Video: Inhalation-Induction Anesthesia New England Journal of Medicine

On October 16, 1846, the world's first inhalation-induced anesthesia
was publicly demonstrated at the surgical amphitheater of the Massachusetts General Hospital.
This epoch-making event changed not only the future of surgery, but even the future of medicine as a whole
.

Today, induction of general anesthesia by inhaling anesthetic gas remains an extremely important technique, especially for children and patients with
intellectual or developmental disabilities or those with pinchophobia.

The New England Journal of Medicine (NEJM) published the video "Inhalation Induction Anesthesia" recorded by Professor Rafael Ortega of Boston Medical Center in the "Clinical Medicine Video" column, including indications, pharmacokinetic characteristics, patient preparation, inhalation induction anesthesia methods and complications
.

We introduce the main content of inhalation-induced anesthesia here and publish Chinese subtitled video
.
Indications

Inhalation-induced anesthesia is most commonly used in children who experience pain or pain during an awake venipuncture, as well as in adults with difficulty obtaining intravenous access and in
patients with intellectual or developmental disabilities or pinchophobia.
Inhalation-induced anesthesia is also used in patients with spontaneous respiratory arrest that may be harmful, such as those with airway or cardiovascular compression due to anterior mediastinal masses
.
However, there is insufficient evidence to support the idea that inhalation-induced anesthesia and maintenance of spontaneous breathing is more effective in preventing airway collapse
than positive pressure ventilation.
Inhalation-induced anesthesia should be considered in patients with multiple drug allergies, as allergic reactions to inhaled anesthetics are extremely rare
.

Although inhalation induction has its own unique value, due to advances in technology such as video laryngoscopy, inhalation induction anesthesia is no longer used
in difficult airway management.
The development of new drugs, such as propofol and dexmedetomidine, and the ability to use sugamadex to rapidly reverse the effects of neuromuscular blockers, also reduces the need for
inhalation-induced anesthesia.

Intravenous induction of general anesthesia is the preferred method of induction and is faster and less
complicated than inhalation induction.
Inhalation induction is contraindicated in patients predisposing to malignant hyperthermia
.
Relative contraindications include hemodynamic instability and elevated intracranial pressure, and inhalation induction
should also be used with caution in patients known to be at risk of aspiration.

Pharmacokinetic characteristics

Commonly used potent inhalation anesthetics such as sevoflurane, are nonionized halogenated ethers, have a small molecular weight, and can easily pass through semi-permeable membranes
by simple diffusion.
Another inhaled anesthetic, nitrous oxide, is an inefficient inorganic compound that acts as an adjunct to potent drugs
.
Nitrous oxide is administered at a much higher concentration than potent inhalation anesthetics and can be rapidly dispersed into a gas-filled space
.
Should
be avoided in patients with pneumothorax, intracranial air buildup, or intestinal obstruction, as well as in patients at risk of venous or arterial air embolism.

The absorption of inhaled anesthetics is a passive process in which the rate of absorption is determined
by the solubility of the gas, the alveolar concentration gradient, and the area and thickness of the alveolar membrane.
Figure 1 shows the anesthetic absorption process, showing the ratio of the anesthetic concentration fraction (FA) to the inhaled anesthetic concentration fraction (FI) in the alveoli over time
.

Figure 1.
Absorption of inhaled anesthetics

The growth rate of the FA to FI ratio depends on the amount of anesthetic entering the lungs and the amount of anesthetic diffused into the blood from the lungs
.
Therefore, during induction, the lower the solubility of the inhaled anesthetic, the faster
the concentration of the drug in the alveoli rises.
Once the inflection point of the curve is reached, the diffusion rate of anesthesia in the alveoli slows down and the ratio of FA to FI gradually approaches 1
.
The FA-to-FI ratio in infants and children rises faster than in adults
.
This difference stems from the fact that infants and children have a greater ratio of alveolar ventilation to functional residual gas than adults
.

A common method of accelerated inhalation anesthesia induction anesthesia is to give an anesthetic higher than the expected target concentration in the alveoli
.
Anesthetic gas enters the systemic circulation from the lungs through the alveoli-capillary membrane, and arterial blood rapidly transports the anesthetic to hyperperfusion organs, such as the brain
.
Over time, anesthetic concentrations in muscle, fat, and other low-perfusion tissues also rise
.

At any point in time, the concentration of an inhaled anesthetic in a tissue depends on the partial pressure of the drug in the alveolar and arterial blood, the blood flow to the tissue, and the solubility
of the drug.
In the United States, the most commonly used anesthetics for inhalation-induction are sevoflurane, which is irritating and poorly tolerated, while nitrous oxide has a weak anesthesia effect and cannot be used alone for inhalation-induced anesthesia
.

Patient preparation

Preoperative visits to the patient or his/her guardian (if one has one) to explain the anesthesia process and describe sensations that the patient may experience during anesthesia induction, such as irritation, dizziness, and disorientation
of inhaled anesthetics.
If the patient is a child, explain the anesthesia procedure
to their parent or guardian.
To alleviate anxiety in children, toys and games
are available.
A paediatrician present, a medical professional who helps children cope with surgery, may be helpful
.
The presence of a parent or guardian helps the patient follow instructions
.
For pediatric patients, it may be beneficial
to introduce them to masks and encourage them to decorate them or apply aromatic odors.

Administration of anxiolytic drugs, such as benzodiazepines, may be helpful
in patients without intravenous access.
If the drug is given before anesthesia, the patient's level
of sedation should be monitored.

