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 ©Copyright
 Published: 29/11/2011

Ventilation Graphics and Respiratory Function Monitoring

 

Reviewed by Carl Kuschel
June
2003
Ventilation Index back Newborn Home
Introduction Waves Loops Examples Key Principles
  • This page contains animated respiratory function graphics.  If you cannot see the animated ventilation waves below, click here to download Flash Player

Introduction

The "art" of ventilation – evaluating clinically what the baby is doing and making changes in ventilation settings – is now being replaced by the "science" of ventilation – looking at data collected by respiratory function monitors and manipulating the ventilator accordingly.

This has only been possible with the sophisticated electronics now available on most ventilators. Respiratory function monitoring involves the integration of information such as airway pressures, flow, and volume to evaluate changes in pulmonary mechanics. Data provided by these systems are ignored at your own peril. Theoretically, being able to tailor the individual baby’s ventilation according to these data should reduce the incidence of complications from ventilation, such as air leak and chronic lung disease. However, no controlled studies have ever been performed to demonstrate the benefit of monitoring.

Each system is slightly different in terms of what is measured. No system directly measures volume but, by the measurement of flow (either directly, as with the Babylog, or indirectly, as with the VIP-Bird), volume can be calculated if as flow (ml/sec) versus time (seconds). This has opened a whole new world to neonatologists who were previously obsessed with pressure (barotrauma). There is animal evidence to suggest that although pressure may lead to lung damage, it is volume leading to overdistension that is far more injurious.

From the volume delivered, compliance and resistance can be calculated. Minute volumes (which truly reflect ventilation) can be also calculated, and it is easier to know how a baby has responded to a change in ventilation or management.

There are three basic parts to respiratory function monitoring. There are "waves", "loops" and indirect measurements (that is, volumes, compliance, and resistance which are calculated by changes in the measurements in pressure and flow).

Waves

The waves are plotted against time. On the horizontal axis is a time scale. On the vertical axis, the parameter studied. The usual parameters are pressure, flow and volume.

Pressure (in blue) is the least useful. It really only tells us what pressure the ventilator is delivering at the point of measurement, and tells us nothing about flow or more importantly volume. However, it is usually permanently on display as these are the settings that have been prescribed. To show how unhelpful it is, place your finger over the end of the ETT flow sensor connector. There will be no flow or volume but the pressure trace will continue unchanged.

Flow (in red) is shown next. Above the time scale is inspiratory flow, below is expiratory flow. From the area under the wave, volume can be calculated. This is useful to detect obstruction, leak (if it is large enough), and to set the inspiratory time appropriately in SIMV or SIPPV.

Volume (in green) is probably the key element – how much air is entering and leaving the lungs (through the ET tube) per breath. This is calculated via flow versus time. It is useful from a quantitative perspective to see the tidal volume (aim for 4-8ml/kg per breath). It is particularly useful to detect leaks, where the volume in expiration will not return to zero as some air is lost around the ET tube.

Loops

There are essentially two loops which are useful – pressure-volume loops and flow-volume loops.

Pressure-volume loops essentially are looking at compliance (change in volume for a given change in pressure). However, they also can give information about over-inflation, prolonged inspiration, and leaks.

The image to the right shows a normal Pressure-Volume loop.  The line moving up from left to right is the inspiratory cycle - as the delivered pressure increases during inspiration, volume increases.

The line moving down from right to left indicates deflation of the lung in expiration with a loss of volume with decreasing pressure.

The green line connecting the points of minimum and maximum inflation is dynamic compliance.  Compliance is the change in volume for a given change in pressure.

Flow-volume loops can give some information about leaks (the volume will not return to zero) as well as resistance.  No images are available yet for flow-volume loops.

Examples

Below are some scenarios explaining what the various loops and waves show.

Inspiratory Time Too Short

In the following images, the inspiratory time has been set too short.

The general form of the pressure wave is unaltered, other than the plateau phase of the peak pressure is shortened.

The flow wave however shows that expiration  starts before a full inspiration has been delivered, and there is a rapid down-slope on the flow wave.

The volume wave is still on an upslope at the end of the inspiration, and has not yet started to plateau.

The inspiratory time should be increased until the inspiratory flow reaches zero and the volume at this time should plateau out also.

