Development of a Bimodal Heart Rate Pattern in Undisturbed Fetal Harbor Seals

Abstract

Fetal and maternal heart rates were studied in unrestrained, pregnant harbor seals during the last third of gestation. Heart rates were recorded while the mothers were resting on land or performing trained simulated dives of up to 2.25 min. Data from resting mothers showed the development of a bimodal or two‐speed fetal heart rate pattern during late gestation. The mean faster and slower fetal heart rates at term were 125 ± 3.7 and 79 ± 3.1 (mean + SEM) respectively. The amount of fetal bradycardia observed increased steadily towards term, and fetal heart rate changes were not correlated with changes in maternal heart rate or maternal respiration. The bimodal fetal heart rate was also seen during the simulated dives, and no decrease in either the faster or slower heart rate was found. Heart rates from resting, unrestrained harbor seal pups were also studied. The pups displayed a bimodal heart rate similar to the fetuses' with the slower rate occurring during breath‐holds. The bradycardia in the pups was equivalent to the slower fetal heart rate. These findings suggest that the regulatory mechanism that determines the apneic bradycardia in young harbor seals during non‐stressful conditions develops in the last quarter of gestation.

Developmental changes in cardiorespiratory patterns associated with terrestrial apnoeas in harbour seal pups

“…Upon weaning, however, several changes occurred. Respiration rate decreased (Fig.·2), approaching adult levels (8-22·breaths·min -1 ; Harrison and Tomlinson, 1960;Bacon et al, 1985). Consistent with dive duration (Greaves et al, in press a), the length of terrestrial apnoeas continued to increase post-weaning (Fig.·3).…”

“…During the last third of gestation, foetal harbour seals exhibited a 'slower' heart rate (79·beats·min -1 ) that was distinct from the 'faster' embryonic rate (125·beats·min -1 ) first observed early in gestation (Bacon et al, 1985). The amount of bradycardia steadily increased as parturition approached (Bacon et al, 1985). This bimodal heart rate pattern continues to develop postpartum (Table·1).…”

“…Breathing frequency did not change throughout the nursing period and fell within the range observed previously for juvenile harbour seals (18-53·breaths·min -1 ; Harrison and Tomlinson, 1960;Påsche and Krog, 1980;Skinner and Milsom, 2001) (C) Proportion of time spent in the low heart rate mode. Cardiorespiratory patterns in harbour seal pups apnoea duration increased during the lactation period, paralleling an increase in dive duration (Greaves et al, in press a), and values were similar to those obtained for resting, captive harbour seal pups less than 2 weeks of age (31·s; Bacon et al, 1985). Upon weaning, however, several changes occurred.…”

“…This bimodal or two-speed heart rate pattern begins to develop in utero (Liggins et al, 1980;Bacon et al, 1985). During the last third of gestation, foetal harbour seals exhibited a 'slower' heart rate (79·beats·min -1 ) that was distinct from the 'faster' embryonic rate (125·beats·min -1 ) first observed early in gestation (Bacon et al, 1985).…”

“…However, it should be noted that even though an animal is lying motionless with its eyes closed, it may not be sleeping; such a state has also been exhibited during periods of quiet wakefulness (Mukhametov et al, 1982). Eupnoea was categorized by regular opening and closing of a pup's nostrils, while apnoea was observed as a breath-hold, during which nostrils remained closed for >10·s following an exhalation (Bacon et al, 1985;Blackwell and Le Boeuf, 1993;Falabella et al, 1999). Since electroencephalographic activity was not recorded during this study, respiratory pauses are referred to as terrestrial apnoeas rather than sleep apnoeas (Castellini et al, 1994b).…”