Anesthesia Monitoring Depth - Stages - Veterinary Anesthesia PDF

VETC2012 VETERINARY ANESTHESIA 1 Anesthetic Monitoring IV: DEPTH of anesthesia Reflexes, Stages, & Planes Laura Couch RVT, MSc, One Health (GradCert) Objectives of this Lecture Introduce terminology, and why we monitor a patient and their depth under anesthesia Provide the...

VETC2012 VETERINARY ANESTHESIA 1 Anesthetic Monitoring IV: DEPTH of anesthesia Reflexes, Stages, & Planes Laura Couch RVT, MSc, One Health (GradCert) Objectives of this Lecture Introduce terminology, and why we monitor a patient and their depth under anesthesia Provide the foundation to understand how we monitor a patient's vitals, reflexes, and other indicators to determine their depth. Introduce the theory on the stages and planes of anesthesia and apply knowledge to determine a patient’s depth and ensure patient is at the most appropriate depth Recall the concepts and devices that can be used to assist in monitoring depth  Textbook Anesthesia & Analgesia for Veterinary Technicians Reading Ch. # 6 Anesthetic Monitoring: page 160-161 Depth (Stages & Planes) of Anesthesia: page 161-164. Determining If the Patient Safe: page 164. o To keep the patient safe and to regulate anesthetic depth Why Monitor?  Identify ‘incidents’ before they lead to accidents  Identify abnormalities to suggest our patient is not safe  To trend changes in vital signs *trends are more valuable than a single set of physiological parameter readings  Assists to identify complications before they occur  To react timely to patient concerns  Reduce adverse effect  Essential physiological systems to monitor: Central nervous system monitoring patient  Depth of anesthesia: to assure appropriate level of unconsciousness, depth of analgesia and immobility (not too light anesthesia. nor too deep for the procedure) cardiovascular system: perfusion (circulation) & oxygenation respiratory (pulmonary) system: ventilation & oxygenation thermoregulatory system Gastrointestinal* system, * esp in ruminants  1. Vital Signs Heart rate Heart rhythm Monitoring Parameters – Respiratory rate and depth recall what Mucous membrane color we monitor Capillary refill time Pulse strength Blood pressure Body temperature 6  2. Reflexes- Involuntary response to stimulus Palpebral, corneal, pedal, swallowing, laryngeal, and papillary light reflexes Indicators of anesthetic depth …We also 3. Body Position – poor positions can cause pain, injury, or nerve damage monitor 4. Equipment – anesthetic equipment and all monitors should be inspected & reliable Patient parameters offer predictable responses to anesthesia at various depths - May be affected 7by drugs, disease, or individual response variation  GOAL of an Appropriate Depth To make sure the patient is at a depth that provides immobility, unconsciousness, and lack of awareness of pain while avoiding conditions that endanger the patient such as hypoventilation, hypoxemia, hypotension, and hypothermia.  Immobility/Muscle Relaxation – no movement Unconsciousness - unaware  Analgesia – no pain  Amnesia – does not remember  Minimize complications All done without compromising patient safety with focus keeping oxygenation, ventilation, perfusion within normal  What is Monitoring Anesthetic Depth?  Subjective monitoring of reflexes (motor & autonomic) is the commonly accepted means of assessing anesthetic depth.  Animals lose consciousness (awareness) long before they lose ability to move spontaneously*  It is very easy to monitor for immobility, & analgesia (sub-cortical activities) * so we do this  Vs very difficult to monitor consciousness (cortical activity requires an EEG – electroencephalography or gas analyzer) * we don’t common do this however it is practiced at referral centers  Sub cortical monitoring uses reflexes and other parameters coming up in this lecture Classifications of Depth: Stages and Planes From Stage I  to Stage IV = a progressive decrease in pain perception, motor coordination, consciousness, reflex responses, muscle tone, and cardiopulmonary function. We aim for Stage 3 Plane 1 or 2. Stages 1-4 Stage 1 = Stage of voluntary excitement (movement) Stage 2 = Stage of involuntary excitement (movement) Stage 3 = Stage of surgical anesthesia with Stage 1 Period of voluntary movement Beginning of induction to loss of consciousness Characterized by: