Intravenous anesthesia

Overview

Anesthesia refers to the use of drugs to alleviate pain during surgery and other treatments. These medications are known as anesthetics. They can be administered by injection, inhalation, topical lotion, spray, ocular drops, or a skin patch. They lead you to lose feeling or consciousness.

Intravenous anesthesia is made up of an induction drug, an opioid, and a neuromuscular relaxant. For anesthesia maintenance, anesthesiologists frequently favor intravenous induction with a mix of inhalational and intravenous drugs. Intravenous induction has various advantages, including the fact that it is quick, reduces patient pain, and is favored by children and most adults.

Intravenous anesthesia definition

Intravenous (i.v.) anesthetics are a class of medicines that rapidly cause anesthesia by reaching large concentrations in the central nervous system.

Etomidate, midazolam, propofol, thiopental, ketamine, and opioid agonists are examples of intravenous (i.v.) anesthetics. The first four drugs work in the CNS by increasing the activity of the inhibitory neurotransmitter -aminobutyric acid (GABA). Ketamine blocks the excitatory neurotransmitter N-methyl-D-aspartate (NMDA) from binding to NMDA receptors, whereas opioid agonists increase opioid receptors.

The ideal intravenous anesthetic drug has a rapid start of action and is swiftly removed from the circulation and CNS, allowing for better management of the anesthetic state (e.g., allowing titration of effect). The ideal agent also preserves important tissues, has other desired pharmacologic effects (for example, an antiemetic effect), does not influence the circulatory system or produce other negative consequences, and is affordable. Propofol is the most commonly utilized induction i.v. anesthetic drug. It is extremely lipophilic and rapidly distributes into the CNS and other tissues, resulting in a rapid commencement of action.

Propofol causes unconsciousness in the time it takes the medication to travel from the injection site to the brain, which is referred to as one "arm-brain circulation time" and lasts less than one minute. Although the medication might induce discomfort at the injection site, the development of anesthesia is gradual. Propofol is metabolized swiftly and extensively in the liver and at extrahepatic locations, resulting in a high rate of total body clearance. The medication has a direct antiemetic action via an unknown mechanism. It is likewise euphorigenic, but does not have the same lingering psychotic effects as ketamine.

Adult patients frequently prefer i.v. induction of anesthesia because it proceeds more swiftly and easily and is linked with less claustrophobia. After having ocular surgery with either inhaled sevoflurane + nitrous oxide or i.v. propofol for induction of anesthesia, 40 patients were asked if they would be willing to receive the same anesthetic agent again.

More propofol patients (90%) were willing to have the same anesthetic again than sevoflurane patients (50 percent ). The disparity might be explained by claustrophobia. Children, on the other hand, frequently choose an inhalation induction of anesthesia due to a phobia of needles. Inhalation induction may also be beneficial in patients who do not have venous access.

Several factors imply that i.v. anesthetics should be used for anesthesia induction but not for anesthesia maintenance. Multiple dosages administered through intravenous injection or continuous intravenous infusion might result in drug buildup and delays in recovery from anesthesia. The increased expense of intravenous therapy in comparison to inhaled therapy is also a factor to consider. Perhaps the most fundamental argument for avoiding the use of i.v. anesthetics for anesthesia maintenance is the lack of a method for continually evaluating the level of anesthesia.

Because sophisticated instruments are available for measuring the concentration of the inhaled anesthetic agent administered to the patient, the use of inhaled anesthetics for maintenance of anesthesia enables greater control of the level of unconsciousness.

The percentage of patients with purposeful movement during surgery was significantly higher with propofol than with either of the inhaled anesthetic agents in 90 patients randomized to receive i.v. propofol or one of two inhaled anesthetic agents (isoflurane or desflurane) for maintenance of anesthesia after induction during an office-based procedure. Movement indicates a shallow depth of anesthesia, which might interfere with and delay operation.

Cellular targets

The principal targets of intravenous anesthetic medicines are very well known; these medications interact with natural neurotransmitter receptors. Neurotransmitter receptors are divided into two types: ionotropic (connected to an ion channel) and metabotropic (linked to a G protein). The majority of neurotransmitters in the CNS have the ability to activate both types of receptors. Most intravenous anesthetics target the ionotropic receptors for -aminobutyric acid (GABA) and glutamate, as well as the metabotropic 2-adrenergic receptor for catecholamines.

Except for ketamine and the 2-adrenergic agonists, it is the principal target responsible for the anesthetic effects (sedation, anxiolysis, hypnosis, and amnesia) of all intravenous anesthetics and sedatives. In recent years, advances in molecular biology and genetic engineering have enabled the determination of structural requirements and receptor subunit specificities for the activities of various intravenous anesthetic medicines.

Intravenous Agents

Sedative hypnotics, muscle relaxants, and opioids are the three broad types of intravenous anesthetic drugs. Anticholinesterases, anticholinergics, and opioid antagonists are some of the other agents. Intravenous anesthetics, like inhalational anesthetics, have a sedative-hypnotic effect that mirrors the agent's CNS concentrations. Intravenously given medications reach vessel-rich organs quickly, such as the CNS, heart, and kidneys.

