## 2 Mechanical Ventilation Modes
### PC-CMV Mode  
Similar to the VC-CMV mode, the PC-CMV mode can be used as a basic mode of ventilatory support. Again, in PC-CMV mode the goal is to maintain an adequate minute ventilation. However, this time the control variable is the airway pressure. Therefore, the doctor/clinician selects airway pressure (P\({}_{\text{aw}}\)), respiratory rate (RR), and inspiration time (T\({}_{\text{i}}\)) (independent variables), from which the machine calculates expiration time (T\({}_{\text{e}}\)), inspiration/expiration ratio (I:E), and, through the patient variables (C\({}_{\text{RS}}\), R\({}_{\text{Insp}}\)), inspiratory peak flow (I\({}_{\text{pf}}\)) and tidal volume (V\({}_{\text{T}}\)) (dependent variables), as shown in Figure 7.  
In PC-CMV mode, inspiratory flow is a consequence of the ventilator's attempt to maintain a pre-set pressure waveform. The inspiratory flow is generated by the pressure gradient between the set pressure and the alveolar pressure (\(\Delta\)P). Its maximum is at the beginning of inspiration when alveolar pressure is equal to PEEP:  
\[\Delta\text{P}=\text{P}_{\text{set}}-\text{ PEEP}, \tag{11}\]  
This \(\Delta\)P maximum allows the inspiratory flow to increase rapidly and to reach its peak (I\({}_{\text{pf}}\)). Subsequently, as alveolar pressure increases gradually during inflation, \(\Delta\)P decreases and, if the inspiration time (T\({}_{\text{i}}\)) is long enough, the inspiratory flow exponentially drops to zero. This is due to the alveolar pressure reaching the value of the pressure set, and  
Figure 6: (a) Linear relationship between pressure and time indicating no recruitment or overdistention; (b) Concave curve indicating overdistention; (c) Convex curve indicating recruitment during the breath. Figure 7: Independent and dependent variables in PC-CMV mode, adapted from [5]. the pressure gradient becomes null. The dynamics of the drop in flow depend on the inspiratory time constant (\(\tau_{\rm i}\)):  
\[\tau_{\rm i}=C_{\rm RS}\;\rm R_{\rm insp}, \tag{12}\]  
Therefore, this time \(\rm P_{\rm i}(t)\) in the equation of motion for the respiratory system becomes constant (\(\rm P_{\rm set}=PIP\)). Using this equation, it is possible to evaluate the tidal volume and the inspiratory and expiratory flow, as follows:  
\[\rm V_{T}=\Delta P\cdot C_{\rm RS}\Big{(}1-e^{-\frac{t}{\tau_{\rm i}}}\Big{)}, \tag{13}\]  
\[\rm\dot{V}_{I}=\frac{\Delta P}{R_{\rm insp}}\Big{(}e^{-\frac{t}{\tau_{\rm i}}} \Big{)}, \tag{14}\]  
\[\rm\dot{V}_{E}=\frac{\Delta P}{R_{\rm exp}}\Big{(}e^{-\frac{t}{\tau_{\rm e}}} \Big{)}, \tag{15}\]  
when in the Equations (14) and (15) the time is zero, then \(\rm\dot{V}_{I}=I_{pf}\) and \(\rm\dot{V}_{E}=E_{pf}\). Again, breaths can be machine-triggered (a) or patient-triggered (b) in this mode, although the waveform displays are different, as shown Figure 8.  
In the PCV mode, the effects of changes in airway resistance and the compliance of respiratory system are very different. Indeed, during inspiration, if \(\rm R_{\rm insp}\) increases, \(\rm I_{pf}\) decreases, \(\rm T_{i}\) is prolonged, and \(\rm P_{\rm plt}\) decreases. If \(\rm R_{\rm insp}\) is very high, \(\rm V_{T}\) decreases. The changes induced by the rise in \(\rm R_{\rm exp}\) during the expiratory phase of the cycle are identical to those observed in the VC-CMV mode. On the other hand, a decrease in \(\rm C_{\rm RS}\) causes the flow  
Figure 8: A display of pressure, volume, and flow waveforms during PC-CMV mode: (**a**) machine-triggered; (**b**) patient-triggered. The theoretical waveforms have been adapted from [44]. to decelerate faster, the inspiratory pause is prolonged, and V\({}_{\text{T}}\) is lowered. This produces faster expiration. All these effects are shown in \[\Delta\text{P}=\text{P}_{\text{set}}-\text{ PEEP}, \tag{11}\]0  
Furthermore, in PC-CMV mode, inspiratory pressure pattern can be a square, a ramp, or an exponential rise. With the square pressure pattern, the inspiratory pressure instantly reaches the PIP and the flow is higher.