Fear, excitement, disorientation, struggling, urination, defecation Increased heart rate and respiratory rate Stage ends with loss of ability to stand and recumbency UNDESIRED! x  Stage 2 Period of involuntary movement “excitement stage” Characterized by: delirium and excitement Breathing irregular Salivation, vomiting, urination, defecation may occur Vocalization, struggling, paddling Increased heart and respiratory rate, pupils dilated, possible nystagmus, muscle tone marked, reflexes present, swallowing reflex present, strong jaw tone (can not intubate), arrhythmias Actions are not under conscious control of the animal Stage ends with muscle relaxation, decreased respiratory rate, and decreased reflex activity UNDESIRED! X ****PASS THROUGH QUICKLY WITH PROPER DRUG ADMINISTRATION Clinical notes on Stages 1 and 2 Vital to know for safe induction of (and recovery from) anesthesia  Both are stages of excitement Both are undesirable stages Both can be shortened significantly by faster onset intravenously administered induction drugs (propofol, alfaxalone, thiopental) versus induction achieved by inhalants. In clinical practice, anesthetic induction is usually completed within 60 seconds. Adjust rate of administration of intravenous induction drugs to shorten (or even bypass) Stages 1 and 2 of anesthesia to minimize the duration of undesirable excitatory signs during induction of anesthesia Induction agent ketamine has different effects during induction Dissociative anesthetic agent: Though completely immobilized, ketamine- anesthetized animals still maintain eye reflexes (nystagmus, spontaneous blink, palpebral reflex) and ability to swallow  Stage 3 Period of anesthesia, DESIREABLE stage! No movement in response to stimulus, progressive loss of reflexes Eye reflexes/position/movement Jaw tone pedal reflex anal tone autonomic (RR, HR, BP) reflexes Stage 3 is Divided into 4 planes We aim for our surgical patients to be in plane 2! Plane 1 okay for non surgical procedures Plane 2 surgical anesthesia Plane 3 too deep, automonic system affected more Plane 4 nearing overdose! Stage 3, Planes 1- 3 Common Terminology Plane 1 = “Light” anesthesia Intubation Good for minor procedures - Not suitable for painful surgery Plane 2 = “Medium”/”Appropriate” surgical anesthesia Optimum depth for most surgical procedures Plane 3= “deep” surgical anesthesia 15  Stage 3 Plane 1  Light Anesthesia  okay at times  Not adequate for surgery  Response to pain present Regular respiratory pattern, no involuntary limb movements Eyeballs centrally positioned, may start to rotate ventrally, pupils partially constricted, decreased pupillary light reflex Endotracheal tube may be passed and connected to gas anesthetic machine due to weak jaw tone and no swallowing reflex Other reflexes are still present but decreased response Simple non-noxious procedure can be performed Stage 3 Plane 2 “Surgical Plabe” Appropriate (medium) plane for surgery Suitable or adequate depth for most surgical procedures Characterized by: Regular and shallow respiration with decreased rate Blood pressure and heart rate mildly decreased Relaxed muscle tone Swallowing reflexes are absent Ventromedial eye rotation !!! Weak or absent palpebral and pedal reflexes Stage 3 Plane 2 (Cont’d) Surgical stimulation may produce: Mild increase in heart rate, blood pressure, or respiratory rate That is okay! Patient remains unconscious and immobile Pupillary light response is sluggish; pupil size is  Stage 3 Plane 3 Deep anesthesia — excessive for most procedures potentially dangerous level of surgical anesthesia! Characterized by: Cardiovascular & respiratory functions depressed: low heart and respiratory rates, decreased tidal volume Reduced pulse strength Increased capillary refill time (CRT) Absent palpebral & pedal reflexes Poor to absent papillary light reflex; central eyeballs; moderately dilated pupils, weak corneal reflex, dry cornea Other reflexes are totally absent Muscle tone is very relaxed – jaw tone absent Abdominal breathing with a rocking motion Stage 3 Plane 4 Early anesthesia overdose Dangerous deep level of anesthesia cardiopulmonary failure imminent Characterized by: Abdominal breathing Fully dilated pupils; dry eyes All reflexes are absent Flaccid jaw tone! Marked depression of the cardiovascular system, pale mucous membranes, increased