Page 350 - Clinical Application of Mechanical Ventilation

316 Chapter 11

side of the waveform (y-axis) is 60 L/min or 1 L/s (60 L/min * 1 min/60s 5 1 L/s).
tidal volume (V T ): Volume
delivered by ventilator during Flow is constant for 1 second, therefore, the width of the rectangle (x-axis) is 1 second
mandatory breaths. and, since length times width equals area, 1 L/s * 1s 5 1 liter tidal volume (V )
T
delivered. Also, constant flow means there is a constant delivery of volume per unit
time. For example, if 0.25s is used as the unit of time, a 0.25 liter of gas is delivered
Constant Peak Flow 3 every 0.25s (Volume 5 1 L/s * 0.25s 5 0.25 L).
I Time 5 Tidal Volume
Since patients exhale what they inhale, it can be stated that area b enclosed under
the expiratory flow wave equals area a under the CFW. If the volume enclosed by
area b during mechanical ventilation is less than area a, then there must either be
a leak in the circuit, some gas has not been expired, or has been trapped in the
patient’s lungs momentarily. The pressure pattern shows a constant rise in lung pres-
sure during the constant flow period as discussed earlier in Figure 11-2.
Figure 11-4 depicts the ideal pressure waveform with details that correspond to
the enclosed square flow wave presented above it. In this example, a 0.5 s pause in
delivery of flow from the ventilator has been set (prolonging inspiratory time T ).
I
inspiratory time T I : Time from
beginning inspiration to end The pause in flow delivery results in a static pressure measurement being maintained
of inspiration and beginning at the same level for 0.5 sec, creating a plateau or pause pressure at the end of the
expiration.
waveform.
For the flow waveform, the double-headed arrow shows that no flow is being
delivered from the ventilator for 0.5 s. During this time period, the inspiratory and
expiratory valves of the ventilator are closed to hold gas volume constant in the

Flow
60 L/min 0.5 sec
Pause
1 sec

40

P AO = P P = P AO 2 P ALV
PIP
TA
P (cm H 2 O) 20 P ALV (Peak) TA
P = Flow 3 Resistance
TA
P
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ALV

1 2
Time (sec)
Figure 11-4 Use of end-inspiratory pause to create and measure peak alveolar pressure (peak
P ALV or plateau pressure) and transairway pressure (P TA ). Peak P ALV or plateau pressure is used to
calculate static compliance; P TA is used to calculate airflow resistance.

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