The impact of the advanced practice nursing role on quality of care, clinical outcomes, patient satisfaction, and cost in the emergency and critical care settings: a systematic review

Background The prevalence of chronic illness and multimorbidity rises with population aging, thereby increasing the acuity of care. Consequently, the demand for emergency and critical care services has increased. However, the forecasted requirements for physicians have shown a continued shortage. Among efforts underway to search for innovations to strengthen the workforce, there is a heightened interest to have nurses in advanced practice participate in patient care at a great extent. Therefore, it is of interest to evaluate the impact of increasing the autonomy of nurses assuming advanced practice roles in emergency and critical care settings on patient outcomes. Objectives The objectives of this study are to present, critically appraise, and synthesize the best available evidence on the impact of advanced practice nursing on quality of care, clinical outcomes, patient satisfaction, and cost in emergency and critical care settings. Review methods A comprehensive and systematic search of nine electronic databases and a hand-search of two key journals from 2006 to 2016 were conducted to identify studies evaluating the impact of advanced practice nursing in the emergency and critical care settings. Two authors were involved selecting the studies based on the inclusion criteria. Out of the original search yield of 12,061 studies, 15 studies were chosen for appraisal of methodological quality by two independent authors and subsequently included for analysis. Data was extracted using standardized tools. Results Narrative synthesis was undertaken to summarize and report the findings. This review demonstrates that the involvement of nurses in advanced practice in emergency and critical care improves the length of stay, time to consultation/treatment, mortality, patient satisfaction, and cost savings. Conclusions Capitalizing on nurses in advanced practice to increase patients’ access to emergency and critical care is appealing. This review suggests that the implementation of advanced practice nursing roles in the emergency and critical care settings improves patient outcomes. The transformation of healthcare delivery through effective utilization of the workforce may alleviate the impending rise in demand for health services. Nevertheless, it is necessary to first prepare a receptive context to effect sustainable change. Electronic supplementary material The online version of this article (10.1186/s12960-017-0237-9) contains supplementary material, which is available to authorized users.


