2. Allergy, immune response and anaphylaxis – May 2015
The human body has a number of defence mechanisms to protect itself against the outside world. The skin, coughing and sneezing form one outer level of barrier. Inevitably some things get in though. Many things are useful and wanted. Some are not. The immune system is part of dealing with those things that are not wanted including bacteria and viruses. Its actions are essential to survival. However when the immune system malfunctions, it can cause as many problems as it fixes. Allergy and anaphylaxis are just such problems.
An antigen is any substance that can trigger the immune system to respond. The first level of response is the antibody system. This is difficult to simplify but essentially is a combination of proteins that identify and tag the antigen then trigger other parts of the immune system to then go about neutralising the antigen. These are the antibodies. This is normal and necessary.
The immune response includes other accompanying actions inside the body. These include opening up (dilation) of the arteries to allow greater blood flow to the antigen. They also include narrowing of the smaller airways (bronchoconstriction) to reduce antigens getting into the lungs and of the gastrointestinal tract to remove any antigen in the stomach. This can mean nausea and vomiting. These three body system functions are part of the normal immune response.
In some people the antigen immune response can at times be excessive or uncontrolled. This is known as an allergic reaction. The antigen that causes this extra response is called an allergen. Allergens can be all manner of substance including bites, food, medicines and chemicals. They can get into the body also in a number of ways including eaten, inhaled or injected.
An allergic response can include features seen on the skin including swelling, rash, itchiness and welts. It may extend to include hayfever, runny nose and conjunctivitis. An allergy is annoying and causes relatively minor problems. It can be managed with antihistamines, eye and nasal drops to reduce the response and relieve the symptoms.
When an allergic reaction becomes severe though it is called anaphylaxis. There is no difference in how it develops only that the body responses are far stronger than in simple allergy. The complications extend beyond what can be seen on the skin and now show up in the other body systems. There is no exact description of how a patient in anaphylaxis will present with but will include one or more of the following:
The blood vessels of the body dilate too much. The blood pressure drops and becomes low. The patient will look red and flushed to go with any other rashes or welts already visible.
The smaller airways narrow even more than normal leading to a whistling sound through them similar to asthma. This makes it difficult for the patient to breathe.
The muscle in the GIT spasms leading to abdominal cramps, nausea, vomiting and diarrhoea.
Arguably the other commonly observed finding is swelling. When blood vessels open up to let more blood flow the smallest blood vessels, capillaries, also open up. Because they are so small this allows fluid in the blood to leak out. Where this happens in large enough amounts blood volume is lost and swelling occurs. Where it happens in the throat it can cause choking and difficulty breathing.
Most patients with anaphylaxis will know they have such a problem and be ready to deal with it. Some have their own medications known as ‘epipens’ to use. An important part of patient assessment is to establish relevant history including previous allergy episodes, to what and how serious the problem became. On occasions the provoking antigen will not be known to the patient. they may not be able to identify what has caused the problem.
On a few occasions the patient will not even have a past history of allergic reactions. The first responder must be prepared to identify a ‘first’ episode just as readily as one that is following many others.
Managing an episode of anaphylaxis is relatively straight forward no matter how the patient presents. Typically the dilation of the blood vessels throughout the body dictate that the patient should be laid flat to return blood back into the main part of the body. Occasionally if the patient is desperately needing to sit up because they are struggling to breathe, they can be allowed to.
The mainstay of treatment of anaphylaxis is to administer adrenaline. This drug does a number of things but addresses the problems very well. Firstly, it constricts all of those excessively opened blood vessels. This returns blood back into the main circulation to where it should be. It also closes down the many capillaries that are allowing blood fluid to leak out and escape causing swelling. Whatever else you do when treating anaphylaxis, adrenaline must be a part of the plan. The way to administer the adrenaline is an injection deep into a big muscle. The epipen is a purpose built needle for doing just this. The thigh or buttock muscle is usually chosen. Firstly it is easier for the patient to administer this injection to themselves in the thigh. Secondly, these big muscles have good blood supply to pick up the adrenaline and return it back into the circulation for effect.
Whilst the blood vessels are dilated, extra blood sits in the arms and legs and for a longer time. This allows more oxygen than normal to leave the blood and be used by the cells. It also creates a problem for the body to replace that extra oxygen when the blood eventually returns to the lungs again. Oxygen administration is important.
If the patient is wheezing from narrowed small airways (bronchospasm) salbutamol can be administered using a nebuliser. It is important not to administer this along with and never instead of adrenaline. On other occasions the patient may be choking from swelling in the throat. Adrenaline can be administered using a nebuliser just like salbutamol. In this way it can act directly on the swelling in the throat. However, also just like salbutamol, an injection of adrenaline must come first in management.
Other options such as antihistamines may or may not be helpful in treating anaphylaxis. Whether or not they are, they are never part of the first response to anaphylaxis and can only ever become something that is considered after the initial treatment. And all patients who have an anaphylaxis episode must go to hospital. Adrenaline as a treatment doesn’t last very long in the body however the allergen can go on causing problems for some time after.
Anaphylaxis can be easy to recognise in many cases. In others it can be more challenging. Wherever there is uncertainty, the assumption should always be that the problem is anaphylaxis. There will usually be skin problems noted such as rash or itchiness. Occasionally these may not be there and indeed they are not essential. There may be swelling or there may not. There may be trouble breathing or maybe not. The blood pressure may be very low from dilated blood vessels or it maybe okay. There may be GIT problems or maybe not. There may be a known trigger that has caused the problem or it may be unknown. There may be previous episodes of anaphylaxis or there may be none. Be open to any combination. Where there is doubt, the safest play is to call the problem anaphylaxis and treat it as such.
Jeff Kenneally www.prehemt.com
Jeff is the author of the 2014 – 2016 Ambulance Victoria First Responder clinical practice protocols and accompanying education program.