Meeting on Antepartum And Intrapartum Management

SAN FRANCISCO – A threshold of 6-cm cervical dilation is more accurate than the conventional 4 cm to determine when a woman enters the active phase of labor, a reevaluation of evidence suggests.

The historical evidence behind the commonly used assumption that 4-cm dilation signals the start of active labor contains methodological flaws, doesn’t match today’s population of pregnant women, and is contradicted by more recent studies supporting the 6-cm threshold, Tekoa King, C.N.M, Ph.D., said at a meeting on antepartum and intrapartum management sponsored by the University of California, San Francisco.

Using the 4-cm threshold, a woman "may just be in the normal latent phase of labor," she said.

Switching to the 6-cm threshold should delay or reduce the use of epidural anesthesia and might lower the high rate of cesarean sections in the United States. "Six centimeters is the new four centimeters," said Dr. King, a certified nurse-midwife and clinical professor of nursing at the university.

The 4-cm threshold for active labor arose out of two studies in the 1950s by Friedman et al. One study plotted the course of labor in centimeters of dilation over time in 500 nulliparous pregnant women. The same investigators profiled 200 women who were considered in the 1950s to have "ideal labor" – meaning term, vertex, singleton pregnancies – but the women commonly received considerable amounts of morphine and may or may not have had instrumented deliveries using outlet forceps. "They wouldn’t be what we would consider ideal today," Dr. King said.

Comparing the two cohorts, investigators in the 1950s found that the "ideal" group had shorter labors. They concluded that women who had been contracting more than 24 hours had a prolonged latent phase, and that the slowest rate of dilation in the active phase of labor was 1.2 cm/hr. Thus was born the "Friedman curve" that underpinned the decades-long dogma that women need to dilate about 1 cm/hr in the active phase of labor.

More recently, other data show that "the Friedman curve was really codified by the way we examined women every 2 hours. If you examine them more frequently, you’re going to get a different curve. Probably what we should be doing is examining them a lot less frequently until they’re 6 cm, and perhaps a little more frequently from 6 cm to complete" dilation, depending on whether or not there are other indications for examination, she said.

A 2002 study by Zhang et al. of 1,329 nulliparous, term, singleton, vertex pregnancies with normal-weight babies, spontaneous onset of labor, and vaginal delivery used a different approach from the Friedman studies. It measured the time between each centimeter change in dilation instead of the time it took to go from 4-cm to 10-cm dilation and then calculating an interval average. The newer study found that it was common to have very slow progress before 7 cm, there was no deceleration phase, and the slowest but still normal rate of cervical dilation was less than 1 cm/hr, with a wide range of variability (Am. J. Obstet. Gynecol. 2002;187:824-8).

To go from 6-cm to 7-cm dilation, for example, took little more than half an hour on average, but it ranged from 0.2 hours to more than 2 hours. "This makes sense," Dr. King said. "What’s really happening between 6 and 7 or 7 and 8 cm? Internal rotation. We often forget that internal rotation and descent station are progress. We just get ourselves fixated on cervical dilation" and end up performing a cesarean section in women for "arrested labor" at a time when they’re having normal progress.

The same investigators followed that with a 2010 study analyzing data on more than 50,000 singleton, vertex pregnancies with spontaneous onset of labor, vaginal delivery and "normal outcome." Again, they used the "repeated measures" approach to estimate the labor curves and to "redefine normal," Dr. King said. They found that the median rate of change was about 2 cm/hr and the slowest rate of normal change was 0.4 cm/hr (Obstet. Gynecol. 2010;116:1281-7).

On Friedman’s curve, what he called the point of inflection (or the beginning of the active phase of labor) was at 4-cm cervical dilation. But Zhang’s curve suggests that the point of inflection when labor starts to progress faster is at 6-cm dilation. "That’s our real world today, and here we are, using 4 cm. Basically, we’re treating women who are in the latent phase of labor as though they were active," she said.

The other insight from the work of Zhang et al. is that dilation progresses faster as the cervix becomes more dilated, not at a steady rate of 1 cm/hr.

