What Are Optimal Cesarean Section Rates in the U.S. and How Do We Get There? A Review of Evidence-Based Recommendations and Interventions

Abstract

Cesarean sections (CSs) are the most commonly performed surgical procedures in the world today. Global epidemiological studies from the last decade suggest that the optimal CS rates in developed countries exist somewhere between 15% and 19%. Despite these findings, CS rates in the United States have remained stable at slightly over 32% over the past 10 years. Using primary and secondary literature published from 2010 to 2015, this review discusses how optimal CS rates were developed. In addition, we define a category of potentially avoidable CS (i.e., those conducted on nulliparous low-risk women who present with vertex infants at term) and explore how CS in this population appear to be one of the main drivers of high CS rates overall. The institutional, provider, and patient-related factors, which may be related to higher-than-recommended rates of CS, particularly those conducted in low-risk women, will be discussed. This review will then delve into clinician and patient-oriented interventions that have been shown to effectively reduce the rate of potentially avoidable CS. Our analysis showed that large-scale, multifaceted interventions that include audit and feedback cycles as well as peer review strategies were the most effective in decreasing rates of potentially avoidable CS. This review concludes with an agenda for future research into interventions that aim to achieve optimal CS rates.

Introduction

Cesarean sections (CSs) were first attempted to save an infant from a dying or dead mother or to extract a dead infant.¹ During the 19th century with the discovery of anesthesia and aseptic surgical technique, CS became a procedure with the potential to also preserve the mother's life.¹ Nevertheless, maternal mortality remained high at the turn of the 20th century with reported rates ranging from 8% to 34%.² It is only in the past century that we have seen the rapid decline of maternal mortality, particularly in the United States, to the current reported rates of about 0.01 to 0.02/1000 total deliveries.² With improvements in both maternal and neonatal outcomes, CS has emerged as the most commonly performed surgical procedure in the world.³

During the early 1980s, CS became safer and more routine, which drove medical experts to define an “appropriate” CS rate that balanced the morbidity and mortality associated with the surgical procedure with the morbidity and mortality benefits to the mother and infant. In 1985, the Lancet published a landmark report by the World Health Organization (WHO) stating there was “no justification for any region in the world to have a CS rate higher than 10%–15%” using data showing countries with the lowest perinatal mortality rates also had CS rates of less than 10%.⁴ Although the estimates derived from this report were based on scarce international data at the time, the 1985 WHO Lancet recommendation of a 10%–15% CS rate is often quoted as the optimal global rate of CS.⁵ This review focuses on recently published data over the last decade exploring the question of the optimal CS rate as well as the potential reasons for the persistence of elevated rates despite this literature. In addition, published interventions aimed at reducing the rate of potentially avoidable CS will be reviewed, with a focus on those that have proven to be most successful.

The Optimal Cesarean Delivery Rate and Its Barriers

Worldwide ecological evidence for an optimal rate

Observational studies conducted in developing countries around the world since 1985 indicate that recommending a 15% CS rate might still represent too high a goal, with some finding evidence that the CS rate needed to avoid maternal death and severe morbidity lay somewhere between 1% and 5% and others showing an association between decreased neonatal mortality and CS rates closer to 10%.^5–7 In 2014, Ye et al. looked at 19 high-income countries in Europe, North America, Japan, Australia, and New Zealand with high access to CS and well-recorded rates of maternal and infant mortality.⁸ After adjusting for life expectancy, education, income per capita, and gross domestic product, Ye et al. noted no significant impact on maternal and infant mortality rates past a CS rate of 10%–15%.⁸

