Session 3 Lung function

Question 1

limitations of pulse oximetry?

Answer 1

movement artefacts- movement of patients will alter readings

Question 2

what index of body size is needed to look up the predicted value for a patient’s FRC?

Answer 2

height

Question 3

measurement from a spirometer used to measure outcome of a child whilst doing an exercise test to test for exercise-induced asthma?

Answer 3

FEV1.0 or PEFR

drop by 15% normally considered cut off for diagnosis, or 20% if PEFR measured

Question 4

reasons for pulmonary function tests?

Answer 4

Diagnosis: used with history and examination
Patient assessment= most common: response to therapy, assessment for compensation e.g. occupational lung disease, serial changes, pre-surgical assessment e.g. if patient going to be given anaesthesia
Research purposes: epidemiology, study of growth, development and investigation of disease

Question 5

potential areas to be investigated in lung function?

Answer 5

lung mechanics/ventilation
gas transfer/mixing
respiratory control e.g. defects cause of cot deaths
pulmonary blood flow
pharmacological/metabolic lung role
other:ciliary function

Question 6

why might a patient be referred to the lab?

Answer 6

unusual cases of common problems
severe case
those who don't respond as expected
history and examination at variance
pattern of illness has changed
dual or multiple pathology

Question 7

describe how simple spirometry is performed

Answer 7

air tight seal made over mouth piece, nose clip prevents any air from escaping. Patient asked to inhale to TLC by taking deepest breath possible- max inspiration, and then forcible exhale as fast and as far as possible, breathing out till RV reached. A plot of volume against time- vitalograph tracing/ vol-time spirogram, produced by continuously measuring volume

Question 8

what is required in spirometry to prevent within-subject variability?

Answer 8

proper performance of test: trained operator, familiar with equipment- set-up, calibration, operation, and experienced with subjects

Question 9

incentive to children to ensure that during spirometry, they breath out as far as possible?

Answer 9

show them a picture of burning candles- they must try and blow out all of the candles so much keep on breathing out until that happens

Question 10

3 main means of a.flow obstruction in intrathoracic airway obstructive diseases?

Answer 10

Excess mucus secretion
Narrowing due to shortening of airway smooth muscle (asthma) and/or inflammation and oedema of the airway lining
Loss of radial traction (emphysema)

Question 11

why is expiration affected more than inspiration in intrathoracic airways obstruction?

Answer 11

during expiration, the intrapleural pressure is greater than the tracheal pressure, compressing the intrathoracic airways and so worsening the obstruction
during inspiration, the pressure in the trachea is greater than the intrapleural pressure, allowing stretching of the airways to help remove the obstruction

Question 12

why is inspiration affected more than expiration in extrathoracic airways obstruction?

Answer 12

expiration: pressure in trachea greater than atmospheric pressure, opening up the larger aiways in the region of weakness
inspiration: pressure in atmosphere greater than pressure in trachea, compressing larger airways in region of obstruction

Question 13

causes of variable extrathoracic airways obstruction?

Answer 13

laryngeal polyp

bilateral VC paralysis

Question 14

describe flow volume curve for a variable extrathoracic airways obstruction

Answer 14

The pattern of the expiratory flow-volume curve is normal, but the inspiratory flow reaches a low plateau value. High pressure in extrathoracic airways distends the airway on expiration, but airways compressed during inspiration in region of obstruction

Question 15

flow volume curve for a fixed extrathoracic airways obstruction e.g. carcinoma of larynx, or obstruction by goitre?

Answer 15

inspiratory and expiratory flow-volume curve abnormal

fixed obstruction prevents larger airways distending on expiration, so both curves exhibit flow plateau at low flows

Question 16

what is an extrathoracic airways obstrucution?

Answer 16

obstruction of trachea above or below VCs

Question 17

conditions for infant lung function testing?

Answer 17

Preterm newborns to approx 18 months
Need for sedation
Medical cover/resuscitation
Warm, quiet, subdued lighting
Infant clean, dry, not hungry or thirsty
Allow plenty of time
Clear explanation for parents

Question 18

why might peak flow come out the same for a number of different VmaxFRC measurements?

Answer 18

peak flow greater than value able to be measured by the machine

Question 19

difference between helium dilution and whole body plethysmography for measurement of lung volumes?

Answer 19

Helium dilution – measures volume of gas that is in communication with the airway opening i.e. the volume that is ventilated
Whole body plethysmography – measures volume of gas in the thorax, including pneumothorax or trapped gas

Question 20

what does N2 washout measure?

