Measurement of respiratory compliance

This chapter is most relevant to Section F3(ii) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "define compliance [and] its measurement". This seems to come up every time the college examiners ask about compliance in general:

• Question 14 from the first paper of 2016
• Question 15 from the first paper of 2014
• Question 7 from the second paper of 2011
• Question 1(p.2) from the second paper of 2008

Most of these SAQs ask for a definition of compliance, as well as methods of measuring compliance.  As for published material, the best free article is probably Scott Harris' article from 2005, mainly because the measurement section there is mercifully brief. A much more detailed overview is afforded by Stenqvist (2003), which is all about the practical means of assessing respiratory mechanics.

In summary, there are several acepted methods of measuring lung compliance:

Supersyringe method of measuring static compliance

This is a classical method of measuring static compliance by gradually and incrementally inflating the lung. The term "supersyringe" refers to an actual syringe, and it earns that superlative because of its humongous size. This one from Hamilton Medical fits up to 2L of gas. The first reference to this thing can be found in a short editorial by Clinton Jenney (1959) which appeared in the "Gadgets" section of Anesthesiology.  In case one wonders what the first supersyringes looked like, the diagram below is stolen straight from that article, together with some delightful scanning artifact. To the right, one can see a grainy photograph of the device being brandished in the attack stance.

In short, this is just an instrument for delivering known, accurate volumes of gas. Though initially intended for the calibration of anaesthetic equipment, it was soon adapted for the purpose of measuring lung compliance in small steps, usually of 100ml. Each time, the respiratory system is given 2-3 second to relax, so that the added volume can redistribute between lung units with different time-constants. When the airway pressure reaches 40 cmH₂O, most researchers have the decency to stop inflating, and trace the expiratory curve by reversing the steps and withdrawing the same volumes of gas. The end result is something like this:

This diagram comes from Harris (2005), where it is not attributed to any reference, and so one might surmise that Scott Harris measured this one himself. The measurement was continued during the inflation steps to demonstrate the utility of waiting a few seconds; after each volume increment, the pressure drops gradually. This method is viewed as the gold standard for the measurement of static compliance, in spite of its limitations. Speaking of which; it is not inconcievable that at some stage the examiners will ask about the limitations of the sypersyringe method of measuring static compliance. In which case, the trainees should list the following points:

• Compressibility of gas is not taken into account, which changes the volume slightly (decreasing it) with increases in pressure. This introduces an inaccuracy into the interpretation of the pressure-volume relationship, because some volume will be lost to this compression, i.e. the lung compliance will look better.
• Temperature changes in the gas are not taken into account;  nor is the addition of humidity. Heated gas can be expected to expand, and room-temperature gas from the supersyringe is going expand somewhat when it is introduced into the warm patient, which will alter the pressure-volume relationship. Specifically, it should increase the pressure slightly, making the compliance look worse.
• The supersyringe measurement takes time. The total measurement process could take a couple of minutes (as each step might take three seconds, and there may be 30-40 steps). Apart from the inconvenience of not breathing during this time, gas will be taken up by the alveolar capillaries - approximately 200-250ml of oxygen, for example. As a consequence, there will be some loss of volume during the measurement process, which will make the compliance look better (i.e. the pressure will appear lower).
• Accuracy may be lost during disconnection.  To use the supersyringe, you will usually need to disconnect the patient from the ventilator, with the possibility that some PEEP and recruitment will be lost (i.e. the supersyringe method will not describe the "true" compliance as it measured with the ventilator). 
• Time-constant of lung units may be longer than expected. Nobody can really say for sure exactly how long one might need to wait between inflations, particularly in the context of heterogeneous lung disease.

