Differences in compliance and resistance occur throughout the lungs. The compliance and resistance values of one lung unit may be entirely different from those of another unit. Thus the characteristics of the lung are heterogenous, not homogenous.
The differences in C L and Raw affect how rapidly the lung units fill and empty. Each small unit of the lung can be pictured as a small, inflatable balloon attached to a short drinking straw. The volume the balloon receives in relation to other small units depends on its compliance and resistance, assuming that other factors are equal.
Normal lung units fill within a normal length of time and with a normal volume. If the lung unit is stiff (low compliance), it fills rapidly. If pressure is applied to stiff lung unit for the same length of time as to a normal unit, a much smaller volume results.
By further contrast, if the balloon (lung unit) is normal but the straw (airway) is very narrow (high airway resistance), the balloon (lung unit) fills very slowly. The gas takes much longer to flow through the narrow passage and reach the balloon. If gas flow is applied for the same length of time as in a normal situation, the resulting volume is smaller.
A: Filling of a normal lung unit
B: A low compliance unit which fills quickly but with less air
C: Increased resistance; the unit fills slowly if inspiration were to end at the same time as in A, the volume in C would be lower.
The length of time lung units require to fill and empty can be determined. The product of compliance (C L ) and resistance (R) is called a time constant. For any value of C L and R, the time constant always equals the length of time needed for the lungs to inflate or deflate to a certain amount (percentage) of their volume. One time constant allows 63% of the volume to be exhaled (or inhaled); 2 time constants allow about 86% of the volume to be exhaled (or inhaled); 3 time constants allow about 95% to be exhaled (or inhaled); and 4 time constants allow 98% of the volume to be exhaled (or inhaled).
Calculation of time constants is important to setting up a ventilator's inspiratory time and expiratory time. An inspiratory time less than 3 time constants may result in incomplete delivery of the tidal volume. Prolonging the inspiratory time allows even distribution of ventilation and adequate delivery of tidal volume. Five time constants should be used for the inspiratory time, particularly in pressure ventilation, to ensure adequate volume delivery. If the inspiratory time is too long, the respiratory rate may be too low to allow sufficient minute ventilation.
An expiratory time less than 3 time constants may lead to incomplete emptying of the lungs. This can increase the functional residual capacity (FRC) and cause trapping of air in the lungs. Using the 95% to 98% volume emptying level (3 or 4 time constants) is adequate for exhalation.
Exact time settings require careful observation of the patient and measurement of end-expiratory pressure to determine which time is better tolerated.
Lung units can be described as fast or slow. Fast lung units have short time constants. Short time constants are a result of normal or low airway resistance and low (decreased) compliance, such as occurs in a patient with interstitial fibrosis. Fast lung units take less time to fill and empty. However, they may require more pressure to achieve a normal volume. Slow lung units have long time constants, which are a product of increased resistance or increased compliance, or both, such as occurs in a patient with emphysema. These units require more time to fill and empty.