Field Management of Chemical Casualties Handbook (TI15D)
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NERVE AGENTS
Nerve agents are considered the primary agents of threat to the U.S. military because of their high toxicity and effectiveness through multiple routes of entry. They are absorbed through the eyes, respiratory tract, and skin.
TOXICITY
The nerve agents are Tabun (GA), Sarin (GB), Soman (GD), GF, and VX. Tables I and II show the toxicities of the nerve agents by inhalation and skin exposure.
The Ct is the product of the concentration (C) of a vapor or aerosol to which one is exposed and the time (t) to which one is exposed to that concentration (C). The units are usually mg/m3 for C and minutes for t. One can be exposed to a Ct of 100 mg-min/m3 by staying in a concentration of 10 mg/m3 for 10 minutes (10x10=100), 20 mg/m3 for 5 minutes (20x5=100), or 5 mg/m3 for 20 minutes (5x20=100). The Ct that will cause a biological effect is constant over a range of C and t. Thus, if a Ct of 100 mg-min/m3 of nerve agent causes shortness of breath, it would be a result of any combination of C and t that produces a product of 100.
The LCt50 is the Ct of agent vapor that will be lethal (L) to half of the population exposed to it. The ICt50 is the Ct that will incapacitate (I) half of those exposed to it. The word “incapacitate” must be defined when using this term. For example, dim vision might incapacitate a soldier for some jobs, in which case the ICt50 will be the Ct needed to cause dim vision. On the other hand, incapacitation might be defined as loss of consciousness and twitching, in which case the ICt50 will be the Ct needed to produce these effects. The ICt50 shown is that causing severe effects, including convulsions.
Table I shows the estimated LCt50, estimated ICt50, and Ct that will cause pinpointing of the pupils (miosis) in half of the population (MCt50). Units of the Cts are mg-min/m3. Table II shows the estimated amounts that will cause lethality in half of the population when placed on the skin.
The LD50 is the dose (D) of agent liquid or solid that is lethal (L) to half of the population exposed to it. The LD50 of VX, when placed on human skin, is the size of a droplet that will cover the width of two columns of the Lincoln Memorial on a Lincoln penny.
TABLE I. Vapor Toxicity mg-min/m3
| Agent | LCt50 | ICt50 | MCt50 | | GA | 400 | 300 | 2-3 | | GB | 100 | 75 | 3.0 | | GD | 70 | UNK | <1.0 | | GF | UNK | UNK | <1.0 | | VX | 50 | 35 | 0.04 |
TABLE II. LD50 on Skin
| Agent | Amount | | GA | 1000 mg | | GB | 1700 mg | | GD | 50 mg | | GF | 30 mg | | VX | 10 mg |
MECHANISM OF ACTION
When a soldier is poisoned by a nerve agent, the action of the enzyme acetylcholinesterase is blocked. The normal function of acetylcholinesterase is to break down or hydolyze the chemical acetylcholine. Acetylcholine is a neurotransmitter, or messenger chemical. Nerve paths, which are divided into sections with gaps between the nerve endings and between the nerve ending and the target organ, are used to pass a command from the central nervous system to various organs. These gaps are crossed by acetylcholine, the messenger, which relays the command on to the next step and finally to the target. Under normal conditions, when the required action at each step is completed, the acetylcholine is broken down by the acetylcholinesterase, thus stopping the action. However, when a nerve agent inhibits the acetylcholinesterase, this enzyme cannot perform its normal function of hydrolyzing the acetylcholine. Acetylcholine then accumulates along the nerve path, and the target organ’s action continues uncontrolled. Muscles become hyperactive and twitch uncontrollably, and glands secrete copiously.
