Assessing pulmonary function I Flashcards
Discuss the role of predicted values and their limitations
- Predicted values are based on individual demographics:
- standing height i.e. without shoes, as a proxy for chest size
- age:
- somatic growth linked to chronological age except in puberty (height and thoracic volume is disproportionate)
- rigidity of chest wall, chest all muscles and elasticity of lung changes with normal ageing. hence lung function decline
- birth sex - fairly accurate, but does not respect gender identity ^[HRT and lung size being researched]
- Limitations: ethnicity ^[mixing, diet changes etc], paediatric and geriatric demographics can often result in a poor fit of predicted values
- large distribution of values
- in a disease resulting in FEV1 decrease, we want to determine only a 5% margin of error for a healthy individual to be below LLN
- defined by 5th percentile
- LLN: mean +/- 1.64*S (z score)
- 5% of healthy patients will have values below LLN - false positive
- 1/20 normals labelled as abnormal ^[avoid over-interpretation]
Accepting Uncertainty
- overlap in healthy and diseased value distribution
- confidence at LLN is lessened
- use other tools
- Bayesian thinking incorporates existing knowledge to calculate conditional probabilities for more reliable conclusions.
- Clinical information and further tests are often needed for accurate assessment. (LF more of a confirmatory tool).
Global Lung Initiative Predicted Equations Recommended
- Utilizes a mean predicted normal value and upper/lower limits of normal based on height, age, birth sex, and ethnicity.
- Derived from Generalised Additive Model of Location Shape & Scale (GAMLSS).
- Incorporates CoV (SD/mean) and any skewness across all ages.
- Minimizes bias due to age, sex, or ethnicity.
- Predicted sets are matched against similar ethnic groups, instrumentation, and measurement techniques.
- Avoid using 80% of predicted as a cutoff to define disease vs. health. But problematic because:
- Underestimation in younger taller individuals.
- Overestimation in older shorter individuals.
- Holds for both ms and fs
- Cannot declare a result “normal,” only confirm that it is “within normal limits” due to unknown individual starting points.
- Comparing results with previous measurements is more valuable than comparing with predicted values.
- e.g. % reduction compared to before
- Helpful for tracking disease progress and therapeutic interventions.
What is the importance of test quality,factors that influence it, and measures to assess it
?
- Equipment performance criteria, validation, quality control
- Patient manoeuvre (can take time, need adjustments), acceptability of results, and repeatability
- Reference values: clinical assessment and quality assessment (feedback into quality control). Also provide results to scientist, and feeds back into quality control
Interpret a spirometry report, describe its indications and the key values derived
- Measures airflow and lung vital capacity during a forced expiratory manoeuvre from full inspiration.
- A different way of describing:
- how much e.g. by FVC or VC
- how quickly e.g. by FEV1
- aim to detect and quantify extent of airway obstruction or lung restriction
- inspiratory manoeuvres also provide clinical information
- Assesses airway hyperresponsiveness with bronchodilator therapy:
- 4 puffs vi MDI and spacer, wait 1-15 minutes for salbutamol
- helps inform management strategies - Assesses airway hyperresponsiveness with direct and indirect challenges:
- methacholine; ^[false positive risk]
- hypertonic saline, mannitol, cardiopulmonary exercise ^[acccounts for airway inflammation, dries, causes bronchoconstriction –> **indicative of asthma]
Spirometry Definitions
- Forced Vital Capacity (FVC): Maximal volume of air expelled from lungs from maximal inhalation.
- indicates volume of lung
- Forced Expiratory Volume in 1 second (FEV1): Maximal volume of air exhaled in first second of FVC maneuver.
- Reflects the mechanical properties of the large and medium sized airways
- Forced Expiratory Ratio (FEV1/FVC or FER%): Ratio of FEV1 to FVC, expressed as a percentage.
- distinguishes obstruction form possible restriction, when FEV1 is reduced
- Two ways of visualising data: spirogram and flow volume curve
- Spirogram and flow volume curve characteristics include FET, FEF25-75%, PEF, FEF25%, FEF75%.
- Volumes and flows reported at Body Temperature and Pressure Saturated (BTPS).
- spirogram is adjusted
Spirometry Interpretation
- Use FEV1/FVC ratio to detect obstruction.
- Use FEV1 as % predicted to grade obstruction severity.
- Use **FVC to assess restriction:
- low FVC or VC in presence of significant obstruction does not necessarily indicate restriction
- **confirm and quantify with Total Lung Capacity measurement in respiratory lab
- Bronchodilator reversibility: Significant reversibility is a ≥12% improvement in FEV1 (FVC) and an increase of at least 200 ml after bronchodilator therapy.
Interpretation Algorithm
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Is FEV1/FVC less than LLN?
- Yes
- Obstruction
- Severity: Use % predicted FEV1
- Mild: 60% – 80%
- Moderate: >40% – 59%
- Severe: ≤40%
- No
- Is FVC less than LLN?
- Yes
- Restriction
- (Refer for confirmation and diagnosis)
- No
- Normal
- Yes
- Is FVC less than LLN?
Identify different ventilatory defects based on spirometry and static lung volume measurements
Obstructive Disorders (Airflow Limiting)
- Obstructive respiratory disorders reduce airflow due to airway narrowing i.e. an inability to blow out quickly
- Reduced maximum expired flows due to:
- airway lumen narrowing e.g. bronchitis and mucus
- airway wall thickening and inflammation: causing reactive airways e.g. asthma
- loss of lung elastic recoil e.g. in emphysema ^[can take up to 20s to fully exhale] and COPD
Note also: overlap syndrome (Asthma and COPD) ^[ACOS]
Restrictive Disorders (Volume Limiting)
- Prevalence of 9-11% in general population measured by spirometry.
