At 36 hours, John’s capillary seal is regained but he shows signs of acute pulmonary oedema. He has an external anteriovenous (AV) shunt ¹ inserted and commences haemodialysis in order to maintain fluid and electrolyte balance while his kidneys convalesce from prerenal acute renal failure (ARF). (John’s case will be reconsidered later in the chapter to address the immunological aspects of organ transplantation).

What are the basic prerequisites for a healthy inflammatory response?

• Adequate supply of water, nutrients and oxygen;

• Adequate drainage and elimination of cellular waste;

• Homeostatic balance and co-operation between the immune, nervous and endocrine systems; and

• A positive psychological outlook ¹.

What raw materials are needed for John to mount an effective immune response? Adequate hydration and nutrition are essential if the inflammatory process/immune system is to be efficient and effective so the following are necessary:

• Water to constantly flush toxins and keep the cellular environment fresh;

• Kilojoules to provide energy for phagocytosis and cell proliferation;

• Essential amino acids for collagen and other protein synthesis;

• Fatty acids and cholesterol for synthesis of steroid hormones and cell membranes; and

• Vitamins and trace elements.

When tissue damage is extensive, the metabolic demands of the body for the inflammatory response and healing purposes can more than double. Unless protein supply matches demand, the body will rapidly catabolise (breakdown) muscles (including the diaphragm in severe cases), and recycle the amino acids to form more urgently-needed proteins such as albumin, fibrin, clotting factors, collagen and immunoglobulins. Matching supply with demand can be challenging, and critically ill patients often require Total Parenteral Nutrition (TPN) in addition to enteral feeds.

An adequate supply of oxygenated blood to body tissues is essential. Any condition that results in cell hypoxia will not only cause the accumulation of lactic acid (pro-inflammatory), but also dramatically reduce the cellular production of adenosine triphosphate (ATP). Without adequate ATP, the Na⁺/K⁺ pumps become less effective. Unable to maintain intracellular fluid and electrolyte balance, cells swell and finally burst, releasing enzymes that then cause further injury to nearby cells. The cycle of tissue destruction causing more tissue destruction will continue until oxygen supply meets cellular demand.

Disorders of the respiratory, cardiovascular and haematological systems have the greatest impact on the delivery of oxygenated blood.

Compared to non-smokers, the healing time for smokers is slow. Nicotine impedes the delivery of oxygenated blood by causing vasoconstriction, and chronic exposure to nicotine causes T cell unresponsiveness ²^,³^,⁴.

Venous and lymphatic drainage is crucial in keeping interstitial fluid clean from cell debris, inflammatory mediators and toxic metabolites. No cell can function properly if it is surrounded by its own (and neighbouring cells’) waste products. Excess interstitial fluid normally drains into lymphatic capillaries, which join with others to form larger and larger lymphatic vessels, eventually emptying into the subclavian veins. En route, the lymph is filtered through a series of lymph nodes, each densely packed with B and T cells ready to react to anything suspicious. In this way, interstitial fluid is constantly cleansed – several times a day. The lymphatic system is fundamental to a properly functioning immune system.

If lymph nodes are diseased (e.g., cancer, parasites) or removed, (e.g., axillary dissection in radical mastectomy), lymphatic drainage is slowed. If the drainage basin (e.g., the affected arm) is suddenly flooded by interstitial fluid (e.g., inflammation-induced oedema from sunburn or bee sting), the lymphatic vessels cannot empty fast enough. The excess pressure causes them to irreversibly dilate, causing valvular incompetence. As a result, interstitial fluid cannot move into the (already full) lymphatic capillaries, so it lies stagnant between the cells. This is lymphoedema, and affected tissues will always be at risk of infection and have poorer healing ability.

The lungs have an extensive lymphatic drainage network which generally serves to keep the lungs ‘dry’, (i.e., free from pulmonary oedema). The lymphatics remove solutes, stray proteins and fluid from the interstitial space in the lungs.

An acute increase in volume or pressure may overwhelm the capacity of the lymphatic vessels to remove the excess fluid, and, as a result, pulmonary oedema occurs.

Why was John at risk of developing pulmonary oedema? John was susceptible to developing pulmonary oedema for several reasons. In burns and most other tissue damage situations, the greater the tissue damage the greater the inflammatory response. As a result of the increase in capillary permeability, fluids and even proteins escape into the interstitium, causing widespread oedema. When the capillary seal is regained, excess interstitial fluid is drained by the lymphatics and returned to the cardiovascular system. This adds to the workload of the heart and kidneys, and during this period pulmonary oedema may occur.

