Page 395 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 361

PIP (from 30 to 40 cm H O) is proportionate to the increase in P ALV (from 25 to
2
(Figure 11-39) A decrease 35 cm H O) that is caused by the reduction in C . The P gradient throughout
in C LT causes the pressure- 2 LT TA
volume loop to shift toward the inspiration (and between PIP and peak P ALV ) is held constant. P is affected by
TA
pressure axis.
changes in resistance, not by the changes in C .
LT
It is essential to note that on some ventilators (e.g., Servo 300), pressure and
volume may be assigned to the y- and x-axis, respectively. A reduction in C would
LT
shift the PVL up and to the left (i.e., toward the pressure axis).
(Figure 11-39) A decrease
in C LT will not affect the P TA
because the PIP- P ALV gradient
(P TA ) remains unchanged. Effect of Airflow Resistance on PVL
Figure 11-40 shows how the PVL is affected by an increase in resistance (dou-
ble-headed arrows and dashed lines). P ALV remains unchanged in this example,
(Figure 11-40) In while inspiratory and expiratory flow-resistive pressure (P ), PIP, and P have all
situations where the airflow TA AO
resistance is increased, the P ALV increased.
remains unchanged while P TA ,
PIP, and P AO are increased.
Lower Inflection Point on PVL

and Titration of PEEP

(Figure 11-41) The Figure 11-41 shows the effects on the PVL if C changes during tidal volume
pressure-volume (compli- LT
ance) loop shows that the delivery and is presented here only as a point of reference. The dashed line in-
initial point of inflection (Ipi) dicates that the slope of the P during V delivery has changed. Historically,
is the compliance point in ALV T
which the alveoli are recruited
(opened) during mechanical
ventilation.

800
P ALV
PIP (1)
600 PIP (2)

Volume (mL) 400 P TA(2) P A0

200 P TA(1)

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10 20 30 40 50
Pressure (cm H O)
2
Figure 11-40 The effects of airflow resistance on the pressure-volume loop during volume-
controlled, constant flow ventilation. An increased airflow resistance causes increase in PIP, inspira-
tory and expiratory P TA , and P AO . Note that the P ALV is unchanged.

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