Spirometry Flashcards
Compliance
Change in volume per change in pressure
Can be dynamic or static
Static Compliance
calculated using end inspiratory occlusion pressure
■ Static compliance = VT/Plateau pressure - PEEP
■ Requires zero gas flow
Reflects elastic resistance of lungs, chest wall
Total Compliance
reflects elastic properties of lungs, thorax, abdomen, breathing system
NMBA: will increase chest wall compliance but not affect lung compliance - if paralyzed, changes in compliance DT alterations in lung compliance
Resistance
Non elastic resistance to breathing: airway flow resiatcne + pulmonary tissue resistance
For given VT, high resistance overcome by lower flow for longer time or higher driving pressure
Pulmonary Tissue Resistance
Pressure required to overcome resistance to gas flow through airways during respiration
factors that Increase resistance
Increased FR
Turbulent flow
Bronchoconstriction/increased SmM
Emphysema
Obstruction
Bronchitis/spasm
Factors that Decrease Resistance
Laminar flow
Increased lung volume
Bronchodilation
Shorter Airways
Decreased Viscosity
Factors that Decrease Resistance
Laminar flow
Increased lung volume
Bronchodilation
Shorter Airways
Decreased Visocisyt
Total Airway Resistance
estimated by (Peak Pressure - Plateau Pressure)
■ Normal 2-5 cmH20
■ Increased resistance: higher peak pressure needed to produce same flow
If inspiratory flow and VT remain constant, but resistance increases, greater difference (Ppeak - PPlateau)
Respirometer
device that measures volume of gas passing during period of time through a location in flow pathway
detect obstructions, leaks, disconnections, apnea, ventilatory failure and high/low volumes in spont breathing/controlled vent
Required: expired VT, minute volume, both
Ventilator Bellows Scale
rough estimate of tidal volume delivered into breathing system
Not an accurate estimate of the volume delivered to patient
● Wasted ventilation DT gas compression, distension of components of breathing system
FGF: additive to VT during inspiration
● Will not be reflected on this scale
Wright Respirometer
Gas enters through outer casing, directed through tangential slots to strike a vane, causing rotation
Vane connected to gear system to hands on dial so that a reading corresponding to volume of gas passing through device registered
Evaluation of the Wright Respirometer
Over-reads at high flows
○ Pulsatile flow adds to over-reading
○ Higher readings with N20 as carrier gas
Under-reads at low flows
Advantages of the Wright Respirometer
● Small size, light weight
● Low dead space - use between patient and breathing system
Disadvantages
● No alarms
● Can be difficult to read
● No respiratory rate
● Does not read bidirectional flow
Spiromed
Electronic respirometer: use with North American Drager breathing systems
Gas flows through monitor forcing a pair of rotors to counter-rotate - rotate in unison with armature containing magnets
Transistors ay 7, 12 o’clock receive pulses from magnets on rotating armatures –> interface panel –> processer
Number of paired pulses related to vol of gas that passes trough sensor over time with # counted during each breath = Vt
Spiromed Accuracy
Programmed to measure TV equal or > 150 mL
○ If < 150 mL, instrument will automatically +2 or more consecutive TV’, reduce the recorded frequency
■ Minute volume remains correct
D-Lite Gas Sampler and Flow/Sensor
–Two-sided Pitot Tube
–Measuring flow via measurement of pressure difference across flow resistor via Bernoulli Equation
MOA Pitot Tube
–Placed btw breathing system and patient
–Two sensing tubes: one faces direction of flow (total pressure measurement), other = opposite flow (static pressure)
–Difference between total, static pressure = dynamic pressure –> Bernoulli equation to estimate flow
Advantages of Pitot Tube
Can determine CO2, O2, anesthetic gases
Derive flows, measured parameters, insp/expiratory VT, minute volumes
Can display FVL, PVLs
Not position dependent, allows bidirectional gas flows, able to be used in both Mapleson and Circle Systems
Variable Orifice Flow Sensory
