Inhalation Anesthesia. Anesthetic Agents.

Inhalation Anesthesia

Posted by Kai Knudsen, Senior Physician in Anesthesia & Intensive Care. Sahlgrenska University Hospital and Jan Peter Bengtsson, Senior Physician in Anesthesia & Intensive Care.
Updated 2018-12-21


The method of anaesthetizing people with gas to give general anesthesia has a long tradition and several different anaesthetic inhalation agents have been used over the years. Generally, halogenated derivatives of ether (inhalation anaesthetic agents) are used alone or in combination with nitrous oxide to pass the patient into a controlled form of unconsciousness – anesthesia. Vaporized inhalation agents such as isoflurane and sevoflurane have a small therapeutic range, so overdose must be avoided and the amount of gas administrated is controlled with high precision via a vaporizer chamber. Inhalation anesthesia has the advantage over intravenous anesthesia by continuously measuring the patient’s end-tidal concentrations/partial pressure in the exhaled air, which in turn reflects the levels in blood and brain. The patient’s needs for anaesthetic agents are age-dependent, but it is also very predictable with small inter-individual variations.

The potency of the anesthetics is expressed by the MAC concept (MAC = minimal alveolar concentration). The term MAC was introduced in 1965 and expressed in percent anesthesia gas at 1 atmospheric pressure (ATM). MAC expresses the minimum alveolar concentration that causes half of a population (experimental animals, subjects or patients) not to respond to defined stimuli, such as a skin incision. MAC is a measure of comparing the same anesthesia’s potency between different populations or the effects of different volatile anesthetics on a particular population. Another expression of gas potency is MAC awake (0.3-0.5 MAC), the concentration required to block voluntary reflexes and perceptive awareness. An advantage of inhalation anesthesia is that in case of accidental overdose, one can eliminate the supply of gas through the exhalation of the patient, which provides good controllability during ongoing anesthesia.

Inhalation Anesthetic Agents Physiological Data

 Nitrous OxideIsofluraneDesfluraneSevoflurane
MAC1.0 (%)1041,26,61,8
MACawake~0,6 MAC~0,4 MAC~0,4 MAC~0,4 MAC
Blod λ (distribution coefficient)0,461,40,450,65
Brain λ0,492,20,551,1
Fat λ1,1701337
Metabolism (%)00,20,025
Airway Irritant0>1,5 MAC>1 MAC0
Liver Injury0<1/106<1/1070
Epileptogen Activity0+0++

The anaesthetic depth is followed primarily by clinical parameters, but technical equipment for anesthetic depth monitoring is becoming increasingly and increasingly common in clinical practice. Normally, the anesthesia is administered after the MAC-values to achieve a MAC of 0.6-1.0, while providing remifentanil with continuous infusion. If instead fentanyl is added in addition to gas, but using gas in the first place, it usually operates on a MAC level between 0.8 and 1.4. The vaporizer of inhalation agents is dosed as a percentage, but the effect is read in the MAC. At about 2 MAC, spontaneous breathing stops and at 3 MAC the heart fails and circulation collapses. Simultaneous use of nitrous oxide reduces the need for vaporized gas and therefore increases the safety margin. Roughly the effect of two combined gases is additive. However, the use of nitrous oxide has, for a number of reasons, been significantly reduced in recent years and is most commonly used in inducing inhalation anesthesia in children and during labor. In prolonged anesthesia, a constant MAC produces a successively higher concentration in the brain, so the MAC should be adjusted slightly over time. The need for opioids and muscle relaxants is greatly reduced during inhalation anesthesia which may be an advantage.

