January 2001 newsletter

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What to tell a diver with a patent foramen ovale

Referees are receiving more enquiries about risk factors for decompression illness, and with recent advances in our understanding of the mechanisms involved, it seems a good time to summarise the PFO evidence.

What is a PFO? The foramen ovale is a hole in the septum between the two atria, and this is open in utero to allow blood to bypass the uninflated lungs. Shortly after birth the hole should close (like a flap valve) but in some cases it may not close completely, leaving a patent foramen ovale.

How common are PFO’s? A post mortem study from the Mayo Clinic found a prevalence of around 20-25% depending on age. This included very small holes which were described as probe patent, measuring only 1-2 mm in diameter. Studies in normal populations have been hampered by detection methods, and conventional echocardiography detected only 20-50% of cases (as compared with transoesophageal echocardiography, the gold standard). Holes big enough to cause significant paradoxical gas embolism are probably only present in 1-2% of the normal population.

How do you look for a PFO? The gold standard is transoesophageal echocardiography but this involves passing a probe down the oesophagus and there is a risk of complications. Recent advances in echocardiography include a new way of processing the information and presenting images, called second harmonic imaging. If intravenous saline is mixed with a small amount of air and mixed or agitated between two syringes using a three way tap, very effective bubble contrast is generated. This is injected through a venflon to opacify the right heart. If there is a PFO then bubbles should cross into the left atrium. Transthoracic echocardiography with second harmonic imaging and provocation manoeuvres such as Valsalva’s, coughing or abdominal pressure will increase the chance of detecting a PFO if present. If bubbles appear late then they may have crossed through an arterio-venous malformation in the lung, or in some cases simply passed through the normal lung circulation. Transcranial Doppler ultrasound can be used as a detection technique but no images of the heart are obtained. It is useful therefore as a screening technique with high levels of sensitivity but provides little anatomical information.

When do you look for a PFO? If a diver has suffered a skin bend, neurological symptoms within recommended decompression limits without an uncontrolled ascent, or dive related migraine. Symptoms usually present 5-10 minutes after surfacing compared with pulmonary barotrauma where symptoms tend to develop immediately.

Can PFO’s be closed? It is possible to close PFO’s percutaneously, usually from the leg using the femoral vein. A sheath is inserted in the vein, and a fine guidewire passed into the right side of the heart, through the PFO and into the left atrium. A device is then fed over the wire and deployed across the septum. It takes approximately 6 weeks for the device to become covered with an endothelial lining, and a repeat contrast echo should be performed to confirm complete closure after this period. The procedure may be carried out under general anaesthetic to allow insertion of a TOE to help site the device, although it is now possible to perform septal closure under local anaesthetic using transthoracic echocardiography. The only medication required for the procedure is aspirin and while local protocols may vary, it is often recommended for at least 6 months. Antibiotic prophylaxis is also recommended during this period.

Can I dive without closing my PFO? The diver should be aware of their high risk of further decompression illness, and subclinical spinal cord damage. A maximum limit of 15 metres should be advised, with particular emphasis on slow controlled ascents, routine safety stops, no-bubble tables (eg DCIEM), and the use of nitrox on air tables. Many divers will choose not to continue diving with these restrictions.

This is a developing area and we will keep you updated as the research continues.

Sinusitis

Active sinusitis increases the risk of barotrauma since inflamed mucosa can obstruct the ostium of the sinus and act as a ball valve. During descent high pressure air may be forced into the sinus, but this will be obstructed as it expands during the ascent and may lead to serious injury. This could include involve the development of a cerebral empyena, pneumocephalus, blindness (because of retinal artery compression), or fifth cranial nerve injury. A recent article describes two divers with a history of sinusitis who developed complications. The first involved a commercial diver who had chronic sinusitis that had been treated with endoscopic surgery. Following a mechanical fault of his full face mask which caused a sudden increase in pressure, he developed severe ear pain which he tried to correct with repeated Valsalva manoeuvres. This led to severe vertigo and disorientation and this gradually resolved after surfacing. Three days later he became pyrexial with a severe headache and was found to have pneumococcal meningitis. He was left with permanent neurological deficits despite treatment. The second case involved a navy diver with recurrent sinusitis and difficulty with ear clearing. He was found to have a mucocele in the ethmoid sinus at endoscopic examination but continued to dive and developed total ipsilateral sensorineural deafness. This eventually resolved completely. The authors advise that divers with recurrent sinus barotrauma should be examined by nasal endoscopy and possibly by CT scan of the sinuses. It may be possible to correct additional intranasal disease such as septal deviation or polyps. Divers should be advised not to dive with active sinusitis. For more information: Neurological consequences of SCUBA diving with chronic sinusitis. Parell GJ, Becker GD. Laryngoscope 2000;110:1358-1360
	Normal CT of sinuses
	CT appearances of chronic sinusitis

Ascent rate and bubble formation

It is well known that rapid ascent increases the risk of decompression illness and evidence continues to gather on the safest ascent rates and optimum decompression profiles. Tables exist that attempt to minimise bubble formation as detected by Doppler ultrasound and these are useful for divers with unexplained decompression illness, or those found to have a patent foramen ovale but but not wishing to undergo closure.

A recent study of 28 recreational divers used Doppler scores of venous bubble formation to assess the safety of two ascent rates in open water conditions to 35 metres. They found a significantly higher bubble score (using the Spencer scale and Kisman integrated severity score, KISS) in divers surfacing to the decompression stop at 17 metres per minute, compared with the group surfacing at 9 metres per minute.

Rapid uncontrolled ascents are a major cause of decompression illness in the UK and divers should already be aware of the risks involved. Dive computers are supposed to help control ascent rates but can be very sensitive to arm movement and posture. Divers may ignore alarm signals during the ascent for this reason and should be advised of the need to maintain a slow controlled ascent to minimise their risk.

For more information:

Carturan D, Boussuges A, Molenat F, Burnet H, Fondarai J, Gardette B. Int J Sports Med 2000;21(7):459-62

Drugs at pressure

Researchers from Pittsburgh, Pensylvania recently assessed the effects of two drugs commonly used by divers(pseudoephedrine to prevent ear barotrauma and dimenhydrinate to control sea sickness) at hyperbaric pressure to look for side effects including psychological dysfunction and cardiac effects. Thirty divers were studied in a hyperbaric chamber using psychometric tests and measuring heart rate responses to the two drugs. This was a double blind, placebo controlled, crossover study.

Pseudoephedrine did not adversely affect any of the seven psychometric scores although there was a non-significant trend to an increase in anxiety scores which may have been partly related to depth. There was a significant increase in heart rate caused by the drug but this was gradually reversed by increasing depth (consistent with the known effects of depth on heart rate). Dimenhydrinate caused a significant decrease in mental flexibility (as shown by trail making tests) but there was no cardiac effect.

The authors conclude that whereas pseudoephedrine appears safe for divers, dimenhydrinate may increase the risk of disorientation or confusion. Dimenhydrinate is not available in the UK but remains available elsewhere. This is consistent with current advice given to divers which also advises trying a test dose of an appropriate preparation before diving to check for side effects including drowsiness. This advice also applies to divers using antihistamines for hay fever prophylaxis and for treatment of allergic rhinitis. Alcholol potentiates any sedating action of these drugs and should be avoided completely. The effect of the drug may be cumulative with repeated doses.

For more information:

Taylor DM, O’Toole KS, Auble TE, Ryan CM, Sherman DR. Pharmacotherapy 2000;20(9):1045-50 and 1051-4