Inhalation Anesthesia – Induction with Gas

Inhalation Anesthesia – General Anesthesia

The method of anesthetizing people with gas, general anesthesia, has a long tradition and several different anesthetic gases have been used over the years. Commonly, halogenated derivatives of ether (inhalation anesthetics) are used alone or in combination with nitrous oxide to put the patient into a controlled state of unconsciousness – anesthesia. Vaporized anesthetic gases such as isoflurane and sevoflurane have a narrow therapeutic range, so overdosing must be avoided and the amount of gas delivered must be precisely controlled via a vaporizer. Inhalation anesthesia has the advantage over intravenous anesthetics in that the patient’s end-tidal concentrations/partial pressures in exhaled air can be continuously measured, reflecting the levels in blood and brain. The patient’s need for anesthetics is age-dependent but also very predictable with small interindividual variations.

The potency of anesthetic gases is expressed using the MAC concept (MAC = minimal alveolar concentration). The concept of MAC was introduced in 1965 and is expressed as a percentage of anesthetic gas at 1 atmosphere pressure (ATM). MAC represents the minimum alveolar concentration that prevents 50% of a population (test animals, subjects, or patients) from responding to defined stimuli, such as a skin incision. MAC is a measure to compare the potency of the same anesthetic gas between different populations or the effects of different anesthetic gases on a given population. Another measure of the potency of gases is MAC-awake (0.3-0.5 MAC), the concentration required to block voluntary reflexes and perceptive awareness. One advantage of inhalation anesthesia is that in the event of an accidental overdose, the gas can be easily eliminated through the patient’s exhalation, providing good control during ongoing anesthesia.

Anesthesia depth is primarily monitored with clinical parameters, but technical equipment for anesthesia depth monitoring is improving and becoming more common in clinical practice. Anesthetic gases are normally dosed according to MAC to achieve a MAC of 0.6-1.0 when remifentanil is given simultaneously in continuous infusion. If fentanyl is used instead of gas but gas is primarily used, a MAC between 0.8 and 1.4 is normally aimed for. The vaporizer for the anesthetic is dosed in percentage, but the effect is read in MAC. At approximately 2 MAC, spontaneous breathing ceases, and at 3 MAC, the heart fails, and circulation collapses. The simultaneous use of nitrous oxide reduces the need for vaporized gas and thus increases the safety margin. Roughly speaking, the effect of two combined gases is additive. However, the use of nitrous oxide has significantly decreased in recent years and is now mostly used for induction of inhalation anesthesia in children and in conjunction with labor analgesia. During long anesthesia procedures, a constant MAC results in progressively higher concentrations in the brain, so the MAC should be adjusted slightly over time. The need for opioids and muscle relaxants is significantly reduced during inhalation anesthesia, which can be advantageous.

Inhalation anesthesia can be divided into controlled breathing anesthesia and spontaneous breathing anesthesia. Spontaneous breathing anesthesia, where muscle relaxants are not used at all, is possible when using gases via a breathing mask, laryngeal mask, or endotracheal intubation. Since the pressure gradient over a laryngeal mask is lower and the risk of gas leakage is therefore lower compared to mask anesthesia, intubation is less frequently required. Spontaneous breathing is not only physiological, but it also provides the correct anesthesia depth compared to controlled positive pressure ventilation. If the anesthesia depth is not correct, i.e., too deep or too shallow, it simply does not work, and it shows and is noticeable on the patient. Noting too deep or too shallow anesthesia is somewhat of an art and requires some routine from the anesthetist. Patient anxiety can be due to both too shallow and too deep anesthesia. An unsatisfactory airway never provides good conditions for good anesthesia and adequate anesthesia depth. A free airway is thus crucial for spontaneous breathing anesthesia, which can sometimes be challenging both with mask anesthesia and laryngeal mask anesthesia. The airway is not even guaranteed to be free when the patient is intubated. Anesthesia depth monitoring can help but is not always reliable.

Since the concentration of anesthetic gas is measurable and the depth of anesthesia is predictable, there is not always a need for other anesthesia depth monitoring such as BIS or “Entropy” in spontaneous breathing anesthesia. Anesthesia depth monitoring can be a useful tool to avoid too deep anesthesia and too slow awakening.

