Cardiac Output during Diving in an Unrestrained Sea Lion

Abstract

AQUATIC mammals and birds experience abrupt and profound cardiovascular changes during water immersion. In seals, for example, a diving bradycardia of one-tenth the normal rate without change in pressure has commonly been observed^1,2. The evidence suggests that this is accompanied by a reduction in cardiac output and widespread vasoconstriction in skeletal muscle^1,3, skin¹ and kidneys⁴. Even in man, a relatively poor diver, the flow of blood in the limbs has been shown to decrease during breath-holding dives⁵. A single determination of cardiac output has been made in a seal during non-diving asphyxia⁶, but direct blood flow measurements are lacking.