DrVirgo
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What is the normal Inspiratory:Expiratory ratio, and what happens in Asthma?
Which parameter changes in Asthma? Inspiration, Expiration, Both, or Neither? HOW?
Does the ratio Increase, Decrease, remain the same?
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| farnsworth
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As asthma is characterized by a (at least partially) reversible airway obstruction, this affects mainly expiratory flow (remember: expiration is usually a complete passive process, requiring no respiratory muscle activity). So in severe asthma (and also in COPD) expiration is prolonged, leading to a decreased I:E ratio.
Adults have a I:E = 1:2. Young children tend to have a I:E of 1:3. A lot of clinician select I:E ratios comparable to these 'physiologic' values when a patient needs mechanical ventilatory support. On the other hand: a patient requiring mechanical ventilation for respiratory or ventilatory failure may need 'unphysiological' ventilatory settings (prolonged inspiratory time, as an injured lung consists of lung areas with long time constants, which are inflated later in the inspiratory cycle -> ventilatory heterogeneities). Mechanical ventilation itself is not physiologic, so why overzealously aiming at physiological values?
Interestingly, increasing the positive end-expiratory pressure (PEEP) in patient w/ asthma (or COPD) requiring mechanical ventilatory can decrease hyperinflation and shorten the expiratory time. Google 'Starling resistor', 'airway stenting by PEEP' (not to confuse w/ airway stenting by introducing metal stents)...
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