The flow, pressure, and volume waveforms produced with a constant flow pattern are shown in Figure 9-2. This is often called square-wave or rectangular-wave ventilation due to the shape of the flow waveform. With the constant flow pattern, the volume (per unit time) is delivered into the lungs equally throughout inspiration. In other words, volume (per unit time) delivery into the lungs at the beginning of inspiration is the same as that at the end of inspiration. Note that airway pressure increases linearly throughout inspiration, following an initial rapid pressure increase due to the resistance through the endotracheal tube. The effect of resistance and compliance on the airway pressure waveform during constant flow volume ventilation is shown in Figure 9-3.
Flow, pressure, and volume waveforms with constant flow, volume-controlled ventilation.
Airway pressure waveforms during constant flow volume ventilation. In each case, the tidal volume is 0.675 L, flow is 40 L/min, and PEEP is 5 cm H2O. The heavy line represents airway pressure and the lighter line represents alveolar pressure. (A) Resistance of 5 cm H2O/L/s and compliance of 50 mL/cm H2O. (B) Resistance of 5 cm H2O/L/s and compliance of 20 mL/cm H2O. Compared to the left panel, peak inspiratory pressure increases, alveolar pressure increases, but the difference between airway pressure and alveolar pressure does not change. (C) Resistance of 20 cm H2O/L/s and compliance of 50 mL/cm H2O. Compared to the left panel, peak inspiratory pressure increases, alveolar pressure is unchanged, and the difference between airway pressure and alveolar pressure is increased.
During volume-controlled ventilation, the inspiratory flow can also be set to a descending ramp. With this pattern, flow is greatest at the beginning of inspiration and decreases to a lower flow at the end of inspiration. Typical flow, pressure, and volume waveforms with a descending ramp are shown in Figure 9-4. Note that most of the tidal volume is delivered early during inspiration and the pressure waveform approaches that of a rectangular shape. A descending ramp flow pattern lengthens the inspiratory time unless the peak flow is increased. The descending ramp flow can be provided in several ways (Figure 9-5). With a complete ramp, flow decreases to 0 at end inspiration. With 50% ramp, the flow at end inspiration is half of the initial flow. Flow can also taper to a fixed, manufacturer-specific level at the end of inspiration (eg, 5 L/min).
Waveforms for descending ramp and constant flows. Note the differences in the shape of the pressure waveform and peak inspiratory pressure.
Full and partial descending ramp flow with volume-controlled ventilation.
Typical pressure, flow, and volume waveforms during pressure ventilation are shown in Figure 9-6. Note the shape of the pressure waveform and descending (exponential) flow pattern. Also note that most of the tidal volume is delivered early in the inspiratory phase. During pressure-controlled ventilation, flow is determined by the pressure applied to the airway, inspiratory effort, airways resistance, and the time constant (Figure 9-7):
Flow, pressure, and volume waveforms during pressure-controlled ventilation.
During pressure-controlled ventilation, the inspiratory flow pattern is determined by airways resistance and respiratory system compliance. (A) Airways resistance of 10 cm H2O/L/s and respiratory system compliance of 20 mL/cm H2O. The inspiratory time is 1.5 seconds and the resulting tidal volume (the area under the flow curve) is 400 mL. (B) Airways resistance of 20 cm H2O/L/s and respiratory system compliance of 50 mL/cm H2O. The inspiratory time is 1.5 seconds and the resulting tidal volume (the area under the flow curve) is 775 mL.
where ΔP is the transpulmonary pressure (difference between airway pressure and pleural pressure), R is airways resistance, t is the elapsed time after initiation of the inspiratory phase, e is the base of the natural logarithm, and τ is the product of airways resistance and respiratory system compliance (the time constant of the respiratory system). The length of zero flow time at the end of inspiration is determined by the inspiratory time; a longer inspiratory time results in more zero flow time.
With many ventilators, it is possible to adjust the time required to reach the peak inspiratory pressure (rise time). The rise time controls the flow at the beginning of the inspiratory phase (Figure 9-8). With a faster rise time, flow is greater at the beginning of inspiration, which may improve synchrony in patients with a high respiratory drive, but at the cost of a higher VT.
Flow, pressure, and volume waveforms for pressure support ventilation.
An inverse ratio can be used in conjunction with pressure-controlled ventilation. This results in pressure-controlled inverse ratio ventilation (PCIRV). This mode has been used in the setting of refractory hypoxemia. Its physiologic effect is to increase mean airway pressure and it is commonly associated with auto-positive end-expiratory pressure (auto-PEEP). The clinical results are determined by the flow pattern, rather than the use of volume-controlled or pressure-controlled ventilation strategies per se. A descending ramp flow pattern with an inspiratory pause can be produced using either pressure-controlled or volume-controlled ventilation. There does not seem to be a benefit from the use of PCIRV on patient outcome.
Despite some clinicians favoring pressure-controlled ventilation in patients with acute respiratory distress syndrome (ARDS), evidence supporting its superiority in this setting is lacking. For the same VT, the same inspiratory time, and a descending ramp of flow with volume-controlled ventilation, the differences in Pao2 between pressure-controlled and volume-controlled ventilation are trivial.
Pressure Support Ventilation
The typical waveform for pressure support ventilation (PSV) is shown in Figure 9-9. When the pressure-supported breath is triggered, the ventilator delivers flow sufficient to reach the set pressure (typically, the pressure support is the amount of pressure added to the PEEP). As with pressure-controlled ventilation, all current generation ICU ventilators allow the initial flow (rise time) to be adjusted with PSV, which controls how quickly the pressure reaches the set target.
Effect of rise time adjustments of waveforms. Note that the faster rise time results in a higher flow at the beginning of inspiration.
The pressure-supported breath should terminate when the patient's inspiratory effort ceases. Premature termination may result in double triggering and a prolonged inspiration may result in the patient activating expiratory muscles in an attempt to end the inspiratory phase. The inspiratory phase stops when the flow decreases to a ventilator-determined level (eg, usually 25% of peak flow). To improve synchrony, many current generation ventilators allow adjustment of the flow at which the ventilator cycles to the expiratory phase (Figure 9-10). To avoid unintentional termination or prolongation of inspiration, redundant systems are used to terminate inspiration. These are typically time or pressure-based. Inspiration is terminated if the time or pressure criteria are met before the termination flow criteria. These redundant features are particularly important if a leak is present or if the patient's respiratory mechanics result in a short or long inspiratory phase. One commercially available ventilator uses measures of lung mechanics and assessment of the airway pressure waveform to automatically determine cycle off criteria using closed-loop feedback control.
Effect of changes in flow termination criteria during pressure support ventilation. Note the effect of a termination flow on inspiratory time.
Proportional-Assist Ventilation, Neurally Adjusted Ventilatory Assist, and Airway-Pressure Release Ventilation
These modes are each pressure-targeted. As such, flow delivery into the lungs is similar to that for pressure control and pressure support ventilation. For proportional-assist ventilation, the inspiratory phase is flow-triggered and flow-cycled. For neurally adjusted ventilator assist, the inspiratory phase is triggered and cycled by the electrical activity of the diaphragm. For airway pressure release ventilation, the time at high pressure is triggered and cycled by the ventilator; the patient's spontaneous breaths are flow-triggered and flow-cycled.