Anatomically Calculated

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Sabrina Kroft
Sabrina Kroft
Registered Respiratory Therapist for 18 years working in all areas of healthcare. Professional interest is the nuances of mechanical ventilation and just learning new/old technology.

The Pre-brief

Anatomical dead space specifically refers to the volume of air located in the respiratory tract that are responsible for conducting air to the alveoli and respiratory bronchioles but do not take part in the process of gas exchange.  These areas include upper airways, trachea, bronchi and terminal bronchi. The anatomical dead space is said to stay about 2ml/kg of ideal body weight for your entire adult life, but in the infant it is larger proportionally to body weight, around 3ml/kg in early infancy.  

Anatomical Factors

Fowler’s dead space measurement is dependent on the patient’s size. Though anatomical dead space is considered a fixed quantity, the patient’s age, posture, and pathology can affect the actual measurement. 

  • Age- extrathoracic causes in infants and children increase dead space compared to adults.
  • Posture/Positioning- Dead space decreases with the supine position increases during the sitting position. 
  • Flow- changes in gas flow can cause dead space volume to change. High flows reduce/clear out decreasing the portion of dead space that is rebreathed.  
  • Pathologies- the constriction or dilation of the bronchioles is thought to cause a variability of anatomical dead space. 

Approximation of anatomical dead space can be done with the patient’s body weight in pounds.  Dead space is an intrinsic part of volume capnography and a significant value for clinical situations. Anatomical dead space is only part of physiological dead space. Understanding the importance of dead space and its indication of overall lung function will prove beneficial to patient outcomes. 

Evaluating and treating

Assessing the dead space throughout ventilation and treating with high flows is a key mechanism for decreased anatomical dead space. Clearance of expired air causes a reduction in dead space, but it is dose and time dependent.  Flow (dose) affects the time (respiratory rate). Selection of proper flow will reduce the respiratory rate. A reduction in respiratory rate has shown to correlate to a reduction in dead space and improved alveolar ventilation.  Adjusting flows to meet the inspiratory demands of the patient and reducing the work of breathing will reduce the rebreathing and improve wash out, thus, decreasing the dead space. Assessment of minute ventilation is a better predictor in the reduction of anatomical dead space with high flow therapy than FiO2. The role of FiO2 in normoxemic and hypoxemic patients has yet to be determined.  

Anatomical dead space is just a piece in physiological dead space. Understanding each piece separately and their role will allow for optimal treatment and improved results.

The Debrief

  • Anatomical dead space is a part of the upper airways and does not take part in gas exchange 
  • Knowing the causes that affect your dead space will improve patient outcomes
  • Clearance of anatomical dead space in acute respiratory distress situations can decrease the patient’s work of breathing

References

  1. Quinn M, St Lucia K, Rizzo A. Anatomy, Anatomic Dead Space. 2021 Feb 7. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan–. PMID: 28723045.
  2. Fowler, Ward S. “Lung function studies. II. The respiratory dead space.” American Journal of Physiology-Legacy Content 154.3 (1948): 405-416.
  3. Brit Long, Stephen Y. Liang, Skyler Lentz, High flow nasal cannula for adult acute hypoxemic respiratory failure in the ED setting, The American Journal of Emergency Medicine, Volume 49, 2021, Pages 352-359, ISSN 0735-6757, https://doi.org/10.1016/j.ajem.2021.06.074.
  4. Möller W, Feng S, Domanski U, et al. Nasal high flow reduces dead space. J Appl Physiol (1985). 2017;122(1):191-197. doi:10.1152/japplphysiol.00584.2016

 

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