What Is an Intensive Care Unit?

July 9th, 2025

An intensive care unit is an area of a hospital in which critically ill people get highly specialized care, such as monitoring of vital functions and advanced life support. In some hospitals, the ICU may be called a critical care unit, an intensive treatment unit or an intensive therapy unit. These terms all mean essentially the same thing. 

Since the care provided in an intensive care unit is so specialized, there are a number of different types of ICUs. Some of the most common include:

• Coronary care or coronary/thoracic units (CCU or CTU), for patients who have had heart surgery or suffered a heart attack or other serious heart-related event.
• Surgical intensive care unit (SICU) for seriously ill patients who have had surgery
• Medical intensive care unit (MICU), for patients who have serious medical issues, such as liver, kidney or respiratory failure
• Long-term intensive care unit (LTAC ICU) for critically ill patients who require long-term care (for example, patients who are in a persistent vegetative state and need chronic ventilator support
• Pediatric intensive care unit (PICU) for critically ill children
• Neonatal intensive care unit, (NICU) for seriously ill and premature newborns

Some hospitals also house other specialized units, such as a burn, trauma or neurosurgery intensive care units, which deliver critical care to patients with life-threatening injuries. 

It’s worth noting, however, that not all hospitals in the U.S. have intensive care units, a deficiency highlighted by the SARS-COV-2 (COVID-19) pandemic of 2020-2022. According to data from KFF Health News, more than half of U.S. counties have no ICU beds at all, and a startling number have no hospital or emergency room. Additionally, the number of ICU beds per capita varies enormously, even within states, with the highest concentration being in wealthy urban areas. Rural and economically depressed urban areas are less likely to have either a hospital or a hospital with an ICU. 

The History of  ICU Care

The concept of the ICU dates back to the early 1950s, when the polio epidemic surged across the globe. The disease had been recognized since the late 1800s, and sporadic epidemics, mostly in the summer months, had claimed thousands of lives. Victims died of respiratory paralysis and bulbar palsy, which made it impossible for them to swallow their own secretions or to breathe effectively on their own. The only treatment available for these critically ill patients was an iron lung or a Cuirass ventilator, both of which used alternating air pressure to stimulate breathing. But there were far too few of these devices available, and they were woefully inadequate at saving lives. As a result, the mortality rate for polio patients with respiratory failure was close to 90%. 

Then, in 1952, the polio epidemic hit Copenhagen, Norway, and the city’s only infectious disease hospital was overwhelmed. The hospital had just one iron lung and six Cuirass ventilators, and there were hundreds of patients in respiratory failure needing care. Desperate for a solution, the chief physician at the hospital, Dr. Henry Cai Alexander Lassen, reached out to a colleague, Dr Bjorn Ibsen, an anesthesiologist. Specifically, Lassen wanted to explore the possibility of using positive-pressure ventilation to treat patients with respiratory paralysis due to polio. The technique had been used to support anesthetized patients in operating rooms for years, and Lassen hoped that Ibsen could provide some guidance on using it to save polio victims’ lives. 

Two days after the two doctors met, Ibsen performed a tracheostomy on a 12-year old girl suffering from polio-induced respiratory paralysis and bulbar palsy. After inserting the tracheostomy tube, he began manually inflating the girl’s lungs with a collapsible bag like those used during surgery. The girl’s condition immediately improved. 

Encouraged by the girl’s response,  Ibsen and Lassen devised a bold plan. They placed tracheostomies in the most seriously ill polio patients in the hospital at the time and recruited medical students to manually ventilate them around the clock — literally standing at the bedside, squeezing a bag delivering a mixture of oxygen and nitrogen to their failing lungs. According to one report, at the height of the epidemic, there were at least 250 medical students caring for over 70 patients at the same time. 

Somewhat astonishingly, the approach was successful. Despite the lack of sophisticated equipment and the dreadful amount of labor involved, the mortality rate for polio patients with respiratory paralysis in Copenhagen dropped from close to 100 percent to about 40 percent within the year. Encouraged, Ibsen proposed that all such patients be housed in a dedicated unit, each with their own nurse. This dedicated unit at Blegdam Hospital in Copenhagen opened in December 1953, becoming the prototype for the modern-day ICU. 