In December of 2015, Molina et al. used country-level CS rates for 178 member countries between 2005 and 2012 and found that maternal mortality was inversely associated with CS rates, up to 7.2 CS per 100 live births or less (adjusted slope coefficient, −68.1; 95% CI, −89.2 to −46.9; p < 0.001).⁹ Importantly, the association between CS rate and maternal mortality was attenuated when the CS rates were between 7.2 and 19.1 per 100 live births. Collectively, these data demonstrated that fewer mothers died when CS rates were between 7.2 and 19.1; however, this effect was significantly reduced after adjusting for covariates when the CS rates rose past 7%. With respect to neonatal mortality, Molina et al. reported that neonatal mortality was lower for countries with increasing CS rate, up to 19.4 CSs per 100 live births (adjusted slope coefficient, −0.8; 95% CI, −1.1 to −0.5, p < 0.001). The composite of this international data thus indicates that better resourced countries may achieve optimal maternal and neonatal mortality and morbidity with CS rates that are lower (i.e., on the order of 10%–15%). However, when studies include low-resource countries all over the world, it appears that higher rates up to 19% may be needed to capture optimal perinatal outcomes. Taken together, we feel these data indicate that CS rates of between 15% and 19% should be expected to optimize both maternal and neonatal mortality in a broad variety of socioeconomic settings around the world. For some low-income countries, where CS rates are less than 1% (such as Chad and Burkina Faso),¹⁰ this may mean raising their CS rates by improving access to obstetric providers and life-saving CS. However, for a high-income resourced country such as the United States, whose CS rates far exceed even the recommended upper limit of 19%, the need to identify drivers of high CS rates becomes paramount.

Which CSs are “potentially avoidable?”

In 2010, a panel of WHO experts estimated that one-third of the 18.5 million CSs performed annually were conducted for nonmedical indications and concluded that CSs performed in these situations should be deemed “unnecessary.”¹⁰ In the United States, the concept of “unnecessary CS” or “potentially avoidable CS” has been reported among women of all ages, races, ethnicities, in every state and across all gestational ages.¹¹ Despite these observations, defining “unnecessary CS” or “potentially avoidable CS” remains a challenge and controversy.^10,11 Accumulating data indicates that current high rates of CS are in large part due to rising numbers of planned elective or nonmedically indicated CS, and as such these are the types of CS that should be labeled “unnecessary” or “potentially avoidable,” terms which are sometimes used interchangeably in the literature.^12–14 Specifically, some have noted that primary CS conducted in low-risk women (i.e., nulliparous women who have term, singleton, vertex infants) account for 50% of the total increase in overall CS rates since 2003.¹⁵ In the United States in 2009, for instance, the overall CS rates peaked at 32.9% and primary low-risk CS rates were 28%.¹⁶ Recent data from the 2014 Center for Disease Control National Vital Statistics Birth Reports indicate the lowest overall CS rates since 2007 (32.2%) with primary low-risk CS declining to 26% of all births.¹⁷ Nevertheless, primary CS in low-risk women account for ∼60% of all CS deliveries in the United States, emphasizing the need to explore both the etiology for such high rates of CS in low-risk women and potential interventions to decrease CS in this low-risk population.^16,17

Some may argue that potentially avoidable CS should also include repeat CS (i.e., CS conducted on women solely due to the fact that they have a history of a previous CS) and CS conducted for breech presentation (i.e., when the infant presents with the feet or buttocks first in the vaginal canal as opposed to the head).^11,18–20 In the United States, only 10% of women with a previous CS will deliver vaginally for their second child, a statistic which highlights the ways in which low-risk women who receive primary CS then increase the count of women later receiving repeat CS.^15,17 An assessment of whether CS conducted for these two particular indications are either potentially avoidable or medically indicated requires a critical review of the outcomes associated with these two indications, which is out of the scope of this review. Thus for the purposes of this review, “potentially avoidable,” “elective” or “unnecessary” CS will refer to those that are conducted on low-risk women as defined above.

Factors Potentially Driving Avoidable Cesareans

Our review indicates that potentially avoidable CS may be related to a myriad of interrelated factors. We have grouped literature that explores institutional factors related to the nature of the hospital institution itself, obstetric provider-related issues or medicolegal issues and literature that discusses patient-related sociodemographic factors.

Institutional factors

Many have found that CS rates are significantly higher among patients delivered in private versus public hospitals.^21–24 In addition, in the U.S. over the last decade, subjective CS indications, such as arrest of dilation and nonreassuring fetal heart tracings, are documented with more frequency, whereas more objectively defined medical indications, such as maternal, fetal, or obstetric conditions, have remained stable.¹⁵ Additionally, hospital scheduling and obstetric provider work schedules have been linked to variations in CS rates as well, with fewer CSs occurring in hospitals where providers cover 12–14 hour in-house shifts compared with hospitals which employ physicians using traditional “on-call” models.²⁵ Similarly, high rates of CSs occurring on Friday afternoons²⁵ in comparison to CSs conducted on weekends highlight bias in response to provider work schedules.²⁶ Specific obstetric provider characteristics have also been cited as potentially contributing to the rise of CS in low-risk women, with male physicians and physicians practicing in university settings being associated with higher rates of CS.²⁶ Finally, increased cases of obstetric malpractice claims and subsequent higher state malpractice premiums have been linked to lower rates of vaginal birth attempts after CS and higher rates of primary and repeat CSs.^27–29