Background
While people of all ages receive emergency and critical care services across the world, the elderly population continues to exhaust a greater proportion of these services [1]. The complexity and acuity of care have heightened with greater prevalence of chronic illness and multimorbidity among older adults [2]. Correspondingly, the demand for emergency and critical care services has increased [1], alongside a concomitant increase in the forecasted workforce requirements for such services [3]. The Accreditation Council for Graduate Medical Education regulations in 2006 in the United States of America (USA) recommends a high-intensity model of care involving 24-h physician coverage [3,4]. This implementation accentuates inadequacies of the healthcare workforce to provide emergent and critical care services. In the USA, it is predicted that, compared to healthcare system's demands, there will be a 22% shortfall of critical care physicians by 2020 and a subsequent 35% shortfall by 2030 [1].
With the impending rise in demand for health services, an effective utilization of the workforce is paramount to ensure high-quality yet cost-effective health service delivery [5]. Across some countries, healthcare workers' wages account for approximately 50% of the total healthcare expenditure [6]. Hence, cost containment strategies will inevitably involve the workforce [7]. Efforts are underway for measures to enhance productivity through increasing the capacity of the workforce.
One potential measure is a greater utilization of nurses in advance practice. The global annual growth of the nurse practitioner (NP) workforce has been estimated to be between three to nine times greater compared to physicians; therefore, of interest to health policymakers is the utilization of NPs and advanced practice nurses (APNs) [8,9]. The nomenclature varies internationally. The "NP" title is used in Australia, Belgium, Canada, Sweden, the United Kingdom (UK), and the USA whereas the "APN" title is used in Switzerland, Singapore, and South Korea [10]. Nonetheless, NPs and APNs (NP/APNs) are registered nurses "who acquired the expert knowledge base, complex decision-making skills and clinical competencies for expanded practice" ( [4], p. 26) and enter the workforce with a master's degree [11].
This advanced practice role was first introduced in the 1960s as a solution to the lack of primary care physicians, to meet the primary care needs of the rural and underserved populations [12]. Primary care has first contact with patients and, subsequently, provides continuity of care within the healthcare system through the coordination of care according to patients' needs [13]. Studies to evaluate the quality of primary care provided by NP/ APNs have been shown to be comparable to that of physicians in terms of effectiveness and safety [14]. To fulfill primary care needs, NP/APNs in this setting are trained generalists who have a breadth of knowledge to render a wide scope of care.
Since the inception of advanced nursing practice in primary care, its role has extended to other healthcare settings such as the acute care. Acute care provides short-term restorative stabilization to patients in unstable chronic conditions and with complex acute and critical illnesses. Acute care encompasses emergency and critical care [15]. Emergency and primary care advanced nursing practice do share similarities in that they serve as first-contact access to healthcare, but the acuity of the patient manifestations delineates the two. Unlike in primary care NP/APNs, emergency NP/APNs are trained to manage patients with acute life-or limb-threatening conditions [15]. In the past decade, greater practice autonomy has been given to NP/APNs in emergency and critical care. This expanded practice allows nurses to assume some medical tasks typically performed by physicians, aiming at not only increasing the access to healthcare and service efficiency but also eventually mitigating the cost of health services.
The development of advanced nursing practice contributed to a service model aiming to respond flexibly to the ever-changing needs of patients [16]. Systematic reviews of studies on the effectiveness and safety of NP/ APN-led primary care have reported positive effects of NP/APN service on clinical outcomes, patient satisfaction, and costs [14,17] These reviews focused on the primary care setting, it may be inappropriate to extrapolate their findings to the emergency and critical care settings since the patient acuity and clinical needs differ among settings.
Nonetheless, reviews evaluating NP services in the emergency and critical care settings exist. However, they have three shortcomings, the first of which concerns their generalizability. Over the past decade, studies have evaluated whether the delegation of medical tasks to NP/APNs in the emergency and critical care settings was feasible and safe. A review of 31 studies on the impact of NPs and physician assistants in such settings reported that their practice was safe and, in some cases, the quality of care was higher than that of physicians [18]. However, only two of the studies were randomized controlled trials (RCTs) [19,20] whereas the rest had small sample sizes and questionable study methodology; these limit the generalizability of the review. A more recent review [21] also reported that NPs do have a positive impact on the quality of care. Nonetheless, the reviews included both NPs and non-nursing healthcare providers, thereby introducing heterogeneity in the synthesis of evidence, making it difficult to assess the true effect of NPs in the intensive care settings [18,21].
The second shortcoming centers on the inconclusiveness of the reviews. One review suggested although NP services in the emergency setting did reduce waiting time and provide care comparable to that of a midgrade physician, the cost of NP services was higher than that of resident physicians [22]. In contrast, another review concluded that the use of NPs reduced the cost of emergency and intensive care services. Further complicating the picture is a recent systematic review that reported an inadequacy of evidence to determine the cost-effectiveness of NP services in emergency departments (EDs) [23]. Consequently, the cost-effectiveness of advanced nursing practice in the emergency and critical care settings has remained inconclusive. Lastly, all existing reviews [18,[21][22][23] elucidating advanced nursing practice in the emergency and critical care settings included only studies published before January 2013, which may be dated.
Considering the existing literature, it is of interest to undertake an updated systematic review on the latest evidence to determine whether advanced practice nursing in emergency and critical care have an impact on the quality of care, clinical outcomes, patient satisfaction, and cost savings. If NP/APNs can indeed provide competent and safe care in these settings, greater access to emergency and critical care services will be available, thereby strengthening the workforce to fulfill the escalating healthcare demands.
Therefore, the main objective of this systematic review is to present, critically appraise, and synthesize the best available evidence on the impact of advanced nursing practice on patients' length of stay, time to treatment or consult, mortality, patient satisfaction, and cost in emergency and critical care settings.

Design
The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were adhered to in the conduct and reporting of this systematic review [24].

Study selection
Published studies and studies which have yet to be published were searched using PubMed, CINAHL, The Cochrane Library, Scopus, Embase, Web of Science, ScienceDirect, Wiley Online Library, and ProQuest Dissertations and Theses Global databases from January 2006 up to September 2016. Only English studies were considered. The search strategy included the keywords, as shown in Table 1, in various combinations for a systematic database search. The search terms and search strategies for each database are included in Additional file 1. The reference lists of all identified studies were also screened. Corresponding authors were contacted for additional information where necessary.

Study eligibility
This review included RCTs, quasi-experimental studies, prospective and retrospective cohort studies. Crosssectional studies and studies without comparison groups were excluded.
The PICO (Population-Intervention-Comparison-Outcome) framework guided the selection process [25]. This review considered studies that included the following: Patients: at least 16 years of age, presenting in EDs, trauma centers, intensive care unit (ICU), or high dependency units, requiring emergency or critical care Nurses: registered nurses in advanced practice role, i.e., APNs or NPs Excluded from the review were studies that examined both adult and pediatric patients requiring emergency or critical care services. Excluded from the review were also studies that examined services provided by physician assistants. This review included studies with interventions which compared the outcomes of the APN-/NP-directed emergency or critical care services with those of the physician-directed care. This review also included studies with interventions which compared the physician-only model of care with APNphysician or NP-physician collaborative model of care.
Studies that had the following outcome measures were included: Patients' length of stay in the emergency or critical care setting Patient mortality Time to consultation or treatment Patients' satisfaction Cost of care The selection of studies was done independently by two of the authors (BW and JL) based on the eligibility criteria. Disagreement during selection was resolved by discussion with a third-party arbiter (WT). The selection process is illustrated in the flow diagram in Fig. 1.