Constant low-flow method of measuring static compliance

The abovestated limitations being taken into account, the major objection to the supersyringe method seems to be that it is annoying to perform at the bedside. You have to stand there, syringing gas into your patient, over some number of precious minutes, and the whole thing is quite cumbersome. Enter the continuous flow method. It was first described by Suratt et al (1981), who stated that, where gas is being blown into a container, the rate of change in pressure is inversely proportional to the compliance:

On the inspiratory limb, this method closely resembles being on a volume control mode of ventilation with a square flow waveform. The ventilator blows a slow stream of gas into the lungs, the pressure rises slowly in accordance to lung compliance, and the respiratory resistance is hopefully not much of an issue because the flow is too slow to generate much turbulence at the airways. Performed with an elegant French ventilator (the César model) by Lu et al (1999), the following mesures courbe pression/volume were generated at a constant débit of 9L/min: 

Sure, it's more convenient than supersyringing the patient, but how does this compare to the gold standard? Manikikian et al (1983) did exactly this. Their stolen graph is presented below, where a supersyringe recording is superimposed over a continuos-flow PV loop.

The jaggedness of the faithfully reproduced tracings and the minimalist elegance of black on black does certainly make these tracings difficult to interpret, and so the point is made more clearly by tracing the original data with a pastel crayon and displacing the result to the right. As one can see, the continuous flow method slightly underestimated the inspiratory static compliance, and slightly overestimated the expiratory static compliance. In other words, because of the contribution from airway resistance, even at the seemingly trivial flow rate of 1.7 L/min, with the continuous flow method the pressure will appear to be higher at any given volume on inspiration, and lower on expiration. 

So, this technique clearly has some limitations which are probably worth knowing about for exam purposes:

• Airway resistance is not 100% eliminated: the constant flow, slow though it might be, still tends to impose a certain resistance, which shifts the inspiratory curve to the right, and the expiratory curve to the left.
• Expiratory flow is difficult to measure: the ventilator has to be specially modified to produce a controlled expiratory flow rate, which is the opposite of what they usually do. In most ventilators there is an expiratory solenoid valve which controls flow to produce the prescribed PEEP, and this mechanism must be altered to produce the desired flow rate with a variable pressure (which then gets recorded for the purposes of measuring compliance). This is sufficiently perverse to void the warranty on most models. 

Multiple occlusion method of measuring static compliance

So,  if one were unwilling to handle a huge 2000ml syringe, or to subject a patient to a fifteen-second inspiration, one might instead choose the multiple occlusion method. This is a simple trick when every ventilator can be trained to do, and which involves performing a supersyringe-like breath hold at different points in different breaths. After such a gold, the ventilator gives a few normal breaths before obstructing the breath at some different volume. By performing a whole series of these measurements over the course of several minutes, an entire supersyringe-like static compliance curve can be determined.

The major advantage of this, provided it is automated, is convenience. However, as everything, this method has several limitations.

• The patient still needs to be sedated and paralysed
• The patient's lung compliance may change over the course of the measurement period, rendering it inaccurate

There is little lietarture out there to describe this method, or to discuss its limitations. The first mention of it appears to be Olinsky et al (1976), who used it in pre-term infants, taking advantages of the transient apnoea which occurs due to the Hering-Breuer reflex. Mehta et al (2003) appear to have used this in adult ARDS patients, concluding that it returns data which is very similar to supersyringe data.

Dynamic measurement of pressure-volume relationships

Dynamic lung compliance, as the name suggests, is measured in the course of a normal respiratory cycle, without interruptions.  Of course, when the air is moving, there is going to be some change in pressure due to airway resistance. This means that "dynamic compliance" is an inaccurate term to describe the relationship being measured (because a component of the pressure you have measured is a part of respiratory resistance rather than compliance).

Anyway, the definition of dynamic compliance is discussed elsewhere. How do you measure this misnamed parameter? Continuously, is the basic answer. It is best explained in terms of ventilator function, as it is the most convenient method of measuring it. Basically, the ventilator blows air into the patient, the volume increases, and because the ventilator carefully measures both pressure and volume, one is able to produce a relationship of volume and pressure over time (i.e. the ventilator waveform graphics)  or a relationship of volume over pressure (the PV loop). Here's an example of a PV loop recorded using an ancient Siemens Servo-I in a SIMV(VC) mode:

In general, provided the patient is breathing sufficiently normally, dynamic compliance measurement is viewed as a satisfactorry surrogate for static compliance.