Physics, monitors, machine Flashcards
What is the equation for the time left before the cylinder is depleted
Divide the pressure left (psig) by 3 and then divide by the flow rate
Example: 500 psig left and 10 L/min
166.67/10 = 16 minutes
2200 psig left and 10 L/min
733.3/10 = 1 hour 13 minutes
Fresh gas flow must be equal to at least what to prevent rebreathe get during spontaneous ventilation while using a Mapleson A circuit?
Minute ventilation
What is a Mapleson A circuit?
Semi-open breathing system at relies on fresh gas flow for CO2 rebreathing
How does a Mapleson A circuit work?
Fresh gas flows to the patient, the APL valve is closed and reservoir bag deflated
Patient exhales and it goes out the APL valve, fresh gas accumulates in the reservoir bag and travels proximally to eliminate all exhaled gas
What is the safest flowmeter arrangement
One with oxygen tube down stream from all gases to prevent hypoxic mixtures
Which gas cylinders have to be measured by weight instead of pressure?
Carbon dioxide
Nitrous oxide
Why do carbon dioxide and nitrous oxide have to be measured by weight?
Because according to the ideal gas law, every time a gas is removed, liquid is vaporized to restore equilibrium
How much does a full e-cylinder of nitrous oxide weigh?
8.8 kg
How much does an empty nitrous oxide cylinder weigh
5.9 kg
What does a capnograph of an incompetent expiratory valve look like?
Looks like rebreathing CO2
The inspiratory segment is elevated
What does the capnograph look like with an incompetent inspiratory valve
Prolonged plateau phase
How is left ventricle contractility measured by echo?
dP/dT = rate of rise of ventricular pressure
The greater the force, the faste the rise
Which of the most commonly used gases has the lowest vapor pressure?
Sevoflurane
What is vapor pressure?
The pressure exerted by the vapor phase of a substance in equilibrium with solid and liquid phases at a given temperature in a closed system
What does a higher vapor pressure mean?
It means that substance is more likely to evaporate at a given temperature
What is the vapor pressure of desflurane
669 mm hg
What is the vapor pressure of isoflurane?
238 mm Hg
What is the vapor pressure of sevoflurane?
157 mm Hg
Fresh gas flows must be greater than or equal to what to prevent rebreathing during controlled ventilation using a Mapleson D circuit?
2-3 times minute ventilation
Fresh gas flows must be greater than or equal to what to prevent rebreathing during controlled ventilation using a Mapleson D circuit?
2-3 times minute ventilation
What causes an abrupt drop to zero in the ETCO2?
- Circuit disconnect
- Kinking of the ETT
- Esophageal intubation
What causes a sudden drop in ETCO2?
Partial airway obstruction
Circuit leak
What would cause a rapid decrease in ETCO2?
Pulmonary embolism
Circuit leak
What would cause a gradual decrease in ETCO2?
Hypothermia
Hyperventilating
What would cause a sudden increase in ETCO2?
Release of tourniquet
Sodium bicarbonate administration
What would cause a gradual increase in ETCO2?
Decrease in minut ventilation Faulty unidirectional valve CO2 absorbent exhausted Thyroid storm Malignant hyperthermia Insufficiency fresh gas flow
Which valve is better visualized with TTE?
Pulmonic due to its anterior position
What are pulse oximetry a poor monitor of?
Ventilation
What does pulse oximetry measure?
Functional hemoglobin saturation
Which color light does oxyhemoglobin absorb?
Blue - less red because this is what is reflected back to us
What gas property influences the flow rate around the annular space of a conventional flowmeter at low fresh gas flows
Viscosity
What is the annular space in the machine?
The space between the float and the wall of the flowmeter tube
What is the Hagen-Poisseuille equation?
Q= (change in Pressure X pi X r4)/8Ln
What gas property influences the flow rate around the annular space at high gas flows?
Density
What happens to partial pressures of gases as temperature decreases?
