Conventional Mechanical Ventilation
There are 174 methods of positive pressure ventilation (1), yet in the 50-plus years since positive pressure ventilation was introduced, the only method that has improved clinical outcomes is low-volume lung protective ventilation (see later), which uses less than traditional levels of ventilatory support (2). What this means is that positive pressure ventilation is much more complicated than it needs to be, and “less is better” (3).
This chapter describes six basic methods of positive pressure ventilation (volume control, pressure control, pressure-support, assist-control, intermittent mandatory ventilation, and positive end-expiratory pressure). These six methods should be sufficient for providing ventilatory support in a majority of patients.
I. Methods of Lung Inflation
A. Volume vs. Pressure Control
There are two basic modes of mechanical ventilation, based on the method used to inflate the lungs. These two methods are depicted in Figure 19.1.
With volume control ventilation (VCV), the inflation (tidal) volume is preselected, and the lungs are inflated at a constant flow rate until the desired volume is delivered. The inspiratory flow rate is adjusted so that the
time for lung inflation is no more than one-third of the respiratory cycle (i.e., I:E ratio of 1:2).
With pressure control ventilation (PCV), the inflation pressure is preselected, and high flow rates are used at the onset of lung inflation to achieve the desired inflation pressure quickly. The flow rate decelerates during lung inflation, and the inspiratory time is adjusted to allow the flow rate to fall to zero at the end of inspiration.
 FIGURE 19.1 Pressure and flow changes during a single ventilator breath with volume control and pressure control methods of lung inflation, at equivalent inflation (tidal) volumes. Changes in airway pressure (Paw) indicated by the solid lines, and changes in alveolar pressure (Palv) indicated by the dashed lines. I = inspiration, E = expiration. |
B. Airway Pressures
Note in Figure 19.1 that the airway pressure (Paw) at the end of inspiration is higher with volume control, but the alveolar
pressure (Palv) at end-inspiration is the same with both methods of lung inflation. This is explained below.
pressure (Palv) at end-inspiration is the same with both methods of lung inflation. This is explained below.
 FIGURE 19.2 Airway pressure profile during volume control ventilation with an inspiratory hold maneuver. See text for explanation. Palv = alveolar pressure, Pres = pressure needed to overcome airway resistance, Pel = elastic recoil pressure (lungs and chest wall). |
With VCV, the airway pressure at the end of inspiration (peak pressure) is the pressure needed to overcome both airway resistance, and the elastic recoil force of the lungs and chest wall. These two components can be separated by briefly holding the inflation volume in the lungs, as demonstrated in Figure 19.2.
During the “inflation hold” maneuver (which typically lasts one second), the peak pressure drops to a steady “plateau” pressure. The difference between the peak and plateau pressure is the pressure needed to overcome airway resistance (Ppeak − Pplateau = Pres), and the plateau pressure is the elastic recoil pressure of the lungs and chest wall (Pplateau = Pel).
Since there is no airflow during the inflation hold ma-neuver, the plateau pressure is equivalent to the alveolar pressure at the end of inspiration (Pplateau = Palv).
With PCV, there is no airflow at the end of the inspiration, so the airway pressure at end-inspiration is equivalent to the alveolar pressure (end-inspiratory Paw = Palv).
C. Alveolar Pressure
The alveolar pressure at the end of inspiration represents the following:
It is the elastic recoil pressure of the lungs and chest wall (Pel in Figure 19.2); as such, it can be used to compute the compliance (C) of the thorax (lungs and chest wall) at a particular tidal volume (VT); i.e.,
The normal thoracic compliance is about 50 mL/cm H2O.
Diffuse infiltrative lung diseases, like the acute respiratory distress syndrome (described in Chapter 17), cause a marked decrease in lung compliance (e.g., to <20 mL/cm H2O), and monitoring compliance can be useful in following the clinical course of such diseases.
It is a reflection of the stress imposed on the walls of the alveoli by the inflation (tidal) volume. An increase in the end-inspiratory alveolar pressure to >30 cm H2O creates a risk for stress fractures in the alveolar-capillary interface, which results in ventilator-induced lung injury (described in Chapter 17, Section II-A) (2,4). Alveolar injury from overdistension is called volutrauma.
It is a reflection of the tendency for overt alveolar rupture, with escape of air into the lung parenchyma or pleural space (i.e., barotrauma).
D. Which Method is Preferred?
Either method of lung inflation can be used effectively, but the following points deserve mention.
One advantage of VCV is the ability to maintain a constant level of alveolar ventilation, despite changes in the mechanical properties of the lungs. With PCV, alveolar ventilation will decrease if there is an increase in airways resistance (e.g., from secretions) or a decrease in lung compliance (e.g., from atelectasis or worsening of infiltrative lung disease).
Another advantage of VCV is the ability to use the lung protective ventilation protocol (see later).
A major advantage of PCV is patient comfort, which promotes synchronous breathing with the ventilator and reduces the work of breathing (5). This has been attributed to the high initial flow rates used during PCV (which are more likely to match the high flow demands of patients with respiratory failure), and the decelerating flow pattern (which promotes more even ventilation of the distal airspaces). A decelerating flow pattern is available for VCV, and has been shown to improve patient comfort (6).
Another stated advantage of PCV is the lower peak airway pressures. However, as shown in Figure 19.1, the end-inspiratory alveolar pressure is the same with PCV and VCV (at the same tidal volume), so the lower peak airway pressures with PCV do not reduce the risk of alveolar overdistension and lung injury. This only occurs when the tidal volume is reduced during PCV.
II. Assist-Control Ventilation
Assist-control ventilation (ACV) allows the patient to initiate a ventilator breath, but if this is not possible, ventilator breaths are delivered at a preselected rate. The ventilator breaths during ACV can be volume-controlled or pressure-controlled.
A. Triggers
Two examples of a ventilator breath during ACV are shown in the upper panel of Figure 19.3.
 FIGURE 19.3 Airway pressure patterns in assist-control ventilation (ACV) and synchronized intermittent mandatory ventilation (SIMV). See text for explanation. |
The ventilator breath on the left begins with a negative pressure deflection, which represents a spontaneous inspiratory effort by the patient. This is a patient-triggered ventilator breath.
The ventilator breath on the right does not begin with a negative pressure deflection, indicating the absence of a spontaneous inspiratory effort by the patient. This is a time-triggered ventilator breath, which is delivered at a preselected rate.Related posts:
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