Inspiratory Pause and I:E Ratio

The I:E ratio

influence of I-E ratio on CO2 clearance and gas trapping
The I:E (Inspiratory:expiratory) ratio is a compromise between ventilation and oxygenation.

By increasing the ratio (and allowing a longer expiratory time) one allows the last dregs of the tidal volume to escape, taking filthy CO2 with it.

This is of benefit in anybody who has hypercapnea because of high air flow resistance.

Inverse ratio ventilation

inverse ratio ventilation

There is a theoretical benefit in decreasing the I:E ratio.

Firstly, the "mean alveolar pressure" increases, because the time spent under pressure increases.

And this is supposed to improve the rate of diffusion by much the same mechanisms as PEEP.

However, this begs the question, why not just use some PEEP?

Also, the prolonged inspiratory phase recruits "long time constant" alveoli, which take longer then average to get recruited.

Furthermore, the comparatively short expiratory phase results in gas trapping, which leads to some "intentional" intrinsic PEEP.

Again, why not just use PEEP?

In any case, the approach of decreasing the I:E ratio and using inverse ratio ventilation have been studied, and the current consensus is that there is no evidence to support its use. It did not improve mortality in this trial, published in 2000.

Inspiratory pause

inspiratory pause

There may be a period during inspiration during which there is no flow, and the patient holds their breath.

During this time, there is loss of resistance due to flow throughout he airways, and there is a redistribution of pressure across the lung, which results in a total loss of elastic energy stored

in the airways, lung tissue and chest wall tissue.

This results in the loss of pressure within the ventilator circuit (of which the patient is a part).

This pressure is what drives passive expiration once the expiratory valve is opened.

So, it stands to reason that a pressure loss results in a decreased expiratory flow. If you already have some bronchospasm and your expiratory flow is already poor, and inspiratory pause could tip you over and you could begin to trap gas, especially if the expiratory phase is short.

Oh's manual estimates the energy loss due to inspiratory pause as 32%. Why such a specific number? Well. There is an article about this stuff in the Journal of Applied Physiology from 1993.

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.

Zavala, Elizabeth et al.Effect of Inverse I: E Ratio Ventilation on Pulmonary Gas Exchange in Acute Respiratory Distress Syndrome Anesthesiology: January 1998 - Volume 88 - Issue 1 - p 35–42