Anesthesia Circuits Flashcards
Dead space
“Dead space” refers to any portion of the airway that does not participate in gas exchange (pharynx, trachea, bronchi)
Dead space in our airway causes all of us to rebreathe CO2 on every single breath
Dead space can also be thought of as any portion of the airway that causes us to rebreathe CO2
“Ventilated, but not perfused”
Anatomic dead space
Anatomic dead space includes the trachea, bronchi, pharynx, etc
2mL/kg in patients in the upright position
Physiological dead space
Physiologic dead space refers to alveolar spaces that receive air but no blood flow, usually from pulmonary alveoli that are destroyed (smokers, elderly, etc.)
What happens to the physiology of smokers?
- Alveolar sac fuse into “blebs” or bullae
- Excess mucous forms in the bronchioles
- Pulmonary capillaries get destroyed
Use lower tidal volumes with smokers and higher respiratory rates
Mechanical dead space
Mechanical dead space includesanesthesia airway equipment (circuit tubing, humidifiers, endotracheal tubes, etc)
Pulmonary shunt
A pulmonary shunt means that, although MOST of the blood passing our alveoli picks up oxygen, SOME of the blood in our body “bypasses” the alveoli and doesn’t pick up oxygen
Usually around 3% of cardiac output
V/Q ratios
“Ventilation,” or “V” refers to “air flow” to an area
Perfusion,” or “Q” refers to ALVEOLAR blood flow
Capital letter is normal, lowercase refers to reduced
V/Q mismatch
Anytime we get a v (less ventilation) or a q (less perfusion), we have a “V/Q mismatch”
What is the most common cause of hypoxia in the recovery room?
The most common cause of hypoxemia in the recovery room is “V/Q mismatch,” most likely due to atelectasis
V/q
In anesthesia, instead of “V/q,” we would say “ventilation without perfusion”
v/Q
In anesthesia, instead of “v/Q,” we would say, “perfusion without ventilation”
Would V/q (normal ventilation, but less alveolar blood flow) represent dead space or a pulmonary shunt?
Dead space
Would v/Q (less ventilation, but normal alveolar blood flow) represent airway dead space or a pulmonary shunt?
Pulmonary shunt
Is this an example of dead space (V/q) or pulmonary shunt (v/Q)?
Pulmonary shunt (v/Q)?
Is this (pulmonary embolism) an example of dead space (V/q)or pulmonary shunt (v/Q)?
Dead space (V/q)
Is this an example of dead space (V/q) or pulmonary shunt (v/Q)?
Pulmonary shunt (v/Q)
Is this an example of dead space (V/q) or pulmonary shunt (v/Q)?
Pulmonary shunt (v/Q)
Will this lead to an increase in dead space (V/q) or pulmonary shunt (v/Q)?
Pulmonary shunt (v/Q)
Is emphysema an example of dead space (V/q) or pulmonary shunt (v/Q)?
Both!
Bullae, along with destroyed pulmonary capillaries
Will spontaneous ventilation under GA lead to an increase in dead space (V/q) or pulmonary shunt (v/Q)?
Pulmonary shunt (v/Q)
A patient has a profound drop in cardiac output (from internal hemorrhage), will this cause more dead space (V/q) or pulmonary shunting (v/Q)?
Dead space (V/q)
Examples of open circuits?
- Nasal cannula
- Insufflation/”blow by”
- Open drop anesthesia
An “open circuit” is open to the atmosphere, meaning that, in addition to going into the patient’s airway, it freely disperses around the face and into the room (because there is nothing to confine the gas to the patient’s airway)
Examples of partial re-breathing circuits?
- Anesthesia circuits
- Semi-open (Mapleson)
- Semi-closed (the kinds on our anesthesia machine) - Oxygen masks
- Simple face mask, “non-rebreather” mask, Venturi (“Venti” mask), self-inflating Ambu bag
Examples of non-rebreathing circuits?
- A T-piece
- A “nonrebreather” mask
Why should you be catious with open circuits?
Since open circuits are primarily used for oxygen delivery, and since the oxygen is dispersed around the face and into the room, and since oxygen is one of the three elements (fuel, ignition source, and oxygen) that kindle fires, there is a higher risk for surgical fires with open circuits, especially if the surgery is around the face.
Contraindicated with facial surgery with drapes
The anesthesia provider needs to communicate with the patient that, because they will not be able to use oxygen, that they won’t be able to give as much sedation
How to estimate the FiO2 with a nasal cannula? Max flow?
21% FiO2+ 4% per 1L/min flow
Max flow: 44% with 6L/min
A patient is breathing a NC running at 2L/min. What is the FiO2?
29%
Normal TOTAL anatomic dead space
2mL/kg or 1/3 of a patients tidal volume
Normal EXTRATHORACIC (nose and pharynx only) anatomic dead space in adults
70-75mL
Endotracheal tube dead space
8.0 ETT = 12.6mL dead space
LMA dead space
Dead space in patients with an LMA is almost half that of patients who are ventilated with a facemask (90mL vs. 162mL)
Y-piece and humidifier on the anesthesia circuit dead space in adult and pediatric
Adult Y-piece = 8mL
Pediatric Y-piece = 4mL
Humidifier = 10-60mL
What is the max Fi02 possible with a nasal cannula?