They decrease because they become more soluble in solution and the partial pressure depends on the amount go gas in gaseous form
What happens to ph with every degree of temperature decrease?
It increases by 0.017
During oscillometric non-invasive BP monitoring, how is the MAP determined?
By the point of maximal amplitude of oscillations
How are systolic and diastolic pressures determined in an oscillometric bp measurement?
By equations that detect the rate of change in oscillations
Which bp measurement is the least accurate?
Diastolic because it is most affected by movement
How many liters of nitrous oxide is in a full tank?
1590L
Why is oxygen always downstream?
Because if there is a leak in the flow meter, gas could leak out of the air flowmeter toward the oxygen flowmeter and patient. If oxygen was upstream it would leak out and be lost to the patient
If there was a leak in the oxygen flowmeter, what would happen if the anesthesia was used?
A lower than expected FiO2 would be delivered if using nitrous oxide too
When does the fail safe alarm go off?
When pressure of oxygen falls below 20-30 psig
What is absolut humidity?
The actual mass of water vapor in a volume of air
What is relative humidity?
The ratio of actual mass of water vapor in a given volume to the maximum it can hold
What does the negative pressure leak test do?
Differentiates between leaks in the machine and breathing system
How much does the BP per 10 cm of height change in comparison with the transducer?
7.5 mmHg per 10 cm of change
How much current does the Line isolation monitor detect?
Greater than 2 mA
Is it possible to deliver a micro shock with a working line isolation monitor.
Yes because it doesn’t detect anything under 2 mA
What are the two causes of the LIM system alarming?
- A faulty piece of electrical equipment was just plugged in
- Too many normal things are plugged in that are leaking current
What happens resistance and compliance of vessels as you get further and further from the aorta?
Resistance increases
Compliance decreases
What does a radial arterial waveform have in comparison with an aortic arterial waveform.
Higher systolic pressure
Wider pulse pressure
More delayed and slurred dicrotic notch
A more pronounced diastolic wave
Why does a radial arterial line have a higher systolic pressure than an aortic arterial line?
Due to all the branch points which reflect centrally and add to the pulse wave
What happens to the delivered concentration of anesthetic gas at higher altitudes through a variable bypass vaporizer?
It increases
What is Dalton’s law?
It states that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the individual gases
How do variable bypass vaporizers work?
They are set to deliver fixed partial pressure of gas for a given level of barometric pressure
So as you ascend and barometric pressure drops, the vaporizer will correct for the barometric pressure and still deliver the same partial pressure, but give a higher concentration of gas
What is a good measure of volatile potency?
Partial pressure
How does a desflurane vaporizer work?
It heats the gas to 39 Celsius to create a certain vapor pressure independent of barometric pressure and then delivers the gas by concentration not partial pressure
What is the saturated vapor pressure of isoflurane?
238
What is the saturated vapor pressure of sevo?
157
What is the saturated vapor pressure of desflurane?
669
What is the concentration of agent inside the vaporizer equation?
Saturated vapor pressure/ barometric pressure
Note: the gas will always be saturated with agent inside the canister
How does pulse oximetry work?
It uses 2 different wavelengths of light to measure oxygenated and deoxygenated blood to give an estimate of SaO2
When the bellows go up, what air is filling them?
The exhaled gases from the patient
When the bellows go down, what air is being expelled and what part of respiration does that correspond with in the patient?
Gases and oxygen from the circuit to the patient
Inspiration
What should you do if the pipeline supply fails?
Turn the gas flows down because driving the bellows with the E-cylinders causes more consumption of gas
Why does a closed circuit machine cause more PONV.
Because of increased inhalation of noxious gases
Is there more heat loss with a closed circuit and why?
No, because each inspired breath has a higher humidity (due to the majority of inhaled gas being what was just exhaled) and lower fresh gas flows can be run in a closed circuit so there is less cool, dry air mixing.
Fresh gas flow is minimized in a closed circuit
How do the inspiratory valves work?
