Ventilator Graphics made easy

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


  • Introduction
  • Basic Terminologies
  • Decreased lung compliance • Suspect/manage
  • Elevated inspiratory and expiratory resistance • Suspect/manage
  • Air hunger or flow deprivation • Suspect/manage
  • Increased work of Breathing due to trigger asynchrony • Suspect/manage
  • Air Leak from ventilator loops • Suspect/manage
  • Auto PEEP and Air trapping • Suspect/manage
  • Increased Airway secretion and Raining circuit • Suspect/manage

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.

Ventilator waveform analysis -

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.

Ventilator graphics can be classified as ...



  1. Scalars
    1. Pressure Time scalar
    2. FLow Time scalar
    3. Volume Time scalar
  2. Loops or curves
    1. Pressure-volume loop
    2. flow-volume loop


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


A normal pressure-volume loop


Normal Pressure-volume loop
Fig.1 - A normal pressure-volume loop on ventilator showing pressure plotted against volume.

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...

• What does the compliance of ventilated lungs mean?

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.


• What is Elastance?

Normal PV loop in mechanical ventilation
Fig.2 - A normal pressure-volume loop showing normal hysteresis, resistive work and elastic work.

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


• What are Resistance and Resistive work?

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...


How to identify decreased compliance of lungs?


Pressrue volume loop showing decreased compliance of lung
Fig.3 - P-V loop showing reduced compliance.

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.

Suspect

  1. RDS, ARDS, 
  2. Pneumothorax, 
  3. CHF
  4. Consolidation, fibrosis, 
  5. pneumothorax, pleural effusion 
  6. Increase in abdominal pressure.

Manage

  1. Identify cause and treat specific causes like pneumothorax, raised intraabdominal pressure.
  2. Titrate PEEP up to or above the lower inflection point.
  3.  Recruitment maneuvers.
  4. Surfactant therapy.

How to Identify Elevated Inspiratory Resistance?


Pressure volume loop with increased inspiratory resistance
Fig.4 - P-V loop showing increasing resistance to inspiratory flow.

Suspect

  1. Correct problems with endotracheal tube-like small size, kinked, obstructed ET, patient biting the ET tube
  2. Mucous plug causing the inspiratory obstruction. (They can cause a dynamic obstruction as they are movable)

Manage

  1. Identify the mechanical obstruction and fix it. 
  2. Proper use of humidifiers and Use of saline nebs for thick and obstructing secretion.
  3. Bronchodilators wherever applicable.

Identifying Elevated Expiratory Resistance.


Pressure volume loop with increased expiratory resistance
Fig.5 - P-V loop showing increasing resistance to inspiratory flow.

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.

Suspect

  1. ETT problems ( small ET size, kinked, obstructed due to mucous plug)
  2. Bronchospasm.
  3. Blocked or malfunctioning expiratory valve.

Manage

  1. Correct ET issues ( change only if required )
  2. Add a bite block
  3. Change expiratory filter if malfunctioning
  4. Deal with increase and thick secretion with nebs and proper humidification.
  5. Increase PEEP to the stent airway wherever applicable (distal bronchomalacia)
  6. Bronchodilators when indicated
  7. Bronchoscopy for the stuck mucous plug, Increase.

How to identify elevated inspiratory and expiratory resistance?


pressure volume loop with increased airway resistance
Fig.6 - P-V loop with increased airway resistance to inspiratory and expiratory flow.

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.


How to identify overinflated or over distended lung.


pressure volume loop in overdistened lung
Fig.7 - P-v Loop in overdistention.

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.

Suspect

  1. Tidal volume set too high in volume control ventilation.
  2. Pressure set too high in pressure control ventilation.
  3. If the compliance of the lung has changed, then the previously set pressure can now result in distention.

Manage

  1. Optimize the tidal volume in VCV.
  2. Optimize Peak pressure in PCV. 
  3. Consider changes in compliance and airway resistance from time to time to keep optimizing your settings.

Identifying Air hunger or flow deprivation from ventilator waveforms. (Flow Asynchrony)


flow asynchhrony on P-V loop
Fig.8 - P-V loop showing flow deprivation /asynchrony.

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.

