Shunt-mediated decompression sickness in a compressed air worker with an atrial septal defect.

We report a compressed air worker who had diffuse cutaneous decompression sickness with pain in his left shoulder and visual disturbance characteristic of migraine aura after only his third hyperbaric exposure. The maximum pressure was 253 kPa gauge with oxygen decompression using the Swanscombe Oxygen Decompression Table. He was found to have a very large right-to-left shunt across a 9 mm atrial septal defect. He had transcatheter closure of the defect but had some residual shunting with release of a Valsalva manoeuvre. Thirty-two other tunnel workers undertook the same pressure profile and activities in the same working conditions during the maintenance of a tunnel boring machine for a total of 233 similar exposures and were unaffected. As far as we are aware this is the first report of shunt-mediated decompression sickness in a hyperbaric tunnel worker in the United Kingdom and the second case reported worldwide. These cases suggest that shunt-mediated decompression sickness should be considered to be an occupational risk in modern compressed air working. A right-to-left shunt in a compressed air worker should be managed in accordance with established clinical guidance for divers.

Impaired consciousness when scuba diving associated with vasovagal syncope.

INTRODUCTION: Drowning is likely to result from impairment of consciousness when scuba diving. Causes include toxic effects of breathing gas, including nitrogen narcosis and oxygen toxicity, and arterial gas embolism. METHODS: Review of the medical records of scuba divers who had impaired consciousness underwater that could not be attributed to toxic effects of breathing gas or arterial gas embolism. RESULTS: Four scuba divers had episodes of impaired consciousness when at shallow depths (8-18 m) underwater. The descriptions of the episodes were very similar. Three had histories of recurrent episodes of vasovagal syncope on land. CONLCUSIONS: Absence of other causes for their impaired consciousness underwater leads to the conclusion that the probable cause was vasovagal syncope.

Risk mitigation in divers with persistent (patent) foramen ovale.

