Low doses of oxytocin can induce foaling at term

Abstract

Summary Levels of the major circulating metabolite of prostaglandin F‐2 α, 13, 14 dihydro‐15‐oxo‐prostaglandin F‐2 α (PGFM) were measured during the induction of foaling using small (2.5–10 iu) intravenous doses of oxytocin. PGFM levels rose rapidly in all animals within 15 min of injection and were associated with typical signs of second stage labour. Because these small doses of oxytocin are effective in successfully triggering parturition it is suggested that higher doses (40–120 iu) used to induce birth in other studies are unnecessary and could be potentially dangerous to the foetal foal. Résumé Les taux des principaux métabolites circulant de la prostaglandine F‐2α, 13, 14 dihydro‐15‐oxo‐prostaglandin (F‐2α PGFM) ont été mesurés durant l'induction du poulinage utilisant de faibles doses (2, 5–10 iu) d'ocytocine intraveineuse. Les taux de PGFM s'élevèrent promptement chez tous les animaux dans les 15 minutes suivant l'injection et furent simultanés de signes cliniques de parturition. Puisque ces petites doses d'ocytocine sont efficaces dans le déclenchement de la mise bas, on pense que les posologies plus élevées (40–120 iu) utilisées dans d'autres études pour induire le poulinage sont inutiles et qu'elles pourraient représenter un risque pour le foetus. Zusammenfassung Man mass die Konzentrationen des hauptsächlichen zirkulierenden Metaboliten von Prostaglandin F‐2×13, 14 Dihydro 15‐oxo‐prostaglandin F‐2X (PGFM)—während der Geburtseinleitung mit niedrigen intravenösen Dosen von Oxytocin (2.5–10 iu). Die PGFM‐Spiegel stiegen rasch innert 15 Minuten bei allen Tieren an und waren mit typischen Zeichen des 2. Wehenstadiums. Weil diese niedrige Oxytocindosen wirksam und erfolgreich verbunden die Geburt auslösen, wird vermutet, dass höhere Dosen (40–120 iu) zur Geburtseinleitung unnötig oder sogar potentiell gefährlich für das Fohlen seien.

Prenatal maturation, the timing of birth and how it may be regulated in domestic animals

“…The situation in the pig is not clear although the size of the fetuses means that less dilatation is required than in ruminants or mares. In the mare the nature of the prepartum changes in cervical structure is unknown, but the speed with which labour can be induced and completed by exogenous agents (Pashen, 1980) suggests that the cervical connective tissue matrix may have already partially ripened well before term.…”

“…A sudden change in the steroid levels will 'switch on' both PG production and the oxytocin receptors, thereby causing an immediate change in myometrial contractility, whereas a long period of exposure to high concentrations of circulating oestrogen with little or no change in progesterone levels might gradually increase the oxytocin receptor population and hence explain the greater sensitivity of the uterus to oxytocin in some species. Certainly the equine myometrium appears to be more active in the last few weeks of gestation than most other species (Haluska et al 1987) and oxytocin will readily provoke the onset of labour (Pashen, 1980).…”

“…On the other hand, 70-100 % success rates have been reported by Bristol (1982) and Rose (1982) using PGF2a or its analogues. It may well be that a combination of PGF2, plus oxytocin is the most logical method of induction of labour in the mare (Leadon et al 1982); certainly a lower dose of oxytocin is generally required if given after fluprostenol and such a regime is likely to have a less violent action on the uterus than oxytocin alone (Pashen, 1980).…”

“…In the last month or so of gestation the relatively high levels of circulating oestrogen and the low concentrations of progesterone together with high levels of progesterone metabolites (Pashen, 1984;Ousey et al 1987) may well create a hormonal environment in which oxytocin receptors are present well before term and consequently endogenous myometrial activity is high (Haluska et al 1987). It is certainly possible to induce labour with oxytocin, with or without additional PGF2a or its analogues, such that a foal is delivered within less than an hour (Pashen, 1980;Rossdale, Ousey, Dudan, Leadon, Cash, Reddy, Silver, Fowden, Broughton Pipkin & Jeffcott, 1984a). The duration of the whole event may be as short as 15 min and as long as 2-3 h.…”

Clinical observations on the course of oxytocinor prostaglandin E2/oxytocin-induced parturition in mares

“…On the other hand a small oxytocin dose not only decreases the risk of complications connected with brutal, abrupt delivery but also allows for precise regulation of the delivery course. There are studies in which oxytocin doses even lower than 5 IU were effective in inducing parturitions in mares but in those cases the time of delivery was hardly predictable (Pashen 1980, Camillo at al. 2000.…”