Social and demographic factors

Several important patient demographic factors have been explored as risk factors for CS with conflicting results. Socioeconomic status described by private versus public insurance,^22–24,26 as well as race, ethnicity, and/or nativity (i.e., immigrant status when delivering) have all been cited as affecting a patient's risk of getting a CS.^23,26,30,31 For instance, Janevic et al. found an association between the ethnicity and nativity of the mother and differential CS rates when looking at women in New York City from 1995 to 2013. Specifically, they noted that African American, African, South Asian, and Caribbean women had higher risks of CS than non-Hispanic white women. After adjusting for maternal age, maternal education, Medicaid status, prepregnancy weight, parity, birth weight and gestational age of infant, birth year, and medical/pregnancy complications, Hispanic Caribbean women in this cohort had the highest adjusted relative risk (RR) of CS (RR 1.27) followed by African American mothers (RR 1.2). Others have found similarly higher rates of CS in non-Hispanic African American mothers as compared with non-Hispanic white mothers.^23,26

Nativity also appears to be a potential determinant of increased CS risk. In Janevic's New York City sample, immigrant women had higher risks of receiving CS, and being foreign born increased women's risk even among similar ethnic groups.³⁰ These findings are similar to those published by Merry et al. in a 2013 meta-analysis of CS rates, which found that migrant people (i.e., people who crossed international borders) in Western countries were more likely to have higher rates of CS than native-born citizens. Risk factors for CS discovered in this meta-analysis included language and communication issues, lower socioeconomic status, poor maternal health, gestational diabetes mellitus, high prepregnancy body mass index (BMI), fetopelvic disproportion, and lack of prenatal care. Merry's group ultimately concluded, however, that there was inadequate evidence to explain the differences noted in CS rates, but that the characteristics of the country receiving the migrants appeared to be important.³¹ This makes sense given the wide disparity in CS rates that exists worldwide. A 2010 WHO report of CS rates in 137 countries found that only about 10% of countries had CS rates between 10% and 15%, with rates in the rest of the countries nearly evenly split above and below this range. Countries with CS rates less than 10% were primarily in Africa and Asia, with Chad and Burkina Faso having rates less than 1%. Conversely, in other countries; CS births represented nearly half of all births, such as in Brazil (46%) and Iran (42%).¹⁰

Finally, patient preferences appear to play a role in the increased prevalence of a CS. Some groups have documented rising number of CSs performed secondary to maternal requests,¹⁵ which may be related to evidence that women with a history of a previous CS have a higher preference for subsequent CSs.³² This phenomenon is present even in middle- to low-income countries such as Bangladesh, where healthcare providers reported feeling significant pressure from families to perform a CS.¹² These findings indicate that even rural families in low-income countries were willing and able to pay for CS in a private healthcare system, thereby increasing the potential risk for higher number of medically unnecessary CSs being performed at families' request.¹²

Evidence-Based Approaches for Reducing Potentially Unnecessary CSs

Published literature on interventions designed to reduce unnecessary CSs can be classified broadly under two main umbrellas: clinician-oriented interventions and patient-oriented interventions (Fig. 1). Clinician-oriented interventions aim to educate, motivate, and incentivize healthcare providers to reduce the number of elective unnecessary CS. Specific clinician-oriented intervention strategies include quality improvement projects such as retrospective analyses of medical records or audits that provide feedback to providers and medical facilities or the creation of standardized hospital policies and practice guidelines related to CS.³³ Clinician-oriented interventions can also include strategies to promote alternative obstetric practices that increase the rate of vaginal births. Patient-oriented interventions are those which target mothers prenatally and during labor through educational and supportive measures in an attempt to improve their readiness for a vaginal birth and reduce their desire for a CS.^33–37 Our review of the literature was limited to clinician and patient-oriented interventions that were focused on reducing potentially avoidable CS as the primary outcome. Our analysis revealed that multifaceted institutional interventions that implement new practice guidelines and use audit/feedback cycles have had the most documented success in reducing high CS rates. Patient-oriented interventions that include longitudinal prenatal education and support also appear to reduce CS rates, although the evidence in their case is less robust.

