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Pediatric Airway Equipment Dead Space

Question# 688

Question received from a paramedic in regards to the length of the "tree" (filter, ETCO2, tube extender) at the end of a SGA - iGel in our case - for pediatrics. Has previously been taught that this should be minimized due to tidal volume of pediatric patients and potential to be rebreathing expelled air leading to less than optimal O2 delivery.

Answer:

After consulting with several subject matter experts (pediatric anesthesia, pediatric critical care paramedicine, neonatology RT, and a neonatology transport physician), we offer the following suggestions:

The number one suggestion offered by the experts was to emphasize the use of appropriately sized equipment. If you do not already carry it, pediatric sized adjuncts should be purchased.

Short of that, it would be reasonable to minimize dead space by removing the tube extender. As a corollary, this may lead to the tube being pulled, so extreme caution should be used to ensure that the airway is secure.

If you are running into ventilation difficulties, you could always just spot check with your EtCO2 for your very little kiddos as dead space from the addition of tubing is an important consideration when ventilating babies/infants, but less of an issue when ventilating older children/adults, as laid out below.

The reason for these suggestions comes down to dead space and the length of the airway “tree”.

Dead space is the volume of ventilated air that does not participate in gas exchange. As you mentioned, since patients are breathing out of the same tube, they could inhale unfiltered exhaled gas. When it comes to dead space, we can categorize dead space as anatomical, physiological, or mechanical.

Anatomical dead space is present in every person, based on our inherent and fixed anatomy (trachea, bronchi, bronchioles), and is not involved in the exchange of gas as there are no alveoli in these structures. While these areas are ventilated, they are not perfused. Oxygen can’t be absorbed, and CO2 can’t be expelled – this is effectively “wasted breath”.

Before we can further examine dead space, we need to first discuss tidal volume – the amount of air that moves in and out of the lungs with each respiratory cycle. It thus needs to include enough volume to fill both the lungs and the dead space. Typically, tidal volume is 8 mL/kg, or about 400-500 mL for an adult.

About 1/3 of each breath that we take is dead space, and dead space is age dependant. In an average adult, this is reported to be ~150 mL, or 2 mL/kg of ideal body weight. In general, pediatric dead space is relatively higher compared to adults. In babies & infants, the reported dead space volume is 3 mL/kg.

As an example, consider a 60 kg teenager. Their tidal volume is 480 mL (8 mL/kg x 60 kg), which means a dead space of 120 mL (2 mL/kg x 60 kg). Subtracting their dead space (120 mL) from their tidal volume (480 mL), we’re left with an alveolar ventilation of 360 mL – or about a can of soda.

Let’s compare this to a 2.7 kg infant, who has a TV of 22 mL (8 mL/kg x 2.7 kg). As above, their dead space is 8 mL (3 mL/kg x 2.7 kg). Subtracting their dead space (8 mL) from their tidal volume (22 mL), we’re left with an alveolar ventilation of 14 mL – or <1 tablespoon. Vastly different volumes, yet if we use the same equipment, you can see the impacts.

Physiologic dead space refers to alveoli that are ventilated, but poorly perfused (they lack capillary blood flow to absorb O2 and eliminate CO2). Physiologic dead space can be fluid and can wax and wane based on rapid changes to cardiac output or pulmonary blood flow.

While we can’t control anatomical or physiological dead space, mechanical dead space IS controllable and should be kept at a minimum.

Mechanical dead space is also an important factor in considering dead space. All our equipment and adjuncts have a baseline volume – from the tube, to the EtCO2, to the filter, to the extender. This dead space can ultimately be thought of as an extension of the anatomical dead space, and the breaths we deliver must cover this dead space to ensure we give an adequate tidal volume, while at the same time balancing and preventing barotrauma or volutrauma. Not an easy task in the prehospital world!

As a real-world example (but please confirm with your specific manufacturers), the filter has a dead space of 45 mL, the EtCO2 reader has 10 mL of dead space, and the tube extenders a volume of 28 mL.

While this might not seem like a lot, the dead space becomes magnified if the tidal volume is baseline low – i.e. in pediatric patients (or short adults). As demonstrated above, if you were to ventilate a 7 kg baby, who has a tidal volume of 50 mL, using these adjuncts, you might not be delivering any fresh air. Couple this with a poor mask seal, or difficulty squeezing the bag, and this hypoventilation is only further exacerbated.

So, what do we do with this information?

As is standard practice, we add adjuncts to the airway tree for a variety of important reasons. An easy acronym to remember the order is B-E-F-T-T: BVM, EtCO2, Filter, Tube (extender), Tube.

The filter is important for several reasons. Not only does it prevent clogging of the EtCO2 in the event that the patient vomits or there’s blood/other secretions, it also filters out humidity, which can impact readings downstream in the long-term ventilated patient. Moreover, it keeps the system “closed” if pieces from the tree fall off, and lastly, it’s more compatible at handoff in the ED as they can connect directly to the filter as it’s a universal adapter (vs. the EtCO2).

Yet, despite the importance of these adjuncts, it’s important to recognize the potential of hypoventilation when using them. Conversely, hyperventilation is a known concern in the prehospital world, so as with anything we do, there’s a risk assessment to do, and a balance to strike.

There are several tools to guide our ventilations, such as monitoring for chest rise, lung sounds, some defibrillators have real time feedback mechanisms, and end-tidal capnography. Interestingly, we can use the capnography waveform to show evidence of mechanical dead space (Case 1: https://www.foamfrat.com/post/2018/12/31/dead-space-and-pediatric-mechanical-ventilation).

In conclusion, dead space is an important consideration when ventilating anyone, but minimizing it is especially critical in our smallest patients for several reasons listed above. Try to minimize it by the above listed means. When in doubt, perform spot EtCO2 checks, and if you're ever worried or have questions, you're always welcome to reach out to the base hospital physicians.

References

Medical Directive Category

Paramedic Practice

Published

18 May 2023

Views

289

Please reference the MOST RECENT ALS PCS for updates and changes to these directives.