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KEY POINTS

KEY POINTS

  • A systematic approach to waveform interpretation is paramount (akin to EKGs interpretation).

  • Recognizing the underlying schemas of ventilator modes will help to understand how they will be affected by patient-ventilator interactions (PVIs).

  • Normal waveforms in a passive patient must be mastered to diagnose resistive and elastic loads and PVIs.

  • Synchrony issues can be analyzed by considering the following phases of the breath: trigger, inspiration, cycle, and expiration.

  • Understanding the goals of mechanical ventilation for a particular patient helps to guide treatment of synchrony problems or physiologic derangements.

INTRODUCTION

Ventilator waveform interpretation is a core skill of critical care medicine. A wide variety of clinically useful information can be ascertained by observing the morphology of the waveforms to complement clinical evaluations. The ventilator waveforms contain readily discernable information regarding the physiology of the respiratory system, the respiratory effort of the patient, and the interactions between the patient and ventilator. In this chapter, a systematic approach to waveform interpretation is presented, based on the ventilator mode, physiology of the respiratory system, and patient-ventilator interactions (PVIs).

WAVEFORMS AND MODES OF MECHANICAL VENTILATION

The first step in interpreting the waveforms is to establish which mode of ventilation is being used. A mode of ventilation is a preprogrammed PVI. Currently, there are almost 500 unique names for modes created by ventilator manufacturers, and these names usually do not convey meaningful information about the features of the mode. The mode named “pressure-regulated volume control (PRVC)” is a classic example; the name implies that pressure is regulated (ie, controlled) and that volume is also controlled. As we show below, the ventilator can only control one variable at a time in the equation of motion (ie, pressure or volume); thus, the name PRVC is ambiguous. Furthermore, the same name for a mode may have different behaviors on different ventilators. Without understanding the foundations of the mode’s function, it is difficult to define the respiratory mechanics and PVIs. Current mechanical ventilators have several modes, all with different features and variables, which make it difficult to know what each does. A taxonomy1,2 to describe mode of ventilation helps the clinician understand what each mode does so that its appropriate use can be deduced (just as with a drug taxonomy). We will highlight some key features of this taxonomy, and define some standard vocabulary, that will help interpret the waveforms. The components of a mode are outlined in Fig. 46-1. We discuss each in relation to interpreting waveforms.

FIGURE 46-1

Structure of a ventilator mode. Every mode is constructed by three basic components. The Control Variable, Breath sequence, and Targeting scheme. The TAG is a way to summarize the components. As an example, conventional “volume-assist control” is described as: control variable is volume (VC), the breath sequence is continuous mandatory ventilation (CMV), and the targeting scheme is setpoint (s). ...

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