For FRC determination, our results show an excellent agreement of the end-expiratory volumes set on the TestChest®, when compared with the gold standard method (CT volumetry). This made it possible to use the TestChest® as a reference volume. Additionally, we demonstrated the accuracy of the TestChest® for the correct simulation of static lung volumes. This may make it an apparently reliable tool for validating volumetric measurement devices. Finally, the FRC determination using the modified N2-Washout/Washin method showed a bias of overestimation of 603 ml. There was a clinically irrelevant influence of the step change of FiO2 on the measurements and a small correction for O2 consumption.
The FRC gives information about the chest wall/lung system because it represents the equilibrium point for the forces of the chest wall and lung and it is a valuable tool for optimizing respiratory settings [9, 19,20,21,22]. Still, it tends to be sidelined behind other parameters, most probably because of technical difficulties in its determination.
The N2-washout/washin technique, initially developed in the beginning of the 1900’s [23], never really gained popularity because of the cumbersome use of mass spectrometry. A later development [24], which relied on O2 and CO2 measurements, needed an unrealistic (and unsafe) step change of 30% in the FiO2 and also a very sensitive flow—gas synchronization. The modified technique from Olegard [10] overcame these limitations by requiring only the small change (10%) of FiO2, without the need for special gas analysers. However, it still showed some degree of FRC overestimation in in vivo settings [10, 12]. Finally, since the EELV relies on a measure of ventilated lung, such measurements can be underestimated in several disease states and in patients with poorly ventilated lung compartments. This is a limitation that may be overcome by the use of a lung simulator.
The TestChest® allows the choice and setting of various parameters of lung mechanics and gas exchange. As such, it may be a valuable tool for teaching and evaluating the performance of devices for physiological measurements. But little is known about its accuracy. The end-expiratory lung volumes that we had set at zero PEEP were almost identical with the CT volumetry. We could, therefore, demonstrate accuracy of the TestChest® for the correct simulation of static lung volumes.
We could also demonstrate that the nitrogen washout/washin technique showed a good correlation with the TestChest®, indicating that the trend of the FRC is trustworthy and can be used in clinical settings. But when we verified the volumes and modified them with the application of PEEP, our results were not in agreement with previous published work. To our knowledge, only one other study [12] compared the nitrogen washin/washout method with the CT scan FRC estimation, in ICU patients. In that study [12], the measurements between the nitrogen washin/washout method and the EELV computed by the CT scan showed a high correlation and a bias of only 100 ml, about 5% of the FRC, when estimated by the formula recommended by the European Respiratory Society [25]. In our case, the bias is much greater, reaching approximately 600 ml, up to one third of the set FRC. While we could reproduce the good correlation between the two measurements, this bias is intriguing. The overestimation of FRC is known for in vivo settings [10, 12], but not in this magnitude. Additionally, the bias is very stable over the measured range of volumes. The TestChest® lacks an oxygen consumption cell. This will lead to an additional 4% of expired oxygen, which artificially dilutes the expired N2, falsely indicating retained nitrogen in the lung. Still, our bias exceeds the expected four percent error by far.
In our setting, the breathing circuit hose had a diameter of 22 mm and a length of 1.6 m, reflecting a volume of 608 ml, which is very close to the given bias. Since gas measurements take place within the ventilator unit and not at the y-piece, apparatus dead space may contribute to the increases in EELV with the N2 method. A third explanation would be incomplete gas mixing in the bellows and a contribution of what is known as the “first-breath problem” [11, 24]. All these explanations are supported by the fact that the bias stays constant over a wide range of measured volumes, therefore representing an artifact of the methodology. Finally, as the TestChest® simulator was set at STPD conditions, the lack of humidified expired air may have introduced a diluting effect on the expired nitrogen. However, our data is the only comparison of the open circuit N2 method with a known volume besides the initial simulation data of Olegard [14].
It has been argued that the absolute value of the FRC is of less interest than its trend during the course of disease or after, for example, a recruitment maneuver [26]. This implies that accuracy is less important than reproducibility. Despite the large bias, changes in end-expiratory volume were tracked very accurately by the N2-washin-washout. The different settings of FiO2 did not influence the accuracy of the measurements.