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Objectives
Draw normal pressure, flow, and volume waveforms for pressure- and volume-controlled ventilation.
Describe the effects of abnormal respiratory system mechanics on pressure, flow, and volume waveforms during pressure- and volume-controlled ventilation.
Discuss the use of flow- and pressure-volume curves during mechanical ventilation.
Describe the use of the stress index during mechanical ventilation.
Describe the use of esophageal pressure to measure pleural pressure during mechanical ventilation.
Discuss the use of intra-abdominal pressure measurements during mechanical ventilation.
Explain how lung volume can be measured during mechanical ventilation.
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It is useful to assess respiratory mechanics in many mechanically ventilated patients using the pressure and volume displays on the ventilator. Additional information can be gained by observing the graphic waveforms of pressure, volume, and flow. In this chapter, mechanics based on the waveform displays of the ventilator, pressure-volume curves, esophageal pressure, intra-abdominal pressure, and measurement of end-expiratory lung volume (EELV) are discussed.
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Some ventilators measure pressure directly at the proximal airway. Others approximate inspiratory pressure by measuring pressure in the expiratory circuit during inspiration and approximate expiratory pressure by measuring pressure in the inspiratory circuit during exhalation.
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With patient-triggered breaths, airway pressure drops below baseline to trigger the ventilator. Active patient effort may continue after the initiation of a patient-triggered breath, which produces scooping out of the airway tracing (Figure 31-1). This suggests that the inspiratory flow of the ventilator should be increased if volume-controlled ventilation is used. Alternatively, pressure-controlled or pressure-support ventilation might be used and the rise time can be adjusted to better meet the patient's flow demand. The depth and duration of the negative pressure deflection prior to a patient-triggered breath indicates the response of the ventilator and the magnitude of the patient effort.
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A typical airway pressure waveform is shown in Figure 31-2. During exhalation, the pressure should be the set positive end-expiratory pressure (PEEP) level. During inhalation, the airway pressure waveform is determined by the flow set on the ventilator and the patient's respiratory demand. With constant-flow volume-controlled ventilation, airway pressure should increase linearly during the inspiratory phase. With pressure-controlled and pressure-support ventilation, airway pressure during inhalation approximates a square wave. The shape of the pressure waveform is also affected by the rise in time setting on the ventilator.
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Although some ventilators measure flow directly at the proximal endotracheal tube, most measure it in the ventilator using inspiratory and expiratory pneumotachometers. Flow measured directly at the airway is not affected by factors such as ...