The awakening time depends on the tissue solubility of the gas/gases and the amount of inhaled anesthetic. Desflurane has about half the solubility of sevoflurane, which in turn is half as soluble as isoflurane. Nitrous oxide is significantly less soluble, especially in fat compared to vaporized gases. Modern anesthetic gases provide a quick and predictable awakening, compared to “MAC awake”.

In the 1990s, great efforts were made to introduce day surgery into clinical practice, primarily for cost-efficiency and faster discharge. Short-acting anesthetics became interesting to quickly wake up and send patients home after surgery. Previously, sevoflurane and desflurane were considered unsuitable due to high metabolism rates with fluoride formation and boiling points at room temperature. The pharmaceutical industry invested heavily in the documentation and marketing of these two gases. This contributed to increased knowledge and interest in inhalation anesthesia in general.

Over decades of use, it has been found that nitrous oxide can cause expansion of gas-filled cavities, inhibition of vitamin B metabolism (clinically relevant primarily in the anesthesia of vegans and certain rare genetic “inborn errors of metabolism”), a slight increase in the incidence of postoperative nausea, and diffusion hypoxia (which presupposes hypoventilation and/or air breathing without oxygen supplementation). Suspected negative effects on the environment and greenhouse effects in the atmosphere also led to a desire to eliminate nitrous oxide from the hospital environment. Since nitrous oxide combined with other vaporized gas constitutes about half of the anesthesia depth, eliminating nitrous oxide would result in significantly higher consumption of sevoflurane and desflurane.

Today, sevoflurane is mainly used in clinical practice in Sweden, but desflurane has also gained widespread use. The risk of kidney damage from sevoflurane feared at its introduction has fortunately not materialized. Both “compound A” from the circle system’s CO2 absorber and fluorides from metabolism could theoretically be nephrotoxic. Ironically, the old combination of isoflurane/nitrous oxide provides a faster awakening than a pure sevoflurane anesthesia. The supposedly quick awakening was the main point of sevoflurane’s launch. An advantage, especially in pediatric anesthesia, is that gas induction is easier to perform with sevoflurane. The patent period for sevoflurane and desflurane has expired, and inhalation anesthesia is no longer a hot topic, allowing alternative but not necessarily better anesthesia methods for patients to gain ground.

Sevoflurane (Sevorane®, Sevoflurane®)

Sevoflurane is a halogenated methyl isopropyl ether. It is an inhalation anesthetic used for induction and maintenance of general anesthesia. Sevoflurane is administered after vaporization via mask, laryngeal mask, or endotracheal tube.

Indication

Induction and maintenance of general anesthesia. Changes in the clinical effects of sevoflurane quickly follow changes in inhaled concentration. Like all inhalation anesthetics, sevoflurane provides a controlled degree of unconsciousness and pain relief (anesthesia) and also lowers cardiovascular function in a dose-related manner. The low solubility of sevoflurane in blood means that the alveolar concentration quickly increases during induction and quickly decreases when the inhalation anesthetic is discontinued. The blood/gas solubility coefficient is 0.68 for sevoflurane.

Dosage

Surgical anesthesia can be maintained with a concentration of 0.5 – 3% with or without the simultaneous administration of nitrous oxide. Normally, a MAC value between 0.8 and 1.6 is aimed for, usually 1.2 – 1.4. As with other halogenated volatile anesthetics, the MAC for sevoflurane decreases when administered in combination with nitrous oxide. The MAC value for Sevorane decreases by 25-50% for adults and about 25% for children when 60-65% nitrous oxide is given simultaneously. As with other inhalation anesthetics, older adults typically require lower concentrations of sevoflurane to maintain surgical anesthesia. In humans, less than 5% of absorbed sevoflurane is metabolized in the liver to hexafluoroisopropanol (HFIP) with the release of inorganic fluoride and carbon dioxide. HFIP is then rapidly conjugated with glucuronic acid and excreted in the urine. The rapid and extensive pulmonary elimination of sevoflurane minimizes the amount available for metabolism.