Since Ibsen first introduced the ICU concept, technological advancements, including mechanical ventilators, invasive monitoring techniques, and advanced life-support systems, contributed to the rapid evolution of ICU care. As of this writing, there are approximately 6,000 ICUs in the U.S. housing a total of about 68,000 beds. 

What Types of Patients Are Treated in the ICU?

Patients are admitted to the ICU with many different types of illnesses and injuries. According to the Society for Critical Care Medicine, the conditions most commonly seen in adults who require ICU care are:

• Respiratory illnesses such as pneumonia that require supplemental oxygen, a breathing tube and a breathing machine
• Acute myocardial infarction, also known as a heart attack
• Stroke due to bleeding or a blood clot in the brain
• Post-operative care following extensive surgery
• Post-operative care following a percutaneous cardiovascular procedure with a drug-eluting stent (a procedure to open a blocked artery in the heart)
• Septicemia or severe sepsis (an overwhelming blood infection)

Additionally, many people are admitted to the ICU for trauma related to an accident, such as a car crash, a fire, or a severe fall, or an intentionally inflicted injury such as a gunshot wound. Poisoning due to drug overdose; heart, respiratory and/or kidney failure; gastrointestinal bleeding; and complications of diabetes may also land a person in the ICU. 

In children, the most common reason for ICU admission is respiratory illness, according to the SCCM. Additionally, over 50% of children who are treated in the PICU have complex chronic conditions, such as cerebral palsy, chromosomal abnormalities and congenital heart defects. Neonates, on the other hand, are mostly admitted to the NICU for low birth weight secondary to prematurity. The usual criteria for admission is birth weight under 1,500 grams (3.3 pounds) and/or severe prematurity requiring ventilator support. Full-term, normal birth weight infants may require intensive care when they inhale amniotic fluid containing meconium, a condition known as meconium aspiration syndrome. Neonatal sepsis and a deadly condition known as necrotizing enterocolitis also require ICU care. 

What Kind of Care Is Provided in the ICU? 

In the ICU setting, patients are cared for by a highly skilled team of caregivers led by an intensivist, a physician who is board-certified in critical care. Unlike other physician specialists, who typically focus on one body system (for example, the heart, the lungs, the kidneys, or the liver), these doctors are skilled at managing multiple comorbidities — medical conditions that occur in the same person at the same time. The intensivist calls on other specialists as the need arises and works with them to coordinate patient care. 

In addition to doctors, the ICU team typically consists of specially trained nurses who are experts in critical care, respiratory therapists, and one or more pharmacists. A physical and/or occupational therapist, a nutritionist,, a social worker and a chaplain are often part of the team as well, and are called in to consult depending on the patient’s needs. 

Once admitted to the ICU, patients typically undergo an array of highly invasive treatments and procedures. According to the Critical Care Innovations Group at the University of California, San Francisco, some of the most common of these are:

• Intubation and mechanical ventilation — a procedure in which the doctor inserts a tube into the patient’s windpipe, which is then attached to a machine that helps the person breathe, or breathes for them if they can’t breathe on their own.
• A central venous catheter —  similar to an intravenous catheter inserted into a small vein in the arm or hand,  a central venous catheter (also called a central line) is a much larger catheter that a doctor inserts into a large vein in the arm or the neck. This larger catheter lets the ICU staff administer large amounts of fluids, medications and/ or blood products to the patient quickly and safely. It also allows the staff to monitor the blood pressure in a large vein that is close to the heart, which makes it easier to titrate medications and fluids effectively.
• Vasopressors —  medicines that help stabilize a person’s blood pressure when they are in shock. Shock is a state in which a person’s blood pressure is too low to maintain blood flow to vital organs, such as the heart, lungs, kidneys and brain. Shock can result from a number of conditions, including infection, heart failure, blood loss, and trauma. 
• Continuous renal replacement therapy, (CRRT or continuous dialysis) —  a procedure in which the patient is attached to a machine that continuously removes excess fluid and toxins from their blood. CRRT is used to treat acute kidney failure, which is a common development in patients who are seriously ill. 