NERVE AGENT EFFECTS
The nerve agent’s mechanism of action is to inhibit the enzyme acetylcholinesterase. Inhibition of this enzyme allows the neurotransmitter. acetylcholine, to accumulate at the nerve endings where it causes excessive stimulation of the target organ. The parts of the body that are affected by excessive acetylcholine accumulation are as follows:- Eyes
- Nose (glands)
- Mouth (glands)
- Respiratory tract
- Gastrointestinal tract
- Cardiac muscle
- Sweat glands
- Skeletal muscle
- Central nervous system
The primary concern of the soldier medic/combat lifesaver when treating the nerve agent poisoned soldier is to provide correct, timely, and lifesaving care. The first step in providing this care is to understand the effects that a vapor or liquid nerve agent exposure has on the soldier.
Eyes.
The eyes will be affected by direct contact with a nerve agent vapor or aerosol. When the route of entry of the agent is through the skin or by ingestion, the effect on the eyes is delayed or may not occur. The main effect of the agent is to cause miosis, or pinpointing, of the pupils. One or both pupils may be pinpointed and unresponsive to light or darkness. Pinpointing causes a complaint of dim vision that is more pronounced in low light conditions. Frontal headache, mild aching around the eye, or severe pains are common complaints in a soldier exposed to a moderate concentration of agent. Twitching of the eyelids may be observed through the protective mask, and the eyes may be reddened. When a light source is used to test for pupillary response, the soldier may complain of an increase in aching behind the eyes due to light sensitivity.
Nose and Mouth.
The secretory glands of the nose and mouth are as sensitive or more sensitive to nerve agent vapor or aerosol than the eyes are. When the soldier is poisoned by nerve agent liquid on the skin or by ingestion, the nose will become affected, but only in response to the whole body (systemic) involvement. When exposed to a nerve agent vapor or aerosol, the nose will begin to run. This effect has been described by patients recovering from accidental nerve agent vapor exposure as "worse than a cold or hay fever" and "like a leaking faucet." Even after low concentrations of agent, rhinnorhea may be severe. The mouth will secrete excessive amounts of saliva that may be so copious that watery secretions run out the corners of the mouth.
Respiratory Tract.
Inhalation of a small amount of nerve agent vapor will cause the soldier to complain of tightness in the chest or shortness of breath (dyspnea). This occurs because the excessive acetylcholine stimulates the muscles in the airways to contract and constrict the airways (bronchoconstriction). As the concentration increases, breathing difficulty will become severe. One or two breaths of a high concentration of nerve agent vapor will cause gasping and irregular respirations within seconds to a minute or two. Cessation of breathing (apnea) can occur within minutes after exposure to a large amount of nerve agent, either by liquid on the skin or vapor.
Excessive bronchial and upper airway secretions caused by stimulation of the airway glands by the excessive acetylcholine will compound breathing difficulty. These secretions can obstruct the airway and cause difficulty in moving air into and out of the lungs with prolonged expiration a noticeable effect.
Gastrointestinal (GI) Tract.
After exposure to a large but sublethal concentration of vapor, the soldier will complain of nausea and may vomit. Also, nausea and vomiting may be the first effects from liquid nerve agent exposure on the skin. The soldier may complain of nausea followed by vomiting, "heartburn," and pain in his abdomen. In addition, the soldier may belch frequently and have diarrhea or involuntary defecation and urination. These effects usually occur within several minutes after vapor exposure. However, after liquid agent exposure on the skin, these effects may not begin for as long as 18 hours after exposure.
Cardiac.
The heart rate can either increase or decrease after nerve agent exposure. Generally, blood pressure will increase, but the blood pressure can rarely be determined in a contaminated area because the casualty and the examiner are in protective gear. The heart rate in nerve agent poisoning will not aid the soldier medic/combat lifesaver in choosing the care needed.
Sweat Glands.
The skin is very permeable to nerve agent. When penetration occurs after either liquid or vapor exposure, localized sweating occurs and progressively spreads over the surrounding skin area as nerve agent is absorbed. The likelihood that the soldier medic/combat lifesaver will be able to observe this sweating is minimal.
Skeletal Muscles.
