Page 382 - Clinical Application of Mechanical Ventilation

348 Chapter 11

120
V (L/min)

1 2 3 4 5 6 7

P (cm H 2 O) 2120

80
a b

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22
1 2 3 4 5 6 7
Time (sec)
Figure 11-28 Constant flow and pressure-time waveforms showing dyssynchronous ventila-
tion as a result of a, inadequate initial peak flow, or b, inadequate mechanical tidal volume to
meet patient demand.

can sustain respiratory muscle weakness or fatigue, and it will prolong mechanical
ventilation because recovery time to regain the strength for spontaneous breathing
is compromised.
The first pressure-time waveform a (Figure 11-28) indicates that a higher ini-
(Figure 11-28) Solid tial flow is needed to keep pace with the patient’s initial flow demands. The initial
pressure tracing a shows in-
sufficient initial flow (low flow-resistive pressure is partially removed, so that the pressure to inflate the lungs
peak flow). Solid pressure has been transferred (imposed on) to the patient. The patient is drawing gas (flow
tracing b shows insufficient
end-inspiratory flow (low tidal demand) from the circuit almost as fast as the incoming gas from the ventilator.
volume). As a result, less pressure is created. Increasing the flow rate to a level greater than
the patient’s demand will provide assistance and restore a normal pressure pattern
(dashed curve).
In waveform b (Figure 11-28), the initial flow demand appears to be met since the
initial rise in pressure is relatively normal. The drop in pressure in this example is at
end-inspiration and suggests that the patient needs more flow (i.e., volume) than is
being supplied. A drop of pressure at end-inspiration means more volume is being
demanded than is being supplied. The ventilator is being time-cycled into expira-
tion before that demand is met.
Sometimes, increasing the peak flow will satisfy the patient’s demands in this
situation because the volume is delivered sooner. This may cause the patient to relax
and breathe in synchrony with the ventilator. Increasing the sensitivity may also

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