Inhalation anesthesia can be divided into anesthesia with controlled breathing and anesthesia with sustained spontaneous breathing. Spontaneous breathing, when muscular relaxants are not used at all, is possible when using gases via facial mask ventilation, laryngeal mask or even endotracheal intubation. Because the pressure gradient across a laryngeal mask is lower and thus the risk of gas leakage less than during mask ventilation, it less often requires intubation. Spontaneous breathing is not only physiological, it also gives a correct level of anaesthetic depth compared to controlled positive pressure ventilation. Otherwise than correct anaesthetic depth, i.e. in case of deep or excessive anesthesia, it simply does not work and it is visible and visible to the patient. How to note to deep or to superficial anesthesia is a bit of a craftsmanship and requires some routine by the anaesthetizer. Anxiety and stress in the patient may be due to both superficial or deep anesthesia. An unsatisfactory airway never gives good opportunities for a good drug delivery and adequate anaesthetic depth. Thus, open airway is essential in the case of spontaneous respiratory breathing, which can sometimes be difficult both in mask ventilation and laryngeal mask ventilation. The open airway is not even guaranteed when the patient is intubated. Anaesthetic monitoring can help but is not always reliable.

Since the drug concentration of anesthetic agents is measurable and the anaesthetic depth is predictable with spontaneous breathing, there is no need for any anaesthetic depth monitoring equipment such as BIS or “Entropy” during routine procedures. However, anaesthetic depth monitoring may be a good aid to avoid too deep anesthesia and slow awakening. The time of awakening is dependent on the tissue/solubility of the gas/gases and the total amount of inhalation anaesthetics administered. Desflurane has about half of the sevoflurane solubility, which in turn is as soluble as isoflurane. The nitrous oxide is substantially less soluble, especially in fat tissues compared with vaporized gases. Modern anaesthetic gases provide a fast and predictable awakening, comparing “MAC awake” levels.

During the 90’s, great efforts were made to introduce day care surgery in clinical practice. This is mainly for cost-efficiency and faster turnovers. It then became important with short-acting agents in order to quickly wake up and dismiss patients after surgery. Previously, sevoflurane and desflurane had been considered inappropriate due to a high degree of metabolism with fluoride formation and boiling point at room temperature. The pharmaceutical industry invested considerable resources in the documentation and marketing of these two gases. This contributed to increased knowledge and interest in inhalation anesthesia in general.

Over decades, it has been found that nitrous oxide has some disadvantages such as it may expand in gas-filled cavities in the body, inhibit vitamin-B metabolism (clinically relevant primarily in vegan diets and in certain unusual genetic inborn errors of metabolism), cause a slight increase in postoperative nausea (PONV), and provoke diffusion hypoxia (which requires hypoventilation and/or air breathing without oxygen supplementation). Even supposed negative environmental impact and greenhouse effects in the atmosphere made it possible to eliminate the nitrous oxide from the hospital environment. Since nitrous oxide in combination with other vaporized gas accounts for about half of the anaesthetic depth, the elimination of nitrous oxide would mean a significantly higher consumption of sevoflurane and desflurane. Desflurane being significantly more dangerous to the environment than sevoflurane.

Today, sevoflurane is mainly used in clinical anaesthesia practice, but desflurane has also been widely distributed. The risk of renal damage caused by sevoflurane at the introduction, fortunately, has not been seen. Both “compound A” from the carbon dioxide absorber of the circular system and the fluorides in the metabolism could theoretically be nephrotoxic. Ironically, however, the old combination of isoflurane/nitrous oxide provides a faster recovery than a pure sevoflurane anesthesia. The alleged rapid recovery was the main point of sevoflurane’s launch. A clear advantage, not least in the case of pediatric anesthesia, is that gas induction by mask is much easier to perform with sevoflurane. The patent time for sevoflurane and desflurane has expired and inhalation anesthesia is no longer a hot topic of debate, why alternative but not necessarily for the patient better anesthesia methods have gained terrain.