The medications' action is mostly terminated by a second redistribution of the pharmaceuticals to muscle groups and less well-perfused parts of the body such as fat and bone. The pharmacokinetics of each patient differ based on body composition, cardiac output distribution, plasma protein level, metabolism, and excretion.

Because of the variety across individuals, it is hard to perfectly predict the impact of intravenous anesthetics, and the anesthesiologist must carefully titrate the anesthetics based on each patient's response. Because the newborn is far more susceptible to the cardiorespiratory effects of intravenous anesthetics than the adult, unit dose of anesthetics is not only ineffective but also dangerous.

Despite their growing popularity and usage in adult anesthesia, most intravenous drugs have not been thoroughly researched in newborns. Animal models provide the majority of the known pharmacokinetic information on analgesics and sedatives in young subjects. The US Food and Drug Administration has addressed this issue, and incentives are being devised for pharmaceutical firms to test novel medications in youngsters.

Propofol, thiopental, and ketamine are the most often utilized intravenous sedative hypnotic drugs. Propofol, in particular, has gained popularity because to its faster onset and termination than thiopental or ketamine. Propofol has lower levels of airway irritation, surgical sedation, and nausea or vomiting.

One disadvantage of propofol is the discomfort associated with intravenous administration. children who require frequent anesthetics have central venous access, which avoids the complications associated with peripheral administration. Inhaling 50 percent N2O significantly reduces the discomfort caused by propofol administration.

Another issue with propofol is the link between long-term infusions in intensive care units and fulminant metabolic acidosis. This might be due to a flaw in the fatty acid oxidation process. Long-term propofol infusions, which were historically utilized in intensive care units, are no longer advised for youngsters.

Although propofol has mainly supplanted the barbiturates thiopental and methohexital, they are still widely used. Thiopental induction is painless and a viable option for propofol when quick emergence is not required. The action is terminated owing to redistribution, however the extended half-life in newborns may be related to decreased clearance. Thiopental is a cardiac depressant and vasodilator, hence administering it to a hypovolemic baby should be done with extreme caution. Methohexital taken rectally at a dose of 25 to 30 mg/kg gives drowsiness for 60 to 80 minutes, allowing for short diagnostic procedures or acting as preoperative sedative in older children.

Ketamine is an injectable anesthetic adjuvant that provides powerful analgesia and drowsiness. It is a phencyclidine derivative that was originally designed for veterinary usage. Ketamine was suggested for pediatric anesthesia shortly after its debut because it gave improved circulatory stability, less respiratory depression, and better preservation of airway protective reflexes.

Ketamine improves cardiovascular stability in newborns who have suffered from shock or heart failure, but it also produces extended drowsiness, a rise in oropharyngeal secretions, and more respiratory depression and airway compromise than was previously thought. Ketamine has been shown to be the most effective anesthetic in the sickest babies with cyanotic congenital heart disease who require prolonged postoperative breathing.

Ketamine is not a good anesthetic option for infants who will be weaned from artificial breathing shortly after surgery. When used as directed, intravenous ketamine titrated to effect results in much faster recovery periods than massive intramuscular boluses of the medication.

In pediatric anesthesia, the most usually utilized opioid is morphine. Morphine offers long-acting, dependable analgesia for infants having major surgery. It is given orally, intramuscularly, or intravenously. Furthermore, in animal studies, the immature blood–brain barrier of a newborn leads in two to three times greater CNS concentrations of morphine in neonates than in adults.

Fentanyl is a stronger, faster-acting opioid than morphine. It is widely used as a supplemental anesthetic to general anesthesia. It is especially beneficial in cardiac anesthesia, where large dosages of 50 to 100 g/kg are highly successful in blocking the stress response to operation while causing little hemodynamic effects. Fentanyl, like morphine, has a longer half-life and lower clearance in newborns than in older children.

The most recent synthetic opioid is remifentanil. It has a very short half-life, does not accumulate during infusions, and is broken down by nonspecific esterases. The 3 to 5-minute half-life and quick elimination may help to prevent the postoperative respiratory effects of other opioids.

The primary disadvantages of remifentanil are that it must be supplied constantly through an intravenous line and that other analgesics must be provided for postoperative pain management. As with other synthetic opioids, bradycardia and chest wall stiffness can be severe, necessitating therapy with vagolytics and muscle relaxants.

Propofol

Propofol has many of the features of an excellent anesthetic induction medication and is popular because it produces anesthesia quickly and easily without causing airway irritation. However, propofol has certain drawbacks, including injection discomfort, uncontrolled movements, temporary apnea, and hypotension following anesthetic induction.

There have been several ways recommended to lessen discomfort on injection, the most successful of which include the use of a wide antecubital vein or pretreatment with lidocaine in combination with venous blockage, which is preferable than changing the formulation of propofol.

