L33: Intraoperative care and support of anaesthetised animal

How is oxygen delivery maintained DO2

What is cardiac output reduced by

• Extremes of HR and disturbances of rhythm

• Poor stroke volume

  • Poor ventricular filling (low preload)

  • Poor myocardial contractility

  • High vascular resistance (afterload)

What is bradycardia like in different the dog cat and horse

Dog; <40 bpm

Cat: <90 bpm

Horse: <25 bpm

What can cause bradycardia

• High vagal tone

• Electrolyte and acid base disturbances (high K+)

• Hypothermia

• Drugs (potent mu agonist opioids, a2 agonists)

• Baroreflex response to hypertension

• Bradyarrhythmias

How do you treat bradycardia and bradyarrhythmias

• Check monitored parameters and anaesthetic depth

• Remember a2 agonist associated bradycardia

• Be aware of raised intracranial pressure as a cause

  • Atropine 0.02-0.04 mg/kg IV

  • Glycopyrrolate 0.005-0.01 mg/kg IV

What are the bpms on tachycardia for dog cat and horse

Dog: >180

Cat: >220

Horse: >50bpm

What can cause tachycardia

• high circulating catecholamines

  • pain, hypertension, hypovolaemia, hypoxia, hypercapnia

• Hyperthermia

• Anaemia

• Drugs

  • Sympathomimetics, parasympatholytics

• Tachyarrhythmias

How do you treat tachycardia and tachyarrhythmias

• Check monitored parameters and anaesthetic depth

• Rule out or treat underlying cause

what are some types of supraventricular arrhythmias

• AV block

  • Define cause

  • Atropine or glycopyrrolate

• Atrial fibrillation

  • Unusual to develop during anaesthesia

In what ways can ventricular arrhythmias occur

• Can occur singly or in runs, or as ventricular tachycardia which may be paroxysmal

How do you approach/treat ventricular arrhythmias

• Try to determine cause and fix it

  • Hypercapnia

  • Hypoxia

  • electrolyte imbalance

• Treat if haemodynamically significant

  • Assess pulse quality and rate

  • Assess blood pressure

  • SpO2, mucous membrane colour and CRT

if you decide to treat ventricular arrhythmias what do you use

• Lidocaine first choice

  • 1-2 mg/kg slow IV bolus (dog and horse)

  • 0.5 mg/kg IV bolus (cat)

• Followed by CRI at 25-100 ug/kg/min

• maximum dose (over 10 minutes)

  • Dogs; 4 (-6) mg/kg

  • Cats; 2 mg/kg

    • Cats sensitive to lidocaine toxicity, may start treatment with beta blocker instead

How do you treat underlying hypovolaemia/hypotension

• Reduce depth of anaesthesia if possible

• Give IV fluids (bolus if necessary)

• inotropes/vasopressors

what fluids are commonly given during anaesthesia

• Crystalloid fluids for relative hypovolaemia

How do you know which fluid type to use

• Depends to some extent on nature of deficit but CSL most common as its balanced

  • Different rates suggest between 2-10 mL/kg/hour

    • Commonly 5ml/kg/hour

  • May need rapid bolus in emergency situation (5-10mL/kg)`

What are colloids good for

• If TP <35g/l or if better IV filling needed

• Plasma

  • FFP, FP as above or if clotting factors required

• Fresh whole blood (FWB)

• Packed red blood cell (PRBC)

• Human serum albumin (HSA)

