Women given spinal anesthetic before delivering by cesarean section may become hypotensive.
According to Ahmed F. Attaallah, MD, assistant professor of anesthesiology at West Virginia University in Morgantown, West Virginia, “Any time spinal anesthesia is given, you are blocking the nerves to the lower half of the body, including the sensory and motor nerves. In addition, autonomic nerves also become blocked, which are involved in the tone of the blood vessels. By blocking these nerves, blood vessels become dilated and blood pressure drops, resulting in hypotension.”
It is important to maintain blood pressure during spinal anesthetic because it can compromise blood supply to the baby and lead to complications in the mother, including nausea and vomiting.
“Maintaining the systemic blood pressure within the normal range is essential for maintaining the perfusion of these vital organs: the brain, heart, and kidneys. In the case of the pregnant woman, maintaining the perfusion to the placenta is important as well [because] if the placental blood supply is reduced, the baby’s perfusion is affected leading to poor outcomes of the newborn,” Attaallah said in an interview.
Phenylephrine Standard of Care for Treating Hypotension
According to Manuel C. Vallejo, MD, professor of anesthesiology at West Virginia University in Morgantown, phenylephrine is the gold standard of care for increasing blood pressure in women having spinal anesthetic during cesarean section because it has been proven effective and safe.
However, research efforts have indicated that another drug, norepinephrine, may be associated with improved cardiac output and other benefits.
“Over recent years, to a large part due to work done by my unit, phenylephrine has become the preferred drug for maintaining blood pressure. However, this drug has a tendency to cause a reflex slowing of maternal heart rate, which is associated with a decrease in the amount of blood the heart pumps to the body (cardiac output). Norepinephrine has less tendency to cause the latter because it has mild heart-stimulating effects (beta effects),” said Warwick D. Ngan Kee, MD, of the department of anesthesia and intensive care at the Chinese University of Hong Kong, Prince of Wales Hospital in Shatin, Hong Kong.
Norepinephrine May Improve Cardiac Output With Fewer Adverse Effects
In a recent study, Ngan Kee and colleagues found that both phenylephrine and norepinephrine were effective for maintaining blood pressure, but using norepinephrine resulted in the heart rate being kept closer to the baseline value. Cardiac output was also greater as compared with phenylephrine.1
“The implication of this is that normal physiology seems better preserved with norepinephrine versus phenylephrine, so potentially it is a better drug to use,” said Ngan Kee.
The investigators also found that the oxygen level measured in umbilical venous blood was higher with norepinephrine, which possibly might reflect greater blood flow to the uterus. However, this was a secondary outcome measure and requires further evaluation. “Ours is a … preliminary study, so the results need to be confirmed by further research,” said Ngan Kee. “Personally, I am actually now using norepinephrine for all my cases, but to make a general recommendation to others requires more research confirmation.”
Currently, Vallejo and Attaallah are conducting a study evaluating the safety and effectiveness of phenylephrine compared to norepinephrine.2
“We are planning on enrolling a total of 80 patients, and to date we have enrolled 61 patients. Our current results will be shown in an abstract that will be presented at the Society of Obstetric Anesthesia and Perinatology [annual] meeting in May in Colorado Springs,” said Vallejo.
The key hemodynamic parameters being evaluated include cardiac output, cardiac index, stroke volume, and systemic vascular resistance.
“Early findings indicate that hemodynamic parameters are comparable with both drugs. However, we found a lower incidence of nausea and vomiting with norepinephrine which was statistically significant. These are our preliminary findings of the study, and we will continue to do further assessment to determine whether this holds up in a larger sample of patients,” said Vallejo.
According to Ngan Kee, it is very important to maintain blood pressure during spinal anesthetic for cesarean section because the baby’s welfare greatly depends on it.
Sustained low blood pressure can be very detrimental to the baby. Although research has shown that using phenylephrine benefits both the mother and baby, norepinephrine may provide a more acceptable alternative because it may have a lower risk of a drop in heart rate as well as a lower risk of nausea and vomiting. However, further research efforts are needed to explore these potential benefits.
Medically reviewed by Pat F. Bass III, MD, MS, MPH
1. Ngan Kee, W, et al. Anesthesiology. 2015; 122 (4): 736-745.
2. ClinicalTrials.gov. Available at: https://www.clinicaltrials.gov/ct2/show/NCT02354833.