Page 617 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 50: Novel Modes of Mechanical Ventilation 437

appropriately adapts to changes in patient position and double- to single- For instance, if the clinician set volume is excessive for patient demand,
lung anesthesia. 48-54 One other study suggested that the I:E algorithm a recovering patient may not attempt to take over the work of breathing
of ASV produced less air trapping in patients with chronic obstructive for that volume and thus support reduction and weaning may not prog-
pulmonary disease (COPD). Longer-duration clinical studies with ASV ress. In addition, if the pressure level increases in an attempt to maintain
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have shown that the algorithm provided adequate ventilator support in an inappropriately high set tidal volume in the patient with airflow
anesthetized patients, 48-51 as well as in patients with respiratory failure. 55 obstruction, intrinsic PEEP (PEEPi) may result. On the other hand, a
More recent evaluations of ASV have focused on its ability to provide patient may receive inadequate support if the clinician set tidal volume is
appropriate lung protective small tidal volumes. Indeed, when respira- not adequate for patient demand. Under these conditions, a patient will
tory system compliance is poor, the ASV algorithm supplies a protective perform excessive work to maintain a patient desired tidal volume all the
low tidal volume ventilator pattern similar to that recommended by the while the inspiratory pressure is being reduced because volume exceeds
ARDS Network. Problems arise, however, when respiratory system the clinician setting. Clinicians need to be aware of the behavior of VS
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compliance is less deranged (eg, patients with milder forms of acute lung under a variety of circumstances to properly use this mode.
injury). Under these conditions, the ASV algorithm tends to deliver tidal
volumes often in excess of 10 mL/kg ideal body weight. The clinical ■ ENHANCEMENTS ON VOLUME FEEDBACK CONTROL
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significance of this is unknown but the potential harm from this should OF PRESSURE-TARGETED BREATHS
be considered by clinicians wishing to use this mode.
Airway occlusion pressure (P ), oxygen saturation (Spo ), 68-70 and end
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2
0.1
NOVEL MODES ADDRESSING IMPROVED PATIENT tidal CO concentrations 73,74 have been incorporated into PRVC and VS
2
VENTILATOR INTERACTIONS mode-control algorithms to adjust either the target V or the breath-
T
delivery pattern. The one system that is commercially available uses end
■ VOLUME FEEDBACK CONTROL OF PRESSURE-TARGETED BREATHS tidal CO and respiratory rate along with the tidal volume to adjust the
2
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As noted previously, pressure-targeted breaths with variable flow fea- applied inspiratory pressure. Known by the proprietary trade name
SmartCare (Maquet systems), the computerized feedback system attempts
tures often synchronize with patient flow demands better than fixed to find an inspiratory pressure that maintains the respiratory rate and
flow, volume-targeted breaths (Fig. 50-2). A drawback to pressure tar- tidal volume in a clinician set “comfort zone.” The end tidal CO serves as
2
geting, however, is that a tidal volume cannot be guaranteed. This may a backup signal to ensure adequate ventilation is occurring. The system
be particularly important if the patient’s respiratory drive is variable is designed to wean the inspiratory pressure to as low a level as possible
and/or lung mechanics are unstable such that a desired minute ventila- within these boundaries and then alert the clinician to perform a sponta-
tion or tidal volume target (eg, 6-8 mL/kg ideal body weight) cannot be neous breathing trial (SBT) when this pressure reaches 9 cm H O.
2
reliably achieved. A number of small observational trials have been done showing that
Over the last two decades, a number of engineering innovations the SmartCare system did indeed keep patients in the clinician selected
have attempted to combine features of pressure- and flow-targeted “comfort zone” for 95% of the time. 73,74 In a larger randomized clinical
breaths by producing feedback algorithms that allow some control trial, this approach appeared to remove ventilator support quicker than
of volume with pressure targeting. The most common approach is to “physician-controlled” weaning. Unfortunately, this control group did
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use a measured volume input to manipulate the applied pressure level not have a protocolized SBT approach and thus may have had support
of subsequent pressure-targeted breaths. 57-62 When these breaths are removal delayed. Moreover, a subsequent trial was unable to duplicate
exclusively supplied with time cycling, the mode is commonly referred the superiority of this automated feedback approach. Even if it is not
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to as pressure-regulated volume control (PRVC) although there are a superior, however, an automated system that is “just as good” as clini-
number of proprietary names (eg, Autoflow, VC+, Adaptive Pressure cians could have applications in settings with rapidly recovering patients
Ventilation). When these breaths are supplied exclusively with patient or low availability of clinicians to make frequent assessments.
triggered, flow cycling characteristics, the mode is commonly referred to When patient efforts occur during the ASV mode described above,
as volume support (VS). Some ventilators will switch between these two the control algorithm continues to try to conform to the minimal work
breath types depending on the number of patient efforts. Both animal tidal volume considerations above and in that sense resembles the feed-
and human studies have shown that these feedback algorithms breaths back features of VS. However, the ASV feedback control is more com-
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function as designed. 60-64 plex than VS in that respiratory system resistance, compliance (and the
Conceptually, the assist-control, time-cycled PRVC mode could be a resulting time constant) modulate the tidal volume target. A number of
useful tool in providing more synchronous lung protective ventilation. studies have evaluated ASV in patients being weaned from mechanical
Specifically, a tidal volume target of 6 to 8 mL/kg could be selected and ventilation. 55,78-83 In general, these studies showed that ASV safely pro-
the ventilator would then automatically adjust the applied inspiratory vided adequate ventilator support and had similar (or faster) weaning
pressure (with its synchronous variable flow feature) to the airway. times as compared with various SIMV and SIMV + PS protocols. These
Indeed, a number of clinical observational studies have demonstrated studies also generally showed fewer ventilator manipulations with ASV.
that this can be done. 65,66 However, one study found that while these Larger trials in patients with different forms of lung injury clearly are
feedback breaths did provide a more reliable small tidal volume ventila- needed to establish the appropriateness of the ASV algorithms in facili-
tory pattern than pure pressure assist control, in a minority of patients, tating ventilator withdrawal.
up to 14% of tidal volumes were above the desired target value.
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Whether this variability is an acceptable tradeoff to improved comfort Proportional Assist Ventilation: Proportional assist ventilation (PAV) is a
during lung protective ventilation needs further study. novel approach to assisted ventilation that uses a clinician set “gain” on
The patient-triggered, flow-cycled, volume-feedback mode VS has patient-generated flow and volume. 84,85 PAV uses intermittent controlled
been evaluated primarily during the ventilator withdrawal process. “test breaths” to calculate resistance and compliance. It can then use
Theoretically, the VS mode could be used to automatically reduce measured flow and volume to calculate both resistive and elastic work.
applied inspiratory pressure as the patient’s ability to breathe improved. The clinician is required to set a desired proportion of the total work that
Conversely, inspiratory pressure would increase if patient effort dimin- should be performed by the ventilator. The ventilator then measures the
ished or respiratory system mechanics worsened. These responses have patient flow and volume demand with each breath and adds both pres-
been demonstrated in several small studies, often involving the rapidly sure and flow to provide the selected proportion of the breathing work.
recovering (eg, postoperative) patient. 67-70 A common finding in these PAV has been compared to power steering on an automobile, an analogy
studies is that the VS mode required fewer ventilator manipulations. that has much truth. Like PAV, power steering reduces the work to turn
Unfortunately, the simplicity of the VS mode may produce problems. the wheels but does not automatically steer the car—the driver must
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