DALLAS -- The more obese a pregnant woman is at the time of delivery, the more likely she will deliver her baby by Cesarean section, according to a study by obstetric anesthesiologists at Duke University Medical Center.
Because of this, the researchers recommend that all obese women who come to a hospital in labor should consider having an epidural analgesia line placed in order to minimize the potential risks of general anesthesia if a C-section becomes necessary. An epidural involves the placement of a thin catheter into the epidural space near the spinal column, which allows physicians to introduce anesthetic agents that can be used to block the pain of vaginal delivery or provide anesthesia for a C-section. In a general anesthetic, the patient is unconscious; when an epidural is used, the patient is awake but cannot feel pain in the lower half of the body.
Anesthesiologists in most cases don't see expectant mothers until just a few hours before delivery, but the type of anesthesia used can directly affect the health of mother and baby. This is especially true of obese women, because physicians encounter more difficulty in successfully administering emergency general anesthesia to overweight patients.
Duke obstetric anesthesiologist Dr. Elizabeth Bell prepared the results of her study for presentation Monday (Oct. 12) at the annual meeting of the American Society of Anesthesiologists.
Bell reviewed the medical records of 2,493 mothers who gave birth at Duke University Hospital and whose height and weight was taken over a 16-month period, and found that 833, or one third, received a C-section. Then she calculated each patient's body mass index (BMI), which is considered the most reliable measurement of obesity, for all the mothers.
"We found that the patients who had C-sections had an average BMI of 53, significantly higher than the average BMI of 31 for those delivering vaginally," Bell said. "Just as importantly, the more obese a patient was at the time of delivery, the more likely she was to have a C-section."
The study was funded by Duke's Division of Women's Anesthesia, which maintains a comprehensive database of all obstetric anesthesic cases at Duke.
"Since we as anesthesiologists are rarely involved in the care of mothers during the course of their pregnancy, we need to be able to make the appropriate anesthetic decision, usually at the last minute," Bell said. "Very large people can be difficult to put under general anesthesia."
When administering general anesthesia, physicians must insert a tube through the voice box into the windpipe which will maintain an open breathing passage for the patient while she is unconscious. To accomplish this, physicians must first administer drugs which in effect "paralyze" patients so they will accept the tube without gagging.
"All pregnant women experience water gains in mucosal tissues and tongue, which makes them swell and prone to bleeding," she explained. "In very large women this problem is amplified, so it can be difficult to place the tube in the right place."
If it then turns out that physicians are unable to place the tube, the patients are still under the influence of the drugs and are unable to take care of themselves until the drugs wear off. The timing is especially critical if the baby is in distress, Bell said.
"Every minute that passes that the baby is not delivered can lead to potential health problems for the baby," she said.
This is complicated by the fact that since the baby pushes the diaphragm upward as it grows, pregnant women are more prone to acid reflux from the stomach, which can come up the throat and down into the lungs. This effect is also amplified in the obese. In terms of actual pounds, the average weight of women in the study who had a C-section was 191 pounds, the average of vaginal delivery was 182.
"Weight alone isn't the best way to compare people, since obviously a tall person may weigh more than a short person without being obese," Bell said. "The BMI corrects for the three-dimensional nature of weight, and it is simple, non-invasive and inexpensive."
BMI, because it takes into account the height of a person, is considered a more accurate measure of obesity, Bell said. It is calculated by dividing body weight by the square of the height. In the general population, a BMI of 30 is considered obese and 35 is considered morbidly obese. A corresponding definition of obesity for pregnant women has not been determined, Bell said. As an example, a patient who is 5'4" weighing 175 pounds would have a BMI of 30.
Joining Bell in the study were Dr. A.J. Hartle and Dr. Peter Dwane, both at Duke.