Commentary from Marvin Gozum, DAN trained physician. Forwarded with
permission.

>--
>--  >-- Yes, this is the mechanism that does it, and unfortunately
>--  >-- is what allows
>--  >-- for the dive industry's RJ Reynolds twist of the facts. Most
>--  >-- PFO's require
>--  >-- the bubbles to raise the pressure to open the flap, so many
>--  >-- people can do
>--  >-- hundreds or even thousands of dives without consequence, and
>--  >-- then end up in
>--  >-- a wheelchair . The morons at DAN call that an "unearned
>--  >-- hit". I call it
>--  >-- lying and denial. 30% of us have it. Do you feel lucky?
>-- Well do ya?
>--
>--
>-- http://www.emedicine.com/ped/topic2494.htm
>--
>--
>-- In 1991, the PFO Issue was put on the map by a DAN physician, Fred
>-- Bove.  The current standing recommendation is:
>--
>-- "
>-- Comment in:
>-- Undersea Hyperb Med. 1999 Spring;26(1):49-50
>--
>-- Risk of decompression sickness with patent foramen ovale.
>--
>-- Bove AA.
>--
>-- Cardiology Section, Temple University Medical School, Philadelphia,
>-- Pennsylvania, USA.
>--
>-- Several reports have described populations of divers with
>-- decompression
>-- sickness (DCS) who have a patent foramen ovale (PFO). The
>-- presence of a PFO
>-- is known to occur in about 30% of the normal population,
>-- hence 30% of
>-- divers are likely to have a PFO. Although observations have
>-- been made on
>-- the presence of a PFO in divers with and without DCS, the risk of
>-- developing DCS when a diver has a PFO has not been
>-- determined. In this
>-- study, Logistic Regression and Bayes' theorem were used to
>-- calculate the
>-- risk of DCS from data of three studies that reported on
>-- echocardiographic
>-- analysis of PFO in a diving population, some of whom
>-- developed DCS. Overall
>-- incidence of DCS was obtained from the sport diving
>-- population, from the
>-- U.S. Navy diving population, and from a commercial
>-- population. The analysis
>-- indicates that the presence of a PFO produces a 2.5 time
>-- increase in the
>-- odds ratio for developing serious (type II) DCS in all three
>-- types of
>-- divers. Since the incidence of type II DCS in these three
>-- populations
>-- averages 2.28/10,000 dives, the risk of developing DCS in
>-- the presence of a
>-- PFO remains small, and does not warrant routine screening by
>-- echocardiography of sport, military, or commercial divers.
>--
>-- ""
>--
>-- We know:
>--
>-- That venous bubbling by doppler [ regardless of the Spencer
>-- Scale] occurs
>-- in a very large fraction of dives, the current Summer issue
>-- of Immersed
>-- reports it exceeds 85% if one does >1 dive/day, and over 67%
>-- of these are
>-- 'high grade' in size;  37% occurred in a single dive, as far
>-- as 90 minutes
>-- post-dive.
>--
>-- Despite the high incidence of venous bubbles in dives,
>-- either well executed
>-- deco or a no-stop, and the 30% incidence of PFOs in the
>-- population, the
>-- incidence of DCS in toto is still < .07% or 7/10,000 as
>-- note, _the status
>-- quo is NOT to screen for PFOs_ among divers in general.
>-- Therefore, if PFO
>-- is a risk factor in diving, the number of cases of emboli should be
>-- proportionate to the proportion of divers similar to the baseline
>-- population risk of PFO ~30%. This is not seen in reality.
>--
>-- As for technical diving, anything increasing the
>-- decompression obligation
>-- increases the production of venous bubbles, and this means
>-- diving deep, and
>-- multiple per day.  Its is unknown if He with nitrox deco
>-- decreases the risk
>-- of venous bubbles versus air, however its assumed logically
>-- that He, nitrox
>-- or 100% 02 significantly diminishes the nitrogen load, and
>-- logically allow
>-- these divers to dive deep and assume a risk akin to
>-- recreational divers for
>-- venous bubbling.
>--
>--  >-- >Sure, that's the point behind not exerting yourself for a
>--  >-- while after
>--  >-- >the in water part of your off gassing.  Until you stop
>-- bubbling,
>--  >-- >anything you do that causes a rise in chest cavity
>--  >-- pressure- bending and
>--  >-- >lifting, straining, etc- can open the PFO and shunt bubbles
>--  >-- into your
>--  >-- >brain.
>--  >--
>--  >-- Unfortunately for those with PFO, a rise in pulmonary
>--  >-- arterial pressure,
>--  >-- and retrograde rise in right atrial pressure secondary to
>--  >-- the pulmonary
>--  >-- embolization of nitrogen/helium bubbles at the
>-- alveolar-capillary
>--  >-- complex would also cause right-to-left shunting through
>-- the PFO - no
>--  >-- exertion required.
