At the 43rd Annual Scientific Meeting of South Pacific Underwater Medical Society going on May 18 – 25, 2014, a key theme is PFO and diving. The keynote speaker is Dr. Peter Wilmshurst, the cardiologist and diving physician who first described the association between PFO and decompression sickness in 1986. Here, he presented his findings in several hundred cases of DCS. His insight into this problem is most valuable and we are looking forward to the publication of a synthesis of his findings.
Endothelium is a single cell layer lining the inner surface of blood vessels. It plays an important role in the regulation of circulation and reaction to various kinds of stress. Endothelial dysfunction is associated with the progression of atherosclerosis of blood vessels and heart disease.
Recently it has been shown that underwater diving may transiently decrease the post-ischemic flow mediated dilation (FMD), which is an indicator of endothelial function. This has caused some divers concern and raised questions in the media about the safety of various aspects of diving.
The FMD is a test that uses the ultrasound Doppler method to measure the diameter of brachial artery (the main artery in the upper arm) before and after a five-minute forearm ischemia caused by external compression with a blood pressure measurement cuff. The average diameter of the brachial artery in adults is about four to five millimeters. After the five minutes of circulation occlusion, the flow through the artery increases and the artery diameter increases for about seven to 10 percent of its initial value. This is called the flow mediated dilatation – FMD.
The extent of FMD may be decreased by various factors like exercise, obesity, hormonal status, diurnal changes, fatty meals, acute/chronic moderate to severe alcohol consumption, etc. On the other hand, dietary factors such as light alcohol consumption, chocolate, and various medications like nitroglycerine and statins may transiently increase FMD.
It has been also shown that FMD is transiently reduced after a single compressed air dive, breathing 60% oxygen at surface, a breath-hold dive and a nitrox dive. Both the increased partial pressure of oxygen and circulating gas bubbles have been suspected as possible causes. Oxygen does affect the metabolism and availability of nitric oxide (NO), which plays an important role in the relaxation of arteries, but the effects of it on the FMD are controversial.
What does it mean for divers?
In general, reported transient changes in FMD are not specific for diving. They appear to be a common endothelial response to a variety of stressors as a part of normal defense mechanisms including self-repair. If the stressors become excessive and exposure chronic, the endothelium may be harmed beyond repair. In extreme cases, dysfunctional endothelium in coronary artery disease may paradoxically cause narrowing of arteries in response to exercise and provoke angina or myocardial infarction. Nothing indicates that diving could reach significance of such an excessive and chronic stressor that it could acutely or in the long-term affect the health of divers through reduced FMD.
So far, only transient decreases of the brachial artery FMD after diving have been reported in asymptomatic divers. FMD is equally affected by repeated breath-hold diving and a single scuba dive on air. Nitrox diving reportedly causes a larger decrease of FMD than air diving. This would be consistent with the dose-dependent effects of hyperoxia; however, hyperbaric oxygen treatment which exposes patients to a much higher partial pressure of oxygen than diving does not appear to affect the FMD. The possible dose-dependent effect of circulating gas bubbles has not been studied, but the greater decrease of FMD observed in nitrox, which is supposed to generate less bubbles, versus air diving is counterintuitive. Most importantly, there is no obvious acute or long-term harm (diseases, increased mortality) associated with reduced FMD after diving.
Having a bite of chocolate or large dose of vitamin C one hour before a dive may prevent the FMD decrease. However, the transient decrease of FMD described so far is not an injury and prevention is not necessary, regardless of how tempting the chocolate is.
Post written by: Petar Denoble, MD, D.Sc.
In a recent report on diving fatalities in Australia during 2009, Lippmann and colleagues identified 12 cases among breath-hold (BH) and 9 cases among scuba divers (SD). Cardiovascular disease was the apparent disabling condition in 3 BH and 3 SD fatalities, and possibly in 4 more BH and 3 SD fatality cases.
In comparison to the period 1977 – 2005 when 18% of deaths were caused by apparent cardiovascular diseases, the proportion in 2009 is much higher and more in-line with what DAN America reported for the period 1992-2003. Authors assume that this is probably due to the increased participation of older divers, which makes the current population of divers in Australia closer in age to the population of recreational divers in the United States. In this series, the age range of the victims who likely died of cardiac cause was 50 to 63 years. Seven out of 12 BH fatalities and four out of nine SC were older than 50 years.
Other causes of disabling conditions in BH were apneic hypoxia (3) and aspiration (2), while in SC fatalities there were two cases of seizures and two of probable cerebral arterial gas embolism (CAGE). Being over-weighted and failure to establish positive buoyancy needed to surface contributed to three deaths.
Besides the cardiac disease that may not be diagnosed previously, many victims may not have been physically fit for diving. Many were overweight and in one case the victim was extremely obese. A medical statement is not mandatory for snorkeling; however, this snorkel operator required a medical statement, but the extremely obese victim failed to declare various medical conditions (asthma, hypertension, arrhythmia and depression) and the medications she was taking. She died quietly, at the surface, a few meters from the boat. The autopsy did not document any apparent cause. While the authors justifiably suggest that in the case of an unfit customer who insists on snorkeling she could be assigned a personal guide, in this particular case even that may not have prevented the fatal outcome.