Unfortunately, the highly invasive nature of ICU care can lead to a host of complications, which can both exacerbate the patient’s underlying condition and create new problems that didn’t exist before. Some of the most common and most serious of these complications are infections, including ventilator-associated pneumonia and blood infections, which often arise from a central venous catheter. Urinary tract infections are also common due to the use of indwelling urinary catheters to measure urine output. These infections typically start in the bladder, but they can spread to the kidneys, where they can cause serious complications, sepsis, and even death. ICU patients who undergo surgery may also develop surgical site infections, especially if the surgery involves the repair of a traumatic injury. 

Nor are infections the only serious complications ICU patients encounter. Acute kidney injury occurs in up to 50 percent of patients, which can progress to acute kidney failure necessitating CRRT. Elderly patients who have heart or liver failure or preexisting kidney disease are at greatest risk, as are patients who receive drugs known to damage the kidneys, such as certain antibiotics, NSAIDS and contrast dyes. Infections, sepsis, shock and the need for ventilator support are also associated with a higher risk of kidney damage in the ICU. According to a report published in the journal Chest in 2018, about 5- 10% of patients who develop acute kidney injury will progress to acute kidney failure, and between 30 and 70% of those patients will die. 

ICU patients may also develop stress ulcers that result in severe gastrointestinal bleeding, and venous thromboembolism or VTE. A life-threatening complication, VTE occurs when a blood clot forms in a vein in the leg, groin or arm (this is known as a deep-vein thrombosis or DVT) and travels to the lung, causing a deadly pulmonary embolism. Today, most ICUs use a variety of methods to try to prevent VTE, including compression stockings, intermittent pneumatic compression devices and  “blood thinning” agents such as heparin. 

Ventilated patients are also at risk of pulmonary barotrauma —  lung damage caused by excessive pressure within the lungs. This may occur due to high-pressure mechanical ventilation or by ventilator settings that are changed too rapidly. In either case, the small air sacs in the lungs that are responsible for gas exchange rupture, causing air to enter the chest cavity and restrict breathing. This is a medical emergency that typically requires the insertion of a chest tube. 

Lastly, but by no means less importantly, medical and nursing errors are common in the ICU due in large part to the complexity of care. According to one study from 2008, the vast majority of these (78%) are medication errors. However, errors involving other aspects of patient care, such as fluid management, maintenance of blood pressure, and communication between caregivers are common as well. Not all of these result in adverse patient outcomes, but many of them do.

Obviously, being cared for in the ICU is fraught with risk, though there is no doubt that — in the right circumstances — ICU care can save lives. Accurate data about the number of patients who are discharged from the ICU alive is difficult to come by, since outcomes vary widely depending on the hospital and the characteristics of patients it serves. But according to an article published in the journal Critical Care in 2013, mortality for all ICU patients has been falling for the last several decades, and now stands at about 12 percent. (Although a more recent analysis places the number at about 22 percent.) It would be foolish, however, to assume that this number applies to all patients admitted to the ICU. Age, illness severity, and diagnosis are important factors that greatly influence a person’s chance of surviving an ICU stay. Still, the numbers suggest that the vast majority of patients who enter the ICU will be discharged alive. 

Sadly, however, the truth is that being discharged from the ICU doesn’t mean that a person is “out of the woods”. According to an analysis of over 40,000 patients who were discharged from the ICU over a seven year period in Wales, about one in five patients died within one year. About half of these patients died before leaving the hospital, and most deaths occurred within three months. The risk of dying was highest in people who had spent the most time in the hospital, those who were elderly, and those who suffered from multiple disease processes. Patients who lived in low-income communities or communities without a local hospital had a greater chance of dying as well.  

In conclusion, the ICU is a vital component of the U.S. healthcare system, specifically designed to care for patients with life-threatening illnesses and complications The conditions treated in these units vary widely, encompassing severe trauma, respiratory distress, cardiac crises, and post-operative complications, among others. While the survival rates fluctuate depending on the condition, severity, and individual patient factors, the ICU’s primary goal remains providing specialized, intensive care to enhance patients’ chances of recovery. The evolution of the ICU is a remarkable testament to the advancements in medical technology, along with the growing understanding of patient care needs and high-quality healthcare standards.

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