How does general anesthesia affect:
Hemodynamics
Respiration
Temperature Regulation

Chemoreceptor Trigger Zone

General anethesia essentially depresses everything:
It causes a drop in blood pressure, a decreased respiratory drive (requiring assisted ventilation), and hypothermia.

General anesthetics also stimulate the CRTZ, leading to nausea and vomiting.  This can be counteracted with ondansetron, metoclopromide, and other CRTZ inhibitors/antiemetics.

Recall that general anethetics are CNS depressants.  When the anesthetic is removed, there can be a rebound tachycardia, hypertension etc. as the autonomic system regains control (the sympathetic system has a tendency to hypersecrete catecholamines when it regains its tone). 

(This is why there is an increased risk of MI after general anesthesia in patients with coronary artery disease)

You can reduce emergence phenomena by administration of opioids as part of the drug regimen.

The MAC is a measure of inhaled anesthetic potency.  It is defined as the minimum concentration of drug in the lungs that prevents movement in response to surgical stimuli in 50% of patients.

It is expressed as the % of lung volume that is occupied by the inhaled anesthetic.

Etomidate.

Etomidate does not cause a drop in BP.  Additionally, it reduces myocardial oxygen consumption (good for patients with coronary disease and risk of MI)

Ketamine is also a good choice for patients at risk of hypotension (b/c of its sympathomimmetic properties), but is not good for patients with coronary artery disease, as it increases myocardial oxygen consumption.

Ketamine

Dissociative anesthesia is where the patient develops analgesia, unresponsiveness, and amnesia, but keeps their eyes open and moves their limbs.

Ketamine may also promote "emergence delirium" characterized by hallucinations.

Recall that ketamine has a few unwanted side effects including "dissociative anesthesia", "emergence delirium" and increased myocardial oxygen consumption. 
However, Ketamine is a potent bronchodilator and causes less respiratory depression than other parenteral anesthetics.  These respiratory benefits are its main indication for use.

Because it has sympathomimmetic effects, it is also a good drug to use for patients in a state of shock.

The general anesthetic Halothane may cause a fulminant hepatitis, termed "halothane hepatitis".  This is histologically identical to viral hepatitis.  Symptoms begin 3-5 days after administration and progress to hepatic failure.  This is associated with a 50% mortality rate.

Why?

Metabolites of haloethane include chloride, bromide and trifluoroacetic acid.  Fluoroacetylation of liver enzymes precipitates an immune response against the liver. 

Halothane is also one of the more potent triggers for malignant hyperthermia, a rare side effect characterized by excessive skeletal muscle contraction, which causes heat production and hyperthermia. (This can result from administration of any inhaled anesthetic).

The halogenated anesthetics are inhaled general anesthetics.  They include Halothane, Isoflurane, and Sevoflurane. 

They may cause Malignant Hyperthermia (muscle rigidity, hyperthermia, acidosis, shock, elevated CK)
Give IV dantrolene = a muscle relaxant that binds the ryanodine receptor and prevents Ca++ release from the sarcoplasmic reticulum.

also remember that all of the inhalational anesthetics have a low therapeutic index (2-4)

Unconsciousness

Immobilization (Neuromuscular blocking drugs)

Analgesia (pain relief)

Amnesia

Decreased ANS response

Propofol

It also has a short elimination half-life, minimizing its hangover effects.

Its short elimination and antiemetic profile make it a good choice for outpatient settings!

Etomidate does not greatly affect the cardiovascular system, and is therefore a good drug to use in patients with cardiomyopathies, ischemic heart disease, or hypertension.

(the inhaled anesthetic, sevoflurane is also acceptable for ischemic patients)

Halothane is the most potent inhaled anesthetic, having an MAC of 0.75%

However, because of its large blood/gas partition coefficient, Halothane takes a long time to exert its effect compared to other inhaled anesthetics.

Halothane is highly soluble in the blood.  This means that a much higher total amount of gas must diffuse across the alveoli before the partial pressure in the blood equals the amount given in the lungs. This, naturally, makes induction slower, as the effective partial pressure is reached more slowly.

Halothane also has a very high lipid solubility so it is stored in fat tissues and slowly diffuses back into the blood for elimination.  Elimination is therefore slow as well.

Isoflurane.

Most inhaled anesthetics increase blood flow to the brain, thereby increasing intracranial pressure.  This is undesireable for neurosurgery.  Isoflurane induces this to a lesser extent than the other inhaled anesthetics.

Enflurane is contraindicated in Epileptics

Do not give this drug to patients with seizures.

Sevoflurane is a rapidly effective drug (low blood solubility so rapid partial pressure equalization)

Recall that a good induction anesthetic has rapid effect and is not an airway irritant.

For example, halothane has a slowwww onset so it is not a good choice.  Desflurane, despite a quick onset, is an airway irritant and therefore not suitable for induction either (though it can be used for maintenance, and is on an outpatient setting b/c of rapid elimination).

100% O2.

N2O rapidly diffusing back out of the blood will displace other alveolar gases, leading to hypoxia.

N2O makes for a good analgesic, but cannot act as a true anesthetic for surgical anesthesia because its MAC > 100%. 

It can be used alone for its analgesic action in dental procedures and parturition, when you don't need a loss of consciousness.

Sodium thiopental is the most commonly used parenteral anesthetic.  It has rapid effect and is used for induction in combination with inhaled anesthetics.  It may also be used as the sole agent in short surgeries.

(Trivia: It is the induction agent used in the lethal injection in the USA.  It is followed by the muscle relaxant Pancuronium, then finally by potassium chloride injection)

Ketamine, which has sympathomimmetic properties, is a useful anesthetic in patients in shock.  It increases HR, CO, and BP though central sympathetic stimulation.

But remember its side effects! Dissociative anesthesia, Emergence delerium, and flashbacks/hallucinations!

Opioids severely depress respiration, requiring mechanical control

Opioids depress the cardiovascular system

Opioids cause profound analgesia and can cause unconsciousness at high doses; however, post-operative recall may occur!

Conscious sedation typically involves the IV anesthetics:

Benzodiazepines, propofol, or opioids. 

It involves analgesia and decreased anxiety without loss of consciousness or airway control.

Benzodiazepines (diazepam, midazolam)

or

Opioids (morphine, meperidine)

Induction is often accomplished with Sodium Thiopental and a loading dose of one or two inhaled anesthetics.

One or two inhaled anesthetics are typically used to maintain anesthesia.

These can be accompanied by adjuvants like NMJ blockers and opioids)

Halothane.  (The blood/gas partition coefficient is 2.3)

A high coefficient means that the speed of induction is much decreased.  Therefore, halothane causes the slowest induction. 

Remifentanil is an ultra-short acting opioid (10min).

In order of duration, the opioids can be remembered as "phasers" (FASR)... as you progress from F to R, the duration phases out (becomes shorter)

LONG

Fentanyl

Alfentanil

Sulfentanil

Remifentanil

SHORT