Measurements of Respiratory System Flashcards
What kind of clinical tests would require measurements of the respiratory system?
- Pulmonary Function Tests (PFT)
- discrete observations on intervals (days or years)
- screening general population
- evaluating changes during diseases
- follow-up after treatment - Patient monitoring
- for trauma, drug overdose, major surgery / disease
- in short term / ICU
What are measurable variables of the respiratory system (5)
- flow (L / min)
ex. tidal flow, peak expiration - gas composition
ex. end-tidal CO2 - lung volume / capacity
ex. spirometry, nitrogen washout, plethysmography - pressure
- (easy to measure at airway opening (nose / mouth))
- challenging to measure pleural (alveoli) pressure - airway resistance
- requires flow and pleural pressure
- used to test for obstruction of flow
Compare and contrast the different types of pulmonary flow (2)
Laminar flow
- particles follow same line of flow
- velocity increases closer to center (parabola)
Turbulent flow
- particles move in different directions
- common in upper airway obstruction
What is a major requirement for flow sensors?
flow stream must pass through and into the instrument
What are some challenges of pulmonary flow sensors?
- Variable and bidirectional flow
- must work for diff breathing rates
- must allow patient to inhale / exhale (should flush CO2 and replenish O2 for multiple-breath measurements)
- imposed pressure at airway opening can cause damage, distortion, or leakage - moisture and condensation
- condensation can impair flow sensor (changes effective area - when heated or above body temperature) - Measurement sensitivity
- sensitive to low pressures
- must consider changes in gas composition and temp that could effect stability of baseline and sensitivity - Minimal Flow resistance requirement
- should not make breathing hard for the patient (no obstruction or produce back pressure) - contamination and hygiene
- consider if it is reusable (sanitizable) or disposable
- heat is used to sanitize BUT must not add excessive heat to inspired gas - Safety
- must not add toxic substances to inspired gas
What are the 4 types of respiratory flow sensors?
- Differential Pressure flowmeter
- Rotating-vane flowmeter
- ultrasonic flowmeter
- thermal-convection flowmeter
Explain how Bernoulli’s equation allows flow meters to measure flow rate
Bernoulli’s equation relates change in pressure (has pressure sensor) to velocity based on the dimensions of the device
- assumes gas is non-compressible, non-viscous, at laminar flow
eq: p + 1/2rv^2 + rgh = const
eq: A_a * v_a = A_b * v_b
p = fluid pressure (static pressure)
r = density
g = gravity
h = elevation
1/2rh^2 = dynamic pressure (kinetic energy)
rgh = hydrostatic pressure (potential energy)
Consider flow meter = two cylinders of diff cross section
Then:
p_a + 1/2r(v_a)^2 + rgh = p_b + 1/2r(v_b)^2 + rgh
then
dP = (1/2)* r* ((v_a)^2 - (v_b)^2)
thus
v_a = sqrt{ [2(dP)(A_b)^2] / {r[(A_a)^2 - (A_b)^2)]} }
What does a peak flow meter measure?
measure peak exploratory flow “PEF”, related to max speed of exhaled air
How does a pneumotachometer measure flow?
- person breathes through disposable mouthpiece
- air goes through a wire mesh (capillary tubes) connected to a differential pressure transducer
- transducer measures the pressure before and after passing the wire mesh
Q = dP / R
Q = flow
dP = diff in pressure
R = fixed resistance of pneumatachometer
Note: R = 8nL / (pi*r^4)
n = dynamic viscosity
Therefore, we can either vary the length (L) or the (r) to vary the resistance (R)
Describe the principle behind rotameters
rotating-vane flowmeters =
- clear tapered glass tube with inscribed scale
- float / bobbin
How does it work:
- air flow enters from bottom of glass tube
- pushes the float / bobbin up
What do you measure:
- the height that bobbin reaches is due to the balancing forces of gravity and pressure from gas
Challenges:
- unidirectional
- humidity (bobbin might get stuck)
Describe the principle behind ultrasonic flowmeters
Describe the principle behind spirometers
Equipment:
- rotating vane
- light source
- photodetector
- processor
How it works
- air flow will rotate the rotating vane (increased air flow = increased rotation)
- this causes obstructions for light as light travels from the source to the photodetector
What do you measure:
- you measure the interruption of light flow to the photodetector
Describe the principle behind ultrasonic flowmeters
Equipment:
- pair of opposite, tangential ultrasound beams with sensors
- processor
Concept:
airflow will:
- slow down ultrasound waves that move in opp dir to flow
- speed up ultrasound waves that move in same dir to flow
Measure:
- the difference in speeds as ultrasound beams travel from source to sensors
Describe the principle behind hot wire anemometer
Equipment:
- heated platinum wire
- processor
Measure:
- flow via electricity input