After exposure to a moderate or large amount of nerve agent, the soldier will complain of weakness and twitching of muscle groups. The twitching can first be noticed at the site of a liquid droplet on the skin. The muscles may show a rippling effect (fasciculations). As the nerve agent effect progresses, muscles can go into a prolonged contraction. However, instead of a prolonged contraction, the large muscle groups may begin unsynchronized contractions that cause the arms and legs to flail about. The hyperactivity of the muscles in these instances leads to muscle fatigue and flaccid paralysis (limp, unable to move). Unless the soldier medic/combat lifesaver aggressively cares for this casualty, he/she will not survive.
Central Nervous System (CNS).
In the case of a large inhalation or liquid dose, the effects are rapid and usually fatal under battlefield conditions. The soldier almost immediately loses consciousness, followed seconds later by seizure activity. Several minutes later, respiration ceases. Without immediate care, this soldier will not survive to reach Level 1 treatment.
When exposed systemically to low amounts of nerve agent, the soldier may complain of generalized weakness.
Understanding when these effects can most occur is critical for the soldier medic/combat lifesaver. The length of time a casualty may be in your care is unknown. It is best to understand what may occur and when, because being surprised by and unprepared for the reactions of a nerve agent poisoned soldier lessens his chances for survival. Tables III and IV show nerve agent effects, the onset time of these effects, and the required self- and buddy-aid.
These tables show the typical time course for mild, moderate, and severe exposures to nerve agent. When a lethal or near lethal exposure occurs, the time to onset of symptoms and maximal severity of symptoms may be extremely brief. If aggressive care is not given to the soldier exposed to a lethal concentration, death can result within five minutes after the appearance of symptoms.
TABLE III. Nerve Agent Effects Vapor Exposure
Mild
Eyes - Small pupils (miosis), Dim vision
Headache
Nose - Runny nose (rhinnorhea)
Mouth - Salivation
Lungs - Tightness in the chest Time of onset: seconds to minutes after exposure
Self-aid: 1 MARK I Kit
Buddy-aid: stand by
Severe
All of the above, plus
Severe breathing difficulty or cessation of respiration
Generalized muscular twitching, weakness, or paralysis
Convulsions
Loss of consciousness
Loss of bladder, bowel control Time of onset: seconds to minutes after exposure
Self-aid: none; soldier will be unable to help himself
Buddy-aid: three MARK I Kits and diazepam immediately
TABLE IV. Nerve Agent Effects Liquid on Skin
Mild/Moderate
Muscle twitching at site of exposure
Sweating at site of exposure
Nausea, vomiting
Feeling of weakness Time of onset: 10 minutes to 18 hours after exposure
Self-aid: 1-2 MARK I Kits, depending on severity of symptoms
Buddy-aid: stand by
Severe
All of the above, plus
Breathing difficulty or cessation of breathing
Generalized muscular twitching, weakness, or paralysis
Convulsions
Loss of consciousness
Loss of bladder and bowel control Time of onset: minutes to an hour after exposure
Self-aid: none; soldier will be unable to help himself
Buddy-aid: three MARK I Kits and diazepam immediately
TREATMENT
The most important care the casualty receives is the care given within the first several minutes after exposure (self-aid, buddy-aid).
Immediate care, including administration of antidotes, can mean the difference between survival and death in a soldier exposed to a nerve agent. It is imperative that every medic/combat lifesaver understand the effects of nerve agents, the time in which effects occur, and the correct steps to take to save the exposed soldier.
Every soldier must know the signs and symptoms of mild and severe nerve agent poisoning and the correct first aid in order to evaluate and provide the appropriate self- and buddy-aid.
SELF-AID AND BUDDY-AID
Timely and correct determination of the type of agent and route of entry causing the signs or symptoms is critical if the poisoned soldier is to survive to reach definitive medical care. Nerve agents will, under most field conditions, be encountered in both the vapor and liquid forms. When nerve agents are encountered and soldiers have donned protective equipment, a hasty self-evaluation for signs or symptoms of poisoning must be conducted. This self-evaluation implies that soldiers know the signs and symptoms of mild and severe nerve agent poisoning, as well as the correct first aid.
