Interpreting the Shape of the Pressure Waveform

The pressure waveform can give one information about the compliance of the different parts of the patients respiratory system.

ventilator pressure waveform

Peak pressure

This is the maximum pressure generated in the circuit.

peak pressure

A rising peak pressure alerts one to the possibility of airway narrowing in some sense, be it the endotracheal tube being kinked (or chewed on), or the ventilator tubing being full of fluid, or the heat and moisture exchanger being waterlogged, or the secretions building up on the inside of the endotracheal tube. Or, the patient might actually be having some sort of bronchospasm.

Airway resistance pressure

This is the pressure produced by the resistance of the whole circuit (the hoses, the endotracheal tube, and the patients own bronchi)

Airway resuistance pressure

This pressure is only present while flow is occurring. It relates directly to the "dynamic" compliance of the chest.

As soon as flow stops, the pressure due to airway resistance drops to zero.

Thus, one can estimate the airway pressure (and thus the degree of airway resistance) from the difference between peak pressure and plateau pressure.

Break point

pressure curve of high and normal airway resistance

This is the point at which the airway resistance pressure has been overcome, and the lungs begin to fill with air.

Break point is determined by the airway resistance pressure. Thus, the higher the flow, the higher the break point. Also, the higher the airway resistance, the higher the break point.

Its very rarely quite so clear cut. Typically a break point is a subtle change of the curvature, where the gradient of the pressure curve changes.

Plateau pressure: alveolar compliance pressure

This is the pressure in the lung after all flow has stopped. It is the alveolar pressure generated by the delivered volume.


It is directly related to the compliance of the lung parenchyma, the "static" compliance.

plateau pressure

"Plateau decay" is the slight drop in pressure during an inspiratory breath hold. It occurs because of the redistribution of gas between different lung units, as well as because of small leaks in the circuit (around the cuff and various tube connections).

pressure corve in poor and in normal lung compliancePlateau pressure is for all intents and purposes the pressure inside the alveoli.


This is the pressure you would need to alter to improve oxygenation.


One can predict that when compliance is poor, plateau pressure will be high, and the pressure gradient will be steep.

Plateau decay

If you introduce a pause after the end of inspiration, the elastic energy stored in the lung tissue and chest wall will dissipate; additionally, a small amount of gas will diffuse out of the respiratory circuit, whether by leaking though gaps in the machinery, or by getting absorbed by the patient. This is the "plateau decay".

This pressure drop only occurs if there is a pause following inspiration; one can imagine why such a pause may not be a good idea. Consider the lung with an airway obnstruction. The elastic recoil of the lung and chest wall is being relied upon to produce pressure which drives air out of the chest. If this elastic recoil is allowed to dissipate, the rate of gas flow out of the lung will be slower, and there will be more gas trapping as a result.

How much energy is lost? Oh's Manual estimates that as much as 32% of the total stored energy can dissipate as a result of an inspiratory pause. Why such a specific number? Well. There is an article about this stuff in the Journal of Applied Physiology from 1993.

Pressure curve gradient

This is the rate of pressure rise. Again, it depends on the rate of inspiratory flow.

pressure gradient

Because the ventilator flow is a controlled known variable, this gradient represents the compliance of the lungs.


Compliance is the change in volume per unit pressure; flow is change volume per unit time. In a patient with poor compliance a smaller change in volume will lead to a greater change in pressure, and the gradient will be more steep.

PEEP

Much of the time, you have some control over what the PEEP is. At the end of expiration, when flow drops to zero, PEEP represents the alveolar pressure.

References

Most of this information comes from only two textbooks. With "Basic Assessment and Support in Intensive Care" by Gomersall et al (as well as whatever I picked up during the BASIC course) as a foundation, I built using the humongous and canonical "Principles and Practice of Mechanical Ventilation" by Tobins et al – the 1442 page 2nd edition.

Jonson B, Beydon L, Brauer K et al. Mechanics of respiratory system in healthy anesthetized humans with emphasis on viscoelastic properties. J Appl Physiol 1993; 75 : 132–40.