Principles of anesthesia

The goal of inhalation anesthesia is to maintain a partial pressure of the anesthetic agent in the alveoli that is sufficient to maintain the required partial pressure of the brain

Stages of anesthesia:

  • Stage 1: analgesia

    • Pa\\in-free

      1. beginning of induction to loss of consciousness, voluntary movements are present

      2. pre-med through part of the induction

  • stage 2: delirium

    1. period of excitement followed by loss of consciousness, nystagmus, and sometimes resp. from induction agent

      pre-med through induction

  • stage 3: Surgical

    • plane 1:

      • regular breathing, poor muscle relaxation, nystagmus gone, active palpebral reflex

    • plane 2:

      • medium anesthesia depth, sluggish palpebral, active corneal reflex, muscle relaxation, adequate analgesia

    • Plane 3:

      • deep surgical anesthesia, some abdominal support to breathing, uneven respiratory character, palpebral reflex gone, week corneal reflex

    • Plane 4:

      • patient is dying

The Brain and the Body

  • Cardiovascular

    • heart rate and strength vs. anesthetics

    • concept of MAC

    • uptake, distribution, and elimination

  • Respiratory

    • VQ mismatch

    • induction, maintenance, and recovery

    • from anesthetic inhalant to blood phrase

Cardiovascular need to remember!

  • SV

    • the volume of blood pumped in one contraction

  • HR

    • number of contractions in one minute

  • CO

    • volume of blood pumped in one minute

    • SV x HR

  • MAC

    • The alveolar concentration of anesthesia at which 50% of the anesthetized patients will not respond to a surgical stimulus

    • Minimum alveolar concentraion

    • Means some may need more and some may need less

  • Blood/ Gas Coefficient

    • the ratio of anesthetic gas in the blood phase to that in the gas phase

There is no inhalation anesthetic, when present in quantities sufficient to produce anesthesia, that will fail to reduce contractility in the mammalian heart

Minimum Alveolar Concentration

  • the alveolar concentration of anesthesia at which 50% of the anesthetized patients will not respond to a surgical stimulus

    • every patient will be different

  • every inhalant has a different value

  • if the inhalant is more soluble it acts less

    • isoflurane

    • sevoflurane

    • desflurane

    • halothane

      • though mostly obsolete

Blood Gas Coefficient: why every inhalant will need to be monitored and adjusted differently

  • blood/gas solubility = the amount of anesthetic dissolved in the blood

  • a high coefficient means there is a large quantity of dissolved

  • the anesthetic that is dissolved in the blood is not available to diffuse into the brain

  • Isoflurane = 1.46 sevoflurane = 0.68

    • more soluble will produce more anesthesia less fast

    • less soluble allows the body to collect more which will take longer

  • this is the characteristic that determines induction and recovery time

MAC and the effect of anesthesia

  • cardiac output and induction time are directly related

    • CO increases, IT increases

    • CO decreases, IT decreases

  • MAC of your inhalant doesn’t change the effect of anesthesia on your patient, but it does affect the technique of how we achieve it

Respiratory need to remember!

  • Tidal volume (TV)

    • amount of gas passed in and out of the lung in one respiratory cycle

  • Mechanical dead space

    • combined inhaled and exhaled air

    • physiological dead space

      • mouth, trachea, etc.

  • Atelectasis

    • collapsed or airless state of the lung

  • resistance pressure

    • how much pressure is pushing up against the diaphragm

  • Inspiration Capacity

    • TV + inspiratory reserve

      • deep breath = reserve cavity

Effects of anesthesia on ventilation

  • The tidal volume of the patient’s respiration decreases

    • focus on how well the patient is breathing, not just the numbers

      • IPPV= intermittent positive pressure ventilation (bagging)

  • Mechanical dead space is increased

  • hypoxic pulmonary vasoconstriction (HPV) is obtunded

    • Reflex of the body

    • Vasoconstriction: constriction of blood pressure, causing increased blood pressure

    • the safety net of the body that saves energy and produces more uptake of oxygen in the body

    • V/Q mismatching occurs

      • Amount of the lung field that will not uptake air as much as other

V/Q Mismatch

  • Under normal, awake conditions ventilation and perfusion are matched so that proper exchange occurs

  • atelectasis creates hypoxic alveoli

  • in patients of greater mass, this problem is more difficult to overcome

    • horses, bovine, bigger sheep or goats, obese dogs and cats

Inhalant induction and maintenance

  • moving from the gas phase to the blood phase takes place by simple diffusion

    • Drop perfume analogy

      • gasses diffuse down a pressure or concentration gradient

      • the larger the gradient the quicker the concentration rises on the lower side of the gradient

  • The anesthesia machine is how this is allowed to happen

    • intermittent dosing device

Two carrier processes of anesthesia

  • anesthesia machine flow meter

    • oxygen flow: carries anesthetic from the vaporizer to the breathing circuit

  • patient ventilation: carries anesthetic from the breathing circuit to the lungs

    • the anesthetic moves from the alveoli to the blood by diffusion

    • the reduced TV and respiration rate generated by the patient necessitates the need for IPPV

The Anesthesia Machine

  • Increase the alveolar concentration

    • increase respirations or gas concentration

      • has a little vaporizer that releases the gas

  • decrease alveolar concentration

    • decrease respirations given via IPPV or gas concentration

  • Affect a change in alveolar concentration

    • changes planes of anesthesia

Changing the gas concentration

  • 1 TC (time constant) = volume/flow

    • 1 TC = 5L/1L per min

    • 1 TC = 5 min

    • 3 TC = 15 mins

Recovery

  • decrease the blood-brain gradient

  • what has been inhaled must be exhaled

  • all forms of the drug must be metabolized

  • Every patient will be different

  • the patient retreats back through the stages of recovery

REMEMBER

there is no safe anesthetics, just safe anesthetists

the anesthetist always doses inhalation agents to effect

therefore attention to the patient is the most important duty of the anesthetist