Pass Machine - Physics/Monitors/Machine Flashcards
O2 E-cylinder volume and pressure
700L, 2200 psi
N2O E-cylinder volume and pressure
1600L, 750 psi
Intermediate pressure w/in machine
45 psi
Final pressure to patient w/in machine
15-25 psi
PISS vs DISS
DISS = wall gas outlets PISS = cylinder connections
Implication of one-way check valve in regards to O2 flush valve and leak test?
ONLY present in older Datex-Ohmeda machines, this check valve prevents any flow from flush valve to go backwards into the machine (vaporizer, flow meters, etc) –> all flush flow goes to patient
If present, then typical positive pressure leak test will ONLY test for leaks in the circle system and NOT the flow meters or vaporizers. Must do negative pressure leak test
During mechanical ventilation, measured FiO2 and tidal volume is HIGHER than expected. What is likely the problem?
Hole in bellows -> O2 that is driving the bellows instead enters the patient
Airflow in bottom vs top of flow meter
Bottom: tubular, laminar, viscosity-dependent (attraction of molecules to each other or to the tube)
Top: orificial, turbulent, density-dependent (number of molecules that get past the blockage)
Why is O2 the last gas to be mixed in?
If there is a hole in any flow meter, at MOST the O2 will leak in the same proportion as the other gases (if the hole is in the O2 flow meter).
What is the relationship between increasing temperature and vapor pressure: linear, curvilinear, sigmoidal, or parabolic?
Curvilinear - as temp rises further, slope of VP increase rises further
In a sevo vaporizer, when the liquid agent converts to gas within the chamber, what happens to the temperature within the chamber?
Why is this important?
It goes DOWN b/c endothermic reaction to cause vaporization
If temp drops, then vapor pressure drops and patient is under-dosed. SO, the vaporizer is temp-regulated such that MORE FGF will be diverted to get saturated with agent as the temp in the chamber goes down.
In very low FGF rates, what happens to vaporizer output? Why?
What about at very HIGH flow rates?
It DECREASES - without enough flow to cause an airstream effect, there will be less pick-up of anesthetic molecules
Still DECREASES - not enough time to completely mix with the anesthetic agent
Explain why you do not change the settings of a variable bypass vaporizer at high altitude?
At high altitude, there is less ambient pressure keeping the gas in liquid form, so you OD the patient in terms of anesthetic molecule #. BUT, dosing is related to partial pressure. SO, even though there is more gas % being delivered, they exert a correlated less partial pressure on the body and thus dosing remains constant
Explain why des vaporizer must be adjusted at altitude?
The gas molecules in the machine are directly injected into the FGF at a set % (NOT picked up by flowing through the vapor). So the number of anesthetic molecules being delivered is constant. BUT that same # of molecules now exerts LESS partial pressure and so you under-dose the patient. SO, you must increase the vaporizer setting to achieve the intended depth of anesthesia.
What is critical temperature of a gas?
Temp at which you cannot liquify a gas despite any amount of pressure
What is critical pressure of a gas?
Pressure at the critical temperature
Absolute humidity vs relative humidity
Absolute: total amount of water dissolved in the air
Relative: % of water molecules in the air compared to the maximum that could be held at a given temp (cold air holds less molecules, thus for any given absolute humidity, relative humidity is higher in cold air)
Open vs Semi-open vs Semi-closed vs Closed circuit
Open: no rebreathing, no reservoir
Semi-open: no rebreathing but has reservoir bag (ambu, mapleson at high flow)
Open and semi-open: FGF >= MV
Semi-closed: some rebreathing (anesthesia machine, mapleson circuits at low flow)
Closed: complete rebreathing, CO2 absorbant, only replace the amount of gases that are used
Why is an ambu bag NOT a mapleson circuit?
Ambu has a non-rebreather valve (all gas exits)
Mapleson has an APL (only some gas exits)
During mechanical ventilation, the capnograph shows an abnormally low plateau phase and quick bump up right at end exhalation. What is this?
Hole in sampling line: during exhalation, room air mixes w/ expired air and decreases the CO2 content. At the end of exhalation, the vent causes positive pressure for inhalation which seals the hole and EtCO2 jumps up
Elevated baseline, otherwise normal capnograph
Dessicated absorbant
Elevated baseline (maybe), shallower slant of inhalational downslope
Inspiratory valve stuck open (some EtCO2 registers during inhalation)
What differs from a stuck inspiratory valve and a stuck expiratory valve?
Stuck expiratory = MUCH higher baseline
Gases that use IR absorption to detect?
