Volume displacement spirometer

Gas volumes may be measured directly by using one of several volume displacement spirometers. The simplest and oldest design, the water-sealed spirometer, uses a hollow cylinder, or bell, which is inverted and lowered into a bucket of water containing a tube for the movement of gas (Fig. 21.4). The bell rises or lowers as gas moves into or out of the gas space trapped between the bell and the... 

A similar approach is used in the dry-seal spirometer. In this case, the bell is sealed to its base with a thin layer of latex (or some other thin and flexible material). As gas is introduced into the bell, the latex prevents its escape and forces the bell to move, as with the water-sealed spirometer. Dry-seal spirometers may be mounted horizontally, and may employ a moving piston instead of a moving bell. Manual and electrical recording are achieved as in the water-sealed spirometer. A diird type of volume displacement spirometer, somewhat less common recently, is the bellows, or wedge, spirometer. In this device, the gas to be measured is contained within a bellows whose expansion is recorded via a pen or a rotational potentiometer. 

Thus, any device capable of measuring either volume or flow can also report the other, given an appropriate time measurement and the needed processing. For this reason, and others given below, flow measuring devices have become popular methods for measuring both volumes and flows (although volume displacement spirometers also can easily calculate and report flow measurements). 

This test requires that some of the gas be circulated through a CO2 removal canister to prevent exhaled CO2 from rising to uncomfortable or dangerous levels during the rebreathing period. Oxygen is added periodically to maintain a constant total system volume at end expiration. Once FRC is obtained, TLC and RV may be calculated, after the patient performs one or more VC maneuvers, as described earlier. The helium dilution system, since it is a closed-system test, is almost always performed using a volume displacement spirometer. It also requires the use of a helium analyzer. 

A third method for measuring lung volumes uses a device known as a body plethysmograph, also referred to as a body box. The body box is a large rigid-walled structure in which the patient is seated, after which the door is closed and sealed completely. In one variety, a small hole in the wall of the cabinet leads to a spirometer or flow sensor. Respiratory efforts within the box cause changes in volume to be recorded on this spirometer as chest wall movement displaces air within the box. In a second variety of body box, there is no hole in the wall of the box and respiratory efforts instead... 

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