What are the typical blood volumes for dogs and cats

dog: 80-90 mL/kg

cat: 50-60 mL/kg

What fluids do you give for 10% total blood volume loss

crystalloids

What fluids do you give for 10-25% total blood volume loss

Colloid

What fluids do you give for >25% total blood volume loss

Blood

What fluid do you give if PCV is <20% or Hb <7 g/dl

Blood or PRBCs

What are some examples of inotropes

• Dopamine

• Dobutamine

What are the high, mid and low rate for dopamine

Low rate: 2-5 ug/kg/min- DA receptor

Mid range: 5-10 ug/kg/min- b1 receptor, positive inotrope

high rate: 10-15 ug/kg/min- affect a1 receptors, vasocontriction

What are some side effects of dopamine

what animals is dopamine usually used in

arrhythmogenic

Dogs and cats

Where does dobutamine act and what is its use

• Acts mainly on B1 receptors

• Positive inotrope with minimal vascular resistance

• Mild chronotropic

• Less arrhythmogenic

• dose of 0.5-10 ug/kg/min

• horses

what are some types of vasopressors

• noradrenaline

• phenylephrine

• ephedrine

• vasopressin

What does noradrenaline do

• B1 and a1 adrenergic effect

• Positive inotrope and vasopressor

• Increase CO and SVR

• Reduced liver, muscles and renal and skin perfusion

• Dose 0.1-1.0 ug/kg/min

what does phenylephrine do

• a1 adrenergic effect

• Vasopressor

• Increased SVR

• Reduced splanchnic perfusion

• Dose 0.002-0.02 mg/kg IV or 1-3 ug/kg/min

What does ephedrine do

• Stimulates endogenous noradrenaline release

• a and b adrenergic effect

• subsequent boluses have diminished effect

• Reduced splanchnic and renal perfusion

• Increased HR, MAP, CO

• Dose: 0.02-0.05 mg/kg IV/IM

what does vasopressin do

• Potent vasopressor on V receptors

• Increased SVR

• Reduced splanchnic perfusion

What are some conditions that need respiratory support

• hypoventilation

• Hypercapnia

• Hypoxaemia

what is hypercarbia

• increased CO2

• Normal range 35-45 mmHg

• May occur due to

  • Hypoventilation

  • Rebreathing of exhaled gas

  • increased BMR

What are some good effects of hypercarbia

• Increased symp tone

• Circulatory stimulation

  • Stimulates respiratory centre

What are the downsides to hypercarbia

• Tachycardia

• Hypertension

• Cardiac arrhythmias

• Increased intracranial pressure

• CV depression at high levels

• Resp acidosis

• vasodilation

Define hypoxaemia

• Arterial PO2 <60 mmHg or SpO2 <90%

What can cause hypoxaemia

• Decreased FiO2

• Hypoventilation (on air/O2 air mix)

• V/Q mismatch and shunt

• CV depression

• Diffusion barrier

• anaemia

• Increased O2 demand

  • Pyrexia, Increased BMR, shivering

How can you help in cases of hypoxaemia and hypercapnia

• check anaesthetic depth

• check airway

• increase FiO2 if possible

• Ensure no rebreathing CO2

• Ventilate

  • intermittent positive pressure ventilation

• Consider use of PEEP

• consider recruitment manoeuvre in horses

• Consider salbutamol in horses

What are the mechanics of ventilation/spontaneous breathing

Inspiration

• expansion of thorax

• Generating negative intrapleural pressure

• draws air into lungs

Expiration

• Intrapleural pressure rises as thorax contracts

but intrapleural pressure remains negative throughout respiratory cycle

negative intrapleural pressure necessary for normal thoracic pump and cardiac output

What are the effects of IPPV

• Intrapleural pressure remain positive/above zero throughout resp cycle

• Decreased venous return decreased cardiac output

  • Worse with high pressures and long inspiratory times

  • Worse in hypovolaemia animals or those in heart failure

what are the IPPV guidelines

• Tidal volume 10-15 mL/kg

• Rate 10-20 breaths/min (horses 6-10 b/min)

• Inspiratory: Expiratory ratio 1:2-1:3

• End tidal CO2 35-45 mmHg

• Peak inspiratory pressure (PIP) <20cm H20 (20-40 cm H20)

• Positive end expiratory pressure (PEEP) <5cm H20

What reflex causes increased intracranial pressure

• Cushings reflex

What is the purpose of increased intracranial pressure : Cushings reflex

• Impending death

• Last attempt to maintain cerebral perfusion

What is in cushings triad

• Increased blood pressure

• bradycardia

• Resp changes if not ventilated

How do you treat increased intracranial pressure

• Hyperventilate as emergency measure (if anaesthetised)

• Mannitol- osmotic effects

  • also reduce blood viscosity → blood flow and oxygen delivery

• Hypertonic saline → osmotic effects

• Furosemide → may be synergistic with mannitol

does shivering and pain increased oxygen demand

yes increased demand in recovery

what are the effects of hypothermia

• MAC reduction (~5% every C)

• Alters pharmacokinetics and pharmacodynamics of anaesthetic drugs → prolonged recovery

• Organ system dysfunction (CV, resp, CNS)

• Increased blood loss → increase clotting times

• Shivering increases oxygen demand in recovery

• increased incidence of SSI (human)

• Delayed wound healing

• Humans report hypothermia as not it to have

How can you treat/give temperature support

• Rebreathing circuits and HME

• Warm operating theatre

• Passive techniques

  • Towels, bubble wrap, reflective blankets

• Active techniques

  • Electric heated blankets, forced warm air blankets, heat lamps, IV fluid warmers, warm abdominal lavage fluids, circulating warm water pads