FIG. 1.

A schematic of the different categories of interventions in the literature aimed at reducing overall rates of cesarean sections.

Clinician-oriented nonclinical interventions

Clinician-oriented nonclinical interventions (CONCI) include obstetric provider education, audit and feedback cycles, clinical practice guidelines, and financial incentives.^33,38–43 Multifaceted large-scale quality improvement (QI) initiatives that employ a combination of CONCI strategies based on the Robson Ten Group Classification System (TGCS) were the most successful in reducing unnecessary CS rates. The TGCS groups pregnant women in “mutually exclusive, totally inclusive, and clinically relevant” categories so as to be able to gauge the relative contributions of each clinical scenario to overall rates of CS and focus CS reduction efforts on groups receiving the most elective CS.⁴⁴ The WHO recommends the use of this system to monitor and compare CS rates across facilities.⁴⁵ In 2010, Scarella et al., a Chilean group, implemented the TGCS at the Van Buren Hospital as a means to audit and analyze their CS rates.⁴³ After presenting the TGCS system to their medical and midwifery staff, they completed a nine-month intervention period of monthly audit and feedback sessions with their staff. Analysis of CS rates from baseline (36.8%) revealed that the overall CS rate was reduced by 10.3% during the first 9 months of the intervention (26.5%), but was only reduced by 5.0% during the subsequent 9-month follow-up intervention period (31.8%). Importantly, Scarella et al. noted that when comparing their baseline rates to their postintervention rates, their entire multifaceted intervention was associated with a statistically significant reduction in the risk of a CS occurring at their hospital (RR = 0.86, 95% CI: 0.76–0.97).⁴³

A Cochrane review in 2011, however, found insufficient evidence that audit and feedback strategies alone were effective in reducing CS rates. Instead, it was the combination of audit/feedback cycles with guideline implementation after education of and support from local opinion leaders which appeared to be successful.³³ This was also seen in a large study published by Ayres-de-Campo et al. (2015), where a national “concerted action” was taken to reduce CS rates throughout Portugal using a multifaceted intervention to promote vaginal births after cesarean sections (VBACs), modify breech deliveries, and reduce labor induction before 41 weeks. Ayres-de-Campo's group implemented educational opportunities and audits of obstetric providers in state-owned hospitals with CS rates >35%. The results of this intervention found that national CS rates decreased 15.4% over the study period of 2000 and 2013 (from 33% to 28%, p < 0.001).³⁸ Similarly, a cluster-randomized controlled trial of 32 hospitals in Quebec reported success with a multifaceted 1.5-year intervention that targeted changing the perceptions and practices of physicians and nurses involved in the decision-making process for CS. Specifically, the rate of CS delivery was reduced from 22.5% to 21.8% among low-risk women in the intervention group with a concurrent change of 23.2% to 23.5% in the control group.⁴⁶ Given the large sample size in this study, the results were statistically significant, although it is arguably questionable whether a reduction of 0.7% (or 1.7% if comparing the final CS rate in the control group with the final rate in the intervention rate) is clinically significant.

In the United States, large-scale multifaceted approaches have been similarly successful. In 2013, Berrien's group published the results of their 39-week project, a collaboration between 33 hospitals that was focused on reducing the rates of early term (i.e., between 36 0/7 and 38 6/7 weeks of gestation) elective CS using individualized interventions.⁴⁰ Although most hospitals relied on the development of new policies, consensus agreements, and educational initiatives for physicians and nurses, Berrien concluded that the most successful interventions consisted of a mandatory peer-review process to approve early term scheduled deliveries without documented medical indications.⁴⁰ They documented a decrease of elective deliveries among scheduled early term deliveries from 23% of all the CS to 16.2% (p = 0.002) over their study period, which they attributed to such “hard stop” interventions.⁴⁰ This hard stop approach has been utilized successfully by other large-scale groups who have implemented the need for similar mandatory second opinions before the scheduling of elective CS.^33,42 The Big 5 State Prematurity Initiative was one such group. A multistate collaborative consisting of representatives from the five most populous states that account for 40% of all births in the United States, this group conducted an intervention focused on reducing the rates of scheduled early term elective CS deliveries using hospital-wide mandatory documentation of deliveries before 39 weeks, an audit/feedback strategy and regular reports that allowed for comparisons within and between hospitals in the Big 5 network.⁴² The authors of the initiative's analyses revealed that after implementation of such policies requiring strict hard stop documentation and intergroup comparisons, elective CS deliveries decreased from 43.5% to 7.1% (an 84% decrease, p < 0.001) over their 12-month study period.⁴²