Answer 20

anatomical dead space volume- volume of conducting airways

Question 21

if helium dilution is used to measure FRC, what is V2 equal to?

Answer 21

V1+FRC
as helium poorly soluble in blood
subject breathes in and out from spirometer, starting at end of a normal expiration, until equilibrium is reached.

Question 22

how can gas mixing be assessed?

Answer 22

single-breath N2 washout

Question 23

why do we need predicted values and where do they come from?

Answer 23

Necessary for interpreting all measurements
Usually based on height
Ideally, each lab would study its own local population and calculate reference values
Where this is not practicable, values are selected from the literature

Question 24

how is a suitable population selected for measuring predicted values?

Answer 24

Ethnicity – should be broadly similar to index population
Age range and height range – as above
Large numbers of males and females
How ‘normal’ are they? (urban/rural? ‘hospital normals’? Do we exclude children with URTI etc?)

Question 25

How can suitable methodology be selected for measuring predicted values?

Answer 25

Equipment should be as similar as possible (e.g. type of spirometer)
Procedure should be similar (e.g. posture)
Analysis of data done the same way (e.g. mean value, or best?)
Check for internal consistency at different heights

Question 26

how can the appropriateness of reference values in population of children be checked?

Answer 26

Study a group of healthy children – do they have data in line with prediction?

Be aware of the effect puberty can have. Decide when to use predicted values from an adult population.

Question 27

pre-requisites for respiratory function testing in children?

Answer 27

 Height and weight
 Age – 4.5 to 5 years is the minimum
o Any younger and they are usually not very cooperative
o After 16 – progress to adult clinic
 Exception: cystic fibrosis patients
• They have to go for regular lung function testing so move to the adult clinics as and when they feel comfortable to do so
 Questions
o Smoking
o Other allergies
o Illness – cough/cold in last 3 weeks
 Usually postponed if that is the case
 Medication
o Ask them to withhold bronchodilators on day of test
o Assess, give bronchodilator and then re-assess

Question 28

how is exercise induced asthma tested for in children?

Answer 28

Measure lung function pre and post exercise- FEV1.0 and PEFR
o FVC is difficult to obtain when patient is panting after exercise
 Treadmill is used
o Not all exercise has the same effect
o Running is the most effective at provoking EIA, and is simple, done before, enjoyable
o You are able to control speed at which the treadmill is operating
o Timing – about 6 minutes
 Start at walk, increase it quite quickly
 Until you reach the appropriate heart rate
• Ensures people of different levels of stress/age/enthusiasm
• Calculate predicted HR by: 230 – age (in years)
o Target: 80% of heart rate within first 3 minutes and maintain it
o But if you can’t use a treadmillyou can use a corridor
 Can be quite scary
o You have to explain to the patient what is going to happen because you actually have to bring on an asthma attack
o 2 people, one on each side of the treadmill, to catch child in case they fall over
 Cardiovascular problems
o Check with cardiologists
 Won’t complete test if lung function is too poor to start with
 Looking for percentage change in FEV1.0
o Should be about 15%, or 20% in PEFR
 Bronchodilator
o Given even if people have no response to exercise
 Treatment: take bronchodilator pre-exercise
 Often a test of exclusion
o Athletes and pushy parents
o Unfit/obese

Question 29

describe the process of whole body plethysmography

Answer 29

Measures changes in volume within an organ or the whole body- Measure thoracic gas volume (functional residual capacity = ERV + RV)) and airway resistance=pressure/flow
 Volume of gas in thorax including trapped gas or pneumothorax
o Used instead of helium dilution
o Particularly good for hyperinflation
 Some parts of the lung are poorly ventilated
 But these areas can be measured using plethysmography
Subject placed in a cabin which is almost air tight and breaths through a pneumotachograph which measures airflow
 A shutter within the device is closed to transiently occlude the external airway
o When shutter closes there is a fixed volume of gas behind
 Temperature kept constant at about 37 degrees
 The patient makes a respiratory effort against the shutter allowing the mouthpiece to directly measure the alveolar pressure change
 The change in thoracic volume can be measured indirectly from the cabin pressure
 Based on Boyle’s law
o Pressure is inversely proportional to volume
 PV = (P + ΔP)(V- ΔV)
 Rearrangement: V = ΔVP
ΔP
o Where
 V = thoracic gas volume – what we want to know
 P = atmospheric pressure (measured from the barometer, must be corrected for water pressure)
• If there is no flow the pressure in the lung = atmospheric
 ΔP = Pressure chance in thoracic volume
• Measure directly by sensor between lips and occlusion
 ΔV = volume change and is measured indirectly
• Volume in chamber is relative to that in the lungs
• Because it is almost air tight
 Once FRC has been obtained we can measure airway resistance
o Get the subject to breath quietly
 Pressure drop across airways is the difference between alveolar and atmospheric pressure
• Measured indirectly from cabin pressure
 Flow is measured by the pneumotachograph
 Confidence levels
o Lung function tests are used for assessment not diagnosis
 Reliability of the test
o Very important that patient has a good technique
 If someone is claustrophobic they will not do the test
o But this is rare in children
o Bigger problem – separation from mothers
 Initially: get child to sit on knee in the booth
 Can usually then get the child to sit in booth alone