Awakening: Awakening generally occurs quickly after Sevorane anesthesia within 5-30 minutes. Therefore, patients may need postoperative pain relief early.

Warning: May trigger malignant hyperthermia in predisposed patients. Caution in severe renal impairment or significantly increased intracranial pressure.

How to Anesthetize with Sevoflurane

Sevoflurane is a halogenated methyl isopropyl ether. It is an inhalation anesthetic used for induction and maintenance of general anesthesia. Sevoflurane is administered after vaporization via mask, laryngeal mask, or endotracheal tube. Sevoflurane provides a controlled degree of unconsciousness and pain relief (anesthesia) and also lowers cardiovascular function in a dose-related manner.

For adult mask inhalation, initiate with 50% oxygen and start with 2% Sevoflurane and 4 liters of fresh gas flow. Then increase by half a percent every third breath. If the patient becomes significantly agitated after a few breaths, the gas can be increased to 5-6%, which can be maintained until the patient relaxes. Then adjust the gas concentration as needed according to the current surgery. Common levels of sevoflurane are around 1-2% with a fresh gas flow of about 4 l/min with at least 30% oxygen. The depth of anesthesia is controlled by the MAC value presented in gas monitoring. With the combination of nitrous oxide/sevoflurane, significantly lower levels of sevoflurane will provide adequate anesthesia depth (0.7-0.9%) compared to using sevoflurane alone in combination with oxygen (1.4-2.0%).

Blood/gas solubility coefficient 0.68

• N₂O reduces the need for sevoflurane by about 25%
• Non-irritating to the airway – suitable for induction
• Uterine relaxation
• Dose-dependent cardio-depression

Physiological effects of sevoflurane

• CO↓
• HF ±0
• SVR↓
• BP↓
• RR↑↑
• TV↓
• pCO2↑

MAC values for Sevoflurane

• Newborn 3.3%
• 25 years 2.5%
• 60 years 1.7%
• 80 years 1.4%

For mask induction in children, the same procedure can be used as for adults to achieve the appropriate concentration of sevoflurane and adequate anesthesia depth. This normally takes at least 5 minutes with spontaneous breathing. Alternatively, the vaporizer can be quickly turned up to very high values (6-8%) so that with just a few breaths (1-3), the child is anesthetized. If the child is ventilated, be aware that they typically receive significantly more anesthetic gas, achieving anesthesia depth faster but also increasing the risk of overdose. Children under one year usually need around 3% sevoflurane without nitrous oxide for the correct anesthesia depth. Between 3 and 12 years of age, they normally need around 2.5% for adequate anesthesia depth. The concentration of sevoflurane can be reduced if the gas is mixed with nitrous oxide by about one percent.

Desflurane (Suprane®)

Desflurane is a halogenated methyl ethyl ether (difluoromethyl-1,2,2,2-tetrafluoroethyl ether). It is an inhalation anesthetic used for the maintenance of general anesthesia via the airways. It is administered into the airways after vaporization via mask, laryngeal mask, or endotracheal tube.

Indication

Induction and maintenance of general anesthesia.

Changes in the clinical effects of desflurane quickly follow changes in inhaled concentration. Like all inhalation anesthetics, desflurane provides a controlled degree of unconsciousness and pain relief (anesthesia) and also lowers cardiovascular function in a dose-related manner. Desflurane should not be used for induction of anesthesia in children due to a high incidence of coughing, breath-holding, apnea, laryngospasm, and increased mucus secretion. The blood/gas solubility coefficient is 0.42 for desflurane. The low solubility of desflurane in blood means that the alveolar concentration quickly increases during induction and quickly decreases when the inhalation anesthetic is discontinued. Desflurane provides a dose-dependent reduction in blood pressure and respiration.

