What is the ICU?

What is intensive care?

Intensive care, also known as critical care, is a multidisciplinary and interprofessional specialty dedicated to the comprehensive management of patients having, or at risk of developing, acute, life-threatening organ dysfunction. Intensive care uses an array of technologies that provide support of failing organ systems, particularly the lungs, cardiovascular system, and kidneys. Although the specialty has developed expertise in the comprehensive management of disorders such as sepsis and the acute respiratory distress syndrome, its common expertise is the pathophysiology and support of organ dysfunction more than the specific management of the diseases responsible for the acute illness; the primary goal of intensive care is to prevent further physiologic deterioration while the underlying disease is treated and resolves.

Training models for physicians providing intensive care have been the focus of another task force of the WFSICCM. In some countries, intensive care is a distinct medical specialty, with a dedicated training program that starts following graduation from medical school. More commonly, aspiring intensivists obtain additional expertise in intensive care after completing specialist training in another discipline, for example, anesthesia, surgery, pulmonology, emergency medicine, or pediatrics. In some countries, the 2 paths oexist. To define intensive care as a multidisciplinary specialty is to recognize that although its practitioners share common expertise in the management of acute organ system insufficiency, they may also come from various specialty backgrounds that provide additional clinical expertise.

Intensive care is not just a clinical specialty but a system of care delivered by a skilled interprofessional team that includes physicians, nurses, respiratory therapists, physiotherapists, pharmacists, microbiologists, social workers, ethicists, spiritual care, and many others.

The definition of an ICU bed varies around the world, and even within a single health care system, and is heavily shaped by public health priorities and regulatory requirements, as well as by economic factors, including resource availability and approaches to hospital and physician reimbursement. A number of professional bodies and health care funders have sought to enumerate the specific features that make care intensive and so differentiated the care provided within an ICU from that delivered throughout a health care facility (Table 1).

Table 1:  Definitions of an ICU

The Intensive Care Society (UK) 2013 An ICU is a specially staffed and equipped, separate, and self-contained area of a hospital dedicated to the management and monitoring of patients with life-threatening conditions. It provides special expertise and the facilities for the support of vital functions and uses the skills of medical, nursing, and other personnel experienced in the management of these problems.
Smith and Nielsen 1999 A service for patients with potentially recoverable conditions who can benefit from more detailed observation and invasive treatment than can safely be provided in general wards or high-dependency areas.
Society of Critical Care Medicine 1999 ICU serves as a place for monitoring and care of patients with potentially severe physiological instability requiring technical and/or artificial life support. The level of care in an ICU is greater than that available on the floor or intermediate care unit
Haupt et al 2003 Provide services and personnel that ensure optimal care to critically ill patients
Royal College of Anaesthestists 2009 Intensive care medicine, also referred to as critical care medicine, is that body of specialist knowledge and practice concerned with the treatment of patients with, at risk of, or recovering from potentially life-threatening failure of one or more of the body organ systems. It includes the provision of organ system support; the investigation, diagnosis, and treatment of acute illness; systems management and patient safety; ethics; end-of-life care; and support of families.
Valentin and Ferdinande 2011 The ICU is a distinct organizational and geographic entity for clinical activity and care, operating in cooperation with other departments integrated in a hospital. The ICU is preferably an independent unit or department that functions as a closed unit under the full medical responsibility of the ICU staff in close concert with the referring medical specialists. It has a defined geographical location concentrating the human and technical resources, such as manpower, professional skills and competencies, technical equipment, and the necessary space.
College of Intensive Care Medicine of Australia and New ZealandMinimum Standards for Intensive Care Units. College of Intensive Care Medicine (2011) 2011 An ICU is a specially staffed and equipped, separate, and self-contained area of a hospital dedicated to the management of patients with life-threatening illnesses, injuries, and complications, and monitoring of potentially life-threatening conditions. It provides special expertise and facilities for support of vital functions and uses the skills of medical, nursing, and other personnel experienced in the management of these problems.
Joint Faculty of Intensive Care Medicine of Ireland and Intensive Care Society of IrelandNational Standards for Adult Critical Care Services, p1-19 (2011) 2011 Critical care units provide life-sustaining treatment for critically ill patients with acute organ dysfunction due to potentially reversible disease. It is the purpose of the unit to support the patient’s failing organs and diagnose and treat the underlying cause. Patients at risk of organ dysfunction due to chronic disease processes may also benefit from critical care in the perioperative period or other temporary reversible circumstance, eg, after trauma.

