Pulmonary fibrosis (PF) is a disease affecting the respiratory system. It is characterized by the thickening and stiffening of lung tissue, leaving scar tissue in the process.
There is limited information about factors affecting mortality in people with PF, and there are currently no models that can reliably predict the risk of mortality in individual patients. However, a number of physiological variables on lung function have been used to assess disease severity and predict survival.
What are physiological predictors?
Physiological factors are the characteristics that a person is born with, such as inherited characteristics like eye color or height.
A series of physiological variables are used to assess disease severity and predict survival in pulmonary fibrosis. The physiologic factors most consistently associated with the prognosis of PF are forced vital capacity (FVC), total lung capacity (TLC), and diffusing capacity of carbon monoxide (DlCO).
In addition, tests such as spirometry, lung volume and gas exchanged are also used to assess physiologic predictors.
Physiological index to predict survival
To assess the combined effect of each individual predictor, a composite physiological index has been developed and is often used. The formula correlates better with disease extent by computed tomography (CT) than only one individual pulmonary function test and is a more accurate predictor of survival.
The index is useful to predict prognosis by measuring changes over time. Research has shown that six- to 12-month changes in FVC and DlCO are highly predictive of ulmonary fibrosis and become more predictive over time.
Clinically significant changes in FVC and DlCO have typically been considered greater than 10 percent in FVC and 15 percent in DlCO. However, even marginal declines in FVC at six months have been linked to a higher risk of mortality. In DlCO, only changes greater than 15 percent were predictive of mortality risk.
Exercise tests to assess physiological severity
Exercise testing is another method for assessing physiological severity of lung disease. This method is more sensitive in detecting abnormalities in oxygen transfer.
As a group, PF patients show limits in exercise tolerance, with a decreased maximal work load, elevated ratio of physiologic dead space over tidal volume (VD/VT) – expressing the ratio of dead-space ventilation (VD) to tidal ventilation (VT) – and abnormal gas exchange.
In fact, exercise gas exchange has been shown to be a sensitive parameter to predict the clinical course of PF: patients with an oxygen volume of less than 8.3 ml/kg per minute at baseline were found to have an increased risk of death.
The six minutes walk test (6MWT), which measures the distance a patient can walk in six minutes, has also become one of the most widely used exercise tests, given its ease of administration and reproducibility.
In addition, the 6MWT allows clinicians to measure heart rate recovery after one minute of rest. Abnormal rates might also indicate a powerful predictor of mortality.
Both distance walked and desaturation have been found to predict mortality in pulmonary fibrosis, but a composite of the two measurements predicted mortality better than the measures taken alone.
Two other exercise tests, the 15-step and the four-minute step tests, add evidence to the usefulness of measuring desaturation during exercise testing for predicting mortality in PF.
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