Zhang et al. also reported that 40% of women who undergo induction of labor get a cesarean section when they are 4 cm dilated. "That’s probably where we really need to start paying some attention to what we’re doing," Dr. King said. Too many cesarean sections are being done at cervical dilations of 6 cm or less, she said.

Other recent data show that women with induced labor need significantly more time to reach 6-cm dilation compared with women with spontaneous labor, but after 6 cm the rate of progression is similar (Obstet. Gynecol. 2012;119:1113-8).

Greater patience with induced labor could reduce the rate of cesarean sections. "These are the women who are getting sectioned," she said.

A separate study suggests that women trying for a vaginal delivery after a prior cesarean section should be assessed by the same progression curves as women without a prior cesarean section (Obstet. Gynecol. 2012;119:732-6).

Clinical variables also have changed since the 1950s in ways that affect the progress of labor. Pregnant women in the United States today are more likely to be obese. A high body mass index prolongs the time it takes for cervical dilation in labor. "We may need a whole new labor curve for these women," Dr. King said.

She reported having no financial disclosures.

[email protected]

On Twitter @sherryboschert

SAN FRANCISCO – A threshold of 6-cm cervical dilation is more accurate than the conventional 4 cm to determine when a woman enters the active phase of labor, a reevaluation of evidence suggests.

The historical evidence behind the commonly used assumption that 4-cm dilation signals the start of active labor contains methodological flaws, doesn’t match today’s population of pregnant women, and is contradicted by more recent studies supporting the 6-cm threshold, Tekoa King, C.N.M, Ph.D., said at a meeting on antepartum and intrapartum management sponsored by the University of California, San Francisco.

Using the 4-cm threshold, a woman "may just be in the normal latent phase of labor," she said.

Switching to the 6-cm threshold should delay or reduce the use of epidural anesthesia and might lower the high rate of cesarean sections in the United States. "Six centimeters is the new four centimeters," said Dr. King, a certified nurse-midwife and clinical professor of nursing at the university.

The 4-cm threshold for active labor arose out of two studies in the 1950s by Friedman et al. One study plotted the course of labor in centimeters of dilation over time in 500 nulliparous pregnant women. The same investigators profiled 200 women who were considered in the 1950s to have "ideal labor" – meaning term, vertex, singleton pregnancies – but the women commonly received considerable amounts of morphine and may or may not have had instrumented deliveries using outlet forceps. "They wouldn’t be what we would consider ideal today," Dr. King said.

Comparing the two cohorts, investigators in the 1950s found that the "ideal" group had shorter labors. They concluded that women who had been contracting more than 24 hours had a prolonged latent phase, and that the slowest rate of dilation in the active phase of labor was 1.2 cm/hr. Thus was born the "Friedman curve" that underpinned the decades-long dogma that women need to dilate about 1 cm/hr in the active phase of labor.

More recently, other data show that "the Friedman curve was really codified by the way we examined women every 2 hours. If you examine them more frequently, you’re going to get a different curve. Probably what we should be doing is examining them a lot less frequently until they’re 6 cm, and perhaps a little more frequently from 6 cm to complete" dilation, depending on whether or not there are other indications for examination, she said.

A 2002 study by Zhang et al. of 1,329 nulliparous, term, singleton, vertex pregnancies with normal-weight babies, spontaneous onset of labor, and vaginal delivery used a different approach from the Friedman studies. It measured the time between each centimeter change in dilation instead of the time it took to go from 4-cm to 10-cm dilation and then calculating an interval average. The newer study found that it was common to have very slow progress before 7 cm, there was no deceleration phase, and the slowest but still normal rate of cervical dilation was less than 1 cm/hr, with a wide range of variability (Am. J. Obstet. Gynecol. 2002;187:824-8).

To go from 6-cm to 7-cm dilation, for example, took little more than half an hour on average, but it ranged from 0.2 hours to more than 2 hours. "This makes sense," Dr. King said. "What’s really happening between 6 and 7 or 7 and 8 cm? Internal rotation. We often forget that internal rotation and descent station are progress. We just get ourselves fixated on cervical dilation" and end up performing a cesarean section in women for "arrested labor" at a time when they’re having normal progress.

