Pharmacodynamics of Anesthesia

Question 1

define “side effect”

Answer 1

• effect other than those specifically desired during the use of a medication
• Often known for drugs

Question 2

Define adverse event

Answer 2

• Unintended and undesired effects secondary to the use of a medication or completion of a medical/surgical procedure producing a new or worsened morbidity, increased hospital stay, or mortality
• Unmanaged possible side effects may produce adverse events

Question 3

Define “medical error (complication)”

Answer 3

• Preventable, undesirable effect of medical care, whether or not it is evident or harmful to the patient
• Unmanaged or mismanaged side effects and/or adverse events may produce medical errors (negligence) during patient management

Question 4

What is the American College of Veterinary Anesthesia & analgesia’s Position on anesthetic monitoring?

Answer 4

• Minimize mortality and morbidity by objective monitoring for hypotension, hypoxemia, and hypercapnia
• Anesthetic providers should provide frequent and continuous monitoring of patients through observation and various monitors while creating record of parameters

Question 5

What is the ACVAA’s recommendations for monitoring anesthetized small animal patients?

Answer 5

• Ensure adequate circulation
• Ensure oxygenation
• Ensure adequate ventilation
• Maintain normothermia
• Maintain legible anesthetic record capturing entire event
• Monitor/ensure safe and comfortable recovering period

Question 6

What are the common cardiovascular side effects/adverse events during anesthesia?

Answer 6

• Hypotension - most common adverse effect reported
  
  • Dogs - 20-30%
  • Cats - 25-35%
  • Mixed pop - up to 60%
• Arrythmias - 6-10%
• Hemorrhage - <2-3%
• Hypothermia - up to 40%+

Question 7

What is the significance of cardiac output?

Answer 7

• Our goal is adequate oxygen delivery to tissues/organs while under general anesthesia
• Organ perfusion/oxygenation is dependent on:
  
  • Cardiac output (CO; L/min) - largest contributor
  • Vascular resistance (VR)
  • Blood pressure (BP)
  • Oxygen carrying capacity ([hgb], %Sat, PaO2)

Question 8

what is responsible for the rhythm and rate of the heart

Answer 8

• Intrinsic cardiac automaticity
  
  • Sinoatrial node (SAN)
  • Atrioventricular Node (AVN)
• Autonomic nervous system
  
  • PNS
  • SNS

Question 9

define bradyarrythmias?

Answer 9

slow abnormal heart rhythm

Question 10

define tachyarrythmia

Answer 10

fast abnormal heart rhythm

Question 11

when do junctional arrythmias most commonly occur

Answer 11

with slow heart rates

Question 12

How are heart rhythms classified?

Answer 12

• Classification based on:
  
  • origin of electrical activity (e.g. sinus, junctional, ventricular)
  • Underlying rate of primary rhythm
  • Underlying rate/frequency of abnormal rhythm

Question 13

What is a sinus rhythm?

Answer 13

• Any cardiac rhythm in which depolarization of the cardiac muscle begins in the sinus node
• To determine if a rhythm is sinus:
  
  • is there a P for every QRS complex
  • Is there a QRS for every P

Question 14

What are the criteria for treating dysrhythmias?

Answer 14

• Is the rhythm affecting cardiac output?
  
  • if the patient hypotensive - treat
• Could the current rhythm progress to a more dangerous rhythm
  
  • If: multifocal VPCs, Ventricular triplets, runs of ventricular tachycardia associated with hypotension, true ventricular tachycardia (Rate > cutoff), V-fib - Treat

Question 15

What is the defined rate for Bradycardia under General Anesthesia? Why these rates?

Answer 15

• HR < 50-80 bpm in dogs
  
  • large - <50-60
  • small - <70-80
• HR < 110-120 bpm in cats
• Allometric scaling:
  
  • A normal HR for one dog, not the same for another
  • Larger dogs have a lower baseline functional HR
  • Smaller dogs have a higher baseline functional HR

Question 16

Why are there special considerations for pediatric animals under general anesthesia

Answer 16

• SNS still developing
• Unable to increase contractility/CO or SVR
  
  • reliant on HR
• Not analog (e.g. a 2mo is more developed than a 1mo, etc)
• Maintain HR in normal-to-tachycardic
• Consider premedicating with anticholinergic meds if <1mo

Question 17

what is the significance of bradycardias/bradyarrythmias?