Data extraction
Data was extracted by one author (BW) and crosschecked by another (JL) for accuracy. Resolution of disagreement was done by discussion with a third-party arbiter (WT). The Joanna Briggs Institute's (JBI) "Data Extraction Form for Experimental/Observational Studies" [26] was adapted to tabulate the characteristics and findings of the studies.

Quality assessment
Two authors (BW and JL) performed the methodological quality assessment independently, based on the "JBI Critical Appraisal Checklist for Randomized Controlled Trials," and "JBI Critical Appraisal Checklist for Cohort Studies" [21]. The RCTs were assessed for their randomization methods, treatment allocation, concealment of treatment groups, and homogeneity of the participants' baseline demographics upon entry of the study. In addition, all studies were appraised for their control of confounding factors, reliability of outcome measures, and suitability of statistical analyses. For this review, a low methodological quality refers to a score assigned to a study of less than 40%, a medium quality refers to one between 40 and 70%, and a high quality refers to one greater than 70%. The findings of any systematic review are only as reliable as the primary data source, upon which the review is based [27]. Hence, studies rated to have low methodological quality (see Additional file 2) were excluded to avoid potentially erroneous conclusions based on the synthesis of poorly conducted studies.

Synthesis
Given the heterogeneity of the interventions and findings in the studies, no meta-analysis was performed. Instead, a narrative synthesis of the studies was done: the analysis was conveyed in prose, alongside tables to outline and explain the results.
In two of the three RCTs, true randomization was used to assign patients to study groups by using computer-generated sequence, thus incurring only low risk of selection bias. In the other RCT, a triage coordinator was present to randomly assign the patients at a planned ratio to either NP-directed care or physiciandirected care. Two of three RCTs, took measures to blind the outcome assessors to treatment assignment, minimizing detection bias. Out of the 15 studies, 14 measured their outcomes in a reliable and valid manner using pre-decided criteria, minimizing reporting bias. The presence of confounding factors was acknowledged in 11 of the 12 cohort studies but only five of them described strategies to deal with it. All the included studies fared poorly in reducing attrition bias. Only three of the 15 studies had complete follow-up or strategies to address incomplete follow-up. Appropriate statistical analyses were chosen in all included studies.

Findings
The study results and statistical conclusions are summarized in Table 4. The details of the individual studies can be found in Table 5. The findings were categorized according to the studies' setting. Studies conducted in emergency and critical care settings measured outcomes such as length of stay, waiting, and patient satisfaction. Outcomes such as mortality and cost were measured only in the critical care setting.

Emergency setting
Length of stay Four out of the 15 studies examined the impact of the advanced nursing practice roles on the length of stay in the emergency setting [28,32,33,42].
NP-directed management of care Two studies [28,33] reported a significant reduction in the length of stay in EDs of patients who were reviewed and treated by NPs when compared to those seen by physicians. However, the shorter time was attributed to the baseline difference in patients' acuity between the groups. The physicians handled patients of higher acuity and complexity than NPs. On the contrary, a multisite study [42], with comparison groups of similar baseline patient acuity, found comparable lengths of stay in EDs when patients with chest pain were managed by either NPs or physicians.
Length of stay in collaborative care involving nurse practitioners One study [32] compared NP-physician collaborative model of care with usual physician-only

Waiting time
Of the 15 studies, six studies [28-30, 32, 33] examined the impact of advanced nursing practice roles on waiting time in the emergency setting.
Time to consultation Only one study [33] reported that patients with minor injuries experienced shorter waiting time (median 14 min) when reviewed by emergency NPs than those reviewed by physicians (median 50 min). The other three studies [28,30] comparing NP-directed care with physician-only care found similar waiting time to consultation in EDs. Another study [32] comparing the NP-physician collaborative care with physician-only care also found similar waiting time to consultation in EDs.
Time to treatment One RCT [29] illustrated that a greater proportion of patients (15.4%) managed by emergency NPs received analgesia within 30 min of arrival at the ED compared to patients managed by physicians (1.6%) (P < 0.001).

