Page 375 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 341

pattern. PSV is pressure-controlled ventilation (Chatburn 2001, 2007) in the spon-
taneous mode of ventilation. Although each breath is mechanically supported by a
set pressure, each breath requires a “spontaneous effort” by the patient throughout
most of inspiration to sustain flow delivery above cycling (termination) level. Thus,
flow, volume, and inspiratory time are primarily under the patient’s control, which
During PSV, the flow, vol- defines the mode of ventilation as spontaneous. However, the pressure can be set
ume, and inspiratory time are
under the patient’s control. high enough to limit virtually all effort by the patient except for triggering the
breath.
If the patient inspires in a relaxed manner as presented in the first set of wave-
forms, gas reaches a peak flow level causing the pressure to reach the preset pressure
support level. The peak flow then tapers to sustain the preset pressure. However, T I
is not constant because the flow will continue until it reaches a ventilator-specific
preset level (e.g., 5% or 25% of initial peak flow) to terminate the flow (and
breath).
For the first pressure waveform example, letter a indicates it is an assist, pressure-/
patient-triggered breath; b indicates that the pressure level set, 15 cm H O, has been
2
quickly reached; and c shows that the pressure target has been sustained throughout
inspiration. The flow wave demonstrates that the ventilator provides flow at a high
level initially to exceed the patient’s flow demands, in order to reach the set pressure
level. The end-flow per breath (dashed line) shows that each wave is flow-cycled into
expiration at 25% (arrows) of the initial peak flow rate. The peak and end-flow may
vary with each breath.
The second flow wave demonstrates that a higher flow is provided to meet the
patient’s increased demand, but the set pressure level is maintained. The third flow
wave shows a lower patient demand and the flow is sustained until 25% of the initial
flow is reached. In comparison of all three waveforms in Figure 11-22, the inspira-
tory flow and inspiratory time can be variable (e.g., d and e in figure), depending on
the patient’s spontaneous breathing effort and lung/airway characteristics.

Adjusting Rise Time during PSV

Figure 11-23 demonstrates PSV at 10 cm H O with PEEP of 5 cm H O. The
2
2
In PSV, the faster the first set of flow and pressure waveforms is ideal. The second flow waveform dem-
initial rise to peak flow (short
rise time), the sooner the onstrates a rapid rise in initial flow that causes a spike in the initial pressure that
preset PS level is reached. This overshoots the set pressure limit. The pressure spike at the beginning of inspi-
may cause the ventilator to
overshoot the PS limit. ration (arrow x) can cause patient discomfort and increased work of breathing
(Dick et al., 1996).
Letters a and b flow waves demonstrate a feature called rise time percent (%),
A slow rise time during which is the percentage of the inspiratory cycle time required to reach the peak
PSV is more comfortable for flow rate and the pressure target set. The vertical dashed lines preceding the flow
the patient.
waveforms show that the rise to peak flow takes a progressively longer period of time.
The dashed lines on the third and fourth pressure waveforms were added to show
that the slope or rise to the set pressure is progressively slower as a result. The faster
the rise in initial flow, the sooner the set pressure is reached. An advantage of a slower
rise to the peak flow rate and set pressure levels is that greater comfort is provided.

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