The same investigators followed that with a 2010 study analyzing data on more than 50,000 singleton, vertex pregnancies with spontaneous onset of labor, vaginal delivery and "normal outcome." Again, they used the "repeated measures" approach to estimate the labor curves and to "redefine normal," Dr. King said. They found that the median rate of change was about 2 cm/hr and the slowest rate of normal change was 0.4 cm/hr (Obstet. Gynecol. 2010;116:1281-7).

On Friedman’s curve, what he called the point of inflection (or the beginning of the active phase of labor) was at 4-cm cervical dilation. But Zhang’s curve suggests that the point of inflection when labor starts to progress faster is at 6-cm dilation. "That’s our real world today, and here we are, using 4 cm. Basically, we’re treating women who are in the latent phase of labor as though they were active," she said.

The other insight from the work of Zhang et al. is that dilation progresses faster as the cervix becomes more dilated, not at a steady rate of 1 cm/hr.

Zhang et al. also reported that 40% of women who undergo induction of labor get a cesarean section when they are 4 cm dilated. "That’s probably where we really need to start paying some attention to what we’re doing," Dr. King said. Too many cesarean sections are being done at cervical dilations of 6 cm or less, she said.

Other recent data show that women with induced labor need significantly more time to reach 6-cm dilation compared with women with spontaneous labor, but after 6 cm the rate of progression is similar (Obstet. Gynecol. 2012;119:1113-8).

Greater patience with induced labor could reduce the rate of cesarean sections. "These are the women who are getting sectioned," she said.

A separate study suggests that women trying for a vaginal delivery after a prior cesarean section should be assessed by the same progression curves as women without a prior cesarean section (Obstet. Gynecol. 2012;119:732-6).

Clinical variables also have changed since the 1950s in ways that affect the progress of labor. Pregnant women in the United States today are more likely to be obese. A high body mass index prolongs the time it takes for cervical dilation in labor. "We may need a whole new labor curve for these women," Dr. King said.

She reported having no financial disclosures.

[email protected]

On Twitter @sherryboschert

SAN FRANCISCO – A threshold of 6-cm cervical dilation is more accurate than the conventional 4 cm to determine when a woman enters the active phase of labor, a reevaluation of evidence suggests.

The historical evidence behind the commonly used assumption that 4-cm dilation signals the start of active labor contains methodological flaws, doesn’t match today’s population of pregnant women, and is contradicted by more recent studies supporting the 6-cm threshold, Tekoa King, C.N.M, Ph.D., said at a meeting on antepartum and intrapartum management sponsored by the University of California, San Francisco.

Using the 4-cm threshold, a woman "may just be in the normal latent phase of labor," she said.

Switching to the 6-cm threshold should delay or reduce the use of epidural anesthesia and might lower the high rate of cesarean sections in the United States. "Six centimeters is the new four centimeters," said Dr. King, a certified nurse-midwife and clinical professor of nursing at the university.

The 4-cm threshold for active labor arose out of two studies in the 1950s by Friedman et al. One study plotted the course of labor in centimeters of dilation over time in 500 nulliparous pregnant women. The same investigators profiled 200 women who were considered in the 1950s to have "ideal labor" – meaning term, vertex, singleton pregnancies – but the women commonly received considerable amounts of morphine and may or may not have had instrumented deliveries using outlet forceps. "They wouldn’t be what we would consider ideal today," Dr. King said.

Comparing the two cohorts, investigators in the 1950s found that the "ideal" group had shorter labors. They concluded that women who had been contracting more than 24 hours had a prolonged latent phase, and that the slowest rate of dilation in the active phase of labor was 1.2 cm/hr. Thus was born the "Friedman curve" that underpinned the decades-long dogma that women need to dilate about 1 cm/hr in the active phase of labor.

More recently, other data show that "the Friedman curve was really codified by the way we examined women every 2 hours. If you examine them more frequently, you’re going to get a different curve. Probably what we should be doing is examining them a lot less frequently until they’re 6 cm, and perhaps a little more frequently from 6 cm to complete" dilation, depending on whether or not there are other indications for examination, she said.