Answer 17

• CO = HR x SV
• Reduced heart rate to a nonfunctional rate = inadequate cardiac output

Question 18

When do bradycardias need to be treated?

Answer 18

• Patient may be:
  
  • hypotensive - indicative of treatment
  • Normotensive - can be tolerated
  • Hypertensive - potential cause of bradycardia

Question 19

What are the causes of Bradycardia/Bradyarrythmias?

Answer 19

• Reduced sympathetic tone
  
  • deep plane of anesthesia (progressive slowing)
  • alpha-2s opioids
• Increased parasympathetic (‘vagal’) tone
  
  • AV conduction disturbances (1st, 2nd, 3rd degree AV blocks)
  • Certain breeds - true brachycephalics
  • Opioids, IPPV, alpha-2s
• Hypothermia
  
  • reduced automaticity of SAN
  • hypothermia induced bradycardia not responsive to anticholinergics
• Hypertension (reflex bradycardia)
  
  • Response to vasopressor, alpha-2s, elevated intracranial pressure (Cushing reflex)
• Myocardial Hypoxia
• Closed pop-off valve
  
  • (⇡intrathoracic pressure, ⇣venous return ⇣CVP ⇡Aortic pressure = reflex bradycardia)
• Hyperkalemia, acute hypercalcemia

Question 20

Why is bradycardia/bradyarrythmia treated? How?

Answer 20

• Why?
  
  • To improve CO
  • To improve BP
  • Certain underlying cardiac diseases
  • Pediatric patients
• How? - determine underlying cause
  
  • Assess depth of anesthesia, adjust as needed
  • Pop-off valve closed - open
  • Hypoxia - check ETT location, increase FiO₂/turn O₂ flowmeter on, if extubated - ensure not obstructed
  • Hypertension
  • Hyperkalemia - Treat
  • Increased vagal tone and hypotension = anticholinergic
• What meds
  
  • Atropine (A)
  • Glycopyrrolate (G)

Question 21

How is Tachycardia/tachyarrythmia defined in general anesthesia?

Answer 21

• HR > 140-180 in the dog
  
  • larger dogs >140
  • Small >180
• HR > 200-220 in the cats
• Underlying cardiac disease will reduce tolerance to extremes in HR
• Allometeric scaling

Question 22

what is the significance of tachycardia/tachyarrythmia?

Answer 22

• Reduced diastolic time
  
  • reduced end-diastolic volume = reduced stroke volume
  • Reduced coronary perfusion
• Increased myocardial work, oxygen consumption
  
  • Oxygen demand > delivery = myocardial ischemia can occur

Question 23

What are the causes of tachycardia/tachyarrythmia under general anesthesia?

Answer 23

• Often indicative of other problems
  
  • Excess sympathetic tone
    
    • Too light a plane of anesthesia (often associated with normal-to-increased blood pressure)
      
      • sudden change (>15%) from baseline prior to stimulus
        
        • Eg HR 80 ⇢ 107 post-incision
    • Severe hypoventilation (high PCO₂) (associated with normal-to-increased blood pressure)
      
      • progressive increase
    • Inadequate analgesia (pain) (associated with normal-to-increased blood pressure)
    • Iatrogenic/sympathomimetics
      
      • Ephedrine, epinephrine, dopamine, dobutamine, ketamine
  • Sepsis, systemic inflammatory response
  • Hypovolemia
    
    • Anesthetized ⇢ baroreceptors may be suppressed = normal reflexive increased in HR may not occur in anesthetized patients
• Pathology
  
  • Cans see number of abnormal rates, rhythms
    
    • sinus tachycardia
    • ventricular tachycardia
    • intermittent ventricular beats
    • paroxysmal supraventricular tachycardia
    • always indicates further assessment, formal ECG evaluation
  • Examples:
    
    • subaortic stenosis, pulmonic stenosis, hypertrophic cardiomyopathy, arrythmogenic right ventricular cardiomyopathy
    • Hyperthyroidism
    • Pheochromocytoma (catecholamine-secreting tumor)
    • Thoracic trauma / myocardial injury / ischemia
    • see box 1 in picture

Question 24

What is the treatment for tachycardia/tachyarrythmia under general anesthesia?

