clinical evaluation of respiratory function Flashcards
What is spirometry?
Spirometry is a common method of quantifying vital capacity, airflow, and the level of airway obstruction present during breathing.
How does spirometry work?
how to interpret results?
Spirometry involves the patients producing a maximum forced expiration into a spirometer, which measures the volume of air passing through over time. This is then plotted on a graph from which calculations and interpretations can be made:
What is FEV1?
what does it correspond to?
FEV1 (forced expiratory volume in one second) is a term used to describe the maximum volume that can be expired during the first second of a maximum forced expiration.
It corresponds to how quickly air can pass through the airways and reflects airway function and health.
What is FVC?
what does this reflect?
FVC (forced vital capacity) measures the maximum volume of an individual can exhale in one breath (after inspiring as much air as possible) and reflects the volume of the lungs that the individual can utilise when breathing.
fev1/fvc
what is indicative of obstructive airways disease?
total lung capacity an individual can exhale in the first second (<80% is indicative of obstructive airways disease)
why fev1/fvc ratio?
what are fvc values compared to?
FEV1 values are normalised by expressing as FEV1/FVC ratio (expressed as a decimal or percentage) to take account of the fact that different individuals have varying rates of healthy airflow depending on their vital capacity (an individual with larger lungs and airways will naturally expire air faster).
Similarly, FVC values are compared to expected healthy values for a person of similar age, height and sex
Obstructive airway diseases
what diseases are these usually?
what indicates this?
what is normally unchnaged and why?
what is the net effect?
Obstructive airway diseases, such as asthma and chronic bronchitis, are indicated by a reduction in FEV1/FVC ratio (<70%). FVC is typically unchanged as lung function is unaffected.
FEV1/FVC <70%
FVC >80%
E.g. Asthma
↑Resistance
hence slower airflow but overal airway capacity is the same (just longer to expire)
Restrictive lung diseases
what diseases are these usually?
what indicates this?
what is normally unchnaged and why?
what is the net effect?
Restrictive lung diseases, such as pulmonary fibrosis, are indicated by a reduction in FEV1 and FVC (<80% expected value), with a relatively normal FEV1/FVC ratio (>70%, i.e. the decrease in FEV1 reflects an overall decrease in lung volume rather than airway obstruction).
FVC < 80%
FEV1/FVC > 70%
E.g. Fibrosis
↓Compliance
ratio of fev1/fvc is the same if no obstruction
Basic interpretation of spirometry
what will indicate obstructive? restrictive? a mix?
FEV1/FVC <70%, FVC = Normal → Obstructive respiratory disease
FVC <80% of predicted volume, FEV1/FVC = Normal → Restrictive respiratory disease
FEV1/FVC <70% and FVC<80% → Mixed obstructive and restrictive respiratory disease
Q: What sort of respiratory disease is indicated by the spirometry readings?
FEV1 = 2.4L (predicted = 3.9L) FVC = 4.3L (predicted = 4.7L)
fev1/fvc = 0.56 therfore airway obstruction fvc = 0.91 hence vital capacity is healthy
spirometry indicates that the patient has obstructive respiratory disease (asthma, bronchititis, cystic fibrosis)
Q: What sort of respiratory disease is indicated by the spirometry readings?
FEV1 = 2.7L (predicted = 4.2L) FVC = 3.0L (predicted = 5.0L)
fev1/fvc = 0.9 therefore healthy airway fvc = 0.6 therefore restrictive lung
restrictive lung disease -> pulmonary firbrosis, emphysema)
Lung compliance
what is the relationship between?
This relationship between the change in lung volume produced by a particular changed in transpulmonary pressure is termed ‘lung compliance’, and essentially describes how easily the lungs can be distended .
What does it mean when lung compliance increases/decreases?
when increased, easier to expand but less recoil
when decreased, it is harder to expand
What respiratory diseases are associated with increased/decreased compliance?
increased = degeneration of elastic fibres hence emphysema decreased = pulmonary fibrosis
Static compliance
what part of the graph is used?
when are lung measurements taken?
when airflow is 0, what is the pressure is equal and what is different?
For static compliance (measurements taken whilst airflow = 0), the steepest part of the curve is used
Lung measurments to determine static compliance are taken at specific lung volumes where the patient pauses inspiration at certain point (therefore airflow falls to zero)
when airflow is 0, pressure of atmosphere is equal to that in the alveoli (only difference in pressure is air in lungs/volume hence gradient)
dynamic compliance
how do you determine this?
what do you measure now and why?
To determine dynamic compliance a patient breaths normally at tidal volume – dynamic complaince respresents the gradient of the line from the end of expiration to the end of inspiration.
as the alveoli pressure is changing all the time, you measure intrapleural pressure now
dynamic compliance - graph
what is the size/fatness of the loop proportional to?
what does airway resistance increase with?
The size of the internal area or ‘fatness’ of the loop is proportional to the level of airway resistance. (how much it deviates from the straight line hence how much force is exerted during breathing)
Airway resistance increases with ↑airway obstruction and ↑speed of airflow therfore more force
Obstructive airway disease e.g. asthma
dynamic compliance graphs
what will have chnaged and what will not have chnaged?
why?
gradient may have not changed but the graph will have got fatter due to airway obstruction hence more forced expiratory breathing
the area contained within the loop (the “fatness” of the loop) is proportional to the level of airway resistance generated (for example it would increase with a forced inspiration/expiration and airway obstruction, and would decrease with a very slow inspiration with low flow rate in the absence of airway obstruction.
Change in gradient for dynamic compliance graphs
what does decreased gradient mean? disease?
increased gradient? disease?
Low compliance (stiff) (e.g. pulomnary fibrosis) A greater pressure change change is required to produce the same change in volume (decreased gradient)
High compliance (floppy) (e.g. emphysema) A smaller pressure change is required to produce the same change in volume (increased gradient)
How do the respiratory system and kidneys function together to regulate and maintain blood pH?
what does the kidney regulate? timeframe?
what does the resp system regulate? timeframe?
ph = log (hco3-)/paco2
Renal regulation of HCO3-
E.g. regulating reabsorbtion/ excretion in glomerular filtrate (timeframe = hours to days)
Respiratory regulation of PaCO2
E.g. regulating ventilation (timeframe = minutes)
The lungs & kidneys maintain blood pH homeostasis by regulating PaCO2 & [HCO3-], respectively
how does lungs regulate? what does it do to change pH?
kidneys? what does it change to regulate pH?
↑Ventilation = ↓PaCO2= ↑ pH ↓Ventilation = ↑PaCO2= ↓ pH ↓ HCO3 excretion = ↑[HCO3-] =↑ pH ↑ HCO3 excretion = ↓[HCO3-] =↓pH (unless PaCO2 or HCO3 changes proportionally, in the opposite direction)
Compensation and mixed disorders
what is compensation?
how is it identified?
The respiratory and metabolic systems can (and do) compensate for disruptions in pH caused by each other. Metabolic acidosis and alkalosis can be compensated by respiratory alkalosis and acidosis, respectively. When compensation occurs, it can be identified by a change in PaCO2 or [HCO3-] that takes place in the presence of a pH disruption that runs counter to the expected change:
respiratory compensation
how is it identified?
Increased PaCO2 in the presence of high pH, or decreased PaCO2 in the presence of low pH = respiratory compensation
metabolic compensation
how is it identified?
Increased [HCO3-] in the presence of low pH, or decreased [HCO3-] in the presence of high pH = metabolic compensation