In some cases victims knew that they had conditions which may turn fatal in diving (seizures, poor physical fitness) but failed to report them. However, most victims who died of cardiac causes in this series were apparently healthy. Age itself is not a disease, but it is associated with increasing incidence of coronary artery disease, which may remain asymptomatic for a long time. The discussion of who should undergo additional medical testing and how often is ongoing with no satisfactory answer in sight.
This paper provides detailed accounts of each accident with an extensive expert comment and thus it makes a worthy read for all divers.
Lippmann J, Lawrence C, Fock Andrew, Wodal T, Jamieson S. Provisional report on diving-related fatalities in Australian waters 2009. Diving and Hyperbaric Medicine. 2013. December; 43(4):194-217.
Post written by: Petar Denoble, MD, D.Sc.
Current recommendations for patients with implantable cardioverter-defibrillators (ICDs) advise against participating in sport that are more vigorous than bowling or golf. These recommendations are based on reasonably estimated hazards of ICD failure to defibrillate, loss of control and injury caused by arrhythmia-related syncope or shock, and damage to the ICD system; however, the data about occurrence of these adverse events was not available. Medical conditions for which ICD is administered vary as well as the age of receivers. Many subjects with ICD are young and otherwise healthy. Participation in sports for some is an important quality of life factor and they choose to participate despite possible risks. The frequency of adverse events and risk of serious injury in such subjects was addressed in a prospective study based on a multinational registry.
Patient-centered care is a basic principle in which the patient establishes what brings quality to his/her life and challenges the physician to provide evidence so the patient can make an informed decision. In this case, that evidence is not established; this provided the ethical justification of a study that reviewed subjects who were enrolled in activities against medical advice. The study protocol had to ensure that it does not appear as an encouragement for subjects with ICD to engage in sports.
The study enrolled 372 athletes with ICDs (age: 10–60 years) already participating in organized (n=328) or high-risk (n=44) sports and followed them prospectively for a median of 30 months. Data was obtained via phone interviews and medical records at baseline, if a shock occurred and every 6 months. Of the enrolled subjects, 33% were women. Sixty subjects were competitive athletes. Running, basketball, and soccer were the most common sports, but some also engaged in skiing (71) and surfing (13), which is considered high risk for syncope and ICD shock-related injuries.
This study found that shocks were not uncommon, but there were no injuries, deaths or need to externally defibrillate. Shocks occurred in 10% of study participants during competition/practice, in 8% during other physical activity and in 6% at rest. Lead malfunctions were not higher than in unselected populations.
In summary, many athletes with ICDs can engage in vigorous and competitive sports without physical injury or failure to terminate the arrhythmia, despite the occurrence of both inappropriate and appropriate shocks. This study also points out subgroups with likely higher risks; it also discusses specific tests and the process of patient evaluation necessary for informed physician advice and patient choices.
For my target audience, it is important to note that no scuba divers participated in this study. Scuba diving is considered a very high risk activity for subjects with ICD because the loss of control due to syncope or shock while underwater is likely to cause drowning. Some subjects with pacemaker, a device that does not provide shock, may be allowed to dive, but it appears that there are few out there since we had a lot difficulty recruiting participants for a survey-based study.
Lampert R, Olshansky B, Heidbuchel H, et al. Safety of Sports for Athletes With Implantable Cardioverter-Defibrillators. Results of a Prospective, Multinational Registry. Circulation. 2013;127:2021-2030.)
More about DAN’s study on diving with pacemakers can be found here:
Post written by: Petar Denoble, MD, D.Sc.
Recently, a group of Italian researchers conducted an underwater Doppler echocardiography study of 18 healthy scuba divers titled “Cardiovascular changes during SCUBA diving: an underwater Doppler echocardiographic study.” The rationale for the study was a concern that body immersion, which induces redistribution of blood from the periphery to the chest, may adversely affect subjects with previous heart disease. The aim of the study was to evaluate cardiovascular changes during immersion using underwater Doppler echocardiography. They found that the left ventricle is enlarged during immersion, an effect that is expected because there is more blood moving into the chest area. In addition, there were some changes in the velocity of the ventricle filling measured, but the significance of this is not clear. These changes were still noticeable when the Doppler echocardiography was repeated immediately after a dive; however, some older studies reported that most changes disappeared within one hour after the dive.
Please note, although the motivating concerns for the study were about divers with pre-existing cardiovascular conditions like hypertension and coronary heart disease, the study involved 18 healthy subjects. While there are quite a few papers that report temporary changes in cardiovascular functions in healthy divers, there are no studies exploring how these temporary changes may acutely affect divers with pre-existing conditions or how long these changes may persist in such divers.
DAN is conducting a study that aims to provide more answers. Cardiac function is evaluated by echocardiography after multiple days of diving. Possible arrhythmias are monitored with a continuous underwater electrocardiogram (ECG) using a specially adapted mini Holter recorder.