Transfer the patient to a stretcher and take the patient to the anesthesia room
.
Minimize noise and unnecessary conversation, young children and people with intellectual or developmental disabilities, may be particularly sensitive
to stimuli, including sound and touch.

The use of masks and other personal protective equipment (PPE) in clinical work is also changing rapidly due to the impact of
the SARS-CoV-2 outbreak and its variants.
Clinicians should follow institutional guidelines
.

equipment

Equipment necessary for inhalation induction includes anesthetic delivery devices with respiratory circuits; Physiological monitors, including non-invasive blood pressure monitors, pulse oximetry, carbon dioxide and electrocardiogram monitors; Attraction facilities; airway management equipment, such as transparent masks, oral or nasal ventilation tracts, laryngoscopes with handles and blades, supraglottic airway equipment and tracheal catheters; First aid drugs, such as succinylcholine and atropine for intramuscular or intravenous injection; Devices for establishing intravenous access; Anesthetic drugs, including inducing drugs and neuromuscular blockers
.
Make sure all devices are working properly and make sure the device you choose matches
the patient's age and body type.

Inhalation induction of anesthesia is performed

Placement of monitors during preparation and induction may cause agitation in some patients
.
The pulse oximetry monitor is installed first, and other physiological monitors can be installed as long as the patient can tolerate it
.

Start giving oxygen, fully opening the adjustable pressure limiting valve to ensure there is no resistance
to the breathing circuit.
Tell the patient that you want to put the mask on his face, be aware that this may cause anxiety and claustrophobia
in the patient.
Instruct the patient to breathe
normally.
Open the evaporator and gradually increase the inhalation concentration
of the anesthetic.

Ensure that the mask is well sealed, and any inhalation of ambient air will dilute the anesthetic concentration, thereby prolonging the induction time
.
Poor sealing can also affect the accuracy of carbon dioxide monitors and spirometry monitors and may expose healthcare workers to inhaled anesthetics
.

With each breath, the patient pays attention to the fog on the mask, the rise and fall of the patient's chest, the color of the patient's face and lips, and the carbon dioxide chart
.
Listen carefully to the frequency and tone
of the pulse oximetry monitor.
The patient's consciousness is gradually lost
.

During anesthesia, patients may be restless and have involuntary movements
.
If these movements are excessive, the patient may need to be restrained
.
Patients may also have irregular breathing, tachycardia, and hypertension
.
During anesthesia, the patient's upper and lower limbs often become stiff, the abdominal wall tension increases, the patient's eyelash reflex disappears, the pupil is enlarged, non-isotropic deviation may occur, conjunctival congestion may occur (Figure 2), and tear secretion increases
.

Figure 2.
Non-co-directional skew

During this period, airway reflexes are highly reactive
.
Attempting to connect more monitors or establish intravenous access may induce laryngospasms
.
If necessary, the airway
should be maintained by raising the chin and holding the jaw.
Consider insertion into the oral or nasal ventilation tract
.
If necessary, adjust the pressure limiting valve to provide positive pressure ventilation
.

Observing the movement of the air storage sac, touching or pressing the air bag simultaneously with the patient's inhalation, can understand the patient's breathing strength and lung compliance
.
To prepare for gastric contents reflux and secretion build-up, this requires sucking fluid from
the mouth.

Continuing to increase the concentration of inhaled anesthetics, the patient's breathing should gradually become more regular, the heart rate becomes slower, the pupils constrict, the gaze becomes co-directional, and the secretion of tears decreases
.

Once the patient reaches a deeper depth of anesthesia, muscle tone decreases, at which point any monitoring is safe and an attempt can be made to establish intravenous access
.
More anesthetic drugs are given, and if necessary, intraairway instruments and endotracheal intubation
are continued.
However, before endotracheal intubation, consideration should be given to reducing or discontinuing the use of inhaled anesthesia to avoid the appearance of unexpected concentrations
during auscultation of breathing sounds.

Once the airway is safe, the inhaled drug concentration can be adjusted so that it can maintain anesthesia
.
For patients who are able to follow the instructions, another method is to induce anesthesia
using a breath.
That is, the patient inhales a high concentration of anesthetic in one breath
.
Either way, nitrous oxide can be used as an auxiliary induction
.
Nitrous oxide is not irritating and accelerates the induction process
of potent inhalation anesthetics.

The breathing circuit is filled with 100% oxygen and 8% sevoflurane
.
Instruct the patient to exhale vigorously and then inhale as deeply as possible directly from the mask
.
Tell the patient to hold their breath for as long as possible and then breathe normally until consciousness disappears
.

Discontinue nitrous oxide before attempting intubation to ensure a high inhaled oxygen concentration, always ensuring that the patient's inhaled oxygen concentration is sufficiently high
.
Continuous monitoring of blood oxygen saturation
.

complication

Inhalation-induced complications of general anesthesia include asphyxia, airway obstruction, aspiration, laryngeal spasm, arrhythmias, hemodynamic instability, and circulatory collapse
.
Clinicians must remain vigilant and prepared to deal with these and other complications
.

For trained physicians, inhalation-induced general anesthesia is a reliable technique
for both children and adults.
For clinicians, it is extremely important
to understand the pharmacology, equipment, and methods.
This allows appropriate treatment to be provided and complications avoided
.
Adequate preparation, careful patient selection and continuous monitoring allow for safe inhalation of potent anesthetics for general anesthesia
.

References

Conley C, Brook K, Louca J, Haber A, Nozari A, Ortega R.
Inhalational induction.
N Engl J Med 2022; 387:e19.

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