Today’s Morning Report from second-year fellow Elen Gusman featured a case of dyspnea with HRCT and PFT findings of air trapping without other parenchymal abnormalities. Below are some of the take-away points from her engaging discussion.
Air trapping – areas of lung parenchyma with less than normal increase in attenuation and lack of volume reduction with expiration (Fleischner Society, 2008)
Image – isolated air trapping due to bronchiolitis obliterans Source: Annals ATS 2014; 11(6):874-881
Expiratory findings:
- Heterogenous hypoattenuation (air trapped) alongside hyperattenuation (normal ventilation)
- Areas of air trapping do not decrease in volume like adjacent normal lung
This was an interesting case. Peter mentioned the term “time constants”, with which many many not be familiar. The time constant is the resistance of a lung region multiplied by the compliance. You will notice the units (R= cmH2O/L/s and C=L/cmH2O) of this multiple is seconds, an estimate of the time to fill or empty.
Emphysema increases the time constant with increased airway resistance and increased compliance. Asthma increases resistance alone, also increasing the time constant. Normal young lungs define the lower limit of time constants, bullae define the upper limits. Lung units with different time constants are the physiologic basis for the heterogeneity Ellen described in expiratory flow and also form the physiologic basis for pendulluft–the exchange of gas between lung regions during respiration. Usually the time constant is short (well less than a second), which is why helium dilution lung volumes measured during a DLCO maneuver approximate TLC in patients without obstructive lung disease.
Thanks for your comment, Scott, and for expanding on ‘time constraints’, a topic with which I was not previously familiar.