The Relationship Between Dive and Pre-Dive Heart Rates in Restrained and Free Dives by Diving Ducks

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Abstract
Dive heart rate was plotted against pre-dive heart rate in forced and voluntary dives and dabbles by restrained and free ducks. The relationship between pre-dive heart rate and the cardiac interval occurring just before or coincident with submersion (first cardiac interval) and the heart rate after 2–5 s submergence (stabilized heart rate) was emphasized. Stabilized heart rate in forced dives by restrained ducks at rest and at the end of a bout of exercise, and heart rate in voluntary dives and dabbles were linearly related on a plot of dive heart rate against the logarithm of predive heart rate. Even the heart rate occurring 2–5 s after ducks were ‘trapped’ under water, compared with the rate immediately before ‘trapping’, fitted on this line. The line was described by the equation y = - 451 + 2461ogx where y is dive (or trapped) and × is pre-dive (or pre-trap) heart rate (r2 = 0.98). The relationship was unaltered by β-blockade with propranolol. Furthermore, nasal blockade with Xylocaine, O2 breathing before submersion, and arterial baroreceptor denervation had no marked effect on the relationship in voluntary and trapped dives. Implantation of stimulating electrodes bilaterally on the cut distal ends of vagal and cardiac sympathetic nerves suggested that in all these dives there is a similar increase in the level of efferent vagal activity during submersion. However, the first cardiac interval in voluntary dives represents a much lower heart rate and therefore higher level of vagal activity. The present data suggest that there is considerable psychogenic modulation of cardiac responses in voluntary diving and only in forced dives, by restrained animals, is cardiac control largely reflexogenic.