In this issue, Anderson and colleagues report follow-up of divers who were found to have a persistent (patent) foramen ovale (PFO) or, in eleven cases, an atrial septal defect (ASD). In most divers diagnosis followed an episode of decompression illness (DCI). The efficacy of closure of the PFO/ASD in preventing future DCI was compared with conservative diving. They reported that in the closure group the occurrence of confirmed DCI decreased significantly compared with pre-closure, but in the conservative group this reduction was not significant.It is believed there are three requirements for a diver to suffer shunt-mediated DCI: A significant right-to-left shunt (usually a large PFO but sometimes an ASD or pulmonary arteriovenous malformation). Venous bubbles nucleated during decompression circumvent the lung filter by passing through the shunt. Target tissues are supersaturated with dissolved inert gas, so that they are able to amplify embolic bubbles. All three are required because DCI does not occur after contrast echocardiography when bubbles cross a right-to-left shunt.Therefore, there are two ways that a diver who has suffered shunt-mediated DCI may continue to dive - either their shunt is sealed or future dives should be so conservative that venous bubbles are not liberated and/or critical tissues are not able to amplify embolic bubbles.PFO/ASD closure will give divers a risk of DCI comparable to the risk in others without a right-to-left shunt, if the procedure adequately seals the shunt. Closure of the shunt will not prevent a diver suffering DCI by other mechanisms, such as when there is arterial gas embolism (AGE) as a result of pulmonary barotrauma or when the dive profile is provocative (e.g., if there is rapid ascent or missed decompression stops). Conservative diving will be effective only if all the dives performed are truly conservative and prevent bubble nucleation and/or amplification.The study by Anderson et al. has a number of serious limitations. The study was small with only 62 self-selected divers, who self-reported outcomes. Eleven divers had not had DCI when their PFO or ASD was detected. Initially 36 divers were classified as closure and 26 as conservative treatment, but six subjects crossed from the conservative group to the closure group. Three of the six dived in the conservative group before having closure and are classified in both groups depending on whether the dives performed were before or after closure. As a result, there were 42 in the closure group and 23 in the conservative group.Randomisation to the treatment groups was not possible and its absence results in imbalance. Because the closure group is approximately twice as large as the conservative group, similar changes in incidence would have a greater probability of achieving statistical significance in the former. Large shunts were present in more than three-quarters of the closure group but fewer than half of the conservative group. The authors have three definitions of a 'large' PFO, so the definition of large was inconsistent. All ASDs were considered to be large.When dealing with small numbers, one needs patient-level data, but that is lacking and may mask inconsistency in management. The divers were investigated and treated in at least 38 hospitals (some divers did not state where they were treated). We do not know what devices were used for PFO/ASD closure, and closure effectiveness varies, or what tests were performed to assess the effectiveness of closure.The primary end-point was not different between the two groups because only two episodes of confirmed DCI occurred in each group. The authors also considered a softer and subjective end-point, possible DCI.Crucially we are not told what the divers in the conservative group were told constitutes a conservative dive and whether it was consistent. Nor are we told whether they followed the advice given. That is important because it appears that incidence of possible DCI increased considerably in only the conservative group, which means either that the advice they were given on what constitutes a conservative dive was flawed, that the divers failed to follow good advice or that they frequently reported innocent symptoms as possible DCI, because knowledge that they had a PFO may have increased their reporting - introducing further bias.There should be assessment of whether DCI after the intervention was shunt-mediated or had another cause. For that assessment, one needs to know details of the dives resulting in symptoms, clinical manifestations and latency of onset.I have investigated 20 divers who had DCI after PFO closure. In five divers, a contrast echocardiogram showed a significant residual shunt. Typically, the diver had their closure procedure by a cardiologist lacking knowledge of diving medicine and no post-closure contrast echocardiogram was performed. In one case, the diver's PFO was closed but they had a residual pulmonary shunt that was not detected. In those cases where there is a significant residual shunt, the dive profiles, clinical manifestations and latencies of onset were typical of shunt-mediated DCI.Three divers, who had PFO closure with no residual shunt, subsequently had neurological symptoms with manifestations consistent with AGE secondary to pulmonary barotrauma. High resolution CT scans of their chests showed pulmonary bullae and emphysema.The remaining divers seen had no residual shunt but had performed highly provocative dives, usually much deeper than 50 metres' sea water (msw). The most recent case that I saw had dived to 102 metres' fresh water (mfw) in a lake at high altitude breathing trimix.In contrast, several hundred divers in whom I diagnosed a PFO and who elected to dive conservatively had not reported further DCI. I advised them that I have never seen shunt-mediated DCI after dives breathing air to depths of 15 msw or less provided no rules were broken. So I set that as the depth limit or allow them to dive to greater depths breathing nitrox so that there are equivalent partial pressures of nitrogen (e.g., 19 msw with nitrox 32 or 23 msw with nitrox 40) provided they use an air decompression table/algorithm. Alternatively, one can dive using the DCIEM recreational air diving table.Recurrence of DCI after PFO closure may be the result of a residual shunt or may have other causes. It is difficult to draw conclusions about the safety of 'conservative' diving unless one knows what the divers were advised constitutes conservative dives and whether they adhered to the advice.

Immersion pulmonary oedema: a cardiological perspective.

It is postulated that immersion pulmonary oedema (IPE) occurs because of combinations of factors that each increase the hydrostatic pressure gradient between the pulmonary capillaries and the alveoli. The factors, by definition, include the effects of immersion, particularly raised central blood volume and hence cardiac filling pressures. Breathing against a negative pressure is important but the magnitude of the effect depends on the relation of the diver's lung centroid to the source of the breathing gas and the breathing characteristics of diving equipment. Other factors are cold-induced vasoconstriction, exertion and emotional stress, but variations of the responses of individuals to these stimuli are important. Hypertension is the most frequent cardiovascular disease predisposing to IPE but other medical conditions are implicated in some patients.