Dosage

Surgical anesthesia can be maintained with a concentration of 2.5-8% with or without the simultaneous administration of nitrous oxide. Usually, 4-6% desflurane is given in the inhaled air. Normally, a MAC value between 0.8 and 1.6 is aimed for, usually 1.2-1.4. In adults, surgical anesthesia can be maintained with a reduced concentration of desflurane when nitrous oxide is used simultaneously. Higher concentrations of desflurane may be indicated. However, the risk of hypoxia should be considered, and nitrous oxide/oxygen administration should be adjusted. The maintenance dose should be gradually adjusted in relation to the clinical effect.

Desflurane is indicated for maintenance anesthesia in infants and children. Surgical anesthesia can be maintained in children with end-tidal concentrations of 5.2 to 10% desflurane with or without the simultaneous use of nitrous oxide. Studies have shown that only 0.02% of absorbed desflurane is metabolized. Only marginal increases in inorganic fluoride can be seen in serum and urine. The rapid and extensive pulmonary elimination of desflurane minimizes the amount available for metabolism.

Awakening: Awakening generally occurs quickly after desflurane anesthesia, within 5-30 minutes. Therefore, patients may need postoperative pain relief early.

Side effects: May cause coughing and airway obstruction and increased mucus production in the airways. Should not be used for induction of anesthesia. Dose-dependent cardio-depression.

Warning: Too rapid an increase of desflurane in the inhaled air can cause airway irritation with bronchospasm and increased mucus secretion. Coughing, laryngospasm, apnea, and bronchospasm may occur. Desflurane is not recommended for induction via inhalation in children. May trigger malignant hyperthermia in predisposed patients. Caution in elevated intracranial pressure.

Anesthesia with desflurane (Suprane®)

Desflurane (DES) can be a natural and good choice of anesthetic gas when rapid awakening from anesthesia is desired. For example, when early mobilization is required, severe sleep apnea, difficult-to-intubate/anomalous upper airways, risky extubation, the need for early “recovery,” long procedures, or severe obesity.

Desflurane is characterized by certain unique properties; it has a sharp odor that causes bronchial irritation and sympathetic activation in an awake, lightly anesthetized patient or with too rapid an increase in DES and also a low solubility in both water and fat. The sharp, irritating odor makes induction (gas induction) with DES impossible and also makes it unsuitable for use in patients with increased bronchial reactivity, i.e., asthmatics, and especially children with obstructive problems, for whom the gas can be considered contraindicated. The generally low solubility in various tissues means rapid saturation in tissues with both normal and low perfusion, which in turn means that DES quickly and more completely leaves the body upon awakening compared to other, more fat-soluble inhalation gases. To achieve DES’s favorable effect on awakening time, it is important that during awakening, normoventilation is maintained and fresh gas flow is increased so that an effective concentration gradient alveolarly is achieved, thus optimal gas clearance is obtained.

General anesthesia with DES in combination with remifentanil (REMI), sometimes called SUPREMI (Suprane + Remifentanil), is preferably started with an initial fentanyl dose (for better-maintained circulation/blood pressure), after which the REMI infusion is started. Target value in TCI mode 4-6-(8) ng/ml. The propofol injection is given when the patient starts to feel the REMI, which he/she can communicate verbally during induction. Intubation is facilitated by a temporary increase in REMI, possibly combined with tracheal Xylocaine spray or the usual dose of muscle relaxants. If the airway is secured with a laryngeal mask, the initial fentanyl dose can be reduced or avoided, and the REMI target value can be reduced.

When the airway is secured with an endotracheal tube or laryngeal mask, mechanical ventilation is started with fresh gas flow (FGF) at 2 l/min and the vaporizer setting on DES at 7%. After 3-5 minutes, FGF is adjusted to low flow, and the DES value on the vaporizer can be reduced somewhat when MAC 0.7-0.8 is reached. Consider further reducing REMI to a target value around 2 ng/ml. Increase the REMI dose to an adequate dose (surgical anesthesia) 90 seconds before the incision. Maintain MAC 0.7-0.8 with desflurane during the surgical maintenance phase and control the depth of anesthesia with the REMI infusion.