Anesthesia with Sevoflurane (Sevoflurane)

Posted by Kai Knudsen, Senior Physician in Anesthesia & Intensive Care. Sahlgrenska University Hospital.
Updated 2019-06-13


In adult inhalation induction, initiate with 50% oxygen and start with 2% Sevoflurane and 4 liters of fresh gas flow. Then increase by half percent of sevoflurane every third breath to achieve an appropriate MAC. If the patient experiences a certain disturbance after a few breaths, the gas can be raised higher up to 5-6%, which can be maintained until the patient relaxes and airway management is under control. Thereafter, the gas concentration is controlled as necessary correlated to the surgical procedure. Common levels of sevoflurane are approximately 1-2% with a fresh gas flow of about 4 l/min with at least 30% oxygen. The anaesthetic depth is controlled by the MAC value presented in the gas monitoring system. With the combination of nitrous oxide/sevoflurane, significantly lower levels of sevoflurane will provide adequate anaesthetic depth (0.7-0.9%) compared with the use of sevoflurane in combination with oxygen (1.4-2.0%).

In the case of anaesthesia induction in children, the same procedure may be used as in adults to achieve a proper concentration of sevoflurane and an adequate anaesthetic depth. This usually takes at least 5 minutes with careful spontaneous breathing. Alternatively, you can quickly turn the vaporizer to very high values ​​6-8% to get the child into anesthesia with only a few breaths (1-3) (The gas tubes may be prefilled at 8% before attached to the child’s ventilation for rapid induction). If you ventilate the child with more than 2.5%, you should keep in mind that you usually get a lot more anaesthetic gas in the child compared to an adult and that you get faster into deep anaesthetic depth, but you can also easily overdose with significant cardiovascular depression. Children around the year usually need to stay at around 3% of sevoflurane without nitrous oxide to get the correct drug depth. Between 3 and 12 years, you usually need around 2.5% for adequate anesthetic depth. The concentration of sevoflurane can be reduced if the gas is mixed with nitrous oxide by about one percent.

Age's effect on MAC for Adults and Children of Different Ages

Age (years)Sevoflurane in oxygen (%)Sevoflurane in 65% N2O/ 35% O2
0 - 1 months*3.3%Undetermined
1 - <6 months3.0%Undetermined
6 months - <3 years2.8%2,0 %**
3 till 12 years2.5%Undetermined
25 years2.6%1.4%
40 years2.1%1.1%
60 years1.7%0.9%
80 years1.4%0.7%
* Newborns after the end of pregnancy. MAC in prematures has not yet been established.
** In pediatric patients 1- <3 years old, 60% N2O / 40% O2 was used.


Anesthesia using Desflurane (Suprane)

Posted by Jan Pålsson, Senior Physician in Anesthesia & Intensive Care. Sahlgrenska University Hospital.
Updated 2019-06-13


Desflurane (Desflurane) may be a relevant and good choice of anesthetic gas when rapid recovery from anesthesia is desirable. For example, in the case of early mobilization, severe sleep apnea, difficult intubation or anomalous upper respiratory tract, risky extubation, early discharge from postop care, long surgery or morbid obesity.

Desflurane is characterized by some unique properties; it has a sharp scent that may provoke some bronchial irritation and sympathetic activation during awakening, or at light anesthesia. Desflurane has a low solubility in both water and fat. The sharp odor prevents inhalation induction with desflurane, and also makes it inappropriate to use in patients with increased bronchial reactivity, i.e. asthmatics and children with obstructive disorders for whom the gas can be considered contraindicated. The generally low solubility in different tissues involves rapid saturation in tissues with both normal and low blood flow, which in turn causes desflurane to leave the body quicker and more completely compared to other, more fat-soluble inhalation anesthetic gases during elimination by respiration. In order to obtain the beneficial effects of desflurane during awakening it is important to maintain normoventilation and increase the fresh gas flow considerably to achieve an effective concentration gradient in the alveoli and thus obtain optimal gas recovery.