CO2, inhaled anesthetics, N2O
Dye that causes the biggest drop in SpO2
Methylene blue (absorbs the most light at pulse ox wavelengths)
Gas that uses paramagnetic sampling to detect? How does it work?
Oxygen - movement of O2 molecules in a magnetic field based on their unpaired electrons
Gas that uses polarographic sampling to detect? How does it work?
Oxygen - reaction w/ water to create charged electrons, then pass through magnetic field and bend toward the cathode (similar to Clarke electrode in ABG analyzer)
What is a galvanic cell?
Similar to Clark electrode, O2 passes through a membrane where it reacts w/ water to create free electrons, which create a current which is measured
What are the 3 molecular reaction steps of CO2 absorption in an absorber with NaOH and Ca(OH)2 (for example)
CO2 + H20 -> H + HCO3
HCO3 + NaOH -> NaCO3 + H2O
NaCO3 + Ca(OH)2 -> CaCO3 + NaOH
SO, Ca(OH)2 is used up and converted to CaCO3
Why are strong bases added to CO2 absorbers like soda lime and Baralyme?
They speed up the initial reaction b/c the strong bases are much more reactive than just Ca(OH)2
What is different about Amsorb compared to soda lime or Baralyme?
Upsides?
Downside?
Amsorb has NO strong hydroxide, JUST Ca(OH)2 (and calcium chloride to help keep the reaction fast and moist)
Decreased compound A, CO, and inhaled anesthetic breakdown
Only has ½ of the CO2 absorption capacity, and is more expensive
How do the dyes work in CO2 absorbers?
As the OH gets converted to the CO3, the pH drops and causes the ethyl violet to turn blue/purple
5 risk factors for creation of compound A or carbon monoxide
- High anesthetic concentration
- Low flow states
- Dessication of absorbent
- High temperature
- Baralyme > soda lime > amsorb
Open vs Closed scavenging systems
Open: holes in the collecting reservoir so that IF the suction tubing gets kinked, etc, the pressure buildup from continued waste gas removal will exit the system via the holes into the room
Closed: no holes, BUT if pressure builds up in the system, there is a positive pressure relief valve that opens into the room
Active vs Passive scavenging systems
Active: constant suction pulling waste gases out of the system – requires a NEGATIVE pressure relief valve so that if the suction is excessive, room air will be pulled into the system and prevent negative pressure pulmonary edema in the patient
Passive: the action of exhalation is the only thing driving the waste gases forward through the system
Explain pH electrode and severinghaus (CO2) electrode
pH electrode: blood sample separated from other control fluid by a glass matrix which absorbs H+ and creates a potential difference that can be measured
Severinghaus: same as pH electrode, but also has a CO2 permeable membrane that causes a change in pH which is then measured by the pH electrode like above
Explain change in paO2, paCO2, and pH when temperature drops
Temp drop -> more O2 and CO2 dissolved -> less partial pressure of O2 and CO2 -> less measured O2 and CO2, AND higher pH b/c less CO2
Should ABG be measured at 37 degrees or patient temperature?
37 degrees, so that it’s a level playing field and you know the true contents of the blood
Explain how the difference in temp thresholds to start sweating vs shivering gets bigger in GA vs Spinal
GA: centrally-mediated reduction/inhibition of hypothalamus function
Spinal: vasodilation -> false signals from periphery that thinks it is warm but the body actually isn’t
Explain radiation heat loss
The body emits photons of energy from higher temperature to lower temperature. This is seen in the infrared spectrum (like night-vision goggles that can see warm creatures). How much this occurs is based on the body surface area. This is why infants get colder easier than adults.
The bear hugger prevents which type of heat loss the most?
Convection - it stills and warms the air immediately adjacent to the body preventing convection heat loss
Best places for temp measurement
PA > TM/esoph/nasophar/bladder high > axillary/bladder low > rectal > skin
Why is there no fade w/ succ?
Depolarization by succ causes lots of ACh in the cleft, so additional twitches will not contribute much extra ACh, thus the twitch heights are about the same
What is the use of double burst stimulation?
Equivalent to the 1st and 4th twitches of TOF but amplifies both, so it is easier to detect the ratio of T4:T1 strength
Why does diaphragm get paralyzed before peripheral muscles BUT also recovers first?
Paralyzed first b/c get more blood flow after a bolus.
Recovers first b/c much bigger so needs more drug to cover the receptors.
Better for defibrillation:
- Inspiration or expiration
- Monophasic or biphasic energy
- Salt or non-salt containing gels
- Bigger or smaller paddles
- Expiration
- Biphasic
- Salt-containing gels
- Bigger paddles
What is easier to cardiovert: AFib or AFlutter?
AFlutter
