Circuits and Flows Flashcards
Anesthetic % delivered to the patient is affected by:
- Vaporizer setting (dial %)
- Anesthetic Circuit
- Patient
FACTORS TO KEEP IN MIND
§Vaporizer setting and location
§Circuit and dilution
§Patient and dilution
§ Uptake and distribution of anesthetic depends on:
- Inspired concentration (up)
- Alveolar ventilation (up)
- Solubility (down)
- Cardiac output (down)
- Alveolar to venous tension difference (down)
how are alveolar concentration and amount of anesthetic in the brain related?
Alveolar concentration reflects the amount of anesthetic in the brain
>directly proportional
how does vaporizer dial setting effect effect anesthetic delivery to the brain?
The higher the dialed %, the faster dilution is
overcome in the circuit and patient
where is the vaporizer located in the circuit and why?
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
in VIC, how does free gas flow effect the delivered % of anesthetic?
§ The higher the fresh gas flow, the lower the delivered % of anesthetic
* DILUTION
in VIC, how do resp rate and depth of breathing effect anesthetic concentration?
- Faster rates and deeper breaths increase the delivery of anesthetic to the patient (Alveolar Ventilation as per Uptake)
- Dial setting does not indicate %
What are the properties of VOC vaporizers?
§ 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)
in VOC, what does higher flow correspond to?
§ 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
in VOC, what does higher flow correspond to?
§ 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
what are rebreathing circuits used with, in terms of other equipment?
- Unidirectional valves
- CO2 absorber
- Lower Fresh gas flows (FGF)
- Preserve humidity and patient temperature
what patients are rebreathing circuits recommended for?
- Patients > ≈10 kg
- arbitrary preference
- NOT an absolute
what patients are rebreathing circuits recommended for?
- Patients > ≈10 kg
- arbitrary preference
- NOT an absolute
what are the types of rebreathing circuits?
§ 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
what is the Bain system? what are the characteristics of its tubing?
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
main 3 differences with non-rebreathing circuits? what do they offer? who are they recommended for?
- 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)
what is circuit dilution affected by?
§ 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
what is fresh gas flow? what is the quantity of O2 required by cells to carry on normal function?
§ Oxygen consumption (VO2)
* Quantity of O2 required by cells to carry on normal function
* 5-10 mL/kg/min
what amount of O2 should be included as FGF in a rebreathing system?
- 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
how do we calculate minimum FGF? What is it for a 20 kg dog?
- 5-10 mL/kg/min
§ 20 kg dog
* 20 kg x 10 mL/kg/min = 200 mL/min
how do we calculate minimum FGF? What is it for a 450kg horse?
- 5-10 mL/kg/min
§ 450 kg horse - 450 kg x 5 mL/kg/min = 2.25 L/min
what values for minimum oxygen consumption should we use for small and large animals and why?
§ 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
why would we use a FGF that is higher than VO2?
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
what FGFs do we more commonly use in rebreathing systems?
100 mL/kg/min immediately after induction
50 mL/kg/min during maintenance
100 mL/kg/min during recovery
what FGFs would we commonly use for a 20kg dog during anesthesia?
- 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
what FGFs would we commonly use for a 450kg horse during anesthesia? what problem will we encouter and how do we deal with this?
- 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)
what is different about FGF in non breathing systems? what additional purpose does it serve?
§ No soda lime absorber, no unidirectional valves
§ CO2 eliminated by means of high FGF
what should FGF be in a non-rebreathing system? eg for a 5kg cat? 20kg dog?
§ 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
when will alveolar concentration equilibriate with brain concentration? what patient characteristics do we nee to keep in mind?
§ 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
During induction and maintenance, the higher the FGF (vaporizer dial >0%):
- Dilution from the system is overcome faster
- Patient is saturated faster
> Alveolar concentration rises faster
> Brain concentration rises faster
Changes in anesthetic % take longer in larger animals. why?
- Dilution from the patient (slower saturation)
