What are commonly used pre anesthetic drugs?

Premedication is the administration of medication before a treatment or procedure. It is most commonly used prior to anaesthesia for surgery, but may also be used prior to chemotherapy.

This article relates to the use of premedication to prepare the patient for anaesthesia and to help provide optimal conditions for surgery. Specific needs will depend on the individual patient and procedure. Purposes may include[1]:

• Reduction of anxiety and pain.
• Promotion of amnesia.
• Reduction of secretions.
• Reduction of volume and pH of gastric contents (to avoid Mendelson's syndrome).
• Reduction of postoperative nausea and vomiting.
• Enhancing the hypnotic effects of general anaesthesia.
• Reduction of vagal reflexes to intubation.
• Specific indications - eg, prevention of infective endocarditis with antibiotics.

Premedication is traditionally given intramuscularly but the oral route is preferred for children and those with bleeding disorders. Premedication is usually given 20 minutes to three hours pre-operatively. Topical anaesthetic creams (eg, EMLA®) are often prescribed for children before cannulation.

The practice of premedication has changed over the last few decades. The use of strongly sedative drugs (eg, morphine and hyoscine) to aid smooth induction and reduce salivation has been abandoned with the advent of modern intravenous and inhalational anaesthetic agents, which have far fewer side-effects and a faster onset of action[2]. Other factors that have reduced the use of a sedative premedication include:

• Increasing use of day-case surgery.
• Same-day admissions - patients often do not get a confirmed bed until just before surgery.
• Changes to the surgical list, making the timing of drug delivery difficult.

The choice of drug(s) used for premedication depends on the procedure, patient and anaesthetic technique. Some patients prefer not to have premedication and potential benefits may be outweighed by potential problems (except for specific indications), especially with day-case surgery. A Cochrane review found no evidence of a difference in time to discharge from hospital following adult day surgery in patients who received anxiolytic premedication[3].

• Careful discussion of the patient's concerns is essential, including at the pre-operative assessment.
• Benzodiazepines are the usual agents used as they provide anterograde amnesia, relief of anxiety and light sedation. If given orally 1-2 hours before surgery they have only a small effect on cardiorespiratory function but large doses can interfere with the speed and quality of recovery. In day-care cases, short-acting benzodiazepines (eg, temazepam) are often preferred. Temazepam is given orally. Lorazepam is longer acting and effective for amnesia. Midazolam is also commonly used, and is associated with a faster recovery time than diazepam, although this may be longer in elderly patients. Diazepam use is not recommended for premedication in children.
• Clonidine (unlicensed) is also increasingly used for sedation, orally or IV.
• Dexmedetomidine is also used as a sedative with anxiolytic and analgesic properties, and clonidine and dexmedetomidine are considered to be effective alternatives to benzodiazepines, particularly in children[4].
• Relieving anxiety and sedation may also be achieved by opioids such as morphine, pethidine and fentanyl.

Analgesic drugs given pre-emptively reduce the required dose of anaesthetic agent and improve patient comfort in the immediate postoperative period. Options used include opioids, paracetamol, non-steroidal anti-inflammatory drugs (NSAIDs) and gabapentin[5].

• NSAIDs are commonly used, particularly in day surgery, unless there are contra-indications.
• Opioids are usually the agents of choice in the presence of acute severe pain. In the absence of pain, some people may experience intense dysphoria. Opioids also cause variable sedation and cardiorespiratory depression. All opioids can cause nausea and vomiting and this may outweigh any beneficial effects. Opioids may also precipitate bronchospasm or anaphylaxis.
• Clonidine given as a premedication has been shown to reduce postoperative pain in children[6].
• Relevant pre-emptive pain relief naturally depends on the nature of the procedure as well as factors relating to the individual patient. 

These may be used to dry secretions in the mouth and airways, and to help reduce vagally mediated bradycardia and hypotension. They are required less commonly than in the past.

• Hyoscine has strong sedative, amnesic and anti-salivation properties. It is a moderately effective antiemetic and potentiates opioids. Intramuscular atropine or hyoscine is therefore often prescribed together with an opioid.
• Hyoscine is the most potent agent available, with the added advantage of amnesia and sedation. However, it can cause significant perioperative confusion in elderly patients.
• Anti-sialogogues (eg, glycopyrrolate intramuscularly or IV) are rarely needed but may be indicated for awake fibre-optic intubation or before ketamine anaesthesia[2]. Anti-sialogogues may cause unpleasant dry mouth.

• Antiemetics are used either to reduce the emetic effects of anaesthetic agents (antihistamines, butyrophenones, hyoscine) or to enhance gastric emptying (metoclopramide).
• Those with a risk of regurgitation of gastric contents or undergoing procedures with a high incidence of nausea and vomiting should receive agents to reduce gastric acidity. Routine use of antiemetics and agents to reduce acidity in those not at risk is not recommended[7].
• An H2-receptor antagonist may be used 1-2 hours pre-operatively and/or oral non-particulate antacids such as sodium citrate 15-30 minutes before induction.

1. British National Formulary (BNF); NICE Evidence Services (UK access only)

2. Steeds C, Orme R; Premedication. Anaesthesia and intensive care medicine Volume 7, Issue 11, Pages 393-396 (November 2006).

3. Walker KJ, Smith AF; Premedication for anxiety in adult day surgery. Cochrane Database Syst Rev. 2009 Oct 7(4):CD002192. doi: 10.1002/14651858.CD002192.pub2.

4. Strom S; Preoperative evaluation, premedication, and induction of anesthesia in infants and children. Curr Opin Anaesthesiol. 2012 Jun25(3):321-5. doi: 10.1097/ACO.0b013e3283530e0d.