The inspiratory valve opens to let fresh gas flow into the patient while the expiratory valve closed to prevent rebreathing of gases
During exhalation, the expiratory valve opens to allow exhaled gas to flow to the absorb and the inspiratory valve closed to prevent exhaled gas from flowing retrograde into the inspiratory limb
What would you do if you have an incompetent inspiratory valve?
Increase your flows
How can you suspect an incompetent inspiratory valve?
CO2 not going back to baseline on the capnograph
Prolonged expiratory plateau
Gradual inspiratory stroke
Exhaled tidal volumes lower than expected as set on ventilator
How should MRI compatible ECG cable be positioned?
Straight and insulated from the patient because can cause thermal injury to a patient
What is acoustic impedance?
A tissue’s capacity for transmitting sound
High = greater ability to reflect sound
When is reflection highest using ultrasound?
When the angle of incidence is 90 degrees
What are the to factors that influence acoustic impedance?
Density and propagation of speed of sound
What are the specifics of pulse oximetry?
It depends on the absorption of red and infrared light
Deoxyhemoglobin absorbs more light than oxyhemoglobin at red wavelengths
Oxyhemoglobin absorbs more light at infrared wavelengths
What IV dye doesn’t mess with pulse oximetry?
Fluorescein because it absorbs light at similar wavelengths as hemoglobin
Why do blue dyes affect pulse oximetry?
Because they absorb significantly more light at 940 nm which is interpreted as a greater concentration of deoxyhemoglobin
Why does red nail polish not mess with the pulse oximetry?
Because it is the color that is pulsed through the tissue at 660 nm as is infrared
Why are closed circuits so much better?
- They allow for less rebreathing
- They allow for low fresh gas flow
- They help conserve heat and humidity
In thermodilution, what are the 2 axes for the graph?
X= time Y= temperature
And remember, by convention, the temperature is a positive deflection even though it represents a decrease in temperature from baseline
What does low cardiac output look like on a thermodilution graph?
High, wide peak
How does thermodilution work?
10 ml of cooled saline is injected into the right atrium and temperature is sensed in the pulmonary circulation
3 measures are taken and averaged due to variability with breathing
What material should not be used with CO2 lasers around?
Polyvinylchloride > silicone > rubber
What ETT materials are most resistant to ignition when using an Nd : YAG laser?
Silicone
Rubber with silver foil and sponge coating
What are the best ETT tubes to use with CO2 laser?
Metal wrapped
Flexible metal
What is the advantage of a microlaryngeal tube?
Same diameter as an ETT, longer, less flexible, large cuff volume, lower pressure
What happens to desflurane partial pressure as the barometric pressure drops?
It decreases
How does the desflurane vaporizer work?
It is heated on the inside to maintain a constant temperature of 39 degrees Celsius so that the same concentration of gas is always delivered at any level above sea level
What is the equation for resetting the dial at a different atmospheric pressure to maintain constant partial pressure of gas?
% concentration X 760 mm Hg/ambient pressure
What happens with variable bypass vaporizers at different altitudes?
They always deliver the same partial pressure of gas because they are not internally pressurized.
What happens to the amount of gas delivered above sea level in a normal vaporizer?
It increases but the partial pressure delivered is the same.
Why does barium hydroxide produce more carbon monoxide?
Less water in the absorbent
CO2 combines with water to form carbonic acid
Which absorbent allows better CO2 absorption?
Soda lime
Why does carbon monoxide form from the absorbents?
It is an exothermic reaction that happens with the volatile agents and the strong bases within the absorbents in the setting of decreased water content
Which inhalational anesthetic produces the most carbon monoxide with absorbents?
Desflurane
Which inhalational anesthetic produces the most heat with absorbents
Sevoflurane
Which absorbent is more likely cause fire and compound A accumulation with sevoflurane?
Barium hydroxide due to its lower water content therefore makes more heat with sevo
How does absorbent work?