What is an ICU?

A global definition of an ICU must acknowledge the wide international variability that currently exists in the capacity to care for the sickest patients in the health care system. In describing the elements that make care intensive and in grouping these to stratify ICUs on the basis of their capacity to provide that care, we recognize that intensive care is not an absolute concept but rather a relative one defined in relation to the realities of a particular health care system that can vary depending on available resources and approaches to care.

The factors that make clinical care intensive and that delineate the provision of that care as separate from routine clinical care can be classified into 5 domains.

Physical space

Although critical care is increasingly delivered outside the physical confines of an ICU, the presence of a discrete geographic locale within a hospital or health center where the sickest patients can be cared for is central to the definition of an ICU. A discrete physical space enables the concentration and efficient sharing of technologies and expertise so that all patients benefit maximally. Detailed design considerations are beyond the scope of this report and are described in greater detail elsewhere.

The physical space of an ICU should be large enough to comfortably accommodate the desired number of ICU beds. Each bed needs access from all sides to allow patient assessment and treatment and effective and reliable implementation of infection control measures. Ideally, each patient should be cared for in a single-bed room. Each room should have a sink and incorporate design features to accommodate and provide easy access to supportive devices such as a ventilator and dialysis machine, as well as monitors. Oxygen and a suction system, preferably from a built-in system, should be available, and a source of natural light is desirable. Individual beds and rooms should be arranged in such a way that they are visible from a central nursing station and, where possible, should have sufficient space to allow family members to visit and even participate in the care of the patient. The ICU ideally will have one or more negative-pressure rooms, with the capacity for isolating patients with airborne infections. Separate areas should be available for mixing and dispensing medications and for cleaning and storing used devices such as bronchoscopes and ventilator circuits.

A central nursing station should reproduce data from individual patient monitors. A capacity to record and interpret patient data over time must be available, whether in paper or electronic record form. Multiple computer stations should ideally be available to provide access to electronic medical records, as well as to facilitate communications and permit access to bibliographic databases. One or more seminar rooms and sleeping facilities for on-call staff should be available in immediate proximity to the ICU. Finally, a waiting area for family members and a quiet room where patient updates can be provided to family and loved ones are central to effective ICU communications.

Support and monitoring technology

The ability to perform continuous monitoring of patient physiologic status is a key factor differentiating intensive care from ward-based hospital care. Monitoring may be noninvasive (transcutaneous oxygen saturation, noninvasive monitoring of heart rate and blood pressure, or continuous electrocardiogram or electroencephalogram monitoring) or invasive (hemodynamic monitoring, monitoring of intracranial pressure). Data should be continuously displayed so that they are readily accessible to all involved in caring for the patient, and recorded so that clinicians can monitor trends and respond appropriately.

Respiratory support available within the ICU may range from supplemental oxygen delivered by mask or high-flow oxygen system to conventional and nonconventional modes of mechanical ventilation; in some ICUs, this may include extracorporeal membrane oxygenation (ECMO) or carbon dioxide removal. Hemodynamic support is largely pharmacologic and guided by data generated from continuous monitoring of hemodynamic parameters; additional modes of support include cardiac pacing, mechanical cardiac support with intraaortic balloon counterpulsation or ventricular assist devices, and ECMO to provide tissue oxygenation. Renal support in the form of intermittent or continuous renal replacement therapy is commonly needed, as is nutritional support provided enterally using a feeding tube or parenterally. There is a continuous need to relieve pain and anxiety and to prevent and treat delirium.

The specific monitoring and support capabilities of a particular ICU will depend both on available resources and on the nature of the patient population served—in hospitals with subspecialty ICUs, a neurosurgical ICU will look different from a burn unit or a transplant ICU.

