The question of the utilization of ICU beds is ultimately one of triage: understanding who is to be admitted to an ICU bed and why, and looking to optimize the use of this expensive resource while providing appropriate care for individual patients. The use of beds is dependent on a number of specific factors: (1) the physical number of ICU beds in a specific ICU, hospital, or system; (2) the ICU bed to hospital bed ratio; (3) other options for care of patients, such as intermediate care units; (3) the casemix of patients cared for in a specific hospital or system, including specific elective surgical patients; (4) hospital culture; and (5) regional culture and norms.
The number of ICU beds: The total number of ICU beds available will clearly have an impact on the use of those beds.7 This may be due to a number of factors related to the absolute number of beds. First, a hospital with only a few (3-4) ICU beds will not have a high volume of critically ill patients and is likely to have a system for transferring their critically ill patients to larger centers.8 Some countries or systems may have a formalized regionalization system (such as for trauma),9 whereas others may be more informal.8 However, such systems mean that small ICUs may tend to care for patients with only a low severity of illness.10,11
Small ICUs or systems of ICUs are also at a disadvantage due to their size, leading to inefficient use of ICU beds. This is due to the concept of queuing theory, which demonstrates that the likelihood of a patient admission being delayed is a function of the occupancy and the total number of ICU beds.12 For example, an ICU that has only 4 beds must operate at 75% occupancy in order to ensure an available bed for the next patient. In contrast, an ICU with 100 beds operating at 75% occupancy has 25 beds available for patients.
The potentially large implications of high occupancy for ICU bed utilization have now been documented in a number of studies. A study by Stelfox et al13 from Canada demonstrated that when beds are not immediately available, more patients have alterations in their goals of care, with no detectable difference in overall mortality, suggesting that occupancy may drive physicians and patients to choose appropriate alternative care paths. However, the knock-on effect of high occupancy may also depend on the country and the overall availability of beds. For example, data from the United Kingdom suggest that many patients are discharged prematurely due to chronic high occupancy and intense pressure regarding new admissions, with worse hospital mortality for the patients discharged prematurely.14,15 In contrast, data from the United States suggest that premature discharges associated with “strained” ICUs do not result in increased mortality, and may consequently represent more efficient use of ICU beds.16
The ICU bed to hospital bed ratio: Little is understood about the ICU bed-to-overall hospital bed ratio and how this affects ICU bed use. We do know that most systems/countries operate on a relatively fixed ratio of ICU beds to hospital beds of approximately 2 to 5 ICU beds for every 100 hospital beds in total, while the United States operates with a very different ratio of 9 to 10 ICU beds per 100 hospital beds in total.7 This high ICU bed to hospital bed ratio in the United States may impact ICU bed utilization by decreasing the threshold for use, particularly if other lower acuity beds become the scarce resource. Data from the United States suggest that 40% of patients admitted to an ICU have monitoring needs only (no active treatment), and that only 35% of these patients are considered at high risk of needing active treatment during the ICU stay.17 Similarly, a study by the Veterans Affairs system in the United States found that many patients (up to 50%) admitted through the emergency room with a predicted risk of death of less than 2% were admitted to ICU beds.18 This is enabled by an overall high availability of ICU beds and a generous ICU bed to hospital bed ratio, as described above, but also may be driven by inadequate nursing or other resources in other parts of the hospital.
Other options for care of patients: The question of other care options for patients is an important one to understand ICU bed utilization. Intermediate care (also called stepdown care, or high-dependency care) has received little attention in the ICU literature. Yet in a survey of 40 hospitals as far back as 1995, 63% of hospitals reported at least one intermediate care unit.19 These types of units provide care at a higher level than available in a general ward, but usually without the ability to provide full organ support, such as mechanical ventilation. Evaluation of their use has mostly focused on elective surgical patients,20,21,22 but, clearly, the utilization of ICU beds in a system depends on the availability of such beds. The availability of intermediate-care beds may impact both the time patients need to spend in an ICU bed prior to discharge and also the need to admit a patient to an ICU bed at all, particularly when the focus is on better monitoring and/or nursing care, rather than full organ support.
Casemix of patients: The casemix of patients admitted to an ICU can vary dramatically.23 This may be driven by the overall availability of ICU beds, but may also be determined by the casemix of patients in a hospital or system. This point is related to the one above regarding intermediate care beds, as some individual patients are more amenable to care in alternate settings. One large driver of the routine utilization of ICU beds is specific surgical programs in a hospital. For example, a hospital that routinely performs liver transplants will use ICU beds for the care of those patients (sometimes) before and (always) after the surgical procedure. Similarly, a hospital that has a large population of oncology patients may expect a certain requirement for ICU beds for patients developing complications from chemotherapy or surgery.24
Specifics of the hospital culture: Use of ICU beds is often driven by the specific culture of a hospital regarding the “perceived” need of an individual patient. For example, in a study of patients admitted to hospitals in New York State with diabetic ketoacidosis, anywhere from zero to 100% of patients received intensive care during the hospitalization.25 Although much of this variation may be driven by the factors described above, such as availability of other appropriate care settings and high-level nursing on wards, at least some of this variation is likely attributable to accepted practices. There are similar findings for other diagnoses, such as patients with carotid endarterectomy.26 One study examined outcomes using their standard practice of admitting almost all (98%) of the patients to the ICU for monitoring after a carotid endarterectomy versus admission based on assessment of risk (22%) and found no difference in outcomes, reinforcing the idea that practices regarding ICU bed utilization may be based on the perceived, rather than actual, need.26
Regional culture and norms: Use of ICU beds may be driven not only by the culture or expectations for care within a specific hospital setting, but also by larger cultural expectations. For example, data from the ETHICUS study carried out in Europe found a large variation in the practice of withholding or withdrawing of treatment before death, depending on the region of Europe,27 and comparison work of cultural expectations in the United States and United Kingdom shows stark differences in expectations and experiences.28 Some of these cultural norms may be codified into laws that then underpin patterns of ICU bed utilization. In some countries, physicians may make decisions regarding escalation of care and/or end-of-life care choices, such as the placement of a do-not-resuscitate order, without much input from families, while other countries or regions mandate that patients or families must agree to the proposed care plan.3