Critical care setting
Length of stay Seven out of the 15 studies examined the impact of the advanced nursing practice roles on the length of stay in the critical care setting [31, 35-38, 40, 41].
NP-directed management of care Comparable lengths of stay in a trauma center was reported in one study [40] where the comparison groups had similar baseline patient acuity. A RCT [31] conducted in a post-cardiac surgery unit where patients required critical care found comparable lengths of stay in hospital between the comparison groups (NP-directed care versus physician-only care). Despite the higher acuity of care required by patients under NP-directed care than those under physician-only care, the discharge outcomes were similar. In addition, a large cohort study [38] reported a significantly shorter length of stay in medical ICUs for patients whose management were led by NPs than those  • A New Zealand study conducted at a single site.
• Registrars took a longer time to see these minor injuries patients as the patients were of higher acuity with comorbidities while the ENP reviewed the straightforward minor injury cases.
• NPs tend to complete patient care on their own while Registrars would delegate discharge or administrative tasks to clerical staff.
• The time recorded electronically might not have been precise in reflecting the patient's transit times. It was possible NPs logged onto the system to review patients faster than Registrars which might have account for the reduced LOS for NP-treated patients.
• An Australian study conducted at a single site.
• Patients in the ENP candidate group were from the Fast Track unit where patients of lower acuity were seen. Patients in the medical officers group were not only from the Fast Track unit. The medical officers might have reviewed more complex cases and hence, required more time.
Roche et al.
• An Australian study conducted at three rural EDs.
• No significant differences between groups in baseline characteristics or acuity, NP service was comparable to that of senior medical officers.
• A Canadian study conducted at a single site.
• The emergency physician group had patients of higher acuity than NP collaborative group yet there was no difference in LOS between groups, possibly implying it was more efficient to do without collaboration with NPs.
• However, the demand for physicians to review lower acuity patients might have reduced with the collaborative NP group, allowing physicians to spend more time with higher acuity patients.

Length of stay (LOS)-Critical Care setting
David et al.
(2015) [35] No significant difference found between study groups in the mean LOS in the inpatient telemetry cardiology unit and ICU, 129.1 ± 96.7 h in NP collaborative group versus 119.1 ± 69.7 h in physician-only group (P = 0.469).
• A USA study conducted at a single site.
• Advocates for the NP collaborative model of care as it provides the unit staff with a consistent point of contact for the multidisciplinary team.
• The NP collaborative model of care allows for NPs to develop expertise for managing a specific group of patients.

Goldie et al.
(2012) [31] No significant difference found between study groups in the mean hospital LOS, 9 ± 6 days in NP group versus 9 ± 14 days in hospitalist group (t test, P = 0.87).
• A Canadian RCT conducted at a single site.
• Total sample size varied during the statistical analysis as there were varying amounts of missing data.
• A much higher proportion of male participants recruited (86% in NP group and 81% in hospitalist group) raised queries about system level factors that might have favored male participants and the general willingness of female patients to participate in research.
• The patient acuity in NP group was higher than that in hospitalist group and yet the groups did not differ in their clinical outcomes.
(2015) [36] No significant difference found between study groups in mean LOS, 4.91 ± 4.53 days in the NP collaborative group versus 6.02 ± 6.74 days in the physician group (Wilcoxon P = 0.1441).
• A USA study conducted at single site.
• After subgroup analysis, significant differences in LOS were found between study groups in patients transferred from another service (Wilcoxon P < 0.0001), patients discharged to rehabilitation facility (Wilcoxon P = 0.0024), patients older than 60 years (Wilcoxon P = 0.0369), or patients discharged on intravenous antibiotics/wound therapy (Wilcoxon P = 0.0171). A significantly lower mean LOS was found in the NP collaborative group.
• In this subgroup of patients, greater communication with multidisciplinary teams, discharge planning, follow-up care coordination and administrative work were required. This demonstrated the value of adopting the NP collaborative model of care.  [37] No significant difference between study groups in the mean ICU LOS, 14.6 ± 9.7 days in NP collaborative group versus 15 ± 11.4 days in non-NP group (P = 0.753).
• A USA study conducted at a single site.
• The comparable ICU LOS between NP collaborative model of care and the model of care without NP might be due to the greater continuity of care rendered by the NP as compared to the rotating coverage of the fellows in the non-NP model of care.
• It might also be contributed by the attending physician's ability to provide expert supervision and direct care of the patients, despite the difference in the composition of the team.
• It could also be because the NP was highly experienced and was familiar with the environment and the patient care demands.
• An USA study conducted at a single site.
• Patients in NP group were solely managed by NPs and the supervising attending physicians and fellows. There was no cross-contamination, the Residents did not interfere with the management of patients in the NP group.
• Even though LOS findings between study group favor the NP group, the lack of clear definition of the role of the acute care NP hinders direct comparison of clinical outcomes with the residents.
• Hospital LOS for NP group was shorter than Resident group as more patients were being discharged straight from the ICU in NP group. It could have been due the differences in patient's diagnosis, social or financial situations, or provider practice paradigm.
• Shorter hospital LOS in NP group did not come at the expense of longer ICU LOS, increased ICU readmissions or post-discharge mortality.
• A higher patient to provider ratio was observed in NP group but the authors were judicious in inferring that NP-led model of care had greater efficiency given the differences in the patients' characteristics between study groups. [40] No significant difference between study groups in mean LOS, 6.5 ± 8.8 days for NP group versus 7 ± 10.8 days for Resident group (t test P = 0.17).

