Page 347 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 313

set of waveforms of the same type, but with characteristic changes made for com-
parison. Both sets of waveforms represent mandatory volume-controlled breaths
or volume-controlled ventilation (VCV). Inspiration in these examples of VCV
is begun or triggered by the ventilator. All breaths during VCV are volume- or
flow-controlled, and ended (cycled into expiration) by the ventilator. These are
also considered examples of volume-controlled ventilation. When a targeted value
reached, such as the volume set, is used to cycle the ventilator into expiration, the
parameter or variable targeted is considered to be limited. And the mode targeting
that variable to cycle the ventilator into expiration can be characterized as limited
as well as controlled. The letters in the graphics represent the various compo-
nents and phase variables of a breath recorded by flow-and pressure-time graphics
(Chatburn, 2001, 2007).

Flow-Time Waveform

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In the first flow-time waveform (Figure 11-2, left - V and P) the letters represent
the four phases of the ventilatory or respiratory cycle, the period of time from the
beginning of one breath and the beginning of the next. The letter a presents the
end of expiration and the beginning of the inspiration where flow is ventilator- or
time-triggered. It is always a positive upward stroke on ventilator graphics. Letter b
marks the inspiration with a peak and constant flow of 60 L/min.
Letter c marks the change from inspiration to expiration where the breath is volume-
or time-cycled into expiration. The inspiratory flow waveforms represent conceptual
or idealized waveforms. The initial flow cannot reach the peak flow level instanta-
neously. No ventilator can perfectly “square off” flow and pressure waveforms as they
are presented in textbooks, or by ventilator graphic software. Realistic waveforms
will have more rounded or slightly jagged corners and variable patterns (so-called
noise) with transitive changes in flow and pressure as inspiratory and expiratory
valves make rapid adjustments in flow rates. Noise, however, is mitigated by reducing
the number of data sampled (measured) per second and digitized by the ventilator’s
hardware for graphic presentation. Approximately 30 to 50 samples of flow or pres-
sure measurements are digitized per second, which creates smoother-appearing lines,
slopes, and curves for graphic representation of waveforms. Higher sampling rates
would be costly. Greater attention to minor fluctuations in measurements and details
is not necessary clinically, nor for graphic presentations in textbooks, to learn the
concepts and major principles involved with waveform analysis. Thus, minor details
to graphics have been omitted for ease of presentation and mathematical analysis.
Clinically relevant exceptions may be presented and explained.
Letter d depicts expiration, the fourth phase of the ventilatory cycle, which is
always to the lower side of baseline or zero flow. Letter e represents the peak expira-
tory flow rate attained (60 L/min), which is assigned a negative value in graphics.
The expiratory flow pattern from the peak level attained to the end of flow is nor-
mally an exponential decay and convex pattern under passive conditions. Letter f
represents the end of a patient’s flow as it returns to baseline, and g is the passive
expiratory pause time in flow until the next breath.

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