Page 384 - Clinical Application of Mechanical Ventilation

350 Chapter 11

high flow demands, and example b suggests a need for increasing the V . In
T
Figures 11-28 and 11-29, signs of respiratory distress would probably result unless
the patient is being sedated.

Changes in Pressure Waveforms during
Respiratory Mechanics Measurement

Figure 11-30 shows another important benefit of graphics: assurance that the re-
spiratory mechanics are being accurately measured. These are the same waveforms
depicted in Figure 11-4 except that the dashed lines a and b (arrows) represent pos-
sible variations in peak P ALV that could result during a pause in flow. Rather than
pressure remaining constant during the pause, pressure either rises above relaxed
Patient-ventilator dys- peak P or descends below it. Pressure rising as in the example (letter a) often
synchrony with rising airway ALV
pressure may occur when the occurs when a patient tries to expire during the pause time. The procedure (pause
patient moves, talks, coughs, or at end-inspiration) is unnatural and patients do not always relax, and the neuro-
expires during the pause time.
logic respiratory drive to breathe may prevent it. Dyssynchronous ventilation may
be occurring as well, and the patient’s breathing pattern does not correspond to
end-inspiration in synchrony with the ventilator. There may be many reasons for an

Flow
60 L/min

1 sec

40

P AO a PIP
P (cm H 2 O) 20 b P ALV (Peak)

P ALV
© Cengage Learning 2014

1 2
Time (sec)
Figure 11-30 Measurements of P TA and P ALV can be affected by the patient’s effort to breathe.
Dotted line a shows patient’s expiratory effort during the pause period. Dotted line b shows inspi-
ratory effort during the pause period.

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