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ICU Capacity Strain: The Need for Standardization of Definitions

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Krzysztof Laudanski, MD, PhD, MA, FCCM; Venu M. Velagapudi, MD

There is an increasing appreciation in the medical community that capacity of the intensive care unit (ICU) is a vital issue in regard to patient recovery, quality of care, and patient and staff satisfaction. The most orthodox definition of capacity is the “ability to provide high-quality care for everyone who is or could become a patient in that ICU on a given day.”1 Capacity is inextricably related to the idea of strain, best defined as a supplydemand mismatch. However, these broad, generic definitions lack granularity and are not easy to operationalize. A recent systematic review of 51 eligible studies (out of a possible 5,297) identified 16 indicators of ICU capacity strain classified across structure, process, and outcome domains.2 The most commonly used indicators are ICU acuity, ICU readmission, after-hours discharge, and ICU census. Definitions were inconsistent across the studies. ICU capacity can also be influenced by institution- and/ or provider-specific patterns. Although such a broad definition is helpful in capturing the idea of capacity as a multidimensional entity, the lack of standardized definitions reduces the number of comparative studies, makes it difficult to identify common bottlenecks, and impedes the analysis of improvements.

Capacity Strain
The multidimensional nature of capacity becomes apparent during any analysis of work flow in the ICU. If the ICU is strained, the insufficient number of beds is usually blamed. Although a physical space to admit the patient with an adequate critical care infrastructure is essential, it is not by any means the only pivotal factor. Further analysis may reveal that a 1:2 nurse-to-patient ratio (or lower) is fixed. Physicians have more flexibility because of their potential ability to provide optimum care to up to 16 patients.3 Advanced practice providers, respiratory therapists, and other critical care clinicians also have more flexibility than nurses in the number of patients they can care for. Therefore, although ICU capacity depends on overall staff allocation, nurse staffing is the primary determinant of resource strain.

While the nurse-to-patient ratio is fixed, respiratory therapists, advanced practice providers, and physicians may be required to care for a variable number of patients with complex critical care issues and diverse needs. Clinician strain escalates with increasing patient census, especially when documentation and regulatory requirements are factored in. Teaching and research necessitate further allocation of resources and contribute to overall strain. Consequently, higher critical care demands reduce capacity and at the same time may result in clinician burnout and unfavorable patient outcomes. Hence, ICU capacity is not limited to how many patients can be admitted and discharged. It is also influenced by downstream factors such as the performance of other hospital units, since ICU patients are often discharged to step-down units or medical units. Hospital patient allocation preferences and policies also impact ICU capacity, especially during times of increasing demand.

Even by this basic analysis, we have identified several definitions of capacity (bed capacity, nurse staffing, physician staffing, and hospital capacity to accept transfers from the ICU) and strain (number of beds, nurse staffing, other clinician staffing, ability to discharge, and burnout). Moreover, other factors that determine ICU strain, such as protocols, staffing patterns, and organizational factors, may not be easily quantifiable. In the context of contingency and disaster planning, ICUs must be prepared for events such as major disease outbreaks or bioterrorism.4 Hence, ICU capacity has to take into account several interactive layers, starting with the patient, moving on to staff, and then to the ladder of organization from unit and hospital to regional and national levels.

Metrics of ICU Strain
ICU strain should be interpreted as a complex, multidimensional entity that can be measured not by a single measure but only by a composite of currently available measures. The current state of affairs calls for evolution of a consensus on: a) definition of ICU strain, and b) data that need to collected to form accurate definitions and standardize indicators that predict ICU strain. The ideal definition of strain capacity indicators should include a clear definition, robust metrics, and linkage to patient and staff outcomes. It is critical that all these steps be quantifiable, preferably in the form of a time series, relevant to patient outcomes and staff performance and satisfaction, and available through existing electronic health records. Such metrics may allow for impartial, objective evaluation of performance individualized to the ICU and clinician and, in the future, may provide inputs to computer-aided decision-making and artificial intelligence systems.

Outcomes of Capacity Strain
The relationship between capacity and clinical outcomes is not straightforward. One study showed that a busier ICU exhibited better patient outcomes, while other research demonstrated some increase in the risk of readmission or mortality when the strain on the ICU was significant.5,6 A strained ICU impacts overall hospital capacity and financial margins as well. Each ICU likely has an individualized strain pattern showing temporal variation contingent on critical care demands at a given point of time.

Future Direction
The importance of capacity strain has been underappreciated by the critical care community and hence has not received the attention and resources it deserves, and has remained at the fringes of mainstream ICU literature. But even this brief review demonstrated the complex nature and clinical relevance of this issue. We propose the initiation of a work group to determine expert consensus on: a) potential candidate variables and data that need to be collected, b) working definition of ICU strain until a more robust data- and outcomedriven definition can be determined c) outcome measures, and d) standardized definitions for indicators and metrics. Ultimately we envision the application of such efforts to the evolution of robust, outcome-driven, and individualized ICU strain parameters. Moreover, quality improvement, ICU operational improvement, prevention of clinician burnout, and regional and federal disaster planning are other attainable goals.

Halpern SD. ICU capacity strain and the quality and allocation of critical care. Curr Opin Crit Care. 2011 Dec;17(6):648-657.
2.           Rewa OG, Stelfox HT, Ingolfsson A, et al. Indicators of intensive care unit capacity strain: a systematic review. Crit Care. 2018 Mar;22(1):86.
3.           California Society for Respiratory Care. Safe Staffing Standards. Watsonville, CA: California Society for Respiratory Care; 2016. Accessed May 24, 2018.
4.           Zilberberg MD, Sandrock C, Shorr A. Swine origin influenza A (H1N1) virus and ICU capacity in the US: are we prepared? PLoS Curr. 2009 Aug 22;1:RRN1009.
5.           Wagner J, Gabler NB, Ratcliffe SJ, Brown SE, Strom BL, Halpern SD. Outcomes among patients discharged from busy intensive care units. Ann Intern Med. 2013 Oct 1;159(7):447-455.
6.           Gabler NB, Ratcliffe SJ, Wagner J, et al. Mortality among patients admitted to strained intensive care units. Am J Respir Crit Care Med. 2013 Oct 1;188(7):800-806.