If you haven’t seen @EM_RESUS recent thread on a scary asthma case, pls read it and see how such a situation should be handled. twitter.com/EM_RESUS/statu
Here, my aim is to delve a bit more into the physiology of BiPAP/CPAP for asthma.
During severe asthma, the rate of lung emptying is markedly slowed, and thus expiration is interrupted by the initiation of inspiration. This retained volume gets “stacked” over the already accumulated, retained air – leading to Dynamic Hyperinflation (DH) or Auto-PEEP.
DH leads to bad conditions: – Breathing takes place at less compliant portion of P-V curve.
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Respiratory muscles at unfavorable position on length-tension curve
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DH increases dead space -> increase required minute ventilation
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Diaphragm blood flow may be reduced 2/2 mechanical stress
Due to these factors, much more energy/effort is required to achieve the same amount of minute ventilation – even beyond the work required to overcome friction loss from obstructed airway. Over time, fatigue leads to hypercapnia, lactic acidosis, etc.
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Q: If hyperinflation is the problem, then why apply positive pressure? Wouldn’t it increase risk for barotrauma?
A: With DH, work is expended mostly on insp phase:
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lung recoil pressure is large (greater load)
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hyperinflated lung is less compliant (more work per vol change)
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muscles are inefficient (more ATP expended w/less results).
Inspiration has a tougher uphill battle.
When pt is fatiguing, IPAP ensures that:
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there is adequate ventilation (to provide O2 to fatigued muscles) and
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muscles encounter a reduced load. As the bronchodilatory effects of meds kick in, then IPAP can steadily be reduced to minimize chance for barotrauma.
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Q: How about EPAP?
A: IMO, setting the positive expiratory pressures to a non-zero value was absolutely right thing to do, though it has more to do w/ having PEEP (positive end-expiratory pressure), than having EPAP (positive pressure throughout the exp phase).
EXAMPLE:
To explain: Let’s say pt has Auto PEEP = 10 cm H2O (i.e., alveolar pressure is +10 at end of expiration)
As pt begins to inhale, diaphragm contracts and pleural pressures decrease to -5 cm H2O.
Q: Will you have any inspiration?
A: No!
The effective alveolar pressure decreases to +5 (AutoPEEP – Pleural Pressure), and since it is still greater than atmospheric pressure (0 cm H2O), there is no effective movement of air into the alveoli.
To actually initiate inspiration, all the inspiratory muscles (diaphragm, external intercostals, accessory muscles) must collectively decrease pleural pressures beyond -10 cm H2O before any air can enter the lung.
This expenditure of energy to bring pleural pressures from 0 to -10 is all wasted. This extra work is called Inspiratory Threshold Load (ITL) – ultimately causing fatigue. ITL leads to the scary scenario where there is no air movement despite seeming insp effort .
Q: What happens when you add PEEP (or EPAP) of +6 cm H2O?
A: During insp, pleural pressures reduce to -5, the alveolar pressure reduces to + 5 (assuming AutoPEEP = +10).
With EPAP , the oral pressure = + 6 which is greater than alveolar pressure of +5 –> Inspiration happens.
PEEP/EPAP can be increased steadily to the AutoPEEP levels, with no real change in the End-Expiratory Lung Volume (EELV), while ITL steadily disappears. This translates to more immediate inspiratory flow after end of expiration.
Q: What if EPAP > Auto-PEEP levels (e.g., +12 > +10)?
A: Lungs cannot expel its contents to the volume seen with +10, but will settle at larger Vol (equilibrated to + 12). That would lead to greater retained volume and thus greater EELV (more DH).
For this reason, Hoffman et al 1989 proposed that PEEP be steadily increased until the EELV begins to increase. Just below this threshold is where PEEP should be set. doi.org/10.1378/chest.
Hoffman used inductive plethysmography to measure EELV, but this is not available on the floors. I would propose titrating PEEP/EPAP according the temporal lag between insp effort and insp flow. The moment that insp effort is met with instant insp should be level set for EPAP.
Q: So why not just give IPAP (BiPAP 15/0)?
A: The problem is that wasted energy a/w ITL is not addressed. During expiration, BiPAP is set to 0, but won’t begin IPAP +15 until it senses onset of insp – when P oral < P atm. This cannot happen until pleural press overcome AutoPEEP.
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In summary, using BiPAP for sev asthma
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Goal for IPAP: assist with ventilation & reduce fatigue. (monitor w/ O2 sats, pCO2, physical exam, etc.)
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Goal for EPAP: minimize ITL. (can be incr to threshold when time lag between onset of insp effort and insp flow is minimized.)