General anesthesia with desflurane in combination with remifentanil (Remi), sometimes referred to as supremifentanil (Suprane + Remifentanil) is suitably started with an initial fentanyl dose by hand (for better maintained circulation/blood pressure), after which the remifentanil infusion is started. Target value in TCI mode is around 4-6- (8) ng/ml. Propofol injection is given when the patient begins to feel the sedative effects of remi, which he/she can communicate verbally during the start of anesthesia. Intubation is facilitated by an increase of remifentanil temporarily (quite high if muscle relaxants is avoided, > 8 ng/ml in TCI), possibly combined with lidocaine spray, of the trachea or usual doses of muscle relaxants. Secure the airway with e.g. laryngeal mask, initial fentanyl dose may be reduced or avoided and the target value of remifentanil reduced.

When the airway is secured with an endotracheal tube or laryngeal mask, mechanical ventilation at a fresh gas flow (FGF) of 2 l/min and gas delivery with desflurane at 7 %. After 3-5 minutes, the FGF is lowered to low flow and the desflurane value of the vaporizer can be reduced slightly when MAC 0.7-0.8 is reached. Consider further reduction of remifentanil to a target value around 2-3 ng/ml. Increase the dose of remifentanil to an adequate dose (surgical anesthesia) 90 seconds before start of surgery. Keep MAC at 0.7-0.8 with desflurane in the anesthesia maintenance phase and control the anaesthetic depth with the remifentanil infusion.

Using MAQUET FLOW-i ventilator with AGC function, the ventilator automatically manages the supply of anaesthetic gas and corrects FGF to the ventilator’s anesthesia gas system during the induction phase, maintenance and wake up. A target value around MAC 0.7-0.8 is normally chosen and it is important that the changed rate in the AGC mode to achieve the required gas concentration is set lower for desflurane than e.g. sevorane, to avoid bronchial irritation. The appropriate value is around 2-3 on the AGC function’s 8-speed speed scale in FGF.

Using desflurane may mean that you as an anesthetist, more regularly evaluate the degree of surgical stimuli and communicate with the surgeon. Water-soluble opioids for postoperative pain relief are given at least 30 minutes before awakening. High remifentanil doses may increase the initial postoperative opioid requirement.

For longer anesthesia and more extensive surgical procedures where long post-operative monitoring is planned, desflurane is combined with the advantage of fentanyl, given in conventional, e.g. repeated doses. Postoperative opioid need can then be reduced.

During recovery, the need for reversal of muscle relaxation is evaluated, remifentanil infusion is stopped and desflurane gas administration is closed. Low FGF is retained and the patient is still normoventilated. After the final dressing is set and possibly bladder scan is done, FGF is increased to 10 l/min with maintained normoventilation (wash-out period). The patient usually wakes up within 2-10 minutes after discontinuation of desflurane. After awakening control and observation of swallowing reflexes, the extubation or withdrawal of laryngeal mask takes place. Awakening with Maquet Flow-i ventilator with activated AGC function means that remifentanil is discontinued as above, but the MAC target value is set to “zero” only when all practical closing moments are complete. The rapid recovery from Desflurane means that the patient quickly gets alert and regain good muscle tone, which may be beneficial if the patient has a restricted upper respiratory tract. Early recovery enables early mobilization and self-relocation from the operating table to bed for continued post-operative care.

Practical advice for anesthesia with desflurane (Suprane)

Laryngeal mask ventilation 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 sec. Then lower remifentanil to 0.08 micrograms/kg/min alternatively in TCI mode with a target value of 4-6- (8) ng/ml for 40-60 seconds, then lower 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, the FGF is lowered to low flow while maintaining MAC 0.7-0.8.
  • Increase remifentanil to an adequate dose (for surgical anesthesia) 90 seconds before beginning of surgery. Normally, 0.2-0.5 micrograms/kg/min is required, or 4-10 ng/ml.
  • If hypotension occurs (waiting for surgical stimuli): tipping of the operating table, administer fluid, ephedrine, atropine. 

