Abdominal Pressure
A method similar to that for PES can be used for gastric pressure (PGA) measurements, which will reflect intra-abdominal pressure (PAb). Abdominal pressure can also be measured via a urinary catheter. The bladder is drained of its content after which 50 ± 100 ml of saline is instilled, and the catheter is clamped distal to the pressure measurement position [66]. Measurements of intra-abdominal pressure obtained from a gastric tube and from bladder pressure have been compared, with the two pressures within 2.5 cmH2O [67].
Lung Volume
Measurements of lung volume variations are usually obtained by integrating the digitized flow signal consisting of a series of data points separated by equal time intervals. A simple method for numerical integration is to calculate the area under the curve defined by the series of measured data. Integration drift represents an important problem. When flow is integrated to yield volume, an upward or downward drift in the volume baseline is invariably seen. This is due to physiological reasons and methodological factors including temperature changes between inspired and expired gas, changes in gas composition between inspiration and expiration, leakages, zero offset in flow calibration, and imperfections in the flowmeter response [68]. It is very difficult, if not impossible, to keep under control all these factors and to avoid drift in volume. Consequently, it is never known how much of the baseline drift in volume is due to drift and how much represents a true change in absolute lung volume.
Monitoring of end-expiratory lung volume (EELV) is essential, however, in the management of patients with ARDS [69]. When dilution techniques are used, mostly for research purposes, the patient breathes in a fixed concentration of helium or methane mixed with oxygen and the concentration in the expired breath can be used to calculate the FRC. An alternative approach is a washout/washin technique using nitrogen or oxygen. By changing the FiO2 abruptly by 0.1, the FRC could be calculated with acceptable accuracy by using standard gas monitoring equipment [70, 71, 72]. When application of PEEP determines EELV to increase, EELV changes can be combined with compliance values in order to differentiate between recruitment and distention [73]. Knowing compliance, the expected change in EELV for a given change in PEEP is compared to the measured EELV to estimate alveolar recruitment at the bedside [74].