Importantly, not all clinician-oriented interventions have proven to be successful in reducing CS rates. Groups that have attempted to implement legislative changes that target medical institutions or attempt financial/insurance reform as their only main intervention have not documented a significant associated decrease in CS rates.^33,47 The lack of success seen with financial reform efforts is surprising given the number of reasons why obstetric providers might be financially incentivized to perform CS over vaginal births. As with most surgical procedures, insurance reimbursement is typically higher for CS compared with vaginal deliveries, a phenomenon that is only accentuated by the fee-for-service compensation models that are common in the United States.⁴⁸ Despite these pervasive financial incentives, attempts to decrease CS rates by changing physician compensation related to CS have not produced statistically significant results. Specifically, this approach has been attempted in California³³ and Taiwan, where the CS rates in the 30% range mirror U.S. rates.^49,50

There have, however, been groups which attempt to use financial incentives as part of their multifaceted approaches for decreasing rates of elective, potentially avoidable CS. For instance, Ayres-de-Campo's national concerted action in Portugal did include the indexing of “an important percentage of hospital funding” to annual CS rates in that hospital, thereby financially incentivizing hospitals to maintain a target annual CS rate. As they could not determine the effect of the individual initiatives separately in their study, it is possible that this financial incentive to control CS rates contributed to their success.³⁸ Similarly, Runmei's three-stage QI program to discourage unnecessary CS in China included a component that deprived surgeons of financial incentives for conducting CSs. Due to their study design, they too were unable to tease out the extent to which the lack of these financial incentives contributed to the reduced CS rates they documented (from 42.4% in 2005 to 36.1% in 2011, p < 0.001).³⁶ Importantly, these authors acknowledge that their success may have been due to the extremely high baseline CS rates in China, which they quote being as high as 70% to 77% in some rural areas and 46% nationwide during their study period.³⁶

Collectively, this literature demonstrates that large-scale multifaceted approaches implemented over a period of 9–12 months with audit, clinical practice guidelines, provider education, and hard stops for scheduling elective CS are effective means of reducing CS rates by anywhere from 2% to 84% depending on the baseline CS rate.

Clinician-oriented clinical interventions

Clinical clinician-oriented interventions are practices aimed at increasing the use of vaginal births over CS. These include increasing trials of labor after CS to allow for more VBACs, modifying breech presentation through external prenatal physical maneuvers, increasing instrumental vaginal births, electronic fetal monitoring (EFM), medical or procedural induction of labor in cases where labor does not initiate naturally, or pharmacological labor augmentation when it is not progressing adequately.^35,38,51 Several groups have found that clinician-oriented interventions which promote such practices reduced the rate of elective CS, but had small or no effect on overall CS rates due to higher contributions of CS for indication.^36,51 For instance, Khunpradit's meta-analysis found that interventions aimed at increasing VBAC rates did not result in a statistically significant reduction in CS rates.³³ Similarly, a meta-analysis of the use of several intrapartum obstetric interventions in low-risk pregnancies between 37 and 42 weeks, which was conducted by Rossignol et al., had negative results.³⁵ Specifically, continuous EFM was linked to higher not lower rates of CS.³⁵ Rossignol's group postulated that if all EFM in low-risk pregnancies were replaced by intermittent auscultation in Quebec, CS rates could be reduced from 23.2% to 20.5% in 1 year.

Another potential clinical intervention is the use of pitocin, a synthetic version of the naturally occurring hormone oxytocin, which stimulates contraction of the uterus. Some studies have found pitocin to be beneficial in reducing CS if used for augmentation of existing labor, but others have noted that when it is used to induce the onset of labor, it is more associated with higher rates of CS.^35,40,42 In the setting of naturally occurring labor, pitocin is considered when natural labor forces do not have enough strength (i.e., Montevideo Units measured by an internal pressure catheter) or do not occur frequently enough to cause cervical change. Pitocin is used in these cases at titrating doses to increase the strength and frequency of contractions to a goal of at least 200 MV units in 10 minutes. This strategy has been shown to decrease CS rates by increasing the forces leading to cervical change and promoting adequate forces during the second stage of labor (i.e., the pushing phase).^52,53 There is also some evidence that waiting even longer to initiate pitocin until the first latent phase is completed and/or the cervix has fully effaced may further decrease the risk of a CS.⁵¹ Overall, there appears to be a need for further studies to evaluate the efficacy of such clinical interventions as a means to reduce current rates of potentially avoidable CS.

