1. “My patient’s peak inspiratory pressure alarm kept beeping, so I decided to decrease the tidal volume.” Did you check the plateau pressure first? Plateau pressure represents the force required to overcome the lung’s elastic recoil. It will increase with decreased compliance, overdistention of the lungs, or hyperinflation. It is important to note the difference between overdistention (too much tidal volume and subsequent alveolar stretching) and hyperinflation (trapped air in the lungs). If the plateau pressure did not change then the lung’s recoil force did not change, and the ventilator alarmed for another reason.
Peak inspiratory pressure (PIP) also reflects the total force required to flow air through the lungs. Increases in PIP without change in plateau pressure may be due to an obstruction to airflow or an increase in airway resistance. In this case, higher PIP levels would not indicate overdistention. Consider increased secretions (suction the airway), bronchospasm (give bronchodilators), and obstruction (check ETT placement).
2. “My patient’s peak inspiratory alarm kept beeping, and I couldn’t find the silent button.” What if the increase in PIP was the result of decreased compliance? If the plateau pressure increased to the same extent that peak pressure increased without a difference between the two (no change in resistance: R = PIP – Pplat/Flow) then decreased compliance is to blame. Factors that decrease compliance include alveolar collapse/atelectasis (measure auto-PEEP and apply PEEP), lung collapse (decompress the pneumothorax), overdistention of the lung (decrease tidal volume), air trapping (reduce I-time or rate), and right mainstem intubation (confirm with CXR).
Please compare pitfall 1 and 2. Note that the PIP alarm in pitfall 1 resulted from increased airway resistance whereas it represents an increase in plateau pressure due to decreased compliance in this example. While these events may appear similar and the alarms sound the same, understand that their etiologies and management are quite different.
3. “Since my asthmatic had extremely high inspiratory resistance, I chose to use a lower inspiratory flow rate.” While a high inspiratory flow may exaggerate the high resistance in asthma and result in increased peak inspiratory pressures, a short inspiratory time is necessary to allow for a prolonged expiratory time. The expiration phase of breathing must be extended so air trapping is avoided. A careful balance must be obtained between adequate inspiratory flow, PIP, and sufficient expiratory times. Longer E-times are necessary to minimize air trapping and hyperinflation. Setting the ventilator to deliver a high inspiratory flow rate shortens I-time (thus, longer exhalation time).
This will increase inspiratory flow and increase PIP. The severe airway resistance seen in asthmatics actually prevents the complete transference of this pressure to the alveoli (Poiseuille’s law) thus averting alveoli from experiencing the full effect of the PIP. Recall the difference between PIP and plateau pressures. If you raise the PIP, there may not be a corresponding change in the plateau pressure after you shorten the I-time and lengthen the expiration because you’ve reduced the auto-PEEP somewhat. As a result, the plateau pressure will tend to decrease. A high peak airway pressure is not necessarily dangerous unless it corresponds to a dangerously high transalveolar pressure (Pplat).
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