Detection of a persistent foramen ovale using echocardiography.

Right-to-left shunts can result in decompression illness in divers and lead to a number of other conditions. Transthoracic echocardiography with intravenous injection of bubble contrast, when performed according to a well-tested protocol by trained personnel, enables the safe, simple, rapid and inexpensive detection of right-to-left shunts, the assessment of the size of the shunts and the differentiation of atrial shunts from pulmonary shunts. This article summarises the author's views on the techniques available and his preferred protocol for transthoracic echocardiography.

Cutis marmorata and cerebral arterial gas embolism.

Dr Kemper and colleagues reported that, when air was injected into the cerebral circulation of pigs, they developed a rash that looked very similar to cutis marmorata of cutaneous decompression illness (DCI) and to livido reticularis. They postulated that cutaneous DCI in divers may be centrally mediated as a result of cerebral gas embolism. It would be helpful if Kemper et al. described the distribution of the rash in their pigs. In divers, cutaneous DCI is generally confined to parts of the body with significant amounts of subcutaneous fat, such as the trunk and thighs, and the rash often crosses the midline. Colleagues and I have reported that cutaneous DCI is commonly associated with significant right-to-left shunts and particularly persistent foramen ovale (PFO). We postulated that the manifestations of shunt-related DCI, whether neurological or cutaneous, are in large part determined by peripheral amplification of embolic bubbles in those tissues that are most supersaturated with dissolved nitrogen (or other inert gas) at the time that emboli arrive. Hence we postulated that cutaneous DCI is the result of amplification of gas emboli that invade cutaneous capillaries. Dr Kemper has kindly sent me a number of the publications from his department on which their report of this skin rash in pigs is based. The aim of their experiments was to produce significant brain injury by means of cerebral air embolism. Their pigs had no tissues supersaturated with inert gas. They were ventilated with a FiO2 of 0.4 and anaesthetised with ketamine and midazolam. They were also given pancuronium and atropine, before air was injected into their cerebral circulation. If their findings in pigs and the resulting hypothesis were applicable to man, it would mean that one could get cutaneous DCI without decompression: one would only need cerebral gas embolism. During contrast echocardiography, I have produced arterial gas embolism in many hundreds of patients with right-to-left shunts and it is certain that some bubbles went into their cerebral circulations, but I have never seen and no patient has reported getting a rash. Nor am I aware of any reports of gas embolism causing a rash like cutaneous DCI without there being tissue supersaturation following some form of decompression. Kemper and colleagues injected between 0.25 and 1 ml·kg⁻¹ body weight of air into the ascending pharyngeal artery (roughly equivalent to human internal carotid artery) of pigs weighing 30-40kg. That immediately produced significant elevation of blood pressure and heart rate suggesting a 'sympathetic surge'. This is similar to the haemodynamic effects that can occur with subarachnoid haemorrhage and some other catastrophic brain injuries. That effect may have been potentiated by pre-treatment with atropine. There was also a considerable increase in intracranial pressure and major adverse effects on cerebral metabolism. Some pigs died quickly and the survivors were killed at the end of the experiment. I suspect that no pig would have survived the experiments without major neurological injury if they had not been killed. Most people with cutaneous DCI have no detectable neurological manifestations at the time that they have a rash. In those that do have neurological manifestations, it is rarely catastrophic. The increases in heart rate and blood pressure reported in the pigs are similar to the effects of a phaeochromocytoma, which can cause livido reticularis in man. Therefore, I wonder whether an alternative explanation for these observations might be that the cerebral injury in the pigs was so massive that the sympathetic surge was comparable to the effects of catecholamine release from a phaeochromocytoma and caused a rash similar to that seen in patients with a phaeochromocytoma.

Comparison of the size of persistent foramen ovale and atrial septal defects in divers with shunt-related decompression illness and in the general population.

