In addition to improving oxygenation, noninvasive ventilation (NIV) can also improve ventilation—the exchange of oxygen for carbon dioxide—in a variety of pulmonary diseases.
First, let's review what ventilation, specifically minute ventilation is so that we can understand how NIV might affect it.
What is minute ventilation?
The minute ventilation of the respiratory system is the amount of air moved in and out by a patient in one minute. It is defined as the respiratory rate (RR) times the tidal volume (VT).
The tidal volume (VT) is the volume of air inspired with each breath, and the respiratory rate (RR) is the number of breaths taken per minute.
Ok, so with that out of the way, let's now take a look at how NIV improves ventilation—in one of two key ways:
- Improves tidal volume (VT)
- Counteracts intrinsic positive end-expiratory pressure (PEEP)
1. NIV improves tidal volume
We know that NIV improves a patient’s ventilation. But how? Noninvasive ventilation is most commonly delivered as a spontaneous mode of ventilation. This means that patients control their intrinsic respiratory rate and the ventilator does not provide a minimum respiratory rate, outside of emergency backup modes. Since NIV does not directly influence the respiratory rate, it must exert its effect on the tidal volume.
Noninvasive ventilation increases the efficiency of breathing, and the added pressure during inspiration augments the volume of air that is inspired—the tidal volume—and increases in tidal volume improve ventilation of the entire lung.
2. NIV counteracts intrinsic PEEP
Another method of improving ventilation relates to the intrinsic pressure created by the airways at the end of expiration, which is termed intrinsic positive end-expiratory pressure, or intrinsic PEEP.
In some disease states, the airways may be constricted making it more difficult for air (specifically carbon dioxide) to be released during expiration, resulting in hyperinflated alveoli and high intrinsic pressure (PEEP). Noninvasive ventilation can counteract this pressure to allow improved expiration, ventilation, and carbon dioxideexchange.
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