Morris et al. (2012)
• A USA study conducted at a single site.
• Although the results are not statistically significant, they were clinically important. The difference of 0.5 days multiplied by the number of patients in NP group (2759) accumulates to a total difference of greater than 1300 patient days.
• A greater proportion of Resident group discharged to other health facilities which was delayed by bed availability. This could be a possible reason for the longer hospital LOS for patients in Resident group.
• Daily multidisciplinary rounds were scheduled in NP group but not in Resident group which could have improved the coordination of patient care, contributing to shorter LOS.

Scherzer et al. (2016) [41]
Significant difference between study groups in mean MICU LOS, 7.9 ± 7.5 days in NP group versus 5.6 ± 6.5 days in Resident group (Wilcoxon P < 0.0001 • A New Zealand study conducted at a single site.
• EM Registrars might have taken a longer time between each patient because they were of higher acuity and complexity compared to patients in NP group.
• The time recorded electronically might not have been precise in reflecting the patient's transit times. It was possible NPs logged onto the system to review patients faster than Registrars which might have account for the lesser wait times for NP-treated patients.
• An Australian study conducted at a single site.
• Lost to follow-up rates was high. The waiting time of patients who left before being seen was not captured.
• Patients in both study groups had similar baseline characteristics.  [28] No significant difference between study groups in median time to consultation, 12 min (IQR 5.5-2.8) in the ENP candidate group versus 31 min (IQR 11.5-76) in medical officer group (Wilcoxon P < 0.001).
• An Australian study conducted at a single site.
• Patients in the ENP candidate group were from the Fast Track unit where patients of lower acuity were seen. Patients in the medical officers group were not only from the Fast Track unit. The medical officers might have reviewed more complex cases and hence, required more time.
Jennings et al.
(2015) [29] Significant difference between study groups in the proportion of patients receiving analgesia within 30 min of ED arrival, 15.4% in NP group versus 1.6% in medical officer group (Chi-square test P < 0.001).
• An Australian study conducted at a single site.
• NP group performed better at complying with the recommended Australian national targets for administering timely analgesia.
• NP provided a hybrid model of care, assimilating nursing, and medical tasks. The NP could perform patient assessment, order and administer the analgesia which reduced the time to treatment.
• An Australian study conducted at a single site.
• No significant differences between groups in baseline characteristics or acuity, NP service was comparable to that of senior medical officers.
Steiner et al.
• A Canadian study conducted at a single site.
• The emergency physician group had patients of higher acuity than NP collaborative group yet there was no difference in waiting time between groups, possibly implying it was more efficient to do without collaboration with NPs.
Waiting time (time to consultation/time to treatment)-Critical Care setting Moran et al. (2016) [39,40] Significant difference between study groups in median door-to-needle time for acute ischemic stroke, 45 min (IQR  in NP group versus 53 min (IQR 45-73) in non-NP group (Mann-Whitney U P = 0.001).
• A USA study conducted at a single site.
• Stroke code care pathway remained the same during the intervention period.
• The reduced time interval between diagnostic imaging and the administration of treatment contributed to the reduction in door-to-needle time.
• NP group was reviewed earlier upon stroke code activation as the NP service was 24/7. Necessary assessments commenced earlier.

