The Un-Measured Variable (Experiment # 2)


When I first learned mechanical ventilation, the importance of watching your plateau pressures (Pplat) was drilled into my skull. If I noticed the peak airway pressures were increasing, I would perform an inspiratory hold and confirm my alveolar pressure was within a safe range (<30). The inspiratory hold removes flow from the equation, and therefore would remove the influences of airway resistance. Let's illustrate this.

The pressures and values we see on a ventilator are measured at the flow sensor, which sits just proximal to the wye of the circuit. In order for us to truly know the alveolar pressure, we would need an esophageal probe to measure the trans-pulmonary pressure (weight of the chest), and subtract it from the alveolar pressure. This is usually not feasible in the retrieval setting.

Currently I use the Hamilton T1, which is a strictly pressure driven ventilator. The breath is delivered utilizing a decelerating flow pattern. This means that as the breath being delivered approaches the pressure limitation, it reduces the flow. I think of it like pre-paying for gas at the pump. The gas comes out fast initially, but as you approach that $16.00 you put in (#medicproblems) it starts to slow down as to not overshoot the $16.00. Because of this technique of delivering a breath, the peak inspiratory pressure should very closely resemble the pressure the alveoli see (Pplat).

For this reason the Hamilton T1 does not have the feature to perform an inspiratory hold. I mean you could give a manual breath and hold it, but the flow will decelerate as soon as it reaches set pressure, and it won't tell you anything the peak inspiratory pressure wouldn't.

The problem I currently see however is when airway resistance (Rinsp or Raw) is present. Lets take a look at the settings below as an example.

This patient just had diminished lung sounds and was intubated for respiratory failure after standard reactive airway treatment. The current peak airway pressure is 30 cmh20. I had to raise the pressure limit to 40, because the ventilator will terminate the breath at 10cmh20 below the alarm pre-set. In order to get adequate exhaled tidal volumes, it is not uncommon for patients with increased airway resistance to need alarmingly high pressures.

How can I estimate how much of this pressure is actually coming out on the other side?

I asked this question on FB and ventilator guru Joe Hylton chimed in with some insight.

Joe is right, as the flow begins to decelerate, it will eventually meet a point in which the PIP matches the Pplat. The issue is that in obstructive patients we rarely have enough inspiratory time to allow this equilibrium to occur. We known that this population will need a longer exhalation time to avoid air trapping. Knowing that volume is equal to pressure over time, we will have to increase the pressure because we don't have the time. Make sense?

This compares two different I:E ratios and how they will affect your ability accurate translate PIP's to Pplats. This should make you feel a little better about that Pinsp of 40 on your obstruction patient.

So now that we know what question we are asking, we can start to break out some calculations. It's important to realize that these equations are rarely performed during actual patient care. However, I feel it important to understand the concepts in regards to resistance.

How to calculate inspiratory resistance (Rinsp or Raw).

The Hamilton T1 provides us with an Rinsp value on monitoring page three. Without obtaining a Pplat, I am not quite sure how this proprietary formula is calculated.The user manual says it uses a statistical technique utilizing the least squares fit (LSF) to perform a breath by breath anaylysis.

Regardless, the way I utilize this value it to get a rough estimate of my pressure loss through airway resistance.

Utilizing the same equation backwards, we can solve for Y which will represent our estimated pressure loss. In the event of elevated peak pressures I will utilize the following to determine compliance vs. obstruction/resistance.

Obstruction/Resistance Problem

Rinsp > than 10 cmh20/l/s

RCexp >1.2

Widening pressure volume loop

Compliance Problem

Cstat < 50 ml/cmh20

RCexp < 0.5

Flattening of the pressure volume loop

I wanted to see how much of the peak pressure is transferred distal to the stenosed appliance replicating airway resistance.. This should allow me to estimate the amount of distal occlusion pressure for a given flow and Rinsp. Check out this experiment.

As predicted it is extremely hard to estimate how much of this peak pressure is being transferred to the alveoli. We know that there is at least some pressure loss through the conduit. For this reason I think it makes sense to utilize a pressure regulated mode in asthmatics. This will allow the monitor to adjust the pressure needed as the patient improves. For some reason I have noted in my experiment log that the manometer end pressure has correlated very closely with the Rinsp. Calculating various options for that correlation does not appear to maintain accuracy.

I am always looking for others to share there experiments as well.