Human resources

The clinical team providing care in an ICU is specially qualified, interdisciplinary, and interprofessional. The care it provides is more intensive than that provided elsewhere in the hospital by virtue of a more intense and more immediate interaction between members of the team and the critically ill patient.

Ideally, members of both the medical and nursing teams will have advanced specialty qualifications in intensive care medicine, as well as ample experience in the care of the critically ill. In addition to physicians and nurses, team members may include nurse practitioners, respiratory therapists who manage the mechanical ventilator, physiotherapists who support mobility and rehabilitation, a nutritionist skilled in the enteral and parenteral feeding needs of complex patient populations, a pharmacist with particular expertise in drug-drug interactions and optimal dosing in the critically ill patient, a social worker who can support the needs of both patient and family, and many others including microbiologists to assist with the diagnosis and management of infection and spiritual care personnel to support patients and families during a time of crisis.

Patient acuity necessitates that medical staff be immediately available to manage emergencies and to make a myriad of decisions in a rapidly changing clinical situation. Even during the day when more medical staff are available, it is important that the number of patients each physician is responsible for is constrained so that adequate attention is given to the needs of each. Continuous nursing care is also essential, and provided at a nurse to patient ratio that is higher than elsewhere in the hospital, and appropriate to the current needs of the particular patient. The optimal ratio will, as well, be dictated by the availability of other support personnel including nursing assistants and respiratory therapists. The nurse to patient ratio may, in some jurisdictions, be dictated by legislation or labor contracts.

Coordination of a large interdisciplinary team requires a well thought out administrative structure with a designated medical director as well as a nursing director who will coordinate staffing and care needs and establish policies and priorities for ongoing patient care.

Critical care services provided

An ICU is also defined by the services it provides beyond the immediate demands of individual patient care.

Within the hospital, the ICU provides the capacity to care for acutely unstable patients—to evaluate, resuscitate, and support the patient using the most advanced facilities that the hospital can provide. However, an ICU also plays a role in the larger health care system. It may serve as a referral center for smaller hospitals that lack ICU facilities. In a larger community, it may provide specialty services not available in other ICUs—neurological support and monitoring, advanced trauma care, management of complex respiratory failure, etc. Conversely, units capable of providing an intermediate level of care, variously known as high-dependency unitsstep-down or step-up units, and intermediate care units, can provide support and monitoring to patients who do not need the full spectrum of ICU supportive care; these are appropriately considered an element of in integrated intensive care program.

Increasingly, the ICU team is taking a larger role in the management of patients on the hospital wards or in the emergency department. Outreach teams provide consultative services on the wards and assist medical and surgical teams in managing clinical situations where more intensive, timely, and coordinated interventions can prevent deterioration and a need for ICU admission. For example, sepsis teams provide support in the initial management and support of unstable patients with sepsis.

Increasingly, survivors of ICU care are being seen following hospital discharge in dedicated follow-up clinics that not only ensure continuity of care following devastating illness but provide important insight into the long-term sequelae of that illness.

Finally, critical care practitioners gain substantial expertise in supporting patients and their families through the process of death and dying when it has become apparent that continuing intensive care can do nothing more than prolong the process of dying, and this skill is frequently harnessed not only during the dying process but also in early discussions to clarify patient wishes and perspectives at the end of life or in the face of a significant risk of death.

Research, education, and quality improvement

A functional ICU, like any other specialized component of the health care system, has an inherent obligation to continuously improve patient care based on an ongoing evaluation of the shortcomings of the care it provides and the changing knowledge base that informs best care. This is the domain of continuous quality improvement. It also has a responsibility to contribute to the processes of evaluating best care on the one hand and sharing that information on the other; these are the domains of research and education, respectively. The extent to which an ICU can engage in quality improvement, research, and education will vary; nonetheless, engagement in these processes should be an aspirational goal of all ICUs for they lead inexorably to better patient care and improved clinical outcomes.


