Critical Illness  Physiologic Reserve

Critical illness occurs when an injury overwhelms physiologic reserve to the extent that life cannot be sustained without outside intervention.

Physiologic reserve decreases with age and chronic disease.

The "Law of Diminishing Reserve": prolonged critical illness depletes physiologic reserve.

Do you ever wonder why you cannot run as fast or regain you breath as quickly as you could when you were 15 years old. Aging? Absolutely, but what is this? Aging involves the slow deterioration, over decades, of physiological functions, which leads to death, when all reserve runs out. The difference between a young person and an old person is how much is left “in the tank”, how much additional activity can be dragged out of organs before they become exhausted. We call this the “physiologic reserve” and it is of core importance in critical care.

The body’s  reserves gradually run down as one ages, and this process is accelerated by the development of intercurrent illness. There are very few single organ diseases. The development of hepatic cirrhosis, atheroma, heart failure, COPD, renal failure etc significantly drains the body’s physiologic reserve. Older patients, malnourished patients, alcoholics and those with intercurrent illness are thus much more likely to succumb to critical illness.

Critical illness is characterized by an acute disruption in physiology, such that the body cannot utilize its reserve to compensate. An example of this is the profound vasodilatation seen in septic shock. Under normal conditions the cardiovascular system is able to compensate for pathologic vascular tone by autotransfusion of extracellular fluid, increasing heart rate and increasing cardiac output. In severe sepsis uncontrolled release of cytokines and nitric oxide along with the presence of a myocardial depressant factor conspire to undermine the body’s compensatory mechanisms, and hypotension and tissue hypoperfusion results. The logical approach is to support the cardiovascular system using fluids and vasopressors until the inflammatory response has relented and the source is under control. Normal physiology is restored.

The ultimate example of loss and restoration of reserve is acute severe asthma. Under normal circumstances an asthmatic may have a peak expiratory flow of 450 to 500 liters per minute. At 200 l/min the patient can breathe normally at rest, but not on exertion, below 100 l/min the patient is really struggling. The 200-450 l/min range represents a comfortable reserve which allows the patient to take exercise, climb the stairs etc. Acute severe bronchospasm annihilates this reserve, but the process, when managed properly, is reversible.

Conversely, a patient with severe COPD may have minimal reserve at rest. Intercurrent illness such as pneumonia will obliterate this and the patient may require mechanical ventilation. Persistent illness, protein catabolism and pulmonary fibrosis may diminish the underlying reserve such that the patient cannot be weaned from mechanical ventilation.

Critical illness should be seen as a multi-system wasting syndrome of varied etiology, whose end result is to deplete the body’s reserves. Critically ill patients cannot be made anabolic, so the purpose of intensive care is to reverse the abnormal physiologic processes, to slow this catabolism, and restore the patient to health. Often this resembles a game of catch-up, correct the problems before the next injury occurs.

The course of critical illness is like a roller coaster of multiple hits and subsequent recovery periods. A patient may, for example, be admitted following emergency surgery, but the course may be complicated by systemic sepsis, acute lung injury, renal failure, nosocomial pneumonia etc. Each subsequent injury makes its successor more likely, and with each hit the physiologic reserve is further depleted.

It is very difficult to die in an intensive care unit. We are able to maintain life almost indefinitely, but at a very high cost – constant suffering. If the patient cannot be weaned off life support such that they can return to a (relatively) normal life, then prolonged intensive care is futile. In the end most patients die due to withdrawal of life support. They die because they have multi-organ dysfunction, loss of physiologic reserve and neuroendocrine exhaustion.

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