Measuring lung compliance

Todd Fraser on 27-06-2011

This weeks blog topic was sent in by Ben Tang, who asks : " Just wondering if anyone could help me with a question on compliance? How do we assess lung compliance at the bedside? In an emergency, bagging the patient via an ET is immediate and helpful. However, in elective settings, what will be a more objective bedside assessment can I use to quantify lung compliance? (1) I normally look at the pressure-volume loops on the ventilator. It is helpful if compliance is very good or very bad. But it is not precise. (2) I note that there is a compliance in the Drager data table. However, I don't know how they calculate it (but I assume it is a dynamic compliance). (3) How do you work out the plateau pressure in Drager? The monitor gives you only peak airway pressure and I am unsure how to identify the plateau pressure as the pressure-time curve resembles a square pattern in SIMV. (4) I sometimes used the inspiratory hold function on the Special Function option (Drager) to work out the plateau pressure. Is this a valid way to do it? In summary, what are bedside options I have in assessing lung compliance? And which one are we measuring...dynamic or static compliance?"


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Benjamin Moran wrote 06-27-2011 10:03:33 pm
My approach to compliance is usually preceded by either a high peak inspiratory pressure (if in volume-controlled mode), or low tidal volumes (if in pressure-controlled mode). To determine whether this is a problem with compliance or resistance, I perform an inspiratory hold manoeuvre. All the ventilators I've worked with have this function. You can then freeze the screen and calculate the plateau pressure. On the Drager, there is a red stop-sign that freezes the screen, and then you turn the knob to scroll back to measure the plateau pressure. Without the exact numbers, you get a pretty good idea whether it's a compliance or resistance issue, but the equations (which you must know for the exam) are:

Dynamic Compliance= Vt/(PIP-PEEP)
Static Compliance= Vt/(PPlat-PEEP)

where Vt= tidal volume, PIP= peak inspiratory pressure, PPlat= plateau pressure and PEEP= positive end-expiratory pressure.

It may be difficult to use it to guide therapy, as you may not know the patient's compliance prior to their ICU stay. We do a quick assessment daily, looking at the patients Vt on the same pressure support, but doing an inspiratory hold gives it a number and makes us look like we are intelligent to our non-ICU colleagues. For me, the greatest utility is in differentiating the aetiology of high airway pressures, and static compliance is the measurement of choice. In terms of using pressure-volume loops, I've always been disappointed as the curves on my patients never look as good as they

Todd Fraser wrote 06-29-2011 08:49:08 am
I guess one thing to remember is that any compliance reading you take from a vent is an average across a lot of lung volumes. Which one is the most important? Does it matter? They are also an average of the compliance of many, many lung units, not all having the same compliance. Finally, is it inspiratory or expiratory compliance that's important?

Technically, static lung compliance is measured in the absence of flow - while the equation you mention Ben is correct in one sense, you can't measure it on a vent in this manner, rather you need a highly calibrated set of bellows that none of us have anymore.

Does all this matter? Perhaps not given our therapies are so crude that we can't tailor them well to the numbers we get anyway...

Nathan from Australia wrote 08-12-2019 07:15:13 pm
For any body reading these comments it is important to note that the above discussion is actually describing "Respiratory System Compliance", not "Lung Compliance".
Total Respiratory System Compliance is defined as;
1/Total Compliance = 1/Lung Compliance + 1/ Chest wall compliance
= 1/200ml/cmH20 + 1/200ml/cmH20
= 100mls/cmH20

Total "Respiratory System Compliance" Can be measured with a Ventilator and Separated into;

Dynamic Compliance= Vt/(PIP-PEEP)
Static Compliance= Vt/(PPlat-PEEP)
Specific Compliance =(Vt/(PPlat-PEEP))/FRC

Specific Compliance accounts for absolute lung size/volume in order to compare for e.g an Adult to a child or an Elephant to a mouse

In order to calculate "Lung Compliance";
=Change in lung Volume/change in pressure (Intrapleural - Alveolar)

A person must have an oesophageal balloon inserted to measure "intrapleural pressure" (oesophageal pressure is a close surrogate)
Then breathe into a closed circuit spirometer to measure the change in volume, stopping periodically with an open glottis to equilibrate with the atmosphere/spirometer.

This enables the change in pressure (Intrapleural - Alveolar) to be calculated
And the change in Volume measured from the spirometer.

Chest wall compliance is done similarly, but the change in pressure is measured as Intrapleural - Atmospheric pressure difference, which is then used in the above formula.

ps. yay ANZCA PEx :-(




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