Ventilator Management

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The overall goals of MV are to provide adequate ventilatory support and reduce work of breathing to the tired asthmatic until time and ongoing phar-macotherapy corrects the airway obstruction. In cases of severe asthma, strategies aimed at minimizing hyperinflation have been shown to be beneficial. To this end, lowering minute volume ( Ve), decreasing inspiratory time (Ti), and assuring patient-ventilator synchrony become the targets of therapy (161).

Both lowered Ve and shorter T aid in abating hyperinflation by lengthening expiratory time and prolonging alveolar emptying. The more powerful determinant of adequate airway emptying is minute ventilation.

Accordingly, attention should be specifically addressed to reduce administered tidal volumes and their frequency. Reasonable initial ventilator settings used to avoid lung hyperinflation are tidal volumes up to 8 cc/kg and respiratory rates between 10 and 12 breaths/min in a volume-control mode (162). In addition, inspiratory flow rate should be set between 70 and 100 L/min to further increase Te (161). Setting flow rates to this level often occurs with the ventilator alarming as the set peak inspiratory pressure (PIP) alarm has been exceeded. Increasing the PIP alarm limit is the correct remedy in this situation. Achieving lower PIPs by decreasing Vt or flow rate has been shown to increase hyperinflation. Conversely, elevated PIPs at the ventilator setting described above are associated with theoretical risks of barotraumas that have never been clinically corroborated (163,164).

More useful measurements of hyperinflation and the consequent risk of barotrauma and hyperinflation include plateau pressure (Pplat) and PEEPi values (162). These measurements are made by implementing inspiratory and expiratory pauses and should be done when the ventilator circuit is uninterrupted by external sources of airflow, i.e., continuous nebulized therapy. Care should be taken in always discontinuing pauses programmed into the routine ventilator sequence after measurements are made. A persistent inspiratory pause will increase the I:E ratio and not only affect Pplat measurement but may also increase the degree of hyperinflation. Reasonable goals of Pplat are below 35 cmH2O with even safer values below 30 cm H2O (153). PEEPi measurements correlate with end-expiratory alveolar volumes, but due to heterogeneity of airway closure, a large proportion of alveolar units may not be in communication with the ventilator at the time of this measurement. This phenomenon likely explains the cases of underestimation of hyperinflation by PEEPi measurements (165,166).

A more accurate measurement of dynamic hyperinflation can be made by quantifying the amount of gas collected from end inspiration during a period of prolonged apnea (up to 60 seconds). This value, end-inspiratory lung volume or VEi, was termed and validated by Tuxen et al. (163,167). While VEI is predictably more prognostic of hypotension than PEEPi measurements, it is more difficult to routinely measure and may more frequently require paralytic use to obtain (Fig. 3).

When ventilation is completely controlled, the addition of ventilator circuit PEEP may worsen hyperinflation (168). In tenuous cases of SAA, sedation should be used to ablate patient-triggered breaths and ventilator PEEP should be set to zero (27). It has been argued that potentially beneficial effects of ventilator administered PEEP are seen as a result of dilating previously collapsed airways, hence allowing improved gas exchange (169); we have not seen this theoretical effect benefit patients and do not employ machine PEEP during the early stabilization of an asthmatic on the ventilator. As patients are aroused from induced coma and resume triggering

Figure 3 Measurements of dynamic hyperinflation. Panel (A): Described by Tuxen et al., end-inspiratory lung volume, VEI, can be measured during a prolonged apnea at the conclusion of a tidal volume delivery. Panel (B): An easier measurement, plateau pressure (pplat) (arrow 1), can be determined by temporarily incorporating an end inspiratory pause into the ventilator circuit (arrow 2). Another clue to the presence of hyperinflation is persistent end-expiratory flow in this example (arrow 3). The magnitude of the auto-PEEP or PEEPi can be quantified by an end-expiratory pause maneuver (not shown). Source: Panel B provided by G. A. Schmidt.

Figure 3 Measurements of dynamic hyperinflation. Panel (A): Described by Tuxen et al., end-inspiratory lung volume, VEI, can be measured during a prolonged apnea at the conclusion of a tidal volume delivery. Panel (B): An easier measurement, plateau pressure (pplat) (arrow 1), can be determined by temporarily incorporating an end inspiratory pause into the ventilator circuit (arrow 2). Another clue to the presence of hyperinflation is persistent end-expiratory flow in this example (arrow 3). The magnitude of the auto-PEEP or PEEPi can be quantified by an end-expiratory pause maneuver (not shown). Source: Panel B provided by G. A. Schmidt.

breaths on the ventilator, prudent use of PEEP should be considered. At this time airflow obstruction has improved significantly, if not completely resolved, and administered PEEP may be acceptable if kept below PEEPi and its addition does not adversely affect measurements of hyperinflation (169,170).

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Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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