Page 359 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 325

V (L/min) 40 I b E

c

1 a 2 3 4 5 6
e f
d
240
P (cm H 2 O) 10

5
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22 1 2 3 4 5 6

Time (sec)
Figure 11-10 Normal flow- and pressure-time waveforms during CPAP (spontaneous breath-
ing with PEEP). The CPAP level is 5 cm H 2 O.

Continuous Positive Airway Pressure

Figure 11-10 is a more detailed example of spontaneous ventilation during 5 cm
(Figure 11-10) In CPAP, H O of continuous positive airway pressure (CPAP). In the flow graphic, letter
airway pressure is above 2
0 cm H 2 O and all breaths are a (arrow) labels the end of expiration and start of inspiration; letter b marks the peak
spontaneous. flow reached (20 L/min) for an idealized sine flow wave created by the patient during
spontaneous breathing; letter c indicates the change from inspiration to expiration;
letter d indicates the peak expiratory flow attained (20 L/min); letter e indicates the
end of expiratory flow; and letter f indicates the pause period before next breath. The
patient creates an inspiratory flow time of approximately 0.75 sec, an expiratory flow
time of 1.25 sec, and an expiratory pause time of 0.75 sec. The dashed lines connect-
ing the flow and pressure recordings in time delineate the total respiratory time into
the inspiratory and total expiratory time periods. As with mechanical breaths, inspi-
(Figure 11-10) In CPAP, ratory effort (negative pressure recorded) triggers flow on demand (demand CPAP)
inspiratory effort triggers
and initiates flow once the once the negative pressure reaches the sensitivity threshold [21 cm H O below
2
negative pressure reaches the CPAP level of 5 cm H O (dashed line)], or the pressure drops during a continuous
sensitivity threshold. 2
flow CPAP. Ideally, gas is delivered to the patient according to the flow demands,
which means that pressure will be maintained at baseline throughout the inspira-
tory phase of ventilation. In this example, the pressure recorded during inspiration
is kept near baseline, indicating patient flow demand is being met. During expira-
tion, as gas is expired into the circuit, a positive pressure is momentarily recorded
until the expiratory flow is too slow or the expiratory circuit and valve resistance is
so low that near-baseline pressure is recorded.

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