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Topics Reviewed: Rapid Sequence Intubation, Induction Agents, Glasgow Coma Scale

Journal Article: Upchurch CP, Grijalva CG, Russ S, et al. Comparison of Etomidate and Ketamine for Induction During Rapid Sequence Intubation of Adult Trauma Patients. Ann Emerg Med. 2017;69(1):24-33.e2.

 

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What is rapid sequence intubation (RSI)?

 

In situations in which a patient’s air passages are at risk of being blocked, a tube can be inserted from the oropharynx into the trachea to maintain an open pathway from the lungs to atmosphere. This process is called endotracheal intubation. Insertion of an endotracheal tube can cause significant patient discomfort and initiate the gag reflex. As a result, medications are administered prior to insertion to sedate and paralyze the patient, a process referred to as induction. Because induction agents prevent the patient from breathing on his/her own, a bag must be used to manually drive air in and out of the tube after insertion. This procedure is called bag ventilation.

 

RSI is a method of more quickly achieving endotracheal intubation, primarily through the use of fast-acting induction agents.

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When is RSI performed?

 

The main advantage of RSI is the reduced need for bag ventilation. This is because the duration of action of fast-acting induction agents is short enough to prevent oxygen levels from getting too low while the patient is unable to breathe on their own. The need for bag ventilation can be further reduced by filling the patients lungs with oxygen beforehand, a procedure called pre-oxygenation.

 

Although the primary purpose of bag ventilation is to deliver air to the lungs, some air can also reach the stomach, and thereby increase the risk for stomach contents going back up the esophagus and into the trachea (which is called aspiration). The introduction of gastric contents into the airway not only compromises airflow into the lungs, but also introduces bacteria into the lungs which can cause pneumonia. The risks are further increased in trauma patients because they often have not fasted prior to the procedure, and therefore have more gastric contents compared to patients who are usually instructed to fast prior to a scheduled surgery.

 

Because induction agents used in RSI have a short duration of action, the patient’s respiratory drive is reduced for a shorter amount of time, and therefore does not require as much bag ventilation.

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What is ketamine?

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Ketamine is a medication used a variety of (usually short) procedures as anesthesia. It is prepared in a solution that can be injected into the bloodstream or into the skin.

 

When injected into the bloodstream, it quickly crosses the blood brain barrier due to its high lipid solubility, which also contributes to its rapid-onset of action. Within the brain, ketamine has numerous different effects but the perhaps most important ones to remember for the purposes of this article are antagonism of NMDA receptors and increased intracranial pressure.

 

NMDA receptors are located on post-synaptic neurons. They are activated by the neurotransmitters glycine and glutamate, which then causes influx of sodium into the post-synaptic neuron. Therefore, antagonism of NMDA receptors causes decreased excitation of neurons and the neurological pathways are a part of. In particular, ketamine antagonizes NMDA receptors of neurons connecting the limbic system to the thalamocortical tracts. Recall that the thalamocortical tracts connect the thalamus (the sensory input relay station of the brain) to the sensory and motor cortices. In broad terms, the limbic system processes emotions and memories. Separation of these two areas of the brain can cause muscular rigidity unresponsive to external stimuli (catalepsy), decreased memory formation (amnesia), and decreased pain sensation (analgesia). Collectively, these features are known as “dissociative amnesia.”

 

The mechanism by which ketamine causes increased intracranial pressure is unknown.

 

 

What is etomidate?

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Like alcohol and benzodiazepines, etomidate is an agonist of the GABA-A receptors in the central nervous system and therefore cause sedation. Etomidate has also inhibits 11-beta hydroxylase, an enzyme found in the adrenal glands and involved in the synthesis of cortisol and aldosterone. Both cortisol and aldosterone aid in reabsorption of water excreted into the kidney tubules (by increasing expression of epithelial sodium channels in the distal convoluted tubule), and therefore prevent excessive fluid loss from the body. Because trauma patients have low fluid volume due to blood loss, inhibition of cortisol and aldosterone synthesis by 11-beta hydroxylase can be an especially concerning side-effect.

 

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What is the Glasglow Coma Scale?

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GCS is a numeric score used to describe a patient’s level of consciousness. It is based on the level of stimuli needed to elicit eye opening as well as verbal and motor responses from the patient (see below). Note the that a completely different set of criteria are used to calculate GCS scores in children.

 

MDCalc has great online tools for calculating GCS scores in adults and children. Be sure to do a quick Google search for them!

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How is the GCS score used?

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In adults, the GCS score is used primarily in verbal communication to provide a general picture of the patient’s level of consciousness. It may be assessed in the ED and regularly throughout a subsequent hospital stay to document the patient’s progression in the patient’s chart. 

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In children, the GCS score can be used for similar purposes, but evidenced-based protocols have also been developed to utilize the score along with physical exam findings to aid physicians in deciding whether to obtain a head CT scan for children being assessed for head trauma.

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What is the Injury Severity Score?

 

ISS is used to quantify the severity of a injuries to a trauma patient. It is based on a qualitative assessment of 9 different body regions and ranges from 0-75. “Major trauma” has traditionally been defined as ISS ≥ 15, which was shown to in early to studies to be associated with 10% mortality. The score has also been shown to be predictive of morbidity and hospital-length of stay. While the ISS is useful in quantifying severity for the purposes of research, its use in clinical decision-making or assessment of prognosis for individual patients remains limited.

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Review contributed by Christopher Ding, UVASOM Class of 2018