Page 348 - Clinical Application of Mechanical Ventilation

314 Chapter 11

Pressure-Time Waveform

The ideal pressure-time waveform that is created under passive conditions of constant
flow ventilation is a step ascending ramp. Letter a on the pressure-time waveform in
Figure 11-2 indicates the beginning of inspiration and corresponds in time to the flow
waveform as indicated by the dashed line connecting the two waveforms. The begin-
ning of the pressure waveform provides information about the triggering variable of
the inspiratory phase of ventilation. There is no patient effort or assist breath (see
Figure 11-27 for an example of an assist breath), which indicates that the initial flow
from the ventilator is time-triggered. The initial flow pushes gas from the ventilator
circuit into the patient’s lungs as it accelerates to peak flow level. Little volume is actu-
ally delivered to the lungs during this initial time period. Only a few milliliters of gas
need to pass through the pneumotachometer at the beginning of the inspiratory limb
of the circuit for the peak flow rate to be attained. In addition, some gas volume is lost
(not delivered to the patient’s lungs) as pressure rises in the circuit because of large bore
tubing compliance (tubing expansion) and compression of gas molecules— resulting
in a higher density or gas per area and gas being “lost” in the circuit during inspiration
and not being delivered to the patient’s lungs. Approximately 2 to 3 mL/cm H O of
2
compressible volume lost is common for disposable adult circuits unless the ventilator
offers volume compensation (MacIntyre et al., 2008).
The initial rise in pressure (the vertical step prior to the linear rise in pressure) is
mostly the result of resistance to flow through the ventilator circuit and endotra-
cheal tube (Tobin, 1994). The back pressure resulting from impedance to ventilation
(flow resistive pressure caused by tubing and airways, and lung tissue recoil pressure)
is graphically recorded by a manometer at the inspiratory valve of the ventilator.
Letter b represents the change in slope on a pressure waveform that occurs once peak
flow is reached. Then, the peak flow level is sustained (constant) throughout inspira-
tion. Once flow delivery from the ventilator becomes constant, there is a relatively
linear rise in the dynamic or airway opening pressure (P ), which closely parallels
airway opening pressure (P AO ): AO
Sum of transairway pressure (P TA ) the linear rise in alveolar pressure (P ALV ) until the peak inspiratory pressure (PIP)
and alveolar pressure (P ALV ). and peak P ALV are reached at end-inspiration.
Flow cannot be constant at both ends of a ventilator circuit, however, because vol-
ume is lost per unit rise in pressure as a result of tubing compliance (expansion) and
alveolar pressure (P ALV ): Pres-
sure required to overcome the gas compression. A loss in volume to the ventilator circuit equates to a loss in flow
elastic recoil property of the lungs. (flow 5 volume per time) to the lungs. There has to be some reduction in flow from
the ventilator end of the circuit when compared to the flow through the patient’s
lungs at the other end of the circuit (carina). Flow is constant on graphics as it leaves
peak inspiratory pressure the ventilator. The reduction in flow from beginning to end-inspiration depends on
(PIP): Highest pressure during the
inspiratory phase. the characteristics of compliance and airflow resistance.
Letter c marks the PIP, the end of inspiration, and the beginning of expiration
where the ventilator is time- and volume-cycled into expiration. The second dashed
line shows that the end of inspiratory flow and PIP are contiguous in time. As
flow exits through the expiratory limb of the circuit, pressure is created by the re-
sistance to flow through the circuit and measured by a manometer at the expira-
tory valve. Pressure subsides as gas is released into the atmosphere. Letter d marks

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