If a MAQUET FLOW-i ventilator with AGC function is used, the ventilator automatically manages the supply of anesthetic gas and corrects FGF to the ventilator’s anesthetic gas system during induction, maintenance phase, and awakening. A target value around MAC 0.7-0.8 is chosen, and it is important that the change rate in AGC mode to reach the required gas concentration is set lower for DES than, for example, sevorane to avoid bronchial irritation. A suitable value is around 2-3 on the AGC function’s 8-speed scale in FGF.

DES may mean that as an anesthesiologist, you more regularly evaluate the degree of surgical stimulus and communicate with the operator. Water-soluble opioids for postoperative pain relief are given at least 30 minutes before awakening. High REMI doses can increase the initial postoperative opioid need.

In longer anesthesias and more extensive surgical procedures where long postoperative monitoring is planned, DES is advantageously combined with fentanyl, given in conventional, as-needed repeated doses. The postoperative opioid requirement can then be reduced.

During awakening, the need for reversal of muscle relaxation is assessed. The REMI infusion is discontinued, and the DES vaporizer is closed. Low FGF is maintained, and the patient is normoventilated. After final dressing and possibly bladder scanning, FGF is increased to 10 l/min with maintained normoventilation. The patient usually wakes up within 2-10 minutes. After waking control and observation of throat reflexes, extubation/removal of the LMA takes place. Awakening with a MAQUET FLOW-i ventilator with activated AGC function means that REMI is closed as above, but the MAC target value is set to “zero” only when all final practical steps are largely completed.

DES’s rapid clearance during awakening means that the patient quickly reaches consciousness and regains good muscle tone, which can be valuable if the patient has a complicated upper airway. Early consciousness allows for early mobilization/self-transfer from the operating table to the bed for continued postoperative care.

How to Anesthetize with Desflurane (Suprane®)

Laryngeal mask anesthesia with assisted or controlled ventilation and continuous infusion of remifentanil

• Start remifentanil in TIVA mode 0.25-0.40 micrograms/kg/min for 40-60 seconds. Then reduce remifentanil to 0.08 micrograms/kg/min or in TCI mode with a target value of 4-6-(8) ng/ml for 40-60 seconds, then reduce remifentanil to around 2 ng/ml.
• Give the induction dose of propofol and secure the airway with a laryngeal mask.
• Start mechanical ventilation with fresh gas flow (FGF) 2 liters/min + desflurane 7%. After 3-5 minutes, reduce FGF to low flow with maintained MAC 0.7-0.8.
• Increase remifentanil to an adequate dose (surgical anesthesia) 90 seconds before the incision. Normally, 0.2-0.5 micrograms/kg/min or 4-10 ng/ml is required.
• Circulation control (while waiting for surgical stimulus): tipping, fluid, ephedrine, atropine.

Intubation anesthesia with controlled ventilation and continuous infusion of remifentanil

• Start remifentanil in TIVA mode 0.30-0.40 micrograms/kg/min or in TCI mode with a target value of 4-6-(8) ng/ml for 40-60 seconds.
• Give propofol and muscle relaxants.
• Reduce remifentanil to 0.15-0.25 micrograms/kg/min or 3-4 ng/ml and secure the airway with endotracheal intubation.
• Start mechanical ventilation with fresh gas flow (FGF) 2 liters/min + desflurane 7%. After 3-5 minutes, reduce FGF to low flow with maintained MAC 0.7-0.8.
• When MAC 0.7-0.8 is reached, consider further reducing remifentanil to about 0.10 micrograms/kg/min, or target value 2 ng/ml.
• Increase remifentanil to an adequate dose (surgical anesthesia) 90 seconds before the incision. Normally, 0.2-0.5 micrograms/kg/min or 4-10 ng/ml is required.
• Blood pressure control (while waiting for surgical stimulus) by tipping, fluid, ephedrine, and/or atropine.

Maintenance phase

• Maintain desflurane with MAC 0.7-0.8 and control the depth of anesthesia with remifentan il infusion. Do not go below 0.15 micrograms/kg/min or target value 3 ng/ml in TCI.
• Evaluate surgical stimulus. Actively communicate with the surgeon.
• Consider giving antiemetics.
• Give postoperative analgesics at least 30 minutes before awakening.

