Cerebral blood flow in the sea lion (Zalophus californianus) during voluntary dives

Recent publications have emphasized the potential generation of morbid cardiac arrhythmias secondary to autonomic conflict in diving marine mammals. Such conflict, as typified by cardiovascular responses to cold water immersion in humans, has been proposed to result from exercise-related activation of cardiac sympathetic fibers to increase heart rate, combined with depth-related changes in parasympathetic tone to decrease heart rate. After reviewing the marine mammal literature and evaluating heart rate profiles of diving California sea lions (Zalophus californianus), we present an alternative interpretation of heart rate regulation that de-emphasizes the concept of autonomic conflict and the risk of morbid arrhythmias in marine mammals. We hypothesize that: (1) both the sympathetic cardiac accelerator fibers and the peripheral sympathetic vasomotor fibers are activated during dives even without exercise, and their activities are elevated at the lowest heart rates in a dive when vasoconstriction is maximal, (2) in diving animals, parasympathetic cardiac tone via the vagus nerve dominates over sympathetic cardiac tone during all phases of the dive, thus producing the bradycardia, (3) adjustment in vagal activity, which may be affected by many inputs, including exercise, is the primary regulator of heart rate and heart rate fluctuations during diving, and (4) heart beat fluctuations (benign arrhythmias) are common in marine mammals. Consistent with the literature and with these hypotheses, we believe that the generation of morbid arrhythmias because of exercise or stress during dives is unlikely in marine mammals.

Section: Discussionmentioning

confidence: 99%

Heart rate regulation in diving sea lions: the vagus nerve rules

Ponganis

McDonald

Tift

et al. 2017

“…At a ƒ H of 20 beats min −1 , cardiac output will be ~36% of resting cardiac output and only about 18% of average surface cardiac output. At these levels of cardiac suppression, most of this flow should be directed towards the brain and heart (Dormer et al, 1977;Zapol et al, 1979).…”

Section: Implications For Blood Flowmentioning

confidence: 99%

Deep-diving sea lions exhibit extreme bradycardia in long-duration dives

McDonald

Ponganis

2014

Heart rate and peripheral blood flow distribution are the primary determinants of the rate and pattern of oxygen store utilisation and ultimately breath-hold duration in marine endotherms. Despite this, little is known about how otariids (sea lions and fur seals) regulate heart rate (f H ) while diving. We investigated dive f H in five adult female California sea lions (Zalophus californianus) during foraging trips by instrumenting them with digital electrocardiogram (ECG) loggers and time depth recorders. In all dives, dive f H (number of beats/duration; 50±9 beats min −1 ) decreased compared with surface rates (113±5 beats min −1 ), with all dives exhibiting an instantaneous f H below resting (<54 beats min −1

“…Some (e.g. Dormer et al, 1977) have even surmised that arterial pressure damping is indeed the very function of the rete. However, because of the massive increase in total vascular cross-sectional area, the retia will, according to Poiseuille's Law, provide a vastly reduced resistance.…”

Section: Discussionmentioning

confidence: 99%

“…Of these, the most popular hypothesis at present seems to be that the retia acts to dampen the arterial pressure pulses (Vogl and Fisher, 1982), and that this for reasons untold should protect the brain against damage during diving (Dormer et al, 1977).…”

Section: Introductionmentioning

confidence: 99%

On how whales avoid decompression sickness and why they sometimes strand

Blix

Walløe

Messelt

2013

SUMMARYWhales are unique in that the supply of blood to the brain is not by the internal carotid arteries, but by way of thoracic and intravertebral arterial retia. We found in the harbor porpoise (Phocoena phocoena) that these retia split up into smaller anastomosing vessels and thin-walled sinusoid structures that are embedded in fat. The solubility of nitrogen is at least six times larger in fat than in water, and we suggest that nitrogen in supersaturated blood will be absorbed in the fat, by diffusion, during the very slow passage of the blood through the arterial retia. Formation of nitrogen bubbles that may reach the brain is thereby avoided. We also suggest that mass stranding of whales may be due to disturbances to their normal dive profiles, resulting in extra release of nitrogen that may overburden the nitrogen 'trap' and allow bubbles to reach the brain and cause abnormal behavior.