INTRODUCTION: Decompression illness (DCI) is associated with a right-to-left shunt, such as persistent foramen ovale (PFO), atrial septal defect (ASD) and pulmonary arteriovenous malformations. About one-quarter of the population have a PFO, but considerably less than one-quarter of divers suffer DCI. Our aim was to determine whether shunt-related DCI occurs mainly or entirely in divers with the largest diameter atrial defects. METHODS: Case control comparison of diameters of atrial defects (PFO and ASD) in 200 consecutive divers who had transcatheter closure of an atrial defect following shunt-related DCI and in an historic group of 263 individuals in whom PFO diameter was measured at post-mortem examination. RESULTS: In the divers who had experienced DCI, the median atrial defect diameter was 10 mm and the mean (standard deviation) was 9.9 (3.6) mm. Among those in the general population who had a PFO, the median diameter was 5 mm and mean was 4.9 (2.6) mm. The difference between the two groups was highly significant (P < 0.0001). Of divers with shunt-related DCI, 101 (50.5%) had an atrial defect 10 mm diameter or larger, but only 1.3% of the general population studied had a PFO that was 10 mm diameter of larger. CONCLUSIONS: The risk of a diver suffering DCI is related to the size of the atrial defect rather than just the presence of a defect.

The role of persistent foramen ovale and other shunts in decompression illness.

A persistent foramen ovale (PFO) and other types of right-to-left shunts are associated with neurological, cutaneous and cardiovascular decompression illness (DCI). A right-to-left shunt is particularly likely to be implicated in causation when these types of DCI occur after dives that are not provocative. It is believed that venous nitrogen bubbles that form after decompression pass through the shunt to circumvent the lung filter and invade systemic tissues supersaturated with nitrogen (or other inert gas) and as a result there is peripheral amplification of bubble emboli in those tissues. Approximately a quarter of the population have a PFO, but only a small proportion of the population with the largest right-to-left shunts are at high risk of shunt-mediated DCI. The increased risk of DCI in people with migraine with aura is because migraine with aura is also associated with right-to-left shunts and this increased risk of DCI appears to be confi ned to those with a large PFO or other large shunt. Various ultrasound techniques can be used to detect and assess the size of right-to-left shunts by imaging the appearance of bubble contrast in the systemic circulation after intravenous injection. In divers with a history of shunt-mediated DCI, methods to reduce the risk of recurrence include cessation of diving, modification of future dives to prevent venous bubble liberation and transcatheter closure of a PFO.

Joint position statement on persistent foramen ovale (PFO) and diving. South Pacific Underwater Medicine Society (SPUMS) and the United Kingdom Sports Diving Medical Committee (UKSDMC).

This consensus statement is the result of a workshop at the SPUMS Annual Scientific Meeting 2014 with representatives of the UK Sports Diving Medical Committee (UKSDMC) present, and subsequent discussions including the entire UKSDMC. Right-to-left shunt across a persistent or patent foramen ovale (PFO) is a risk factor for some types of decompression illness. It was agreed that routine screening for PFO is not currently justifiable, but certain high risk sub-groups can be identified. Divers with a history of cerebral, spinal, inner-ear or cutaneous decompression illness, migraine with aura, a family history of PFO or atrial septal defect and those with other forms of congenital heart disease are considered to be at higher risk. For these individuals, screening should be considered. If screening is undertaken it should be by bubble contrast transthoracic echocardiography with provocative manoeuvres, including Valsalva release and sniffing. Appropriate quality control is important. If a shunt is present, advice should be provided by an experienced diving physician taking into account the clinical context and the size of shunt. Reduction in gas load by limiting depth, repetitive dives and avoiding lifting and straining may all be appropriate. Divers may consider transcatheter device closure of the PFO in order to return to normal diving. If transcatheter PFO closure is undertaken, repeat bubble contrast echocardiography must be performed to confirm adequate reduction or abolition of the right-to-left shunt, and the diver should have stopped taking potent anti-platelet therapy (aspirin is acceptable).