Circuits and Flows

Question 1

Anesthetic % delivered to the patient is affected by:

Answer 1

• Vaporizer setting (dial %)
• Anesthetic Circuit
• Patient

FACTORS TO KEEP IN MIND
§Vaporizer setting and location
§Circuit and dilution
§Patient and dilution

Question 2

§ Uptake and distribution of anesthetic depends on:

Answer 2

• Inspired concentration (up)
• Alveolar ventilation (up)
• Solubility (down)
• Cardiac output (down)
• Alveolar to venous tension difference (down)

Question 3

how are alveolar concentration and amount of anesthetic in the brain related?

Answer 3

Alveolar concentration reflects the amount of anesthetic in the brain
>directly proportional

Question 4

how does vaporizer dial setting effect effect anesthetic delivery to the brain?

Answer 4

The higher the dialed %, the faster dilution is
overcome in the circuit and patient

Question 5

where is the vaporizer located in the circuit and why?

Answer 5

In (VIC- vaporizer in the circuit)
-Between the inspiratory one-way valve and patient
-Rarely used

Out (VOC- vaporizer out of the circuit)
-Most commonly used due to safety and accuracy

Question 6

in VIC, how does free gas flow effect the delivered % of anesthetic?

Answer 6

§ The higher the fresh gas flow, the lower the delivered % of anesthetic
* DILUTION

Question 7

in VIC, how do resp rate and depth of breathing effect anesthetic concentration?

Answer 7

• Faster rates and deeper breaths increase the delivery of anesthetic to the patient (Alveolar Ventilation as per Uptake)
• Dial setting does not indicate %

Question 8

What are the properties of VOC vaporizers?

Answer 8

§ Precision vaporizers that deliver the concentration dialed on them)
* Dilution caused by the anesthetic circuit and patient will affect the speed at which FA approaches FI (FA/FI)

Question 9

in VOC, what does higher flow correspond to?

Answer 9

§ The higher the flow, the faster dilution is overcome
* More molecules of inhalant are delivered to the patient which helps with saturation of tissues and equipment

Question 10

in VOC, what does higher flow correspond to?

Answer 10

§ The higher the flow, the faster dilution is overcome
* More molecules of inhalant are delivered to the patient which helps with saturation of tissues and equipment

Question 11

what are rebreathing circuits used with, in terms of other equipment?

Answer 11

• Unidirectional valves
• CO2 absorber
• Lower Fresh gas flows (FGF)
• Preserve humidity and patient temperature

Question 12

what patients are rebreathing circuits recommended for?

Answer 12

• Patients > ≈10 kg
• • arbitrary preference
• NOT an absolute

Question 13

what patients are rebreathing circuits recommended for?

Answer 13

• Patients > ≈10 kg
• • arbitrary preference
• NOT an absolute

Question 14

what are the types of rebreathing circuits?

Answer 14

§ Circle circuit:
* 2 limbs joined by “Y” piece

§ F-circuit:
* Co-axial system
* Inspiratory limb is the inner tube * Expiratory limb is the outer tube

Question 15

what is the Bain system? what are the characteristics of its tubing?

Answer 15

Non-rebreathing circuit
* Co-axial system
* Inspiratory limb is the inner tube
* Notice is the fresh gas flow tubing
* Expiratory limb is the outer tube

Question 16

main 3 differences with non-rebreathing circuits? what do they offer? who are they recommended for?

Answer 16

• No valves
• No CO2 absorber
• Higher FGF to eliminate CO2

§ They offer:
* Less resistance (no valves)
* Higher FGF may result in lower body T and less humidification (cool and dry O2)

§ Recommended for:
* Patients < ≈10 kg (some recommend lower BW)

Question 17

what is circuit dilution affected by?

Answer 17

§ Circuit needs to be saturated by delivered concentration from vaporizer
§ Affected by:
*Size of circuit
* Hoses, reservoir bag, sodalime canister
* Higher impact in larger circuits (as per Large Animals)
*Fresh gas flow dialed in flowmeter
*Vaporizer setting

Question 18

what is fresh gas flow? what is the quantity of O2 required by cells to carry on normal function?

Answer 18

§ Oxygen consumption (VO2)
* Quantity of O2 required by cells to carry on normal function
* 5-10 mL/kg/min

Question 19

what amount of O2 should be included as FGF in a rebreathing system?

