Stages of Anesthesia
The defined “Stages of Anesthesia” is an old concept created by Dr. Arthur Guedel in 1937 that described the physiological changes patients would experience while being anesthetized with ether. Dr. Arthur Guedel is also known for creating multiple airway devices that we still use today. These include the oropharyngeal airway, the laryngeal mask airway (LMA), and a cuffed intratracheal tube. Our ultimate landing spot is what we call “general anesthesia”, or a medically-induced coma with loss of protective reflexes. While under general anesthesia, patients will not respond to painful stimuli and often need some type of airway device (ie. LMA or endotracheal tube) to support their ventilation. It is also common to have some degree of cardiovascular depression requiring vasopressors. Prior to the development of modern day halogenated inhaled anesthetics and intravenous anesthetics, getting to the depth of general anesthesia was incredible dangerous from a cardiopulmonary perspective. These newer medications, along with new technology to monitor hemodynamics and ventilation, have made general anesthesia incredible safe. And although we no longer use ether, you will still hear Anesthesiologists refer to “Stages of Anesthesia”. Let’s go through the 4 stages.
Stage 1: Analgesia/Induction
This process can start in the pre-operative area, where a patient may receive a benzodiazepine (ie. Midazolam) or an alpha-2 agonist (ie. Dexmedetomidine) to help calm their nerves. Patients may also benefit from an opiate like fentanyl or hydromorphone in the pre-operative holding area if they are experiencing pain. During this stage, patients are sedated but awake enough to talk and will likely respond to verbal stimuli. Breathing may become slowed. Usually there are no major changes in hemodynamics. Depending on what medications you use, this stage may come with amnesia.
Stage 2: Excitement/Delirium
This is one of the most dreaded stages of anesthesia. With this stage comes disinhibition, delirium, uncontrolled movements, loss of eyelash reflex, hypertension, and tachycardia. Although we typically don’t see this stage when a patient falls asleep due to our fast acting IV anesthetics, we commonly see this stage when waking someone up after surgery. During this stage, airway reflexes become hypersensitive to stimulation. Therefore, we typically wait for patients to get past this stage prior to extubation. Extubating during this stage can lead to laryngospasm, which is when the vocal cords close in a protective manner to prevent aspiration. However, the patient is not awake enough to cough and open their vocal cords. This can lead to hypoxia and hypercarbia with fatal consequences if not addressed quickly.
Stage 3: Surgical Anesthesia
Finally your patient has arrived at the desired depth of anesthesia. This stage technically has 4 planes. Plane 1 is when your patient is still spontaneously breathing with regular respirations. Plane 2 is when you start to see intermittent pauses in breathing. You will also notice loss of corneal and swallowing reflexes. Plane 3 is complete relaxation of intercostal and abdominal muscles along with loss of pupillary reflex to light. This is considered the “true ideal surgical anesthesia” plane. And plane 4 is defined by irregular respirations and paradoxical rib cage movement.
Stage 4: Overdose
As the name describes, this stage is when you are delivered more anesthesia than what is required for that particular patient undergoing that particular surgery. Skeletal muscles are extremely flaccid and pupils are fixed and dilated. It is common to see severe hypotension at this point requiring vasopressors. Without cardiopulmonary support, this stage has potential to progress and end with death.