Sensors at both connections to CO2 absorber used to measure inspiratory, expiratory flows
Can be used to generate pressure, flow volume loops
Main advantage = used by ventilator to compensate for changes in FGF
MOA Variable Flow Orifice
Sensors on both sides of circle system
Plastic (miler) flap placed across direction of gas flow - opens at increasing gas flows
Two sensors, transducer inside AxM measure proximal and distal pressure to flap
Volume calculated from these flows
Sensor on inspiratory side connected to pressure sensor so breathing system pressure is measured
Fixed Orifice Flow Sensor
■ Restrictor and two pressure sensors (on either side of the restrictor)
■ Zeroing valve compensates for pressure sensor drift
Respirometer: Ideal position in breathing system
Best location = C btw patient, breathing system (at y piece)
Advantages: Respirometer at Y Piece
● Readings not affected by breathing system leaks, expansion of breathing system components, gas compression
● Can measure both inspired and expired volumes
Disadvantages: Respirometer at Y Piece
● Will increase dead space
● water condensation may be a problem
● Increased likelihood of damage, disconnection or tracheal tube kinking
Where respirometers normally found?
exhalation limb upstream or downstream of unidirectional valve
● Can detect reverse flow, malfunctioning uni-directional valve
● Disconnection upstream of exp valve will detect change in volume
● Disconnection that prevents exhaled gases from passing down the exhalation tubing an apnea alarm will occur
● Can read accurately during spontaneous respiration
If respirometer downstream of absorber…
volume of gas measured will be decreased by the amount of CO2 absorbed
If respirometer on inspiratory side…
■ Will display high readings as gas that does not inflate the patient’s lungs will also pass through it
■ May not detect disconnection during controlled ventilation
Advantages of respirometer placed downstream of inspiratory valve and upstream of expiratory?
■ Measurement of both inspiratory and exhalation volumes to produce flow-volume and pressure-volume spirometry loops
Placement of Respirometer in Mapleson System
Place between the patient connection port and the patient
● Place in expiratory limb in small patients to avoid increased dead space
Low Peak Pressure - Causes DT Patient Factors
● Leaking trach tube cuff
● Extubation
● Increased compliance
● Reduced resistance
Low Peak Pressure - Mechanical factors
● Disconnection or major leak in breathing system
● Obstruction upstream of pressure sensor
● Faulty or poorly set/unconnected ventilator
● Failure of gas or power supply to ventilator
● Malfunctioning scavenge
● Increased compliance
● Reduced resistance
● Suction device mistakenly placed within gas flow pathway
Sustained Elevated Pressure
● Accidental activation of O2 flush
● Occlusion/obstruction of the expiratory limb
● Improperly adjusted APL valve
● Occlusion of scavenging system
● Malfunctioning ventilator
● malfunctioning/incorrectly placed PEEP valve
High Pressure
● Airway obstruction
● Reduced compliance
● Increased resistance
● O2 flush activation
● occlusion/obstruction of expiratory limb
● Scavenger malfunction
● Patient coughing/straining
Factors that can decrease PIP
● High compliance, leaks, low insp flow rates, high resp rates, low I:E ratios, Low TV and low FGF
Subambient Pressure
■ Can be generated by a patient attempting to inhale against a collapse rebreathing bag or increased resistance (blocked inspiratory limb)
Paw Measurement Site
Best = at y piece
More distant measuring site from patient, less useful as estimate of Paw
Typical Locations for Paw Measurement
Immediately downstream of inspiratory valve
Upstream to peep valve
Downstream to expiratory valve
Occlusion in circuit = low-pressure distal to obstruction, high pressure proximal to it
Expiratory Flow Rate
Rate at which gas is exhaled by the patient expressed as volume per unit of time.
Expiratory Phase Time
Time between start of expiratory flow and the start of inspiratory flow
sum of the expiratory flow and expiratory pause times
