For Dr. Cynthia Smith, listening to hearts is part of the job. Her patients just happen to be dolphins. In 2016, Smith was monitoring the health of a specific population of bottlenose dolphins in Barataria Bay, Louisiana as part of the Natural Resource Damage Assessment following the Deepwater Horizon oil spill. During a physical examination, Smith and her team discovered something concerning. A healthy dolphin heart makes a syncopated beat, just like a human. But instead of the classic “lub dub” sound, some of these dolphins had a heartbeat with an extra sound, a “lub dub shhh.” It turns out, many of the dolphins in Barataria Bay that Smith was studying had heart murmurs.
“We were noticing that we were hearing a lot of heart murmurs, and that was intriguing because of all the results coming from the bird and fish research about oil cardiotoxicity,” said Smith, National Marine Mammal Foundation Chief Medical Officer and Co-Principal Investigator of the Consortium for Advanced Research on Marine Mammal Health Assessment (CARMMHA), a part of the Gulf of Mexico Research Initiative. “We started to wonder, have we been missing something or is this a new problem that is emerging?”
Part of the problem was the lack of understanding about dolphin heart health. While we know much about the heart health of humans, dolphins are trickier to assess, owing to their marine lifestyle. In order to properly assess dolphin heart health, Smith and a team of CARMMHA scientists and veterinarians, including two veterinary cardiologists, needed to enhance dolphin cardiology for field ready deployment. In 2018, they would return to Barataria Bay, but before a thorough assessment could be completed there would need to be a major overhaul of procedure. Even listening to the heart using a stethoscope, an assessment technique called auscultation, had never before been described.
“Previously, there was no published information regarding the technique of ausculting dolphin hearts or regarding prevalence of murmurs in dolphins,” said Dr. Barbara Linnehan, National Marine Mammal Foundation Deputy Director of Medicine and former Postdoctoral Veterinary Fellow on the CARMMHA cardiac project. “Early studies even referred to the dolphin heart as practically inaudible.”
So, how do you go about defining a whole medical discipline for a wild animal that spends its entire lifetime underwater? In this case, the CARMMHA team got lucky. The United States Navy has a population of dolphins that live in the San Diego Bay, cared for by a mixed team of military, government, and contractor personnel, including NMMF veterinary and training staff. These dolphins have regular health assessments and are trained to react to the signals of trainers. Some have medical histories stretching to over 50 years, which serve to show how heart health evolves over a dolphin’s lifetime. These Navy dolphins were about to become key to figuring out whether the dolphins of Barataria Bay had heart problems related to the oil spill or not.
The Navy dolphins helped veterinarians perfect listening to the heart in a controlled way, in the name of new auscultation techniques. In the past, subtleties in the heart’s natural sound were masked by sounds from the water, saltwater damage to the stethoscopes, movement of the dolphins, and other errors on the listeners' part. Figuring out what position to hold the dolphin was only one part of effectively listening to the heart, and it turns out having the dolphin lay on its left side and listening between the two flippers is the best position in the trained Navy dolphins. The team then determined six places along the dolphin’s chest where a stethoscope would be placed to capture the sounds of each of the heart’s chambers.
“There is a lot of preparation and creativity required to waterproof our equipment as much as possible. We ruined a lot of stethoscopes with salt water in the process!” said Linnehan.
Listening to the heart is only one way to get a picture of heart health. Next, CARMMHA researchers attempted an assessment technique never before performed on a wild dolphin. They were about to try to get a picture of the heart and all its valves.
An echocardiogram is essentially an ultrasound image of the heart. The setup is much like an ultrasound at the doctor’s office, though in this case the dolphin is held by trained experts in the water and the machine is positioned right on the edge of a boat. Using a probe, high frequency sound waves are projected on the heart which then bounce back to the receiver. When the sound waves hit heart tissue they bounce back in different ways depending upon the structure of that tissue. The receiver then processes the sound data into a picture. Using this picture experts can figure out how well blood flows in each of the heart valves.
Once the echocardiography techniques were perfected on the Navy dolphins, it was time to try them out on the wild dolphins of Barataria Bay—an exciting and rewarding experience. “The whole team felt a sense of accomplishment in performing the first heart ultrasounds on wild dolphins, especially with such an important population as the Barataria Bay dolphins who have been so heavily impacted by the Deepwater Horizon oil spill,” said Dr. Forrest Gomez, National Marine Mammal Foundation Director of Medicine and Co-Investigator on CARMMHA.
After capturing and safely assessing 34 dolphins in Barataria Bay during the month of July in 2018 an interesting trend began to emerge. All of the dolphins had heart murmurs. A prior trip to Sarasota Bay, where the population of dolphins was not impacted by the oil spill, showed similar results, as did research including the Navy dolphins. It would appear that the dolphin heart makes extra sounds due to its immense strength—in a way they are musical.
“Essentially, their athletic hearts pump blood out so quickly that we can hear it with a stethoscope in the form of a murmur,” said Linnehan.
But that wasn’t the only result that piqued the team’s interest. After assessing blood flow in all the dolphins in Barataria Bay, two of them had heightened blood pressure in the right side of their hearts. This is a sign of pulmonary hypertension, which has never been diagnosed in any wild whale, dolphin, or porpoise. It is possible that the hypertension is a result of chronic pulmonary disease due to oil exposure. Barataria Bay dolphins were also found to have thinner ventricular (heart) walls compared to the Sarasota Bay dolphins. It is possible that the thin heart walls and hypertension are results of chronic cardiac and pulmonary disease due to oil exposure.
While it was a relief to find that prevalent heart murmurs were most likely a sign of a healthy, athletic heart, further examination would show that dolphin hearts weren’t necessarily exempt from damage from an oil spill. For an animal that relies on a strong heart to chase speedy prey, a diagnosis of cardiac disease or pulmonary hypertension could be devastating. Further research is needed to investigate the cardiac abnormalities identified in this population and how they are linked to oil exposure. For now, a celebration of current achievements is more than earned. Perhaps, Smith says it best, without funding, “I don’t know when dolphin heart health would have been addressed.”
The Ocean Portal receives support from the Gulf of Mexico Research Initiative (GoMRI) to develop and share stories about GoMRI and oil spill science. The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization, and remediation technologies.
For more information, visit http://gulfresearchinitiative.org