In this case study, John developed acute renal failure because of the prolonged time his kidneys went without adequate perfusion during shock. His decreased ability to make urine, combined with the return of excess interstitial fluid to the cardiovascular system, overloaded the capacity of the pulmonary lymphatics and caused the development of pulmonary oedema.

The immune system constantly treads a fine line between being over-vigilant and hyper-responsive (resulting in hypersensitivity and autoimmune disorders), and under-active and hypo-protective (resulting in immunodeficiency syndromes and a heightened susceptibility to infections and cancer). Appropriate balance and coordination between the various elements of the immune system, including appropriate levels of cellular activity, cytokine release and hypothalamic function are of the essence for normal function ⁵.

NON-SPECIFIC IMMUNITY – FIRST AND SECOND LINES OF DEFENCE

The three major elements of non-specific immunity are

1. Physical Defences

a. Barriers (anatomical and mechanical) are skin and mucosae (which line the respiratory, gastrointestinal and urogenital tracts) physically separate the external environment from the underlying tissues, and provide protection to internal structures. In addition, areas of the body subject to abrasion (e.g., skin or the mucosal layer of the rectum and vagina) are lined by stratified squamous epithelium and continually undergo desquamation, where the top layer of cells is shed to reveal the next layer of cells.

Since burns cause the most severe loss of skin integrity, overwhelming infection remains the major cause of death and strict adherence to the principles of hygiene are important from the very onset of injury.

Any condition that causes shock can result in the loss of integrity of the gastrointestinal mucosae. Compensatory mechanisms for shock include vasoconstriction of vessels supplying non-essential tissues – such as the gastrointestinal tract. If vasoconstriction is severe and prolonged, some cells in the blood-depleted areas become hypoxic, swollen and die. This causes defects in the anatomical barrier of the mucosae, and bacteria from within the gut wall can gain access to the capillaries and blood. This is called ‘translocation of gut bacteria’, and partly explains why critical illness and severe shock often result in septicaemia.

b. Oscillation of bronco-pulmonary cilia moves mucus laden with inhaled micro-organisms, chemicals and dust, towards the mouth for expulsion or swallowing.

Intubation temporarily hampers or renders useless ciliary function. Ciliary damage can be acquired, such as through inhaling cigarette smoke.

Primary Ciliary Dyskinesia is a genetic disorder (thought to be caused by mutation of a single gene, inherited in an autosomal recessive manner), where ciliary motion is erratic and uncoordinated ⁶. Affected individuals have a lifetime history of chronic sinusitis, bronchiectasis, chronic cough, and recurrent pneumonia.

c. Reflexes like coughing and sneezing help to expel foreign matter from the respiratory tract.

d. Fever – in response to many viral and bacterial infections, macrophages release IL-1 and IL-2, which raise the hypothalamic temperature set-point. This may limit bacterial and some viral multiplication.

2. Chemical defences

In addition to flushing and diluting potential pathogens, bodily secretions (tears, saliva, gastrointestinal secretions, sweat) and plasma contain chemicals that help to defend the body against microorganisms:

a. Salt and fatty acids, secreted by glands in skin, inhibit the growth of streptococci, fungi and most Gram-negative bacteria.

b. Gastric acid – the acid concentration in the stomach is such that it effectively kills most bacteria. Antacids raise the stomach pH and make a person more susceptible to gastrointestinal infection.

c. Lysozyme, an enzyme in secretions such as saliva and tears, breaks down the cell walls of some bacteria (especially Gram positive).

d. Tumour necrosis factor (TNF), which is produced primarily by monocytes and macrophages, suppresses viral replication and activates phagocytes.

e. Complement components and their products cause destruction of microorganisms directly, or with the help of phagocytic cells. Complement comprises a system of at least 20 proteins, which promote immune function and destroy foreign substances. Complement is self-regulating and also serves to limit immune function, as the molecules of the complement system help to remove immune complexes and prevent their accumulation ⁷.

f. Transferrin, which is mainly produced in the liver, and lactoferrin, present in neutrophil granules and most secretions, deprive organisms of iron and also have a role as immune system modulators.