A 2002 study by Zhang et al. of 1,329 nulliparous, term, singleton, vertex pregnancies with normal-weight babies, spontaneous onset of labor, and vaginal delivery used a different approach from the Friedman studies. It measured the time between each centimeter change in dilation instead of the time it took to go from 4-cm to 10-cm dilation and then calculating an interval average. The newer study found that it was common to have very slow progress before 7 cm, there was no deceleration phase, and the slowest but still normal rate of cervical dilation was less than 1 cm/hr, with a wide range of variability (Am. J. Obstet. Gynecol. 2002;187:824-8).

To go from 6-cm to 7-cm dilation, for example, took little more than half an hour on average, but it ranged from 0.2 hours to more than 2 hours. "This makes sense," Dr. King said. "What’s really happening between 6 and 7 or 7 and 8 cm? Internal rotation. We often forget that internal rotation and descent station are progress. We just get ourselves fixated on cervical dilation" and end up performing a cesarean section in women for "arrested labor" at a time when they’re having normal progress.

The same investigators followed that with a 2010 study analyzing data on more than 50,000 singleton, vertex pregnancies with spontaneous onset of labor, vaginal delivery and "normal outcome." Again, they used the "repeated measures" approach to estimate the labor curves and to "redefine normal," Dr. King said. They found that the median rate of change was about 2 cm/hr and the slowest rate of normal change was 0.4 cm/hr (Obstet. Gynecol. 2010;116:1281-7).

On Friedman’s curve, what he called the point of inflection (or the beginning of the active phase of labor) was at 4-cm cervical dilation. But Zhang’s curve suggests that the point of inflection when labor starts to progress faster is at 6-cm dilation. "That’s our real world today, and here we are, using 4 cm. Basically, we’re treating women who are in the latent phase of labor as though they were active," she said.

The other insight from the work of Zhang et al. is that dilation progresses faster as the cervix becomes more dilated, not at a steady rate of 1 cm/hr.

Zhang et al. also reported that 40% of women who undergo induction of labor get a cesarean section when they are 4 cm dilated. "That’s probably where we really need to start paying some attention to what we’re doing," Dr. King said. Too many cesarean sections are being done at cervical dilations of 6 cm or less, she said.

Other recent data show that women with induced labor need significantly more time to reach 6-cm dilation compared with women with spontaneous labor, but after 6 cm the rate of progression is similar (Obstet. Gynecol. 2012;119:1113-8).

Greater patience with induced labor could reduce the rate of cesarean sections. "These are the women who are getting sectioned," she said.

A separate study suggests that women trying for a vaginal delivery after a prior cesarean section should be assessed by the same progression curves as women without a prior cesarean section (Obstet. Gynecol. 2012;119:732-6).

Clinical variables also have changed since the 1950s in ways that affect the progress of labor. Pregnant women in the United States today are more likely to be obese. A high body mass index prolongs the time it takes for cervical dilation in labor. "We may need a whole new labor curve for these women," Dr. King said.

She reported having no financial disclosures.

[email protected]

On Twitter @sherryboschert

SAN FRANCISCO – For much of the past decade, most pregnant women in the United States have not had access to nitrous oxide for analgesia during labor because the only company that sold a nitrous oxide machine for obstetrics in this country stopped making it.

This year, though, "laughing gas" for labor pain is back.

The Nitronox system delivers a fixed mixture of 50% oxygen and 50% nitrous oxide that is safe, effective, inexpensive, simple, and popular with many laboring women, said Judith T. Bishop, C.N.M., M.P.H. Physician supervision is not needed for its use, she added at a meeting on antepartum and intrapartum management sponsored by the University of California, San Francisco (UCSF).

Sherry Boschert/IMNG Medical Media

Other nitrous oxide systems commonly are used for labor analgesia in the United Kingdom, Canada, Australia, and Scandinavia, and are available in Japan and Israel, but the gas has never caught on extensively in the United States for obstetrics. "I’ve been doing kind of a road show for nitrous oxide for about 7 years now," said Ms. Bishop, professor of obstetrics and gynecology and reproductive sciences at the university. "Ironically, during the entire period that I’ve been enthusiastically sharing my 20 years of experience with nitrous oxide use at UCSF, the nitrous oxide equipment appropriate for use in labor has been unobtainable" in the United States.