Question 30

process of helium dilution?

Answer 30

 Measures volume of gas that is in communication with the airway opening
o Volume that is ventilated
v1c1=v2c2
so v2=v1c1/c2
Need something to absorb carbon dioxide and keep oxygen levels normal

Question 31

process of single breath N2 washout?

Answer 31

 Single breath
o Breathe out to residual volume
o Maximal inspiration of 100% pure oxygen
o Breathe out slowly
 Monitor Nitrogen concentration
 If all regions of the lungs are evenly ventilated, the slope of Phase 3 will be flat. If some parts empty and fill more readily than others (i.e. ventilation is uneven), there will be an increase in [N2] during Phase 3

Question 32

describe how pulse oximetry works

Answer 32

Consists of two light-emitting diodes
o Each light is of a different wavelength - red light (lower wavelength) and infrared
 And a photodetector
 The emitters and photodetector are positioned opposite to each other across a finger
 Oxygenated and deoxygenated haemoglobin absorb different amounts of light as they absorb different light wavelengths.
o Different levels of light reach the photodetector
 Arterial blood pulsates
o This modulates the amount of light that is absorbed
 Other fluids do not pulsate so do not have this effect
 The pulse oximeter isolates the pulsatile portion of the incoming signal (i.e. arterial blood) and from the ratio of red and infrared absorption calculates the relative amounts of oxygenated haemoglobin and deoxygenated haemoglobin
 Hence giving saturation

Question 33

what light does oxygenated Hb absorb in pulse oximetry?

Answer 33

absorbs higher wavelength light (infrared)

so appears red

Question 34

what light does deoxygenated Hb absorb in pulse oximetry?

Answer 34

absorbs lower wavelenght light (red light)

so appears blue

Question 35

where are pulse oximeters found?

Answer 35

	Most wards, espec. resp and cardiology
	Ambulances
	Theatres
	A&E
	ICU
	Home
o	COPD patients on home oxygen
o	Advantages: Easy to use, portable, small in size and weight, runs of battery rather than mains.

Question 36

what happens if patient holds their breath in pulse oximetry?

Answer 36

It won’t alter the reading because of the sigmoid binding relationship of haemoglobin so it takes quite a large fall in oxygen levels before saturation falls

Question 37

why must paitient with pulse oximeter be monitored even if on O2?

Answer 37

Because of the effects of oxygen on respiratory disease (type 2 failure), correct their low O2 that was their drive to breathe, so may stop breathing

Question 38

why does skin pigmentation have no effect on saturations in pulse oximetry?

Answer 38

pulse oximeter only looking at pulsatile part of bflow which is same

Question 39

upper and lower limits of adult HR?

Answer 39

140/55

Question 40

upper and lower limits of O2 saturation in adults?

Answer 40

100/85

Question 41

how do limits for HR and O2 saturationm differ between adults and children?

Answer 41

higher limits for HR in children, but saturations the same

Question 42

effect of exercise on pulse oximetry?

Answer 42

The movement of the person may make it difficult to pick up a regular signal so saturation will alter erratically
o Same principle applies if a patient is fitting
o And if they have removed the pulse oximeter or turned over in bed

Question 43

why can O2 saturation measured in pulse oximetry be less than 100% in a healthy person?

Answer 43

effect of CO binding to Hb

presence of small amounts of abnormal Hbs e.g. in thalassaemia

Question 44

most important factor in getting good results in spirometry?

Answer 44

good technique

Question 45

considerations in spirometry and where are spirometers found?

Answer 45

If technique is good the results will be reproducible
	You need 3 decent recordings 
o	Otherwise disregard
	A word of warning about FVC/FEV1.0
o	If both change the ratio will remain the same
	Home use
o	Heart-lung transplant patients
	Also found in
o	Outpatients
o	On respiratory wards
o	GP surgery’s