Management of Hypotension

Question 1

What is Organ perfusion/oxygenation dependent on?

Answer 1

• Oxygen carrying capacity (CaO2)
• Cardiac Output (CO; L/min)
• Vascular resistance (VR)
• Blood pressure (BP)

Question 2

How is Oxygen Carrying capacity (CaO2) monitored?

Answer 2

• Packed cell volume/hematocrit
• Pulse Oximetry (% hemoglobin saturated with oxygen)
• Healthy dog/cat on 100% O2, not commonly deranged (?)

Question 3

Why is cardiac output not monitored?

Answer 3

• Would be gold standard in hemodynamic monitoring
  
  • CO directly related to organ perfusion
  • Takes into consideration: HR, Rhythm, Contractility, Preload (Volume status) afterload, etc
• BUT:
  
  • Involved/can cause harm to measure
  • Expensive
  • Takes specific training
  • Global perfusion parameter, does not specifically indicate adequate individual perfusion

Question 4

What is Vascular resistance

Answer 4

Question 5

What is blood pressure proportional too?

Answer 5

• Proportional to CO (⇡CO = ⇡BP)
• Proportional to VR (⇡VR = ⇡BP)

Question 6

What is the problem with regional vascular resistance?

Answer 6

• Complicates organ perfusion/Oxygenation
• Determines individual organ perfusion

Question 7

How common in hypotension during anesthesia?

Answer 7

• Most common anesthetic-related complication reported, with incidence of:
  
  • Mixed health population of dogs - 58%
  • Healthy dogs - 7.5 - 32%
  • Healthy cats - 8 - 30%
• A healthy dog/cat will generally not become hypotensive when in an adequate plane of anesthesia?

Question 8

What complications can hypotension cause?

Answer 8

• In people:
  
  • Acute Kidney injury
  • Acute coronary syndrome
  • Increased all-cause mortality 1 year post anesthetic event
• Cats - 2.6x increased odds of death

Question 9

What is Systole?

Answer 9

Stroke volume ejection

Question 10

What is diastole

Answer 10

Affected by: Vessel elastance and Smooth muscle tone

Question 11

What is Hypotension?

Answer 11

• Definition:
  
  • Systolic <90mmHg
  • MAP <60 mmHg
• MAP - average blood pressure presented to organs over time
• Organs-specific minimum perfusion pressure
  
  • Ex: Brain, Kidneys = 50 mmHg
• SBP - Map = ~30-40mmHg
  
  • Where the 90mmHg SBP target originates
  • Can vary depending on morbidities

Question 12

What are the monitoring methods for Blood Pressure?

Answer 12

• Direct:
  
  • Artery catheterization
  • Accurate, real-time
  • Ideal for unstable, high-risk patients
  • Not practical in healthy patients - time & Money
• Indirect:
  
  • Non-invasive
    
    • Doppler flow probe
    • Oscillometric
  • Intermittent monitoring
  • Cheaper, easier to have in workflow
  • Modest accuracy/precision when applied correctly

Question 13

Why is indirect monitoring of blood pressure only modestly accurate?

Answer 13

• Easily influenced by technical error
  
  • Cuff sizing is important
    
    • cuff width needs to be 40% of circumference of where it will be placed
    • Why 40? - Ensures cuff balloon makes it all the way around the limb
  • Placement
    
    • Tight placement, lightly taping around to keep velcro attached
    • Mid-antebrachium ideal (NOT over carpus)
      
      • Base of tail or hind limb - OK

Question 14

How does occlusion based technology work to monitor blood pressure?

Answer 14

• Doppler-
  
  • Inflate cuff with sphygmomanometer
    
    • Release cuff pressure 3-5 mmHg/second
    • First sound = SBP
  • an estimate
• Oscillometric-
  
  • Automated inflation/deflation several times
  • MAP is the only measred value
  • Algorithm estimates SBP and DBP

Question 15

What are the considerations of to keep in mind for Indirect Blood pressure monitoring?