Michael Civitello, a salesman for the company that makes the Nitronox system, said the equipment went out of production during changes involving corporate mergers, not for reasons related to the product itself. Parker Hannifin Corporation’s Porter Instrument Division decided to return Nitronox to the market when it realized it still had a sales niche and advocates such as Ms. Bishop built increased interest in its use, he said in an interview at the company’s booth at the meeting. The new system costs approximately $5,000.

Perhaps 20-30 more hospitals and birth centers are expected to be offering nitrous oxide for labor by the end of this year, predicted Ms. Bishop and Mr. Civitello.

Women in labor at UCSF have been offered nitrous oxide for more than 30 years with no break in service because the gas delivery systems were built into the hospital, and are being built into a new UCSF hospital that’s under construction. Ms. Bishop searched and was able to find only three other U.S. hospitals with the ability to offer nitrous oxide during labor: the University of Washington, Seattle ("although they had largely forgotten about it," she said); a hospital in Lewiston, Idaho; and Vanderbilt University in Nashville, Tenn., which got tired of waiting for a nitrous oxide machine to return to the market and bought two used machines on eBay in 2011, Ms. Bishop said.

Data from Vanderbilt from June 2011 to May 2013 show an epidural rate of 40% in its midwifery service, compared with approximately 85% in the rest of the university practice, she said. Twenty percent of women in labor initiated nitrous oxide, and approximately 45% of those converted to epidural analgesia.

Data from 5,987 term singleton pregnancies at UCSF during 2007-2011 show an epidural rate of 76%. Nitrous oxide analgesia was started in 14% of deliveries, 41% of which converted to epidural analgesia.

Those numbers do not include other uses for nitrous oxide on labor and delivery units, she added, including analgesia for women experiencing laceration repair, retained placenta, Foley balloon placement, vaginal exams, and blood draws or IV placement.

For labor, nitrous oxide is an adjunct for pain relief and is not meant to replace other analgesia alternatives, Ms. Bishop said. Its use may allow the woman to delay or avoid using narcotics or epidural anesthesia. Nitrous oxide may be especially useful for women who want an epidural but can’t have one because they arrived at the hospital too late, they have a contraindication such as low platelet levels, or an anesthesiologist is unavailable to administer an epidural.

Another good use of nitrous oxide is for teenage mothers who are "out of control and can’t handle a needle in the back" for epidural analgesia, added Tekoa L. King, C.N.M., M.P.H., also of UCSF.

"There’s an antianxiety effect as well as an analgesic effect," Ms. Bishop said.

Data suggest that about half of women find nitrous oxide to be effective analgesia, better than the satisfaction rate for opioids in labor. That’s "no surprise," because opioids are not very effective in labor, she said. "Bathtubs are rated much more highly than opioids."

Women who report being satisfied with nitrous oxide may not show a decrease in pain scores, she added. With nitrous oxide, they say, "It still hurts, but I don’t care."

Inhaling the gas typically provides some degree of pain relief in less than a minute, and the effect dissipates after another breath or two. Since the first study of its use in labor in 1880, nitrous oxide has proved to be safe, Ms. Bishop said. It does not build up in the mother or fetus, and does not seem to affect contractions, labor progression, or the ability to push. It can be used through the second stage of labor, and there’s no evidence that it affects newborns or breastfeeding.

"You can’t kill somebody with 50/50 nitrous oxide and oxygen," she said.

In the United States, the woman initiates and controls the gas flow through a mask, with the negative pressure from inhalation opening a demand valve that stops gas flow when inhalation ceases. Excess nitrous oxide is scavenged out by suction. It’s meant for intermittent, not continuous, use.

Dosimeter badges worn by obstetrics nurses at UCSF consistently show that staff exposure to nitrous oxide is less than 2 parts per million in an 8-hour period, far below the 25-ppm limit set by the National Institute for Occupational Safety and Health.

It’s important to counsel family members who are trying to be "helpful" that only the woman should hold the mask to her face so that she controls the gas flow. Not all women find it helpful, and some may experience dizziness, drowsiness, or nausea, although those effects usually occur with higher doses of nitrous oxide, not the 50/50 blend with oxygen, Ms. Bishop said.

Usually, the nitrous oxide is more effective if the woman breathes it just before a contraction starts instead of waiting for a contraction, but each woman will find what works for them.