- CO2 combines with water in the absorbent to make carbonic acid
- Carbonic acid reacts with NaOH (strong base) to form sodium bicarb, water, and heat
- Then sodium bicarb reacts with CaOH to form calcium carbonate and sodium hydroxide
What does the PADSS use? (Post anesthesia discharge scoring system?
Scores from 0-2 for all categories
- Pain
- Hemodynamic stability
- Surgical site bleeding
- PONV
- Activity level
Why does helium create laminar flow?
It is the least dense
What is the viscosity of helium compared to air and oxygen?
The same
Why is helium used as a treatment for reactive airway disease?
It’s low density allows for more laminar flow
What is Boyle’s law?
For a given mass, at constant temperature, the pressure time the volume is constant
PV= C
What is Charles and Gay Lussac’s law?
For a given mass, at constant pressure, the volume is proportional to the temperature
What is Dalton’s law of partial pressures?
In a mixture of gases, the pressure exerted by each gas is the same as if each gas occupied the volume alone
What does laminar flow depend on?
Viscosity
Hence annular flow depends on viscosity at low flow gas
When does ETCO2 show signs of VAE?
0.5 ml/kg of air
How much does N2O increases the bubble at 50% nitrous?
It doubles it
How much does N2O increases the bubble at 75% nitrous?
It quadruples it
What are the NIOSH regulations for N2O scavenging?
Must be less than 25 ppm
What are the NIOSH scavenging guidelines for halogenated agents with N2O?
0.5 ppm
What are the NIOSH scavenging guidelines for halogenated agents without N2O?
2 ppm
What does a time constant equal?
Volume of the container/flow through the container
What is the % change of 1 time constant?
63%
What is the % change of 2 time constants?
86%
What is the % change in 3 time constants?
95%
What does the time constant of the circuit?
Volume of the circuit/FGF
What does a line isolation monitor do?
Continually monitors the impedance from all lines to ground and indicates wha
How is ETCO2 measured?
Infrared spectrophotometer where a wavelength of infrared light is passed through a gas sample and the amount of energy detected is inversely proportional to gas partial pressure
What does a Ground fault current interrupter do?
Shuts off the power to the faulted machine to prevent electric shock to a person standing in the field (who offers a low resistance pathway for the current).
What does a line isolation monitor do?
Alarms when one machine creates a current above 5 mA, this is called a first fault
Note: you need 2 faults to create a circuit and thus electrocution
Do GFCI or LIM prevent microshock?
No
Which Mapleson circuits require FGF to be 2-3X minute ventilation to prevent rebreathing?
Mapleson D, E, F
What Mapleson circuit is the most efficient for spontaneous breathing?
A
Which Mapleson circuits are most efficient for controlled ventilation?
D,E,F
What is saturated vapor pressure?
The partial pressure of a substance at which the liquid and vapor states are in equilibrium
How is end tidal CO2 measured?
By infrared spectrophotometer where a wavelength of infrared light is passed through a gas sample and the amount of energy detected is inversely proportional to the gas partial pressure
Which gases are measured with infrared?
CO2, N2O, volatiles
Which gases do not absorb infrared?
O2, N2, xenon
What is a closed circuit?
Where all total fresh gas flow is equal to oxygen consumption and inhalational anesthetic metabolism.
No gas is vented to a scavenger
All gas is rebreathed
To minimize FGF - only oxygen is used
What are semi-open circuits?
There are no expiratory/inspiratory valves, no CO2 absorbent
Rely on FGF to prevent rebreathing
FGF has to be greater than or equal to what in controlled ventilation using Mapleson D circuit to prevent rebreathing?
1-2 times minute ventilation
What should you do if a power failure happens during surgery?
Switch essential equipment to the red face plate electrical supply system
What is different about the Bain circuit?
It has coaxial delivery of FGF
Requires higher gas flows to prevent rebreathing (2.5X minute ventilation)
Lower volume of tubing
Faster equilibration to changes in inspired gas concentrations
What is Mapleson A better for?