Awakening phase

• Ensure that any local pain relief or block has been given. Assess the need for reversal of muscle relaxation.
• Five minutes before extubation (during dressing), stop remifentanil and desflurane administration while maintaining fresh gas flow (FGF).
• After bladder scan, increase FGF to 10 l/min and mechanically ventilate out the gas until the patient wakes up. Use a timer! The patient usually wakes up within 2-10 minutes.
• Communicate with the patient and check the throat reflex during extubation/removal of the laryngeal mask.
• Utilize the patient’s alertness for self and early mobilization/transfer to bed.

Isoflurane (Isoflurane®, Forene®)

Isoflurane is a halogenated ethyl methyl ether. It is an inhalation anesthetic used for induction and maintenance of general anesthesia via the airways in a closed breathing system. It is usually administered after vaporization via mask, laryngeal mask, or endotracheal tube.

Indication

Induction and maintenance of general anesthesia. Changes in the clinical effects of isoflurane quickly follow changes in inhaled concentration. Like all inhalation anesthetics, isoflurane provides a controlled degree of unconsciousness and pain relief (anesthesia) and also lowers cardiovascular function in a dose-related manner. The low solubility of isoflurane in blood means that the alveolar concentration quickly increases during induction and quickly decreases when the inhalation anesthetic is discontinued. The blood/gas solubility coefficient is 1.4 for isoflurane.

Dosage

Surgical anesthesia can be maintained with 1-2.5% isoflurane in oxygen/nitrous oxide. A higher concentration, 1.5-3.5% isoflurane, is necessary if administered with pure oxygen. For cesarean section, 0.5-0.75% isoflurane in oxygen/nitrous oxide is recommended. As with other halogenated volatile anesthetics, the MAC for isoflurane decreases when administered in combination with nitrous oxide. MAC decreases with increasing age. The MAC value for isoflurane decreases by about 50% for adults and about 25% for children when 60-65% nitrous oxide is given simultaneously. As with other inhalation anesthetics, older adults typically require lower concentrations of isoflurane to maintain surgical anesthesia. The biotransformation of isoflurane is minimal in humans. On average, about 95% of administered isoflurane is found in exhaled air. The rapid and extensive pulmonary elimination of isoflurane minimizes the amount available for metabolism. Awakening: Awakening generally occurs quickly after isoflurane anesthesia. Therefore, patients may need postoperative pain relief early.

Side effects

Should not be used for induction of anesthesia. Dose-dependent cardio-depression.

Warning

May trigger malignant hyperthermia in predisposed patients. Caution in significantly increased intracranial pressure. Rare cases of hypersensitivity (including contact dermatitis, rash, dyspnea, wheezing, chest discomfort, facial swelling, or anaphylactic reaction) have been reported, primarily in connection with long-term occupational exposure to inhalation anesthetics, including isoflurane.

Nitrous Oxide N₂O

Nitrous oxide is a medical gas with good analgesic effects and moderate anesthetic effects. Nitrous oxide was discovered in the 1700s and came into medical use in the mid-1800s due to its analgesic and sedative properties. By the late 1800s, nitrous oxide began to be used for analgesia during childbirth and dental extractions and other minor surgical procedures. Nitrous oxide was a standard agent in anesthesia throughout most of the 20th century. During the 2000s, the use of nitrous oxide decreased but can still be of great benefit when used correctly.

Nitrous oxide effects N₂O

• Blood/gas solubility coefficient 0.42
• Minimal effect on circulation and respiration.
• BP ±0
• TV↓
• RR↑
• MAC: 105%. During induction 30-70 volume % with O2 + possibly another anesthetic gas.

