Session 3 Lung function Flashcards
limitations of pulse oximetry?
movement artefacts- movement of patients will alter readings
what index of body size is needed to look up the predicted value for a patient’s FRC?
height
measurement from a spirometer used to measure outcome of a child whilst doing an exercise test to test for exercise-induced asthma?
FEV1.0 or PEFR
drop by 15% normally considered cut off for diagnosis, or 20% if PEFR measured
reasons for pulmonary function tests?
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
potential areas to be investigated in lung function?
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
why might a patient be referred to the lab?
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
describe how simple spirometry is performed
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
what is required in spirometry to prevent within-subject variability?
proper performance of test: trained operator, familiar with equipment- set-up, calibration, operation, and experienced with subjects
incentive to children to ensure that during spirometry, they breath out as far as possible?
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
3 main means of a.flow obstruction in intrathoracic airway obstructive diseases?
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)
why is expiration affected more than inspiration in intrathoracic airways obstruction?
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
why is inspiration affected more than expiration in extrathoracic airways obstruction?
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
causes of variable extrathoracic airways obstruction?
laryngeal polyp
bilateral VC paralysis
describe flow volume curve for a variable extrathoracic airways obstruction
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
flow volume curve for a fixed extrathoracic airways obstruction e.g. carcinoma of larynx, or obstruction by goitre?
inspiratory and expiratory flow-volume curve abnormal
fixed obstruction prevents larger airways distending on expiration, so both curves exhibit flow plateau at low flows
what is an extrathoracic airways obstrucution?
obstruction of trachea above or below VCs
conditions for infant lung function testing?
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
why might peak flow come out the same for a number of different VmaxFRC measurements?
peak flow greater than value able to be measured by the machine
difference between helium dilution and whole body plethysmography for measurement of lung volumes?
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
what does N2 washout measure?
anatomical dead space volume- volume of conducting airways
if helium dilution is used to measure FRC, what is V2 equal to?
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.
how can gas mixing be assessed?
single-breath N2 washout
why do we need predicted values and where do they come from?
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
how is a suitable population selected for measuring predicted values?
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?)
How can suitable methodology be selected for measuring predicted values?
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
how can the appropriateness of reference values in population of children be checked?
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.
pre-requisites for respiratory function testing in children?
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
how is exercise induced asthma tested for in children?
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
describe the process of whole body plethysmography
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
process of helium dilution?
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
process of single breath N2 washout?
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
describe how pulse oximetry works
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
what light does oxygenated Hb absorb in pulse oximetry?
absorbs higher wavelength light (infrared)
so appears red
what light does deoxygenated Hb absorb in pulse oximetry?
absorbs lower wavelenght light (red light)
so appears blue
where are pulse oximeters found?
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.
what happens if patient holds their breath in pulse oximetry?
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
why must paitient with pulse oximeter be monitored even if on O2?
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
why does skin pigmentation have no effect on saturations in pulse oximetry?
pulse oximeter only looking at pulsatile part of bflow which is same
upper and lower limits of adult HR?
140/55
upper and lower limits of O2 saturation in adults?
100/85
how do limits for HR and O2 saturationm differ between adults and children?
higher limits for HR in children, but saturations the same
effect of exercise on pulse oximetry?
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
why can O2 saturation measured in pulse oximetry be less than 100% in a healthy person?
effect of CO binding to Hb
presence of small amounts of abnormal Hbs e.g. in thalassaemia
most important factor in getting good results in spirometry?
good technique
considerations in spirometry and where are spirometers found?
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
