Marine mammals such as seals and dolphins can develop arrhythmias -- problems of irregular heartbeats -- during their deep dives in search of food, researchers say.
Scientists at the University of California, Santa Cruz, say they've found evidence of cardiac rhythm complications in Weddell seals and bottlenose dolphins as they hold their breath during deep dives.
When diving, marine mammals undergo what's known as a dive response, where their heart rate drops sharply in a bid to conserve oxygen.
However, when they locate and begin to chase underwater prey their heartbeat needs to increase to keep up with the increased physical effort.
These differing signals can result in erratic cardiac rates and rhythms, or arrhythmias, the researchers report in the journal Nature Communications.
"The heart is receiving conflicting signals when the animals exercise intensely at depth, which often happens when they are starting their ascent," says lead author Terrie Williams, a professor of ecology and evolutionary biology. "We're not seeing lethal arrhythmias, but it is putting the heart in an unsteady state that could make it vulnerable to problems."
Depending on dive depth and level of physical effort, the heart rate of the animals can vary, sometimes switching quickly between a reduced rate, known as bradycardia, and an elevated rate, called tachycardia, Williams says.
"We tend to think of marine mammals as completely adapted to life in the water. However, in terms of the dive response and heart rate, it's not a perfect system," Williams says. "Even 50 million years of evolution hasn't been able to make that basic mammalian response impervious to problems."
To gather data, the researchers built monitoring devices to record the heart rate, frequency of swimming strokes, and the depth and duration of dives.
The monitors were used on trained dolphins in both diving pools and in open water, and were also attached to Weddell seals swimming beneath Antarctic ice.
The monitors detected arrhythmias in more than 70 percent of the animals' dives, the researchers say.
They add that their findings may be relevant to humans, who also have a dive response, though it's less pronounced than that of marine mammals.
In humans, the response is triggered as the face comes in contact with cold water.
In 2010, a study of triathlon events showed that 90 percent of deaths of participants occurred during the swimming segment of the races.
"It may be that the same conflicting signals we saw in dolphins and seals are causing arrhythmias in some triathletes," says Williams, who is collaborating with triathlon organizers to help reduce such incidents during the races.