Inspiratory Time Too Long

In the following images, the inspiratory time has been set too long.

The general form of the pressure wave is unaltered, other than that the plateau phase of the peak inspiratory pressure is lengthened.

The flow wave however shows that expiration starts well after inspiratory flow has ceased and there is a horizontal line along the time axis before the downward expiratory phase begins.

The volume wave is flattened over the final 0.1seconds of the inspiration, with no extra volume delivered for the longer breath.

The inspiratory time should be decreased until the inspiratory flow reaches zero and the horizontal component along the time axis is abolished.

Leak

A leak is present when the amount of gas inspired does not equal the amount of gas expired.

As per the previous images, the pressure wave form is unaltered.

The flow wave shows a normal inspiratory flow wave above the line (with severe leaks, the inspiratory wave may not return to zero flow).  The expiratory flow also looks normal or near-normal but the area of the expiratory part of the flow is less than the area of the inspiratory part.

The key wave however for detecting leaks is the volume wave.  With a significant leak, less gas will come out past the sensor than is going in, so the volume wave does not return to zero at the end of expiration but instead resets at the start of the next ventilator breath.

Some of the volume and flow wave forms can take on bizarre shapes if the leak is huge.

The solution to this, if ventilation is affected, is to change the tube to a larger size.  This may not be possible (for example, you would not put a size 3.0 ETT in a 650g infant even if the leak was this large), in which case other strategies may have to be developed. Another option is to extubate to CPAP.

In huge leaks however, there may be positive flow into the baby despite the ventilator cycling through expiration and the flow wave may rise above the zero axis.  This, in some ventilators, can predispose to a situation of "auto-cycling" or "auto-triggering" where the ventilator thinks that the baby is breathing but it is in fact detecting flow from the leak.  This results in the ventilator delivering a few breaths in close succession, then pausing. This can be manually compensated for in some ventilators.

Note that autocycling from leaks does not occur in the Babylog as it compensates for the leak in deciding when to trigger.  But it will autocycle particularly if there is water in the ventilator tubing.


Overdistension

In the pressure-volume loops, overdistension can be detected by flattening of the last part of inspiration - that is, as the lungs become full, further application of pressure does not result in any appreciable increase in tidal volume.  This is recognised as "beaking" on the loop.

An alternative way of objectively measuring this is via the C20/Compliance ratio - that is, the compliance in the last 20% of inspiration (C20) should be greater than 80% of the value of the dynamic compliance of the entire breath.  A C20/Cd ratio <0.8 suggests overdistension

Overdistension may be a result of:

  • too high peak inspiratory pressures
  • too long an inspiratory time
  • too high peak end expiratory pressure (PEEP)

The appropriate strategy to combat this will depend on the underlying reason.


Autocycling and Autotriggering

Auto-triggering

In this example, the ventilator essentially interprets positive flow in expiration (from continued flow of gas into the baby because of a leak or pressure/flow fluctuations in the ventilator tubing) as a breath and triggers inappropriately.  This may take the form of several breaths together, followed by a pause while the ventilator compensates for the fast respiratory rate. The way to fix this is to adjust the assist sensitivity up to above the residual flow in expiration.

Although the Babylog compensates for a leak, auto-triggering can occur when there is water in the ventilation tubing.  In the Babylog, in PSV or SIPPV mode, it results in a very high respiratory rate and the Panting Alarm will sound.  One option in this situation is to increase the trigger sensitivity so that the baby has to do more work for a breath to be recognised as such.

Key Principles

There are some key principles to using respiratory function monitoring:

  • It helps to have a basic understanding of respiratory physiology.
  • Look at what the baby is telling you about their lung function. What they tell you now may be quite different in 5 minutes.
  • Think about the effect of any change you make to ventilation because of the monitoring. What effect did the change in inspiratory time have on volume delivered? Did turning the peak inspiratory pressure up make any impact on volume or did the compliance decrease?
  • Have some default settings – if you are turning the inspiratory time down, have a limit where you are uncomfortable (e.g. you might set the lower limit of inspiratory time to 0.25 seconds instead of the default 0.35).
  • Stimulate your brain to think about what the baby is telling you!
  • Don't use the graphics as your sole tool in decision making - it should be used in conjunction with looking at the baby, the blood gases, and all of the other information at your disposal.