and acute arterial occlusion: rare or
> unsuspected?
> Author: Chaikof-E-L. Campbell-B-E. Smith-R-B-3rd.
> Source: JOURNAL OF VASCULAR SURGERY, (1994 Sep) vol. 20(3): 377-84
> Abstract: PURPOSE: The high prevalence of clinically silent venous
> thrombosis and the presence of a patent foramen ovale (PFO)
> in up to 35% of the general population suggests that
> paradoxical emboli may be the cause of an ischemic stroke or
> a peripheral thromboembolic occlusion more often than is
> presently considered. This study was undertaken to review
> our experience with presumed paradoxical embolism. METHODS:
> Hospital records were reviewed for all patients diagnosed
> with both a documented PFO and a thromboembolic event
> between January 1970 and June 1993. Patients with a
> ventricular or an atrial septal defect or a probable
> pulmonary arteriovenous fistula were excluded. RESULTS: The
> presumptive diagnosis of paradoxical embolism was made in
> seven patients. There were five men and two women, with a
> median age of 43 years. Four patients were admitted with an
> acute cerebral ischemic event, and in three others
> hospitalization was prompted by the development of an
> acutely ischemic limb (two upper extremity; one lower
> extremity). In none was there evidence of angiographically
> significant peripheral or extracranial atherosclerotic
> occlusive disease. Symptoms suggestive of pulmonary emboli
> were noted in two patients, and in only one patient was
> there evidence on physical examination of a deep venous
> thrombosis. Before 1988 the diagnosis of paradoxical
> embolism had been made in only one patient after postmortem
> examination. All six patients who were discharged were
> available for follow-up (mean 20 months; range 6 to 60
> months). There was one late death from lung cancer.
> Recurrent paradoxical emboli have not been documented during
> the follow-up period. CONCLUSIONS: The incidence of
> presumed paradoxical embolism has increased dramatically in
> the recent past as a consequence of our improved ability to
> unequivocally detect PFO with associated physiologic
> shunting. The suspicion of this heretofore "rare" event
> should be raised, particularly in the young or middle-aged
> adult diagnosed with an acute thromboembolic event. Until
> the risk of recurrent ischemic events in the presence of a
> PFO is better defined, we currently recommend closure of the
> foramen ovale after a significant or recurrent paradoxical
> embolus. Otherwise, the selective use of intracaval
> filters, antiplatelet therapy, and oral anticoagulation
> remain undefined.
>
> Title: Comparison of transesophageal and transthoracic
> echocardiography with contrast and color flow Doppler in the
> detection of patent foramen ovale.
> Author: Belkin-R-N. Pollack-B-D. Ruggiero-M-L. Alas-L-L. Tatini-U.
> Source: AMERICAN HEART JOURNAL, (1994 Sep) vol. 128(3): 520-5
> Abstract: We directly compared the utility of agitated saline solution
> contrast echocardiography and color flow Doppler with both
> transthoracic and transesophageal echocardiography in the
> detection of patient foramen ovale (PFO). Forty-three
> patients referred for contrast echocardiography and
> transesophageal echocardiography were prospectively studied.
> Three were excluded because of technically inadequate
> contrast, and two were excluded because of hemodynamically
> significant atrial septal defect. The remaining 38
> patients, who ranged in age from 19 to 73 years, were
> referred for cerebrovascular events (31), peripheral embolus
> (5), atrial septal aneurysm (1), and suspected atrial septal
> defect (1). With either contrast or color flow Doppler, PFO
> was detected by transthoracic imaging in 9 (24%) of 38
> patients compared with 20 (53%) of 38 with transesophageal
> echo. PFO was present in 1 (3%) of 38 by TTE color flow, 9
> (24%) of 38 by TTE contrast, 17 (45%) of 38 by TEE color
> flow, and 14 (37%) of 38 by TEE contrast. Discordant
> findings with TEE were the result of contrast-positive,
> color-negative results in 3 patients and color-positive,
> contrast-negative results in 6. With TEE contrast used as a
> diagnostic gold standard, other techniques detected PFO with
> the following sensitivities, specificities, and positive and
> negative predictive values: TEE color flow 79%, 75%, 65%,
> 86%, respectively; TTE contrast 50%, 92%, 78%, 76%,
> respectively; and TTE color flow 7%, 100%, 50%, 65%,
> respectively. Thus PFO is detected more frequently with
> TEE. TEE contrast and color flow Doppler yielded discordant
> findings in a minority of patients, probably as a result of
> intrinsic limitations in each technique.(ABSTRACT TRUNCATED
> AT 250 WORDS).
>
> Title: Impact of transesophageal echocardiography on the
> anticoagulation management of patients admitted with focal
> cerebral ischemia.
> Author: Hata-J-S. Ayres-R-W. Biller-J. Adams-H-P-Jr.
> Stuhlmuller-J-E. Burns-T-L. Kerber-R-E. Vandenberg-B-F.
> Source: AMERICAN JOURNAL OF CARDIOLOGY, (1993 Sep 15) vol. 72(9):
> 707-10
> Abstract: Transesophageal echocardiography (TEE) improves the
> diagnostic accuracy of transthoracic echocardiography in the
> identification of potential cardiac sources of embolus.
> However, there are few studies of the impact of TEE on the
> medical management of patients with focal cerebral ischemia.
