More precisely, it’s the ‘transpulmonary Pplat’ that’s the most direct marker of lung stress, but that’s a discussion for another day. If the peak pressure is reported to be high, the next question should be – what is the plateau pressure (Pplat)? A high plateau pressure should raise concerns about lung injury. (Remember, resistive component = resistance x flow if flow is zero, resistive component is zero.) This is nothing but plateau pressure (Figure 1). This can be achieved by measuring airway pressure after applying a post-inspiratory pause in airflow. Hence, to get any information about the potential for ventilator-induced lung injury, we have to filter the resistive component and tease out the elastic component of PIP.

For simplicity, we will ignore the effect of chest wall for now.

On the other hand, elastic pressure is “spent” on the elastic structures: lung and chest wall. Resistive pressure is dissipated entirely in parts of the respiratory system that contribute to resistance (mainly small airways). Straight off the bat, it should become obvious that the resistive component of PIP should have no bearing on lung injury. For simplicity, let’s assume a PEEP of zero so that elastic pressure = driving pressure = plateau pressure.Ĭomponents of peak inspiratory pressure: further analysis Note that clinically, elastic pressure (elastance x volume) is nothing but driving pressure (plateau pressure – PEEP). ARDS, IPF) and with high set tidal volume. This is increased in cases of increased lung and/or chest wall elastance (e.g. (b) Elastic (static) component: The product of respiratory system elastance and volume. higher set flow or shorter I-time for a given tidal volume). This can be higher due to an increase in resistance (e.g. (a) Resistive (dynamic) component: The product of respiratory system resistance and flow. Peak inspiratory pressure = (Resistance x Flow) + (Elastance of respiratory system x Tidal volume) + PEEP The equation of motion in a passive mechanically ventilated patient with square waveform would be as follows: In summary, ventilatory pressure has two distinct components (Figure 1): At this point, it would be helpful to brush over the previous articles on the equation of motion ( Part 1 & Part 2 ). So, is PIP directly related to lung injury? To answer this question, it is important to carefully study how the PIP is ‘spent’.

For simplicity, the following discussion is limited to passive ventilation in a paralyzed patient. This engineering feature is a testament to the potential dangers of high ventilatory pressures.īy definition, peak inspiratory pressure (PIP) is the highest pressure delivered by the ventilator at any instant during a single ventilatory cycle. The ventilator also cycles the inspiration off when the upper limit of PIP (set by the user) is reached. It is perhaps not widely appreciated that the peak pressure alarm in ventilators is not simply an alarm. It is inherently true that extremely high pressures delivered by the ventilator may lead to lung injury. It is natural to have some inherent dread about high PIP. Nonetheless, it is reflective of how some of us feel about peak inspiratory pressure (PIP). I would love to take credit for the title of this article but this is stolen from this excellent narrative review.

Pip in mechanical ventilation