The Foundations of Synchrony:
Respiratory Cycle Time & I:E Ratio
The PreBrief
This post is the first in a series about improving synchrony between the ventilator and the patient. Many times, when there are synchrony issues between the ventilator and the patient, the knee jerk reaction is to simply increase sedation. We know that high levels of sedation mean that the patient stays on the ventilator longer and has more undesired ventilator events. [1]
Our first step in increasing synchrony should not be to try to force the patient to synchronize with the ventilator. Our first step should be to optimize the ventilator to synchronize with the patient.
When using Assist Control mode of ventilation, two of the most common settings that are adjusted to increase synchrony are Inspiratory Time (or Inspiratory Flow) and Respiratory Rate. Changing these two settings will alter the patient’s I:E ratio. Before we can discuss how to adjust these settings to promote synchrony, you must understand how to calculate the correct I:E ratio using respiratory cycle time and inspiratory time.
The I:E ratio is calculated on the ventilator; why do we need to be able to do the calculation?
In Assist/Control Mode of ventilation, you must calculate these values based on the total respiratory rate. Many ventilators only give you these values based upon the set respiratory rate. It is imperative that you be able to calculate them on your own.
Respiratory Cycle Time – The Time for One Full Breath.
When using Assist Control mode of ventilation, two of the most common settings that are adjusted to increase synchrony are Inspiratory Time (or Inspiratory Flow) and Respiratory Rate. Changing these two settings will alter the patient’s I:E ratio. Before we can discuss how to adjust these settings to promote synchrony, you must understand how to calculate the correct I:E ratio using respiratory cycle time and inspiratory time.
The I:E ratio is calculated on the ventilator; why do we need to be able to do the calculation?
In Assist/Control Mode of ventilation, you must calculate these values based on the total respiratory rate. Many ventilators only give you these values based upon the set respiratory rate. It is imperative that you be able to calculate them on your own.
Respiratory Cycle Time – The Time for One Full Breath
This one is easy. To calculate the Respiratory Cycle Time, divide 60 by the total respiratory rate. It really is that easy.
- For a RR of 20: 60/20=3. Respiratory Cycle time is 3 seconds.
- For a RR of 15: 60/15=4. Respiratory Cycle time is 4 seconds.
- For a RR of 12: 60/12=5. Respiratory Cycle time is 5 seconds.
Before you can calculate an I:E ratio you must know the time allowed for each breath.
I:E Ratio – Leave time for the air to leave!
To calculate the I:E ratio, you need to know the Respiratory Cycle Time, the inspiratory time, and the Expiratory time.
“I” is almost always set at 1. Occasionally we do allow the inspiration to last longer than exhalation and then the E is set at 1, but that is a discussion for another day.
To calculate the “E” for the ratio, take the expiratory time (Respiratory Cycle Time – Inspiratory Time) and divide it by the inspiratory time.
Example 1
For a RR 20 and I-Time of 1 second
- Respiratory Cycle time = 60/20 = 3 seconds
- I-Time = 1 second
- E-Time = 3 seconds – 1 second = 2 seconds
“I” is set as 1
“E” = 2/1 = 2
In this example, I:E 1:2
Example 2
For a RR 15 and an Inspiratory Time of 0.8 seconds,
- Respiratory Cycle time = 60/15 = 4 seconds
- I-Time = 0.8 seconds.
- E time = 4 seconds – 0.8 seconds = 3.2 Seconds.
“I” is set as 1
“E” is 3.2/0.8 = 4
In this example I:E is 1:4
What do all these numbers mean and why do we care?
Normal, not intubated humans typically maintain an I:E ratio around 1:4. When we breathe faster, our I:E ratio approaches 1:2 or in severe tachypnea, it may reach 1:1 (Think Kussmals respirations of a DKA patient).
An unnatural I:E ratio will be quite uncomfortable for the patient and cause them to “buck” the ventilator [2]. On your patient with a normal respiratory rate, an I:E ratio of 1:3-1:4 is ideal [3]. For your tachypneic patient, the I:E ratio may be 1:2 or even approach 1:1. If we allow the inspiratory time to exceed the exploratory time, our patients will be extremely uncomfortable and will likely require high levels of sedation and possible neuromuscular blockade.
In patients intubated secondary to obstructive lung pathology (Asthma, COPD), extra time is needed to exhale. This patient’s I:E ratio will need to be 1:5, 1:6, or even longer to allow them to fully exhale.
What is next?
Over the coming weeks, we are going to take a much closer look at promotion of synchrony. We will discuss PRVC setting on the ventilator and how its use can promote a more natural breathing experience. We will look at sedative options to optimize comfort and minimize time on the ventilator. Finally, we will look at optimizing synchrony in some specific pathologies and physiologic states such as obstructive lung disease and severe tachypnea.
References
- Klompas M, Li L, Szumita P, et al. Associations Between Different Sedatives and Ventilator-Associated Events, Length of Stay, and Mortality in Patients Who Were Mechanically Ventilated. Chest2016;149:1373-9. 10.1378/chest.15-1389
- Subirà C, de Haro C, Magrans R, Fernández R, Blanch L. Minimizing Asynchronies in Mechanical Ventilation: Current and Future Trends. Respir Care. 2018 Apr;63(4):464-478. doi: 10.4187/respcare.05949. Epub 2018 Feb 27.
- Roberts, J. Roberts and Hedges’ Clinical Procedures in Emergency Medicine and Acute Care. 7th Edition, Elsevier, Inc. 2019 (160-180.e2)
Thanks for the brief review.
But sometimes the patient went into assynchrony secondary to is air hunger so we might forcefully (increasing) the inspiratory time to achieve patient demand as long as the safety pressure is not allowing to give further volume. What I mean sometimes we are taking the patient away from his normal physiological set to solve his problem.
Completely agree! There will be a post addressing this in the coming weeks.