Answer 15

• Both technologies overestimate hypotensive blood pressure
• If using 90/60 mmHg threshold for Doppler SBP/Oscillometric MAP respectively ⇢ Will miss some hypotensive patients
• Doppler - treat for hypotension if <95mmHg
• Oscillometric - Treat for hypotension if MAP <65mmHg

Question 16

What causes Hypotension?

Answer 16

• Reduced VR
  
  • direct vasodilation - Isoflurane, sepsis, ACEIs/Amlodipine
  • Indirect vasodilation - reduced sympathetic tone (opioids, inhalants, alpha-2 agonists, trazadone)
• Reduced CO
  
  • Reduced contractility - Isoflurane, DCM, hypovolemia, sepsis
  • Reduced forward stroke volume - DMVD, HCM
  • Reduced HR - opioids, alpha-2 agonists, reduced sympathetic tone
• Reduced intravascular volume
  
  • Relative volume deficit - Vasodilation*
  • Absolute - Vomiting, reduced/no intake, hemorrhage

Question 16

What is the approach to treating a Hypotensive patient?

Answer 16

• `Patient’s baseline states (frame perspective)?
  
  • volume deficit, pediatric, comorbidities?
• Assess/optimize patient’s depth if too deep and assess for user error in BP measurement
  
  • (Re-take BP in 5 min to confirm BP not correcting)
• Treat suspected direct cause
  
  • Bradycardia = anticholinergic
  • volume deficit = volume
  • Neither of the later = vasopressor
  • Unsure? = vasopressor (rarely wrong)
• Allow intervention time to work (~5min)
  
  • Corrected bradycardia &/or still hypotensive = repeat/administer vasopressor
• Effective Treatment can target any main factor because all are influenced by the drugs used
  
  • Vascular resistance
  • Cardiac output
  • Intravascular volume

Question 17

Why are IV fluids used during surgery

Answer 17

• Maintenance IVF up to 30 ml/kg/hr:
  
  • Do not prevent inhalant-induced hypotension
  • Do not alter blood pressure throughout the anesthetic course
  • Do not affect microcirculatory perfusion (vessels < 20um)
• Main Benefits:
  
  • maintain a patent IVC/port for medication administration
  • Address volume deficits, over time

Question 18

What maintenance rates are used? Why?

Answer 18

• 5 ml/kg/hr for most dogs/cats
  
  • higher if concurrent deficit
• Used due to convention more than anything else
  
  • we know it isn’t doing anything

Question 19

Why are IV boluses used?

Answer 19

• To address calculated deficits
• To assess if patient will be fluid responsive, if volume deficit known
• To bridge time until another treatment is started/works (atropine, Glyco, NE infusion

Question 20

What is the normal heartrate of a small animals under general anesthesia?

Answer 20

• Allometric scaling
  
  • A normal HR for one dog is not the same for another
  • Larger breed dogs have lower baseline/resting HRs
  • Smaller breed dogs have higher baseline HRs
• Examples:
  
  • Great Dane:
    
    • HR of 40-50 is not bradycardic
    • HR <40 and hypotensive? may need an anticholinergic
    • HR >100-120 = tachycardic
      
      • And hypotensive? - suspect volume deficit
  • Yorkie:
    
    • HR 40-50 is bradycardic: anticholinergic unless normotensive
    • HR 100-120 normal
    • HR >160-180 tachycardic

Question 21

What is the suspected complication when a dog presents with tachycardia and hypotension

Answer 21

volume deficit

Question 22

What are the vasopressors used in vet med?

Answer 22

• Dopamine
• Dobutamine
• Ephedrine
• Epinephrine
• Norepinephrine
• Phenylephrine

Question 23

How are CRIs calculated?

Answer 23

• Desired dose/kg/min X weight = total dose of drug per minute
• Total dose per minute X 60min (1hr) = Total dose each hour
• How many hours are needed?
• Divided by drug concentration and add to any volume of your choice for infusion