5. Steinberg AC, Schimpf MO, White AB, et al; Preemptive analgesia for postoperative hysterectomy pain control: systematic review and clinical practice guidelines. Am J Obstet Gynecol. 2017 Sep217(3):303-313.e6. doi: 10.1016/j.ajog.2017.03.013. Epub 2017 Mar 27.

6. Lambert P, Cyna AM, Knight N, et al; Clonidine premedication for postoperative analgesia in children. Cochrane Database Syst Rev. 2014 Jan 28(1):CD009633. doi: 10.1002/14651858.CD009633.pub2.

7. American Society of Anesthesiologists Task Force; Practice Guidelines for Preoperative Fasting and the Use of Pharmacologic Agents to Reduce the Risk of Pulmonary Aspiration: Application to Healthy Patients Undergoing Elective Procedures: An Updated Report on Preoperative Fasting and the Use of Pharmacologic Agents to Reduce the Risk of Pulmonary Aspiration. Anesthesiology. 2017 Mar126(3):376-393. doi: 10.1097/ALN.0000000000001452.

Before anesthesia, a group of drugs are used to prepare the patient, administered from half an hour before the surgery to the night before, known as pre anesthetic medication.

Aims

1.      Relief of anxiety and apprehension

As patient is going for operation, he is apprehensive.

2.      Amnesia

To produce amnesia, so that the patient does not remember what he is going through.

3.      Supplement analgesia

Different general anesthetics produce different levels of anesthesia, so used to supplement or augment analgesia.

4.      Decrease secretions

Given to decrease the secretions in the mouth and gastric secretions (as laryngospasm, problem in metabolism, gastric aspiration might occur)

5.      Decrease vagal stimulation

Especially when viscera are entered, leading to cardiac arrest.

6.      Anti-emetic effect

Used to prevent nausea, vomiting, providing anti-emetic effect pre and post-operatively.

7.      Decrease acidity / aspiration

Decrease acidity to avoid aspiration of gastric acids.

8.      Reduce dose

Overall, decrease the requirement of dose of general anesthetic required (for muscle relaxation, analgesia).

Timing & Route of administration

They are administered half an hour to one hour before surgery. Administered parentally as patient is not taking anything before surgery.

Groups of Preanesthetics

1. Sedative / Hypnotic / Anxiolytics

–  Benzodiazepines (still commonly used)

·         Diazepam

·         Lorazepam

·         Midazolam

–  Barbiturates (not used much)

·         Secobarbital

·         Pentobarbital

Characteristics

–  Use

They decrease the anxiety, make the person relax and drowsy, providing amnesia.

–  Duration

There is difference in the duration of action

Diazepam         72 hours

Lorazepam        24 hours

Midazolam        4 hours

–  Depressant Effect

Barbiturates have more depressant effect on CVS and respiration, so are less used, but when depressant effects are required, these barbiturates can be given.

2. Opioid Analgesics

– Morphine

– Pethidine

– Fentanyl & its congeners

Characteristics

There is a difference in the duration of action. All can be given parentally. Fentanyl is also available as transdermal patches.

– Use

Given to provide sufficient analgesia during procedure and post operatively.

– Disadvantages

a.      Awakening

Patient has delayed awakening after operation and takes longer time to come out of anesthesia.

b.      GIT

Morphine causes contraction of smooth muscles, so may cause constipation because of constriction of sphincters.

c.        Respiratory System

In asthmatics, can precipitate asthma.

d.       CVS

Excessive hypotension

e.        Urinary

Benign prostatic hypertrophy, leading to urinary retention.

f.         Eye

As morphine causes constriction of pupil, so sign of shock (dilated pupil) is obscured in the patients administered.

3. Antiemetics

–  Metoclopramide

–  Antihistamines

–  Phenothiazines –Promethazine used

– 5HT3 Receptor Blockers

•Ondansetron

•Tropisetron

•Granisetron

•Characteristics

– Use

Antiemetics enhance gastric emptying. They are used to prevent nausea, vomiting in patients. Most commonly used is Metoclopramide. Antihistamines are also used for this purpose. They are not only antiemetic but also used for:

1. Sedative property
2. Relieving anxiety
3. Anti-cholinergic effect

Patients having cancer chemotherapy have some vomiting. 5HT receptor blockers are used to protect. Nowadays Ondansetron is also in use in patients as pre-anesthetic.

Metoclopramide

Acts both centrally and peripherally.

Acting as dopamine antagonist, acts on medullary vomiting center, producing anti-emetic effect.

Enhances gastric emptying so that gastric components are passed earlier, preventing gastric aspiration.

4. Anticholinergics

Three drugs are in use as preanesthetic

– Atropine

– Hyoscine

– Glycopyrronium

Characteristics

Effects produced in patients:

1. Decreased secretions, preventing laryngospasm
2. Avoiding problems during intubation
3. Also suppress secretions in GIT, decreasing the chances of aspiration.

– Use

There are chances of vagal stimulation especially during thoracic and abdominal surgeries, which may lead to cardiac arrest, so given to avoid vagal stimulation.

– Differences

Atropine has more effect on vagal stimulation.

Hyoscine and Glycopyrronium have more effects on decreasing secretions.

Hyosine crosses BBB, producing CNS effects.

Glycopyrronium cannot cross BBB, thus has no CNS effects

Combinations

These are the different preanesthetic medications administered before surgery.

These are usually used in combination as required, depending on clinical status, type of operation and duration of operation.

In emergency, along with these proton pump inhibitors like Omeprazole and H2 blockers like Cimetidine are advised to decrease gastric secretions.

These drugs decrease the chances of aspiration pneumonia.

Browse all articles on Drugs Acting on Central Nervous System