Answer 19

• Quantity of O2 required by cells to carry on normal function
• 5-10 mL/kg/min
  § This is the minimum amount of O2 that should be
  included as FGF in a rebreathing system
• F-circuit, Circle system
• Usually, this flow rate is exceeded for safety and to increase the number of molecules of inhalant anesthetic carried by it

Question 20

how do we calculate minimum FGF? What is it for a 20 kg dog?

Answer 20

• 5-10 mL/kg/min

§ 20 kg dog
* 20 kg x 10 mL/kg/min = 200 mL/min

Question 21

how do we calculate minimum FGF? What is it for a 450kg horse?

Answer 21

• 5-10 mL/kg/min
  § 450 kg horse
• 450 kg x 5 mL/kg/min = 2.25 L/min

Question 22

what values for minimum oxygen consumption should we use for small and large animals and why?

Answer 22

§ Metabolism is greater in smaller patients, therefore use the higher value of VO2 (5-10 mL/kg/min)
ie. use 10 for small and 5 for large

Question 23

why would we use a FGF that is higher than VO2?

Answer 23

FGFs more commonly used
§ Higher flows than VO2
* To overcome dilution* and to reach dialed anesthetic % faster**, so that FA approaches FI faster

§ *Dilution from system and patient

§ ** A higher FGF allows for vaporization of a larger amount of inhalant anesthetic, even though the same % is present
* 1L/min at 2% isoflurane versus 5L/min at 2% isoflurane

Question 24

what FGFs do we more commonly use in rebreathing systems?

Answer 24

100 mL/kg/min immediately after induction
50 mL/kg/min during maintenance
100 mL/kg/min during recovery

Question 25

what FGFs would we commonly use for a 20kg dog during anesthesia?

Answer 25

* Induction * 20 kg x 100 mL/kg/min = 2 L/min * Maintenance * 20 kg x 50 mL/kg/min = 1 L/min * Recovery * 20 kg x 100 mL/kg/min = 2 L/min

Question 26

what FGFs would we commonly use for a 450kg horse during anesthesia? what problem will we encouter and how do we deal with this?

Answer 26

* Induction * 450 kg x 100 mL/kg/min = 45 L/min * Maintenance * 450 kg x 40 mL/kg/min = 18 L/min * Recovery * 45 L/min § Flowmeters reach only 10 L/min in Large Animal Anesthetic machines * Therefore, the above flows cannot be achieved § Instead, we use FGF that are closer to VO2 approximate values § Recommended FGF in full size LA * Induction * 500 kg x 20 mL/kg/min = 10 L/min * Maintenance * 500 kg x 10 mL/kg/min = 5 L/min * Recovery * 500 kg x 20 mL/kg/min = 10 L/min § This emphasizes that VO2 is all is needed; however, time to achieve vaporizer settings % are a factor due to dilution especially in larger patients (Patient Dilution)

Question 27

what is different about FGF in non breathing systems? what additional purpose does it serve?

Answer 27

§ No soda lime absorber, no unidirectional valves § CO2 eliminated by means of high FGF

Question 28

what should FGF be in a non-rebreathing system? eg for a 5kg cat? 20kg dog?

Answer 28

§ Flows * 100-300 mL/kg/min * Greatly exceed VO2 * Applicable and feasible in smaller patients with flowmeter capabilities and cost of anesthesia * 5kgcat * 5kgx200mL/kg/min=1L/min * 20 kg dog * 20 kg x 200 mL/kg/min = 4 L/min

Question 29

when will alveolar concentration equilibriate with brain concentration? what patient characteristics do we nee to keep in mind?

Answer 29

§ Patient needs to be saturated by anesthetic * Alveolar concentration will only equilibrate with brain concentration when tissues are mostly saturated § Larger patients take longer to equilibrate (saturate) than smaller ones * More obvious comparing a horse/cow to a dog/cat

Question 30

During induction and maintenance, the higher the FGF (vaporizer dial >0%):

Answer 30

* Dilution from the system is overcome faster * Patient is saturated faster > Alveolar concentration rises faster > Brain concentration rises faster

Question 31

Changes in anesthetic % take longer in larger animals. why?

Answer 31

* Dilution from the patient (slower saturation)