Page 261 - Clinical Application of Mechanical Ventilation

Initiation of Mechanical Ventilation 227

Example 2 Using I Time to Change the I:E Ratio
At f of 16/min, the I time
needed for an I:E ratio of Given: f 5 16/min
1:4 is 0.75 sec. The E time is
(3.75 2 0.75) sec 5 3 sec.
Desired I:E Ratio 5 1: 4
Calculate: The I time needed for an I:E ratio of 1: 4

Solution: Since f 5 16/min, time for each breath
5 60 sec/16 or 3.75 sec

I Time 5 Time for Each Breath 3 [I Ratio / Sum of I:E Ratio]
5 3.75 sec 3 [1/(1 1 4)]
5 3.75 sec 3 [1/5]

5 3.75 sec/5
5 0.75 sec
I Time % and I:E Ratio. Some ventilators (e.g., Hamilton Veolar) permit the I:E ratio
to be preset, usually by setting an I time % (percent inspiratory time). In these
ventilators, the flow rate is automatically adjusted by the ventilator to maintain a
constant I:E ratio regardless of changes in tidal volume or frequency.
The I time % and I:E ratio equivalent are listed in Table 8-12. For other I:E ratios
not listed in the table, they may be calculated by following Example 3:

Example 3 Using I Time % to Set the I:E Ratio
The I time % needed for an
I:E ratio of 1:3.5 is about 22%. Given: Desired I:E Ratio 5 1:3.5

Calculate: The I time % needed for an I:E ratio of 1:3.5
I Ratio
Solution: I Time % =
Sum of I:E Ratio

1
=
(1 + 3.5)
1
=
4.5
= 22%

Flow Pattern

Most modern ventilators offer different inspiratory flow patterns. Although there
are subtle variations, the principal flow patterns are (1) square (constant) flow pat-
tern, (2) accelerating (ascending) flow pattern, (3) decelerating (descending) flow
pattern, and (4) sine wave flow pattern. The waveforms for each of these flow pat-
terns are shown in Figure 8-1.
The square flow pattern may be used initially upon setting up the ventilator. This
flow pattern provides an even, constant peak flow during the entire inspiratory

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