Tasty Morsels of Critical Care 071 | Non invasive ventilation in the ICU
Welcome back to the tasty morsels of critical care podcast. Oh Chapter 37 is dedicated to NIV in the ICU and is probably worth some time given that this is a common respiratory support both in the ICU and throughout the hospital. Many of the benefits of NIV are similar to those seen with ventilation with the blue plastic tube through the vocal cords.For example you still get: * positive airway pressure which recruits alveoli and improves oxygenation * improved alveolar ventilation which improves minute volume and lowers CO2 * reduction in work of breathing as the machine is doing some of the work * stabilisation of the chest wall eg in rib fractures * reduction in transmural LV pressures acting as a sort of poor man’s IABP (more on that later) The big advantage of course is that you get all the positives but avoid the blue plastic tube through the cords and all the hassle and complications that come with that. But it’s not all unicorns and rose petals, the mask itself has a tendency to macerate the face over time and patients who are already feeling breathless and suffocating often don’t take kindly to having a plastic mask shoved over their face. Even if they do tolerate the mask it is frequently difficult to make a decent seal and maintain that lovely positive mean airway pressure that you’re looking for. And while i did wax lyrical about the potential positives of positive pressure ventilation at the beginning of the post, it seems only fair to point out the negatives of positive pressure ventilation. It is clear that positive pressure ventilation is non physiological and is known to cause its own form of lung injury when applied through a plastic tube through the cords. The alveoli only see the pressure and care not which device it’s being delivered through, so there’s no good reason why NIV wouldn’t cause similar problems. This of course brings up the unanswered and quite entertaining controversy over P-SILI or patient self induced lung injury that hit its zenith during the worst days of the COVID-19 pandemic. There were back and forth letters in the journals between some of the heaviest hitters in the ventilation world bouncing back and forth whether they actually believed self induced lung injury was a thing. Now this is not the post to explore it, but perhaps suffice to say that someone sitting with a resp rate of 30 for a week on 80% O2 and a PEEP of 10 on NIV may well be undergoing some of the same lung stress that any typical ventilated ARDS patient may be undergoing. NIV is not necessarily a free pass. When it comes to modes, the names are, as ever, confusing and baffling. Overall they split into some kind of CPAP mode where airway pressure is constant throughout the respiratory cycle and a mode with pressure support set above the PEEP where the pressure increases above the baseline CPAP when the patient inspires. To make matters worse there’s no clear consensus in how the numbers are described. For example, our portable, single limb circuit, ward based NIV machines use the terminology EPAP and IPAP to describe the pressures with both numbers starting from zero. for example 10/5 would be a CPAP of 5 with an additional 5cmH2O pressure support whenever the patient expires. On an ICU vent this would be described as 5/5. When would you reach for NIV over say one of the aforementioned blue plastic tubes through the cords? Well there are a number of now well established indications where it is entirely appropriate to try and temporise with NIV rather than just putting the tube in. I’ll give a brief summary of a few of them below: Pulmonary oedema. * the heart is poor, the lungs are wet and heavy and the sats are low. The patient is crying out for some CPAP. How might it help,