Mortality-Critical Care setting
Hoffman et al.
(2006) [37] No significant difference between study groups in ICU mortality, 2% in NP collaborative group versus 2% in non-NP group without treatment limitation (Fisher's exact test P = 1.0).
• A USA study conducted at a single site.
• The comparable ICU mortality between NP collaborative model of care and the model of care without NP might be due to the greater continuity of care rendered by the NP as compared to the rotating coverage of the fellows in the non-NP model of care.
• It might also be contributed by the attending physician's ability to provide expert supervision and direct care of the patients, despite the difference in the composition of the team.
• It could also be because the NP was highly experienced and was familiar with the environment and the patient care demands.
• An USA study conducted at a single site.
• Cross-contamination was minimized, the Residents did not interfere with the management of patients in the NP group. Patients in NP group were solely managed by NPs and the supervising attending physicians and fellows.
• The 90 days and beyond information on the patient's outcome strongly suggests that the quality of NP services in the ICU setting is high.
Moran et al.
• A USA study conducted at a single site.
• Stroke code care pathway remained the same during the intervention period.
• The involvement of the NP in the stroke code team did not change the overall tPA treatment rate of acute ischemic stroke patients because the final decision to treat lies with the stroke physician.
Scherzer et al.
• A USA study conducted at a single site.
• Patients in NP group were older, more chronically and critically ill than patients in Resident group yet the MICU and hospital mortality in both groups were comparable.
under physician-only management. Patients in the NPdirected group also had lower odds (odds ratio 0.87, P < 0.001) of longer hospital stays. Interestingly, a higher patient-to-provider ratio was observed in the NPdirected group but the authors [38] were judicious in inferring greater efficiency in NP-directed care given the Table 5 Findings of studies (Continued) No significant difference between study groups in hospital mortality, 24.4% in NP group versus 24.8% in Resident group (adjusted odds ratio 0.7, P = 0.072).
• Administrative data was used to calculate the risk of mortality. Furthermore, only a subset of the patients had their ICU mortality score calculated. An exhaustive comparison to national data was not done. Outcomes were only compared within the single institution.
Skinner et al.
• A UK study conducted at a single site. Significant difference between study groups in overall rating categories. A higher proportion (68%) of patients in the NP group rated their care as excellent compared to the doctor group (50%) (Fisher exact test, P = 0.02).
Significant difference between study groups in total patient satisfaction score, median score 23 (IQR 20-24) in NP group versus median score 21 (IQR 16-24) in doctor group (Students t test, P = 0.002).
• An Australian study conducted in a single site.
• Loss to follow-up rates were high. The satisfaction level of these patients was not captured.
Roche et al. (2017) [42] No significant difference between study groups in patient satisfaction of care at the occasion-of-service (Fisher's exact test, P = 0.96). No significant difference between study groups in patient satisfaction of care at follow-up (Fisher's exact tests, P = 0.98).
• An Australian study conducted at a single site.
• Evidence to show that majority of the patients were highly satisfied (88.5%) with NP services in the ED and was sustained over time (30 days [31] No significant difference between study groups in mean overall patient satisfaction score, 103 ± 11 in NP group versus 97 ± 14 in hospitalist group (independent t test, P = 0.10).
• A Canadian RCT conducted at a single site.
• Although there was no significant difference between groups in overall patient satisfaction, patients rated NP services significantly higher on several patient satisfaction items.
• The NPs were rated to perform better at teaching, answering questions, listening and pain management.
• These are the forte of NPs, consistent with the NP goals and education, which are grounded in nursing.
• The overall patient satisfaction score of NP group was higher than in the hospitalist group though not statistically significant, it was plausible Type II error happened, and a larger sample size would have generated statistical differences in the overall score. • A USA study conducted at a single site.
• Despite the longer ICU LOS in the collaborative NP group, the overall hospital charges observed was comparable to that of the resident group.
• Resource utilization was similar in both groups, supporting the contention that NPs are cost-effective healthcare providers.
Skinner et el. [34] Annual staffing cost of NP and junior residents was £933 344 with the usual model of care and £764 691 with the collaborative NP model of care.
• A UK study conducted at a single site.
• A reduction of staffing costs was observed.
• Uncertain of how cost analysis was done.
differences in the patients' characteristics between comparison groups.
Collaborative care involving nurse practitioners All included studies that compared NP-physician collaborative model of care with usual physician-only model of care found similar lengths of hospital stay [35][36][37]41] between the comparison groups. However, in one study [36], after subgroup analysis, a significantly shorter length of stay was found in the physician-NP collaborative group for patients transferred from another service (mean difference 6.54 days, P < 0.0001), patients discharged to rehabilitation facility (mean difference 2.63 days, P = 0.0024), patients older than 60 years (mean difference 1.80 days, P = 0.0369), or patients discharged on intravenous antibiotics/wound therapy (mean difference 3.93 days, P = 0.0171). The management of such patients warrants greater communication with multidisciplinary teams, discharge planning, care coordination, and administrative work were required; in this niche, NPs are familiar with such tasks and can competently perform them [43].

Waiting time
Time to treatment Only one study [39] examined the impact of advanced nursing practice roles on waiting time in the critical care setting. The study [39] [34,37,41] which compared the NP-physician collaborative care with physician-only care.