Patient-oriented interventions

Patient-oriented interventions can be divided into: (1) educational prenatal public health measures focused on changing patients' perspectives of CS, and (2) supportive intrapartum interventions aimed at increasing a woman's chances of progressing through labor without need for surgery.^33,34,47 In 2010, Hodnett et al. identified nine randomized controlled trials which found that offering social support to at-risk pregnant women was associated with a reduction in CS births (n = 4522; RR 0.87, 95% CI 0.78–0.97). Hodnett's group concluded that the additional support may be lessening mothers' anxiety and in this way increasing the likelihood of normal vaginal birth, but the exact mechanism for the decreased CS rates is not clear.³⁴ Others have found that prenatally disbursed information packets or decision aid booklets were not sufficient to decrease CS rates, but serial prenatal preparatory sessions, especially those focused on relaxation training, appeared to lead to reductions in women pregnant for the first time subsequently undergoing CS deliveries.³³ These findings, however, have not been replicated by all, with a few large studies in Spain showing no significant association between the provision of free antenatal group maternal education and CS rates.^37,54

As far as intrapartum interventions, one group demonstrated a large potential for reducing obstetric interventions through the use of what they termed “upper cerebral pain control” (UCPC) techniques that include nonpharmacological methods of labor pain control, such as continuous one-on-one support of the laboring woman (i.e., through the presence of a doula), breathing techniques, self-hypnosis, and the use of birth balls.⁴⁷ This group's findings were supported by a subsequent systematic review which concluded that UCPC measures had the potential to reduce CS deliveries by almost one-third (29.6%) based on pooled results.³⁵

Conclusions and Directions for Future Research

Despite data from large-scale, worldwide ecological studies suggesting that optimal CS rates are between 15% and 19%, current CS rates both worldwide and even here in the United States far exceed these recommended optimal rates, with the most recent documented CS rates in the United States exceeding 30%. Reasons for the continued high prevalence of this obstetric practice are complex and appear to intertwine institutional, demographic, and sociocultural factors. Nevertheless, there is a substantial body of literature of interventions aimed at reducing current CS rates down to more recommended levels, primarily by decreasing the number of CS that are offered to and performed on low-risk primiparous women whose pregnancies reach term without major medical complications and whose infants are not in breech position.

From our review, multifaceted clinician-oriented interventions that combine audit and feedback cycles with the implementation of new practice guidelines by local opinion leaders can effectively reduce CS rates if implemented over at least a 9–12-month period. Interrelated to this strategy is the “hard stop” technique that requires second opinion or peer review for decisions to proceed with elective CS, particularly in low-risk women. In addition, there is some burgeoning evidence that certain patient-oriented interventions such as longitudinal social support through home visits and continuous individualized labor support may also be effective at reducing CS.

Importantly, there is a current dearth of concrete data regarding the costs associated with these evidence-based CS reduction strategies, and in particular, with the interventions that deployed research nurses or midwives over various regions of coverage for home visits. It is quite possible that the economics of such interventions would be prohibitive, particularly to smaller hospitals, clinics, or private practices. These financial questions must therefore be better explored in future research. In addition, moving forward, it will be important to better understand how obstetric providers in different settings decide to proceed with a CS. As stated in the 2015 WHO statement on CS rates, it is more important to offer sections to women who need them rather than aiming for a specific rate.⁴⁵ Thus, research regarding the indications used to choose CS for low-risk women may more clearly illuminate strategies to promote vaginal births in this population and thus avoid potentially unnecessary CSs.

Acknowledgments

The authors appreciate the help of Olivia Carney with the preparation of this article and the financial support of the Robin Hood Foundation to complete this project. They also thank Dr. David Allison for his contributions to this project. The authors declare no conflicts of interest.

Author Disclosure Statement

No competing financial interests exist.