> The records of 52 consecutive, hospitalized patients
> undergoing both TEE and transthoracic echocardiography for
> suspected cardiac source of embolus were reviewed to
> determine the influence of TEE on the decision to
> anticoagulate patients. Of 52 patients, 39 had focal
> cerebral ischemia (transient ischemic attack, n = 9; acute
> cerebral infarction, n = 30). In 4 of these 39 patients
> (10%), the TEE results changed the management of
> anticoagulation. In 19 of 39 patients (49%), the TEE
> results helped confirm anticoagulation decisions, and in 16
> (41%), the results had no effect on anticoagulation
> decisions, because of overriding clinical information. Ten
> of the latter 16 patients had TEE evidence for a possible
> source of an embolus, but were not anticoagulated; 5 of
> these were poor candidates for long-term anticoagulation,
> and the others had right-to-left shunting across a patent
> foramen ovale or an interatrial septal aneurysm. Clinical
> variables (atrial fibrillation, TEE findings and pre-TEE
> anticoagulation status) were considered as possible
> predictors of post-TEE anticoagulation status using logistic
> regression analysis; the strongest predictor of post-TEE
> anticoagulation status was pre-TEE anticoagulation status (p
> < 0.0005). Despite the selection of patients presumed to
> receive maximal benefit from TEE, this study suggests that
> TEE findings are not predictive of subsequent
> anticoagulation management. However, TEE is at least
> confirmatory of anticoagulation decisions in most cases.
>
> Title: Femoral vein delivery of contrast medium enhances
> transthoracic echocardiographic detection of patent foramen
> ovale.
> Author: Gin-K-G. Huckell-V-F. Pollick-C.
> Source: JOURNAL OF THE AMERICAN COLLEGE OF CARDIOLOGY, (1993 Dec)
> vol. 22(7): 1994-2000
> Abstract: OBJECTIVES. We postulated that femoral vein delivery of
> contrast medium because of streaming, might enhance
> precordial echocardiographic detection of patent foramen
> ovale. BACKGROUND. Although precordial contrast
> echocardiography is widely used to diagnose patent foramen
> ovale, this method is limited by poor sensitivity. Previous
> investigators have demonstrated enhanced detection of atrial
> defects by the dye-dilution technique after delivery of
> contrast medium into the inferior rather than the superior
> vena cava. METHODS. Transthoracic contrast examinations
> were performed in a randomly selected group of 70 patients
> (without previous history of cerebral or systemic embolus)
> undergoing cardiac catheterization. Paired contrast agent
> injections (10 ml dextrose in water/0.25 ml air) were
> administered from an upper extremity vein and femoral vein
> in each patient during spontaneous respiration, cough and
> Valsalva maneuvers. Studies were interpreted by an
> experienced echocardiographer unaware of the sequence and
> site of injections. Positive studies were
> semiquantitatively graded from +1 (minimal left ventricular
> opacification) to +4 (intense left ventricular
> opacification). Catheterization and echocardiographic
> assessment of patent foramen ovale were compared in 21
> subjects. RESULTS. Patent foramen ovale was detected
> significantly more often during femoral vein versus upper
> extremity contrast delivery (23 of 70 patients [prevalence
> 33%] vs. 9 of 70 patients [prevalence 13%], p < 0.001). The
> intensity of left ventricular opacification was also greater
> during femoral vein contrast injection. Precordial
> echocardiography combined with femoral contrast delivery was
> significantly more sensitive than cardiac catheterization
> for assessment of patent foramen ovale (8 of 21 patients vs.
> 2 of 21 patients, p < 0.05). CONCLUSIONS. Femoral vein
> contrast delivery significantly enhances the ability of
> precordial contrast echocardiography to diagnose patent
> foramen ovale. Physiologic patency of the foramen ovale is
> more common (prevalence 33%) than previously documented.
>
> Title: The cough test is superior to the Valsalva maneuver in the
> delineation of right-to-left shunting through a patent
> foramen ovale during contrast transesophageal
> echocardiography.
> Author: Stoddard-M-F. Keedy-D-L. Dawkins-P-R.
> Source: AMERICAN HEART JOURNAL, (1993 Jan) vol. 125(1): 185-9
> Abstract: A patent foramen ovale may result in paradoxical
> embolization and serious morbidity. Thus a sensitive method
> to diagnose a patent foramen ovale is important. It is
> unknown whether the cough test or the Valsalva maneuver is
> superior in delineating right-to-left shunting through a
> patent foramen ovale during contrast transesophageal
> echocardiography. Thus we studied 73 consecutive patients
> (53 men and 20 women), aged 54 +/- 16 years (range 18 to 79
> years), during elective transesophageal echocardiography.
> Contrast transesophageal echocardiography was performed from
> a four-chamber view during quiet respirations, Valsalva
> maneuver, and cough test. In the entire group the incidence
> of a patent foramen ovale was higher during the cough test
> (32/73) as compared with the Valsalva maneuver (24/73, p <
> 0.025) and quiet respirations (18/73, p < 0.005). All
> subjects with a patent foramen ovale during the Valsalva
> maneuver had a positive contrast transesophageal
> echocardiogram during the cough test. In subjects (n = 55)
> without a patent foramen ovale during quiet respirations,
> the incidence of a patent foramen ovale was higher during
> the cough test (15/55) as compared with the Valsalva
> maneuver (9/55, p < 0.05). In a subgroup (N = 17) of
> patients with nonhemorrhagic stroke (n = 11), transient
> ischemic attack (n = 2), or peripheral embolus (n = 4), the
> cough test had a higher yield (9/17) in delineating a patent
> foramen ovale as compared with the Valsalva maneuver (7/17)
> but did not reach statistical significance. These data
> demonstrate that the cough test is superior to the Valsalva
> maneuver in delineating a patent foramen ovale during
> contrast transesophageal echocardiography.(ABSTRACT
> TRUNCATED AT 250 WORDS).
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