Toward a classification of ICUs

We propose a categorization of ICUs based on a variety of parameters that reflect the potential of the unit to provide excellent and expert care to the most acute seriously ill patients. Stratification is based on 12 variables:


  1. Availability of skilled medical personnel
  2. Availability of skilled nursing personnel
  3. Availability of other specialists—respiratory therapists, physiotherapists, pharmacists, nutritionists, etc.
  4. Capacity to monitor acutely ill patients
  5. Availability of resources for the support of failing organ function
  6. Design and structure of the physical space
  7. Integration with ICU outreach services—in the emergency department and hospital ward, as well as services for follow-up of discharged patients
  8. Presence of formal educational and professional development services for staff
  9. Presence of dedicated house staff and role as a center for training expert personnel
  10. Capacity for research and quality improvement activities
  11. Role in acting as a referral service for the hospital, the community, and the country
  12. Ability to scale up services in response to a natural or human-made disaster or pandemic outbreak.

Most frameworks for classifying ICUs use a numeric scale, with 3 being the top level. We have followed this precedent and classify ICUs as primary, secondary, and tertiary (Table 2).

Table 2: A proposed classification of ICUs

Level 1 Level 2 Level 3
Therapeutic capacity Physiologic stabilization and short-term support of mild organ dysfunction Basic support of failing organ function Complex, comprehensive support and management of organ dysfunction
Personnel Physicians with some experience in critical care available at least during the day Physicians with ICU training or comparable experience present during day and available at night Physicians with formal ICU training on call 24/7; immediate in-hospital availability of medical staff with ICU experience
Experienced nurses provide 24/7 care Nurses have extra training or comparable experience in critical care and provide 24/7 care Nursing staff with specialist ICU training provide 24/7 care
Other personnel available Variable inclusion of allied health personnel—respiratory therapists, physiotherapists, dieticians, pharmacists, etc—as part of ICU care team Allied health personnel—respiratory therapists, physiotherapists, pharmacists, dieticians, etc—as regular members of ICU team
Nurse-patient ratio higher than on ward; preferably 1:4 or 1:3 (1 nurse for 4 patients) Nurse-patient ratio appropriate to patient needs but usually no less than 1:3 Nurse-patient ratio appropriate to patient needs and no less than 1:2
Daily rounds; ad hoc structure Formal daily ICU rounds with physicians and nurses Formal multidisciplinary ICU rounds daily and as needed based on patient complexity and acuity
Variable engagement in critical care continuing professional education Engagement in continuing professional education Regular engagement in continuing medical/nursing education
Variable access to other medical specialties in hospital Ready access to respirologists, nephrologists, cardiologists, infectious disease specialists, general surgeons Rapid access to and variable engagement of full complement of medical and surgical consultant specialists
Monitoring capacity Noninvasive or minimally invasive monitoring—transcutaneous oxygen saturation, cardiac monitoring, urine output Invasive monitoring of blood pressure and central venous pressures as dictated by patient status Advanced hemodynamic monitoring (cardiac catheterization, ultrasonography, etc); advanced monitoring of pulmonary, cerebral, and other physiology as directed by clinical needs
Blood gas analyzer immediately available Blood gas analyzer and stat lab associated with ICU
Unit design and organ support Dedicated geographic area Dedicated geographic area with central monitoring station Dedicated geographic area with individual patient care areas and central monitoring station
Capacity for oxygen therapy and noninvasive respiratory support Basic mechanical ventilatory support, pharmacologic support of cardiovascular function, intermittent renal replacement therapy, parenteral nutrition Advanced ventilator and hemodynamic support, continuous renal replacement therapy, capacity for tracheostomy and other basic surgical procedures
Capacity for isolation of patients needing contact or airborne precautions
Integration within the hospital Defined geographic area only Ad hoc interactions with other acute care areas such as emergency department Outreach team(s), integration with step-down or high-dependency unit; close collaboration with emergency department
Research and education Ad hoc activity Organized educational activities for staff Formal educational programs for staff
Basic quality improvement program Formal quality improvement program Formal quality improvement program
Ad hoc engagement in clinical research Active involvement in clinical research
Training of residents and fellows as available
Responsiveness to regional and societal needs Ad hoc only, but available and responsive in event of disaster Serves as resource for critically ill patients within hospital Referral resource for community and district hospitals and for other ICUs
Formal policy outlining criteria for patient transfer to higher level ICU Disaster preparedness plan and capacity

