Page 105 - Clinical Application of Mechanical Ventilation

Classification of Mechanical Ventilators 71

A B C D E
50 50 50 50 50
Pressure (cm H 2 O) 0 0 1 2 3 4 0 0 1 2 3 4 0 0 1 2 3 4 0 0 1 2 3 4 0 0 1 2 3 4

1.0 1.0 1.0 1.0 1.0
Volume (L)
From R. L. Chatburn (1991). Respir Care, 36 (10). Used with permission.

0 0 0 0 0
0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4

100 100 100 100 100

Flow (L/min) 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4

–100 –100 –100 –100 –100
Figure 3-17 Theoretical output waveforms for (A) pressure-controlled inspiration with
rectangular pressure waveform, identical to flow-controlled inspiration with an exponential-
decay flow waveform; (B) flow-controlled inspiration with rectangular flow waveform, identical to
volume-controlled inspiration with an ascending-ramp flow waveform; (C) flow-controlled inspi-
ration with an ascending-ramp flow waveform; (D) flow-controlled inspiration with a descending-
ramp flow waveform; and (E) flow-controlled inspiration with a sinusoidal flow waveform. The
short dashed lines represent mean inspiration pressure, whereas the longer dashed lines denote
mean airway pressure (assuming zero end-expiratory pressure). For the rectangular pressure
waveform in A, the mean inspiratory pressure is the same as the peak inspiratory pressure. These
output waveforms were created by (1) defining the control waveform (e.g., an ascending-ramp
flow waveform is specified as flow 5 constant 3 time) and specifying that tidal volume equals
644 mL (about 9 mL/kg for a normal adult); (2) specifying the desired values for resistance and
compliance (for these waveforms, compliance 5 20 mL/cm H 2 O and resistance 5 20 cm H 2 O/L/
sec, according to ANSI recommendations); (3) substituting the above information into the equa-
tion of motion; and (4) using a computer to solve the equation for pressure, volume, and flow and
plotting the results against time.

and flow values rising above the horizontal axis for inspiration and falling back to
the baseline during expiration. The ideal waveforms are represented in Figure 3-17.
Careful observation and assessment of waveforms during mechanical ventilation
can provide useful information for the clinician. Waveforms can assist the clini-
cian in the detection of inadvertent PEEP, the patient’s ventilatory work, resistance
and compliance changes, as well as many other events or changes. Some ventilators
are able to present pressure versus volume waveforms to assist in minimizing the
patient’s work of breathing. Still other ventilators can present flow versus volume
waveforms, to aid in the assessment of airway obstruction and the effectiveness
of bronchodilator therapy during mechanical ventilation. As waveforms become
widely used, their usefulness will approach that of the ECG tracing in the assess-
ment of the heart.

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