Intubation anesthesia with controlled ventilation and continuous infusion of remifentanil

  • Start remifentanil in TIVA mode 0.30-0.40 microgram/kg/min alternatively in TCI mode with target value 4 – 6 – (8) ng/ml for 40-60 seconds
  • Give propofol and muscle relaxants in adequate doses.
  • Lower remifentanil to 0.15-0.25 micrograms/kg/min alternatively 3-4 ng/ml as well as secure the airway through endotracheal intubation.
  • Start mechanical ventilation with fresh gas flow (FGF) 2 liters/min + desflurane 7%. After 3-5 minutes, the FGF is lowered to low flow while maintaining MAC around 0.7-0.8.
  • Once MAC 0.7-0.8 is reached, further reduction of remifentanil should be considered to be about 0.10 micrograms/kg/min, alternatively targeting at 2 ng/ml.
  • Increase remifentanil to an adequate dose (for surgical anesthesia) 90 seconds before beginning of surgery. Normally, 0.2-0.5 micrograms/kg/min is required, or 4-10 ng/ml.
  • Blood pressure control (in anticipation of surgical stimuli) by tipping the table, fluid, ephedrine, and/or atropine.

 Maintenance anesthesia

  • Maintain desflurane with MAC 0.7-0.8 and control the anaesthetic depth with remifentanil in infusion. Do not go lower than 0.15 micrograms/kg/min or the target value 3 ng/ml in TCI.
  • Evaluate every surgical stimuli.
  • Communicate actively with the surgeon.
  • Consider giving antiemetics.
  • Give postoperative analgesics no later than 30 minutes before awakening.

Recovery period

  • Make sure that local anesthesia or blockade has been given. Evaluate the need for reversal of muscle relaxants.
  • Five minutes before extubation (at dressing of the wound), the remifentanil and desflurane supply is closed with retained fresh gas flow (FGF).
  • After performing bladder scan – increase the direct FGF to 10 l/min and ventilate the gas mechanically until the patient wakes up. Please use the timer clock! Normally, the patient wakes within 2-10 minutes.
  • Communicate with the patient and check the swallowing ability during extubation/withdrawal of laryngeal mask.
  • Take advantage of the patient’s alertness for self-mobilization and early movement to bed.

Desfluran (Suprane)

Posted by Kai Knudsen, Senior Physician in Anesthesia & Intensive Care. Sahlgrenska University Hospital.
Updated 2019-06-13


Desflurane is a halogenated methyl ethyl ether (difluoromethyl-1,2,2,2-tetrafluoroethyl ether). It is an inhalation anaesthetic agent used for the maintenance of general anesthesia via the respiratory tract. It is supplied in the respiratory tract after gasification, via laryngeal mask or an endotracheal tube.

Indication

Induction and maintenance of general anesthesia. Changes in the clinical effects of desflurane follow rapid changes in inhaled concentration. Like all inhalation anaesthetics, desflurane delivers a controlled degree of unconsciousness and analgesia (anesthesia), and desflurane also decreases the cardiovascular function in a dose-related manner. Desflurane should not be used for anesthesia induction in children due to the high incidence of coughing, respiratory arrest, apnea, laryngospasm and increased secretion of the sebum. The blood/gas solubility coefficient is 0.42 for desflurane. The low solubility of desflurane in blood causes the alveolar concentration to rapidly increase upon induction and rapidly decreases upon discontinuation of the inhalation anaesthetic agent. Desflurane provide a dose-dependent reduction of blood pressure and breathing..

Dosage

Surgical anesthesia can be maintained at a concentration of 2.5-8% with or without simultaneous use of nitrous oxide. Usually 4-6% of desflurane is given in the inspiratory air. Normally, a MAC value is aimed at between 0.8 and 1.6, usually 1.2-1.4. In adults, surgical anaesthetic depth can be maintained with a reduced concentration of desflurane when nitrous oxide is used simultaneously. Higher concentrations of desflurane may be indicated. However, one should take into account the risk of hypoxia and adjust nitrous oxide/oxygen supply. The maintenance dose should be adjusted gradually in relation to the clinical effect. Desflurane is indicated for maintenance anesthesia to in neonates and children. Surgical anaesthetic depth can be maintained in children with end-tidal concentrations of 5.2 to 10% desflurane with or without simultaneous use of nitrous oxide. Studies have shown that only 0.02% of the absorbed desflurane is metabolized. Only marginal increases in inorganic fluoride can be seen in serum and urine. The rapid and extensive lung elimination of desflurane minimizes the amount available for metabolism. Generally, awakening is rapid after desflurane anesthesia for 5-30 minutes. Patients may therefore need postoperative pain relief early.