- air flow will cool down wire –> requires more electricity to reheat to specific temperature
2 types
- simple
- direction sensitive (contains 2 wires with 1 being sheltered by bar on one side –> can only be cooled by air in a specific dir)
Pros:
- (lab settings) - capable of measuring flow using gases other than room air
Describe the principle behind infrared spectroscopy
Molecular footprint:
- gasses with dissimilar atoms (CO2) can absorb infrared light at specific wavelengths
- energy absorbed will increase temp of gasses
Beer’s law:
[] = 1 / (A*L) ln(Pin / Pout)
A = absorption coefficient
L = length through gas
Pin = power entering sample
Pout = power per unit area transmitted
Equipment:
- infrared light source
- airway (patient to ventilator)
- IR optical bandpass filter
- infrared detector (output inversely proportional to [gas])
used for capnometry (End-tidal CO2 Meter):
- measures amount (volume) of CO2 in exhaled air (end-tidal)
- (used by first responders to determine how well heart is working)
What are the absolute lung volume measurements (4)
- Total Lung capacity (TLC)
- Functional Reserve Capacity (FRC)
- amount of lung volume left after normal exhalation - Reserve Volume (RV)
- amount of lung volume left after full exhalation - Closing Capacity (CC)
- maximal lung volume at which airways (bronchioles) begin to close
What are the relative lung volume measurements (5)
- inspiratory Capacity (IC)
- maximum capacity of lungs during inhalation - Expiratory Reserve Volume (ERV)
- maximum capacity of lungs during exhalation - Vital Capacity (VC)
- difference between total lung capacity and reserve volume - Tidal Volume (V_t)
- volume of lungs for normal breathing - Closing volume (CV)
- difference between FRC and CC
What are the 2 ways to measure lung volume?
- Measuring changes in lung volume
- spirometry: measures gas passing through airway opening
- plethysmography: measure changes in gas space within body - Measuring absolute volume of lung
- nitrogen washout
A spirometer is used during normal breathing. What lung volume does it measure?
Tidal volume (normal breathing)
- cannot measure absolute lung volume
Nitrogen washout experiment starts at the end of a quiet expiration following normal breaching of air. What volume does it measure?
Functional reserve capacity
Which respiratory equipment is used for measuring absolute volume of the lung?
a. Spirometry
b. Nitrogen Washout
c. Full body Plethysmography
Both Nitrogen Washout and Full body plethysmography
Describe how spirometry detects changes in lung volume and what is its clinical use
Measures gas passing through the airway:
- (old method is using a spirometer to mechanical integrate accumulate gas)
- (new method is to electronically integrate output of flow meter at mouth)
Accurate and continuous:
- only for small compression of gas in lungs (slow relaxed breathing)
Clinical use:
- find lung volume for pulmonary function tests
Describe the mechanical components of spirometry
- rigid bell
- air-tight via water seal
- encloses soda-lime canister (prevents buildup of CO2)
- encloses oxygen chamber connected to tubing and mouth piece - counterweight
- patient breathes in: moves up
- patient breathes out: moves down - kymograph drum
- (analog) draws the lung volume graph based on position of counterweight
Describe the procedure of Nitrogen Washout Experiment
- Subject breathes in non-N2 gas mixture (sometimes pure O2)
- Subject will exhale mixture of N2, O2, and CO2 into spirometer
(starts at end of quiet expiration following the breaching of air
- keeps pumping non-N2 gas until all the N2 is washed out - lung volume calculated based on change in N2 moles.
What are the two assumptions made when using a nitrogen washout experiment
- assumes no diffusion of N2 into alveolar gas from lung tissue and pulmonary capillary blood
- allows us to related the change in number of moles of nitrogen at the mouth to the models of nitrogen in spirometer
- ideal gas law
- allos us to relate change in volume to molar change (PV = NRT)
What does full body plethysmography measure and how is it used in combination with other respiratory measurements?
Measures: (indirectly)
- lung volume and its changes
- alveolar pressure
- airway resistance
Complements Spriometry:
- measures absolute lung volume values such as:
1. Residual volume
2. Total lung capacity
How does full body plethysmography compare to nitogen washout?
More accurate
- accounts for all air in the lungs (even trapped gas in poorly ventilated areas)
Explain how Fully Body Pressure Plethysmography works
- person sits inside box w/ pressure tranducsers, shutter, and pneumotachograph
(V_plethysmography = V_tissue+ V_lung + V_box)
Boyle-Mariotte’s law:
PV = const; assumes const T
- shutter closes
- results in zero air flow to measure lung volume
V_lung = dP_box / dP_mouth * V_box
- shutter opens
- measures changes in alveolar pressure
R = - dP_lung / dQ
