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2.8 Mode controls
2.8.1 TTV Control Mode (Targeted Tidal Volume)
The aim of TTV is to prevent over distension of the alveoli and thereby limit volutrauma. By
targeting a selected tidal volume, and achieving and maintaining the tidal volume with
pressure ventilation, also limits barotrauma.
Targeted tidal volume can be applied to all modes of pressure cycled ventilation, provided
that a flow sensor is in situ. TTV cannot be applied in HFO or HFO+CMV.
Once an applicable mode has been selected, and the following parameters have been set
by the user i.e. PEEP, PIP, Ti, Rate and O
2
, the user may then choose to apply Targeted
Tidal Volume.
Once TTV has been activated and a target tidal volume selected the following occurs:
The PIP initially selected becomes the maximum PIP available (MAX PIP). To achieve the
targeted tidal volume only a percentage of the MAX PIP may be used. The MAX PIP is not
delivered to the patient unless the targeted volume is not being achieved. The Ti set
becomes the MAX Ti when TTV is applied. The time taken to achieve the targeted tidal
volume may be shorter than the MAX Ti. This is because the required volume is delivered
before the Ti is reached. The actual or measured Ti is displayed on the right hand side of the
screen and with in the eyebrow on the parameter setting. If the measured Ti is felt to be too
short, then the user may “slow down” the wave shape, using the wave shape button, to
prolong the breath to increase the measured Ti
Practical Considerations. The targeted tidal volume is delivered on a breath by breath basis.
This means that there is not a variable tidal volume in the initial phase of TTV.
However if the resistance of the lungs increases, the measured PIP will increase up to the
MAX PIP allowed, to achieve the targeted tidal volume. No more than the MAX PIP will be
delivered to the patient unless increased by the user.
Conversely, as compliance increases the measured PIP automatically decreases to
maintain the targeted volume. In effect weaning down the pressures as the lungs improve.
If volume cannot be achieved, the MAX Ti can be increased to allow for the delivery of the
targeted tidal volume. The breath then becomes time limiting as opposed to volume limiting.
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