Nitrous oxide use at UCSF increased by 50% after the university expanded the privileges of certified nurse-midwives in 2007 to include initiation of the gas mixture, instead of having to call an anesthesia resident. Now the university is moving toward a standing order allowing registered nurses to initiate nitrous oxide use, similar to a standing order for fentanyl initiation. "I think that’s going to be a huge improvement," Ms. Bishop said.

Ms. Bishop reported having no financial disclosures.

[email protected]

On Twitter @sherryboschert

SAN FRANCISCO – For much of the past decade, most pregnant women in the United States have not had access to nitrous oxide for analgesia during labor because the only company that sold a nitrous oxide machine for obstetrics in this country stopped making it.

This year, though, "laughing gas" for labor pain is back.

The Nitronox system delivers a fixed mixture of 50% oxygen and 50% nitrous oxide that is safe, effective, inexpensive, simple, and popular with many laboring women, said Judith T. Bishop, C.N.M., M.P.H. Physician supervision is not needed for its use, she added at a meeting on antepartum and intrapartum management sponsored by the University of California, San Francisco (UCSF).

Sherry Boschert/IMNG Medical Media

Other nitrous oxide systems commonly are used for labor analgesia in the United Kingdom, Canada, Australia, and Scandinavia, and are available in Japan and Israel, but the gas has never caught on extensively in the United States for obstetrics. "I’ve been doing kind of a road show for nitrous oxide for about 7 years now," said Ms. Bishop, professor of obstetrics and gynecology and reproductive sciences at the university. "Ironically, during the entire period that I’ve been enthusiastically sharing my 20 years of experience with nitrous oxide use at UCSF, the nitrous oxide equipment appropriate for use in labor has been unobtainable" in the United States.

Michael Civitello, a salesman for the company that makes the Nitronox system, said the equipment went out of production during changes involving corporate mergers, not for reasons related to the product itself. Parker Hannifin Corporation’s Porter Instrument Division decided to return Nitronox to the market when it realized it still had a sales niche and advocates such as Ms. Bishop built increased interest in its use, he said in an interview at the company’s booth at the meeting. The new system costs approximately $5,000.

Perhaps 20-30 more hospitals and birth centers are expected to be offering nitrous oxide for labor by the end of this year, predicted Ms. Bishop and Mr. Civitello.

Women in labor at UCSF have been offered nitrous oxide for more than 30 years with no break in service because the gas delivery systems were built into the hospital, and are being built into a new UCSF hospital that’s under construction. Ms. Bishop searched and was able to find only three other U.S. hospitals with the ability to offer nitrous oxide during labor: the University of Washington, Seattle ("although they had largely forgotten about it," she said); a hospital in Lewiston, Idaho; and Vanderbilt University in Nashville, Tenn., which got tired of waiting for a nitrous oxide machine to return to the market and bought two used machines on eBay in 2011, Ms. Bishop said.

Data from Vanderbilt from June 2011 to May 2013 show an epidural rate of 40% in its midwifery service, compared with approximately 85% in the rest of the university practice, she said. Twenty percent of women in labor initiated nitrous oxide, and approximately 45% of those converted to epidural analgesia.

Data from 5,987 term singleton pregnancies at UCSF during 2007-2011 show an epidural rate of 76%. Nitrous oxide analgesia was started in 14% of deliveries, 41% of which converted to epidural analgesia.

Those numbers do not include other uses for nitrous oxide on labor and delivery units, she added, including analgesia for women experiencing laceration repair, retained placenta, Foley balloon placement, vaginal exams, and blood draws or IV placement.

For labor, nitrous oxide is an adjunct for pain relief and is not meant to replace other analgesia alternatives, Ms. Bishop said. Its use may allow the woman to delay or avoid using narcotics or epidural anesthesia. Nitrous oxide may be especially useful for women who want an epidural but can’t have one because they arrived at the hospital too late, they have a contraindication such as low platelet levels, or an anesthesiologist is unavailable to administer an epidural.

Another good use of nitrous oxide is for teenage mothers who are "out of control and can’t handle a needle in the back" for epidural analgesia, added Tekoa L. King, C.N.M., M.P.H., also of UCSF.

