Briefly outline the role of non-invasive ventilation in the management of a 24 year old woman who presents with acute severe asthma.
NIV includes CPAP, BiPAP and PAV. Non-invasive ventilation has been used with success in acute severe asthma but there are no RCTs to support its use. In theory it assists the patient by decreasing the inspiratory work, decreasing expiratory work and improving V/Q mismatch. The potential negative effects are numerous and include claustrophobia, agitation, gastric distension, dys-synchrony, increased expiratory work and hyperinflation.
Decreasing the pressure change that needs to be generated to initiate respiration in the presence of auto-peep may decrease inspiratory work. Expiratory work may be decreased by opposing dynamic airway compression and allowing more complete expiration with less gas-trapping and hyperinflation. Experimental work in induced asthma suggests that CPAP mainly acts to unload inspiratory muscles.
Since all these factors cannot be anticipated in an individual, the role of NIV is best found by testing the patient’s response to titrated therapy eg starting with 5 cm CPAP and titrating IPAP and EPAP.
There is of course no role in respiratory or cardiac arrest or in the patient who is unable to cooperate or protect the airway.
Probably the best reference for this answer is the article by Teke et al (2015), which discusses the mechanisms in some detail. The SAQ asked to "briefly outline", rather than critically evaluate, the use of NIV in asthma.
Role of NIV in asthma is basically to decrease respiratory workload (usually by by 30-70%)
(see also the chapter on Non-invasive mechanical ventilation)
Interestingly, the college answer makes the comment that the use of NIV may increase the expiratory workload. This would be the consequence of PEEP (or rather ePAP), seeing as it is workload during expiration when the bilevel pressure is at the lower level. By increasing airway pressure at the mask, the expiratory pressure gradient from alveoli to the mouth is decreased, which decreases expiratory flow. Specifically, the peak tidal expiratory flow rate is reduced by this. On the other hand, end-expiratory flow rates are increased by PEEP (as the result of the abovementioned airway splinting and effects on dymanic compression). Generally, it appears that beyond a certain PEEP no further improvement in end-inspiratory flow is seen (because the airways are already maximally splinted open, and no further increases in pressure will produce any improvement in the degree of splintedness). However, as you keep cranking up the PEEP, the mouth-alveoli gradient will keep changing, and the peak expiratory flow will keep decreasing. Thus, at low PEEPs the splinting mechanism is the dominant influence, and at high PEEPs the pressure gradient mechanism is dominant.
From this it follows that the ideal level of PEEP is one which achieves the maximum expiratory flow rate. If the PEEP is high enough, the beneficial increase in end-expiratory flow rate is counteracted by the decrease in peak expiratory flow rate. Shivaram et al (1987) determined that this is something asthmatics find uncomfortable at higher CPAP levels. Ergo, for every asthmatic, at any given moment there is some optimal PEEP level at which the balance of these opposing influences achieves maximal expiratory flow, and this magic PEEP is indeed "best found by testing the patient’s response to titrated therapy".
Medoff, Benjamin D. "Invasive and noninvasive ventilation in patients with asthma." Respiratory care 53.6 (2008): 740-750.
Murase, K., et al. "Non-invasive ventilation in severe asthma attack, its possibilities and problems." Panminerva medica 53.2 (2011): 87-96.
Gupta, Dheeraj, et al. "A prospective randomized controlled trial on the efficacy of noninvasive ventilation in severe acute asthma." Respiratory care 55.5 (2010): 536-543.
Op't Holt, Timothy B. "Additional Evidence to Support the Use of Noninvasive Ventilation in Asthma Exacerbation." Respiratory care 58.2 (2013): 380-382.
Carson, Kristin V., Zafar A. Usmani, and Brian J. Smith. "Noninvasive ventilation in acute severe asthma: current evidence and future perspectives." Current opinion in pulmonary medicine 20.1 (2014): 118-123.
Teke, Turgut, Mehmet Yavsan, and Kürsat Uzun. "Noninvasive ventilation for severe acute asthmatic attacks." Journal of Academic Emergency Medicine 14.1 (2015): 30.
Shivaram, Urmila, et al. "Effects of continuous positive airway pressure in acute asthma." Respiration 52.3 (1987): 157-162.