Recovery

Recovery is generally very rapid after desflurane anesthesia with awakening within 5-10 minutes. Patients may therefore need postoperative pain relief early.

Side effects

Desflurane may trigger cough and airway obstruction as well as increased airway mucous production. It should not be used for induction of anesthesia. It has dose dependent cardiac depression.

Warning

To rapid increase desflurane in the inspiratory air can cause respiratory irritation with bronchospasm and increased mucus secretion. Cough, laryngospasm, apnea and bronchospasm may occur. Desflurane is not recommended for induction by spontaneous breathing in children. May trigger malignant hyperthermia in predisposed patients. Caution in patients with elevated intracranial pressure.


Nitrous Oxide (N2O)

Posted by Kai Knudsen, Senior Physician in Anesthesia & Intensive Care. Sahlgrenska University Hospital.
Updated 2019-06-13


Nitrous oxide is a medical gas with good analgesic effects and moderate anaesthetic effects. The nitrous oxide was discovered in the 18th century and came into medical use in the mid-19th century through its analgesic and sedative properties. Towards the late 1800s, nitrous oxide began to be used for analgesia in childbirths, as well as in tooth extraction and other minor surgical procedures. Nitrous oxide has been a standard anesthetic agent for almost all of the 20th century. During the 21st century, nitrous oxide has decreased in use but may still be useful when used properly.

Indication

Maintenance of general anesthesia. Analgesics in childbirth and minor surgical procedures such as tooth extraction. Prehospital analgesics. Nitrous oxide gives an additive effect when combined with most other anaesthetics, both intravenous and inhaled anaesthetics. MAC value is set to 105%. Nitrous oxide potentiates the effect of other inhalation anaesthetics in such a way that the concentration of these can be significantly reduced to maintain the same anaesthetic extent (MAC value). In this way, anesthesia with nitrous oxide and other inhalants produces less hemodynamic effects than without nitrous oxide. Nitrous oxide has dose-dependent effects on sensory experiences and cognitive functions that begin at 15% by inhalation. Concentrations over 60-70% by inhalation give rise to unconsciousness. Nitrous oxide has dose-dependent analgesic properties that are clinically noticeable at end-tidal concentrations around 20% by inhalation. The blood/gas solubility coefficient is 0.46 for nitrous oxide. The low solubility of nitrous oxide in blood causes the alveolar concentration to rapidly increase upon induction and rapidly decreases upon discontinuation of the inhalation anaesthetic agent. Nitrous oxide provide a rapid saturation of the blood and reaches equilibrium faster than other inhalation anaesthetics.

Dosage

In general anesthesia, nitrous oxide is usually used in concentrations between 35-70% by volume in a mixture with oxygen and when necessary other anaesthetic agents. Usually oxygen/nitrous oxide is given in the mixture 1:2 or 1:1. Nitrous oxide as sole anaesthetic agent is usually not sufficiently potent to create surgical anesthesia but should therefore be combined with other anaesthetic agents when used in general anesthesia. Nitrous oxide is rapidly eliminated from the body after short-term inhalation and the effect on psychometric functions usually declines approximately 20 minutes after completion of administration.

Recovery

Generally, recovery occurs rapidly after nitrous oxide anesthesia. Patients may therefore need postoperative pain relief early.