• Interferons regulate growth and differentiation and exert immunological control. Alpha interferon is a small protein produced by virus infected cells which non-specifically inhibits all other virus replication by blocking protein synthesis. Interferons also activate macrophages and natural killer (NK) cells. Alpha interferon is currently being used to treat conditions as diverse as hepatitis C, melanoma and renal cancer ⁸.

Why are interferons not more widely used in the management of viral infections? The answer is that interferons are expensive to produce, are administered by injection and unfortunately cause unpleasant side effects such as flu-like symptoms, nausea and headache.

3. Defensive cells

The following cells are involved in non-specific cellular defence mechanisms, and will thus attack a wide range of invading organisms. The most important defensive cells of the non-specific immune system are the neutrophils, macrophages and natural killer (NK) cells (not to be confused with T cytotoxic cells).

• Neutrophils (polymorphonuclear leucocytes) are the most important phagocytes in bacterial destruction. Their granules and lysosomes contain degradative enzymes. In a non-specific immune response, neutrophils are the first cells to move out of dilating blood vessels as they are most easily deformed. Neutrophils respond to chemotactic factors or chemotaxins (chemicals released by bacteria and dead tissue cells) and move towards the area of highest concentration. They engulf the offending cell/particle, enclose it within a membrane, and release hydrolytic enzymes. Because this process (phagocytosis) consumes so much energy, the neutrophils’ glycogen reserves become depleted and they soon die. When neutrophils die and their contents are released, remnants of their enzymes cause liquefaction of closely adjacent tissue. The accumulation of dead neutrophils, tissue fluid and other cell debris forms pus (often described as ‘purulent discharge’).

• Macrophages, derived from monocytes, are large mononuclear phagocytes. They are found in many body tissues and remove dead cell debris as well as attacking bacteria and some fungi. Since macrophages have greater glycogen reserves than neutrophils, they are able to replenish their lysosomal contents and therefore have a longer active life. Macrophages and other phagocytic cells such as the dendritic cells are known as antigen presenting cells (APC) because they display portions of what they have ingested on their surfaces and in doing so, communicate with and stimulate the specific immune system.

• Natural killer (NK) cells are non-specific aggressive lethal lymphocytes which have no immunological memory but attack and kill on contact anything which is recognised as foreign or abnormal. They target tumour cells and infectious microbes, particularly viruses. Granules inside NK cells contain cytolytic proteins such as perforin. These cells are involved in graft rejection and pose a major problem to transplant patients. NK cells that have been activated by lymphokines (such as interferon-gamma and IL-2) kill their targets more efficiently.

Low NK cell activity seems to be associated with stress ⁹ of any type, poor nutrition, overwork, acute or chronic disease, emotional trauma and bereavement.

Numbers of NK cells are reduced in cancer, severe viral infections, acquired immunodeficiencies and some autoimmune diseases. A drop in NK cells also occurs after significant blood loss.

HEALING AND THE INFLAMMATORY RESPONSE

Case study

Kim Casey, 5, had a splinter in her finger. Her father removed most of the wood, but the finger became sore, red, swollen and warm. The following day a small amount of pus was evident on the Band-Aid^®, but two days later there was only a barely visible scar.

WHY THE INFLAMMATORY RESPONSE IS IMPORTANT TO SURVIVAL

Whenever there is tissue damage, be it from a period of hypoxia (e.g., myocardial infarction), physical trauma (e.g., traumatic brain injury) or invasion by a pathogen (infection), the innate and non-specific response of the body is to inflame the affected area. Inflammation is the process by which the body attempts to heal itself, and essentially consists of two stages:

• Removal of damaged tissue and offending substances; and

• Replacement with scar tissue.

Although the classical clinical manifestations of inflammation (redness, heat, oedema, pain and loss of function) may be uncomfortable, they nevertheless signal immune activity and responsiveness.

However, for someone who is severely immunodeficient, a simple splinter wound might lead to a life-threatening infection.

It is important to note that fever is not always a reliable indicator of infection. The elderly and those with immunodeficiencies cannot mount an adequate inflammatory response and thus may not display the classic systemic manifestations of infection such as fever. They are often misdiagnosed because they may only present with symptoms like confusion.

Small scale injuries (e.g., Kim’s splinter) elicit a smaller immune response than large scale injuries (e.g., John’s extensive burns), but the underlying process is similar, regardless of the cause or extent of tissue damage. The ability to generate an appropriate inflammatory response is crucial for healing and repair.

