As discussed on the previous page a comprehensive system for grading the severity of airway obstruction using lung function tests should be applicable in asthma and COPD, take into account complicating restrictive lung disease, and be applicable to any ethnic group. The dimensionless FEV1/VC ratio meets this criterion. Hence, whether there is airway obstruction is decided from either
In adults the above ratio declines with age. Therefore age is taken into account, and the z-score = standard deviation score (SDS) used. In children and adolescents the according to some authors. The FEV1/FVC measurement displaces the FEV1/VC measurement that requires a separate procedure and is more time consuming. The second important reason is that predicted values recently published in vast majority refers to FEV1/FVC, not to FEV1/VC
An FEV1/VC ratio below the age and height adjusted 5th percentile of healthy subjects is regarded as evidence of airway obstruction. It is classified as:
There are several grading systems recommended by various organizations. The above is in agreement with that of the BTS.
The classification into mild, moderate or severe airway obstruction is based solely on spirometric data; it is not based on clinical criteria. The correlation between spirometric and clinical findings is generally poor.
Identification of the type of ventilation disorder
There are four types of ventilation disorders, but only one of them is possible to be diagnosed in a spirometry test. Airway obstruction is characterized by a decrease in FEV1/FVC below the lower limit of normal. The remaining types are:
All the mentioned above require the assessment of volumetric indices, which can not be measured in spirometry (eg TLC - total lung capacity). The result of spirometry can only suggest the existence of restrictive pattern in the absence of obstructive features and reduced FVC, but verification should be performed (lung volumes). It should be noted that non-specific disorders in spirometry will be similar to restrictive pattern. It should be cautioned against the common practice of diagnosis based on a spirometric test of a mixed type of ventilation disorders (coexistence of obstructions and restrictions). In the case of obstruction, the accompanying reduction of FVC is usually not due to restriction, but frequently is associated with hyperinflation and increased RV.
Assessment of the degree of ventilatory disturbances
Because FEV1 is reduced in all types of ventilatory disorders, regardless of their cause (in obstruction and restriction), it is an universal index of lung ventilatory efficacy. Although it has been proposed more than 10 years ago, the FVC not FEV1 value is usually still used in cases of restrictive pattern, even in clinical trials. The use of FEV1 seems to be more reasonable as in cases when both components of disorders occur, the reduction of FEV1 will in some sense be the result of the sums of these influences and processes.
The severity of disturbances is assessed by classifying the FEV1 expressed as %predicted due to arbitrarily determined and specified in the ATS/ERS recommendations intervals corresponding to certain degrees of severity of the disorder (table below from ATS/ERS 2005 Interpretative strategies for lung function tests).
This classification method is also flawed. Due to the dependence of age-related values (lower in older, larger in younger) and distribution pattern in the population, the same percentage (eg. 60%) in young people means much more severe disorder than in older people (where it may even be the limit of normal ). Figure below explains why %pred. also in grading system is biased.
The new simple, easily memorized and clinically valid classification system was proposed (Eur Respir J 2014; 43: 505–512).
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