Breathing Systems Flashcards
What 4 factors affect rebreathing?
Fresh gas flow, apparatus dead space (from Y to alveoli), breathing system design (closed vs open vs semi-closed) and empty space (circuit, canister, reservoir bag, ventilator).
Name 3 effects of rebreathing?
Heat and moisture retention, altered carbon dioxide and agent concentrations.
How do you avoid hypercarbia with increased dead space?
Minute ventilation must increase (alveolar ventilation = VE - VD).
Name 4 common features of non-rebreathing circuits.
Low resistance, less dead space/empty space, lack of unidirectional valves and lack of CO2 absorption.
Name the most popular non-rebreathing circuit and what feature makes it so.
Mapelson-D. Excess gas scavenging is easy and most efficient during controlled ventilation.
2 factors affecting resistance
Width and length of tubing
Why should a spontaneously breathing patient under anesthesia on a vent always be assisted?
Any breathing circuit creates resistance to gas flow and increases work of breathing.
Advantages of rebreathing
Cost reduction, increased warmth and humidity of inspired gases and decrease of OR contamination.
What is higher FGF associated with?
Higher FGF is associated with less rebreathing.
The higher the FGF the more gas concentration in circuit will resemble that at the common gas outlet.
What does the Bain modification to the Mapelson involve?
Placement of the FGF through the expired gases to heat inspired gases.
Advantages of NRB system
Inexpensive, rugged, excellent method to deliver positive pressure ventilation, less dead space, in Bain system inspiratory limb heated by exhaled gases, low resistance, lightweight, changes in fresh gas concentrations result in rapid changes, no CO2 or compound A.
Disadvantages of NRB
High FGF required (1.5 times MV) with resulting pollution and economic waste, inspired heat and humidity low, requires frequent adjustment in FGF, not for pt with MH history ( inability to inc FGF enough to blow off CO2.
Reasons for decline of NRB
Modern efficient ventilators, conservation of heat with rebreathing circuits and waste gas management.
Advantages of a circle system
Economical: expired oxygen and anesthetic vapor are re-circulated and reused.
Fresh gas flow and anesthetic agent utilization are minimized compared with a NRB system.
Humidifies inspired gas, preserving heat and moisture of the pt.
Disadvantages of the circle system
Complex, less portable, opportunities for disconnect, unidirectional valves may malfunction, inc dead space, inc “empty space” (longer diffusion time).
Describe the basic configuration of a circle system.
Fresh gas inlet, inspiratory limb 1-way valve, inspiratory tubing, patient Y piece, expiratory tubing, expiratory limb 1-way valve, reservoir bag, APL valve, CO2 absorbent.
Semi-closed circuit features
O2 delivery exceeds O2 consumption and excess gases are eliminated through the APL valve constantly.
Semi-closed circuit compared to closed circuit advantages and disadvantages.
Advantages of semi-closed over closed circuit: safer and more rapid change in anesthesia concentration.
Disadvantages: less economical, heat loss, more pollution.
Differences between semi-closed vs semi-open
Semi-closed: conveys excess gas to scavenging system, humidify cation, low FGF.
Semi-open: excess gas released into air, no humidification, high FGF (1.5 x MV).
2 functions of the APL valve
User-adjustable valve that releases gases to a scavenging system, used to control pressure in the breathing system.
Describe basic characteristics of carbon dioxide absorbents.
Gas tends to travel along periphery of canister and inlet. Does not support bacterial growth. Each canister should last 20-30 hours. Is approx 55% granules, 45% air space. Smaller granules absorb more but dust and cake so usually a mix of large and small packaged.
Carbon dioxide absorbent canisters have what % of water?
14-19%. H2O as moisture is essential for CO2 absorption to occur.
What’s the granule size for common CO2 absorbents?
4-8 mesh size of granules.
Describe the chemical reaction involving CO2 and soda lime.
- CO2 + H2O = H2CO2 (and heat)
- H2CO3 + 2 NaOH = Na2CO3 + CO + 2 H2O + heat
- Na2CO3 + CaCO3 + 2 NaOH
Signs of canister exhaustion
Rise in HR (later fall), increased RR, respiratory acidosis, dysrhythmias, signs of SNS activation (flushed, sweating), inc bleeding, inc ETCO2
Function of the expiratory valve on the circuit.
Closes to prevent rebreathing of exhaled gas that still contains CO2.
Identify measurement units for airway pressure.
kPa or cmH2O
List functions of pressure gauges.
Used to measure circuit pressure between inspiratory and expiratory valves. Higher pressures reflect change in compliance or resistance, lower pressures reflect circuit leak, pressure higher than 20 cmH2O opens the esophageal sphincter (mask induction or anesthetic.
List 3 uses of the reservoir bag.
Provides a means for delivering positive pressure, can serve as a monitor for spontaneous ventilate, allows the use of lower fresh gas flows, protects the pt from excessive pressure in the breathing circuit.
List 3 characteristics of a closed system.
O2 supply = metabolic consumption, no waste of O2, should be used with close attention to FiO2 and no nitrous, extremely economically as far as fresh gas and agent, retains more heat and humidity, less environmental pollution.
Hazards of breathing circuits
Circuit disconnection (leading cause of critical incidents), most common disconnection is at the Y piece, leaks are also a source of failure to ventilate.
Component of the machine check that verifies an intact circuit.
To detect leaks and disconnects circuit must be checked by inflating and testing ability to hold positive pressure.