We just conducted our first field study involving 25 volunteers and plan to do five more trips to study up to 120 divers by the end of 2014.
Photography and post by: Petar Denoble, MD, D.Sc.
We’ve just recently returned from the first field trip for the left ventricular hypertrophy and risk of cardiac death in divers study in Bonaire (August 31-September 7). For the first time, we field tested the study logistics and the protocol. The dive trip was organized by Down Under Surf & Scuba in Raleigh, N.C. Out of 36 subjects in the group, 25 were qualified participants over 40 years of age and actively participated in the study. Most of the baseline testing was completed prior to the trip in four sessions: two at DAN Headquarters and two at the dive shop. Five new subjects joined us from other parts of the country and were pre-screened upon arrival to Bonaire.
The Buddy Dive Resort was our central study location. They were very accommodating and allowed us to use their main classroom as a lab. The classroom was at the waterfront where most divers enter and exit water or board the boat for the boat dives.
The plan was to get every participant scanned by echo twice after a full day of diving and once in the morning after a night’s rest. We took take a resting electrocardiogram (ECG) to monitor heart rate variability after a full day of diving on select dive days. All divers carried the Holter Monitor during at least one dive and were asked to record the depth and time of all their dives.
The compliance was nearly perfect. In fact, the only delays were related to travel time back from some remote dive sites. We completed a total of:
- 100 echo scans
- 50 resting ECGs
- 24 underwater Holters
- Nearly 500 recorded dives
The research team included cardiologist Dr. Douglas Ebersole, two professional echo cardiographers Brandy Emory of Lakeland Clinic and Lisa Caudill of Duke and myself. They did an excellent job maintaining the tight schedule and were well received by all of the participants. They even had time to join participants on some dives.
Scott Powell, the manager of Down Under Surf & Scuba and Rochelle Wright, a DAN Member Services specialist, managed all of the logistical challenges so we were able to complete our study. We’re very thankful for their support as well as the overwhelming support from the participants. We appreciate your participation and thank you for helping us to work toward improving diver safety.
Photography and Post by: Petar Denoble, MD, D.Sc.
Decompression sickness (DCS) is a condition that may result from quick decompression, which may occur when diving or flying. One mechanism involved in DCS is the passage of venous gas emboli (VGE or “bubbles”) to the arterial side of circulation; this is known as arterialization.
Until recently, arterialization was considered a rare event except when there is a passage in the heart wall because of a patent foramen ovale, (PFO), atrial septal defect or ventricular septal defect. In people with normal hearts it was previously thought that when venous gas passed through the narrow pulmonary capillaries, the potential for VGE was eliminated. It is known that some pathways allow up to five percent of venous blood to bypass pulmonary capillary filters, but the caliber of these bypasses was considered too small to allow VGE to pass through. Occasionally, some bubbles would arterialize, but it was considered a rare event. Recently, however, several authors have reported postdive VGE arterialization, but the true incidence of this phenomenon was not known.
Our colleagues of University Split, Ljubkovic M., et al. studied VGE arterialization and published two papers. The first, “Determinants of arterial gas embolism after scuba diving” (http://jap.physiology.org/content/112/1/91.long), reports results of laboratory testing and postdive findings. They tested 34 subjects by injecting saline with air bubbles in an arm vein and used echocardiography to monitor for bubble passage to the left side of the heart. In 23 out of 34 subjects, the transpulmonary passage of bubbles was observed at rest or after mild exercise. Nine subjects with confirmed arterialization in lab conditions also experienced arterialization after a field dive. All nine had large amounts of VGE in their right heart (VGE grade of 4B or greater). In subjects with no arterialization in lab conditions, there was no arterialization postdive either despite five of them having VGE grade 4B.
Authors concluded that “Postdive VGE arterialization occurs in subjects that meet two criteria: 1) transpulmonary shunting of contrast bubbles at rest or at mild/moderate exercise and 2) VGE generation after a dive reaches the threshold grade.”
It is important to notice that none of the nine divers with echocardiographically detected arterialization had any symptoms or signs of DCS or cerebral arterial gas embolization (CAGE). There is also no clear evidence concerning long-term consequences of chronic embolization in divers without history of manifested DCS or CAGE. Read the Alert Diver article, “Effects of diving on the brain” to learn more.
The significance of these findings is dubious. In the first place, it is now clear that a certain level of arterialization occurs more often than previously assumed and proven. One of the reasons may be in increased resolution of new generations of echocardiography machines, which enables us to detect smaller VGE than before. Second, it is reasonable to assume that occurrence of DCS in cases of VGE arterialization depends on the size and quantity of VGE, but the threshold values are not known. Third, a loose relationship between the presence of PFO and DCS may be due to not accounting for transpulmonary bubble passage.
Thus, we are looking forward to results of a prospective study conducted by Germonpre, P. and colleagues, which relates to the presence of VGE in carotid artery (accounting for PFO and transpulmonary passage) to DCI.
Post written by: Petar Denoble, MD, D.Sc.