Caution

Gas filled body cavities may expand due to the ability of the nitrous oxide to diffuse. As a result, nitrous oxide is contraindicated in patients with symptoms of pneumothorax, pneumopericardium, gas embolism, severe head injury, ileus or airfilled intestines. In case of suspicion or lack of vitamin B12 or symptoms consistent with the influence of methionine synthetase, substitution therapy with vitamin B should be given to minimize the risk of adverse events/symptoms related to methionine synthetase inhibition such as; leukopenia, megaloblastic anemia, myelopathy and polyneuropathy.


Isoflurane (Isofluran, Forene)

Posted by Kai Knudsen, Senior Physician in Anesthesia & Intensive Care. Sahlgrenska University Hospital.
Updated 2019-06-13


Isoflurane is a halogenated ethyl methyl ether. It is an inhalation anaesthetic agent used for induction and maintenance of general anesthesia via the respiratory tract in a closed respiratory system. It is usually supplied after vaporization, by laryngeal mask or endotracheal intubation.

Indication

Induction and maintenance of general anesthesia. Changes in the clinical effects of isoflurane follow rapid changes in inhaled concentration. Like all inhalation anaesthetics, isoflurane gives a controlled degree of unconsciousness and analgesia (anesthesia) and in addition reduces cardiovascular function in a dose-related manner. The low solubility of isoflurane in blood causes the alveolar concentration to rapidly increase upon induction and rapidly decreases upon discontinuation of the inhalation anaesthetic agent. 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. In Caesarean section, 0.5-0.75% isoflurane is given in a blend with oxygen/nitrous oxide. As with other halogenated volatile anaesthetics, MAC for isoflurane decreases when given in combination with nitrous oxide. MAC is reduced with increasing age.

The MAC value of isoflurane decreases by approximately 50% for adults and approximately 25% for children when 60-65% nitrous oxide is given at the same time. As with other inhalation anaesthetics, elderly patients usually require lower concentrations of isoflurane to maintain surgical anesthesia. The biotransformation of isoflurane is minimal in humans. On average, about 95% of isoflurane is present in the exhaled air. The rapid and extensive lung elimination of isoflurane minimizes the amount available for metabolism.

Recovery

Recovery is generally rapid after isoflurane anesthesia with awakening within 5-30 minutes. Patients may therefore need postoperative pain relief early.

Side Effects

It should not be used for induction of anesthesia. There is a dose dependent cardiac depression.

Caution

Isoflurane may trigger malignant hyperthermia in predisposed patients. Caution with greatly increased intracranial pressure. Rare cases of hypersensitivity (including contact dermatitis, rash, dyspnéa, wheezing, chest discomfort, facial swelling or anaphylactic reaction) have been reported, especially in the context of prolonged occupational exposure to inhalation anaesthetics, isoflurane included.


NO (Nitric Oxide)

Posted by Kai Knudsen, Senior Physician in Anesthesia & Intensive Care. Sahlgrenska University Hospital.
Updated 2019-06-13


NO acts as a signal substance in the body and accounts for a variety of physiological functions, including dilatation of smooth muscles in blood vessels.

Indication

Pulmonary hypertension. Severe ARDS. NO in the inhaled air can improve the oxygenation in severe ARDS with hypoxia and give a partial decrease in blood pressure in the pulmonary circulation without affecting systemic blood pressure. Large individual differences between different patients in the therapy. NO as a therapeutic agent in gaseous form is available for strictly controlled and limited clinical license use in intensive care. NO participates in so widely different functions as vasodilation, immune system regulation and regulation of cell respiration. NO formed in vascular endothelium is of great importance in the regulation of blood flow and counteracts aggregation of platelets and white blood cells.

Dosage

The doses of NO tested at ARDS have varied between 5 and 40 ppm in the respiratory air in closed respiratory system via ventilator hoses.

Contraindications

Treatment with sildenafil (Viagra)

Warning

May cause methemoglobinemia.


