Anaesthetising the Pregnant Mare and Techniques for Caesarean Operation

Anaesthesia for pregnant mares
Anaesthetic challenge and risk increase throughout pregnancy and reach a maximum at term because of a) increasing fetal size and b) ongoing physiological adaptations in the mare. However, different problems exist in different trimesters; in people the risk of spontaneous abortion diminishes with gestation. The teratogenic effects of anaesthesia are theoretically greatest in the organogenetic stage (1st trimester) of pregnancy. Most drugs are unlicensed for use in pregnant animals, not because they are teratogenic, but because they have not been tested in pregnant animals. Despite its effects on nucleic acid synthesis, N2O is not teratogenic and is not contraindicated in the first trimester. As pregnancy enters the second trimester, adverse haemodynamic effects of anaesthetics (and surgery) on the utero-placental unit may lead to premature placental separation, i.e. abortion. This risk increases as pregnancy advances. Owners should be informed of this for medico-legal reasons. The combined risk to the mare and fetus is a minimum during the second trimester, which is in theory the safest time for nonobstetrical operations. As pregnancy advances to term, rapid increases in fetal size and nutritional demand initiate major physiological changes in the dam. Other maternal changes occur in preparation for parturition itself. The net effect on the dam is a diminution of functional reserve and an increased risk from anaesthesia (see later). Much is made of the abortifacient effects of sedative drugs - particularlyα2 agonists - and anaesthetics. However, endogenous glucocorticoids released in response to stress, i.e., anxiety/pain, inadequate anaesthesia/surgical stimulation/nociception, post operative pain, hypotension, hypoxia and hypercapnia also promote myometrial excitability. Consequently, adequate doses of theoretical abortifiacients may be safer than inadequate doses that fail to prevent a stress response. [...]