- Can be intrapulmonary or extrapulmonary
- lung: resection, collapse, fibrosis, CHF, thickened pleura, tumour
- pleural cavity: engaged heart, tumour, effusion,
- chest wall and musle: NM disease, paralysed diaphragm, ascites, scleroderma. pregnancy, obesity ^[BMI 40-45], kyphoscoliosis
- muscle: neuromuscular disease ^[MND, muscular dystrophy, myasthenia gravis], old polio, paralysed diaphragm
Ventilatory Defects (Volume-Time)
- Different ventilatory defects include normal, obstructive, and restrictive patterns.
Ventilatory Defects (Flow Volume)
- Spirometry provides measurements for obstructive, restrictive, and mixed ventilatory defects.
What information can you gather from flow volume curve shape?
Coving in OLD
Shrink in RLD
Flow Volume Loop “Abnormal” Examples
Clinical Conditions:
- Variable extrathoracic lesions
- Vocal cord paralysis
- Subglottic stenosis
- Neoplasm (primary hypopharyngeal or tracheal, metastatic from primary lesion in lung)
- Goitre
- Variable intrathoracic lesions
- Tumor of lower trachea (below sternal notch)
- Tracheomalacia
- Fixed lesions
- Fixed neoplasm in central airway
- Vocal paralysis with fixed stenosis
- Fibrotic structures
What is the clinical significance of maximal inspiratory and expiratory pressures?
Respiratory Muscle Pressures
- Assesses respiratory muscle weakness:
- particularly diaphragm “pump” failure.
- assists with treatment need in ventilation
- Non-invasive expiratory and inspiratory pressures.
- Measured at mouth, and sum of pressure of respiratory muscles and passive elastic recoil of the respiratory system
- Maximal Inspiratory Pressure > 80 cmH2O considered normal.
- Large variation in predicted values.
- Learning effect and fatigue are major issues.
- Best 3 measures have to be < 20%
List some indications and contraindications for LFT
- Diagnosing and assessing patients with respiratory symptoms (cough, wheeze, phlegm, dyspnoea)
- Assessing smokers, those exposed to pollution, and those with family history of respiratory disease
- Differentiating between respiratory and cardiac causes of breathlessness ^[exam questions]
- Screening high-risk populations for respiratory disease
- Evaluating treatment response
- **Pre-operative risk assessment for anesthesia and abdominal/thoracic surgery (e.g. pulmonary exercise tests)
Absolute contraindications for testing:
- Harmful effects on thoracic and abdominal tissues due to maximal pressures generated in thorax impact (discomfort) ^[caution if aneurysm, risk of rupture]
- impact on BP: large swings causing stresses on other tissues e.g. limbs, head e.g. dizziness and blackout
- active communicable diseases
- expansion of chest wall and lungs
Risk assessment has four categories:
- DO NOT PROCEED
- Proceed, with extreme caution
- Proceed, with caution
- Typically safe
Describe effect of aging on lung volume changes
Ageing causes:
- Loss of lung elastic recoil and alveolar attachments both conspiring to increase the volume associated with the start of small airways closure
- Reduced chest wall compliance due to rib cage stiffening balances the inward recoil of the lung at higher FRC
obs
Discuss how TLC is interpreted
- Comparison of TLC against LLN and cut-off: severity based on % predicted.
- < LLN and 70% of predicted: Mild restrictive ventilatory defect
- < 70% and > or = to 60%: Moderate
- < 60% and > or = to 50%: Severe - TLC > upper limit of normal may indicate lung hyperinflation.
- Consider larger than expected lung subdivisions in interpretation.
- Air trapping defined as disproportionate increase in residual volume (RV) or RV/TLC ratio.
- RV/TLC ratio of > 120%: Mild air trapping
- RV/TLC ratio of > 140%: Moderate
- RV/TLC ratio of > 160%: Severe
Describe how static lung volumes are measured
**
Measured by:
- Body Plethysmography (Gold standard)
- Inert gas dilution (Helium or Nitrogen Washout)
Parameters:
- Measured:
- Functional Residual Capacity (FRC): Represents balance of lung and chest wall elastic recoil properties.
- Inspiratory Capacity (IC) or Expiratory Reserve Volume (ERV)
- Vital Capacity (VC)
- Derived:
- Total Lung Capacity (TLC): Lung dominates recoil pressure
- Residual Volume (RV): Chest wall dominates associated recoil pressure
- RV/TLC ratio
Body plethysmography
- Tidal breathing is determined at FRC, a shutter is closed and the patient gently inhales and exhales (pants) against the closed shutter for 3 seconds
- The expansion and decompression of the air in the lungs causes small volume (change in box volume) and pressure changes in both the lungs and the body plethysmograph
- FRC is derived using Boyles Law (P1.V1 = P2.V2) from mouth pressure (change in alveolar pressure) and box pressure
- IC and VC are measured, to derive TLC and RV respectively
Describe bronchial provocation testing
- Airway irritability can be increased in asthma: spirometry may be within normal range, without reversibility to bronchodilators
- Assessment of airway irritability or hyper-responsiveness using spirometry (esp % fall in FEV1) confirms diagnosis.
- Results describe provocation dose and grade severity of disease.
- Methods:
- Mannitol Challenge
- Methacholine
- Hypertonic saline
- Exercise
- Eucapnic hyperventilation
- PD15 = 155 mg (15% fall in FEV1)