Answer 24

• Assess depth of anesthesia
  
  • adjust (increase vaporizer, increase O2 flow for 3-5 min)
  • reassess in 3-5 min
• Assess analgesics given thus far - more may be indicated
  
  • if depth is appropriate and not hypoventilating, consider additional analgesic
• Assess ventilatory status
  
  • EtCO2 >60mmHg ⇢ potentially contributing to increased HR
  • Correct hypoventilation IPPV via mechanical or manual
• Assess volume deficit
  
  • administer crystalloid bolus (especially if hypotensive) 10-20ml/kg over 10-15 min
• Iatrogenic cause:
  
  • reduce admin of sympathomimetic agents, titrate accordingly
  • Assess patient body temperature and remove heat source if hyperthermic
• Unknown etiology:
  
  • patient potentially having primary cardiac disease leading to tachycardia
  • patient have reason for systemic inflammatory response, source of sepsis
  • If hypotensive, consider reducing HR directly via fluid bolus, opioid bolus IV, esmolol, etc.

Question 25

What are Ventricular Extrasystoles under GA?

Answer 25

• Electrical activity within the heart originating from a location other than the SAN, atria, or AVN
  
  • wide and bizarre complexes, no p-wave
• Ventricular Escape Rhythm
  
  • follows a long pause in following sinus rhythm and acts to prevent cardiac arrest (AVN takes over)
  • rhythm can be entirely made of escapes also
  • present with slower rates most commonly
• Ventricular Premature Complexes (VPCs)
  
  • A premature self-initiated ventricular discharge

Question 26

What are Ventricular Escape Rhythms ?

Answer 26

• Associated with bradycardia. AVN or Bundle of His taking over cardiac electrical function due to sinus node inactivity
• Inherent rate of SAN v. AVN in dogs, cats?
• May or may not be concurrent with hypotension (eg. reflex bradycardia)
• Causes similar to causes of bradycardias, rule-out causes and implement treatment as you would for bradycardia
• Do NOT give lidocaine - will abolish escape rhythm
  
  • escape rhythms are good
  • lidocaine could produce atrial standstill, sinus arrest

Question 27

What are causes of Ventricular Premature Complexes (V-tach)

Answer 27

• Myocardial injury, disease
• Pain
• Hypoxia
• Acidosis, hypoventilation
• Severe hyperthermia
• Drugs:
  
  • inhalants, alpha-2 agonists, acepromazine, opioids
• Electrolyte disturbances:
  
  • hyperkalemia, hypomagnesium, hypocalcemia

Question 28

What is Ventricular Tachycardia?

Answer 28

• A fast rhythm arising form inappropriate electrical activity in the ventricles
  
  • Monomorphic - all the complexes look similar, as they are originating within the same area of the heart
  • Polymorphic - the complexes look different (no symmetry) and are originating from multiple foci within the heart

Question 29

What is Ventricular Fibrillation?

Answer 29

Disorganized electrical activity in the heart that causes the ventricles to quiver rather than produce effective cardiac output

Question 30

How are VPCs, V-Tach treated?

Answer 30

• Identify, address contributing cause if possible
• Treat if:
  
  • hypotensive
  • Multifocal VPCs
  • Runs associated with hypotension
  • V-tach
  • R-on-T
• Lidocaine:
  
  • 2mg/kg IV, assess if resolution/improvement
  • Repeat dose if needed
  • Consider starting lidocaine CRI 50-100mcg/kg/min