8. Nitrate therapy for first responders – July 2015
Glyceryl Trinitrate (GTN) comes in various presentations for use including transiderm patch applied to the skin, externally applied paste, an orally administered spray and fluid to inject into veins. Some of these may be encountered in paramedic use or hospital transfers.
The most common form seen by pre-hospital responders is likely the tablet form anginine. These can be bought from any pharmacy and are common patient possession. Patients may use the alternative presentation of liquid spray also easily administered into the mouth. Patients with known heart disease typically have a use plan from their doctor such as sit down and self administer a tablet every five minutes until symptoms resolve. Usually there will be an instruction to call for an ambulance after two or three tablets if the problem does not resolve.
The square shaped tablet is administered sublingually (under the tongue) or buccal (in the cheek). The tablet is not chewed or swallowed but to sit in the mouth and absorb through the oral mucosa. If it is chewed the drug will release from the tablet too quickly with dramatic effect. If the tablet is swallowed it will not be taken up at all. Instead it will be destroyed by the gastric actions and be ineffective.
Tablets usually contain 600mcg, the maximum dose permitted for any administration. They are kept in small brown bottles usually containing 30 other tablets when opened.
Indications allow circumstances to consider treating. GTN actually has a number of medical uses including ischemic heart disease, acute hypertension and even premature labour. For first responders the only indication for administration is for the patient complaining of chest pain or discomfort suspected to be of cardiac origin. The drug dilates blood vessels making it easier for the heart to contract blood out and so reduce its workload.
Contraindications are situations that overrule indications and stop administration, usually for patient safety. GTN has a large number of contraindications despite ease of purchase by patients and common use in self administration. The contraindications associated with GTN essentially stem from it causing blood pressure to drop.
The first contraindication is known hypersensitivity. This is not unique to this drug and exists for any drug. It needs to be clearly stated with some drugs where hypersensitivity is either more common or causes more alarming complications. GTN can act quickly and significantly on some patients producing uncomfortable responses including large drops in blood pressure or heart rate leading to patient collapse. Establish if a patient has taken GTN previously before administering any tablet and what the effect was on them. If the effect was problematic then administration of GTN should be avoided.
The second contraindication is a systolic blood pressure of less than 110mmHg. This is arbitrary but is a point of safety for the patient. Many patients will have a blood pressure higher than this. This blood pressure may be part maintained by constriction of peripheral blood vessels. Since the heart has to contract and eject blood against this, to lessen this constriction will make it easier for the heart to pump blood. This easing of work will ‘rest’ the heart and reduce the ischemia pain. This theory works very well in practice except if the blood pressure is too low. A systolic blood pressure below 100mmHg systolic may not fill the coronary arteries. This makes the balance worse not better. A blood pressure line at 110mmHg systolic means little risk of GTN causing hypotension and worsening the problem.
The third contraindication is concurrent use of a ‘Viagra’ type product such as Levitra or Cialis. Typically taken by men for sexual dysfunction each causes vasodilation. The combined effect can lead to hypotension. These drugs are not identical with differing duration of action time. The only way to determine if these drugs are being taken is to ask and this must form a part of the history prior to GTN administration. This may cause discomfort for first responder and patient but is essential for safety. Ask discreetly regardless of age or gender.
The fourth and fifth contraindications refer to low or fast heart rate. A fast heart rate has less time for blood to flow into the ventricle with each contraction. If the heart beats too fast not enough blood will enter the ventricle each beat for the next pulse. Blood pressure will drop. Administering GTN causes even less blood to flow to the heart and worsens the problem. A heart rate of 150 is an arbitrary line. This is where patient safety is at risk if exceeded. As heart rate slows there is more time for ventricular filling and better blood ejection but to a point. Once the heart rate becomes too slow the ventricle will have time to fill but there will not be enough heart beats to maintain blood pressure. Administration of a GTN tablet will worsen this situation. Safe administration of GTN requires a suitable safe heart rate.
Apart from the GTN contraindications there aren’t many other situations to be wary of it. The elderly patient should not be underestimated. There is no clear definition of ‘elderly’ but patient’s over sixty should be so considered. The concern is that as people get older their cardiovascular, nervous, renal and respiratory systems all become less effective. They are more susceptible to diseases and drug actions. If the blood pressure drops too much the patient may not have the ability to self recover. Drugs are less effectively metabolised. There are also other medications that cause further interference with body responses and new medications administration. Do not withhold treatment in the elderly but be cautious using smaller drug doses of GTN. The older the patient, the greater the assumed patient risk.