CRT Flaccid muscle tone Stage 4 Period of anesthetic overdose Characterized by: Cessation of respiration Circulatory collapse Corneal reflex absent Anus dilated, feces/urine may be voided Death Resuscitate immediately to save the patient  Judging Anesthetic Depth Monitor as many variables as possible No one piece of information is reliable by itself Each animal will respond in its own unique way to anesthesia 22  Assessment of Anesthetic Depth: Reflexes Conscious animals have protective reflexes Decreased reflexes to stage III, plane 3 level anesthesia (when there are few to none) Reflexes evaluated Swallowing, laryngeal, pedal, palpebral, corneal, papillary light reflex Reported as present, decreased, or absent 23  Clinical note on Stages & Planes Place importance on Stage 3 (Surgical Stage) and its planes during maintenance of general anesthesia Consider the global picture (motor reflexes, autonomic reflexes, anesthetic dose, patient pathophysiology, etc.) in assessing depth of anesthesia and possible causes of depression of autonomic reflexes You must be able to distinguish Planes I, II and III from each other while monitoring an anesthetized patient  Be cognizant of species differences in display and applicability of signs of anesthesia Use the least possible dose of anesthetic drugs to cause adequate unconsciousness, muscle relaxation and analgesia with the minimal adverse autonomic (cardiorespiratory) effects …more Clinical notes on Stages Be cognizant of effects of non-inhalation drugs used for modern anesthesia (balanced anesthesia) Autonomic reflexes may appear depressed much earlier Ventilatory depression (hypoventilation) from opioids or induction drugs, e.g. Propofol (induction apnea) Hypoventilation can be easily treated by manual (IPPV) or mechanical ventilation Hypotension might also appear at lighter planes due to adverse effects of other anesthetic drugs (e.g. acepromazine, propofol) The anesthetized patient doesn’t necessarily have to be too deeply anesthetized to be hypotensive Reflexes are: Involuntary response to stimulus Examples of reflexes: Palpebral (eye blink), pedal, swallowing, laryngeal, corneal and papillary light reflexes for anesthetic depth assessment we will often (q5min or less) asses medial and lateral palpebrals! Swallowing Reflex A normal response to food or saliva in the pharynx Monitored by viewing the ventral neck region Present in sedation or light surgical anesthesia Lost in medium surgical anesthesia This may occur with sedation/pre-medication at time at the patient must be intubated if they loose their swallow reflect as they can not protect their airway Returns just before the patient regains consciousness Smooth muscle movement Used to determine when to pull the endotracheal 27 tube Larynge al Reflex Epiglottis & vocal cords close immediately when larynx touched by an object Prevents tracheal aspiration Observed during intubation if animal is in the light plane of anesthesia Makes intubation difficult Especially in cats, pigs, and small ruminants 28 May cause laryngospasm in cats, pigs, and small ruminants Overview of stages and clinical application Motor (~ somatic) reflexes Eye Reflexes Palpebral reflex: absent from Stage 3, Plane III onwards :. We want a very slight palpebral for stage III plane II Corneal reflex: Only absent during anesthetic overdose Pupillary light response: (not so practical during anesthesia) Palpebral Reflex The blink reflex in response to a light tap on the medial or lateral canthus May be elicited by lightly stroking the hairs of the upper eyelid Present in light anesthesia Often lost during medium anesthesia, although the exact point varies Slow palpebral response in horses indicates adequate surgical anesthetic depth Ruminants tend to have a slightly stronger reflex than horses 31 Palpebral Reflex (Cont’d) Assess both medial and lateral! Gentle. Slowly. Compare sides. *memorize 32 Note Pupil Size 33 Indicators of Anesthetic Depth Pupil size example s …Overview of stages and clinical application Other Eye activities  Eyeball position: ruminants, dogs & cats Ventro-medially rotated in Plane II Stage 3; centrally positioned in Planes I and III of Stage 3 ** Memorize eye positions we continuously assess these!  