Suspect

  1. Low set tidal volume
  2. Very high Ti
  3. Trigger set too high (less sensitive trigger)
  4. Inadequate sedation.

Manage

Identify the problem and solve it.


The equation for Flow is...

Flow = TV / iTime 

So to increase flow, you can either increase either TV or decrease the Ti whichever is appropriate in the given ventilator scenario.



How to identify increased work of Breathing due to trigger asynchrony?


trigger asynchrony in ventilator
Fig.9 - Trigger asynchrony on the PV loop.

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.

Suspect

  1. Trigger set too high (less sensitive trigger)
  2. Inadequate sedation
  3. Spontaneous breathing in scenarios where it is inadvisable.

Manage

  1. Optimize the trigger sensitivity as per patient comfort 
  2. Deepen sedation where applicable
  3. Paralysis where appropriate.

Identifying Air Leak from ventilator curves and loops.


Pressure volume loop showing leak
Fig.10 - P-V loop showing leak.

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.

Suspect

  1. ET leak due to small size, wrong placement. 
  2. Detached or loose ventilator tubes
  3. Loose water trap
  4. Sensor not in place
  5. Any attachment like nebulizer left loose or open in the circuit.
  6. Damaged circuit.

Manage

While some leak is permissible, Identify the cause of a leak and manage is the leak is able permissible level and affecting ventilation. The use of cuffed ET can help.


How to identify auto-PEEP or Air trapping?


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.

auto-peep on flow volume loop
Fig.11 - Flow-Volume loop showing air trapping.

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.


flow volume curve showing leak in ventilator circuit
Fig.12 - Flow volume loop in air leak.

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.

Suspect

  1. Extrinsic factors like Increased expiratory resistance in the circuit (small ET tube, excessive secretions) 
  2. High set TV and insufficiently set expiratory time resulting in inadequate emptying.
  3. Partially blocked ETT 
  4. Intrinsic factors like the early collapse of unstable alveoli/airways during exhalation, acute or chronic airflow limitation (Ashtama)
  5. ARDS
  6. Asynchronous ventilation due to inadequate sedation.

Manage

  1. Identify extrinsic causes like blocked airway, small ET size, and resolve.
  2. Decrease minute ventilation either by decreasing TV or RR (MV=TV x RR) whichever feasible for the case, simply speaking If you put less air into the lungs each minute, the patient has to exhale less air and, therefore, there is less potential for air-trapping.
  3. To provide more time for the patient to exhale so increase E time. Decreasing the rate to low normal will automatically increase the expiratory time if inspiratory time is kept constant.
  4. Add optimal PEEP. Removing PEEP in spontaneously triggered ventilation will increase WOB. So you have two choices. Either take away the spontaneous trigger by deepening sedation and paralyzing or optimize the PEEP at 80% of the total PEEP. 
  5. Bronchodilators in case of bronchial asthma or reactive airway obstruction. Ventilation is not a solution in bronchial asthma but a bronchodilator is.  
  6. Adequate sedation and or paralysis to ensure patient-ventilator synchrony, simply doing this sometimes solves the problem.


How to identify increased Airway secretion and Raining in the circuit?


P-V loop showing condensed water in ventilator circuit
Fig.13 - Pressure volume loop showing condensed water in the ventilator circuit .

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)

Suspect

  1. Airway Secretions.
  2. Condensed Water in the Circuit.

Manage

  1. Endotracheal Suction
  2. Clearing water from tubing and water traps

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

To make it simple, if you see noisy lines, check the circuit for water first, If it's clean, suction should be the next thing to do. Remember even the smallest amount of water can be sufficient to act as a trigger.

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.

Recommended Reading

  1. R Scott Harris. Pressure-Volume Curves of the Respiratory System. Respir Care 2005;50(1):78–98.
  2. Frank Rittner, Martin Doring. Curves and loops in mechanical ventilation. Drager.
  3. Albert L. Rafanan. Ventilator Waveforms: Interpretation. Asia pacific society of respirology. 
  4. Ventilator graphics focused more on scalars (pdf)

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auhtor

About Author

Ajay Agade | DNB FNB Fellowship in Pediatric pulmonology

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