Primary (level 1) ICU

The most basic level ICU is staffed by medical doctors who have experience in critical care but may not have formal training. They are readily available at least during the day, and medical expertise is accessible at night. Nurses have some ICU experience with or without additional training, and the nurse to patient ratio is higher than it is on a regular ward of the same facility. Other specialists are available within the hospital but not necessarily a part of the ICU team. The unit has the capacity to monitor vital signs noninvasively on an intensive basis, to monitor transcutaneous oxygen saturation, and to provide continuous electrocardiogram monitoring. Organ support is limited to the provision of supplemental oxygen by mask or by noninvasive positive pressure ventilation or short-term invasive mechanical ventilation; simple mechanical ventilation may also be available for the care of stable chronically ventilated patients. A level 1 ICU consists of a dedicated space within the hospital. There is no formalized outreach service, and educational activities for staff are sporadic; no formal training program for intensivists is in place. The unit has a basic quality improvement program, but may not regularly participate in research, and is a referral service for patients from within the hospital only. There is no formal plan for surge capacity management. A plan outlining criteria for transfer of patients to higher-level more specialized centers should be in place.

Secondary (level 2) ICU

A level 2 ICU is staffed by physicians with specialty training in medicine, surgery, anesthesia, pediatrics, emergency medicine, and other disciplines who may also have formal ICU training; alternatively, staff may have dedicated intensive care training. Staff are present or immediately nearby during the day, and promptly available by phone or in person to handle crises during evenings and weekends. Nursing care is provided around the clock by nurses having some special qualifications in intensive care; the nurse to patient ratio is higher than in other areas of the hospital but typically no less than 1:3. Allied health care professionals are part of the ICU team or readily available, and a commitment to interprofessional practice is reflected in regular multidisciplinary rounds. Continuous monitoring of vital signs is available, including the invasive monitoring of blood pressure, central venous pressure, and transcutaneous oxygen saturation (Spo2). A blood gas analyzer should be immediately available. Modalities of organ support include endotracheal intubation and positive pressure ventilation, the administration of vasoactive medications, and renal replacement therapy. Facilities are available for family members, and educational programs for staff are in place. A level 2 ICU has a formal quality improvement program, with regular review of practices and complications. It serves as a referral center for local hospitals that lack dedicated ICU facilities.

Tertiary (level 3) ICU

A level 3 ICU provides state-of-the-art care to the sickest critically ill patient. It is staffed by medical doctors with specialty training in critical care—either as a stand-alone specialty or following full training in another specialty—with 24-hour in-house coverage by a staff physician, nurse practitioner or equivalent, or critical care trainee. Nursing care is provided by nurses with additional training in critical care, and the nurse to patient ratio is typically 1:1 or 1:2, at least during the day, and guided by patient severity. Additional trained staff contribute to the care of patients, including respiratory therapists, physiotherapists, pharmacists, nutritionists, and microbiologists. Multidisciplinary rounds are conducted daily. A full spectrum of ICU monitoring and support is available, including invasive mechanical ventilation, invasive hemodynamic monitoring and support, and renal replacement therapy. Other specialty services such as ECMO and invasive neurologic monitoring may be available. A level 3 ICU frequently serves as a teaching unit for intensive care trainees and participates actively in quality improvement activities and in clinical research. It serves as a regional referral center for critically ill patients and is expected to have in place plans for pandemics and other emergency situations where an acute increase in demand might be expected. The unit has isolation facilities for patients needing contact or airborne precautions. It has an integrated outreach program that can provide support to patients on the ward or in the emergency department. Ideally it has a formal ICU follow-up program.

Ref: Marshall JC, Bosco L, Adhikari NK, Connolly B, Diaz JV, Dorman T, Fowler RA, Meyfroidt G, Nakagawa S, Pelosi P, Vincent JL, Vollman K, Zimmerman J, What is an intensive care unit? A report of the task force of the World Federation of Societies of Intensive and Critical Care Medicine, J Crit Care. 2017 Feb;37:270-276.