Sevofluran (Sevorane, Sevofluran)

Posted by Kai Knudsen, Senior Physician in Anesthesia & Intensive Care. Sahlgrenska University Hospital.
Updated 2019-06-13


Sevoflurane is a halogenated methyl isopropyl ether. It is an inhalation anaesthetic agent used for induction and maintenance of general anesthesia. Sevoflurane is administered after vaporization, by laryngeal mask or endotracheal intubation.

Indication

Induction and maintenance of general anesthesia. Changes in the clinical effects of sevoflurane follow rapid changes in inhaled concentration. Like all inhaled anaesthetics, sevoflurane provide a controlled degree of unconsciousness and analgesia (anesthesia) and, in addition, reduces cardiovascular function in a dose-related manner. The low solubility of sevoflurane in blood causes the alveolar concentration to rapidly increase upon induction and rapidly decreases upon discontinuation of the inhalation anaesthetic agent. The blood/gas solubility coefficient is 0.68 for sevoflurane.

Dosage

Surgical anesthesia can be maintained at a concentration of 0.5 – 3% with or without simultaneous administration of nitrous oxide. Normally, a MAC value is sought between 0.8 and 1.6, usually 1.2-1.4. As with other halogenated volatile anaesthetics, the MAC for sevoflurane decreases when given in combination with nitrous oxide. The MAC value for sevoflurane decreases by 25-50% for adults and about 25% for children when 60-65% nitrous oxide is given at the same time. As with other inhalation anaesthetics, elderly patients usually require lower concentrations of sevoflurane to maintain surgical anesthesia. In humans, < 5% of absorbed sevoflurane is metabolized in the liver to hexafluoroisopropanol (HFIP) with the release of inorganic fluorine and carbon dioxide. HFIP is then conjugated rapidly with glucuronic acid and excreted in the urine. The fast and extensive lung elimination of sevoflurane minimizes the amount available for metabolism.

Recovery

Recovery is generally rapid after sevoflurane anesthesia with awakening within 5-30 minutes. Patients may therefore need postoperative pain relief early.

Caution

Sevoflurane may trigger malignant hyperthermia in predisposed patients. Caution in severe renal impairment or greatly increased intracranial pressure.


Oxygen (O2)

Posted by Kai Knudsen, Senior Physician in Anesthesia & Intensive Care. Sahlgrenska University Hospital.
Updated 2019-06-13


Oxygen is essential for human cell respiration and is needed for normal aerobic cell metabolism. Normally the content is about 21% in air, but it varies with partial pressure. Oxygen is always given in all closed ventilator systems, at least in 21 percent. Oxygen suppresses nausea secondary to hypoxia. Normal saturation in arterial blood is 95-98%. Lower than 90% saturation is at risk of organ damage. Less than 70% is tolerated only briefly. Less than 50% is immediately life threatening. Supernormal values ​​give rise to vasoconstriction. It may cause alveolar hypoventilation.

Dosage

By nasal cannula, facial mask, Optiflow high-flow system or via a closed respiratory system (on a ventilator).

Standard dose

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

Caution

Caution in respiratory insufficiency and hypoventilation. Caution after treatment with bleomycin in chemotherapy.

The time a gas bottle with oxygen (bomb) is sufficient at different flows and pressures.
Size of the gas bottlePressure (bar)2 l/min3 l/min5 l/min 10 l/min
1 liter2001 hour 40 min1 hour 30 min20 min
1501 hour 15 min50 min30 min15 min
10050 min33 min20 min10 min
5025 min17 min10 min5 min
2,5 liter2004 hours 10 min2 hours 45 min1 hour 40 min50 min
1503 hours 2 hours 1 hour 15 min38 min
1002 hours 1 hour 20 min50 min25 min
501 hour 50 min25 min13 min
5 liter2008 hours 20 min5 hours 30 min3 hours 20 min1 hour 40 min
1506 hours 15 min4 hours 10 min2 hours 30 min1 hour 15 min
1004 hours 20 min2 hours 45 min1 hour 40 min38 min
502 hours 1 hour 20 min50 min25 min