Spontaneous respirations
What is diluent flow?
The gas that bypassed the volatile chamber
What happens to vaporizer output when it is moved from sea level to Denver?
Higher output
What happens to vaporizer output when a vapor with higher pressure is put in a vaporizer set at a lower pressure (isoflurane for sevo)
Lower output
How does using a Mapleson circuit speed the rate of rise of Fi?
Vc is lower and the smaller circuit causes less of a dilution all effect
No CO2 absorbent
Less plastic/rubber to absorb anesthetic
What is the difference in a Mapleson A circuit?
The FGF enters near the breathing bag, not near the patient connection
Most efficient system in spontaneous breathing
What is the difference in Mapleson E circuit?
Does not have a breathing bag, but a long piece of tubing that acts as a reservoir or expiratory limb
No rebreathing occurs during controlled ventilation because FGF fills the lungs
WHat is the difference in Mapleson F circuit?
Allows for scavenging gas
Equally efficient of spontaneous and controlled ventilation
What is the equipment ground wire for?
It is for prevent micro shocks from current leaks
What is an equipment ground wire?
A low impedance wire that allows leakage current to pass through to prevent buildup of leakage current.
What is the line isolation monitor useful for?
Preventing macro shock
What does the line isolation monitor measure?
The total leakage current of the nongrounded isolate power circuit
What is the ground fault circuit interrupter for?
Preventing macro shock
It monitors current imbalance in grounded circuits and interrupts them if there is an imbalance
What is used to measure the partial pressure of oxygen and require a power source?
Clark electrode - polarographic
What are the three ways you can measure the partial pressure of oxygen?
- Galvanic
- Paramagnetic (electromagnetic field that attracts oxygen molecules
- Polarographic
What is the galvanic method?
Oxygen molecules enter th electrode and react with The lead anode.
The reaction produces electrons that move to the gold anode
The amount of electrons moving across is proportional to the oxygen concentration
What is the polarographic method?
Oxygen molecules react with silver anode producing electrons and go toward a platinum cathode
- needs a battery (unlike Galvanic) so it’s faster
What is the severinghaus electrode used for?
Measuring pCO2
- has an internal ph sensor
What is IR absorption used to detect?
Volatile, N2O, CO2
Why are the unidirectional valves close to the Y piece?
To limit the amount of back flow there is if there is a leak
Why is the APL valve positioned before the absorber?
To prevent the loss of fresh gas
Why is the reservoir bag positioned on the expiratory limb?
To reduce expiratory resistance
Why should the fresh gas outlet be positioned between the absorbed and the inspiratory valve?
So the patient always gets fresh gas that is not diluted
What is a semi-open system?
High FGF with no rebreathing
Where should the APL valve NOT be positioned to prevent rebreathing in a traditional circuit?
Between the patient and the inspiratory valve
Where can the FGF not enter to prevent rebreathing in a traditional circuit?
Between the patient and the expiratory valve
Where must a unidirectional valve be positioned to prevent rebreathing?
Between the patient and the inspiratory and expiratory limbs of the circuit
How does the O2 analyzer work and what is it influenced by?
Paramagnetic analyzer
It is influenced by water vapor because water creates a magnetic field directly opposite of an imposed magnetic field –> reads O2 levels that are falsely low
What is a semi-closed system?
Lower FGF with some rebreathing
What is a closed system?
FGF matches patient consumption with complete rebreathing
How do you decrease the time constant?
By decreasing the capacity of the circuit OR
Increasing the FGF
What is the equation for the time constant?
Vc/FGF
What is the time constant for isoflurane?
3-4 minutes, following first order kinetics, equilibration with the brain will take 10-15 minutes
What is the time constant for N2O, Des, sevo?
2 minutes
Why are newer absorbent so better than older?
They lack strong bases that react with volatile to make carbon monoxide
