Because gas-trapping is potentially associated with significant adverse events in severe asthma, clinicians must be vigilant for its development and employ strategies to limit it. Understanding how gas-trapping occurs is the first step in developing such strategies. These strategies include controlled hypoventilation (reduced tidal volumes [less gas to exhale] and reduced respiratory rates [longer expiratory time]), relieving expiratory flow resistance (frequent airway suctioning if necessary, bronchodilators, steroids, large-bore endotracheal tube), reducing inspiratory time by increasing the inspiratory flow rate or incorporating nondistensible tubing, and reducing the need for high minute ventilation by decreasing carbon dioxide production (e.g. sedation/paralysis, controlling fever/pain). The application of external PEEP in severe asthma remains a controversial topic. It could theoretically decrease the work of breathing and hence carbon dioxide production, while limiting gas-trapping by splinting the airways open [29
]; however, in practice there are situations in which the application of external PEEP may increase total PEEP and worsen gas-trapping.
Assuming that appropriate medical therapy to alleviate airflow obstruction has been administered (i.e. inhaled beta agonists, inhaled ipratroprium bromide, steroids, with/without intravenous magnesium sulphate, etc.), by far the most effective method of decreasing dynamic hyperinflation/gas-trapping is to reduce the minute ventilation [31
]. Reducing the minute ventilation by adjusting the tidal volume, frequency, or set pressure on the ventilator may result in carbon dioxide retention. In this setting the controlled use of 'permissive hypercapnia' is generally considered well tolerated [33
]. Permissive hypercapnia that maintains a pH above 7.20 or an arterial carbon dioxide tension below 90 mmHg has gained widespread acceptance [27
]. Permissive hypercapnia has been used successfully in mechanically ventilated patients with status asthmaticus [33
Expiratory time can be lengthened by using higher inspiratory flow settings (70–100 l/min) during volume cycled ventilation, using a shorter inspiratory time fraction, reducing respiratory rate, and eliminating any inspiratory pause. Prolongation of expiratory time has been shown to decrease dynamic hyperinflation in patients with severe asthma, as is evident by decreased plateau pressures [37
]. The magnitude of this effect becomes relatively modest when the baseline minute ventilation is 10 l/min or less and when the baseline respiratory rate is low [37
]. It should be emphasized that while modifying the I/E ratio is important in fine tuning the amount of gas-trapping, the single most effective way is by reducing minute ventilation [6
Applying adequate sedation and analgesia is a fundamental step in lowering the production of carbon dioxide and subsequently ventilatory requirements. Sedation and/or paralysis may also allow the clinician to avoid patient–ventilator dysynchrony and facilitate strategies to limit gas-trapping in the most severe of cases. It is beyond the scope of this review to recommend which agents or protocols are best for this. The use of neuromuscular blocking agents should be limited to short periods of time and only when absolutely necessary in patients with severe asthma who are not achieving synchrony with other agents. Although neuromuscular blocking agents effectively promote synchrony, lower the risk for barotrauma, reduce lactate accumulation [38
] and reduce oxygen consumption and carbon dioxide production, their prolonged use, particularly when combined with steroids, can lead to prolonged paralysis and/or myopathy [39
The addition of extrinsic PEEP in the setting of auto-PEEP may reduce work of breathing and possibly even prevent gas-trapping by splinting the airways open [29
]. In terms of reducing the work of breathing, the addition of extrinsic PEEP in patients with dynamic hyperinflation would theoretically reduce the inspiratory muscle effort required to overcome auto-PEEP and initiate an inspiration. It has been demonstrated that in patients with chronic obstructive pulmonary disease more than 40% of inspiratory muscle effort can be expended to overcome auto-PEEP [41
], and that adding extrinsic PEEP can attenuate the inspiratory muscle effort needed to trigger inspiration and improve patient–ventilator interaction. In these patients extrinsic PEEP must be titrated individually, with an average of 80% of the auto-PEEP being tolerated before the plateau pressures and total PEEP begin to increase. Such an approach is only useful in those patients who are breathing spontaneously and capable of triggering the ventilator. In addition, extrinsic PEEP may prevent airway collapse (which could lead to occult auto-PEEP) by splinting the airways open. If this is the case then extrinsic PEEP would be most useful only in the most severe of cases, including those patients who are not spontaneously breathing. It should be noted that extrinsic PEEP has also been shown to be effective at preventing ventilator-induced lung injury in other forms of lung injury and hence may be of added benefit in this situation. In practice, however, adding extrinsic PEEP in some patients with severe asthma has been shown to worsen auto-PEEP [43
]. As mentioned above, it is occasionally difficult to measure auto-PEEP reliably, and if the extrinsic PEEP is greater than the auto-PEEP then gas-trapping will likely worsen. This has led some to recommend minimizing the use of extrinsic PEEP or not using it at all [35
] in the ventilation of patients with severe asthma. If extrinsic PEEP is to be used, then careful bedside observation with a clear understanding of how the benefits (reductions in auto-PEEP) and adverse effects (worsening gas-trapping) would manifest is mandatory.