13. Lung function measurement Flashcards
How would you assess and measure a patient’s lung function?
The aim of pre-operative lung function testing to identify patients at high risk of perioperative pulmonary complications, in order to try and reduce these risks through patient preparation, targeted anaesthetic, and surgical techniques and planning for the appropriate level of post-operative care (e.g. HDU/ITU).
If patient risk is high and the risk cannot be reduced, the risk–benefit ratio for surgery needs to be carefully evaluated and a decision has to be made as to whether to proceed.
Evaluation of a patient’s pulmonary function requires correlation of history, examination findings and relevant investigation results in conjunction with the nature of proposed surgery.
Clinical
History and examination findings ma
y provide valuable information about
a patient’s respiratory function.
Pertinent history should include
history of
pre-existing lung disease (e.g. COPD, asthma, pulmonary fibrosis),
smoking history (quantified via pack-year history), exercise tolerance,
respiratory symptoms (cough, sputum production, wheeze),
number and frequency of hospital admissions with respiratory problems,
and current treatment regimen
(e.g. bronchodilators, steroids, supplemental oxygen).
Risk factors for pulmonary complications include the following
Patient factors:
> Age >70 years
History of lung disease
BMI >30
Smoking history >20-pack year
Surgical factors:
> Upper abdominal surgery
> Thoracic surgery
> Open vs. laparoscopic procedures
Investigations
Investigations should be targeted to the patient,
i.e. based on clinical assessment and the
nature of the planned surgery.
Investigations that will have
little or no clinical impact should be avoided.
For example, a CXR in a 20-year-old ASA1 patient presenting for femoral hernia repair would be
inappropriate
However, a CXR may well be indicated in a 20-year-old patient with cystic fibrosis presenting for the same procedure.
Peak expiratory flow rate (PEFR) >
Provides a simple method to measure airways obstruction, particularly in asthmatic patients
.
> Normal range values are dependent on age, sex and height.
20-year-old female height 1.60 m PEFR 433 L /min
20-year-old male height 1.83 m PEFR 654 L /min
Arterial blood gas (ABG) analysis
> Allows evaluation of gas exchange by providing essential information about the state of oxygenation, acid–base balance, chronicity and severity of respiratory failure.
> It is useful to have baseline arterial blood gases, especially for patients
undergoing surgery,
where there will inevitably be perioperative
changes in gas exchange,
as it allows for easier interpretation
of these subsequent changes.
Spirometry >
This is the timed measurement of
dynamic lung volumes
during forced expiration and inspiration.
> Measurements –
forced vital capacity (FVC),
forced expiratory volume in one second (FEV1)
and the ratio of these two volumes (FEV1/FVC).
> Measurement of maximum expiratory flow
over the middle 50% of the vital capacity (FEF25–75%) is a sensitive index of small airway function.
> Measurements of forced maximal flow during expiration and inspiration flow can be made as a function of volume thus generating a flow volume loop, the shape of which also contains information of diagnostic value.
Interpretation of spirometry data
> The presence of ventilatory abnormality
can be implied if any of
FEV1, FVC or FEV1/VC ratios are
outside the reference ranges.
> Interrelationships of the various measurements are important diagnostically:
• FEV1/FVC <80%
obstructive ventilatory defect
(e.g. asthma/COPD)
• FEV1/FVC >80%
constitutes a restrictive ventilatory defect
(e.g. pulmonary fibrosis/kyphoscoliosis).
> It is routine practice to quantify the
degree of reversibility of an
obstructive defect by measuring spirometry
before and after the administration of a bronchodilator.
Generally, an improvement in
FEV1 of 200 mL or more
infers significant reversibility
if the baseline FEV1 is <1.5 L ,
as does an improvement of >15%
if the FEV1 is >1.5 L .
Flow volume loops
> Constructed from spirometric data.
Note that expiratory flow is above the x-axis,
whereas inspiratory flow is represented
below the x-axis.
> Analysis of the flow volume loops can be diagnostic (see Fig. 13.1).
Cardiopulmonary exercise testing (CPET)
> Non-invasive and objective method
of evaluating both cardiac and pulmonary functions.
> Cycle ergometry is the
most common mode of exercise.
> Safe procedure,
with a risk of death between 2 and 5 per 100 000
exercise tests performed.
> Computerised test provides a
breath-by-breath analysis of respiratory
gas exchange at rest
and during a period of exercise,
the intensity of which is increased incrementally
until symptoms limit testing or the
patient reaches maximal levels.
> Information on airflow, O2 consumption, CO2 production and heart rate is collected and used for computation of other variables such as oxygen uptake and the anaerobic threshold.
> Primarily determines if the patient
has normal or reduced maximal
exercise capacity ( VO2 max).
Reduced VO2 max can further suggest
probable causes.
> Used to define which organ systems (pulmonary or cardiac) contribute to a patient’s symptoms of exertional dyspnoea and exercise intolerance and to what extent.
> The anaerobic threshold may
also be measured using CPET.
> More sensitive for detecting early or subclinical disease and is establishing a key role in pre-operative assessment of high-risk patients undergoing major surgery.
Respiratory muscle strength
> Assessed globally by
measurement of maximum mouth pressures.
Maximum inspiratory mouth pressure measurements reflect the force generating
capacity of inspiratory muscles.
> Measurements are taken during maximum inspiratory effort against an occlusion at residual volume (where mechanical advantage of inspiratory muscles is greatest)
or at FRC.
Carbon monoxide diffusing capacity
DLco or transfer factor
> Measure of the ability of gas
to transfer from alveoli to red blood cells
across the alveolar epithelium
and the capillary endothelium.
> Depends not only on the area
and thickness of the blood–gas barrier
but also on the volume of blood in the pulmonary capillaries.
The distribution of alveolar volume and ventilation
also affects the measurement.
> Measured by sampling end-expiratory gas for carbon monoxide (CO) after a patient inspires a small amount of CO, holds their breath and exhales.
> Measured DLco should
be adjusted for alveolar volume
(which is estimated from dilution of helium)
and the patient’s haematocrit.
> DL co is reported as mL/min/mmHg
and as a percentage of a predicted value
