When we see patients in the emergency department, medical ward, or intensive care unit who are tachypneic and using accessory muscles, we clinicians typically comment that they have increased work of breathing. We see this extra effort as work, and quickly plan on how to diagnose the problem and ease our patient’s breathing. We reach for a tool like noninvasive ventilation (NIV) to improve our patient’s oxygenation or ventilation, but, as a side-effect, NIV changes their pulmonary physiology in a way that makes breathing easier and more efficient.
Let's review the three ways NIV can change standard pulmonary physiology:
- Improves functional residual capacity (FRC)
- Improves lung compliance
- Removes lung water
1. NIV improves functional residual capacity (FRC)
By providing positive pressure throughout the respiratory cycle, NIV improves the FRC.
Functional residual capacity is the volume of air left in the lung after the end of a typical breath.
Typically, exhalation against atmospheric pressure is a passive process that allows the lung to empty significantly. But by providing positive pressure during expiration, the breath ends earlier due to the cessation of passive recoil at a pressure higher than atmospheric pressure, therefore the FRC is greater.
An improved FRC will allow alveoli that were completely closed previously to remain open and participate in gas exchange. And areas of the lungs that were well-perfused before without adequate ventilation will have improved V/Q matching and improved oxygenation.
2. NIV improves lung compliance
A secondary benefit of the recruitment of more alveoli is improved lung compliance—the change in volume of the lung due to a change in pressure.
Low FRC is associated with atelectatic (closed) alveoli. When alveoli remain open, the lung remains on the ideal portion of the compliance curve. On this steep portion of the curve, changes in volume can be achieved with smaller changes in pressure, reducing pressure-related injury of the lung.
Warning: Increasing positive pressure too much can cause overdistension of alveoli.
3. NIV removes lung water
Another way in which NIV can help improve pulmonary physiology of the diseased lung is through the removal of lung water.
Alveoli that were otherwise full of fluid, say in the case of pulmonary edema, have a thicker interstitium which increases the distances gas must travel to the neighboring blood vessel.
Positive pressure can redistribute fluid into the neighboring bronchial interstitium, allowing for a shorter distance for diffusion between the alveolus and blood vessel.
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