Cardiac responses to snout immersion in trained dogs

Drivers of the dive response in pinnipeds; apnea, submergence or temperature?

Long and deep dives in marine mammals are enabled by high mass-specific oxygen stores and the dive response (DR), which reduces oxygen consumption in concert with increased peripheral vasoconstriction and a lowered heart rate during dives. Diving heart rates of pinnipeds are highly variable and modulated by many factors, such as breath holding (apnea), pressure, swimming activity, temperature, and even cognitive control. However, the individual effects of these factors on diving heart rate are poorly understood due to the difficulty of parsing their relative contributions in diving pinnipeds. Here, we examined the effects of apnea and external sensory inputs as autonomic drivers of bradycardia. Specifically, we hypothesized that 1) water stimulation of facial receptors would—as is the case for terrestrial mammals—enhance the dive response, 2) increasing the facial area stimulated would lead to a more intense bradycardia, and 3) cold water would elicit a more pronounced bradycardia than warm water. Three harbor seals (Phoca vitulina) and a California sea lion (Zalophus californianus) were trained to breath-hold in air and with their heads submerged in a basin with variable water level and temperature. We show that bradycardia occurs during apnea without immersion. We also demonstrate that bradycardia is strengthened with both increasing area of facial submersion and colder water. Thus, we conclude that initiation of the DR in pinnipeds is more strongly related to breath holding than in terrestrial mammals, but the degree of the DR is potentiated autonomically via stimulation of facial mechano- and thermoreceptors upon submergence.

Life under water: physiological adaptations to diving and living at sea

This review covers the field of diving physiology by following a chronological approach and focusing heavily on marine mammals. Because the study of modern diving physiology can be traced almost entirely to the work of Laurence Irving in the 1930s, this particular field of physiology is different than most in that it did not derive from multiple laboratories working at many locations or on different aspects of a similar problem. Because most of the physiology principles still used today were first formulated by Irving, it is important to the study of this field that the sequence of thought is examined as a progression of theory. The review covers the field in roughly decadal blocks and traces ideas as they were first suggested, tested, modified and in some cases, abandoned. Because diving physiology has also been extremely dependent on new technologies used in the development of diving recorders, a chronological approach fits well with advances in electronics and mechanical innovation. There are many species that dive underwater as part of their natural behavior, but it is mainly the marine mammals (seals, sea lions, and whales) that demonstrate both long duration and dives to great depth. There have been many studies on other diving species including birds, snakes, small aquatic mammals, and humans. This work examines these other diving species as appropriate and a listing of reviews and relevant literature on these groups is included at the end.

Diving bradycardia: a mechanism of defence against hypoxic damage

A feature of all air-breathing vertebrates, diving bradycardia is triggered by apnoea and accentuated by immersion of the face or whole body in cold water. Very little is known about the afferents of diving bradycardia, whereas the efferent part of the reflex circuit is constituted by the cardiac vagal fibres. Diving bradycardia is associated with vasoconstriction of selected vascular beds and a reduction in cardiac output. The diving response appears to be more pronounced in mammals than in birds. In humans, the bradycardic response to diving varies greatly from person to person; the reduction in heart rate generally ranges from 15 to 40%, but a small proportion of healthy individuals can develop bradycardia below 20 beats/min. During prolonged dives, bradycardia becomes more pronounced because of activation of the peripheral chemoreceptors by a reduction in the arterial partial pressure of oxygen (O2), responsible for slowing of heart rate. The vasoconstriction is associated with a redistribution of the blood flow, which saves O2 for the O2-sensitive organs, such as the heart and brain. The results of several investigations carried out both in animals and in humans show that the diving response has an O2-conserving effect, both during exercise and at rest, thus lengthening the time to the onset of serious hypoxic damage. The diving response can therefore be regarded as an important defence mechanism for the organism.

The evolution of asphyxial defense

From the time animals became dependent upon molecular oxygen as an integral part of their energy-producing processes, they have remained in the shadow of acute asphyxial threat--the blocking of respiratory exchange resulting in the intracellular triad of hypoxia, hypercapnia and acidosis. The most commonly occurring precipitant of acute asphyxia has always been the transfer between air and water environments. Over the last one hundred years studies on a wide range of living organisms, from single cells to complex multicellular organisms like mammals, have demonstrated the presence of well-defined metabolic and cardiovascular-respiratory mechanisms for protecting living things against acute asphyxia. Single-celled animals depend upon anaerobiosis and secondarily hypometabolism. In addition to these processes, animals with gills or lungs utilize "passive" protection such as increased oxygen storage and the "dynamic" cardiovascular adjustments of bradycardia and selective ischemia. These latter changes decrease overall oxygen consumption and hence utilize the oxygen stores in the most economical way to protect the cardiac and cerebral tissue, which are most sensitive to hypoxia and vital to continued survival of the organism. In this article an attempt is made to place these processes into an evolutionary context. As through a glass darkly we glimpse asphyxial defense running like a paleophysiological thread through hundreds of millions of years, being accentuated here and muted there, depending upon the particular needs of individual species.