44% with 6L/min
What is the limit Fi02 on a patient with facial drapes? How do you prevent C02 from accumulating under the drape?
If Fi02 is over 30% there can be a fire
Place the breathing circuit under the drape, turn air to 15L/min, insufflate the air around the patients face, and create a path under the drape for the air (and C02) to escape!
What are three types of partial rebreathing circuits?
- Semi-closed partial rebreathing circuits/circle system (our machines)
- Semi-open (Mapleson) partial rebreathing circuits
- Partial rebreathing oxygen masks (simple face mask, venturi mask, nonrebreathing mask, ambu bag)
What are two ways we can adjust the extent to which gases are rebreathed?
- APL valve
- Adjusting the FGF rate
If you want to provide general anesthesia with an endotracheal tube or LMA outside the OR, what are your two options?
- Transport an anesthesia machine to the remote location you’ll be in (must have at minimum wall supply of oxygen/if no scavenging use TIVA)
- Use a “Mapleson circuit”
Which Mapleson circuit is best for SV?
Best for CV?
What are Mapleson circuits primarily used for?
- Deliver 02; not anesthetics
- Used outside the OR or during transport of patients from one hospital location to another
Advantage of Mapelson Circuits?
- Ability to be hooked up to endotracheal tubes or LMAs, in which we can now use TIVA outside the operating room
- They can provide positive pressure ventilation
Disadvantages of Mapleson Circuits?
A lot more dead space, much greater potential to rebreathe CO2 because:
- Only one tube for inhalation and exhalation
- No CO2 absorber
- No inspiratory and expiratory valves
Since the biggest concern with Mapleson circuits are rebreathing, what are three ways we can mimimize it?
- Use a higher fresh gas flow
- Opening the APL valve
- Shortening the circuit volume
Best for spontaneous ventilation (SV)
Worst for control ventilation (CV)
Best for control ventilation (CV)
Worst for spontaneous ventilation (SV)
Mapleson D
What’s a Bain circuit?
The Bain circuit is basically a Mapleson D circuit, but with the fresh gas flow inside the breathing limb
CoAxial design, inspiratory inner tube and expiratory outside inner tube
What is a Mapelson E circuit?
Ayre’s T-piece
Advantages/Disadvantages of T-piece
Advantages: Virtually no rebreathing in this circuit
Disadvantages: Positive pressure ventilation is not possible, it can only be used on spontaneously breathing patients
What is a Mapelson F circuit?
This is basically a T piece that allows positive pressure ventilation because it has a breathing bag
Jackson-Rees’ Modification
What are the components of a semi-closed circuit?
- Circuit tubing
- Elbow adapter
- Inspiratory & expiratory unidirectional valves
- CO2 absorber
- Breathing bag
- Humidifier
What are the different types of anesthesia circuit tubing?
- Circuit with inhalation and exhalation tubing
- “Coaxial circuit” In a coaxial circuit, the inspiratory lumen (purple) is inside the expiratory lumen (clear)
Does stretching out the semi-closed anesthesia circuit cause a patient to rebreathe more CO2?
No
What will dried out C02 absorbers do?
- Degrade all volatile agents into carbon monoxide, desflurane the most
- Accelerate the degradation of Sevoflurane into compound A
At what flow rates is the C02 absorber basically negligible?
>5L/min
Recommended humidity in the OR?
“A relative humidity of 50 to 60% inhibits bacterial growth and decreases the potential for static electricity.”
“AIAAAH recommends a humidity range of 30 to 60% for the OR.”
Simple Face Mask (SFM)
Fi02 of 40% at 5L/min and 60% at 10L/min
Venturi (Venti) Mask
Delivers specific Fi02 that ranges from 24-60%, specific adapter determine the Fi02
Maximum Fi02 can be up to 60% with the right adapter
“Nonrebreathing” Mask
Fi02 of 80’s at 10L/min and 90’s at 15L/min
Some rebreathing does occur, since you cannot get to 100%
The main purpose of supplemental oxygen is to compensate for:
- Anesthetic induced hypoventilation
- Atelectasis
These supplemental oxygen devices are commonly used during:
- Sedation (MAC) anesthesia
- Transport from the OR to the PACU
- In the PACU
What are the clinical uses of an Ambu bag in anesthesia?
- Mask ventilation for a patient in an emergency
- Transferring mechanically ventilated patients from the OR to the PACU or ICU
Steps to take when transporting a patient with an Ambu bag
- Call respiratory therapy to bring a ventilator to the PACU
- Ventilate the patient with a self inflating Ambu bag during transport
- Place the patient on the ventilator in the PACU until they are ready to be extubated
- Administer a propofol drip if the patient is paralyzed
What does the Hagen Poiseuille Equation tell us?
- Adding dead space (length) to a circuit increases airway resistance
- Resistance can be minimized if:
- The diameter of the equipment is bigger
- The length of the equipment is shorter
How do you determine compliance loss in the breathing circuit?
Peak inspiratory pressure (cmH20) times the compliance (mL/cmH20)
