The series of illustrations in this post demonstrate the different types of dead space encountered in clinical practice and interventions to address them. Dead space is a concept that we generally discuss in the context of mechanical ventilation – this short series of graphics won’t get into too much detail on that, but we can point you towards our free book on the idea: www.rykerrmedical.com/vent-management. The goal here is simply to review the different types of dead space, demonstrate them graphically, and point out specific interventions for each of the various types. And then real quick before we dive in: if you’re not familiar with the idea of dead space in general, it’s the concept that there is a difference between the quantity of air breathed in during a breath (tidal volume) and the volume of air that actually participates in gas exchange at the alveolar level (clinically relevant tidal volume). Hopefully the components of that volume of air will be clearer after reviewing the following illustrations.
The first type of dead space is anatomic dead space. Anatomic dead space represents the volume of gas inside of the body during a breath that cannot participate in gas exchange because it fills the conducting airways and doesn’t have the opportunity to interface with the alveoli. It essentially extends from the mouth to the terminal bronchioles where we begin to see alveolar sacs. One intervention to reduce this space is to use airway devices which eliminate anatomic dead space in the upper airway. Another way to reduce anatomic dead space when there is a need to increase minute volume (or minute ventilation, the volume of air moved in one minute) is to increase the volume of each breath (tidal volume) instead of the quantity of breaths (respiratory rate or frequency). This is due to the idea that the amount of anatomic dead space per breath is relatively fixed, so introducing more breaths adds dead space while adding volume to existing breaths simply adds to alveolar tidal volume.
The next type of dead space is alveolar dead space. Alveolar dead space refers to the volume of air within alveoli that is unable to fully participate in gas exchange due to some restriction of blood flow to those alveoli. Essentially the air within those under-perfused alveoli is wasted or not able to be used to its full capacity. Interventions to address alveolar dead space depend on the cause of the under-perfusion and are shown on the right hand side. Another term noted in this graphic is physiologic dead space. Physiologic dead space is the sum of anatomic and alveolar dead space and describes all of the dead space within a patient’s body.
The final type of dead space to demonstrate is mechanical dead space. Mechanical dead space, also known as equipment or apparatus dead space, is an additional quantity of dead space created by the devices we add into the system in order to administer treatments and interventions. Mechanical dead space is most relevant when using smaller tidal volumes (such as with pediatric patients), as the effect of mixing expired air with incoming air is more pronounced the smaller breaths are. Fixes include removing unnecessary items from a circuit and using appropriately sized pediatric devices.
And then to wrap it all up, here’s one last graphic that combines the various types of dead space into one visual:
Further reading on the idea:
And one final closing thought: if you’re interested in creating content like this, we’d be happy to both help and share via our platform. The plan is that we’ll eventually be able to award continuing education to folks who contribute in this way. For more on all of that, check it out at www.rykerrmedical.com/coned.
Addendum, Sep 2021: The portion of the pie-charts labeled “clinically relevant tidal volume” had been previously notated as “alveolar tidal volume,” but we made this edit because we realized that such a label technically isn’t 100% accurate. Alveolar tidal volume includes both volume utilized (clinically relevant tidal volume) and volume wasted (alveolar dead space), so we needed a new descriptive term. Not sure if there’s a better term or one that already exists out there in the universe, but we’ll ask around and update this as needed.