"There’s an antianxiety effect as well as an analgesic effect," Ms. Bishop said.

Data suggest that about half of women find nitrous oxide to be effective analgesia, better than the satisfaction rate for opioids in labor. That’s "no surprise," because opioids are not very effective in labor, she said. "Bathtubs are rated much more highly than opioids."

Women who report being satisfied with nitrous oxide may not show a decrease in pain scores, she added. With nitrous oxide, they say, "It still hurts, but I don’t care."

Inhaling the gas typically provides some degree of pain relief in less than a minute, and the effect dissipates after another breath or two. Since the first study of its use in labor in 1880, nitrous oxide has proved to be safe, Ms. Bishop said. It does not build up in the mother or fetus, and does not seem to affect contractions, labor progression, or the ability to push. It can be used through the second stage of labor, and there’s no evidence that it affects newborns or breastfeeding.

"You can’t kill somebody with 50/50 nitrous oxide and oxygen," she said.

In the United States, the woman initiates and controls the gas flow through a mask, with the negative pressure from inhalation opening a demand valve that stops gas flow when inhalation ceases. Excess nitrous oxide is scavenged out by suction. It’s meant for intermittent, not continuous, use.

Dosimeter badges worn by obstetrics nurses at UCSF consistently show that staff exposure to nitrous oxide is less than 2 parts per million in an 8-hour period, far below the 25-ppm limit set by the National Institute for Occupational Safety and Health.

It’s important to counsel family members who are trying to be "helpful" that only the woman should hold the mask to her face so that she controls the gas flow. Not all women find it helpful, and some may experience dizziness, drowsiness, or nausea, although those effects usually occur with higher doses of nitrous oxide, not the 50/50 blend with oxygen, Ms. Bishop said.

Usually, the nitrous oxide is more effective if the woman breathes it just before a contraction starts instead of waiting for a contraction, but each woman will find what works for them.

Nitrous oxide use at UCSF increased by 50% after the university expanded the privileges of certified nurse-midwives in 2007 to include initiation of the gas mixture, instead of having to call an anesthesia resident. Now the university is moving toward a standing order allowing registered nurses to initiate nitrous oxide use, similar to a standing order for fentanyl initiation. "I think that’s going to be a huge improvement," Ms. Bishop said.

Ms. Bishop reported having no financial disclosures.

[email protected]

On Twitter @sherryboschert

SAN FRANCISCO – For much of the past decade, most pregnant women in the United States have not had access to nitrous oxide for analgesia during labor because the only company that sold a nitrous oxide machine for obstetrics in this country stopped making it.

This year, though, "laughing gas" for labor pain is back.

The Nitronox system delivers a fixed mixture of 50% oxygen and 50% nitrous oxide that is safe, effective, inexpensive, simple, and popular with many laboring women, said Judith T. Bishop, C.N.M., M.P.H. Physician supervision is not needed for its use, she added at a meeting on antepartum and intrapartum management sponsored by the University of California, San Francisco (UCSF).

Sherry Boschert/IMNG Medical Media

Other nitrous oxide systems commonly are used for labor analgesia in the United Kingdom, Canada, Australia, and Scandinavia, and are available in Japan and Israel, but the gas has never caught on extensively in the United States for obstetrics. "I’ve been doing kind of a road show for nitrous oxide for about 7 years now," said Ms. Bishop, professor of obstetrics and gynecology and reproductive sciences at the university. "Ironically, during the entire period that I’ve been enthusiastically sharing my 20 years of experience with nitrous oxide use at UCSF, the nitrous oxide equipment appropriate for use in labor has been unobtainable" in the United States.

Michael Civitello, a salesman for the company that makes the Nitronox system, said the equipment went out of production during changes involving corporate mergers, not for reasons related to the product itself. Parker Hannifin Corporation’s Porter Instrument Division decided to return Nitronox to the market when it realized it still had a sales niche and advocates such as Ms. Bishop built increased interest in its use, he said in an interview at the company’s booth at the meeting. The new system costs approximately $5,000.

Perhaps 20-30 more hospitals and birth centers are expected to be offering nitrous oxide for labor by the end of this year, predicted Ms. Bishop and Mr. Civitello.

