In the ICU/CCU blood tests are usually done at least once a day. They measure such things as how the kidneys are working, cardiac markers (to make sure the heart is healthy) and levels of potassium (K+) and calcium (Ca++) as well as other elements. These are some of the indicators of how the body is working and can show the intensive care specialist how well a patient's body is coping with their illness. Intra-arterial and intravenous lines (tubes placed in arteries and veins) are often used to monitor the body and, once established, allow rapid, reliable and pain-free access for repeated blood tests. Some conditions will require multiple repeated blood testing every few hours.

Patients with critical illness commonly develop problems with their hearts and circulation. Various factors are involved, some related to the primary disease while others are secondary effects. Problems include changes in: the distribution and volume of body fluid, the condition of the blood vessels and the ability of the heart to pump blood around the body. Treatment for cardiovascular problems may include fluids therapy and a wide range of medicines to control the heart rate, cardiac function and blood pressure.

Respiratory failure occurs when the respiratory system is no longer able to provide enough oxygen requirements or remove enough carbon dioxide from the body. Hypoxia (not enough oxygen is reaching the tissues) may occur unless there are interventions. Large amounts of carbon dioxide may also build up in respiratory failure. Mechanical Ventilation This is the use of a ventilator (sometimes called a life support machine) to do the breathing for a patient experiencing respiratory failure. The ventilator fills the lungs with air, thereby providing oxygen to, and removing carbon dioxide from, the body via the lungs. Usually the ventilator delivers oxygen directly into the airway of the patient. This is done using an endotracheal tube which is a plastic tube that is passed through the mouth into the larynx (the top of the trachea or windpipe). Conscious patients are usually given a medication to make them sleepy or unconscious and a muscle relaxant to help them relax while the tube is inserted. Sometimes people may be ventilated for a short or a long time. Noninvasive Positive Pressure Ventilation Some patients may receive ventilation without needing intubation, with the breathing support being delivered via a sealed mask applied to the face. However noninvasive ventilation is useful only in some circumstances and in some patients.

A nasogastric tube is often inserted at the same time as the endotracheal tube. The nasogastric tube is inserted into the stomach via the nose. This tube enables removal of gastric fluids and/or the delivery of nutrition.

Kidney (or renal) failure is when a patient's kidneys are unable to remove wastes and excess fluid from the blood. The likelihood that the kidneys will get better depends on what caused the kidney failure. Kidney failure is divided into two general categories, acute and chronic. In acute (or sudden) kidney failure, when kidneys stop functioning due to a sudden stress, they might be able to start working again. However, when the damage to the kidneys has been continuous and has worsened over a number of years, as in chronic renal failure (CRF), then the kidneys often do not get better. This can be compounded if a patient has an acute illness. There are a number of causes of acute renal failure and in intensive care patients there is often more than one factor that contributes to its development. Some patients may require continuous renal replacement therapy (dialysis) for a period of time.