## 2 Mechanical Ventilation Modes
### VC-CMV Mode  
In volume-controlled ventilation, the patient receives a pre-set tidal volume (V\({}_{\text{T}}\)) determined by the pre-set inspiratory peak flow rate (I\({}_{\text{pf}}\)) over a set inspiratory time (T\({}_{\text{i}}\)). \[\text{V}_{\text{T}}=\text{I}_{\text{pf}}\ \text{T}_{\text{i}}, \tag{4}\]  
This mode is used when a precise minute ventilation is therapeutically essential to the care of the patient [43]. Minute ventilation is entirely determined by the ventilator settings, as the patient is unable to initiate spontaneous breaths. Therefore, the doctor/clinician selects tidal volume (V\({}_{\text{T}}\)), respiratory rate (RR), and peak inspiratory flow rate (I\({}_{\text{pf}}\)) (independent variables), in addition to PEEP, \(\text{TiO}_{2}\), and pressure airway limit (P\({}_{\text{aw,limit}}\)). Based on these independent variables, the ventilator automatically calculates the dependent variables, namely expiration time (T\({}_{\text{e}}\)), and then inspiration/expiration ratio (I:E) and total time cycle (TCT). \[\text{T}_{\text{e}}=\frac{60}{\text{RR}}-\text{T}_{\text{i}}, \tag{5}\]  
\[\text{I}:\text{E}=\frac{\text{T}_{\text{i}}}{\text{T}_{\text{e}}}, \tag{6}\]  
\[\text{TCT}=\text{T}_{\text{i}}+\text{T}_{\text{e}}, \tag{7}\]  
Airway pressures (peak, plateau, and mean) are not directly adjusted, and are dependent on the equation of motion for the respiratory system through the patient variables (C\({}_{\text{RS}}\), R\({}_{\text{insp}}\), R\({}_{\text{exp}}\)). The correlation between independent, dependent, and patient variables in VC-CMV mode is shown in Figure 2.  
The airway pressure measured during the inspiration pause (absence of flow) is referred to as the plateau pressure (P\({}_{\text{plt}}\)), and is a measure of the alveolar pressure, as the \begin{table}
\begin{tabular}{c c c c} \hline
**Ventilation Mode** & **Trigger Variable** & **Limit Variable** & **Cycle Variable** \\ \hline Controlled & Ventilator & Ventilator & Ventilator \\ Assisted–Controlled & Patient & Ventilator & Ventilator \\ Supported & Patient & Ventilator & Patient \\ Spontaneous & Patient & Patient & Patient \\ \hline \end{tabular}
\end{table} 
Table 2: Classification of the mechanical ventilation modes based on phase variables, adapted from [5]. Figure 2: Independent and dependent variables in VC-CMV mode, adapted from [5]. pressure drop due to airway resistance is zero at zero flow. This is based on the equation of motion for the respiratory system:  
\[\mathrm{P_{plt}}=\mathrm{P_{0}}+\frac{\mathrm{V_{T}}}{\mathrm{C_{RS}}}, \tag{8}\]  
On the other hand, the inspiratory resistance (\(\mathrm{R_{insp}}\)), which can be calculated only in VC-CMV mode when the inspiratory flow pattern is constant (square), allows the machine to evaluate the PIP as follows:  
\[\mathrm{PIP}=\mathrm{P_{plt}}+\ \mathrm{R_{insp}}\mathrm{I_{pf}}, \tag{9}\]  
Mean airway pressure, which is defined as the average pressure at the airway opening over a given time interval [44], is generally higher for pressure-controlled modes than volume-controlled modes (at the same tidal volume) due to the differences in the shapes of the airway pressure waveforms. Indeed, it is evaluated as:  
\[\mathrm{\overline{P}_{aw}}=\mathrm{k(PIP-PEEP)}\bigg{(}\frac{\mathrm{T_{i}}}{ \mathrm{T_{i}}+\mathrm{T_{e}}}\bigg{)}-\mathrm{PEEP} \tag{10}\]  
where (\(\mathrm{k}=1\)) for rectangular waveform and (\(0.5\leq\mathrm{k}\leq 1\)) for other waveforms. Modern ICU ventilators provide graphic displays in two formats: waveforms and loops. Waveform displays show pressure, volume, and flow on the vertical axis, with time on the horizontal axis. Loop displays show one variable plotted against another (flow-volume or pressure-volume). Breaths, during VC-CMV, can be machine-triggered (a) or patient-triggered (b) and the waveform displays are different, as shown in Figure 3.  
The area of the pressure curve under the baseline is proportional to the work the patient does on the ventilator, and the area above is the work the ventilator does on the patient.