Age-related changes in autonomic control: the use of beta blockers in the treatment of hypertension

Functional decrements in autonomic control and reflex activity in the elderly resemble the effects of beta-adrenoreceptor blockade. This arises partly from an age-related decrease in intrinsic beta-adrenoreceptor sensitivity and partly from effector changes associated with degenerative processes such as arteriosclerosis. In the elderly, compensatory adjustments in cardiovascular control result from both sympathetic and parasympathetic dysfunction. The characteristics of aging in autonomic nervous control are examined in relation to the treatment of essential hypertension by beta blockers in the elderly. Increased cardiac output with exercise depends more on increased intracardiac volume than on sympathetic modulation of heart rate in older people. Baroreceptor-dependent and renin blood pressure responses are diminished. The cold pressor response, which is found to be greater in the elderly than in young adults, is abolished by alpha and not beta blockers. Blood viscosity and blood platelets also increase in moderately cold conditions, and a beta blocker with vasodilator and antiplatelet activity may therefore be useful. Trigeminal cardiorespiratory reflex responses to facial cooling evoke a higher blood pressure but smaller bradycardia in old people. These constraints of autonomic nerve function on the use of beta blockers for treating hypertension are imposed on a background of altered drug pharmacokinetics and pharmacodynamics in the elderly.

Disorders of cardiac conduction accompany the dive reflex in man

The faces of 19 healthy subjects were immersed in water at the temperatures varying between 10 degrees and 30 degrees C and at different degrees of lung inflation. Several abnormalities of cardiac conduction were noted. They occurred with the greatest frequency in the coldest water and when there was relatively little air in the lungs at the moment of immersion. There was fragmentary evidence, confirmatory of earlier studies, that heart-rate slowing was accentuated by fear and that very little slowing occurred when the subject was distracted or preoccupied. Various conduction abnormalities were recorded. The most striking finding was the wide difference from person to person in the occurrence of conduction disturbances under more or less comparable circumstances. Moreover, the patterns of conduction alterations, differing from person to person, were nevertheless relatively consistent from dive to dive for the same individual. To ascertain whether or not such idiosyncratic responses may have prognostic significance calls for a long-term, prospective study.

Interaction of fastigial pressure response and depressor response to nasal perfusion

The fastigial nucleus pressor response (FPR) and the nasal-perfusion diving response were elicited alone and simultaneously to observe the net effect on cardiovascular variables. The fastigial nucleus was electrically stimulated in 14-chloralose-urethane anesthetized dogs and the FPR was characterized by a transient tachycardia with sustained elevations in arterial pressure, left ventricular pressure and maximal dP/dt. The tachycardia was buffered by baroreceptor reflexes during the elevated arterial pressure of the FPR. All variable of the FPR, however, were reduced by superimposition of the diving response upon the FPR. Heart rate was most sensitive to depression by the nasal perfusion which elicited a bradycardia as much as 70 beats/min below the control rate. The nasal-evoked diving response was discussed with respect to the trigeminal depressor response which results from direct stimulation of the spinal trigeminal complex. Algebraic cancellation of the FPR and dive responses is considered along with anatomical and electrophysiological evidence which suggests that these two responses, as well as the baroreceptor reflex, could be integrated at a common site. This site may be the medullary nucleus of the solitary tract which receives projections from trigeminal and glossopharyngeal nerves and from the fastigial nucleus.

Apnoea and bradycardia from submersion in "chronically" decerebrated cats

In "chronically" but not in acutely decerebrated cats, submersion of the head caused apnoea and marked bradycardia, associated with a maintained or slightly raised arterial pressure. Since these reflex adjustments, though very reproducible, occurred with a varying latency and could be induced also by nasal injection of water, they appeared to be, at lest in part, elicited from the upper respiratory passages. Thus, a terrestrial mammal, reputed to shun any form of immersion, can exhibit adjustments during head submersion, similar to those in habitually diving species. This response pattern is basically organized at the lower brainstem level.

Evaluation of a neurogenic rapid coronary dilatation during an excitatory response in conscious dogs

The present study was undertaken to evaluate the mechanisms of coronary adaptation to sudden changes in myocardial oxygen demand that occur during excitement. An excitatory response was evoked either by electrical stimulation of the hypothalamic defence area or by noise (discharge of a fire-arm). Continuous measurement of the oxygen saturation in coronary venous blood was used to judge, whether an increase in coronary flow was adequate to match an increased myocardial oxygen demand. During the excitatory response heart rate, cardiac output and coronary flow increased. However, the increase in coronary flow was not adequate to meet the increased metabolic requirement as indicated by a decrease in coronary venous oxygen saturation. In dogs with experimental atrioventricular block, and with heart rate controlled by external pacing, a rapid coronary dilation occurred during the excitatory response and was accompanied by an increase in coronary venous oxygen saturation. This rapid coronary dilation was abolished by beta-adrenergic blockade. The pattern of coronary flow and coronary venous oxygen saturation that occurred during the excitatory response in normal dogs could be mimicked in dogs with atrioventricular block by increasing the ventricular pacing rate. However, when identical increases in heart rate were induced either excitement or by external pacing, the drop in coronary venous oxygen saturation was significantly larger in the paced series. This demonstrates, that an increase in heart rate is responsible for the transient decrease in coronary venous oxygen saturation during the excitatory response. From these experiments it is concluded that a rapid neurogenic dilation of the coronary vessels occurs during the excitatory response. Under normal conditions this rapid neurogenic dilation is masked by the effect of the accompanying increase in heart rate on extravascular coronary resistance.

Nasal-cardiopulmonary reflexes: a role of the larynx

A brief review of the literature from otorhinolaryngology and the basic sciences shows the existence and role of nasal-cardiopulmonary reflexes in animals and man. There is ample evidence that odors, fluids and mechanical stiumlation of the nasal mucosa will induce changes in the lungs and cardiovascular system. The proposition that nasal obstruction also produces cardiopulmonary changes is briefly reviewed. The suggestion is made that one of the functions of the nose is to act as an expiratory brake. the removal of this brake could result in changes in laryngeal resistance that lead to poor ventilation.