In this video, we will try to
Explain the stress index
And how we can use it in the ventilatory management of ARDS
Let's start with the relationship of volume to pressure of the respiratory system
This graph shows that the pressure volume
Ratio is not linear. In general it is sigmoid in shape with two extremities
where elastance
Is greater and a relatively
Linear intermediate zone in which elastance is lower
These three segments are separated by the lower inflection point
and the upper inflection point
These two points allow us to identify
pressures at which derecruitment or
over-distinction begin
So the first zone is the zone of decrecruitment where the elastance is high
But it is decreasing with time
The second zone is
the zone that is
that starts after the upper inflection point
And the elastance is high, but it is increasing with time
The middle zone is the zone where the elastance is
Lower, but it is constant
It has been postulated that tidalr volume should occur in this central zone of
the PV tracing
between the lower inflection point and the upper inflection point
In this context, it is postulated that in volume control mode of ventilation
with a square wave of the flow
linearity of the pressure curve during inspiration
indicates that the tidal volume occurs between the two inflection points of the PV curve
In order to analyze this phenomena the stress index was defined and
Can be calculated by the computer
using this equation
This equation determines the pressure increase between two points
So we'll take the first point time zero and the second point at time one
And we'll determine the increase in the pressure
required to overcome
number one the lung elastance which is indicated by the a
By the constant a here and the airway resistance
which is indicated by the
Constant b in this formula here
However a simple visual inspection of the tracing can demonstrate this phenomenon
You can see if the pressure-time
curve is linear it indicates that the stress index is close to 1
And that means the elastance is constant
If it is more than 1 it indicates that the elastance is increasing with time and that
indicates over distension
So we'll explain that in this examples here
So the first one is
Where the tracing is straight
You can see the pressure
Time curve has a straight linear increase in the pressure
the stress index here equals to 1
And that indicates that the tidal volume occurs in the
low-risk ventilatory area
it is the area in that lies between the lower inflection point
and the upper inflection point and that's what you want to see in the management of
ARDS on mechanical ventilation
In contrast to this example here
where the pressure dressing is convex
You can see that here
corresponding to the lower part of the PV curve and
Indicating probable lung derecruitment. The stress index here is o.7
You can see this is outlined in the
two columns here and the calculation of the stress index by the computer is
0.7
So this patient is ventilated in the area with the derecruitment of the lung
In this example here, the stress index
is more than one and
that corresponds to a concave pressure tracing you can see how the pressure
Tracing is concave here this indicates probable over-distension
The patient is ventilated at the
Right end of the PV curve, and there is a possibility of our distinction here
so if you put them all together
The stress index of one with as linear pressure time curve
indicates ventilation in the
constant elastince zone which lies between the lower and the upper inflection point which is optimal
the stress index of less than one
With the convex
Pressure over time curve
indicates that the lung is ventilated at
An area of derecruitment
and requires
to increase the peep for that patient so he can move that tidal volume
towards the middle part of the PV curve
stress index of more than one
with concave pressure tracing
Indicates that you are over distending the lung, and you need to decrease the tidal volume for the patient. Thank you