Anesthesia is a reversible condition due to the effect of anesthetic drugs that cause a reduction or complete loss of response to pain or other sensations such as consciousness and muscle movements during surgery or other invasive procedures that can be painful. William Yancey, MD, is the anesthesiologist in Houston highly-qualified in offering pain relief at Yancey Pain & Spine, administering anesthesia to make invasive procedures comfortable for patients.
There are two main types of anesthesia:
- General anesthesia, which makes the whole body lose feeling, movement, and consciousness. Drugs that are used to induce this type of anesthesia are called general anesthetics.
- Local anesthesia, which numbs only a specific targeted treatment body area. Drugs used to induce local anesthesia are called local anesthetics.
Anesthesia performed with general anesthetics occurs in four stages:
Stage 1: Induction
This period involves a patient going from a state of consciousness to a state of unconsciousness.
At this stage, the presence of inhibitory neurons in the central nervous system causes increased excitement, involuntary muscle movement, increased heart rate, blood pressure, and respiration.
Stage 3: Surgical anesthesia
At this stage, there is a gradual loss of muscle tone and reflexes. The patient is fully unconscious, responsive to surgery, and has regular breathing. Surgery can ideally take place in this stage. Careful monitoring is necessary to prevent stage four.
Stage 4: Medullary paralysis or overdose
At this stage, respiratory and cardiovascular failure occurs, which leads to death if the patient is not revived quickly.
The mechanism of action of general anesthetics
Multiple sites and different mechanisms are responsible for the effects of general anesthetics.
- At the microscopic level, the action of general anesthetics on the thalamus and reticular activating system leads to reversible loss of consciousness.
- The action on the hippocampus, amygdala, and prefrontal cortex causes amnesia.
- The action on the spinal cord causes immobility and analgesia.
General anesthetics can be divided into three groups at the molecular level depending on their abilities to produce unconsciousness, immobility, and analgesia.
- Intravenous agents
Etomidate, Propofol, and Barbiturates are much more potent at producing unconsciousness rather than immobility or analgesia. These drugs are commonly used in the induction phase. However, etomidate can cause adrenal suppression and transient skeletal muscle movements, propofol is known to cause respiratory depression and hypotension, while barbiturates can cause apnea and bronchial spasms.
- Intravenous agents
Ketamine and inhalation agents such as nitrous oxide, xenon, and cyclopropane produce significant analgesia. However, their ability to produce unconsciousness and immobility is relatively weak and because of that, these drugs are typically used in the maintenance phase of anesthesia. When it comes to side effects, Ketamine can cause hypertension and tachycardia. Nitrous oxide and cyclopropane are known to cause dizziness, nausea, and vomiting.
- Halogenated volatile anesthetics
This group has a more diverse mechanism of action that produces immobility and includes Halothane, Enflurane, Isoflurane, Sevoflurane, and Desflurane.
Unlike the other commonly used general anesthetics, this drug has a unique ability to produce sedation and analgesia without the risk of respiratory depression. Side effects of dexmedetomidine include hypotension.
Pharmacology of local anesthetics
Unlike general anesthetics, these drugs produce a transient loss of sensory perception, especially those of pain in a localized area of the body without having unconsciousness. Due to their distinct chemical properties, local anesthetics can pass through the neuronal membrane and bind to a specific receptor at the opening of the voltage-gated sodium channel, thus preventing sodium influx and the initiation of action potentials leading to loss of sensations in the area supplied by the nerve. Most commonly used local anesthetics include bupivacaine, lidocaine, mepivacaine, Procaine, Popivacaine, and Tetracaine. When appropriately used, these agents are generally very safe. However, systemic toxicity can cause seizures and cardiac arrhythmias.