## 2 Mechanical Ventilation Modes  
A mode of mechanical ventilation may be defined, in general, as a predetermined pattern of patient-ventilator interaction [41]. In respiratory mechanics, it is useful to think of patient-ventilator interactions in terms of a mechanical system consisting firstly of a resistive element (a rigid tube) followed by an elastic element (a balloon), which in turn are connected to a mechanical ventilator (a piston), as shown in Figure 1.  
An in-series mechanical arrangement means that at any time (t), the pressure that is applied to the tube inlet (the airway pressure, \(P_{\text{i}}(\text{t})\)) is equal to the sum of three pressures, the initial alveolar pressure at the beginning of the inspiration (\(P_{0}\)), which can be atmospheric pressure or greater than atmospheric pressure (called positive end-expiratory pressure, PEEP), an elastic pressure (\(P_{\text{el}}(\text{t})\)), and a resistive pressure (\(P_{\text{res}}(\text{t})\)):  
\[P_{\text{i}}(\text{t})=P_{0}+P_{\text{el}}(\text{t})+P_{\text{res}}(\text{t}), \tag{1}\]  
The elastic pressure is the pressure needed to expand the lungs and chest wall, whereas the resistive pressure is that needed to generate a certain gas flow rate within the airways. These two pressures can then be expressed as a function of the parameters of respiratory mechanics, namely the elastance of the respiratory system (\(E_{\text{RS}}\)) and the airway resistance to flow (\(R_{\text{insp}/\text{exp}}\)). Therefore, \(E_{\text{RS}}\) reflects the elastic characteristics of the respiratory system and is the inverse of the compliance of the respiratory system, \(C_{\text{RS}}=1/E_{\text{RS}}\). The inspiratory resistance (\(R_{\text{insp}}\)) describes the opposition to a flow of gas entering the respiratory system during inspiration, and is primarily made up of the resistance of the airways and the endotracheal tube. The expiratory resistance (\(R_{\text{exp}}\)) is higher than the inspiratory resistance due to the shape of the airway tree. Therefore, assuming linear system behaviour, as shown in [7], and knowing the parameters of respiratory mechanics, the inlet pressure-time profile can be computed for any piston stroke volume (V) and flow (\(\dot{V}\)) setting:  
\[P_{\text{i}}(\text{t})=P_{0}+\frac{V(\text{t})}{C_{\text{RS}}}+R_{\text{insp }/\text{exp}}\ \dot{V}(\text{t}), \tag{2}\]  
Let us then move on to the concepts of breath and breath sequence. A breath, which can be mandatory, assisted, or spontaneous, is one cycle of positive flow (inspiration) and negative flow (expiration) defined in terms of the flow-time curve. A mandatory breath is a breath in which the start or end of inspiration (or both) is determined by the ventilator,  
Figure 1: Mono-compartment model of the respiratory system connected to ventilator using mechanical analogues. independent of the patient's condition. An assisted breath is a breath in which the ventilator does some portion of the work of breathing. A spontaneous breath is a breath in which the patient him/herself initiates and finishes the inspiration, independent of any ventilator settings for inspiratory and expiratory times. Thus, based on these types of breath, we can speak of different types of breath sequence, namely continuous mandatory ventilation (CMV), continuous spontaneous ventilation (CSV), and intermittent mandatory ventilation (IMV). The former is also commonly known as assist control (when the breaths are patient-triggered), and is a breath sequence in which spontaneous breaths are not permitted between mandatory breaths. The second is a breath sequence in which all the breaths are spontaneous. The latter is a breath sequence in which spontaneous breaths are allowed between mandatory breaths [41].