Tables III and IV show methods of exposure, resulting signs or symptoms, and self-aid or buddy-aid to be rendered. It must be stressed that timely and correct first aid actions are critical to enhance the casualty's chances for survival.
It must be emphasized during training that, when the effects progress to more than one organ system, the situation is moving rapidly from a mild to a severe exposure. The buddy's aid in determining this change becomes critical. As the change occurs, the remaining MARK I Kits and one diazepam autoinjector must be administered.
Self- or buddy-aid must be promptly followed by Level 1 medical care.
SOLDIER MEDIC/COMBAT LIFESAVER TREATMENT OF NERVE AGENT POISONING
The Level 1 care provider (medic) must rapidly determine the following:- extent of the poisoning
- what medications have been administered
- complications induced by the poisoning and/or resulting from conventional wounds
PROTECTIVE POSTURE DURING TREATMENT
First, protect yourself by donning MOPP Level IV.
CASUALTY DECONTAMINATION
Next, assist the casualty in performing decontamination of exposed skin in the following order:- face
- neck area
- chest area
- abdomen
- arms and hands
- other exposed skin areas
Performing this decontamination eliminates nerve agents on the skin surface that could continue to absorb into the skin causing a "time release" effect of symptoms.
TREATMENT GUIDELINES
The treatment guidelines provided below assume that the soldier medic/combat lifesaver is certain that nerve agent poisoning has occurred. Use of atropine in the absence of nerve agent will cause the casualty to experience sweat inhibition and heat storage problems in a warm climate.
DRUG THERAPY
Atropine is the drug of choice for treating nerve agent poisoning. It will dry secretions, (including those in the airways), reduce bronchoconstriction, and decrease gastrointestinal motility.
Atropine will not relieve miosis and will not relieve muscle twitching or spasms.
Mild and Improving Symptoms
Observation is all that is needed for the casualty with mild symptoms such as rhinorrhea, slight or recovering breathing difficulty, or excessive salivation that is decreasing. In the casualty with mild symptoms that appear to be clearing, the one MARK I Kit administered during self-aid, followed by observation for several hours, will normally be all that is needed.
Pain in the eyes, twitching of the eyelids, redness, and miosis cannot be treated in the field setting by the soldier medic/combat lifesaver. However, at the battalion aid station (BAS), eye pain can be controlled with atropine eye drops. These conditions, although annoying, are not life threatening.
Severe Symptoms
If the casualty has severe symptoms involving two or more major organ systems (systemic) (gastrointestinal, skeletal muscle, respiratory, etc.), the first step is to administer all three MARK I Kits and diazepam. Diazepam should always be administered when the three MARK I Kits are given together. Additionally, more atropine (2 mg, Atropen) should be given every five minutes until secretions decrease or the casualty is breathing easier (or it is easier to ventilate him). A total of 15 to 20 mg of atropine may be required in the first 3 hours after the onset of symptoms.
Atropine. If the casualty is unconscious and in respiratory difficulty, three MARK I Kits and diazepam should be given immediately, followed by additional atropine as described above. Over the next 5 to 15 minutes, 10 to 15 mg of atropine may be needed. Atropine administered with the autoinjector will show some effectiveness in three to five minutes. During the time the atropine takes to reach maximum effect, the constriction and secretions in the airway and feeling of “tightness in the chest” will begin to decrease. Atropine will have a drying effect on salivation and rhinnorhea. Atropine (2 mg) should be administered at three to five-minute intervals until the casualty can tell the soldier medic/combat lifesaver that it is easier to breathe or manual ventilation becomes easier. Observe the casualty for indications that the atropine can be discontinued.
Discontinue atropine when:- Secretions of the mouth, nose, and lungs are minimized.
- The casualty tells you that breathing is easier, or it is easier to administer assisted ventilation.