Patient satisfaction
Of the 15 studies, only one examined patient satisfaction in the critical care settings [31]. The study developed a new self-reported tool to measure patient satisfaction and found similar scores when comparing NP-directed care with physician-only care. Nonetheless, the study [31] reported that NPs performed better than physicians in teaching, answering questions, listening, and pain management. This finding was akin to the study [30] conducted in the ED which assessed the healthcare provider for completeness of care, politeness of service provider, explanation and advice given, waiting time, and comprehension of discharge instruction.

Cost
Three of the 15 studies reviewed the impact of the advanced nursing practice roles on cost [34,36,41], all of which compared NP-physician collaborative care with physician-only care in the critical care setting. One study [41] reported that despite a longer ICU stay for patients in the NP-physician group than for those in physicianonly group, there was no significant difference in the observed charges between them. This supports the contention that involving NPs in the management of the critically ill can lead to cost savings. The other two studies [34,36] had results that demonstrated cost savings in the NP-physician group compared to physician-only group. One of them concluded that an annual staffing cost of approximately £170 000 could be saved when physicians worked with NP in managing ICU patients.

Discussion
With population aging and the consequent global epidemic of chronic diseases, healthcare demands will only rise. Accordingly, nurses in advanced practice can add value and increase access to healthcare by, potentially strengthening the healthcare workforce. Nonetheless, the expansion of role and autonomy of nurses will lead to concerns of patient safety and clinical outcomes. Through the narrative synthesis of the available evidence from Australia, Canada, New Zealand, UK, and USA, nurses in advanced practice appear to generate clinical outcomes comparable to those of physicians in the emergency and critical settings. Generally, in the ICU setting, the involvement of NPs in managing the critically ill allowed for greater continuity of care [37], as NPs did not have to be on frequent rotation coverage as junior physicians. Hence, NPs developed greater familiarity with the environment and patient demands than the physicians who were constantly on rotation. The involvement of NPs also provided the unit's staff with a consistent point of contact for the multidisciplinary team [35]. When daily multidisciplinary rounds were initiated by NPs, the coordination of care was shown to improve [40]. Providing effective care coordination is a forte of nurses [10]. Care coordination requires interpersonal communication and collaboration. As nurses can establish more personal and tangible relationships with patients than do physicians [44], they perform better in care coordination. The value of NPs was exemplified when the patient care required crossdisciplinary communication, discharge planning, followup care, and administrative work. With NPs' involvement, patients' length of stay was shortened [36]. Apart from delivering efficient care, nurses in advanced practice will get to develop expertise for managing specific groups of patients through assigned responsibilities [35].
One of the prioritized quality-of-care indicators in the emergency setting is the time from arrival to first assessment by physician [45]. This review has demonstrated that NPs were capable of rendering emergency care services as timely [28,32] as, if not faster [33] than, physicians. The addition of nurses in advanced practice in the emergency settings enabled physicians to pay greater attention to patients of higher complexity and acuity, thereby, improving access to prompt emergency care.
Time to treatment is also a priority in emergency care. The time to first administration of analgesia is an important quality-of-care indicator in EDs [45]. There are national targets in place to improve this aspect of care. In Australia, New Zealand, and the USA, the national target for time to analgesia is 30 min from time of arrival [46,47] and, in the UK, it is 20 min [48]. When compared with physicians, NPs were observed to have greater adherence to the recommended targets for administering analgesia in a timely fashion [29]. In their provision of a hybrid model of care amalgamating nursing and medical tasks, NPs are trained to perform patient assessment and, in some countries, have prescription rights. These factors contributed to a shortened time to treatment in the emergency setting for patients [29].
The experience of the patient is highly valued in the healthcare system [49]. This review showed that patients' level of satisfaction was not dependent on whom but how the care was delivered [30,31]. NPs were rated to perform better at patient education, answering queries, listening, and pain management than physicians [31]. These are the strengths of NPs, consistent with the NP goals and education, which are grounded in nursing [43,50].
Cost savings are an important outcome measure in evaluating the feasibility of any new service model [51]. Findings from this review suggest greater cost savings with the implementation of the advanced nursing practice role in emergency or critical care [34,36,41]. However, judicious interpretation of the evidence is recommended. A fair synthesis of the cost savings in the included studies could not be performed as they had been done in different countries. The varying financial and funding models make it difficult to synthesize the findings. Furthermore, none of the studies in this review performed any cost-effectiveness analysis.
The existing evidence has demonstrated the positive impact of advanced nursing practice roles in the emergency and critical setting, it is then of benefit to examine the necessary conditions for its implementation and receptivity. According to Pettigrew et al.'s "receptive contexts for change" framework ( Fig. 2), there are eight dynamically linked factors which influence the receptivity to change [52]. Three of which are especially apparent in the studies featured in this review. They are namely the presence of environmental pressure, supportive organizational culture, and managerial-clinical relations.
