Veterinarians and researchers know general anesthesia in horses can be risky, with reported mortality rates ranging from 0.12-1.6%. They also are well-aware that pregnancy can exacerbate those risks and possibly reduce live-foaling rates. What they’re less certain about, however, is when is the safest time during gestation to recommend elective surgeries (such as to remove masses or repair wounds).

Veterinary anesthesiologist John A.E. Hubbell, DVM, MS, Dipl. ACVAA, and colleagues at Rood & Riddle Equine Hospital in Lexington, Kentucky, recently reviewed the literature to develop recommendations on when and when not to pursue elective surgeries in mares to maintain pregnancy. He presented the team’s findings at the 2017 American Association of Equine Practitioners Convention, held Nov. 17-21 in San Antonio, Texas.

Research Says …

Hubbell first reviewed what veterinarians and researchers know about equine pregnancy, how it impacts anesthesia, and how sedation and anesthesia affect pregnancy.

Pregnancy—Mares are pregnant for an average of 320 to 370 days. He reviewed the basic timeline of events:

  • Around Day 6 or 7 after ovulation, the conceptus begins to move around inside the uterus until it becomes fixed Day 15-17. During this early stage, progesterone (a type of progestogen, or hormone that has pro-gestational activity) produced by the corpus luteum (the structure formed after the ovarian follicle releases the egg) maintains the pregnancy.
  • At Day 35, endometrial cups form at the base of the uterine horn containing the conceptus, where it forms an accessory corpus lutea that produces additional progesterone to maintain pregnancy until Day 100. At this point, placental progestogens take over that role until the mare foals.
  • “The majority of fetal growth occurs in the final three months of gestation (Day 220 to foaling), at which point the mare’s abdomen usually becomes noticeably distended,” Hubbell said. “As the mare’s abdomen is expanding, the foal takes up an increasingly larger amount of space, growing approximately 1 kilogram (roughly 2 pounds) every two days.”

Live foaling rates reported in Central Kentucky range from 62% to 87%, he said.

Researchers have also studied pregnancy loss rates, which are higher prior to 40 days of gestation, Hubbell said

How pregnancy affects anesthesia—Hubbell said there’s little knowledge about this subject in horses, but there’s more research in other species. Scientists have shown that anesthetized women early in pregnancy have increased tidal volume (the amount of air inhaled and exhaled with each breath) and respiratory minute volume (the amount of gas exhaled per minute). Anesthetized late-term pregnant women, on the other hand, have a decreased residual respiratory volume—the amount of air remaining in the lungs at the end of respiration.

“Similar changes would be expected in anesthetized pregnant mares in late gestation, particularly when they are positioned in dorsal recumbency (lying down on their back),” he said.

How sedation and anesthesia impact pregnancy—Scientists know more about this subject in horses.

In one study researchers administered the sedative detomidine to 10 pregnant mares weekly from Day 14 to 60 of gestation and then monthly until foaling. One mare aborted on Day 167 of gestation and another was euthanized after developing a large colon torsion (twist). Six of the eight live foals born were normal, while the other two suffered from torticollis (abnormal neck twisting) and bilateral (in both of a pair of limbs) upward fixation of the patella (a stifle condition). Still, he said, “the authors concluded that there were no specific adverse effects of the detomidine.”

However, he acknowledged that all known anesthetic techniques have the potential to decrease cardiac output, depress arterial pressures, and cause respiratory depression (hypoventilation, or slow and ineffective breathing), which could lead to fetal acidosis (abnormally high blood acidity), hypoxia (inadequate oxygen supply), and hypercarbia (abnormally high blood carbon dioxide levels).

Researchers haven’t performed extensive studies specifically evaluating the effects of anesthesia on pregnant mares undergoing elective surgeries, Hubbell said, so veterinarians must use findings from studies on mares anesthetized on an emergency basis. Some examples of results include:

  • “The largest retrospective study of anesthetic morbidity and mortality in horses shows that mares in the third trimester of pregnancy were at increased risk, with two of 16 mares dying of anesthetic complications,” he said;
  • Based on results from a series of studies, researchers concluded that intravenous agents resulted in fewer cases of fetal acidosis and hypoxia than inhaled anesthetics;
  • In four studies in which researchers evaluated colic surgery’s effects on subsequent foaling rates, Hubbell said, foaling rates for surviving mares were 80%, 79.4%, 66.7%, and 53.8%; and
  • “One study examined the relationship between the timing of colic surgery and gestational age, finding that mares pregnant 16 to 39 days at the time of surgery had a lower foaling rate than mares pregnant for greater than 40 days at surgery,” he said.

What does it all mean?

Based on the research the team reviewed, Hubbell recommended that if general anesthesia must be performed on pregnant mares, the safest time window is Days 40-220 of gestation.

“The decision to anesthetize a pregnant mare is made on a case-by-case basis,” Hubbell said. “Factors to consider include the general health of the mare, her age (an increased anesthetic risk on its own), the anticipated duration of the procedure, and any previous history of foal loss. In most cases the risk of anesthetizing the mare for surgery must be weighed against the risk of leaving a condition untreated. An additional factor to consider, if surgery is postponed, is the difficulty in dealing with a mare and her foal at the time of surgery, particularly when the owners desire to get the mare rebred at the earliest opportunity.”