IMMUNE SYSTEM RESPONSE TO TISSUE DAMAGE

When cells are damaged, normally hidden parts of the cell membrane and intracellular components are suddenly exposed and released to the local environment. This initiates a chain of events, including the synthesis of many pro-inflammatory cytokines such as prostaglandins (PG) and leukotrienes (LT). Irritated mast cells (abundant in skin and respiratory mucosae), as well as neutrophils, basophils and platelets, release a mixture of other pro-inflammatory molecules including histamine, heparin and bradykinin.

PRO-INFLAMMATORY CHEMICALS DURING INFLAMMATION

• Histamine (H) is a powerful vasodilator released by mast cells and basophils. It increases vessel permeability and contributes to pain by sensitising nerve endings to other inflammatory mediators. Histamine also causes smooth muscle contraction of the bronchi.

• Prostaglandins (PG) are local hormones derived from membrane fatty acids. PG are highly potent substances that are not stored but rather produced as needed by cell membranes in virtually every body tissue. They have diverse actions dependent on cell type and may cause smooth muscle contraction, stimulate nociceptors, function as chemotaxins and promote the inflammatory response. Although highly potent, they are rapidly inactivated in the systemic circulation and the inactive metabolites are excreted primarily in urine.

• Leukotrienes (LT) are local, slow acting vasoactive substances released largely by white blood cells. LT are potent bronchoconstrictors and cause airway wall oedema. LT also attract eosinophils into the tissues and amplify the inflammatory process. Some asthma preventer drugs, such as montelukast (Singulair), act by blocking LT.

• Serotonin (5-hydroxytryptophan, or 5-HT) is capable of increasing vascular permeability, dilating capillaries and producing contraction of nonvascular smooth muscle.

• Bradykinin (BK) stimulates bare sensory nerve endings and is the most potent mediator of pain.

• Interleukins (IL) are chemical messengers released from lymphocytes and macrophages. They play a role in the inflammatory response (Il-5, IL-9, IL-11, IL-22), stimulate haemopoiesis (IL-3) and allow communication between and stimulation of B lymphocytes (Il-1, IL-2, IL-4, IL-5, IL-6, IL-10, IL-13), T lymphocytes (IL-1, IL-2, IL-7, IL-16) and NK cells (IL-2, IL-12)⁵.

WHAT IS THE PATHOPHYSIOLOGY?

The relationship between the physiological events involved in inflammation, the clinical manifestations and the beneficial effects of the inflammatory process are shown in Table 13.1.

TABLE 13.1 PHYSIOLOGICAL EVENTS UNDERLYING THE CLINICAL MANIFESTATIONS AND BENEFITS OF INFLAMMATION

Underlying Physiology	Clinical Manifestation	Benefit
1. Cytokines relax vascular smooth muscle and cause vasodilation 2. The increase in blood flow, together with an increase in capillary permeability, makes the area hyperaemic	Redness	1. Increases phagocytes such as neutrophils and macrophages and clotting factors in affected area 2. Increase the supply of O2 and nutrients needed for repair
1. Increased metabolic activity by leucocytes and fibroblasts → an increase in the production of heat as a waste product of biochemical reactions → local warmth 2. Increased blood to area brings metabolic heat	Warmth	1. If microorganisms are present, the local increase in temperature may inhibit their growth or make them more vulnerable to attack 2. Haemoglobin alters its shape, releasing O₂ to the tissues more easily than normal 3. Many biochemical reactions become faster
Cytokines (especially histamine and PG) increase capillary permeability, making them more “leaky”. The increased volume of interstitial fluid is oedema	Oedema	The influx of fluid dilutes: 1. Harmful chemicals and toxins in the area 2. Metabolic acids caused by anaerobic metabolism
1. Inflammatory cytokines (especially PG and BK) are released from damaged cells → irritate sensory nerves → perceived as pain 2. Oedema → presses on nerves → perceived as pain, numbness, tingling etc	Pain	1. Warning system that tissue is damaged 2. Inhibits movement and limits further damage
1. Cytokines can directly damage and inflame nearby muscle cells or motor nerves 2. Pain induces muscle guarding 3. Swelling and oedema impair function	Loss of function	1. Inhibits movement and limits further damage

HOW SCAR TISSUE IS FORMED

Following the neutrophils, monocytes leave the blood stream. Local macrophages are attracted to the area and ingest and destroy bacteria and cell debris. Monocytes and macrophages secrete cytokines (IL-1 and TNF), which stimulate the proliferation of

• Endothelial cells and the formation of new capillary networks; and

• Fibroblasts, which secrete collagen (protein) bridges to form a scar.