Environmental pressure can be especially pivotal in creating favorable conditions for change. When considering environmental pressure, besides the entire healthcare system, the political context of the country has an integral role in defining the environment [53]. Political influence, a large environmental pressure, was evident in the studies conducted in the USA [35][36][37][38][39][40][41] and UK [34]. In the USA, the implementation of the Affordable Care Act in 2010 was a catalyst for the development of more efficient healthcare delivery models to cope with the projected influx of new patients. In the UK study [34] featured in this review, political influence was also observed. The enactment of provisional immigration laws for physicians outside of the European Union and the European Working Time Directive has make it more difficult to support safe staff-to-patient ratios in the critical care setting. The political context of the country created an environmental pressure which consequently compelled the institutions [34][35][36][37][38][39][40][41] cited in this review to capitalize on nurses in advanced practice and experiment with new models of care delivery.
The environmental pressures trigger the development of a supportive organizational culture to effect change to ease the pressure [52]. A supportive organizational culture strives to promote staff engagement [53]. Staff engagement involves autonomy to be extended, and it was apparent in the included studies. In this review, the NPs were given greater autonomy to either practice independently [28-31, 33, 34] or collaborate [32,[35][36][37][38][39][40][41] with physicians at greater extents in the emergency and critical care settings. In these studies, the institutions' willingness to take risks and evaluate new workforce utilization strategies possibly led to the successful implementation of the advanced practice nursing role [53].
Effective managerial-clinical relations is also a crucial factor in leveraging institutional change [53]. In the study conducted in Canada [31], the authors attributed the success observed in the NP role implementation in the postoperative cardiac surgery unit to the support from and collaboration between the administrators and clinical staff.
As the NP role was fairly new in the study's [31] setting then, it was necessary to involve individuals at all levels in the NP role implementation to optimize its success [54]. One approach to facilitate effective managerial-clinical relations is through adopting a distributed model of leadership [55], which encourages collaboration between the administrators and clinical staff. The distributed leadership approach is known to be most efficacious where job roles are mutually dependent [56]. The implementation of advanced practice nursing roles in the emergency and critical care settings involves mutually dependent job roles and so will benefit from the distributed leadership approach. The distributed leadership approach utilizes a bottom-up process, where individuals working in the setting-of-interest participates in decision-making [55]. Using this approach creates the notion of co-construction, which avoids the overreliance on a dominant individual, increasing the likelihood for sustainable change [54].
The quality and coherence of policy is one factor in the receptive context framework [52] which was not discussed in the included studies but is vital in the implementation of the NP/APN role. The lack of coherent policy to define the roles and professional boundaries of advanced nursing practice can cause healthcare administrators to be apprehensive about the implementation of healthcare models where NP/APNs are given more autonomy and responsibilities [57][58][59]. State law governs advanced nursing practice and define supervisory requirements [60]. Often, the legal frameworks lack clarity on the legal accountability of physicians, should nurses under the physicians' supervision commit errors harmful to patients [61,62]. Professional indemnity is closely associated to legislative boundaries [63]. The successful implementation of the NP/APN role hinges on the institution of relevant regulatory frameworks and credentialing systems to guide policy implementations and educational establishments [64]. It, therefore, reiterates the importance of having coherent policies to define roles and professional independence of nurses in advanced practice.

Limitations
The meta-analysis of the outcomes was not done to present the combined effect of estimates on the impact of advanced nursing roles in the emergency and critical care settings. Yet, to perform a meta-analysis would be inappropriate as the included studies were heterogeneous in designs, interventions, and outcome measures. The heterogeneity of studies was expected as the professional boundaries of nurses differ across countries. However, a review of the impact of advance nursing practice across countries is still valuable.
A limitation in all studies is the poor definition and description of the scope of advanced nursing practice. In addition, preparatory training for nurses to assume advanced practice was rarely discussed. The level of theoretical knowledge and clinical competence of the nurses Finally, despite the search across nine international databases, this review included papers in only English; relevant papers not published in English might have been omitted.

Conclusion
Capitalizing on nurses in advanced practice to increase patients' access to emergency and critical care is appealing and beneficial. This review suggests that the implementation of the NP/APN role in the emergency and critical care settings improves patient outcomes. The transformation of healthcare delivery through effective utilization of the workforce may alleviate the impending rise in demand for health services. Nevertheless, it is necessary to first prepare a receptive context to effect sustainable change.

Additional files
Additional file 1: Search strategy.