Page 421 - ACCCN's Critical Care Nursing

398 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

Pressure-regulated Volume Control Neurally-adjusted Ventilatory Assist
Pressure-regulated volume control, available on the Servo Neurally-adjusted ventilatory assist (NAVA) is available
300 and Servo I (Maquet, Solna, Sweden), uses a ‘learning on the Servo-I ventilator (Maquet, Solna, Sweden) and
period’ to establish a patient’s compliance that guides uses the electrical activity of the diaphragm to control
regulation of pressure/volume. During the learning patient–ventilator interaction. 163 Electrical activity of the
period, four test breaths of increasing pressure are deliv- diaphragm, measured using an oesophageal catheter,
ered. The ventilator regulates inspiratory pressure based should result in optimal patient–ventilator synchrony
on the pressure/volume calculation of the previous breath as it represents the endpoint of neural output from the
and the clinician-determined target tidal volume. To respiratory centres and thus is the earliest signal of
maintain the target tidal volume during ongoing ventila- patient inspiratory trigger and expiratory cycling. Pres-
tion, the ventilator continues to adapt the inspiratory sure delivered to the airways (P aw ) is proportional
pressure in response to changing compliance and to inspiratory diaphragmatic electrical activity using a
resistance. clinician determined proportionality factor set on
the ventilator. 164 NAVA provides breath-by-breath assist
Airway Pressure Release Ventilation and in synchrony with, and in proportion to, respiratory
165
Although clinical data on NAVA is currently
Biphasic Positive Airway Pressure demand. 164,166-168 this mode shows promise for improving
limited,
Airway pressure release ventilation (APRV) and biphasic patient–ventilator synchrony.
positive airway pressure (BiPAP) are ventilator modes
that allow unrestricted spontaneous breathing inde- VENTILATOR GRAPHICS
pendent of ventilator cycling, using an active expiratory
valve that allows patients to exhale even in the inspi- Analysis of ventilator graphics provide clinicians with the
ratory phase. 147,148,155,156 Both modes are pressure-limited ability to assess patient–ventilator interaction, appropri-
and time-cycled. In the absence of spontaneous breath- ateness of ventilator settings and lung function.
ing, these modes resemble conventional pressure limited,
time-cycled ventilation. 157 In North America the acronym Scalars: Pressure/time, Flow/time,
BiPAP® is registered to Respironics non-invasive Volume/time
ventilators (Murrayville, PA). Therefore ventilator com- Many mechanical ventilators now offer integrated graphic
panies have developed brand names such as BiLevel displays as waveforms that plot one of three parameters,
(Puritan Bennett, Pleasanton, CA, GE Healthcare, pressure, flow or volume, on the vertical (y) axis against
Madison, WI) Bivent (Maquet, Solna, Sweden), DuoPaP time, measured in seconds, on the horizontal (x) axis
(Hamilton Medical, Rhäzüns, Switzerland), PCV+ referred to as scalars. Examination of scalars can assist
(Dräger Medical, Lübeck, Germany) or BiPhasic (Viasys, with assessment of patient–ventilator synchrony, patient
Conshocken, PA) to describe essentially equivalent triggering, appropriateness of inspiratory/expiratory
modes. Ambiguity exists in the criteria that distinguish times, presence of gas trapping, appropriateness and ade-
APRV and BiPAP. When applied with the same I : E quacy of flow, lung compliance and airway resistance and
ratio, no difference exists between the two modes. circuit leaks. 169,170
APRV as opposed to BiPAP, however, is more frequently
described with an extreme inverse ratio and advocated Pressure vs time scalar
as a method to improve oxygenation in refractory
hypoxemia. 158 The morphology of this waveform depends on the
breath target (volume or pressure) and the breath type
(mandatory or spontaneous). Pressure–time waveforms
171
Automatic Tube Compensation reflect airway pressure (P aw ) during inspiration and expira-
Automatic tube compensation (ATC) is active during tion and can be used to evaluate peak, plateau and end
spontaneous breaths and compensates for the work of inspiratory pressures as well as inspiratory and expiratory
breathing associated with artificial airway tube resistance times and appropriateness of flow (see Figure 15.5).
via closed-loop control of continuously calculated tra- Pressure–time scalars vary in appearance depending on
cheal pressure. 159,160 During spontaneous inspiration, a the control variable (volume vs pressure). In volume-
pressure gradient exists between the proximal and distal control breaths, the inspiratory waveform continues to
ends of the artificial airway due to resistance created by rise until peak airway pressure is achieved at the end of
the tube. A reduced pressure at the proximal end of the inspiration. In pressure control breaths, the inspiratory
tube means a patient needs to produce a greater inspira- waveform reaches its peak at the beginning of inspiration
tory force (greater negative pressure) to generate an ade- and remains at this elevation until cycling to expiration.
161
quate tidal volume. Higher flow rates generate larger Spontaneous triggering of ventilation can be identified by
pressure gradients and greater resistance. ATC requires examination of the pressure–time scalar at the beginning
the airway type and size to be entered into the of inspiration. A small negative deflection indicates
ventilator program as well as the percentage of automatic patient effort. When pressure-triggering is used, a breath
tube compensation (ATC) to be applied. It appears to is triggered when the pressure drops below baseline.
have most use in reducing the work of breathing for The depth of the deflection is proportional to patient
patients with high respiratory drive who require high effort required to trigger inspiration. A flow-
inspiratory flow. 162 triggered breath occurs when the flow rises above baseline,