Cave: Pneumothorax, pneumopericardium, gas emboli , head injury, ileus, B12 deficiency

Indication

Maintenance of general anesthesia. Analgesic for childbirth and minor surgical procedures such as dental extractions. Prehospital analgesic. Nitrous oxide has an additive effect when combined with most other anesthetics, both intravenous drugs, and inhalation anesthetics. The MAC value is stated to be 105%. Nitrous oxide potentiates the effect of other inhalation anesthetics in such a way that their concentration can be significantly reduced to maintain the same anesthesia depth (MAC value). Thus, anesthesia with nitrous oxide and other inhalation agents has less hemodynamic impact than without nitrous oxide. Nitrous oxide has dose-dependent effects on sensory perceptions and cognitive functions that begin at 15 volume percent. Concentrations above 60-70 volume percent cause unconsciousness. Nitrous oxide has dose-dependent analgesic properties that are clinically noticeable at end-tidal concentrations around 20 volume percent. The blood/gas solubility coefficient is 0.46 for nitrous oxide. The low solubility of nitrous oxide in blood means that the alveolar concentration quickly increases during induction and quickly decreases when the inhalation anesthetic is discontinued. Nitrous oxide provides rapid blood saturation and reaches equilibrium faster than other inhalation anesthetics.

Dosage

For general anesthesia, nitrous oxide is usually used in concentrations between 35-70 volume percent mixed with oxygen and, when necessary, other anesthetic agents. Typically, an oxygen/nitrous oxide mixture of 1:2 or 1:1 is given. Nitrous oxide alone is usually not potent enough to create surgical anesthesia and should therefore be combined with other anesthetic agents when used for general anesthesia. Nitrous oxide is rapidly eliminated from the body after short-term inhalation, and its effects on psychometric functions usually subside about 20 minutes after the administration is stopped.

Awakening

Awakening generally occurs quickly after nitrous oxide anesthesia. Therefore, patients may need postoperative pain relief early.

Warning

Gas-filled cavities can expand due to the ability of nitrous oxide to diffuse. As a result, nitrous oxide is contraindicated in patients with symptoms of pneumothorax, pneumopericardium, gas emboli, severe head injury, ileus, or distended intestines. In case of suspected or deficiency of vitamin B12 or symptoms consistent with methionine synthase inhibition, vitamin B supplementation should be given to minimize the risk of side effects/symptoms related to methionine synthase inhibition such as; leukopenia, megaloblastic anemia, myelopathy, and polyneuropathy.

NO (Nitric Oxide)

NO functions as a signaling molecule in the body and is responsible for various physiological functions, including dilating smooth muscle in blood vessels.

Indication

• Pulmonary hypertension.
• Severe ARDS.

Inhaled NO can improve oxygenation in severe ARDS with hypoxemia and provide a partial reduction in blood pressure in the pulmonary circulation without affecting systemic blood pressure. There are significant individual differences in therapeutic response among patients. NO as a gas-form medication is available for strictly controlled and limited clinical license use in intensive care. NO is involved in functions such as vasodilation, immune defense, and regulation of cell respiration. NO formed in vascular endothelium plays a major role in regulating blood flow and counteracting platelet and white blood cell aggregation.

Dosage

The doses of NO tested for ARDS have varied between 5 and 40 ppm in inhaled air in closed breathing systems via respirator tubes.

Contraindications

Treatment with sildenafil (Viagra).

Warning

May cause methemoglobinemia.

Oxygen (O₂)

Oxygen is essential for human cellular respiration and is necessary for normal aerobic cell metabolism. Normally, the concentration is about 21% in the air, but it varies with the partial pressure. Oxygen is always administered in all closed ventilator systems, at least 21%. Oxygen counteracts nausea secondary to hypoxia. Normal saturation in arterial blood is 95-98%. Saturation below 90% increases the risk of organ damage. Saturation below 70% is tolerated only briefly. Saturation below 50% is immediately life-threatening. Supranormal values cause vasoconstriction. Can cause alveolar hypoventilation.

Dosage

Administered via nasal catheter, cannula, breathing mask, or through a closed breathing system.

Standard dose

Normally given at 30-40% in inhaled air. 2-17 l/min during spontaneous breathing. With the Optiflow system, oxygen can be administered at even higher flows up to 60 l/min. In the treatment of COPD with respiratory insufficiency, oxygen is usually administered in low doses, 0.2-1 l/min, which requires a specially calibrated flow regulator.

Cave

Caution in respiratory insufficiency and hypoventilation. Caution after treatment with cytostatics of the bleomycin type.