Critical illness is a condition where life cannot be sustained without invasive therapeutic interventions.
A wide variety of diseases may lead to critical illness; however the number of interventions required is limited.
 
A high ratio of nurses to patients is characteristic of intensive care units.
Many doctors and nurses have a very poor understanding of what constitutes an intensive care patient: they are not merely standard medical or surgical patients, sicker than normal, perhaps plugged into ventilators. All intensive care patients fit into one of the following categories:
     Patients admitted to intensive care for intensive monitoring, in anticipation of possible aggressive interventions: this is the coronary care model.
    Patients admitted to units which act as extensions of the post-operative recovery room, allowing abnormal perioperative physiology to reverse, with or without modulation of the normal stress response. Post operative cardiac care is an example of this model.
     Patients who require very intense nursing care, which would not be available elsewhere: an example of this is a burns unit.
    Patients who do not necessarily require life sustaining treatments, but whose physiology is taken under control in order to prevent organ injury: neurosurgical critical care.
     Patients who have minimal physiologic reserve, and who undergo acute potentially reversible injury, requiring life support until the abnormalities have been reversed and reserve restored: this is the archetypical medical intensive care patient (COPD with pneumonia requiring mechanical ventilation).
     Patients who undergo an massive disruption to their physiology, due to an overwhelming stress response to injury, or inadequate compensation to the response: this is the patient frequently seen in surgical intensive care units - major trauma or sepsis such as pancreatitis.
 
It is important that you are able to differentiate between the types of patients that you look after in ICU: for routine post operative surgical patients - fluid balance, analgesia and heart rate control (control over the stress response) may be the over-riding priorities; rather than feeding, for example. It is also important to realize that patients admitted under one category may enter another: a patient following coronary bypass surgery may develop severe sepsis or acute lung injury.
 
It is important to differentiate patients who are in critical care units from those with "critical illness", which is characterized by acute loss of physiologic reserve.
 
The patients in groups 5 and 6 have "critical illness": their admission to ICU has followed an injury which has depleted endogenous reserves, and death is inevitable without life supporting interventions. These patients do not follow predictable courses of illness, such as "the ebb and flow paradigm", originally described by Cuthbertson.
 
In many cases the course of illness is prolonged, and the underlying causes difficult to discern. Indeed there appears to be great interpatient variability - two patients with the exact same injury may follow different paths: one may follow the standard stress response - acute compensation, followed by hypermetabolism and catabolism and, after 4 to 7 days, resolution with fluid mobilization and anabolism. The second patient may rapidly develop multi organ failure and remain in intensive care for a prolonged period of time.
 
We do not know why this occurs; there is some evidence of  a hereditary component. To look at this another way, the standard stress response has evolved as the body's mechanism to save itself and deal with major injury: the greater the injury, the greater the response. Conversely, an overwhelming response, which will lead to death without life support, cannot be considered "normal".