What is the difference between spirometry and chest movement measurements

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Subscripts I and E refer to inspired and expired quantities. If TV e is measured when the gas mixture exits the ET tube, this is the correct reading. The conditions can change further as the gas mixture cools down some degrees before it enters the ventilator.

Thus, in ventilator spirometry, the expired tidal volume would again be different. Even though all the calculated tidal volumes in this example are different, they are all correct, too. Patient spirometry values reflect more accurately what is entering and exiting the ET tube than ventilator spirometry values. This was a simplified example. The effects of airway leaks, compliance of the circuit, and condensation of water were not considered.

To obtain as precise measurements as possible, the measurement site should be selected so that possible sources of error will be minimized. The nearer to the lungs the measurements are made, the more accurate the results are. Indeed, pressure measurement at the caudal end of the ET tube has been shown to be more accurate than measurement in the patient airway 3. However, this technique has not gained wide popularity because it requires an extra catheter inserted into the ET tube. When the measurement site is taken farther away from the lungs, differences between measured quantities and those actually delivered to the lungs arise.

Possible error sources include:. There may be leaks in the circuit between the ET tube and the ventilator. In that case only a part of the gas delivered by the ventilator actually enters the ET tube.

In patient spirometry such leaks can be easily detected. However, leaks can also occur in the patient airway, meaning that a part of the gas does not enter the lungs but escapes through the airway.

The effect of gas conditions was described in the previous section. Technically, this is not an error source as long as the user is aware of the measurement and reporting conditions. All the gas that is fed into the patient circuit does not reach the lungs even when leaks are not present.

Resistance of the tubes causes pressure loss; the situation is analogous to electric potential decrease in a circuit due to electric resistance. If the resistance grows, for example, due to increase in tubing length or decrease in its diameter, the pressure losses also increase. During inspiration, a part of the gas mixture remains compressed in the tubing and does not reach the patient. During expiration, the pressure drops and gas expands, and all the delivered gas flows back to the ventilator.

When a pressure is applied to an elastic tube, the walls will expand and increase the volume of the tube. As a result, a part of the gas delivered by the ventilator remains in the tubing and does not reach the patient. Pressure decrease due to resistance of the tubes, compression of gas in the circuit, and compliance effects all depend on the amount of tubing between the patient and the measurement site.

In patient spirometry the measurements are made just before the gas mixture enters or exits the ET tube. The resulting parameters are, thus, less prone to these error sources. Compression and resistance losses depend on the tubing used. Such losses may be compensated with algorithms, but the adjustments are not always effective.

The configuration of the tubing may vary and the actual tubing may also change. These error sources are not present in patient spirometry. A number of studies have been published on the accuracy of spirometry measurements made in different locations.

Table 1 summarizes some findings on tidal volume parameters. The general finding is that ventilator spirometry produces values that differ substantially from values recorded in patient spirometry. These studies suggest that tidal volumes should be measured at the patient airway, regardless of the compliance compensation.

Al-Majed et al. Heulitt et al reported that the ventilator overestimated tidal volume and, after compliance correction, underestimated it. There is neither theoretical nor empirical evidence that would support the view that ventilator spirometry could be consistently more accurate than patient spirometry. In patient spirometry, less error sources are present. Differences between the methods are most apparent when using small tidal volumes, the lung compliance is low, airway resistance is high, or when the tubing configuration is changed.

Results on pressure parameters in general are more ambiguous. We present preliminary results from our own tests on pressure parameters.

We studied the differences in pressure parameters obtained using ventilator spirometry and patient spirometry. D-lite and Pedi-lite sensors were used in the tests. Ventilator spirometry was obtained using two different ventilators. A neonatal, pediatric, and an adult patient was simulated, using both pressure- and volume-controlled modes. Values obtained from the ventilators and from the respiratory module were compared against reference values obtained from the artificial lung.

We present results with data from both ventilation modes and all simulated patient groups pooled together. In Figure 3, the black bars correspond to patient spirometry errors and the grey bars to ventilator spirometry errors. Our results are in agreement with the theoretical considerations and support the view that using patient spirometry provides, in general, more accurate values than ventilator spirometry. From a theoretical point of view, patient spirometry is less prone to measurement errors that are caused by physical effects in the patient circuit.

The closer to the lungs the measurement is made, the more accurate the obtained parameters reflect the situation in the lungs. There are also published data suggesting that patient spirometry is more accurate than ventilator spirometry. A number of special circumstances exist when patient spirometry is the favored system, for example, using small tidal volumes and patients with increased respiratory system impedance.

Our tests concur with the theoretical considerations and the published data. Patient spirometry proved more accurate than ventilator spirometry in PEEP and peak pressure measurements. However, the test data was recorded in laboratory conditions and covered a wide range of simulated patients, making it hard to draw decisive conclusions based on them.

In this measurement scheme, less error sources are present than in measurements made further away inside the ventilator. See all resources.