>--  >--
>--  >-- All the more reason to get tested if you're doing
>-- technical diving.
>--
>--
>-- While many mechanisms are possible, to cause pulmonary
>-- artery hypertension
>-- from bubbles alone suggests a massive plethora of bubbles.
>-- By the time
>-- this happens, the patient would probably be dead from
>-- cardiac failure, less
>-- likely from embolization from a bubble transit through a
>-- PFO, induced by
>-- the resistance the bubbles produce to the flow of blood in
>-- general ... as
>-- was described in the book, "Last Dive" for the death of Chris Rouse.
>--
>-- A Valsalva or coughing after bubbles are intentionally
>-- injected into the
>-- venous system is done routinely in a bubble contrast
>-- echocardiogram to view
>-- bubble transits in suspected PFO. Despite that, patient's don't
>-- stroke.  This test suggests the large quanitity and size of
>-- the bubbles is
>-- vital to cause harm.
>--
>-- You can still have shunting from within the lungs itself,
>-- either congenital
>-- OR acquired lung trauma [barotrauma, smoking damage, age related
>-- weaknesses.]  This allows bubbles to squeeze through the
>-- lungs and removes its filtering effect.
>--
>-- Finally, to repeat, there hasn't been a correlation between
>-- venous bubbling
>-- and DCS.
>--
>-- Assuming you do do the WKPP/DIR recommendation of determing
>-- your existence
>-- of a PFO before embarking on a technical dive.  What do you
>-- do if you find
>-- a PFO?  If divers have been diving with a PFO without
>-- incident, would you
>-- repair an asymptomatic PFO to lower a 'theoretical' risk for
>-- AGE with
>-- diving or stop diving?  An echocardiogram is ~$1000 and the
>-- cath will set
>-- you back $10,000 plus endure a 0.5% risk of death, a risk
>-- higher than the 3
>-- fold likelihood [~.21%] of AGE from PFO from a baseline DCS
>-- risk of .07%.
>--
>-- The standing recommendation for PFO repair is if it
>-- demonstrates harm IN
>-- INDIVIDUAL PATIENTS.  If a diver developed a DCS3 hit under
>-- a NSL dive or
>-- proper dive with a deco procedure, was treated in the
>-- chamber and recovered
>-- to dive again, a PFO can be screened for and may then be
>-- considered for
>-- repair should a patient wish to pursue diving.
>--
-- -----Original Message-----
-- From: Trey [mailto:trey@ne*.co*]
-- Sent: Friday, June 08, 2001 5:51 AM
-- To: Rodriguez; Wendell Grogan
-- Cc: Bruce Sherman; Quest@Gu*. Com; Techdiver List
-- Subject: RE: Not opening PFO's, waqs RE: Repairing PFO's
--
--
--
-- Yes, this is the mechanism that does it, and unfortunately
-- is what allows
-- for the dive industry's RJ Reynolds twist of the facts. Most
-- PFO's require
-- the bubbles to raise the pressure to open the flap, so many
-- people can do
-- hundreds or even thousands of dives without consequence, and
-- then end up in
-- a wheelchair . The morons at DAN call that an "unearned
-- hit". I call it
-- lying and denial. 30% of us have it. Do you feel lucky? Well do ya?
--
-- -----Original Message-----
-- From: Rodriguez [mailto:mikey@ma*.co*]
-- Sent: Friday, June 08, 2001 1:09 AM
-- To: Wendell Grogan
-- Cc: Trey; Bruce Sherman; Quest@Gu*. Com; Techdiver List
-- Subject: Re: Not opening PFO's, waqs RE: Repairing PFO's
--
--
-- At 09:17 PM 6/7/2001 -0400, Wendell Grogan wrote:
--
-- >Sure, that's the point behind not exerting yourself for a
-- while after
-- >the in water part of your off gassing.  Until you stop bubbling,
-- >anything you do that causes a rise in chest cavity
-- pressure- bending and
-- >lifting, straining, etc- can open the PFO and shunt bubbles
-- into your
-- >brain.
--
-- Unfortunately for those with PFO, a rise in pulmonary
-- arterial pressure,
-- and retrograde rise in right atrial pressure secondary to
-- the pulmonary
-- embolization of nitrogen/helium bubbles at the alveolar-capillary
-- complex would also cause right-to-left shunting through the PFO - no
-- exertion required.
--
-- All the more reason to get tested if you're doing technical diving.
--
-- -Mike Rodriguez
-- <mikey@mi*.ne*>
-- http://www.mikey.net/scuba
-- Pn(x) = (1/(2^n)n!)[d/dx]^n(x^2 - 1)^n
--

--
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