Some of the side effects of propofol can be reduced by combining it with adjuvant medications, the most frequent of which being midazolam. Midazolam 0.1 mg/kg pretreatment permitted anesthesia to be produced with a lower propofol dosage while simultaneously attenuating the resulting hemodynamic alterations.

A significant reduction in propofol dose can be accomplished even when midazolam is provided up to 10 minutes before anesthesia induction, an approach that can also make the induction experience more pleasant for the patient. This strategy, however, may cause healing to be delayed. Even while midazolam 0.03 mg/kg reduced the needed propofol dose by half, it severely affected psychomotor recovery, despite the fact that waking times were not delayed.

Induction using short-acting opioids, such as alfentanil, can enhance induction quality and ease of laryngeal mask airway (LMA) installation, but at the price of increased hypotension and extended apnea. 188 Similarly, fentanyl decreased propofol dosage and improved LMA installation circumstances, but it also extended respiratory depression. 

An early bolus of 30 mg propofol lowered the overall propofol induction dosage to a level comparable to 2 mg midazolam. This approach, known as propofol autocoinduction, can lower propofol dosage and hypotension to levels equivalent to midazolam pretreatment while without delaying recovery. Propofol coinduction can also be conducted with intravenous lidocaine, in accordance with the ideals of opioid reduction and better recovery following surgery. 

How you prepare?

General anesthesia relaxes the muscles in your digestive tract and airway, preventing food and acid from flowing into your lungs from your stomach. Always follow your doctor's advice when it comes to avoiding food and drink before surgery.

Fasting is normally required starting six hours before surgery. You may be allowed to drink clear fluids until a few hours before the procedure.

During your fasting period, your doctor may advise you to take some of your normal prescriptions with a little sip of water. Consult your doctor about your drugs.

Some drugs, such as aspirin and other over-the-counter blood thinners, may need to be avoided for at least a week before your treatment. These drugs have the potential to cause difficulties during surgery.

Some vitamins and herbal medicines, such as ginseng, garlic, Ginkgo biloba, St. John's wort, kava, and others, have been linked to surgical problems. Before your operation, talk to your doctor about the dietary supplements you use.

If you have diabetes, discuss any changes to your medications during the fasting period with your doctor. You will not normally take oral diabetic medication on the morning of your procedure. If you use insulin, your doctor may advise you to take a lower amount. If you have sleep apnea, talk to your doctor about it. During and after surgery, the anesthesiologist or anesthetist will need to closely check your breathing.

What are the levels of sedation?

The amount of sedation a patient receives is determined by a number of factors, including the type of treatment being performed and how your body reacts to anesthetic. The type of anesthetic you receive may also be affected by your age, medical condition, and health habits. It is critical that a medical anesthesiologist be engaged in your anesthetic treatment, regardless of the amount of sedation. A physician anesthesiologist is a doctor of medicine who specializes in anesthesia, pain management, and critical care medicine.

The main levels of sedation are:

• Minimal – Although minimal sedation can help you rest, you will most likely remain awake. You'll comprehend your doctor's queries and be able to respond as well as follow orders. This degree of sedation is usually used when your doctor requires you to be present throughout the treatment.
• Moderate – During the process, you will feel tired and may possibly fall asleep. Some of the method may or may not be fresh in your mind.
• Deep – You will not be completely unconscious, but you will sleep through the process and have little or no recall of it.

Risks of anesthesia

Overall, general anesthesia is quite safe; most patients, including those with substantial health difficulties, may safely endure general anesthesia. In reality, your risk of problems is more directly tied to the sort of treatment you're having and your overall physical health than to the type of anesthetic you're using.

Older persons or those with major medical conditions, especially those undergoing more extensive treatments, may be at greater risk of postoperative disorientation, pneumonia, or even stroke and heart attack. The following conditions can increase your risk of problems after surgery:

• Smoking
• Seizures
• Obstructive sleep apnea
• Obesity
• High blood pressure
• Diabetes
• Stroke
• Other medical conditions involving your heart, lungs or kidneys
• Medications, such as aspirin, that can increase bleeding
• History of heavy alcohol use
• Drug allergies
• History of adverse reactions to anesthesia

Conclusion 

Intravenous anesthetics are a class of fast-acting drugs intended to create a condition of reduced consciousness or full drowsiness. Propofol, etomidate, and ketamine are examples of commonly used intravenous anesthetics.

Total intravenous anesthesia (TIVA) is a general anaesthetic method that employs a mix of medications administered only through syringe pump via the intravenous route, without the use of inhaling agents.

There is a compelling reason for using TIVA in some patient circumstances when inhaled anesthetic administration is difficult or undesirable, or where typical anesthetic delivery methods are unavailable or unsuitable. In other circumstances, using TIVA might make the process more efficient and beneficial to the patient.

Because of its potent dissociative, sympathomimetic, and analgesic properties, ketamine is used extensively in emergency care. Thiopental, a barbiturate, lowers intracranial pressure and is therefore effective in patients with excessive intracranial pressure and/or head trauma. While the properties and adverse effects of intravenous anesthetics vary depending on the drug, they always have a significant hypnotic effect.