Practical Anesthetic Management of Mares in the Last Month of Gestation

Abdominal surgery in a late-term pregnant mare is never just about the mare. Every anesthetic decision directly affects the fetus, and that makes perioperative management far more demanding than routine equine anesthesia. 

Studies report viable foal survival rates of only 45–80% following abdominal surgery in pregnant mares, highlighting the delicate balance between maternal stabilization and fetal safety. The biggest fetal threat during anesthesia? Intrauterine hypoxia and asphyxia^1,2. 

Why Late-Term Mares Are High-Risk Anesthetic Patients 

In the final trimester, the enlarged gravid uterus creates major physiologic challenges during inhalation anesthesia—especially in dorsal recumbency. 

The uterus compresses¹: 

• The caudal vena cava → reducing venous return and preload 

• The abdominal aorta → reducing uterine blood flow 

• The diaphragm → impairing ventilation and gas exchange 

The result is a dangerous combination of¹: 

• Hypotension 

• Poor oxygenation 

• Hypercarbia 

• Reduced uteroplacental perfusion 

• Potential fetal distress 

Even with mechanical ventilation and 100% oxygen, maintaining adequate maternal oxygenation can be difficult. 

The Blood Gas Changes You Cannot Ignore 

One of the most clinically relevant findings in late-term mares under anesthesia is persistent respiratory compromise. 

Key findings reported during anesthesia: 

Hypercarbia is particularly important because fetal PaCO2 closely mirrors maternal PaCO2. As maternal carbon dioxide rises, fetal acidosis and cardiac depression may follow. 

The study showed that by 75 minutes of anesthesia, mares demonstrated significant hypercarbia and worsening acidemia—strong indicators that prolonged anesthesia increases fetal risk^1,3,4. 

Dorsal Recumbency: The Hidden Problem 

Dorsal recumbency significantly worsens cardiopulmonary compromise in pregnant mares. 

Common consequences include¹: 

• Atelectasis 

• Ventilation-perfusion mismatch 

• Reduced functional residual capacity 

• Decreased preload 

• Negative central venous pressure 

• Reduced stroke volume 

Interestingly, cardiac output remained relatively stable because mares compensated with tachycardia. However, this compensation did not fully correct hypotension. 

Clinically, this means a mare may appear cardiovascularly “stable” while uteroplacental perfusion is still inadequate. 

What Happens to the Fetus? 

The fetal response closely reflected maternal instability. 

Observed fetal changes included^1,3: 

• Progressive fetal bradycardia 

• Reduced fetal activity 

• Correlation with maternal acidosis and elevated PaCO2 

• Evidence of transient fetal distress 

The study found that as maternal heart rate and PaCO2 increased, fetal heart rate decreased significantly. 

This reinforces a critical practical point: 

Fetal compromise during anesthesia is often secondary to poor maternal ventilation and perfusion—not simply anesthetic drug exposure. 

Practical Anesthetic Pearls for Equine Practitioners 

Minimize anesthesia duration 

Prolonged inhalation anesthesia increases respiratory and cardiovascular compromise. 

Avoid severe hypotension 

Maintaining adequate mean arterial pressure is essential for uterine perfusion. Dobutamine support may be necessary throughout anesthesia. 

Aggressively monitor ventilation 

Pay close attention to: 

• PaCO2 

• PaO2 

• Blood pH 

• End-tidal CO2 

Normocapnia should be a priority¹. 

Reconsider positioning when possible 

Dorsal recumbency substantially worsens venous compression and pulmonary compromise in advanced pregnancy. 

Monitor the fetus whenever feasible 

Serial fetal heart rate assessment can provide early warning of fetal distress. 

The Good News: Fetal Distress May Be Reversible 

Despite significant trans-anesthetic fetal compromise, all foals in the study were ultimately born at term without complications. 

Fetal heart rates normalized after maternal recovery, suggesting that rapid correction of maternal hypoxemia, hypercarbia, and hypotension can reverse fetal distress if addressed early. 

Take-Home Message 

Late-term pregnant mares undergoing inhalation anesthesia represent a true cardiopulmonary balancing act. The anesthetic challenge is not only keeping the mare stable—but maintaining adequate uteroplacental oxygen delivery throughout the procedure. 

For equine surgeons and anesthetists, the major warning signs are: 

• Rising PaCO2 

• Falling pH 

• Persistent hypotension 

• Progressive fetal bradycardia 

Careful monitoring, shorter anesthetic times, aggressive cardiovascular support, and optimized ventilation remain the keys to improving both mare and foal outcomes. 

References 

1. Brito PH, Ferreira MA, Rusch E, Arantes JD, Carregaro AB, Valadão CA, Ghantous GF, Dória RG. Anesthesia for non-obstetric surgery during late term pregnancy in mares. Plos one. 2024 Nov 22;19(11):e0313563. https://doi.org/10.1371/journal.pone.0313563  

2. Upadya M, Saneesh PJ. Anaesthesia for non-obstetric surgery during pregnancy. Indian Journal of Anaesthesia. 2016, 60:234–41. https://doi.org/10.4103/0019-5049.179445  

3. Hubbell J.A.E. and Muir W.W. Monitoring Anesthesia. In: Muir W.W. and Hubbell J.A., Eds., Equine Anesthesia Monitoring and Emergency Therapy, Saunders Elsevier, St. Louis. 2009. p.149–170. https://doi.org/10.1016/b978-1-4160-2326-5.00008–0  

4. Taylor PM, Clarke KW. Anaesthesia in Special Situations. In: Taylor PM, Clarke KW. Eds., Handbook of equine anaesthesia. Saunders, Edinburgh. 2007, p. 177–207. https://doi.org/10.1016/B978-0-7020- 2835-9.50012–0