Excessive production of collagen leads to the formation of a raised keloid scar. Trauma to the skin, both physical (e.g., surgery) and pathological (e.g., lesions arising from acne vulgaris and varicella (i.e., chickenpox) is the primary cause identified for developing keloids. There are also familial tendencies and racial differences in the prevalence of keloid formation. The highest incidence of keloid scarring occurs in black Africans. The genetics of keloid formation are currently under investigation, and several genes seem to be involved ¹⁰^,¹¹.

However, due to the rising popularity in body piercing, there has been an increase in keloid formation of ‘unusual’ parts of the body such as the umbilicus and genitalia.

DAMPENING THE INFLAMMATORY RESPONSE

Problems arise if the inflammatory response is too strong, too weak, misdirected or otherwise damaging for the host, (e.g., allergic rhinitis, gastroenteritis, hepatitis or cellulitis). Basically, the inflammatory response may be dampened if it is excessive or when there is no significant protective benefit to the host.

Acute anaphylaxis¹² refers to a severe allergic reaction to a foreign protein or molecule such as penicillin, IV contrast media, bee or wasp venom, and certain foods (most notably, peanuts). Upon a second exposure to the allergen sensitised mast cells and basophils release large amounts of histamine. A cascade of biochemical and physiological events follows, which includes the involvement of many pro-inflammatory chemicals including PG and LT. Excessive vasodilation and increased capillary permeability lead to a dramatic drop in blood pressure, while bronchial smooth muscle contraction, airway oedema and mucus production cause acute respiratory distress.

In addition to providing support for the respiratory system (assisted ventilation, intubation) and cardiovascular system (IV fluids), the primary drug treatments for acute anaphylactic reactions are IV adrenaline and H1 antihistamines such as diphenhydramine (Benadryl, Unisom). (Corticosteroids are mainly effective in preventing biphasic i.e. delayed reactions, and are not considered as a first-line treatment).

ANTIHISTAMINES IN THE REDUCTION OF INFLAMMATION

Antihistamines antagonise histamine at H1 receptors and limit the effects of histamine. Unfortunately, many of the older drugs block histamine receptors in the CNS and cause drowsiness. Second-generation antihistamines such as loratadine (Claratyne, Claratin) and fexofenadine (Telfast, Allegra) have less ability to penetrate the CNS and are therefore non-sedating. These drugs are widely used as over-the-counter (OTC) medications for the control of allergic rhinitis.

OTHER INFLAMMATORY DRUGS IN THE REDUCTION OF INFLAMMATION

In Australia there are over 8 million prescriptions annually for non-steroidal anti-inflammatory drugs (NSAID)¹³; these drugs work by inhibiting the enzyme cyclooxygenase (COX), and in so doing, decrease the production of prostaglandins and leukotrienes. Some types of prostaglandins (those involved in the COX-1 pathway) are needed on a continuous basis and are stimulated by routine physiological events. The COX-1 prostaglandins stimulate physiological body functions, such as the production of protective gastric mucus and platelet maturation.

In contrast, the COX-2 pathway is induced by tissue damage/inflammation, and the resultant prostaglandins are pro-inflammatory. Inhibition of the COX-2 pathway dampens the inflammatory response, reduces oedema and alleviates pain.

Older NSAIDs such as aspirin (Asprin), indomethacin (Indocid), ibuprofen (Nurofen) and naproxen (Naprosyn, Naprogesic) inhibit both COX-1 and COX-2 pathways, thus lowering the production of the maintenance and pro-inflammatory prostaglandins. While the anti-inflammatory benefits come from the inhibition of the COX-2 pathway, many of the undesirable effects (such as susceptibility to gastric erosion) result from blockade of the COX-1 pathway. Second-generation NSAIDs such as celecoxib (Celebrex), selectively inhibit the COX-2 pathway (expressed in synovial cells and macrophages), and are thus becoming popular for the symptomatic control of arthritis, sports injuries and other forms of inflammation (see Figure 13.2).