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Hemodynamic Compromise in Trauma

 

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Dead space is something we need to always look at and calculate when dealing with our ventilator patients.  We can calculate dead space in 2 different ways.

1. 1 mL/pound of ideal body weight (neonates, pediatrics, small adults)

2. 150 mL/breath (normal sized adults)

Understanding the difference between minute ventilation (VE) and alveolar minute ventilation is important.  All that’s great. But how does that affect our patients?  How does it affect our patients with hemodynamic instability?  Do we treat our trauma patients different than an ARDS patient?

Let's attempt to answer some of these questions.

Smaller breaths have a higher percentage of dead space, which adds to intrathoracic pressure but does not participate in gas exchange.  Thus, for a given ventilation need (alveolar ventilation, or the volume that actually exchanges CO2 and O2) it's more efficient to use larger and much slower breaths. 

When looking at the difference seen with patient’s receiving respiratory rates of 8 – 12 per minute, it doesn't make a huge difference, but this is gigantic when you look at intrathoracic pressures with rates of 20-30 breaths per minute as seen with the ARDS net approach, metabolic acidosis and with patients over-breathing the ventilator.

Many questions have been raised regarding this.  Let's dive into a few!

1. Is there a point where the larger tidal volumes actually add more to intrathoracic pressure than they contribute to gas exchange?

  Yes, but this doesn't occur until the "Upper Inflection Point" is reached.  In a normal set of lungs, that's not for 1.5-2 liters (way more than we would be giving). 

 

2. What is an upper inflection point?  What is a lower inflection point?

            The upper inflection point is the amount of volume that the lungs can be filled with so as to not have barotrauma or alveolar damage. 

 

The lower inflection point is the minimum amount of volume needed to inflate the alveoli so as to not have atelectasis trauma/collapse.

In an ARDS patient, (which is where most ventilator recommendations are derived), the upper inflection point can be much less than one liter.  “That's how we reconcile our recommendations for larger/slower breaths against the "lung-protective" strategies (which were derived for ARDS) that use smaller/faster breaths” (Davis, Dan MD; Air Methods Medical Director/Scientific Advisor, Dec 2014)

They actually are very comparible when you consider the upper inflection point being much lower in ARDS. 

In a hemorrhaging trauma patient, most often they will have non-diseased lungs.  Alveolar recruitment isn’t an issue, nor is the oxygenation diffusion shunts seen in ARDS.  So the hypoxia aspect isn’t the issue. The issue is the aspect of dead space and the increased intrathoracic pressure associated with that dead space and high (f).

The primary challenge in ARDS is oxygenation due to the huge diffusion gradient shunts seen with the diseased lungs.  Remember, oxygen is perfusion-dependent.  In other words, it may be worth the hemodynamic compromise from elevated intrathoracic pressure to avoid hypoxia in ARDS.  In a bleeding trauma patient, this may not be true.

Listen to the podcast:  Ventilator Strategy - Hypotension

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