Ventilator Graphics made easy

Signals come out of the patient, go into the ventilator, and pop up on the screen Warren Sanborn

Managing mechanical ventilation and ventilator Graphics is an area of Intensivist. However, as pediatric residents, we often see mechanically ventilated patients & many of us are involved in ventilation monitoring directly.

Front-note  •  During my residency, I made small sketches with sidenotes for easy reference. This helped me to understand and remember the ventilator graphics, especially during my NICU posting. Comparing them with what is seen on the ventilator screen helped me to identify the issues and rectify them. It is only later, that I discovered how complex the subject is, and therefore I strongly recommend further reading in detail.

Analysis of vent waveforms gives us an insight into the patient’s respiratory dynamics in real-time, helps to fine-tune the setting, and above all help, identify patient-ventilator asynchrony.  Simply, it is our pulmonary function tests on ventilated patients.

These sketches illustrate common problems that we all face in PICU and NICU and can be a handy tool for troubleshooting.

Let us start our ventilator graphics cheat sheet with some super simples basics.

Remember - This covers only ventilator loops / Curves here and leaves the scalars for another day. Let us start with a normal pressure-volume loop.

The pressure is plotted on X-axis against Volume on Y-axis. TV is tidal volume, and it's the highest point achieved on Y-axis. PIP or peak inspiratory pressure is the highest point (pressure) achieved on X-axis.

Observe initially, the volume rises with rising pressure slowly, and then eventually, at one point, there is a rapid increase in volume with a relatively lesser rise in pressure. This area where volume change rapidly is marked with the dotted red circle in the image and is called a lower inflection point.

Based on this, let us try to understand...

If you have to define how compliant the lung is, it will be defined as the ability to increase the lung volume with an increase in pressure, which means with a more compliant lung, more volume can be achieved at lower pressure, right !!

In short, compliance is the ease of expansion.

And hence...

Compliance = change in volume/change in pressure.

Elasticity is a property of coming back together, a property to recoil back to its original shape and size. Hence Elastance is the ease of lung deflation. 

Imagine a thick vs thin rubber band. A thick rubber band will easily recoil back to normal shape than a thin one. But at the same time, it will be difficult to stretch it, so it is less compliant. Therefore Elastane is exactly opposite to resistance and can be put in an equation as,,,

Compliance = 1/ Elastance

Resistive work

Normally When the flow travels through the ventilator during mechanical breath in either direction, it faces resistance by the tubing, human airway, and everything through which it travels until it reaches the alveoli. The work done to overcome this resistance is nothing but resistive work or resistance. The area colored with red is resistive work.

Inspiratory Resistance

The resistance encountered during the phase of inspiration is Inspiratory resistance.

Expiratory Resistance

During exhalation in mechanical breath (which is passive just like a normal breath) again, the flow faces resistance, this is expiratory resistance (ER), and work done to overcome the expiratory resistance is elastic work. It is colored blue in the images.

Still not clear about elastic work? A normal breath does not require active muscle contraction during exhalation. The required energy for this comes from the elastic energy which is stored during force exerted by inspiratory muscle to expand alveoli. Thus elastance is a property to collapse. It is exactly the opposite of compliance!

Moving on to the Interpretation of the various problem from the shape of the pressure-volume loops and curves...

Notice the slowly rising volume on Y-axis with rising pressure. The Change in volume is small as compared to the change in pressure, this is reduced compliance. The lower inflection point as we described above will be shifted up. Simply look at the PV loop it looks like a falling leaf i.e. loop lays more horizontal.

When the PV loop widens towards the left, it is expiratory resistance. The increased resistance to expiratory gases will result in an increase in WOB as it will need active exhalation to empty the lung. Normally it is a passive process.

If resistance is increased for both inspiratory and expiratory gases, the combination of the above two images will be seen on the pressure-volume graphic on the screen. This will make the PV loop look like a box as in the above image.

After a certain point, the linear relation between pressure and volume is lost, which means the change in volume is negligible as compared to the change in pressure. 

Increasing the pressure now will not increase the tidal volume further.  The resultant PV loop will now look like a bird beak at the end. This is over-distention.

The separation between inflation and deflation limbs is called hysteresis of the P-V loop. 

When the flow doesn't meet the inspiratory demand from the patient, It will cause a “Figure Eight” appearance at end-inspiration. 

In modern ventilation, the flow is controlled by the ventilator (autoflow) based on set parameters like weight, TV, inspiratory time, minute ventilation. As the patient’s demand begins to outstrip the flow delivery of the ventilator, the pressure starts to decrease (A) while volume continues to increase. 

Exhalation, therefore, becomes slightly positive at the beginning (B) and then assumes a normal configuration as the lungs empty ultimately giving the loop shape of the digit 8.

The left side of image 10 shows a normal patient-triggered breath, a small fish-tail appears before the beginning of inspiration indicating that the breath is triggered by the patient.  

In trigger synchrony, the patient has to put more effort or create more negative pressure to trigger a ventilator breath (the ventilator now starts giving inspiratory flow). The more effort required, the more will be the work of breathing.

The larger the fish-tail, the higher the patient's effort to trigger the breath and so is the work of breathing. Such patients may eventually tire out despite ventilator support. As they have to fight more to get this support. 

It looks like a Figure of  '8' at the bottom unlike in flow synchrony (figure 8),  where it appears at the top of the loop.

The expiratory limb suddenly disappears or looks like suddenly dropping to baseline not following the expected trajectory. Simply the Pressure-volume loop fails to close.

Pressure-Volume loop does not visually provide a very distinctive interpretation for air trapping, even though we are not discussing the F-V loop here, for identification of air trapping or auto-peep it is worth to discuss it. In the F-V loop, flow replaces pressure on Y-axis.

Here, the expiratory limb is not returning from where it started. The area with red dots represents the volume that is not emptied during expiration, with every single breath there will be residual volume trapped inside the lungs resulting in what is called dynamic overinflation (eg asthma). 

Compare this with the following flow-volume loop which shows air leaks in the circuit, it looks similar however, the expiratory limb touches the baseline earlier than it normally should, as some of the ventilator air is already leaked out. In image 12, the area inside the expiratory loop is smaller than is expected. Thus...

The expiratory tidal volume is smaller than the inspiratory Tv. The area with black dots represents the volume of leaked air.

Coming back to our discussion on auto-PEEP.

While PEEP helps in the patient's work of breathing, auto-peep increases patient efforts. It simply is a result of dynamic over inflammation which means the amount of air going in every time is not coming out in the same amount, thus trapping more and more amount of air inside the patient's lung.

This causes a “noisy” signal on both scalars and loops and is the result of turbulence created by flow in the water or secretion either in ET tube or circuit.

Identifying it may help in doing secretion before they organize and block the airway. It also helps in avoiding repeated suction. The secretion and water in the circuit keep it dangling which results in unnecessary triggers by the ventilator causing patient-ventilator asynchrony. (trigger asynchrony)

Simply emptying the tube of condensed water is all that is required for smother ventilation.

Now, all you have to do is to draw these on paper (which is a good practice) or save them as pdf and keep with you when you are around the ventilator or note them in your daily round diary in NICU and PICU.

Remember this covers only P-V loops, I feel the scalars are more important especially the Flow time scalars, following links might give more insight for that, and hopefully, I will come up with them sometimes.

Author

Ajay Agade | DNB(Pediatrics), FNB(Pediatric Intensive Care), Fellowship in Pediatric pulmonology and LTV

Ajay is a Paediatric Intensivist, currently working in Pediatric Pulmonology & LTV at Great Ormond Street Hospital NHS, London