Women in labor at UCSF have been offered nitrous oxide for more than 30 years with no break in service because the gas delivery systems were built into the hospital, and are being built into a new UCSF hospital that’s under construction. Ms. Bishop searched and was able to find only three other U.S. hospitals with the ability to offer nitrous oxide during labor: the University of Washington, Seattle ("although they had largely forgotten about it," she said); a hospital in Lewiston, Idaho; and Vanderbilt University in Nashville, Tenn., which got tired of waiting for a nitrous oxide machine to return to the market and bought two used machines on eBay in 2011, Ms. Bishop said.

Data from Vanderbilt from June 2011 to May 2013 show an epidural rate of 40% in its midwifery service, compared with approximately 85% in the rest of the university practice, she said. Twenty percent of women in labor initiated nitrous oxide, and approximately 45% of those converted to epidural analgesia.

Data from 5,987 term singleton pregnancies at UCSF during 2007-2011 show an epidural rate of 76%. Nitrous oxide analgesia was started in 14% of deliveries, 41% of which converted to epidural analgesia.

Those numbers do not include other uses for nitrous oxide on labor and delivery units, she added, including analgesia for women experiencing laceration repair, retained placenta, Foley balloon placement, vaginal exams, and blood draws or IV placement.

For labor, nitrous oxide is an adjunct for pain relief and is not meant to replace other analgesia alternatives, Ms. Bishop said. Its use may allow the woman to delay or avoid using narcotics or epidural anesthesia. Nitrous oxide may be especially useful for women who want an epidural but can’t have one because they arrived at the hospital too late, they have a contraindication such as low platelet levels, or an anesthesiologist is unavailable to administer an epidural.

Another good use of nitrous oxide is for teenage mothers who are "out of control and can’t handle a needle in the back" for epidural analgesia, added Tekoa L. King, C.N.M., M.P.H., also of UCSF.

"There’s an antianxiety effect as well as an analgesic effect," Ms. Bishop said.

Data suggest that about half of women find nitrous oxide to be effective analgesia, better than the satisfaction rate for opioids in labor. That’s "no surprise," because opioids are not very effective in labor, she said. "Bathtubs are rated much more highly than opioids."

Women who report being satisfied with nitrous oxide may not show a decrease in pain scores, she added. With nitrous oxide, they say, "It still hurts, but I don’t care."

Inhaling the gas typically provides some degree of pain relief in less than a minute, and the effect dissipates after another breath or two. Since the first study of its use in labor in 1880, nitrous oxide has proved to be safe, Ms. Bishop said. It does not build up in the mother or fetus, and does not seem to affect contractions, labor progression, or the ability to push. It can be used through the second stage of labor, and there’s no evidence that it affects newborns or breastfeeding.

"You can’t kill somebody with 50/50 nitrous oxide and oxygen," she said.

In the United States, the woman initiates and controls the gas flow through a mask, with the negative pressure from inhalation opening a demand valve that stops gas flow when inhalation ceases. Excess nitrous oxide is scavenged out by suction. It’s meant for intermittent, not continuous, use.

Dosimeter badges worn by obstetrics nurses at UCSF consistently show that staff exposure to nitrous oxide is less than 2 parts per million in an 8-hour period, far below the 25-ppm limit set by the National Institute for Occupational Safety and Health.

It’s important to counsel family members who are trying to be "helpful" that only the woman should hold the mask to her face so that she controls the gas flow. Not all women find it helpful, and some may experience dizziness, drowsiness, or nausea, although those effects usually occur with higher doses of nitrous oxide, not the 50/50 blend with oxygen, Ms. Bishop said.

Usually, the nitrous oxide is more effective if the woman breathes it just before a contraction starts instead of waiting for a contraction, but each woman will find what works for them.

Nitrous oxide use at UCSF increased by 50% after the university expanded the privileges of certified nurse-midwives in 2007 to include initiation of the gas mixture, instead of having to call an anesthesia resident. Now the university is moving toward a standing order allowing registered nurses to initiate nitrous oxide use, similar to a standing order for fentanyl initiation. "I think that’s going to be a huge improvement," Ms. Bishop said.

Ms. Bishop reported having no financial disclosures.

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