Lacrimation: a sign of light anesthesia - increase inhalation anesthetic dose to return to ideal surgical anesthesia  Nystagmus: a sign of very light anesthesia – will require a small dose of intravenous induction drug to Indicators of Anesthetic Depth: Eye Position Ensure you open both lids equally, compare each side. Look at both eyes …eye position may differ from R to L, anisocorias (pupils of differ size),or nystagmus could be present  Pupillary Light Reflex (PLR) – not so practical under anesthesia because it is lost very early in anesthesia Constriction of pupils in response to bright light shined on one retina Present in light and medium anesthesia; absent in deep anesthesia Dazzle reflex Blink response to bright light shined on retinas Same significance as PLR 38 Corneal Reflex not generally used, not as practical in small animall anes (unreliable) or can damage eye but use if concerned Retraction of eyeball within orbit and/or a blink in response to corneal stimulation Touch the cornea with a drop of sterile saline or artificial tears Most useful in large animals; difficult to elicit in small animals Present in light and medium anesthesia; absent in deep or excessive anesthesia Used primarily to determine if a LA patient is too deep 39 Indicators of Anesthetic Depth Jaw Tone: can be described as: present/tight, appropriate/mild, loose/relaxed -4 th would be absent or flaccid which suggest deep or overdose Assess amount of resistance of opening jaw while securing the maxilla or canines and gently opening the mandible to its full potential. Ensure the ADS does not rotate or slip!!!! We aim to have a mild/loose appropriate level of jaw tone  Flexion or withdrawal of limb in response to squeezing, twisting, or pinching a digit or pad Used in small animals only Varies from subtle muscle contraction to full withdrawal of limb Varies with depth of anesthesia Present in light anesthesia Absent in medium anesthesia Requires a high intensity stimulus …Overview of stages and clinical application …More Motor (~ somatic) Reflexes to assess depth Purposeful (spontaneous) movement of extremities (limbs or neck) Implies patient not in surgical stage (S3P2), but Stage 2 YIKES! No dancing!  Quick intervention!!! required to return patient back to surgical anesthesia  Be cognizant of involuntary movement of some muscle groups (face, shoulder blade) that might occur with some anesthetic drugs like propofol  Pedal (withdrawal) reflex: small animals Present in Plane 1, weak in Plane I, & absent in Plane III of Stage 3 Anal sphincter tone / reflex: used for large animals & very helpful! The IDEAL anesthetic depth for surgery/noxious stimuli Non-responsive to surgical stimulus – stage 3 plane 2  Weak palpebral reflex  Weak jaw tone  Weak pedal  Weak anal tone  Eyeballs rotated ventro-medially ruminants, dogs (some), cats (some)  Stable autonomic reflexes (RR, HR, BP, Temp)  Too Light (left) vs Too Deep (right) < Stage II plane I > Stage II plane 3  Movement of limbs / neck in response to surgical  Absent palpebral reflex stimulus  Absent pedal reflex  Blinking  Absent corneal reflex  Increased tear production (lacrimation)  Dry wrinkled cornea (fishy eye)  Nystagmus  Bradypnea, apnea,  Corneal reflex present  Bradycardia,  Tachypnea, tachycardia hypotension Unreliable Signs of Anesthetic Depth Unremarkable autonomic response to stimulation (unrelated to stimulation)  Changes in heart rate - may change in response to anesthetic depth, but also to compensate for stroke volume changes  Changes in blood pressure  more reliable than heart rate changes, but also influenced by other drugs  Changes in respiratory rate or depth of breathing  the 2 also compensate for one another  can be influenced by onboard drugs How to control depth  Know what stage/plane patient is at, & know which they should be at  Do you need to lighten or deepen your patient?  Know what drugs are on board, what procedure/stimulus is planned  Know what MAC is for your patient/species, & the anesthetic agent in use  Keep patient safe and at Stage 3 Plane 1 or 11 for proceduers!  Adjust anesthetic inhalant concentration accordingly  MAC and adjustments will be discussed in upcoming lectures  MAC = Minimum alveolar concentration (MAC) is defined as the expired (end-tidal alveolar) concentration of an inhalation anesthetic agent that prevents purposeful movement in response to a supramaximal noxious stimulus in 50% of observed subjects.  r a xt e