Barbiturates are medications that help calm and depress the central nervous system. This review will discuss the mechanism of action of Barbiturates, what conditions they treat, nursing implications, side effects, and patient education.
Barbiturates Pharmacology Nursing Review
Barbiturates are a class or group of medications that depress the central nervous system (CNS). Therefore, they produce a calming and sedative effect to the central nervous system.
How to recognize them? Most generic names end with barbital or ital with an exception to a few like Primidone (seizures).
- Phenobarbital (seizures, increased ICP)
- Pentobarbital (veterinary medicine or capital punishment)
- Secobarbital (prior to surgery)
- Butalbital (migraines in combination with acetaminophen and caffeine)
- Methohexital (anesthesia)
*This class of medications are not as commonly prescribed for anxiety or insomnia as in the past because they have a very low therapeutic index (meaning there is a small threshold between the therapeutic dose and a toxic dose) which has contributed to many accidental overdoses in the past. Instead, they’ve been replaced by Benzodiazepines for these conditions.
How do they work….mechanism of action?
Barbiturates amplify the effects of the neurotransmitter GABA (gamma-aminobutyric acid) by influencing GABAA receptors. GABA is an inhibitory neurotransmitter.
What is a neurotransmitter? It’s a chemical agent produced and released by a neuron that TRANSMITS messages to a specific structure that it was designed to target via that target’s special receptors. Many times the target is another neuron.
Neurotransmitters can be grouped as being excitatory or inhibitory. Some neurotransmitters can do both depending on where they are acting in the body. However, GABA is one of the main types of neurotransmitters in the body that is an inhibitory neurotransmitter.
What’s the difference between excitatory and inhibitory neurotransmitters?
Excitatory: they excite and increase the potential of an action… like the neuron firing and the message being carried out
- If we were treating anxiety or seizures, we wouldn’t want to excite the neurons even more but calm them down.
Inhibitory: they impede and decrease the potential of an action by preventing an action…hence stopping the message from being carried out….this helps create a CNS depressant state or calming effect
The neurotransmitter GABA binds to GABA receptors. Many GABA neurons are located in the limbic system, which is the place in our brain that helps us process our emotions and the way we behave. Now there are two types of GABA receptors: GABAA and GABAB receptors
Barbiturates influence GABAA receptors.
GABAA receptors are ligand-gated or also called ionotropic receptors. These terms describe how the receptor works to complete its job. When these GABA receptors are influenced by Barbiturates and binding occurs, a channel opens to allow the movement of ions into the cell, specifically chloride (CL-). This is going to majorly affect how the nerve fires (hence it will inhibit action potential).
Now let’s take a closer look at these types of receptor and how they work to depress the CNS.
GABAA receptors have the following characteristics that allow them to work:
The receptor is designed with 5 sections. Notably called subunits:
- 2 alpha “α” subunits
- 2 beta “β” subunits
- 1 gamma “γ” subunit
In between these subunits is a hole (pore) that contains a channel. And when the channel opens due to binding, it allows chloride to move through the channel into the cell. This causes hyperpolarization and is responsible for inhibiting potential action…hence providing calm, tranquilizing effects.
It has various binding sites: Notably two sites for the neurotransmitter GABA (found between each alpha and beta subunit) and a binding site for the Barbiturate (on the alpha or beta subunit).
So, what happens? Neurons release the neurotransmitter GABA. GABA rains down onto the GABAA receptors and finds its binding site between the alpha and beta subunits. Now if a Barbiturate has been administered its chemical substance that is hanging out finds its special binding site at the alpha or beta subunit. Then due to this binding the channel opens and Chloride flows down into the cell and hyperpolarizes it.
Sounds familiar to how Benzos worked right? Yes, but Barbiturates cause the channel to stay open longer so that more negative chloride ions freely flow. BENZOS cause the channel to open more often rather than stay open longer.
Now an interesting thing about barbiturates is that in very high dosages they can influence GABAA receptors (hence open the chloride channel) without the help of the neurotransmitter GABA, which will allow the flow of chloride via the channel and cause hyperpolarization of the cell. Benzos can’t do this.
Now let’s talk about nursing responsibilities, side effects, and education pieces for the patient. In order to help us remember the main concepts let’s remember the word:
Barbiturates and Benzos differences:
- They each bind at different locations on the GABAA receptors.
- High doses of barbs can influence the receptor without the GABA neurotransmitter binding.
- Barbs have a low therapeutic index, no antidote for overdose (Benzos have an antidote called Flumazenil)
- Benzos have replaced Barbs for conditions like anxiety and insomnia.
Abuse, addiction, dependence high risk (long-term usage, history of substance abuse or suicidal ideations)
Recognize Overdose Signs and Symptoms…no antidote: supported with mechanical ventilation (most people die from respiratory failure), dialysis to help remove medication from blood, and activated charcoal may be used. Educate the patient to avoid alcohol and other CNS depressants.
- Loss of consciousness (coma)
- Eye pupils pinpoint and then progress to dilation and fixed as it progresses and body shuts down
- Tachycardia (weak)
- Have cold and sweating skin
- Absent reflexes
- Low and shallow respirations
Be aware of patient safety
- Barbiturate make patients feel very relaxed, drowsy, euphoric, and they won’t be thinking clearly. Inform the patient about safety measures such as help with ambulation, bed in lowest position with side rails up, no important decisions to be made while under the influence of this medication, or driving or operating any type of machinery or appliances that take concentration.
- In addition, inform about safety risks related to accidental overdose, addiction, withdrawal, and tolerance.
Index low (therapeutic) monitor serum drug levels…ex: range for Phenobarbital: 15-40 mcg/mL
Tolerance can develop: this occurs when the person cannot obtain the same effects from the drug and needs a higher dosage to get the same effects, which can lead to accidental overdose/death…happens with abuse or long-term usage.
Stopping drug abruptly, especially after long-term use can lead to withdrawal signs and symptoms. In severe cases a patient can experience these signs and symptoms as soon as 24 hours without the medication.
- CRAVES (mild to severe)
- Vomiting and nausea
- Elevated temperature
- Sleep problems
- —can progress to shock and then death
Drug Facts on Barbiturates. Dea.gov. (2021). Retrieved 8 April 2021, from https://www.dea.gov/factsheets/barbiturates.
Skibiski J, Abdijadid S. Barbiturates. [Updated 2020 Nov 20]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539731/