Pralidoxime Chloride. (2-PAMCl) in the autoinjector (600 mg, 2 ml) is the second drug for use in nerve agent poisoning cases. The 2-PAMCl removes nerve agent from the enzyme acetylcholinesterase. The 2-PAMCl (included in the MARK I Kit) must be used as early as possible. If symptoms are severe, involving two or more organ systems (for example, the lungs and gastrointestinal tract), all three MARK I Kits and diazepam should be given immediately. Additional 2-PAMCl autoinjectors are not administered until an hour later. If severe signs or symptoms still persist one hour after using the three MARK I Kits, three additional 2-PAMCl autoinjectors should be administered. More than two sets of three 2-PAMCl (six total) must not be used. Excess 2-PAMCl may harm the casualty by dangerously raising the blood pressure.
Discontinue the use of 2-PAMCl after symptoms of respiratory distress have eased.
Diazepam in the 10-mg autoinjector is the drug adopted by the U.S. military for use in controlling convulsing patients. The doctrine for its use instructs the soldier to administer one diazepam autoinjector to his buddy immediately after using the third MARK I Kit in severe poisoning cases. Diazepam is not for self-use. It should be given only to severe casualties, and severe casualties cannot self-administer it. The key to increasing the effectiveness of the diazepam is administering it before convulsions begin. Again, when two or more organ systems become involved, one diazepam autoinjector should be administered along with the three MARK I Kits to lessen the convulsive activity the soldier may experience.
The soldier medic/combat lifesaver may administer a second and third diazepam autoinjector using the guidelines below.
After the first injection (buddy-aid):- Observe the casualty for about ten minutes.
- Turn the casualty on his/her side to facilitate breathing.
- Pad areas to prevent other injuries.
- Restrain if necessary.
- If still convulsing after ten minutes, give the second diazepam autoinjector.
Following the second injection (medical aid):- Observe the casualty for five to ten minutes.
- If still convulsing after five to ten minutes, give a third diazepam autoinjector.
VENTILATION
Although the use of pyridostigmine pretreatment will decrease the need for assisted ventilation in nerve agent casualties, the need will arise, on occasion, for assisted ventilation in some severe nerve agent casualties. Aggressive airway maintenance and the use of assisted ventilation will greatly increase the casualty's chances for survival.
Providing assisted ventilation in a potentially contaminated environment is possible using the Resuscitation Device Individual Chemical (RDIC) (see chapter on equipment). By using this device, the soldier can survive to reach the Level 1 care facility where mechanical ventilation can take over. Without this aggressive, far-forward resuscitation, the soldier will not survive.
PRETREATMENT
The U.S. military has adopted the policy of pretreating soldiers against the nerve agent's effect on acetylcholinesterase with pyridostigmine. Each soldier in the combat theater of operations is issued one package of pyridostigmine tablets. Each blister pack contains 21 tablets, and each tablet contains 30 mg of pyridostigmine. The soldier takes the pretreatment only on order from the unit commander. When ordered, one tablet is taken orally every eight hours. If a scheduled dose is missed, it will not be made up; the soldier will take one tablet at the earliest opportunity to begin the next eight-hour interval. The soldier will discontinue taking the tablets on order from the unit commander. The pretreatment should not be taken on a continuous basis for longer than 14 days.
Pyridostigmine bromide shields the acetylcholinesterase enzyme from the full effects of GD. It prevents GD from permanently and irreversibly binding the enzyme, which it would otherwise do in two minutes. Pyridostigmine enhances the efficacy of 2-PAMCl in GD casualties. The pretreatment does not increase the effectiveness of treatment for GB, GF, or VX. These nerve agents also become irreversibly bound to acetylcholinesterase but require many hours to do so, and the binding does not affect therapy.
Pretreatment alone will not protect the soldier and does not reduce the effects from the nerve agent. Pretreatment is not an antidote. When used in conjunction with the MARK I Kit, pyridostigmine enhances the effectiveness of the MARK I Kit against GD only. It is critical that the soldier medic understand that the effect of the pretreatment will have no effect on the severity of nerve agent poisoning symptoms. Therefore, an aggressive approach to care is still warranted. |
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