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Objectives
Describe the significance of peak inspiratory pressure (PIP), plateau pressure (Pplat), and auto-PEEP.
List factors that affect PIP, Pplat, and auto-PEEP.
Calculate airways resistance, respiratory system compliance, and mean airway pressure.
List causes of abnormal airways resistance and respiratory system compliance.
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Pulmonary mechanics are frequently measured on mechanically ventilated patients. Some such as PIP and Pplat are recorded as part of patient-ventilator system checks. Others can be easily made at the bedside with no equipment but that available on the ventilator (eg, airway pressure, flow, and volume).
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Assessment of Mechanics During Mechanical Ventilation
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A typical airway pressure waveform during volume-controlled ventilation (VCV) is shown in Figure 30-1. With VCV, pressure increases as volume is delivered. If a constant flow pattern is chosen, there should be a constant increase in pressure during inspiration. With a descending ramp flow pattern, the inspiratory pressure waveform will be more rectangular. PIP varies with resistance, flow, tidal volume, respiratory system compliance, and PEEP.
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An end-inspiratory pause of sufficient duration (0.5-2 seconds) allows equilibration between proximal airway pressure and alveolar pressure (Palv). This measurement should be made on a single breath and removed immediately to prevent the development of auto-PEEP. During the end-inspiratory pause, there is no flow and a pressure plateau develops as proximal airway pressure equilibrates with Palv (Figure 30-2). The pressure during the inspiratory pause is the Pplat and represents peak Palv. Because it reflects Palv, Pplat should usually be kept at less than 30 cm H2O and always should be kept as low as possible.
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The difference between PIP and Pplat is due to the resistive properties of the system (eg, pulmonary airways, artificial airway), and the difference between Pplat and total PEEP is due to respiratory system compliance. The measurement of Pplat is valid only if the patient is passively ventilated—active breathing invalidates the measurement. The measurement is also not valid if leaks are present.
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During pressure-controlled ventilation (PCV), PIP and peak Palv may be equal due to the flow waveform with this mode of ventilation (Figure 30-3). With PCV, flow decreases during inspiration and is often followed by a period of zero flow at end inspiration. During this period of no flow, proximal airway pressure should be equal to Palv. If flow does not reach zero before the end of inspiration during PCV, an end-inspiratory pause maneuver is needed to determine Pplat. With all other factors held constant (eg, tidal volume, compliance, PEEP), Palv is identical for volume- and pressure-control ...