Chapter 32: Waveforms: Scalars and Loops

Objectives

1. Draw normal pressure, flow, and volume waveforms for pressure- and volume-controlled ventilation (PCV and VCV).

2. Describe the effects of abnormal respiratory system mechanics on pressure, flow, and volume waveforms during PCV and VCV.

3. Discuss the use of flow- and pressure-volume curves during mechanical ventilation.

4. Describe the use of the stress index during mechanical ventilation.

5. Discuss the use of the occlusion pressure (P_0.1) to set an appropriate level of ventilator support.

Introduction

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 are discussed.

Scalars

Pressure

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.

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 upward concavity of the airway tracing (Figure 32-1). This suggests that the inspiratory flow of the ventilator should be increased if volume-controlled ventilation (VCV) 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.

Typical airway pressure waveforms are shown in Figure 32-2. 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 VCV, 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 time setting on the ventilator.

Flow

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 circuit leaks and the compressible volume of the ventilator circuit.

Typical airway flow waveforms are illustrated in Figure 32-3...