P.Germonpré^1,4, C.Balestra ^1,2,3, L.Kitoko³, Ph. Unger² .

1: DAN Europe research Division; 2 : Université Libre de Bruxelles; 3: Haute Ecole Paul Henry Spaak, Bruxelles; 4: Centre for Hyperbaric Oxygen Therapy, Military Hospital Bruxelles.

DAN EUROPE RESEARCH

INTRODUCTION

Patent Foramen Ovale (PFO) has been suggested as a possible cause for decompression sickness (DCS) in divers. The mechanism would be a paradoxical arterial embolisation of nitrogen bubbles after dives that would not normally lead to DCS. Previous studies indicated that there was a very significant correlation between cerebral DCS and the presence of a large PFO (Grade 2: > 20 bubbles passing through the patency during the first 3 cardiac cycles). (1)(2)(3).

Although the evidence to support PFO as a possible cause for "undeserved DCS" is strong, it is still based on relatively small-scale retrospective studies (less than 100 individuals), and no prospective study has been done yet.

Because of the relatively low incidence of DCS in sports divers (approx. 1 in 10.000 dives) (4), the setup of a prospective study would require the screening of a large number of divers (considering divers who perform an average of 50 dives per year, the observation of 20 DCS episodes per year would require 4.000 divers to be examined). Obviously, contrast trans-oesophageal echocardiography (TEE) is not appropriate for this purpose, due to its relative invasiveness and unpleasantness, its time and personnel consuming nature and its cost. For these last same reasons, other methods of detecting a right-to-left shunt (e.g. Transcranial Doppler) are equally unsuited. Therefore, a more convenient screening method needs to be defined.

Any chosen screening method should ideally comply to the following prerequisites:

• Easy to perform by a general physician (it could thus be included in the annual "fitness to dive test").
• Rapidly executable, with a minimum of third person assistance required
• of an acceptable level of unpleasantness to the subject
• low cost
• high specificity, i.e. a low level of false negative tests.

Contrast Carotid Doppler examination (CCD) complies with the first 4 prerequisites formulated above and could be a useful tool in large scale screening of sports divers. Several TEE studies have indicated that a good correlation may exist but no formal validation has been done until now (5)(6) . The present study aims to validate Contrast Carotid Doppler as a means of detecting paradoxical bubble embolization through a PFO.

METHODS

Prospective, semi-blinded comparative evaluation between early Doppler signal detection in the carotid artery after injection of saline contrast medium in a large antebrachial vein, and the subsequent TEE examination for detection and semiquantification of a PFO, if present.

Thirty-three patients from a general cardiologist population, who undergo TEE for a reason independent of the present study, participated in the study.

Exclusion criteria were:

• patients who are unable to adequately perform abdominal straining or Valsalva manoeuvres (7) as explained further,
• patients who have to be sedated for the TEE examination.

A large bore venous catheter was placed in an antebrachial vein, connected to a three-way valve system with two 10cc syringes attached, as well as a connection to a 500cc saline perfusion bag (fig.1).

Carotid doppler examination was performed by means of a vascular 8 Mhz doppler probe. Signals were monitored on a earcuff in order to prevent noise interferences on the subject or the experimenter.
A good signal was located on the common carotid artery, and the probe was held in the correct position by an assistant throughout the detection-experiment (fig. 2).

For the experiment, 9.5cc of saline and 0.5cc of air was pushed through and fro in the double- syringe system for 10 times, and then injected rapidly into the vein. The appearance of short (less than 3 seconds) series of "clicks" in the carotid doppler signal, within 5 seconds of the end of the injection, was considered to be due to the passage of contrast bubbles in the carotid artery (either via an intracardiac or pulmonary shunt), and classified the patient as "positive". This maneuver was repeated 2 times in rest, and two times after abdominal straining.

For this, the patient was asked to exert abdominal pressure, by respiratory block after maximal inspiration (if this maneuver will produce sufficient pressure to interfere with intrathoracic pressures, it should produce moderate bradycardia that can be heard by the doppler manipulating assistant). After 10 seconds, a duration considered sufficient to pool enough blood in the peripheral parts of thorax, rapid injection of saline contrast medium was performed, and at the end of the injection, the patient was instructed to suddenly release the air from the lungs. Care was taken to ensure correct positioning of the Doppler probe during these maneuvers, particularly in relation to the straining maneuvre, where the Doppler signal tended to be less audible.

After this, a standard contrast TEE was performed, as described in an earlier study (1), and the patency of the foramen ovale was semi-quantificatied in: Grade 0 (no bubble passage), grade 1 (< 20 bubbles, either at rest or after straining maneuver) or grade 2 (> 20 bubbles, either at rest or after straining maneuver). Special attention was given to false respiratory contrast (by performing some straining maneuvers without injection of contrast before and after the actual contrast study) and "late" contrast appearance, more than 4 heartbeats after the injection (which may be attributed to pulmonary shunts).

The results were analyzed in order to obtain sensitivity and specificity of this method in comparison to TEE, which is still considered the "gold standard" in PFO detection.

RESULTS

Thirty-three patients participated in the study. Contrast TEE study revealed 8 cases of PFO (24%) which is consistent with the known prevalence of PFO in the normal population. The Carotid Doppler examination was positive in all 8 patients with PFO (sensitivity of the method: 100%). Three false positive results were detected (specificity: 88%). In these, TEE did not reveal possible transpulmonary passage. No false negative detection happened.

CONCLUSIONS

Although the technique may seem subjective in nature, this semi-blinded comparative study shows an excellent sensitivity and specificity of CCD compared to TEE. CCD may be considered as a screening method for detecting PFO in general population and thus can be used for a prospective low cost, low invasive study on the diving population.

A multicentric prospective study, running over four to five years, using this methodology may be necessary in order to determine the relative risk of PFO in diving.

REFERENCES

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3. Wilmshurst PT., Byrne JV., Webb-Peploe MM. Relation between interatrial septum and decompression illness in divers. Lancet (1989); 11:1302-1306.
4. DAN Europe data. Proc XXI Annual Congress of EUBS, Milano, 1996
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