A whole square tablet contains 600mcg. This is a ‘usual’ dose frequently used by patients who have safely used GTN previously. It can be repeated according to protocols as required.
Each tablet has a score across it. This allows it to be broken to allow half dose administration. Where there are concerns regarding patient age or no previous administration then half doses of drug can be safely employed.
Medication safety point: Ensure gloves are worn at all times when administering tablets. Not only is this tablet to be placed into the patient’s mouth, but the drug can be absorbed through your own skin. This can lead to unwanted side effects. Break the table in half and offer it to the patient to pick up from your gloved hand or place it into the patient’s own hand for self administration. Discard the unwanted half tablet and do not return it to the bottle for use by other patients.
The side effects of GTN relate to the actions of the drug. Some dilation of peripheral blood vessels is desired but too much causes unwanted hypotension. This is arguably the most significant GTN side effect.
GTN does not have direct action on the heart rate. Any changes that happen are usually indirect reaction of the patient’s sympathetic nervous system to changes in blood pressure caused by vasodilation. This dilation of blood vessels can also occur in the patient’s head causing headache. Headache is a common but transient side effect.
The main GTN action is to cause relaxation and hence dilation of the arteries and veins. This leads to blood pooling in the peripheral circulation with two direct results. The first is that less blood will be returned to the heart. The less blood returned the less to be ejected with the next contraction. This directly reduces the work of the heart. Less work means reduced demand for oxygen.
The second direct result is when the heart contracts and ejects blood it must push out against blood already in circulation. If peripheral vessels are dilated, then resistance to each contraction is reduced. If the circulation is dilated with GTN, the heart has an easier job of ejecting blood with each subsequent heart beat.
When managing the patient with cardiac chest pain, glyceryl trinitrate (GTN or anginine) should be administered. This drug is not an analgesic drug. Instead it causes blood vessels to relax and dilate. The effect of this is that some blood is pooled in the feet. The reduced amount of blood returning to the heart means that there is less blood for the heart to push around the body. Less blood to be pushed by the heart means less work for the heart to do. This in turn reduces the heart’s need for oxygen. Of course local guidelines and protocols should be followed at all times when administering any medication. The patient must be assessed for effectiveness or problems. If the GTN causes problems such as collapse or hypotension, it should be removed from the patient’s mouth as soon as practicable.
GTN is a safe drug but can be significant in its actions. Before administration, ensure that the systolic blood pressure is at least 110mmHg. GTN can drop the blood pressure and anything lower than 110 will not be beneficial to the perfusion of the arteries of the heart. A reasonable blood pressure is needed to ensure that blood actually flows through the coronary arteries.
If the patient has not had any GTN previously only administer half doses to ensure the effects are not too excessive for the patient. It is always better safe than sorry. You can always administer more drug but it may be difficult to take it back out. If the patient has had GTN previously and had unwanted and unpleasant side effects then do not risk any further administration. If the patient has a sudden collapse or hypotension occurs the patient must be made to spit the tablet out. No GTN should be administered if this happens.
If the pain is not relieved by the first GTN administration, then further tablets may also be required. As long as the first tablet has not lowered the systolic blood pressure below 110 or produced any unwanted side effects then the first dose of tablet can be repeated every five minutes. Five minutes is the time it takes for each tablet to become active and effective.
It is always worth administering GTN first when treating cardiac chest pain. GTN is not an analgesic but reduces pain through its beneficial actions on the heart. Inhaled methoxyflurane, intranasal fentanyl or even injected morphine or fentanyl are analgesics and reduce the ability to be aware of the pain. If the pain persists despite the administration of one or more GTN tablets then this direct analgesia should be considered. Direct analgesia is most meaningful the more severe the pain the patient is complaining of. There is little advantage in treating mild pain less than two in severity but a lot of potential advantage in treating severe pain.
GTN may continue to be administered even after analgesia is commenced. The two drugs are providing different actions. Analgesia is also most appropriate if GTN is not safe to be administered making it the only remaining option.
Jeff Kenneally www.prehemt.com
Jeff is the author of the 2014 – 2016 Ambulance Victoria First Responder clinical practice protocols and accompanying education program.