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Objectives

  1. Describe the significance of peak inspiratory pressure (PIP), plateau pressure (Pplat), driving pressure, and auto-PEEP.

  2. List factors that affect PIP, Pplat, and auto-PEEP.

  3. Calculate airways resistance, respiratory system compliance, and mean airway pressure.

  4. List causes of abnormal airways resistance and respiratory system compliance.

Introduction

Pulmonary mechanics are frequently measured on mechanically ventilated patients. Some such as peak inspiratory pressure (PIP), plateau pressure (Pplat), and driving pressure 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).

Assessment of Mechanics During Mechanical Ventilation

Airway Pressure

A typical airway pressure waveform during volume-controlled ventilation (VCV) is shown in Figure 31-1. 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, PEEP, and patient effort.

Figure 31-1

A typical airway pressure waveform during volume ventilation.

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 31-2). 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 28 cm H2O provided that pleural pressure is not increased, and always should be kept as low as possible.

Figure 31-2

Plateau pressure is determined using an end-inspiratory pause.

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 (driving pressure) 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 (eg, circuit leak or bronchopleural fistula).

During pressure-controlled ventilation (PCV), PIP and Pplat may be equal due to the flow waveform with this mode of ventilation (Figure 31-3). With PCV, flow decreases during inspiration and may be followed by a period of zero flow at end inspiration. During this period of no flow, proximal airway pressure should be equal to Pplat. If flow does not reach zero ...

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