VENTILATOR GRAPHICS MADE EASY

Easy way to learn Ventilator Graphics
By Dr Ajay Agade
Signals come out of the patient, go into the ventilator, and pop up on the screen
Warren Sanborn

Graphics are considered as a area of super-specialist, but every post graduate who ventilates and monitor ventilated babies should be well verse with these, otherwise ventilation will become a blind procedure. Just monitoring ventilated parameters on screen is not sufficient, Ventilator graphics gives us insight of patient’s respiratory dynamics in real time.

The vent graphics itself is a very exhaustive topic and understanding graphics by no means can be achieved simply by reading this post.  It requires thorough knowledge of lung mechanics and machine characteristics and this post is no way a short cut to that.
However
I have just tried to provide simple hand sketched VENTILATOR Graphics that illustrates common problems that we all face in our unit during post graduates days. In fact many of us are managing PICU and NICU patients, for those, this may come as handy tool plus this can be a theory question as well.
Ventilator graphics are broadly of two types
  1. The Scalars
  2. The Loops
Loops can be further divided as
  1. Pressure Vs Volume loop
  2. Flow Vs Volume loop
Only loops are discussed here, you need to go through scalars too, and may be i will post about them some other time but for now only loops

Normal pressure volume  loop looks like this.
On X axis pressure  is plotted against Volume on Y axis. TV is tidal volume, and its the highest point achieved on Y axis. PIP is peak inspiratory pressure  which is highest point (pressure) achieved on X axis.

Now you see how slowly the volume rises with increase in pressure initially and then suddenly it gives away, with a sharp increase in volume even with lesser rise in pressure. ( See the area of give away as a circle.) The point at which this change happens is called as lower inflection point.

If you have to define how compliant the lung is, it will be defined as the ability to raise the volume with rise of pressure, that means more the compliance, more volume can be achieved at lower pressure and vice versa, right !!
Compliance = change in volume/change in pressure
When the flow travels in during mechanical breath, it faces resistance, this is inspiratory pressure  (IR) and therefore area colored with red  is resistive work. When exhalation happens in mechanical breath (which is passive just like spont breath) again the flow of air meets resistance, this is expiratory resistance/ ER and area colored with blue is elastic work.

Still not clear about elastic work ? Tidal breathing does not require active muscle contraction during exhalation. The required energy for this is provided by 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 opposite to compliance.

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

Suspect:
RDS ARDS,
Pneumothorax,
CHF, Consoliation, fibrosis, pneumothorax, pleural effusion
Increase in abdominal pressure

Manage:
Identify cause and treat
Titrate PEEP upto or above  lower inflection point
Recruitment
Surfactant therapy
Manage specific causes like pneumothorax, raised intrabdominal pressure

Elevated Inspiratory Resistance
PV loop bulges to the right, it’s inspiratory resistance , look at the resistive work, it is increased, results in increase WOB.

Suspect:
ETT problems (too small, kinked, obstructed, patient biting),
Mucous plug causing inspiratory obstruction

Manage:
Identify above causes and fix it
Bronchodilators

Elevated Expiratory Resistance
PV loop widens towards left, it’s expiratory resistance and will result in increase WOB due to need to active exhalation.

Suspect:
ETT problems (too small, kinked, obstructed, mucous plug),
Bronchospasm,
Expiratory valve blocked/damp/ Malfunctioning
Manage:
Identify cause and fix it, drain water, change ETT, add a bite block,
Change exp filter,
Bronchoscopy for mucous plug,
Increase PEEP (stent airways),
Bronchodilators

If both resistance is increased in both inspiration and expiration then PV loop will like like a box like this

OVERDISTENSION / HYPERINFLATION
After certain point, the linear relation between pressure and volume is lost, The change in volume is negligible as compared to change in pressure. PV loop looks like bird beak at the end. This is over-distention

Suspect:
Vt set too high (volume control ventilation),
Pressure set too high (pressure control ventilation)
If suddenly the compliance of lung has changed, then the previously correctly set pressure can now result in distention
Manage:
Optimize set TV,
Optimize peak pressure for target tidal volume, consider changes in compliance and airway resistance

AIR HUNGER / FLOW DEPRIVATION
Little separation between inflation and deflation limbs (small hysteresis) of P-V loop
“Figure Eight” appearance at end-inspiration. in modern ventilation the flow is controlled by machine based on set parameters like weight, TV, inspiratory time, minute ventilation.
w. 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 becomes slightly positive at the beginning (B) and
then assumes a normal configuration as the lungs empty.

Suspect:
Inadequacy of flow due to Low set tidal volume, high Ti,
Trigger set too high (less sensitive)
Manage:
ID the problem and solve
To increase flow, we can increase either TV or decrease the Ti as,
Flow = TV / iTime 
(so either increase TV or decrease I time, remember even too less I time will also cause flow asynchrony if patient still wants to inspire)

INCREASE WORK OF BREATHING
A small pig tail before the begining of inspiration indicates the presence of patient trigger for that breath in case the child is spontaneously breathing. It looks like a Figure of eight at bottom (unlike in air hunger where it appears at the top of loop).

The bigger the pig tail, the higher the patients effort to trigger the breath and so is the work of breathing. Such patient may eventually tire out despite of support.

Suspect:
Trigger set too high (less sensitive trigger)
Manage:
Optimise trigger sensitivity, adequate sedation

AIR LEAK
The expiratory limb suddenly disappear or seems like suddenly dropping to baseline. ( Pressure-volume loop fails to close )


Suspect:
ET leak (small size, wrong placement), Tubings detached or loose, loose water trap,
Sensor not in place, anything left loose or open in circuit
Manage:
Identify cause of leak in circuit and manage
Replace with bigger/ cuffed ETT if leak is not permissible

AIR TRAPPING (Auto-PEEP)
Flow volume loop showing air trapping or auto PEEP in ventilator graphics

Suspect:
Extrinsic factors like Increased expiratory resistance in the circuit (small ET tube, excessive secretions)
High set TV and insufficiently set expiratory time
Partially blocked ETT
Intrinsic factors like early collapse of unstable alveoli / airways during exhalation, acute or chronic airflow limitation (Ashtama), ARDS
Manage:
Identify extrinsic cause and resolve (Suction, ETT size)

Decrease minute ventilation either by decreasing TV or RR (MV=TV x RR) whichever feasible, 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.

Second, you can provide more time for the patient to exhale so increase E time.

Add optimal PEEP ( The increase work done against auto PEEP is negated if you add extrinsic PEEP reducing the PEEP is not the solution here.)
Bronchodilators in case of bronchial asthma, adequate sedation and paralysis to ensure patient ventilator sunchrony, simply doing this sometimes solve the problem.

AIRWAY SECRETION / RAINING CIRCUIT
This causes a “noisy” signal on both scalars and loops and is result of turbulence created by flow in water of secretion either in ET or circuit, Identifying it may help in doing secretion before they organize and block the airway. It also helps in avoiding routine suction.

To make it simple, if you see noisy lines, check the circuit for water first, If its clean, suction should be the next thing to do. Remember even a smallest amount of water can be sufficient to act as a trigger. The movement in tubings cause changes in pressure or flow and may trigger the ventilator to give breath which will result in trigger asynchrony.

Suspect:
Airway Secretions, Water in the Circuit
Manage:
Suction, clearing water from tubing and water trapps

Finally, all you have to do is  to draw these on paper and paste them near ventilators (if allowed) or note them in your daily round diary.
So that's simple ! isn't it ?
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