Dynamic Methods

Compared to static or quasi-static methods, dynamic methods are continuous and can be used as ventilator treatment is applied. Therefore, they are more methods of monitoring rather than of measurement of the mechanical properties of the respiratory system, and their lack of accuracy is counterbalanced to some degree by their capacity to follow trends.

In the past, various methods have been proposed to analyze respiratory mechanics during ongoing mechanical ventilation [31, 32, 33, 34]. The term “dynamic compliance” refers to the calculation of compliance under these conditions and is calculated by dividing the tidal volume by the peak airway pressure (minus PEEP). Dynamic compliance, so calculated, will always be lower than static compliance, as the former includes resistive pressure components, and therefore the term is incorrect. Therefore, methods that consider also the nonlinear mechanical behavior of the respiratory system [35, 36] are preferable.

Among these methods, in the SLICE method [37, 38, 39], the tidal volume of each breath is divided into different volume slices of equal size (Fig. 14.2). Multiple linear regression is then performed for each slice separately, solving the equation of motion (eq. 14.4). Flow-dependent pressure decrease along the artificial airway is also corrected.

A common aspect of all dynamic methods is that a specific model of the respiratory system is considered, and the mechanical properties are estimated in real time by best fitting the model with the data typically available in real time (typically, pressure, and flow) using different algorithms. A wide range of physiologically and clinically relevant models of respiratory mechanics and parameter estimation algorithms and methods have been proposed [4, 40-45], ranging from very simple, like the equations of motion described in the previous section of this chapter, to complex descriptive models. Simpler models are limited in their abilities to describe all respiratory mechanics, while more complex models can suffer from non-identifiability [46].

The dynamic methods are able to analyze the functional mechanics of the respiratory system under its actual ventilatory pattern as chosen by the caregiver. If any of the settings is being varied (e.g., during an incremental PEEP trial [39]), the dynamic analysis may serve to monitor the resulting changes in respiratory mechanics. Although these important advantages are offered by dynamic methods, only static and quasi-static methods are currently used in the clinical practice on ARDS patients. Static measurements are generally considered ideal, and the ventilator settings are based on them. Typically, ventilation is set with an end-inspiratory pause giving quasi-static conditions, so that an adequate plateau pressure indicates the maximal alveolar pressure. Intermittent application of a prolonged hold in expiration gives information about intrinsic PEEP.

Nevertheless, it is still an open question whether static measurements are truly superior to dynamic measurements. In fact, there is a discrepancy between the static methods used for research and the dynamic measurements of respiratory mechanics which are obtained under real clinical condition.

Very probably, the development of robust methods able to reliably minimize the effects of endotracheal tube and airway resistance will determine a wider use of dynamic methods in the future.