I have timed how long it takes for general anesthesia to kick in. The time taken for anesthesia to kick in depends on two factors
- The route through which the medicine is given. This is either through a face mask or intravenous route.
- Age of the patient.
Let’s jump to find out more.
By Dr. Rajeev Iyer MBBS, MD, FASA, Anesthesiologist
Does anesthesia knock you out instantly?
I will explain this based on whether the anesthesia is given via the face mask or through the intravenous route.
Face mask: When going off to sleep with a face mask via anesthesia gas, the speed of anesthesia kicking in (aka anesthesia induction) depends on the size of the patient. In general, the smaller the age it takes less time. This means when I have compared a 1-year-old baby and a 16-year-old teen, it takes like a minute or so for anesthesia to kick in, whereas it may take 3-4 minutes or longer in a 16-year-old teen.
The anesthesia kick-in time is generally the time for loss of eyelash reflex. The anesthesia gas also correlates with the lipid solubility of each anesthetic agent. Going off to sleep with anesthesia gas can cause unnatural movements in many patients/children.
Intravenous route: When going off to sleep with the intravenous route, it takes less than a minute for the anesthesia to kick in. This is true irrespective of age.
How does anesthesia work so fast?
General anesthesia blocks the transmission of electrical signals in the brain. According to a recent study, general anesthesia does it by preferentially affecting the higher frequency impulses that are required for movement and has minimal impact on major things like breathing.
The electrical disruption happens quickly enough to get people off to sleep within minutes. This research study was performed with an anesthesia gas called Isoflurane. Although this theory is directly applicable to the anesthesia gas, a similar phenomenon (although not fully known) happens with intravenous medicine.
Why is there a difference between ages with the face mask?
A face mask delivers anesthesia gas. The anesthesia gas comes as a liquid and is placed in a specialized anesthesia gas tank called a vaporizer.
The gas first goes into the lungs. Then from the blood into other organs of the body. It has to saturate all the organs/tissues like skin, fat, liver, kidneys, and finally the brain. Once enough anesthesia enters the brain, the anesthesia starts kicking in.
At younger ages, the size of the baby is small. In other words, their body surface area is less and hence the amount of anesthesia gas saturation is less as compared to older people who have a bigger body. Hence, a bigger size is more body surface area and more tissues to saturate.
How long does it take for IV anesthesia to work?
The most commonly used IV anesthesia is propofol. The propofol works in less than 1 minute typically like 40-45 seconds. This is true for all ages.
There are a few intravenous different anesthesia medicines. These are some of the intravenous medicines that are available. I have personally used all these medicines as an anesthesiologist.
- Fospropofol – This is a newer form of propofol that is water-soluble. It is not commonly used at present. Propofol is the most common intravenous agent used today.
- Midazolam – Although technically not anesthesia, used in appropriate doses can cause sedation.
- Pentobarbital – This is mostly used for sedation, not as general anesthesia
- Thiopental – I have listed this last since this is not commonly used in many countries. It is not available for anesthesia use in the USA.
|Medicine||Route of administration||Age||Anesthesia Kick in time (loss of eyelash reflex)|
|Anesthesia gas (Sevoflurane)||Face mask||Children||1-2 minutes|
|Intravenous||Propofol||Children & Adults||Less than 1 minute (40-45 seconds)|
|Ketamine||Children & Adults||10-30 seconds|
|Etomidate||Children & Adults||Less than 1 minute|
|Dexmedetomidine||Children & Adults||This is dose dependent.|
|Thiopental sodium||Children & Adults||15-20 seconds|
|Midazolam||Children & Adults||This is dose dependet|
When can the operation be started after anesthesia kicks in?
After the initial few minutes of anesthesia, additional things are done. If you are off to sleep with a mask, then an intravenous line is placed. Well, there may be some exceptions to this where anesthesia can be done only with a mask with no intravenous line. But in general, an intravenous line will be placed.
Following this, the following additional medications may be administered.
- Opioids like fentanyl, and morphine for pain relief
- Muscle relaxants like vecuronium and rocuronium provide relaxation for surgery and help with breathing tube placement.
Following a few minutes after muscle relaxant administration (typically 3-5 minutes), a breathing tube is placed. The breathing tube is commonly checked with the following: This is not in any particular order. I personally use all of these to verify the breathing tube position.
- Fogging of the endotracheal tube
- Chest rise
- Listening to breath sounds with a stethoscope
- End-tidal carbon dioxide
What anesthesia gases are available?
The anesthesia gases are referred to as volatile anesthetics. These are the following gases available presently. All of these gases come in a liquid form and become a gas as I have explained above.
- Halothane (Less commonly used presently)
- Nitrous Oxide – Nitrous oxide is not used as a sole anesthetic by anesthesiologists. This is used mostly as a supplement during general anesthesia. It is used as a sole agent for Dental sedation by dentists.
- Xenon – I have not seen this being used on patients. Maybe it is used, I don’t have personal experience with this. The cost of Xenon and limited availability has prevented the widespread use of Xenon for general anesthesia.
The common gas used to get anesthesia started is Sevoflurane and Halothane. Desflurane and Isoflurane are not used due to their pungent smell and the potential for causing complications.
Is there any other way anesthesia medicine is administered?
The anesthesia medicines are primarily administered only as gas or intravenously. However, there are other ways anesthesia can be initiated in certain patients. This is done by either placing a mask or putting an intravenous line to administer the anesthesia medicines:
- Intramuscular – Usually ketamine is administered this way
- Intranasal – Usually Dexmedetomidine or midazolam is administered this way
The table above is a summary of how long it takes for anesthesia to kick in based on the medicine and route of administration. There may be variations with the dose that is being used. A higher dose increases the speed and a lower dose reduces the speed by a small margin. This is especially true for intravenous medicines.